Space Nuclear Propulsion for Human Mars Exploration (2021) / Chapter Skim
Currently Skimming:
3 Nuclear Electric Propulsion
3 Nuclear Electric Propulsion
Pages 30-54
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 30... ...
High power NEP systems require heat rejection radiators with large surface areas to provide adequate cooling, and, as power levels increase, the size and mass of the heat rejection subsystem has the potential to dominate over other subsystems. Heat rejection at high temperatures reduces the radiator area since radiation increases proportionally to the fourth power of the absolute temperature of the radiator.1 High-temperature operation thereby increases performance, but it becomes a challenge for other aspects of the system.
|
From page 31... ...
distributed over relatively large distances to the electric propulsion (EP) subsystems.
|
From page 32... ...
Unfortunately, no relevant-scale component, subsystem, or system testing was performed before NASA terminated the program in 2005 after reevaluating its budgetary priorities. NASA supported research to advance thruster technologies relevant to megawatt electric power levels in parallel with the above programs, including the 200 kWe mercury ion thruster tested in 1968; fundamental research on 1 to 10 MWe pulsed MPD and pulsed inductive thrusters; 250 kWe steady-state MPD thrusters; and 100 kWe Hall, radiowave-driven magnetized electrothermal (VASIMR®)
|
From page 33... ...
NEP technologies, designs, and M&S tools related to HEU fuels, power conversion, heat rejection, and thrusters have been developed for 100 to 200 kWe NEP systems; some of these technologies could be scaled to the megawatt electric power level. Developing an NEP system for the baseline mission will likely involve the use of multiple NEP modules which, in the aggregate, will provide the total propulsive power.
|
From page 34... ...
The most relevant power conversion technologies are as follows: • Static -- Thermoelectric converters -- Thermionic converters13 • Dynamic -- Brayton cycle engines -- Rankine cycle engines -- Stirling cycle engines The level of development and the potential performance of these technologies varies widely, and none have been tested to the power levels required for a MWe-class NEP system in an appropriate operating environment, even if multiple power conversion units are used to meet total power and system reliability requirements. Thermoelectric converters have a long history in space nuclear fission systems, particularly with the SNAP program and the SP-100 program.
|
From page 35... ...
The TFE, however, required fuel temperatures on the order of 1800 K, which introduced additional structural material concerns for the reactor.17 The characteristics of the most recent power conversion technology tests relevant to space power systems are shown in Table 3.1. As shown, the demonstrated power levels for the different options vary widely, as they were not intended for use in high power, low specific mass systems.
|
From page 36... ...
This design used Ti/water heat pipes in a loop panel configuration and was designed to operate at temperatures of 500 K Multiple heat pipes on a single representative panel were tested in vacuum in 2010.18 The projected specific mass of the heat rejection subsystem for this 200 kWe system was 10.1 kg/kWe (about half of the total system specific mass required for the baseline mission)
|
From page 37... ...
Subsequent estimates of specific mass with direct-drive scaling for a 1 MWe NEP cargo vehicle, using 50 kWe Hall thrusters, were on the order of 1 kg/ kWe for the PMAD subsystem.21 Electric Propulsion Thrusters EP systems have been used for spaceflight for decades, but to date the available power level has been limited to kilowatt-electric, not megawatt electric, and the source of power has been solar panels. Of the various thruster types that have been used, the two most likely to provide the required performance and lifetime capabilities for Mars missions at the required power levels are ion thrusters and Hall thrusters.
|
From page 38... ...
is injected from plasma the plenum and travels toward the discharge cathode Discharge Ion hollow beam cathode Step 3 -- Electrons impact Negative Positive Step 4 -- propellant atoms to grid grid Ions are pulled out create ions (shown of the discharge in blue) chamber by the ion optics FIGURE 3.3 Ion thruster.
|
From page 39... ...
NUCLEAR ELECTRIC PROPULSION 39 Anode Magnet Coils Xenon Dielectric Walls Electric Field Magnetic Field Cathode Xenon Power Supply Power Supply FIGURE 3.4 Hall thruster.
|
From page 40... ...
TABLE 3.2 Examples of State-of-the-Art Hall and Ion Electric Propulsion Thrusters and Power Processing Units (PPUs) Thruster Thruster Power PPU α Thruster Type (kWe)
|
From page 41... ...
Existing thruster concepts such as Hall thrusters and ion thrusters can meet Isp and efficiency requirements, but thruster power levels must increase by an order of magnitude compared to current and near-term solar electric propulsion (SEP) flight systems.
|
From page 42... ...
Curren t FIGURE 3.5 Magnetoplasmadynamic thruster. SOURCE: Electric Propulsion and Plasma Dynamics Laboratory, Princeton University.
|
From page 43... ...
Helicon coupler ionizes propellant RF g ene rato r Step 3. Superconductor generates magnetic field that confines plasma FIGURE 3.6 VASIMR® thruster.
|
From page 44... ...
Achieving a specific mass of 20 kg/kWe for the entire NEP system scaled for the baseline mission is a significant challenge that drives the reactor, power conversion, and heat rejection subsystems to higher operating temperatures, and drives EP subsystems to efficient power distribution, processing, and thrust production. The multiple subsystems of an NEP system must demonstrate adequate performance and reliable operation of interconnected subsystems across all phases of mission operations as well as unexpected transients during abnormal operating conditions.
|
From page 45... ...
Operating temperatures for the power conversion subsystems tested to date are at the minimum acceptable level to meet NEP needs. Brayton energy conversion technologies are more advanced than other types, but they introduce new types of risks, and demonstrated power levels for space-qualified systems are orders of magnitude below that required for a 1 to 2 MWe system.
|
From page 46... ...
Electric Propulsion Subsystem Thruster performance requirements are to some extent dependent on power system specific mass and power levels. As specified in Chapter 1, the Isp goal is 2,000 s or more, with thruster efficiencies greater than 50 percent in order to provide enough acceleration for the power levels, payloads, and trip times.
|
From page 47... ...
Laboratory models have been tested to address this scaling, including the use of multiple concentric channels.28 For the ion thruster, the annular ion thruster mitigates grid spacing issues by providing a central support to the grids.29 For the Hall thruster, multiple, nested channels have been tested to 100 kW power levels.30 Both concepts have been tested only for short periods of time and further testing is needed. MPD and VASIMR® thrusters, while considered to be better able to process high power, also require higher powers to operate efficiently.
|
From page 48... ...
This approach could substantially reduce PPU specific mass; however, only laboratory simulations of direct drive have been performed, with laboratory power supplies supplying the other low voltage and power components needed by a thruster, and without a full assessment of control during transients. For instance, the simulated direct drive of an ion thruster by a Brayton conversion device was only for the 1100 V thruster beam power; other thruster components such as cathodes were operated using laboratory power supplies.34 Additionally, system reliability and fault protection requirements for flight systems will increase the PPU mass.
|
From page 49... ...
However, full retirement of risk for the reactor subsystem and its components would require ground testing in a representative nuclear environment. With sufficiently rigorous M&S and ground testing, it may be feasible to conduct the first test of a fully integrated, full-scale NEP system during the first cargo mission.
|
From page 50... ...
Potential tests could include, for example, a single electrically heated power conversion unit and heat rejection panel in a vacuum facility with relevant sink temperature, connected by a PMAD system to a thruster operating in another vacuum chamber. This would likely require several large vacuum chambers, as the heat and plasma loads of 50 to 100 kWe power conversion and thruster units would likely overload a single existing facility.
|
From page 51... ...
To develop a nuclear electric propulsion (NEP) system capable of executing the baseline mission, NASA should rely on (1)
|
From page 52... ...
52 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030 2031 2032 2033 2034 2035 2036 2037 2038 2039 2040 2041 Launch of NASA's Power Trade and Propulsion Element Cargo Mission Development and Launch study Cargo Mission Flight Vehicle HEU Development, Integration, and Launch Cargo Mission vs HALEU Safety Assessments and Nuclear Systems and Mission System Launch Approval Trade Study PDR CDR Power System Design (w/ reactor) Reactor M&S Fuel/Clad Development Reactor Fab Reactor Ground Test Power Materials Power System Fab Development Human Mission Development and Launch Power Electronics PMAD Fab Mars Human Human Mission Flight Vehicle Exploration Development Development, Integration, and Launch NEP Power System Ground Test Mission #1 Heat Pipe/Panel Heat Rejection Development Panel Fab Safety Assessments and Nuclear System M&S Systems and Mission Launch Approval Thruster/PPU/Electronics EP Fab Development PDR CDR Thruster M&S EP Acceptance Test EP Life Test Safety Analyses FIGURE 3.7 Nuclear electric propulsion development roadmap for the baseline mission, with a 2039 launch of the first human mission.
|
From page 53... ...
NASA should invigorate technology development asso ciated with the fundamental nuclear electric propulsion (NEP) challenge, which is to scale up the oper ating power of each NEP subsystem and to develop an integrated NEP system suitable for the baseline mission.
|
From page 54... ...
However, testing thrusters at power levels above 50 kWe, particularly for in-space performance and lifetime, will challenge existing vacuum facility capabilities.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.