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Commercial Space Vehicle Emissions Modeling (2021)

Chapter: 4 Database of Emissions Indices

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Suggested Citation:"4 Database of Emissions Indices." National Academies of Sciences, Engineering, and Medicine. 2021. Commercial Space Vehicle Emissions Modeling. Washington, DC: The National Academies Press. doi: 10.17226/26142.
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Suggested Citation:"4 Database of Emissions Indices." National Academies of Sciences, Engineering, and Medicine. 2021. Commercial Space Vehicle Emissions Modeling. Washington, DC: The National Academies Press. doi: 10.17226/26142.
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Suggested Citation:"4 Database of Emissions Indices." National Academies of Sciences, Engineering, and Medicine. 2021. Commercial Space Vehicle Emissions Modeling. Washington, DC: The National Academies Press. doi: 10.17226/26142.
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Suggested Citation:"4 Database of Emissions Indices." National Academies of Sciences, Engineering, and Medicine. 2021. Commercial Space Vehicle Emissions Modeling. Washington, DC: The National Academies Press. doi: 10.17226/26142.
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Suggested Citation:"4 Database of Emissions Indices." National Academies of Sciences, Engineering, and Medicine. 2021. Commercial Space Vehicle Emissions Modeling. Washington, DC: The National Academies Press. doi: 10.17226/26142.
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Suggested Citation:"4 Database of Emissions Indices." National Academies of Sciences, Engineering, and Medicine. 2021. Commercial Space Vehicle Emissions Modeling. Washington, DC: The National Academies Press. doi: 10.17226/26142.
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Suggested Citation:"4 Database of Emissions Indices." National Academies of Sciences, Engineering, and Medicine. 2021. Commercial Space Vehicle Emissions Modeling. Washington, DC: The National Academies Press. doi: 10.17226/26142.
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Suggested Citation:"4 Database of Emissions Indices." National Academies of Sciences, Engineering, and Medicine. 2021. Commercial Space Vehicle Emissions Modeling. Washington, DC: The National Academies Press. doi: 10.17226/26142.
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Suggested Citation:"4 Database of Emissions Indices." National Academies of Sciences, Engineering, and Medicine. 2021. Commercial Space Vehicle Emissions Modeling. Washington, DC: The National Academies Press. doi: 10.17226/26142.
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Suggested Citation:"4 Database of Emissions Indices." National Academies of Sciences, Engineering, and Medicine. 2021. Commercial Space Vehicle Emissions Modeling. Washington, DC: The National Academies Press. doi: 10.17226/26142.
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Suggested Citation:"4 Database of Emissions Indices." National Academies of Sciences, Engineering, and Medicine. 2021. Commercial Space Vehicle Emissions Modeling. Washington, DC: The National Academies Press. doi: 10.17226/26142.
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Suggested Citation:"4 Database of Emissions Indices." National Academies of Sciences, Engineering, and Medicine. 2021. Commercial Space Vehicle Emissions Modeling. Washington, DC: The National Academies Press. doi: 10.17226/26142.
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Suggested Citation:"4 Database of Emissions Indices." National Academies of Sciences, Engineering, and Medicine. 2021. Commercial Space Vehicle Emissions Modeling. Washington, DC: The National Academies Press. doi: 10.17226/26142.
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Suggested Citation:"4 Database of Emissions Indices." National Academies of Sciences, Engineering, and Medicine. 2021. Commercial Space Vehicle Emissions Modeling. Washington, DC: The National Academies Press. doi: 10.17226/26142.
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Suggested Citation:"4 Database of Emissions Indices." National Academies of Sciences, Engineering, and Medicine. 2021. Commercial Space Vehicle Emissions Modeling. Washington, DC: The National Academies Press. doi: 10.17226/26142.
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Suggested Citation:"4 Database of Emissions Indices." National Academies of Sciences, Engineering, and Medicine. 2021. Commercial Space Vehicle Emissions Modeling. Washington, DC: The National Academies Press. doi: 10.17226/26142.
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Commercial Space Vehicle Emissions Modeling 29 4 Database of Emissions Indices The commercial space vehicle emissions model uses emissions indices to estimate the total amounts of the various pollutants emitted by space vehicles. As discussed in Section 3, emissions indices are the factors that relate the amount of propellant burned to the amount of each pollutant emitted by a rocket engine. Equation (6) demonstrates the direct quantitative relationship between the propellant mass burned and the pollutant mass emitted. The emissions index for a specific pollutant reports the outcome of the complex series of chemical reactions that occur within the rocket engine and exhaust plume as a single number. The following sections discuss the detailed development of the database of emissions indices for the commercial space vehicle emissions model:  Section 4.1 provides a quantitative definition of emissions indices for space vehicles,  Section 4.2 describes the methods used to calculate the primary emissions indices at the nozzle exit plane,  Section 4.3 describes the methods used to estimate the final emissions indices due to the formation of secondary emissions in the rocket exhaust plume, and  Section 4.4 presents the primary and final emissions indices for the first-stage rocket engines of current and in-development commercial space vehicles. 4.1 Definition of Emissions Indices In the commercial space vehicle emissions model, the emissions index for a specific pollutant is defined as �EmissionsIndex � = Pollutant Mass Emitted (g) Propellant Mass Consumed (kg) (7) which has units of grams of pollutant emitted per kilograms of propellant consumed (abbreviated as g/kg). These units are adopted from AEDT, in which the emissions indices for aircraft are defined as the grams of pollutant emitted per kilograms of fuel consumed. However, the emissions indices for aircraft and spacecraft are different: emissions indices for aircraft are defined relative to fuel mass, whereas emissions indices for spacecraft are defined relative to propellant mass. The propellant includes the fuel plus the oxidizer. The difference between the emissions indices for aircraft and spacecraft arises from the physical differences between jet engines and rocket engines. Jet engines carry their fuel on board the aircraft and ingest oxygen from the surrounding air, whereas rocket engines carry both their fuel and oxidizer on board the spacecraft. Thus, the emissions indices for both aircraft and spacecraft are defined based on the substance carried on board the vehicle.

Commercial Space Vehicle Emissions Modeling 30 4.2 Primary Emissions Indices As discussed in Section 2.1, the primary emissions are the chemical species present at the nozzle exit plane due to processes that occur inside the rocket engine. If high-quality predictions or measurements of the primary emissions indices for a space vehicle are publicly available in the literature, the published values are used in the commercial space vehicle emissions model. However, the primary emissions indices are not publicly available for most commercial space vehicles because they are considered proprietary or have not been measured. Instead, the primary emissions indices for most vehicles in the commercial space vehicle emissions model were predicted using the computer program Chemical Equilibrium with Applications (CEA). CEA was developed by Bonnie McBride and Sanford Gordon at the NASA Glenn Research Center for the purpose of calculating the chemical equilibrium composition and thermodynamic properties of any chemical system [109]. A key application of CEA is the prediction of theoretical rocket engine performance. CEA performs calculations at several locations inside a rocket engine, including the combustion chamber, throat, and nozzle exit plane. The results at each location include [110]:  Thermodynamic state (e.g., temperature, pressure, and molecular weight),  Mach number,  Sea level and vacuum specific impulse, and  Chemical composition (i.e., mole fractions or mass fractions of the product species). The mass fractions and the emissions indices are both defined relative to the propellant mass consumed. Thus, the primary emissions indices are directly proportional to the mass fractions of the product species at the nozzle exit plane calculated by CEA. Figure 19 shows a flowchart of the method used to estimate the primary emissions indices for the commercial space vehicle emissions model. The input parameters required by CEA to predict theoretical rocket engine performance are the propellant (fuel and oxidizer) species, mixture ratio, combustion chamber pressure, and nozzle area ratio. These parameters were obtained or estimated for the first-stage rocket engines of numerous commercial and historical launch vehicles. The primary emissions indices for these rocket engines were then calculated using CEA. Figure 19. Overview of the method for estimating the primary emissions indices.

Commercial Space Vehicle Emissions Modeling 31 CEA implements sophisticated and well-validated models for calculating the chemical equilibrium of complex chemical systems. However, the theoretical rocket engine performance calculations are based on numerous simplifying assumptions [111], including:  One-dimensional forms of the continuity, momentum, and energy conservation equations,  Ideal gas law,  Complete and adiabatic combustion,  Homogeneous mixing,  Chemical equilibrium of the products in the combustion chamber,  Isentropic expansion in the nozzle, and  Instantaneous equilibrium or frozen chemical composition during expansion in the nozzle. These simplifying assumptions may introduce errors into some of the primary emissions indices calculated by CEA. For example, the formation of black carbon is known to depend on nonequilibrium processes and inhomogeneous mixing [8], which violates the assumptions of CEA. Thus, CEA cannot accurately predict the primary emissions indices of black carbon and other minor chemical species that form due to nonequilibrium processes or incomplete combustion. Despite these limitations, CEA’s simplifying assumptions are generally reasonable for predicting the major products of combustion, which account for the largest primary emissions indices. 4.3 Final Emissions Indices As discussed in Section 2.1, the chemical species in the high-temperature rocket exhaust plume may continue to react with each other and with the surrounding air to produce secondary emissions. Thus, the primary emissions indices predicted by CEA at the nozzle exit plane are not the final emissions indices used in the commercial space vehicle emissions model. The formation of secondary emissions in a rocket exhaust plume is a complex process involving finite- rate chemical kinetics, non-isentropic shocks and expansion waves, and turbulent dispersion. Advanced numerical models are required to accurately model the secondary emissions indices. Exercising these numerical models, such as the Joint Army Navy NASA Air Force (JANNAF) Standardized Plume Flowfield (SPF-III) code [112], is beyond the scope of this project. Instead, prior computations and measurements reported in the literature were leveraged to develop first-order estimates for the secondary emissions. Figure 20 shows a flowchart of the method used to estimate the final emissions indices for the commercial space vehicle emissions model. Prior studies have shown that the formation of secondary emissions depends on the chemical composition of the rocket exhaust plume as well as the altitude. Thus, the first-order estimates for the secondary emissions require the primary emissions indices and altitude as input parameters. Due to the sparsity of high-quality data in the literature, the first-order estimates neglect the effects of other parameters such as the temperature and velocity of the exhaust plume and interactions between multiple plumes. Additionally, the uncertainty in the first-order estimates increases at high altitudes because no emissions measurements have been conducted above the lower stratosphere.

Commercial Space Vehicle Emissions Modeling 32 Figure 20. Overview of the method for estimating the final emissions indices. In the following sections, the development of the first-order estimates for the secondary emissions and the resulting final emissions indices are described for each of the major pollutant species emitted by rocket engines:  Water vapor,  Carbon monoxide and carbon dioxide,  Alumina,  Chlorine-containing species,  Nitrogen oxides, and  Black carbon. Although the first-order estimates are subject to potentially large uncertainty, they are based on the best available data in the literature. Furthermore, they provide a general method that can be applied to estimate the altitude-dependent final emissions indices for any commercial space vehicle. 4.3.1 Water Vapor Previous studies [14, 15, 19] have shown that nearly all of the hydrogen (H and H2) at the nozzle exit plane is converted to water vapor (H2O) in the high-temperature exhaust plume due to chemical reactions with oxygen (O2) molecules from the surrounding air. Additionally, the hydroxyl (OH) at the nozzle exit plane typically reacts with hydrogen in the exhaust plume to form H2O. The H2O present at the nozzle exit plane remains nearly unchanged through the exhaust plume. Based on these results, the final emissions indices for H, H2, and OH are assumed to be zero. Furthermore, the final emissions index for H2O under these assumptions is given by 𝐸𝐸𝐸𝐸𝑓𝑓(𝐻𝐻2𝑂𝑂) = 𝐸𝐸𝐸𝐸𝑝𝑝(𝐻𝐻2𝑂𝑂) + 𝑀𝑀𝑀𝑀(𝐻𝐻2𝑂𝑂) 𝑀𝑀𝑀𝑀(𝐻𝐻) 𝐸𝐸𝐸𝐸𝑝𝑝 (𝐻𝐻) + 𝑀𝑀𝑀𝑀(𝐻𝐻2𝑂𝑂) 𝑀𝑀𝑀𝑀(𝐻𝐻2) 𝐸𝐸𝐸𝐸𝑝𝑝(𝐻𝐻2) + 𝐸𝐸𝐸𝐸𝑝𝑝(𝑂𝑂𝐻𝐻) (8) where 𝐸𝐸𝐸𝐸𝑓𝑓 is the final emissions index, 𝐸𝐸𝐸𝐸𝑝𝑝 is the primary emissions index, and 𝑀𝑀𝑀𝑀 is the molecular weight. The molecular weight ratios in Eq. (8) are required because the reactions between the hydrogen in the exhaust plume and the oxygen from the surrounding air add the mass of the oxygen molecules to the rocket exhaust products. However, the emissions index is defined relative to the propellant mass consumed, which does not include the mass of the oxygen molecules from the surrounding air. Thus, the final emissions index for H2O may be greater than the sum of the primary emissions indices for H, H2, OH, and H2O.

Commercial Space Vehicle Emissions Modeling 33 No molecular weight ratio is needed for the hydroxyl term in Eq. (8) because the hydroxyl is assumed to react with hydrogen from the exhaust plume. This assumption violates the earlier assumption that the hydrogen in the exhaust plume reacts solely with oxygen molecules from the surrounding air. However, the amount of hydroxyl at the nozzle exit plane is typically much lower than the amount of water vapor or hydrogen, so the error introduced by this assumption is negligible. 4.3.2 Carbon Monoxide and Carbon Dioxide Previous studies [14, 19] have shown that most of the carbon monoxide (CO) at the nozzle exit plane is oxidized to form carbon dioxide (CO2) in the high-temperature exhaust plume due to chemical reactions with oxygen (O2) molecules from the surrounding air. However, the rate of oxidation decreases at high altitudes because fewer oxygen molecules are present in the surrounding air. Thus, a larger fraction of CO remains in the plume at high altitudes. The first-order estimates for the final emissions indices of CO and CO2 are based on previous studies that calculated the emissions downstream of the Space Shuttle solid rocket motors [14] and a solid rocket motor proposed by TRW Inc. [19]. Figure 21 shows the results from the literature at several different altitudes. The vertical axis presents the final mass fraction of CO relative to the combined mass of CO and CO2. Figure 21. Mass fraction of CO relative to the combined mass of CO and CO2 as a function of altitude [14, 19]. An exponential fit through the points shown in Figure 21 gives the final emissions index for CO as 𝐸𝐸𝐸𝐸𝑓𝑓(𝐶𝐶𝑂𝑂) = min�𝐸𝐸𝐸𝐸𝑝𝑝(𝐶𝐶𝑂𝑂), 0.0025𝑒𝑒(0.067 𝑘𝑘𝑘𝑘⁄ )ℎ�𝐸𝐸𝐸𝐸𝑝𝑝(𝐶𝐶𝑂𝑂) + 𝐸𝐸𝐸𝐸𝑝𝑝(𝐶𝐶𝑂𝑂2)�� (9) where ℎ is the altitude in kilometers, and the maximum value cannot be greater than the primary emissions index for CO.

Commercial Space Vehicle Emissions Modeling 34 Based on Eq. (9), the final emissions index for CO2 is given by 𝐸𝐸𝐸𝐸𝑓𝑓(𝐶𝐶𝑂𝑂2) = 𝐸𝐸𝐸𝐸𝑝𝑝(𝐶𝐶𝑂𝑂2) + 𝑀𝑀𝑀𝑀(𝐶𝐶𝑂𝑂2) 𝑀𝑀𝑀𝑀(𝐶𝐶𝑂𝑂) �𝐸𝐸𝐸𝐸𝑝𝑝 (𝐶𝐶𝑂𝑂) − 𝐸𝐸𝐸𝐸𝑓𝑓(𝐶𝐶𝑂𝑂)� (10) The ratio of molecular weights in Eq. (10) is required because external mass is added to the rocket exhaust products by the reactions between the CO in the plume and the O2 from the surrounding air. Even though Eqs. (9)–(10) were developed based on data from solid rocket motors, the oxidation of CO to CO2 occurs for carbon-containing liquid propellants, too. Thus, these first-order estimates are applied to all rocket engines with carbon-containing propellants. 4.3.3 Alumina Previous studies have shown that alumina (Al2O3) is emitted from solid rocket motors as particles [8] and that the amount of alumina at the nozzle exit plane remains unchanged through the rocket exhaust plume [14, 15]. Thus, the final emissions index for alumina is equal to the primary emissions index calculated using CEA: 𝐸𝐸𝐸𝐸𝑓𝑓(𝐴𝐴𝑙𝑙2𝑂𝑂3) = 𝐸𝐸𝐸𝐸𝑝𝑝(𝐴𝐴𝑙𝑙2𝑂𝑂3) (11) CEA predicts that alumina is emitted from solid rocket motors as liquid or solid particles. In fact, alumina is the dominant source of particulate matter for solid rocket motors (liquid propellants do not contain aluminum and hence do not emit alumina particles). However, little is known about the size distribution of the particles, though the mean particle size has been shown to scale with the diameter of the nozzle throat [8]. Because commercial solid rocket motors tend to be large, and based on the lack of additional information, it is assumed that all of the alumina particles are PM10 (particulate matter less than 10 µm in diameter). 4.3.4 Chlorine Species The primary emissions indices predicted by CEA show that, for solid rocket motors, a significant amount of hydrogen chloride (HCl) and a small amount of atomic and diatomic chlorine (Cl and Cl2) are present at the nozzle exit plane (liquid propellants do not contain chlorine and hence do not emit chlorine-containing species). Previous studies have shown that HCl is partially converted to Cl and Cl2 in the rocket exhaust plume and that the amount of Cl and Cl2 production increases with altitude [18- 20]. However, the reactions that partition chlorine atoms between HCl, Cl, and Cl2 are complex, and inconsistent quantitative results are presented in the literature. Figure 22 shows the mass fractions of HCl, Cl, and Cl2 relative to the total mass of chlorine-containing molecules. The data points are based on calculations from previous studies of the Space Shuttle solid rocket motors [15, 18], a proposed TRW solid rocket motor [19], and the Titan IV solid rocket motor [20]. The prior studies agree that an increasing amount of HCl is converted to Cl and Cl2 at higher altitudes, but the computed mass fractions differ by a factor of 2–3 at an altitude of 30 km. Furthermore, the Titan IV results at an altitude of 40 km demonstrate that the conversion of HCl to Cl and Cl2 is non-monotonic with altitude. Thus, a first-order model for the emissions indices of HCl, Cl, and Cl2 based on these results would have high uncertainty.

Commercial Space Vehicle Emissions Modeling 35 Figure 22. Mass fractions of HCl, Cl, and Cl2 relative to the total mass of chlorine-containing molecules as functions of altitude [15, 18-20]. Instead, a single emissions index is calculated to represent all chlorine-containing species. The total chlorine emissions are often reported as a single value in environmental documents, such as the Evolved Expendable Launch Vehicle EIS [23, 24]. The results predicted by CEA provide the individual primary emissions indices for HCl, Cl, and Cl2, and no additional chorine is added in the plume. Thus, the final emissions index for all chlorine-containing species is given by 𝐸𝐸𝐸𝐸𝑓𝑓(𝐶𝐶𝑙𝑙𝑥𝑥) = 𝐸𝐸𝐸𝐸𝑝𝑝(𝐻𝐻𝐶𝐶𝑙𝑙) + 𝐸𝐸𝐸𝐸𝑝𝑝(𝐶𝐶𝑙𝑙) + 𝐸𝐸𝐸𝐸𝑝𝑝(𝐶𝐶𝑙𝑙2). (12) where Clx represents the sum of all chlorine-containing species. 4.3.5 Nitrogen Oxides (NOx) The most commonly used liquid propellants are liquid oxygen as the oxidizer and liquid hydrogen, RP-1 (a highly refined form of kerosene), or methane as the fuel. Since none of these propellants contains nitrogen, NOx cannot form inside liquid-propellant rocket engines unless impurities are present in the propellant. Impurities are typically minor and are not considered in the commercial space vehicle emissions model. Unlike liquid propellants, most solid, hypergolic, and hybrid propellants contain significant amounts of nitrogen. These propellants may produce a small amount of NOx inside the rocket engine due to nonequilibrium processes and incomplete combustion. Regardless of the type of propellant, NOx forms outside the rocket engine due to reactions between the high-temperature rocket exhaust plume and the nitrogen (N2) in the surrounding air. For solid, hypergolic, and hybrid propellants, the amount of NOx formed in the exhaust plume at low altitudes is typically larger than the amount of NOx formed inside the rocket engine. Thus, the final emissions index for NOx is mostly due to the secondary emissions.

Commercial Space Vehicle Emissions Modeling 36 Figure 23 shows calculations of the NOx secondary emissions index from previous studies of the Space Shuttle [15, 18] as well as the Atlas V RD-180 and Delta IV RS-68A liquid rocket engines [23, 24] at various altitudes. These literature sources demonstrate that NOx production in the exhaust plume decreases with altitude and eventually becomes negligible in the stratosphere. Figure 23. Secondary emissions index for NOx as a function of altitude [15, 18, 23, 24]. An exponential fit was created based on the calculations performed by Leone and Turns [18] for the Space Shuttle. The NOx emissions index at sea level was set equal to the mean value for the Atlas V and Delta IV engines to produce a more conservative estimate based on more recent data. This fit represents a first-order estimate for the production of NOx in the exhaust plume as a function of altitude. Although the exponential fit neglects the effects of temperature, velocity, and interactions between multiple plumes on the production of NOx, it represents the best estimate based on available data. The first-order estimate of the final emissions index for NOx is given by 𝐸𝐸𝐸𝐸𝑓𝑓(𝑁𝑁𝑂𝑂𝑥𝑥) = 𝐸𝐸𝐸𝐸𝑝𝑝(𝑁𝑁𝑂𝑂𝑥𝑥) + (33𝑔𝑔 𝑘𝑘𝑔𝑔⁄ )𝑒𝑒−(0.26 𝑘𝑘𝑘𝑘⁄ )ℎ (13) where ℎ is the altitude in kilometers. This first-order estimate is applied to all rocket engines in the engine database, regardless of propellant type.

Commercial Space Vehicle Emissions Modeling 37 4.3.6 Black Carbon Black carbon (BC), also known as soot, is produced inside rocket engines by incomplete combustion of carbon-based propellants. Prior observations have shown that LOX/RP-1, solid, and hybrid propellants produce black carbon, and it is expected that LOX/methane engines will produce some black carbon as well. However, the nonequilibrium chemistry and inhomogeneous mixing processes involved in the formation of black carbon are still not completely understood [8], so little is known about the quantity of black carbon produced by different types of rocket propellants. The only publicly available quantitative studies on black carbon emissions from rocket engines are for LOX/RP-1 propellants. These studies report values for the black carbon final emissions index that vary from 1–30 g/kg [8]. The black carbon emissions indices for different types of rocket propellants are unknown. However, it is well known that black carbon at the nozzle exit plane is partially oxidized to CO and CO2 due to afterburning in the high-temperature exhaust plume. Since oxidation requires O2 molecules from the surrounding air, the amount of oxidation decreases at higher altitudes. Figure 24 shows results from the few studies in the open literature that reported estimates of the final emissions index for black carbon at specific altitudes [8, 31, 32]. All of these results are for the Atlas II rocket, which used LOX/RP-1 propellant in a gas-generator cycle rocket engine. An exponential fit was created based on these results at altitudes above sea level. The fit was clamped to a minimum value of 1 g/kg, which is the lowest value for the black carbon final emissions index reported in the literature, and a maximum value of 25 g/kg, which is the average of the maximum values reported by several different studies [8, 32]. Figure 24. Final emissions index for black carbon in LOX/RP-1 engines as a function of altitude [8, 31, 32].

Commercial Space Vehicle Emissions Modeling 38 A reasonable estimate for the primary emissions index of black carbon is given by the final emissions index at high altitudes, where afterburning is negligible. The results shown in Figure 24 suggest that the primary emissions index of black carbon is 25 g/kg for LOX/RP-1 propellants. Due to afterburning, the final emissions index of black carbon is reduced to as little as 4% of the primary emissions index at low altitudes. Thus, the first-order estimate of the final emissions index of black carbon is given by 𝐸𝐸𝐸𝐸𝑓𝑓(𝐵𝐵𝐶𝐶) = 𝐸𝐸𝐸𝐸𝑝𝑝(𝐵𝐵𝐶𝐶) max �0.04, min�1, 0.04𝑒𝑒(0.12 km⁄ )(ℎ−15 km)�� (14) where the primary emissions index of black carbon is assumed to be 25 g/kg for LOX/RP-1 propellants. Due to a lack of data, this estimate neglects differences between gas-generator and staged- combustion engine cycles, which are expected to produce different amounts of black carbon. No quantitative data are available in the open literature to estimate the black carbon emissions indices for other types of rocket propellants. One study [33] assumed that solid rocket motors produce similar amounts of black carbon as LOX/RP-1 engines, so the black carbon primary emissions index is assumed to be 25 g/kg for solid rocket motors, too. Due to a lack of data, the same estimate is also applied to hypergolic and hybrid propellants. Table 6 summarizes the assumed values of the black carbon primary emissions indices for different types of rocket propellants. As indicated by Table 6, LOX/methane engines are expected to produce less black carbon than LOX/RP-1 engines. Results from the automotive industry have shown that, under optimal conditions, black carbon emissions from internal combustion engines can be reduced by more than 80% by switching from gasoline to natural gas, which is primarily composed of methane [113]. However, rocket engines are limited by performance constraints that favor fuel-rich combustion, so results from internal combustion engines may not be directly transferable to rocket engines. Additionally, several prominent LOX/methane rocket engines currently under development utilize staged- combustion cycles, whereas the LOX/RP-1 engines shown in Figure 24 use gas-generator cycles. The staged-combustion cycle is expected to generate less black carbon than the fuel-rich gas-generator cycle, but the reduction in black carbon emissions has not been quantified. Since no other quantitative data are available, the black carbon primary emissions index for LOX/methane engines is assumed to be 20% of the value for LOX/RP-1 engines based on the reduction observed for internal combustion engines. Table 6. Primary emissions index of black carbon for different types of rocket propellants. Propellant EIp(BC), g/kg Notes LOX/Hydrogen 0 LOX/Hydrogen propellants do not contain carbon LOX/RP-1 25 Based on the data shown in Figure 24 LOX/Methane 5 Based on results from internal combustion engines [113] Solid (APCP) 25 Based on a single assumption in the literature [33] Hypergolic 25 Assumed due to lack of data Hybrid 25 Assumed due to lack of data

Commercial Space Vehicle Emissions Modeling 39 The first-order estimate of the black carbon emissions index for LOX/RP-1 engines is already highly uncertain, and the estimates for the other propellants have an even greater amount of uncertainty. Future measurements will be required to reduce the uncertainty and provide more accurate black carbon emissions indices for different types of propellants. 4.4 Emissions Indices for Commercial Space Vehicles The methods described in Section 4.2 and Section 4.3 were applied to estimate the primary and final emissions indices for current and in-development commercial space vehicles. The final emissions indices are the values needed for the commercial space vehicle emissions model because they describe the pollutants that a space vehicle ultimately emits into the atmosphere. However, the final emissions indices vary with altitude, whereas the primary emissions indices are a property of the rocket engine alone. Thus, the primary emissions indices are stored in the internal fleet database of the commercial space vehicle emissions model, and the final emissions indices are calculated from the primary emissions indices and the user-specified altitude profile using the first-order estimates presented in Section 4.3. The following sections provide estimates of the primary and final emissions indices for multiple commercial space vehicles and other representative launch vehicles in the fleet database. 4.4.1 Primary Emissions Indices As discussed above, the primary emissions indices are stored in the internal fleet database of the commercial space vehicle emissions model. Table 7 lists the primary emissions indices for the first- stage rocket engines of numerous launch vehicles in the fleet database. Every orbital-class commercial space vehicle that was built or launched in the United States in 2019 [114] is included in Table 7. Several other licensed [46] and in-development commercial space vehicles are also listed. Although the Proton rocket is not a U.S. launch vehicle, it provides reference values for hydrazine (N2O4/UDMH) propellants. Additionally, the Space Shuttle and Saturn V are included for historical interest and validation purposes.

Commercial Space Vehicle Emissions Modeling 40 Table 7. Estimates of the primary emissions indices, in grams of pollutant emitted per kilogram of propellant consumed, for first-stage rocket engines. Propellant Vehicle Engine H2O H2 OH CO2 CO Al2O3 HCl Cl NOx N2 LOX/Hydrogen Delta IV RS-68A 965 35 0 – – – – – – – New Shepard BE-3 965 35 0 – – – – – – – Space Shuttle SSME 959 35 0 – – – – – – – LOX/RP-1 Antares 230 RD-181 284 6 0 470 240 – – – – – Atlas V RD-180 284 6 0 470 240 – – – – – Electron Rutherford 278 9 0 375 337 – – – – – Falcon 9/Heavy Merlin 1D 263 12 0 352 372 – – – – – Saturn V F-1 250 15 0 335 399 – – – – – LOX/Methane New Glenn BE-4 439 11 0 360 189 – – – – – Starship Raptor 452 2 2 492 51 – – – – – APCP (HTPB) Atlas V AJ-60A 71 27 0 18 228 358 209 5 0 82 Delta IV GEM-60 55 30 0 13 251 357 205 5 0 81 Minotaur IV SR-118 71 27 0 18 228 358 209 5 0 82 Minotaur-C Castor 120 71 27 0 18 228 358 209 5 0 82 Pegasus XL Orion 50SXL 71 27 0 19 228 359 209 4 0 82 APCP (PBAN) Minotaur I M55A1 130 13 6 48 188 300 194 22 1 94 Space Shuttle RSRM 93 21 0 34 241 301 212 3 0 87 N2O/HTPB SpaceShipTwo RocketMotorTwo 100 1 0 240 99 – – – 4 558 N2O4/UDMH Proton RD-253 290 4 0 289 69 – – – 0 348 As discussed in Section 4.1, the emissions indices for space vehicles are expressed in grams of pollutant emitted per kilogram of propellant consumed. Since the primary emissions are formed due to processes that occur solely within the rocket engine, the primary emissions indices must sum to 1,000 grams of pollutants emitted per kilogram of propellant consumed. Due to rounding, the primary emissions indices listed in Table 7 may not exactly sum to 1,000 g/kg. Furthermore, Table 7 omits minor chemical species, such as atomic hydrogen (H) and diatomic chlorine (Cl2), that are formed in quantities of 1 g/kg or less. The primary emissions indices for the following launch vehicles listed in Table 7 were taken from publicly available literature sources:  SpaceShipTwo primary emissions indices were reported in the Environmental Assessment for SpaceShipTwo at Mojave Air and Space Port [26]; and  Space Shuttle primary and secondary emissions indices were reported in the Environmental Impact Statement for the Space Shuttle program [14].

Commercial Space Vehicle Emissions Modeling 41 However, high-quality predictions or measurements of the emissions indices are not publicly available for the remaining launch vehicles. Thus, most of the primary emissions indices listed in Table 7 were estimated using CEA. As discussed in Section 4.2, CEA is subject to uncertainties arising from the accuracy of the user- supplied input parameters. CEA calculates the emissions indices based on the propellant species, mixture ratio, chamber pressure, and nozzle area ratio. These parameters are not publicly available for some of the rocket engines listed in Table 7 and had to be estimated instead. The estimates are particularly uncertain for rocket engines that are currently under development, such as the Blue Origin BE-4 and SpaceX Raptor. Furthermore, CEA is based on simplifying assumptions that introduce additional uncertainties into the primary emissions indices. For example, CEA cannot accurately predict the primary emissions indices of minor chemical species, such as complex hydrocarbons and black carbon, that form due to nonequilibrium processes or incomplete combustion. Thus, unburned hydrocarbons and black carbon are not listed in Table 7. However, CEA’s simplifying assumptions are generally reasonable for predicting the major products of combustion, which account for the largest primary emissions indices. 4.4.2 Final Emissions Indices The final emissions indices are calculated within the commercial space vehicle emissions model using the first-order estimates developed in Section 4.3. The first-order methods require only the primary emissions indices from the fleet database and the user-specified altitude profile to estimate the final emissions indices. Since these calculations are performed internally by the commercial space vehicle emissions model, end users will not need to calculate the final emissions indices directly. However, examples of the final emissions indices are instructive for understanding the pollutants that a space vehicle ultimately emits into the atmosphere. Thus, this section presents estimates of the final emissions indices at two different altitudes for the first-stage rocket engines listed in Table 7. Final Emissions Indices at Sea Level Table 8 lists the final emissions indices at sea level for the first-stage rocket engines, and Figure 25 shows these values graphically for selected rocket engines. The emissions indices are expressed in grams of pollutant emitted per kilogram of propellant consumed. The sum of the final emissions indices may be greater than 1,000 g/kg if the reactions between the high-temperature exhaust plume and the surrounding air add mass from the air to the plume. Conversely, the final emissions indices listed in Table 8 may sum to less than 1,000 g/kg because minor product species are not included in the table. Additionally, any nitrogen molecules (N2) emitted by the rocket engine are considered to be part of the surrounding air. A comparison between Table 7 and Table 8 reveals major differences between the primary and final emissions indices due to the chemical reactions in the high-temperature exhaust plume. For example, hydrogen (H2) is converted into water vapor (H2O), carbon monoxide (CO) is oxidized to carbon dioxide (CO2), and NOx forms due to afterburning in the high-temperature exhaust plume at sea level.

Commercial Space Vehicle Emissions Modeling 42 Table 8. Estimates of the final emissions indices, in grams of pollutant emitted per kilogram of propellant consumed, for first-stage rocket engines at sea level. Propellant Vehicle Engine H2O CO2 CO Al2O3 Clx NOx BC LOX/Hydrogen Delta IV RS-68A 1277 – – – – 33 0 New Shepard BE-3 1277 – – – – 33 0 Space Shuttle SSME 1272 – – – – 33 0 LOX/RP-1 Antares 230 RD-181 338 850 2 – – 33 1 Atlas V RD-180 338 850 2 – – 33 1 Electron Rutherford 362 911 2 – – 33 1 Falcon 9/Heavy Merlin 1D 374 944 2 – – 33 1 Saturn V F-1 384 970 2 – – 33 1 LOX/Methane New Glenn BE-4 539 661 1 – – 33 0 Starship Raptor 469 571 1 – – 33 0 APCP (HTPB) Atlas V AJ-60A 318 381 1 358 213 33 1 Delta IV GEM-60 329 413 1 357 210 33 1 Minotaur IV SR-118 318 381 1 358 213 33 1 Minotaur-C Castor 120 318 381 1 358 213 33 1 Pegasus XL Orion 50SXL 318 381 1 359 214 33 1 APCP (PBAN) Minotaur I M55A1 270 348 1 300 216 34 1 Space Shuttle RSRM 281 418 1 301 215 33 1 N2O/HTPB SpaceShipTwo RocketMotorTwo 109 397 1 – – 37 1 – – N2O4/UDMH Proton RD-253 325 397 1 – – 33 0

Commercial Space Vehicle Emissions Modeling 43 Figure 25. Estimates of the final emissions indices, in grams of pollutant emitted per kilogram of propellant consumed, for selected first- stage rocket engines at sea level.

Commercial Space Vehicle Emissions Modeling 44 Final Emissions Indices at 40 km Table 9 lists the final emissions indices at an altitude of 40 km (25 mi) in the upper stratosphere for the same first-stage rocket engines. A comparison between Table 8 and Table 9 reveals that some of the final emissions indices vary as the density of oxygen (O2) molecules in the surrounding air decreases at higher altitudes. Due to decreasing oxidation, the final emissions indices of carbon dioxide and NOx decrease with altitude. Conversely, decreasing oxidation causes the final emissions indices of carbon monoxide and black carbon to increase with altitude. These physical processes are captured in the commercial space vehicle emissions model through the first-order estimates developed in Section 4.3. Table 9. Estimates of the final emissions indices, in grams of pollutant emitted per kilogram of propellant consumed, for first-stage rocket engines at an altitude of 40 km (25 mi). Propellant Vehicle Engine H2O CO2 CO Al2O3 Clx NOx BC LOX/Hydrogen Delta IV RS-68A 1277 – – – – 0 0 New Shepard BE-3 1277 – – – – 0 0 Space Shuttle SSME 1272 – – – – 0 0 LOX/RP-1 Antares 230 RD-181 338 811 26 – – 0 20 Atlas V RD-180 338 811 26 – – 0 20 Electron Rutherford 362 872 26 – – 0 20 Falcon 9/Heavy Merlin 1D 374 904 26 – – 0 20 Saturn V F-1 384 930 27 – – 0 20 – – LOX/Methane New Glenn BE-4 539 631 20 – – 0 4 Starship Raptor 469 541 20 – – 0 4 APCP (HTPB) Atlas V AJ-60A 318 368 9 358 213 0 20 Delta IV GEM-60 329 399 10 357 210 0 20 Minotaur IV SR-118 318 368 9 358 213 0 20 Minotaur-C Castor 120 318 368 9 358 213 0 20 Pegasus XL Orion 50SXL 318 368 9 359 214 0 20 APCP (PBAN) Minotaur I M55A1 270 335 9 300 216 1 20 Space Shuttle RSRM 281 403 10 301 215 0 20 N2O/HTPB SpaceShipTwo RocketMotorTwo 109 378 12 – – 4 20 – – N2O4/UDMH Proton RD-253 325 378 13 – – 0 0

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Federal Aviation Administration (FAA) regulations require the licensing of spaceports and launch vehicles, which includes the assessment of environmental impacts.

The TRB Airport Cooperative Research Program’s ACRP Web-Only Document 51: Commercial Space Vehicle Emissions Modeling presents a user-friendly tool for practitioners to estimate the emissions associated with commercial space vehicle activity.

Supplementary materials to the document include an Emissions Example Information & Users Guide, the RUMBLE application, and a RUMBLE User Guide.

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