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2019-2020 Assessment of the Army Research Laboratory (2021)

Chapter: 6 Propulsion Sciences

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Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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6

Propulsion Sciences

The Panel on Propulsion Sciences at the Army Research Laboratory (ARL) conducted its review of selected research and development (R&D) projects of the ARL propulsion sciences research core competency during a virtual meeting on June 8-11, 2020. The research areas reviewed were platform power, platform design and control, and intelligent maneuver.

As described by ARL, the Combat Capabilities Development Command (CCDC) ARL’s research investments in the propulsion sciences core competency are focused on gaining a greater fundamental understanding of advanced propulsion and platform technologies to inform and enable enduring cross-domain maneuver of air and ground (manned and autonomous) operations in multi-domain operations (MDO).1 This serves as a key enabler of innovative vehicle configurations and subsystem architectures that are critical to the future Army’s movement and maneuver. Knowledge gained through fundamental and early applied research efforts will lead to technologies that overcome the constraints on design, fabrication, integration, control, propulsion, and vehicle intelligence to support reliable mobility and ensure power projection superiority for the future Army. The propulsion sciences core competency builds on fundamental pillars of science and technology (S&T) to conduct research in manned and unmanned Army air and ground platforms that are critical to the future Army’s movement, sustainment, and maneuverability to “operationalize science for transformational overmatch.”

The goals of the propulsion science core competency area are to discover, innovate, inform, and transition S&T-enabled capabilities that significantly increase the force effectiveness of the Army’s air and ground manned and unmanned systems in future MDO and cross-domain maneuvers.

The vision of the propulsion science core competency area is to create disruptive and game-changing vehicle-centric technologies for the Army through utilization of ARL and partner expertise and facilities in combustion physics, propulsion sciences, aeromechanics, intelligent mechanics, vertical take-off and landing (VTOL) design and analysis, aviation component and structural materials, autonomous systems, robotic mobility and manipulation, and state-of-the-art modeling and experimental facilities.

To realize this vision, the propulsion sciences core competency is organized by ARL around three primary research areas in platform power, platform design and control, and intelligent maneuver. Multidisciplinary approaches explore the intersections and span all three research areas in order to create disruptive and game-changing vehicle-centric technologies to inform the Army’s future vertical lift platforms, next-generation combat vehicles, and robotics and autonomous systems.

PLATFORM POWER

The platform power research area focuses on understanding and exploiting innovations in energy sources, storage, generation, conversion, transmission, distribution, and management. The goal of this

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1 See https://www.arl.army.mil/what-we-do/foundational-research-competencies/, accessed August 25, 2020.

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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research is to provide power and propulsion technologies and configurations to enhance the Army operational effectiveness of both manned and unmanned air and ground systems through improved reliability and efficiency, increased power densities, expanded range of fuel sources, and reduced power source acoustic signatures, all of which are critical in ensuring Army power projection superiority.

Accomplishments and Advancements

Overall, the quality of the briefings and posters in the platform power group was very good. The overall research program is commendable, with high scientific quality. ARL has made remarkable progress in implementing the Army Research Laboratory Technical Assistance Board (ARLTAB) recommendations from the 2018 assessment in certain areas, such as establishing collaborative relations with universities and national laboratories in the study of ignition and combustion characteristics at altitude conditions for various fuels and fuel blends of interest. Collaborative efforts have helped to improve the technical quality of the program and made use of state-of-the-art experimental facilities and models to achieve the objectives. The program now reflects a better balance between experimental and theoretical approaches, the latter including computational efforts. ARL would now benefit from seeking to improve the in-house capabilities in these areas via interactions between ARL researchers and their collaborators.

The program on tribology and lubrication science is well formulated and aimed at understanding fundamental physics. This program has demonstrated commendable use of experimental facilities in fundamental research. Application of this capability to study the failure mechanisms for fuel pumps is an excellent application of fundamental capability in resolution of relevant problems—for example, operation using different fuels and under different atmospheric conditions.

The research program on development of coatings for austere environments addresses a very relevant problem; however, its objectives are overstated.

There were several excellent presentations by tenured and early-career researchers; this indicates a positive, material response to the ARLTAB 2018 recommendation to pursue developing a robust workforce pipeline.

Effects of Fuel Blending on Ignition and the Impact for Simplified Ignition Models

ARL has made very good progress in implementing the recommendations provide by the ARLTAB in 2018. In particular, the research program in ignition behavior of blended and alternative fuels has made remarkable progress in establishing collaborations with Sandia National Laboratory (SNL) in experimental as well as computational efforts. The group has collaborations with Argonne National Laboratory (ANL) and university partners as well. Collaborative efforts have helped improve the technical quality of the program and made available state-of-the-art experimental facilities and models to achieve the objectives. The program now reflects a better balance between experimental and theoretical approaches, the latter including computational efforts. The research team is improving in its experience level and is making good use of the complementary capabilities available from SNL. There is room for improvement in the technical quality of the work via a clearer articulation of the motivation and challenges. Collaborations with universities have helped to cover overlapping pressure-temperature phase space. There needs to be a demonstrated effort to co-publish with collaborators in first-level journals as a matter of raising the technological image and impact of ARL research.

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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Spray Breakup and Atomization in a Combusting Environment

Applying X-ray imaging to study spray breakup at the nozzle exit under realistic combustion conditions is an exciting new area of research for ARL. The presentation clearly laid out the inadequacies of current laser and other optical diagnostic techniques. The ability to visualize how spray evolves after leaving the nozzle is exciting. This experiment generated a tremendous amount of data owing to high speed and spatial resolution. The present data analysis methods need to be improved to integrate the various data acquired into a dynamic and multidimensional description of reaction inception, and rate of heat evolution in multiphase combustion conditions.

Understanding Energy-Assisted Ignition and Its Sensing and Control Methods

This work represents a part of a larger set of projects arrayed around diversifying the fuel supply to have the ability to use a wide array of fuels that might be available in different places or from different sources. This particular project is based on the fact that some fuels have lower ignitability, and some method of coercing those fuels to burn is necessary in compression ignition (e.g., diesel) engines. The method used is a hot surface, which can reduce the time for a fuel charge to ignite and generate heat. The hot surface ignition principles could align the heat release profile appropriately with crank angle to generate optimal power conditions for diesel combustion conditions. This work is part of a set of projects dealing with fuel for compression ignition engines. The objective of this project is to sense whether the engine or even a specific cylinder is igniting properly and to light up the ignition assistant if it is not.

The ARL group is collaborating with an appropriate selection of university investigators. It is a broadly based investigation by ARL into the use of fuels that is easy to employ and would be necessary for military engines in the future.

Nonintrusive Ignition and Combustion Sensing for Engine Feedback Control of Variable Energy Assisted Compression Ignition Engines

This work focused on unmanned aerial systems (UASs) and their respective fuels. The challenge is the wide range of combustion characteristics for multifuel-capable engines. To address this challenge, new sensor technology is being developed to capture combustion properties of reacting systems in nonintrusive ways such that the harsh environment of the combustor does not influence or destroy the sensor and measurements. The work scope replaces an in-cylinder pressure sensor with an outside situated vibration sensor. The advancement here is in making measurements that correlate pressure readings to vibration readings. The outcome is very promising in that some fuels depending on cetane number and enthalpy release behavior correlate very well with the pressure sensor measurements—that is, start of combustion based on pressure aligns with vibration. This is innovative work. Much data still need to be collected and analyzed and applied to real-time engines. This work could benefit from a more rigorous data analysis.

Multi-Injection Mixture Preparation and Ignition in Unmanned Aerial Vehicle Intermittent Combustion Engine Conditions

This poster presentation complements the technical briefing discussed above. The two presentations together do a good job of providing a more complete picture. The comments above encompass both presentations.

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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Development of a Hybrid-Electric Propulsion Optimization Tool

This work focused on developing an optimization tool for the propulsion system of hybrid-electric vehicles. The approach was to develop optimization algorithms that consider empirical and theoretical models of components within the engine system—engine, motor, energy storage, and thermal management. Preliminary results suggest great sensitivity to the thermal management system. The presentation shows only simulation results. Some simulations were validated by experimental engine results at altitude. Future work needs to develop more experimental observations on which to test simulation predictions.

Design and Analysis of Non-Contact Magnetic Gears

This work is done in collaboration with Texas A&M, a well-known expert in magnetic gears. The topic is not, in itself, novel, as magnetic gears have been a topic of interest for some years. However, the objective of the ARL project is to improve the power-to-weight ratio. The team has chosen to use Halbach array magnets to eliminate the need for back iron.

The choice of Texas A&M as a partner is good. The investigator is well qualified to do this work, and the collaborators in academia are among the most well known in the field of magnetic gears. This background and the ARL uniqueness were adequately articulated.

Numerical and Experimental Investigation of Vibrations in Compact Aviation Turbomachinery

This work is about a failure that occurred in a turbo compressor operating at altitude—it appeared to be a high-cycle failure of compressor blade tips, likely because of vibration. The vibrations of the blades were sensed by one of the three accelerometers, and a separate sensor determined rotor position so that individual blades could be identified.

This experimental work is of high quality, comparable with other leading laboratories, and the investigator is very well qualified. The equipment used in the experimentation is of high sophistication. This work is integrated with other investigations at ARL.

Aluminum Superalloy Structures and Processing Science for Propulsion

This is a metallurgy-based study of materials for use in low production volumes for unmanned systems. The materials were subjected to higher combustion pressure and duty cycles that increase stresses. Commercial alloys based on Al lose strength under these thermal conditions. New ternary mixtures are being considered and produced based on Al, Ce, Zr, Mg, Cu, Mn, and others. Results suggest that Al-Ce-Mg properties have good potential. Results reveal mechanisms for strengthening that have implications beyond this application. The approach has been successful based on the multiscale attention and investigation of the microstructure. Two specialty techniques bring new insights into this work: in situ X-ray measurements of crystallization during laser fusing of metal powder beds—that is, metal 3D printing—and atomic probe microscopy that adds nanometer-scale information.

This work was very well formulated, and results show great potential for this application and the science of materials in general.

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
×

Surface Characterization and Modeling of the Interfaces Within High-Pressure Fuel Pumps

High-pressure fuel pumps have many sliding interfaces, which are susceptible to wear damages, and the only lubricant is the fuel. Thus, as fuels are varied toward lower viscosity with inferior lubricity, the wear can be more severe. This work performed modeling combined with component-level experiments. One of the unique capabilities of ARL is an in-house experimental fuel stand that can test commercial high-pressure (2000 bar) fuel pumps, which is a big asset for this work.

Determination of Method for Tribological Experiment on Ultra-Hard Coatings in Low-Viscosity Fuels

This work is related to other investigations into the use of alternative fuels in reciprocating engines. This investigation is of hard surface coatings to reduce wear in the parts of high-pressure fuel pumps. At this stage, standard tribological methods were employed, and certain coatings were investigated with a few fuel surrogates.

The investigator was a rising graduate student. The objective of the project was limited to developing an understanding of how hard materials wear under conditions of different fuels and atmospheres. The presentation was clear and has yielded useful results. One interesting aspect was that the tribological experiment was in a glove box so that oxygen content of the surrounding atmosphere could be controlled.

No collaborators outside ARL were identified. However, this work is just getting started. The work appears to be of high standard, comparable to what would happen in top universities, and the equipment employed was good and of current state of the art.

Challenges and Opportunities

The state-of-the-art experimental facilities and innovative approaches have generated a large amount of data. A valuable investment would be to build up in-house data analytics capabilities to better harness new knowledge from these results.

Before delving into the technical details, the researchers need to have a good understanding of current traditional competing technologies. Such knowledge will lead to clearer articulation of the proposed approach.

Hybrid-electric drive encompasses power electronics and electromechanics, and is an important component of propulsion science. This area needs to be integrated in the program portfolio.

Dimensionless quantities Re, We, and Oh were used in several presentations, which is an improvement over previous ARLTAB reviews. ARL could use dimensionless numbers more broadly, beyond fluid dynamics and across other propulsion science programs.

All the programs have followed the ARLTAB recommendations from the 2018 assessment to augment modeling components to complement experimental investigation. A caution on numerical simulations is that they can be forced to agree with experimental data and lose grounding to the physical world. ARL also needs to maintain and develop core competency capabilities in house in addition to collaborations with external organizations.

Spray Breakup and Atomization in a Combusting Environment

The team needs to apply data analysis tools to analyze the large amount of data and describe heat release physics using a dimensionless number. This will permit generalization of the results.

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
×

Development of a Hybrid-Electric Propulsion Optimization Tool

The optimization algorithm was not discussed, so a technical assessment of the approach cannot be made. While empirical assessments of performance and optimization of a multicomponent complex system has merit; this research needs to be compared to experimental findings to assess the validity of the approach.

Design and Analysis of Non-Contact Magnetic Gears

The group needs to further examine its assumptions because it is known that Halbach arrays produce useful flux densities somewhat below those of radially magnetized magnets with back iron, and that Halbach arrays take up the whole periphery of their region and so are a bit heavier. That said, Halbach arrays do not require back iron. On the other hand, the required back iron may not be very thick for a radially magnetized multipole array, and the stronger flux density from a radial magnet will have an impact on developed force density.

Combustion-Based Solid-State Portable Power

This research is on system-level modeling to understand energy flow in a proposed device to burn liquid fuel to generate electricity. The rationale behind this proposed device is for quiet power generation to charge batteries, for example, in unmanned vehicles. The proposed device relies on two steps of energy transformation—burning fuels to heat an emitter (chemical to thermal energy) followed by conversion to electricity of the thermal emission with a thermal photovoltaic (TPV) cell (thermal energy to electricity). There is also a filter between the photoemitter and the photoreceptor, but it was not clear what that filter is. Owing to the inefficiency of these transformations, the combined system efficiency is about 5 percent. The maximum combined system efficiency reported in the literature is 5 percent. Most devices are around 1 to 2 percent. ARL’s target system efficiency, which is conservative and based on what has been demonstrated for the individual components that would compose a TPV system, is 10 percent.

Comparison with traditional generators (chemical to mechanical to electrical energy), for example, using a Stirling engine, needs to be made to articulate the proposed approach. While the focus was at the system level, explanations on the selected materials and components would have been helpful for the panel to understand and evaluate the project.

Developing Methods for Understanding Material Behavior in Fuel-Lubricated Interfaces to Match Fuel Pump Observations

This is a new ARL effort to understand how different fuels affect the wear properties of an engine. There is a clear difference between testing done in air versus in nitrogen, as well as among different fuels. In addition, many electron microscopy images were shown, but acquiring 3D analysis such as the depth and roughness of the scars (for example, profilometry or optical interferometry) would add value to the research.

Numerical and Experimental Investigation of Vibrations in Compact Aviation Turbomachinery

While the overall program related to turbochargers involves collaborators at highly ranked universities and industrial firms, it was not apparent that this experiment had any collaborators outside

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
×

ARL. The researchers need to review the comments provided later under the turbocharger aeroelasticity at altitudes program, as many of those comments are applicable here.

Durable, Stable Ceramic Materials for Propulsion Systems in Austere Environments

This work is to develop sand-phobic ceramic thermal and environmental coatings for turbines. The material proposed is yttrium stabilized zirconia (YSZ) infused with Gd2O3 particles, leveraging YSZ’s mechanical properties and rare earth oxide’s resistance to sand. The preliminary results are not solid enough to draw any conclusions about the promises of these materials—reaction products were not observed, which might be owing to inadequate reaction conditions, and accumulation results do not appear to correlate with GdO concentration. Hence, the project is far from the stated objectives of 25 percent improvement in efficiency and 40 percent in durability. ARL has engaged key stakeholders in this project—the Department of Energy (DOE), Pratt & Whitney, Rolls Royce, Naval Air Systems Command, and the Environmental Protection Agency (EPA). The recent strategic environmental research and development (R&D) program will help advance the project. Two suggestions for improvement are (1) surface chemical properties such as composition (e.g., using depth-dependent X-ray photoelectron spectroscopy or secondary-ion mass spectrometry) and surface energy (e.g., using high-temperature contact angle measurements) need to be better characterized; and (2) testing needs to be conducted with different kinds of sand.

Diagnostic Signal Features from Aircraft Propulsion Bearings in Accelerated Aging Experiments

This project developed a large volume of data indicating features of aging that affect the performance of bearing materials. The cited work in this area did not appear to have any domestic collaborators. ARL needs to recruit and develop U.S. expertise in the area of crack detection in bearings.

Turbocharger Aeroelasticity at Altitudes

This program aims to develop computational models of unmanned aerial vehicle (UAV) turbochargers to predict and remove the onset of fluid-structure interactions causing turbocharger failure. The problem is very relevant, as the turbocharger failures have resulted in UAV crashes. The approach adopted in this program includes a high-fidelity simulation tool encompassing the entire turbocharger—that is, compressor, turbine, rotor shaft, and housing. This model uses computational fluid dynamics and finite element analysis (FEA) methods. The presentation also points to a reduced order aeroelastic lumped parameter (ALP) model. No details of this ALP were provided. The presentation provides three references that form the basis for this model. However, the cited literature is focused on stall and surge in compressors, and it is not clear how that would relate to aeroelastic behavior of turbine blades in the turbocharger. This program needs a focused review of the approach. The two publications listed within this presentation are titles for two conference presentations that are scheduled to occur at some time in the future. The two titles are also almost identical. The researchers need to strive for publications in high-quality journals.

Numerical Methods for Modeling Complex Composite Architectures in Representative Volume Element Development

This work focused on developing a model to accurately predict material responses based on complex architecture. The goal is to move toward greater weight reduction in future vertical lift applications. The

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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study was numerical in nature to down-select experimental approaches that would otherwise be expensive and time consuming. The challenge is that complex loading conditions in a gear or transmission require quasi-isotropic triaxial braided matrix—but geometric modeling of braids without penetrations is difficult. The approach is to use microscopic images to represent architectural defects that lead to non-quasi-isotropic behavior and use novel thermal expansion and compression challenges of high-volume fraction triaxial braids with a linear elastic material model. Results are preliminary and show some promise in comparison to experimental measurements of moduli for isolated directions. Continued development of this model needs to take place with attention to the mesocopic scale that may reduce simulation errors.

PLATFORM DESIGN AND CONTROL

The platform design and control research area focuses on fundamental research that enables the development of the highly maneuverable and tactically adaptive platforms. The research is expected to impact a wide array of vehicle technologies, cutting across the ground, air, and maritime domains, as well as vehicle classes from large- to micro-scale device technologies.

The platform design and control program had nine technical briefings and eight posters presented during the panel meeting. The research topics reviewed included conceptual design and sizing tools for next-generation unmanned rotorcraft configurations and propellers, rotor control methods, trajectory guidance and control, fluid and structural dynamics and fluid-structure interaction, and reconfigurable and tailorable structures.

Accomplishments and Advancements

Overall, the platform design and control research program adds considerable value to the body of R&D knowledge. The laboratory leadership is very connected to projects, objectives, and deliverables. ARL has acted effectively to address the comments and recommendations made by ARLTAB in its 2018 assessment. ARL’s focus on modeling small-scale unmanned systems is necessary to develop and employ these emerging machines, and to identify, assess, and pursue relevant breakthrough technologies.

The materials and structures research, with an emphasis on incorporating artificial intelligence (AI), machine learning (ML), and deep learning (DL), represents pioneering of complex, high-risk areas of basic research. This demonstrates that discovery can be disruptive and can have wide-ranging implications for science and innovation in synthesis, and this research needs to continue. Several topics presented represent ARL’s extended strategy to explore disruptive topics outside the current core competencies and to identify promising future directions for the laboratory.

While some work is not balancing theory with experiments and simulations, a particular exemplar is the microstructure deep learning that is developing AI/ML approaches by fusing simulation and experimental data. The molecular dynamics modeling in both the computational design of shape memory polymer actuation and the tailorable and multifunctional dynamic polymer networks can provide data to implement AI methods. Dynamic polymer networks research is a unique combination of morphing, self-healing, and shape memory. The non-equilibrium molecular motor research is on the leading edge of bio-hybrid basic research and will help inform ARL in shaping its future portfolio.

The emerging R2C2 facility is a highlight for unmanned aerial system (UAS) experimentation, capitalizing on the opportunity to offer local capability to develop autonomy for the extreme UAS vision. The nascent intelligent mechanics initiative can be disruptive, exploring the historical distinctions between proaction and reaction, such as adaptation, reconfiguration, and control versus deformation and failure.

There are a number of talented early-career researchers who are capable in their disciplines, and they will benefit from mentoring and experience to engender a greater system-level perspective. For example, cross-discipline rotations can be implemented and the technical breadth needs to be expanded.

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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Tilt Rotor Aeroelasticity Analysis

This is the analysis portion of a combined computational and experimental program to assess the capability for predicting the dynamic instability boundaries for tilt rotor aircraft. The experimental portion of the program has been delayed owing to the COVID-19 pandemic. Dynamic instabilities can lead to catastrophic failure of wind tunnel models, as has happened for prototype rotorcraft in the past. This combined computational and experimental program addresses these critical issues.

Predictions of the dynamic instability boundaries have been made using potential flow aerodynamic models with various representations of the rotor inflow. In addition, several methods have been assessed for extracting frequency and damping of critical system modes from the computational data.

Stacked Rotor Aeromechanics and Acoustics

This is a good exploratory study with improvements observed for metrics including increased efficiency, a thrust change that could be used for control, and reduced acoustics emissions.

Hybrid Design and Analysis of Rotorcraft (HYDRA)

This research constitutes an excellent shift of applicable systems analysis from what was largely empiricism to a far more physics-based approach. Such a shift was necessary for comparative studies of new design precepts and has already proven useful in that regard.

Conceptual Design of UAS with Dissimilar-Size Rotors

HYDRA improves upon the National Aeronautics and Space Administration (NASA) Design and Analysis of Rotorcraft conceptual design tool for Group 1-3 UAS by adding physics-based rotor blade and airframe weight models, support for airfoil tables, increased fidelity rotor power modeling, and rotor/wing/airframe interactional aerodynamics. It combines FEA, blade element momentum theory, a powerful vehicle trim function, and semi-empirical power plant models. This effort opens the aperture on design of distributed propulsion rotorcraft by modifying the HYDRA tool to assess performance of a broad design space of rotor geometry and operating conditions.

The modeling predicted performance improvements available with dissimilar rotors and provided guidance on how the rotor dissimilarity decisions differ with scenarios description (e.g., extent of hovering), but awaits validation through correlation with experimental data.

Common Research Configuration

The common research configuration (CRC) initiative seeks to create a reference vehicle concept for scalability studies across disciplines such as design, aeromechanics, acoustics, and flight control. An additional goal of the initiative is to facilitate collaborations with research partners.

The team has performed a UAS design space exploration to select the quad bi-plane tail sitter configuration for maximum value and as best suited for a research environment. The conceptual design analysis has been performed for a notional scenario at different scales. Additionally, 3 lb and 20 lb configurations have been fabricated and flight-tested to acquire performance data to support analytical efforts.

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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The use of CRC both for in-house research projects and by multiple research partners has shown that CRC is proving to be invaluable to create a community of interest to advance collaboratively the state of the art in Group 2‐3 UAS platform and component technologies. It is excellent that the reference models for Group 1-3 UAS have been utilized by universities investing their research funds, and these relationships need to be continued and enhanced with national laboratories, commercial industry, and other governmental agencies.

Sizing and Performance Tool

This study focuses on physics specific to air launched effect (ALE) class propellers and development of a computational tool for rotor sizing, performance, and optimization analysis to support the ALE concepts, trades, and analyses (CTA) study.

Performance for the two rotors, the ALTIUS UAS propeller and the TRV-150 UAS rotor, were measured to provide data for analysis tool validation. The ALTIUS UAS propeller data were test data from CCDC Aviation and Missile Center and ARL provided computational data. For TRV-150, the data were test data and computational data. Wind tunnel tests of this rotor with various tunnel speeds, rotor speeds, and rotor pitch settings have been completed. A hover test stand has been set up at the University of Maryland, and testing of a matrix of rotor speeds and pitch angles was in progress.

Performance of both rotors was computed using both FUN3D and Helios computational fluid dynamics (CFD) analyses. The CFD analyses provide data for validation of the rotor sizing, performance, and optimization analysis tools for the cases where test data are not available and where flow details are difficult to measure. A rotor performance analysis tool, JBLADE, is evaluated using the test data and CFD analysis results. Based on the discrepancies identified in the evaluation, the JBLADE code will be enhanced to allow a high-accuracy rotor performance analysis over a broad range of operating conditions. The ALE CTA will be performed using the improved tool. This is a good example of using both experiment and simulation and leveraging the strengths of each.

Automated CFD Process for Airfoil Aerodynamics Characterizations

This research produced a method that provides a fast, robust grid for any CFD code and is therefore very valuable. The work was an initial step on the path to incorporating and improving the requisite physics. The meshing achieves structured mesh efficiencies with unstructured mesh robustness.

Turbulence Effects on Airfoil Performance at Low Reynolds Number

UAVs operate at lower altitudes, at low Reynolds number, and at low speeds, which induces separated flow. This work studies the impacts of free stream turbulence of a scale similar to the boundary layer thickness on separation control. The work makes good progress and finds that these effects are analogous to boundary layer transition—that is, imposed turbulence has similar effects to body-generated turbulence in delaying separation.

Tailorable and Multifunctional Dynamic Polymer Networks for Adaptive Structures

This effort explores the viability of engineering materials at the molecular level to make mechanically relevant structures. Employing highly adaptive multifunctional systems with tailorable response and structural elements that are configured based on environment, the research investigates whether a stimuli-

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
×

responsive dynamic molecule can leverage the intrinsic flexibility of the polymer backbone to achieve on-demand reconfigurability.

This promising research on modeling molecular dynamics is very impressive and a highlight of the laboratory, exploring the unique combination of morphing, self-healing, and shape memory. The early experimental results show promise and could be a game changer. The team could further use the molecular dynamics simulations to provide additional data to implement AI methods.

Non-Equilibrium Molecular Motor-Based Mechanical Actuators: Modeling the Attraction Between Two Charges in an Ionic Solution

This directed biology research is leveraging new techniques in microscopy to inform improved modeling of contractile mammal muscle tissue. The modeling of protein-protein attraction is being used to determine what forces are responsible for guiding molecular motors to their binding sites. This modeling is intended to enable the move from empiricism to first principles, to set the conditions for disruptive design for innovation in robotics.

The researcher brings a strong background in modeling nonlinear dynamical systems, and has developed a substantial understanding of the literature on molecular motors. This good, basic research seeks to establish a path to bio-hybrid robotics, bio-functionalism, and living devices. This research is informing ARL whether to include this in its future research portfolio. Success is not ensured, but this high-risk/high-reward research is worthwhile.

Microstructure Deep Learning

It is well known that material microstructure directly influences the physical properties of a material system; however, the specifics of these interactions can be difficult to quantify because they can be nonlinear and difficult to predict. This research focuses on applying modern deep learning (DL) techniques on scanning electron microscopy material microstructure images in an attempt to quantify relationships between material processing parameters, corresponding microstructure, and subsequent material properties. DL-based approaches applied to material systems are validated on simulated microstructure images of TiAlZrN ultra-hard nanostructured coatings and subsequent processing parameters. This effort is a good example of fusing the simulation data together with the experimental data to develop an AI/ML-based approach. The researchers need to add some theoretical results into the learning for a better understanding of the mechanisms involved.

Parallelized Fluid Structure Interaction for Optimized Aero-Structural Design

By prescribing a small family of structural deformation shapes, the pressure distribution can be computed from a CFD code. Also, by prescribing a small family of aerodynamic pressure distributions, the structural deformation can be computed from a finite element (FE) code. Using these separate fluid and structural data sets to create reduced order models (ROMs) for the flow and structure, these fluid and structure ROMs may then be combined to compute very efficiently the fluid structure interaction of the fluid structural system. This is particularly efficient and straightforward for static, linear analysis. Only static cases have been treated to date, and for the range of parameters investigated, the system response is in the linear range. This work is making good progress.

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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Deep Learning of Nonlinear Dynamics from Pixel Measurements

Deep learning attempts to extract patterns and models from large data sets. Even when the uncertainties in the data are not well understood, certain general principles, which the data ought to satisfy, are known. For example, conservation of energy might be required, or the system must obey Hamilton’s principle, or certain symmetries need to hold. In this work, these general principles are imposed as constraints on the data and on any models that are inferred from the data.

To date, simple examples have shown promise, and a structural beam experiment is planned to provide the next step in this research.

Computational Design and Shape Memory Polymer Actuation for Reconfigurable Aero-Structural Design

A coarse-grained molecular dynamics (MD) computation has been used to identify promising polymer material mixes that can be used as actuators. The materials have been used in experiments performed by university partners to measure desired macroscopic properties—for example, modulus of elasticity. While the MD simulations do not provide quantitative agreement with the experimental measurements, there is qualitative agreement and thus the MD simulations provide a useful guide to selecting promising material combinations.

Modeling and Control Methods for Future Vertical Lift Rotorcraft Fatigue Reduction

Rotorcraft undergo constant vibratory excitation owing to unsteady air loads. Controlling the vibratory loads is essential for improved performance and safety. This research seeks to develop rotorcraft loads analysis methods with fatigue damage estimation and fatigue reduction controllers for future rotorcraft. The research emphasizes the development of a simulation-based predictive capability by coupling a state-of-the-art rotorcraft loads prediction tool with fatigue damage analysis to aid in the development of a fatigue mitigation controller. Extensive modeling, simulation, and testing are conducted to evaluate the effectiveness of the fatigue mitigation controller and its impact on the primary flight controls to ensure that flight performance, quality, and safety are preserved.

A computer simulation using state-of-the-art models for aerodynamic loads, structural response, and an active control system was performed. It showed the possibility of significant reduction in structural stresses and hence extension of fatigue life. This is an excellent systems-level compendium of earlier government work on integrated vehicle health management and digital twin with maintenance-based design and flight.

Challenges and Opportunities

The materials work is fundamental, demonstrates significant discovery, and can be disruptive. It is discovery science. However, some work does not include all three components of theory, experiments, and modeling and simulations working closely together.

The review included an array of research on small UAS system-level assessment tools to support inserting high-fidelity modeling and expanding capability to a broader array of concepts. The platform modeling understandably does not involve scientific hypotheses, but it needs to be conceived and executed in way that will support research.

An overall approach to developing and integrating all of the various platform modeling and tools was not provided. There needs to be a tighter coupling. So what is needed is to develop a coherent strategy to

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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ensure that the tools are appropriately connected and converging, moving in parallel toward a robust capability that supports the laboratory objectives.

A number of opportunities and challenges apply in general across the air platform tool development portfolio, and an overarching plan can address them globally. The focus on rapid UAS development at small scale, with limited research on large aircraft, may not enable the laboratory to address adequately the stated objectives to assess whether technology scales, and to explore future increments for the current large aircraft developments.

The tools do not appear to be specifically focused on decision metrics such as detection and availability. ARL needs to identify and implement improved decision metrics. Specifically, weight is not necessarily the only appropriate success metric, especially for small UASs. The UAS research needs to recognize the issues, opportunities, and constraints associated with considering the off-board ecosystem and metrics such as sustainment, transportation, storage, warfighter survivability, and supply chain. While the tools are not anticipated to include these factors, consideration of them may result in small but effective and affordable changes that can support system-level decisions.

In general, the platform modeling is not exploring disruptive or visionary component technology and approaches—for example, aero-elastically tailored tilt rotor wings. There is an opportunity to be deliberate with the tools to serve as a means to assess the value of breakthrough technologies—for example, shape memory polymers.

There has been some work to insert high-fidelity models and modules into the methodology to address the multidisciplinary system needs, but it is not a systemic objective throughout. ARL needs to consider a systematic insertion of high-fidelity modeling to capture complex behaviors. The research efforts on aerodynamics and aeromechanics needs to increase the emphasis on accounting for unsteady effects, turbulence, viscous effects, boundary layer transition, and the difficulty with multiple length and time scales. This will enable ARL to address difficult UAS questions such as unsteadiness, gusts, complex environments, and interactional aerodynamics. These will all be critical, with intent to focus on exploration of small, unique UASs in extreme conditions and environments. ARL needs to emphasize use of dynamic and nonlinear extreme problems.

The design and control developments have weak ties to, and interactions with, the platform power activity. A tighter integration with the versatile tactical power and propulsion (VICTOR) activity needs to be undertaken.

There was very limited discussion of interfacing with other Department of Defense (DoD) laboratories or industry for similar or comparable platform design tools. Correlation of the methods and tools with other stakeholders is critical to ensure trust.

The communication regarding the tools appears limited to conference papers, with very few peer-reviewed journal papers. Model validation continues to be an ad hoc process in many of the projects. Some of the efforts lack discussion of prior relevant DoD research and interactions with researchers. The system development tools in general are not embracing AI/ML/DL into the approaches.

ARL needs to consider a more robust exploration of combining design and control. For example, ARL could consider structural deformation as an opportunity for control, and combine the evaluation of planned and intentional adaptation and control authority with unplanned deformation and failures. In addition, ARL needs to leverage integration and system tools to assess value of research thrusts.

The UAS in extreme conditions such as weather, terrain/buildings, and induced disturbances especially in degraded environments (e.g., obscured, jammed, and gusty) is considered a significant area of research to pursue, as the commercial sector may not make investments to work on issues unique to the Army. ARL needs to insert considerations for extreme conditions into all research, and explore means to enable experimentation of extreme environments and conditions.

A noteworthy example of a next-generation facility that allows for experimentation with extreme environmental conditions is the unique $35 million (CAD) WindEEE wind facility at Western University

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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in London, Ontario, in operation for several years.2 Other more affordable facilities are available, such as the Cal Tech drone aerodynamics facility.3 Although they are less capable, they offer the capability for testing conditions not possible in conventional wind tunnels.

Masking as a susceptibility feature is a platform consideration closely related to intelligent maneuver, so interactions with that research area need to be explored.

The focus on computational speed could be considered not just for optimizing during a nonrecurring design effort, but be used downrange for real-time configuration design adaptation, such as aggregating and disaggregating on-demand or for sustainment and degraded mode operation.

Other avenues for research could include disaggregating functions and capabilities (e.g., actuation) as is being done for platform capabilities such as swarms, machine ideation, and advanced actuation (e.g., passive and active flow control, smart materials, and plasma).

Tilt Rotor Aeroelasticity Analysis

The plan to use computational fluid dynamics (CFD) in the computations has been deferred for now. Various research groups in the national and global community have developed ROM methods for Euler and Navier-Stokes flows, and there is an opportunity to use such ROM methods in future work for this program using CFD-based computational models. In addition, the methodology developed for extracting frequency and damping of critical modes from computational data could be used for the future wind tunnel test program as well.

Stacked Rotor Aeromechanics and Acoustics

The researchers need to continue optimization studies over a greater number of independent variables and continue attempts to obtain better definition of the causative physics. In addition, the researchers need to explore the Defense Advanced Research Projects Agency (DARPA)/Bell/Boeing triple deuce stacked rotor research. In addition, the researchers need to address control dynamic response, dynamics, power plant implications, and design point and control margins, and focus on detection as the applicable metric for acoustics.

Hybrid Design and Analysis of Rotorcraft (HYDRA)

The researchers need to continue, as available machine capability allows, improving the physical fidelity. The researchers need to address structural stiffness, unsteadiness and turbulence effects of drone/drone and drone/environment interactions, and assessment of degraded modes and failures. In addition, the researchers need to consider utilizing the tool to assess the application value of ARL technologies.

UAS Transitioning Trajectory Guidance and Control

This effort is developing a simulation of the flight dynamics of a tail-sitter aircraft with low computational cost using reduced order modeling with wake-interference based on momentum theory.

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2 See https://windeee.com/, accessed August 14, 2020.

3 See https://www.caltech.edu/research/research-facilities, accessed August 14, 2020.

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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The research will use high-fidelity models and experiments to identify and validate relevant vehicle constraints; derive a differentially flat, low-order trajectory planning set of equations of motion for inflight trajectory planning; and simulate an optimized trajectory planning and controlled transition in real time. Then, additional constraints will be added such as acoustics and obstacle avoidance. The research focuses on a quadrotor biplane tail sitter to leverage data available from other ongoing research.

Based on the history of high-speed VTOL aircraft on the vertical and/or short take-off and landing (VSTOL) wheel, for which adequate control and authority through conversion and flight transition was predominantly the critical flaw, this course of research is important with the range of high-speed and long-endurance VTOL configurations.

It is important to recognize that this effort to use model-based control for optimal autonomous platform transition between flight states—more robust, smoother, quicker, more efficient, more reliable, and more predictable—is not specifically about behaviors, but about optimizing platform operation. It is also important to recognize that the transition of this capability for large-scale tail-sitter platforms needs to occur to industry platform developers.

The research focuses on a quadrotor biplane tail sitter. However, at all times the ability to expand beyond this configuration needs to be manifest. The effort needs to identify how this modeling to define a well-designed tail-sitter inner loop is extensible to enable sophisticated autonomous capabilities for a wide range of high-performance VSTOL platforms with complex dynamics.

The effort needs to move beyond just 2D trim to consider control authority in the presence of full 3D effects, transients (e.g., gusts), component dynamics, and the relevant characteristics of dynamic components.

The relationship to the HYDRA platform assessment and synthesis tool needs to be established and emphasized. In addition, the capability to use this tool to develop design guidance to inform and refine new platform designs (e.g., required downwash and wing aspect ratio combinations) needs to be considered.

The plan for wind tunnel validation is needed, to validate the use of a simple dynamics model, and model-free control.

Common Research Configuration

The plan to include flight dynamics needs to also include structural dynamics, because Group 3 aircraft are an order-of-magnitude and a half bigger than Group 2 aircraft and dynamics considerations are in play. Weight is not an appropriate success metric, especially for small-scale UASs. The research needs to include environmental conditions, especially degraded conditions such as obscured, jammed, and gusty.

Conceptual Design of UAS with Dissimilar-Size Rotors

With the concerns regarding the tool focusing specifically on rotorcraft, the transition to the National Ground Intelligence Center for use appears early and needs to be accompanied by a thorough accounting of the level of confidence in the methodology and the tool based on validation and verification. ARL needs to ascertain what organization has the authority to accredit such a tool.

The effect of the dissimilar rotors on the control system and dissimilar torque on the power plant motors and motor controllers needs to be explored, especially with the use of semi-empirical motor models. Exploring high-fidelity modeling and integration with the VICTOR activity is needed.

The impact of 3D effects and transients needs to be explored.

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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Sizing and Performance Tool

The researchers need to explore opportunities to include more industry participation, as a minimum in an advisory role.

Isolated pusher prop wind tunnel testing appears to consider only body effects for tares and interference, not the interactional and interference effects on the propeller model (as JBLADE does not have a body model), so there is a need to explore the effect of the body, with CFD or an empirical model. It appears that the tool selection was based on a narrow scope. No future experiments were discussed. The researchers need to continue iterating both modeling and experiment.

Automated CFD Process for Airfoil Aerodynamics Characterizations

As available machine capability increases, the researchers need to move on to additional levels of high- fidelity physics modeling to finalize optimization over the parameter space. Also, the researchers need to explore speed variations between blades and 3D effects.

Turbulence Effects on Airfoil Performance at Low Reynolds Number

This work suggests that atmospheric boundary layer turbulence will delay separation at UAS flying conditions. The literature indicates that turbulence in the atmospheric boundary layer scales with altitude and that such scales are thought to be too large to significantly delay separation at most UAS flying conditions. Therefore, further research is required to finalize to what extent atmospheric boundary layer turbulence scales will delay separation at UAS size and flying conditions. In addition, the researchers need to extend the wind tunnel studies to oscillating airfoils and to the effects of atmospheric turbulence gusts using active turbulence generation.

Reconfigurable Aero-Structures

This research explores structural adaptation using additive manufacturing to achieve span morphing of a fixed-wing UAS. The premise is that adapting the configuration can offer a unique balance of capabilities, such as long endurance of high wingspan with the capability to enter or traverse narrow spaces with short span. The stated need is a design tool to assess the relevance of adaptable structures.

There was very limited discussion of the breadth of previous similar span-morphing DoD research, and so relevant prior research needs to be reviewed.

This research is focused on generating empirical data using available resources, and has only recently begun to develop models for coupling with experiment. The research vision could be expanded beyond adaptation alone to consider how it could benefit other technologies, such as exploring bandwidth to understand capability to enhance the application of flow control.

The research is constrained by conventional engineering design perspectives, and needs to explore the impacts and opportunities with moving beyond these. For example, the challenge to achieve sufficient structural stiffness to preclude deformation could be replaced with the consideration of deformation as a means of control. In addition, the research could explore synergies of adaptation with other advanced approaches, such as flow control.

In addition to generating experimental data, the researchers need to explore theory accompanied by modeling for a robust research effort. Simulation needs to be in hand to shape and substantiate experimentation. A broader perspective on the means of construction and adaptation needs to be considered, beyond mechanical kinematics of additive manufacture for wingspan variation.

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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The opportunities and constraints of specific applications need to be considered, because these affect the adaptation needs and therefore feasible solutions. Notable examples are reversibility—deploy and retract yields different approaches than deployment alone—and control bandwidth (e.g., adaptive/slow versus active/rapid).

Tailorable and Multifunctional Dynamic Polymer Networks for Adaptive Structures

There was limited discussion of modeling. The research needs to ensure a balance of theory, modeling, and experimentation.

It is critical to consider fatigue properties, and the potential to extend the capability of shape memory to multiple cycles.

While the effort focuses on the polymer to perform as a self-actuated adaptive structure, a very valuable interim achievement would be a bi-modal adaptive structure that relies on external actuation, so these applications could be considered.

The promise of self-healing of the weaker chemical bonds that fail first is enticing, but the value proposition must acknowledge the design paradigm for structural materials and assemblies that fasteners and bonds need to be stronger than the substrate.

The research needs to explore a broad range of controllable stimuli to change molecular states, beyond thermal and a bit of pH, to include light and current and electromagnetics, perhaps in concert.

Non-Equilibrium Molecular Motor-Based Mechanical Actuators: Modeling the Attraction Between Two Charges in an Ionic Solution

This effort appears to be exploring from the technology side to get to an understanding of biology; it could consider also exploring the biology to get to technology.

The research could examine whether there is enough fidelity available in the experiments to use in AI/ML/DL training.

The understanding of specific phenomena—for example, non-equilibrium systems and electric double layers)—is appreciated, but the path from empiricism to deterministic models of biological actuators is unclear. There was no discussion of how, once understanding of phenomenology and science is in place, the results could turn to synthesis of capability. Ongoing research needs to emphasize the recognition that molecular mechanics requires nonreductionist multiphysics modeling.

Microstructure Deep Learning

The methodology works on data from finite element models. A major question yet to be answered is the capability to operate on experimental data.

As this research continues, it needs to consider information beyond the individual slice, in physical depth exploring 3D microstructural details, and to relate observations to changes in properties and failure mechanisms.

This small effort is appropriately pioneering a promising frontier, and while early, it needs to include application-specific metrics as possible in the research planning and execution to enhance the opportunity for transition.

The research is seeking to recognize and correlate macro material properties with observed microstructures. The research plan could address the feasibility of determining the materials and the processing required to achieving desired properties.

In addition to the nominal, universal material microstructure details, the research needs to also consider the prevalence and impact of local issues or imperfections owing to processing or other causes.

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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Parallelized Fluid Structure Interaction for Optimized Aero-Structural Design

In principle, the methodology can be extended to nonlinear and dynamic system responses, but the effort required is substantial. There is a significant effort in the national and global research community to address these issues, and there is an opportunity to leverage this effort.

Deep Learning of Nonlinear Dynamics from Pixel Measurements

To provide a compelling case and demonstration of the value of this approach, ultimately systems that are more complex will need to be considered. More challenging opportunities to use this approach would include inferring mathematical models for biological systems from experimental data or constructing ROMs for complex fluid systems using data from computational fluid dynamics modeling.

Computational Design and Shape Memory Polymer Actuation for Reconfigurable Aero-Structural Design

There is an opportunity for further advances in molecular dynamics (MD) simulations both by considering a wider range of material candidates and by improving the computational efficiency of the MD simulations. Reduced order modeling methods developed for fluid and macro structural systems might prove to be useful here to improve computational efficiency of the MD simulations.

Modeling and Control Methods for Future Vertical Lift Rotorcraft Fatigue Reduction

Experimental evaluation of the computer simulations is important, and such experiments are planned. The actuation method assumed may not be feasible or practical, and will limit the capability to perform experiments, so there is a need to consider the actuation approach to ensure achievability. Data from prior relevant studies could be explored, such as the Boeing Smart Materials Actuated Rotor Technology (SMART) rotor test in the NASA National Full-Scale Aerodynamics Complex 40 × 80 wind tunnel for acoustics with simple moving average actuated mid-span tabs, that was tested in both open loop and closed loop configurations.

INTELLIGENT MANEUVER

The intelligent maneuver (IM) research area focuses on fundamental research that enables effective teaming of soldiers and unmanned vehicles to conduct maneuver operations. Topics include enhancing the autonomous capabilities of unmanned systems such as global positioning system-denied navigation, agile all-terrain mobility, interaction with the environment, and advanced agent-agent and human-agent teaming concepts. This research thrust focuses exclusively on vehicle systems in real-world environments and complements research conducted within the information and computational sciences core competencies.

This assessment of the IM program is based on information presented in a set of briefings and posters prepared by the ARL team engaged in the IM research activities and associated interactions with the researchers. The panel also carefully considered supporting documents provided by the ARL. Research presented to the panel, included projects from autonomous mobility and collaborative maneuver IM areas.

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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Accomplishments and Advancements

In general, research presentations and posters were very well prepared and presented. These presentations, associated reports, and interactions have provided a useful and clear overview of the various projects and activities undertaken by a team of highly talented researchers engaged in a broad range of important IM projects. Researchers engaged in various IM projects were also observed to come from different stages of their careers, including individuals in the early parts of their careers, those in their mid-careers, and a good number of senior experienced individuals. This variety is a noteworthy improvement over what was observed during previous program reviews. Researchers have expertise in electrical, mechanical, and computer engineering and as well as physics, mathematics, and computational sciences. There is a renewed emphasis on enhancing experimental facilities, including a broad range of robotic platforms, multimodal sensor suites, and powerful simulation platforms as well as expanded research test areas for field studies and evaluations. IM projects include both basic research and applied research projects. Many projects have more of an applied flavor, but basic research is also being pursued. There was a good balance between theory, simulation, and experimentation, including the use of platforms. It was also evident that meaningful, long-term, and continuous engagement is maintained with academic institutions that offer expertise in key, new research topics.

Ground Mobility: An Autonomy Stack Perspective

This research is conducted as a part of the ARL intelligent unmanned ground system teaming technologies activities. The mobility stack architecture provides a carefully designed standard for effective collaboration and efficient research progress. This project has successfully engaged multiple researchers during design, development, and experimentation phases. Future field studies will allow evaluation of the key functionalities.

Reflexive Learning for Enhanced Adaptive UAS Capabilities

The tilt rotor development versus quadcopter introduces a new capability—the ability to change attitude and apply force without lateral motion. When added to a quadrotor small-unmanned aerial system (sUAS), the tilting and force capability showed promise over pure quadrotor variants, enabling a new level of control. This development offers capabilities beyond a standard UAS, potentially aiding future designs, and could lead to disruptive, game-changer functionalities. The interplay between modeling, simulation, and testing in this project was excellent.

Legged Locomotion in Resistive Terrains

This research provides a clear understanding of how to properly frame and analyze problems of legged robots traversing resistive terrain, which is realistic during practical situations. It also offered a novel framework for optimization of the cost of transport. There were exceptional contributions with significant potential and excellent use of experimental data to deduce principles of operation.

Legged Locomotion and Movement Adaptation Robotic Platform

Traversing rough terrain with an intelligent legged robot is a challenging and an important engineering and AI challenge. This project is part of a large activity involving multiple research teams.

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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The ARL IM team’s presentation provided a very clear discussion of every aspect of the problem. The optimization of leg geometry against cost of transportation was very relevant. The team’s expertise, focus, and good work ensure continued success.

Calibrating Complex Sensor Suites

Sensor calibration is a critically important and challenging task with direct influence on the success or failure of real-world problems. As numbers, types, and tasks associated with intelligent robots increase, calibration becomes that much more complicated. Calibration-related research activities at ARL IM is outstanding, and the attention to this aspect is commendable. The researchers have identified key challenges and have developed sophisticated calibration algorithms for multi-modal sensor suites. This R&D effort is rigorous, with evaluations using real-world systems. This research has definite potential to support the ultimate goal of accurate multi‐modal online calibration in natural environments.

Recognition of the Role of the “Human” in Deployment of Intelligent Systems

It is refreshing to note important research undertaken in this area by a number of research teams at ARL. The following projects bring out distinctive contributions of these research activities.

Integrating Soldier-Robot Dialogue Software with ARL Autonomy Stack

Speech is a natural communication modality that allows a warfighter to command and interact with a robot with minimal training. This project is pursuing transition of the autonomous speech and dialogue processing software with the ARL robot autonomy code stack. A core component of this integration is a message bridge that handles bi‐directional communication between the natural language interface and robot software.

Classification-Based Approach for Robust Warfighter-Robot Dialogue

This early-stage project presents an architecture that supports autonomous back‐and‐forth communications in the form of natural language dialogue between warfighters and robots. With dialogue, warfighters can issue verbal instructions to robots, while robots can provide status updates and ask for clarification. This research has good ties into the autonomy stack. The research required collection of a sizable corpus of training data on Army-specific scenarios with untrained users. This project showed a solid start on a program for systematic analysis of speech interactions between a warfighter and an intelligent robot.

After-Action Review Technology for Human-Autonomy Teaming

The global after-action review allows multi‐agent—humans and autonomy—teams to review their performance and understand the outcomes of actions and decisions from a more global, top‐down team perspective. This technology provides the crew with a tool to replay an operation, while identifying vehicle behaviors, weapon system and sensor behaviors, crew behaviors, specific environmental features, as well as crew/autonomy interactions. This tool will enhance teaming by supporting communication throughout the entire team, developing situation awareness, and facilitating team trust—thus providing a

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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method for observing and evaluating context‐specific, environmental, and social factors, as well as calibrating team trust in autonomy‐enabled systems.

Challenges and Opportunities

The New Test Area (R2M2) is an opportunity to develop test grounds and data sets for air and ground vehicles in a variety of terrains.

In the area of human-robot communication, ARL needs to consider realistic constrains such as multiple speakers and ambient noise. In the area of human-autonomy teaming, ARL needs to consider team/crew leadership roles, both by the leader in setting the goals, and then by conducting command and control.

ARL could leverage the new test sites to generate and share standard data sets, thereby getting other groups to work on ARL-relevant problems. For example, ARL could make the mobility stack more widely usable for other groups. Sharing and wider use will also provide unique opportunities for the development of “open” data sets, performance metrics, benchmarks, and sharable modules for experimentation. These efforts will promote collaborations and would enhance research quality and productivity. It will be highly valuable if such field studies are planned and designed utilizing arrays of sensors and instrumentation to accurately capture and record states of all agents, situational dynamics, and states. This data collection will provide naturalistic data sets to develop machine learning algorithms, evaluation, and validation protocols. There needs to be more emphasis on research on intent or behavior prediction, naturalistic studies, data augmentation, annotation, transfer learning.

Other opportunities include R&D for harnessing drones (tele-operated and AI controlled) for scouting (using multi-spectral sensors), for light weapons fire, and for target designation. Given the potential complexity of sensor and intelligence inputs overwhelming field soldiers and leaders, it is necessary to harness AI/ML for targeting analysis and prioritization, as well as creating the “big picture” for enemy dispositions and operations.

Scientific research outcomes and progress needs to be carefully evaluated and needs to be reproducible by others. This need for reproducibility requires the use of appropriate metrics. The importance, utility, and criticality of performance metrics needs to be emphasized. Development of proper metrics and benchmarks, at various levels of signal, information processing, and semantic levels needs be a priority topic.

ARL leadership and program managers could address the matter of certification of autonomous systems for field operations. It is recognized that this issue appears underdeveloped, but lack of certification protocols could constrain the nature of experimentation and field trials. Emphasis on the needs and nature of the cyberspace wherein the developed systems operate is critical as a capability nears transition into the field. Recognizing the importance of cybersecurity, related considerations need to be made in the early stages of new projects—it is in general undesirable to introduce cybersecurity considerations and design modifications at the end-stage of system development. More emphasis on projects related to reliability, fault tolerance, and security at the system as well as the module level would be beneficial.

IM research pursued at ARL needs to ultimately introduce novel systems for enhanced effectiveness during end-use. These systems would naturally be composed of multiple subsystems or modules. It is necessary that a clear systems perspective in design, development, evaluation, and assessment be emphasized beyond what was provided in the presentations.

ARL needs to engage and involve the end-user into the laboratory and field activities as early as possible. In addition, ARL needs to develop and use benchmarks in guiding projects and in assessing work that is assigned to academic partners and contractors.

Research projects need to have a clear definition, justification, and characterization of success and failure of the systems, algorithms, and experiments. Failures are often just as valuable as successes, as long as there is a systematic and careful postmortem phase. Iterative refinement is thus an important

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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guiding principle, especially in experimental research where the ability to learn from failures is key. Along these lines, it would be beneficial to researchers to lay out clearly the path to success in various projects. Questions such as those listed below could be useful in the development and assessment of research progress:

  • How do the pieces fit together for robust human-robot interactions?
  • How much effort will be required before modeling will be useful?
  • How will results in multi-agent coordination scale to real-world applications?
  • What is the path from basic research in learning Gaussian processes to real applications?

ARL needs to have more researchers publishing in high-impact factor journals and conference proceedings. Some of these papers may be published jointly with academic partners. ARL researchers need to take on primary author responsibilities. High-quality publications and associated citations are important metrics to guide scientific efforts and progress.

OVERALL QUALITY OF THE WORK

Most of the research is of high quality, but it can be further improved. The use of dimensionless numbers could be extended across all disciplines and programs, as this approach makes the results more broadly applicable. Data analytic tools could be used to analyze the large amount of data collected. Now that ARL has established collaborations with national laboratories and universities, it needs to pay attention to knowledge transfer. Fellows and early-career researchers need to be mentored to understand how their work fits in the larger picture.

Most presentations are of good quality, but they can be further improved. The presentations need to clearly articulate the objectives in quantitative terms. Some of the presentation slides were overcrowded with undersized graphics and text. Appropriate size of fonts and graphs need be used for ease of understanding. All images and graphs in the presentation could be clearly explained. The title could clearly summarize the presentation.

Platform Power

Overall, the quality of the briefings and posters in the platform power was very good. The overall research program is commendable, with high scientific quality. ARL has made remarkable progress in implementing the ARLTAB recommendations from the 2018 assessment in certain areas, such as establishing collaborative relations with universities and national laboratories in the study of ignition and combustion characteristics at altitude conditions for various fuels and fuel blends of interest. Collaborative efforts have helped improve the technical quality of the program and made use of state-of-the-art experimental facilities and models to achieve the objectives. The program now does reflect a better balance between experimental and theoretical approaches, the latter including computational efforts. ARL would now benefit from seeking to improve the in-house capabilities in these areas via interactions between ARL researchers and their collaborators.

The program on tribology and lubrication science is well formulated and aimed at understanding fundamental physics. This program has demonstrated commendable use of experimental facilities in fundamental research. Application of this capability to study the failure mechanisms for fuel pumps is an excellent application of fundamental capability in resolution of relevant problems—for example, operation using different fuels and under different atmospheric conditions.

The research program on development of coatings for austere environments addresses a very relevant problem; however, its objectives are overstated.

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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There were several excellent presentations by journeyman fellows and early-career researchers, clearly following the ARLTAB 2018 assessment feedback on building up the workforce pipeline.

Platform Design and Control

Overall, the platform design and control research program adds considerable value to the body of R&D knowledge. The laboratory leadership is very connected to projects, objectives, and deliverables. ARL has acted effectively to address the comments and recommendations made by ARLTAB in its 2018 assessment. ARL’s focus on modeling small-scale unmanned systems is necessary to develop and employ these emerging machines, and to identify, assess, and pursue relevant breakthrough technologies.

The materials and structures research, with an emphasis on incorporating artificial intelligence (AI), machine learning (ML), and deep learning (DL), represents pioneering of complex, high-risk areas of basic research. This is demonstrating that discovery, can be disruptive, and can have wide-ranging implications for science and innovation in synthesis, and this research needs to continue. Several topics presented represent ARL’s extended strategy to explore disruptive topics outside the current core competencies and to identify promising future directions for the laboratory.

While some work is not balancing theory with experiments and simulations, a particular exemplar is the microstructure deep learning that is developing AI/ML approaches by fusing simulation and experimental data. The molecular dynamics modeling in both the computational design of shape memory polymer actuation and the tailorable and multifunctional dynamic polymer networks can provide data to implement AI methods. Dynamic polymer networks research is a unique combination of morphing, self-healing, and shape memory. The non-equilibrium molecular motor research is on the leading edge of bio-hybrid basic research and will help inform ARL to shape its future portfolio.

The emerging R2C2 facility is a highlight for unmanned aerial system (UAS) experimentation, capitalizing on opportunity to offer local capability to develop autonomy for the extreme UAS vision. The nascent intelligent mechanics initiative can be disruptive, exploring the historical distinctions between pro-action and reaction, such as adaptation, reconfiguration, and control versus deformation and failure.

There are a number of talented early-career researchers who are capable in their disciplines, and they will benefit from mentoring and experience to engender a greater system-level perspective.

Intelligent Maneuver

In general, research presentations and posters were very well prepared and presented. These presentations, associated reports, and interactions have provided a useful and clear overview of the various projects and activities undertaken by a team of highly talented researchers engaged in a broad range of important IM projects. Researchers engaged in various IM projects were also observed to come from different stages of their careers including individuals in the early parts of their careers, those in their mid-careers and a good number of senior experienced individuals. This variety is a noteworthy improvement over what was observed during previous program reviews. Researchers have expertise in electrical-, mechanical-, and computer engineering and as well as physics, mathematics, and computational sciences. There is a renewed emphasis on enhancing experimental facilities including a broad range of robotic platforms, multi-modal sensor suites, and powerful simulation platforms as well as expanded research test areas for field studies and evaluations. IM projects include both basic research and applied research projects. Many projects have more of an applied flavor, but basic research is also being pursued. There was a good balance between theory, simulation, and experimentation including the use of platforms. It was also evident that meaningful, long-term, and continuous engagement is maintained with academic institutions that offer expertise in key, new research topics.

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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RECOMMENDATIONS

In general, the platform modeling is not exploring disruptive or visionary component technology and approaches—for example, aero elastically tailored tiltrotor wings. There is an opportunity to be deliberate with the tools to serve as a means to assess the value of breakthrough technologies—for example, shape memory polymers. ARL needs to address the potential applicability to related work in other fields, particularly in the commercial industry. While the unique challenges presented by military vehicles is recognized, at the more fundamental levels the modeling, simulation, physical tools, and physics can indeed be common between military and commercial applications, and they need to be explored further.

There has been some work to insert high-fidelity models and modules into the methodology to address the multidisciplinary systems needs, but it is not a systemic objective throughout. ARL needs to consider a systematic insertion of high-fidelity modeling to capture complex behaviors. The research efforts on aerodynamics and aeromechanics needs to increase the emphasis on accounting for unsteady effects, turbulence, viscous effects, boundary layer transition, and the difficulty with multiple length and time scales. This will enable ARL to address difficult UAS questions such as unsteadiness, gusts, complex environments, and interactional aerodynamics. These will all be critical with intent to focus on exploration of small unique UAS in extreme conditions and environments. ARL needs to emphasize use of dynamic and non-linear extreme problems.

Recommendation: The Army Research Laboratory (ARL) should consider a systematic insertion of high-fidelity modeling to capture complex behaviors. ARL should emphasize use of dynamic and non-linear extreme problems.

The tools do not appear to be specifically focused on decision metrics such as detection and availability. ARL needs to identify and implement improved decision metrics. Specifically, weight is not necessarily the only appropriate success metric, especially for small UAS. The UAS research needs to recognize the issues, opportunities, and constraints associated with considering the off-board ecosystem and metrics such as sustainment, transportation, storage, soldier survivability, and supply chain. While the tools are not anticipated to include these factors, consideration of them may result in small but effective and affordable changes that can support system-level decisions.

ARL leadership and program managers could address the matter of certification of autonomous systems for field operations. It is recognized that this issue appears underdeveloped, but lack of certification protocols could constrain the nature of experimentation and field trials. Emphasis on needs and nature of the cyberspace wherein the developed systems operate is critical as a capability nears transition into the field. Recognizing the importance of cybersecurity, related considerations need to be made in the early stages of new projects—it is in general undesirable to introduce cybersecurity considerations and design modifications at the end-stage of system development. More emphasis on projects related to reliability, fault tolerance, security, at the system as well as the module level would be beneficial.

IM research pursued at ARL need to ultimately introduce novel systems for enhanced effectiveness during end use. These systems would naturally be composed of multiple subsystems or modules. It is necessary that a clear systems perspective in design, development, evaluation, and assessment be emphasized. Systems perspective needs to include survivability and availability of the systems.

Recommendation: The Army Research Laboratory (ARL) should pursue a clear systems perspective in design, development, evaluation, and assessment including considering the eventual certification of autonomous systems by coordinating with the relevant entities for field operations, introduce cybersecurity considerations, and design modifications at the early stage of system development.

Suggested Citation:"6 Propulsion Sciences." National Academies of Sciences, Engineering, and Medicine. 2021. 2019-2020 Assessment of the Army Research Laboratory. Washington, DC: The National Academies Press. doi: 10.17226/26325.
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The National Academies of Sciences, Engineering, and Medicine's Army Research Laboratory Technical Assessment Board (ARLTAB) provides biennial assessments of the scientific and technical quality of the Army Research Laboratory (ARL). These assessments include the development of findings and recommendations related to the quality of ARL's research, development, and analysis programs. 2019-2020 Assessment of the Army Research Laboratory reviews the following research core competencies of ARL: human sciences, network and information sciences, computational sciences, materials and manufacturing sciences, and propulsion sciences. This biennial report summarizes the findings of the ARLTAB from reviews conducted in 2019 and 2020.

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