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Commercial Space Vehicle Emissions Modeling 45 5 Model Validation The commercial space vehicle emissions model is based on the best available data in the literature. Nevertheless, the model must be validated before it can be applied to analyze the environmental impacts of commercial space operations. The key results of the emissions model that require validation are the propellant burn report and the emissions inventory. The following sections discuss the completed and proposed validation efforts for the commercial space vehicle emissions model: ï Section 5.1 describes the model validation results based on the limited data sources available in the literature, and ï Section 5.2 proposes a future model validation plan that leverages advanced computational modeling and field measurements to fill major gaps in the literature. 5.1 Model Validation Results Although the commercial space vehicle emissions model was developed and validated using the best available data in the literature, the availability of high-quality emissions data for current commercial space vehicles is extremely limited. The scarcity of data leads to uncertainty in the emissions model. Future emissions measurements of commercial space vehicles are needed to fill the major gaps in the literature and further reduce the uncertainty in the emissions model. The following sections describe the sources of uncertainty in the emissions model and the validation efforts that were performed. End-to-end model validation was performed using the Space Shuttle because it has the most complete and well-documented emissions data in the literature. The results of this model validation effort suggest that the emissions model has lower uncertainty than the emissions data presented in most environmental assessments of commercial space vehicles. The commercial space vehicle emissions model is based on the best available emissions data and provides the most accurate means to analyze the environmental impacts of commercial space operations. 5.1.1 Sources of Uncertainty The accuracy of the commercial space vehicle emissions model depends on the accuracy of the input parameters from the fleet database and user-specified operational data. The input parameters that affect the propellant burn report and emissions inventory are discussed below. Propellant Burn Report From Eq. (5), the amount of propellant burned by a space vehicle during each trajectory segment depends on the following parameters: ï Propellant mass flow rate, and ï Trajectory segment duration. The duration of each trajectory segment is calculated from the user-specified trajectory. Users may also provide the time-varying propellant mass flow rate as part of the operational data. If both parameters are provided by the user, the accuracy of the propellant burn report is completely
Commercial Space Vehicle Emissions Modeling 46 governed by the accuracy of the user-specified operational data. If the user does not provide the time-varying mass flow rate, the nominal mass flow rate and burn time from the fleet database are used to calculate the propellant mass burned. In such cases, the accuracy of the propellant burn report depends on the accuracy of the engine performance data in the fleet database. Initial validation efforts for the engine performance data in the fleet database are discussed in Section 5.1.2. Emissions Inventory From Eq. (6), the amount of each pollutant emitted by a space vehicle during each trajectory segment depends on the following parameters: ï Amount of propellant burned, and ï Emissions index for the pollutant. As discussed in Section 4, the emissions index for a pollutant reports the outcome of a complex series of chemical reactions as a single number. However, the methods used to estimate the emissions indices are subject to many simplifying assumptions. Thus, the emissions indices are the parameters in the commercial space vehicle emissions model with the highest uncertainty. Initial validation efforts for the primary emissions indices are described in Section 5.1.3, and the final emissions indices are discussed in Section 5.1.4. A full validation of the emissions inventory is presented in Section 5.1.5. 5.1.2 Engine Performance Data Manufacturers typically provide time-varying propellant mass flow rates for the preparation of environmental documents in support of launch vehicle operator licenses. However, if the user does not provide the time-varying mass flow rate, the nominal mass flow rate and burn time from the fleet database are used to calculate the propellant mass burned. As discussed in Section 2.2, the nominal propellant mass flow rate of a rocket engine may be estimated from the following parameters: ï Sea level thrust and specific impulse, or ï Total propellant mass and burn time. Such estimates are subject to uncertainty arising from the following sources: ï These engine performance parameters are not publicly available for all commercial rocket engines, especially engines that are currently under development. ï The total propellant mass typically includes excess propellant margin that is not consumed during ascent, which will result in an overestimate of the average mass flow rate. ï The mass flow rate is assumed to remain constant over the burn time, but the actual mass flow rate may fluctuate if the rocket engine throttles up or down during the operation. Our team reached out to engine and vehicle manufacturers to confirm the values of the sea level thrust, specific impulse, total propellant mass, burn time, and propellant mass flow rate in the fleet database. However, no manufacturers responded to our request for information. Thus, the engine performance parameters for most commercial space vehicles in the fleet database are subject to uncertainty.
Commercial Space Vehicle Emissions Modeling 47 The uncertainty in the propellant mass flow rates in the fleet database can be partially quantified by comparing the initial and average mass flow rates listed in Table 2. The initial mass flow rate is estimated from the sea level thrust and specific impulse, and the average mass flow rate is estimated from the total propellant mass and burn time. Thus, these estimates are independent of each other. The median percentage error between the initial and average mass flow rates in the fleet database is approximately 4%. This excellent agreement provides confidence in the estimates of the propellant mass flow rates for the commercial space vehicles in the fleet database. 5.1.3 Primary Emissions Indices As discussed in Section 4.2, the primary emissions indices are calculated using CEA if they are not available in the literature. CEA requires the following input parameters: ï Propellant species, ï Mixture ratio, ï Combustion chamber pressure, and ï Nozzle area ratio. These engine parameters are not publicly available for all rocket engines, and they had to be estimated for some of the engines in the fleet database. Our team reached out to engine and vehicle manufacturers to confirm the values of these parameters, but no manufacturers responded to our request for information. CEA is highly sensitive to errors in the propellant species and mixture ratio, somewhat less sensitive to errors in the nozzle area ratio, and relatively insensitive to errors in the combustion chamber pressure. Thus, errors in the estimated engine parameters may lead to errors in the calculated primary emissions indices. Additionally, CEA is based on numerous simplifying assumptions, which introduce another source of uncertainty into the primary emissions indices. For example, CEA assumes chemical equilibrium and complete combustion, so it cannot accurately predict the primary emissions indices of minor chemical species that form due to nonequilibrium processes or incomplete combustion. Despite these limitations, CEAâs simplifying assumptions are generally reasonable for predicting the major products of combustion, which account for the largest primary emissions indices. The primary emissions indices calculated by CEA were validated compared to published emissions indices for the Space Shuttle , as described below. Space Shuttle Primary Emissions Indices The 1978 EIS for the Space Shuttle program  includes the primary emission indices for both the SSMEs and RSRMs. The emissions indices in the EIS were computed using the best available computational models at the time . The results represent a validation case to assess the effects of CEAâs simplifying assumptions on predictions of the primary emissions indices. The primary emissions indices from CEA and the EIS  are listed in Table 10 for the SSME and RSRM. The CEA predictions for the SSME demonstrate excellent agreement compared to the EIS, and the CEA predictions for the RSRM match the EIS results reasonably well. The following observations describe some of the differences between CEA and the EIS for the RSRM:
Commercial Space Vehicle Emissions Modeling 48 ï CEA predicts a somewhat higher amount of H2O and lower amount of H2 compared to the EIS, which suggests that the chemical reaction may not achieve equilibrium in the rocket engine. ï The ratio of CO2 to CO is approximately 1 to 4 for the CEA predictions, whereas the ratio is approximately 1 to 7 for the EIS results. The ratio of CO2 to CO is a measure of the completeness of combustion, where higher amounts of CO2 relative to CO indicate that combustion is closer to completeness . ï The partitioning of chlorine species between HCl and Cl is different for the CEA and EIS results. However, this difference does not affect the final results of the emissions model because the sum of all chlorine species is reported as Clx. These results demonstrate that the assumptions of chemical equilibrium and complete combustion are not entirely valid for rocket engines. However, the primary emissions indices presented in the Space Shuttle EIS  are based on models that are subject to uncertainty, too. For example, the EIS did not model minor chemical species such as black carbon or other unburned hydrocarbons. Thus, this validation case for the Space Shuttle demonstrates that the CEA predictions match the best available data in the literature reasonably well. Although the primary emissions indices predicted by CEA are subject to uncertainty, they provide the most accurate estimates currently available for commercial space vehicles. Table 10. Primary emissions indices, in grams of pollutant emitted per kilogram of propellant consumed, for the Space Shuttle. Species SSME RSRM CEA Ref.  CEA Ref.  H2O 966 959 129 93 H2 34 35 13 21 H 0 0 1.2 0 OH 0 0 7.1 0.2 CO2 â â 47 34 CO â â 189 241 Al2O3 â â 299 301 HCl â â 192 212 Cl â â 23 3 NOx â â 0.7 0 5.1.4 Final Emissions Indices The previous section showed that the primary emissions indices predicted by CEA are subject to uncertainty, but they are the best available estimates for the commercial space vehicle emissions model. As discussed in Section 4.3, the first-order estimates for the final emissions indices are subject to higher uncertainty, particularly at high altitudes and for minor combustion species. However, few detailed studies of the final emissions indices for space vehicles are reported in the literature.
Commercial Space Vehicle Emissions Modeling 49 Furthermore, most of the results that are reported were used to develop the first-order estimates, so these sources do not provide independent validation results for the final emissions indices. The remaining literature sources investigated the effects of alumina, chlorine, or NOx emissions from solid rocket motors on stratospheric ozone, but the rockets involved in those studies have since been retired. Thus, high-quality computations or measurements that are needed to validate the final emissions indices for commercial space vehicles are unavailable in the literature. 5.1.5 Emissions Inventory Uncertainty in the engine performance data and emissions indices described in the previous sections could lead to errors in the propellant burn report and emissions inventory. Ultimately, the emissions inventory is the final result that must be validated before the commercial space vehicle emissions model can be applied to analyze the environmental impacts of commercial space operations. However, no high-quality emissions inventories that include all secondary emissions species for current commercial space vehicles are available in the literature. Overview of the Emissions Inventory Validation The Space Shuttle EIS  contains the most complete and well-documented emissions inventory in the literature. The Space Shuttle used both liquid-propellant rocket engines and solid-propellant boosters during ascent, which provides an excellent validation case. Additionally, the Space Shuttle trajectory, engine performance, and emissions indices are publicly available. Thus, even though it is not a current commercial space vehicle, the Space Shuttle was used to perform the end-to-end validation of the commercial space vehicle emissions model. The emissions inventory for the Space Shuttle was calculated under the following assumptions, which were necessary to compare the results to the Space Shuttle EIS : ï The primary emissions indices from the Space Shuttle EIS, which are the emissions indices listed in Table 7 and stored in the fleet database, were used instead of CEA calculations; ï The altitude profile was based on the STS-124 trajectory shown in Figure 15 with several seconds of hold-down time added to the start of the trajectory; ï The nominal propellant mass flow rates of the SSME and RSRM were assumed to remain constant; ï The burn time of the RSRM was decreased to 112.5 seconds to match the results of the Space Shuttle EIS; ï Both RSRMs and only one of the three SSMEs were modeled to match the results of the Space Shuttle EIS; and ï The altitudes of the atmospheric layers were adjusted to match the Space Shuttle EIS.
Commercial Space Vehicle Emissions Modeling 50 Results of the Emissions Inventory Validation Figure 26 shows the comparison between the emissions inventory calculated by the commercial space vehicle emissions model and the emissions inventory published in the Space Shuttle EIS . The pollutant masses in metric tons are presented in the surface boundary layer, troposphere, and stratosphere based on the altitude bands defined in the Space Shuttle EIS. Overall, the calculated pollutant masses of the major emissions species demonstrate close agreement to the Space Shuttle EIS. Since the mass of alumina and the total mass of chlorine species remain unmodified in the exhaust plume, the results for these species provide confidence in the trajectory segment durations and burn time. The following observations describe the validation results for the other pollutant species: ï The close agreement for water vapor validates the assumption that all hydrogen and hydroxyl at the nozzle exit plane is converted to water vapor in the exhaust plume. ï The amount of carbon monoxide is underpredicted at all altitudes, and the amount of carbon dioxide is overpredicted above the surface boundary layer. These results suggest that the first-order estimates for CO and CO2 may overpredict the degree of oxidation that occurs in the exhaust plume. ï The amount of NOx is overpredicted in the surface boundary layer, reasonably predicted in the free troposphere, and underpredicted in the stratosphere. These results suggest that the first-order estimate for the amount of NOx formed in the exhaust plume may decrease too rapidly with altitude. Although the emissions inventory in the Space Shuttle EIS was based on the best available computational models at the time the EIS was written, these models are also subject to uncertainty. Additionally, the Space Shuttle EIS did not include calculations of black carbon, so the first-order estimate for the black carbon emissions index could not be independently validated. Despite the remaining uncertainty, the results of this validation effort suggest that the commercial space vehicle emissions model is reasonably accurate to analyze the environmental impacts of commercial space operations.
Commercial Space Vehicle Emissions Modeling 51 Figure 26. Comparison between the emissions inventory calculated by the commercial space vehicle emissions model and the emissions inventory published in the Space Shuttle EIS .
Commercial Space Vehicle Emissions Modeling 52 5.2 Future Model Validation Plan As discussed in the previous section, no high-quality emissions inventories that include all secondary emissions species for current commercial space vehicles are available in the literature. Thus, additional work is needed to fully validate the commercial space vehicle emissions model. As described in the following sections, the future model validation approach should include both high- fidelity modeling and field measurements at different altitudes. 5.2.1 Background The secondary emissions formed by the chemical reactions in the high-temperature rocket exhaust plume depend on multiple factors, including: ï Initial conditions at the nozzle exit plane, ï Meteorological conditions of the surrounding air, ï Chemical composition of the exhaust plume and surrounding air, ï Turbulent mixing between the exhaust plume and surrounding air, ï Incoming solar radiation, and ï Chemical kinetic processes and reaction rates. These factors are complex and interrelated. For example, the turbulent mixing between the exhaust plume and surrounding air depends on the pressure and velocity of the ambient air. Similarly, the chemical reaction rates are governed by the ambient temperature and incoming solar radiation. Thus, the formation of secondary emissions is strongly dependent on the ambient environment surrounding the rocket. Since launch and re-entry vehicles traverse the entire vertical extent of the atmosphere, the ambient conditions vary from sea level temperature, pressure, and relative humidity at the ground to near- vacuum conditions in orbit. Additionally, the local concentrations of reactive species such as ozone and radicals change dramatically between different atmospheric layers. As a result, the secondary emissions produced by commercial space vehicles vary significantly with altitude. Furthermore, the relative environmental importance of each pollutant varies with altitude. Thus, the commercial space vehicle emissions model must be validated at ground level and across a range of altitudes to ensure the results are appropriate for applications ranging from local air quality analyses to global impact assessment models.
Commercial Space Vehicle Emissions Modeling 53 5.2.2 High-Fidelity Modeling High-fidelity computational models should be leveraged as the first step to validate the final emissions estimates for commercial space vehicles across a range of altitudes. The following high- fidelity codes may be useful for future model validation: ï The Rocket Exhaust Effluent Diffusion Model (REEDM)  or the AERMOD modeling system  can be used to model the dispersion of the ground cloud formed at the launch site due to a rocket launch. AERMOD is the dispersion model used in AEDT. ï The Two-Dimensional Kinetics (TDK), Solid Propellant Rocket Motor Performance Prediction (SPP), and SPF-III codes can be used to model the chemical kinetics and turbulent mixing of the flow inside the rocket engine and in the high-temperature exhaust plume [112, 117]. For example, Denison, et al. applied SPP and SPF to calculate the secondary emissions as a function of distance behind the nozzle exit plane of a proposed solid rocket motor . ï Loci/CHEM is a Computational Fluid Dynamics solver that simulates three-dimensional, turbulent, chemically reacting flows, and it can be used to perform extremely high-fidelity simulations of the flow inside the rocket engine and in the exhaust plume , as shown in Figure 27. These high-fidelity models simulate the complex chemical kinetics and turbulent mixing processes between the high-temperature exhaust plume and surrounding air that produce the secondary emissions. High-fidelity modeling is significantly more cost effective than measurements for obtaining results across a wide range of vehicles, propellants, and altitudes. Thus, high-fidelity modeling could provide excellent validation cases for the first-order estimates of the final emissions indices. Figure 27. Simulation of a space vehicle and exhaust plume in Loci/CHEM.
Commercial Space Vehicle Emissions Modeling 54 5.2.3 Field Measurements Although high-fidelity modeling is recommended as the first step for future model validation, even advanced computational models must be validated based on measurements. Unfortunately, only a limited number of rocket emissions measurements have been conducted to date. A review of these past measurements provides a starting point for the design of future measurements to validate the commercial space vehicle emissions model. Measurements performed during the RISO study [41, 42] in the late 1990s and early 2000s provided important data for rocket emissions at altitude. However, the study measured only a limited number of launch vehicles and propellant types, and most of the measured vehicles are no longer in service. Additionally, measurement technologies and platforms have advanced considerably since the RISO study. Current technologies could provide more accurate measurements, especially for emissions of NOx and PM at altitude. Confidence in the commercial space vehicle emissions model would be greatly increased by conducting new measurements on current commercial rocket engines using state-of-the-art measurement methodologies. Measurement Campaign Roadmap Since emissions measurements can be complex and expensive, a high-level roadmap for the measurement campaign should be developed before any measurements are conducted. The roadmap will ensure the measurement campaign is optimally designed to: ï Validate the commercial space vehicle emissions model, ï Expand the database of emissions indices, and ï Provide confidence in the high-fidelity chemical kinetics, turbulent mixing, and atmospheric dispersion models. Measurements of trace species that are not primary combustion products would be most useful since they are not as well understood as the species that directly affect rocket performance. Even minor emissions species, such as NOx and black carbon, may have major environmental impacts on the local air quality near launch sites and on global climate change due to emissions at high altitudes. Understanding which chemical species have the largest environmental impacts will guide the development of the measurement campaign roadmap. The roadmap should consider a hierarchy of approaches for measurement locations and types of measurements. Measurements at the ground and at different altitudes are required to fully validate the emissions model. Ground-based measurements should be conducted first since they are the least resource-intensive, and they enable the validation of the emissions model for species that are crucial for the preparation of environmental documents. As the ground-level measurements become more mature and sophisticated, higher-altitude measurements should be conducted to validate the changing chemical dynamics that occur in the exhaust plume at higher altitudes. A conceptual roadmap for future model validation, including high-fidelity modeling and measurements at different altitudes, is shown in Figure 28. The following sections describe the proposed ground-based and high- altitude measurements in greater detail.
Commercial Space Vehicle Emissions Modeling 55 Figure 28. Conceptual roadmap for future model validation. Ground-Based Measurements The first ground-level emissions measurements should be conducted in association with scheduled rocket engine testing activities. Since the operation of rocket engines requires significant resources, a dedicated test for emissions measurements would be prohibitively expensive. Significant cost savings would be achieved by coordinating emissions measurements with commercial rocket engine manufacturers and government laboratories who perform rocket engine performance tests. For example, Figure 29 shows an engine test at the NASA Marshall Space Flight Center. Emissions measurements at engine test stands would be useful for validating emissions indices and measurement techniques. Figure 29. Rocket engine test at the NASA Marshall Space Flight Center.
Commercial Space Vehicle Emissions Modeling 56 In addition to emissions measurements at engine test stands, ground-based emissions measurements should be conducted at rocket launch sites. The emissions produced by a rocket during launch may differ from the emissions produced by a rocket engine on a test stand due to the following factors: ï Orientation of the rocket engine, ï Plume impingement on the ground, ï Plume interactions between multiple engines, ï Rocket engine power settings, ï Reactions with local chemical compounds, ï Deluge water at the launch pad, ï Dispersion time, and ï Initial ascent. Ground-based measurement techniques at launch sites include both remote sensing and standalone monitoring. Remote sensing techniques rely on phenomena such as spectral reflection or absorption to measure specific pollutants at varying distances from the rocket. For example, LIDAR-based instrumentation at a single location could be used to measure the ultraviolet reflection from small particulate matter. Instruments at two locations may be needed to measure the concentrations of CO, CO2, and NOx based on infrared laser absorption. The advantages of remote sensing include the ability to gather large numbers of samples in the relatively short duration of the launch, measure different portions of the plume from a single measurement location, and characterize the plume dispersion parameters. Although remote sensing offers numerous advantages for ground-based emissions measurements, standalone instrumentation provides higher resolution than remote sensing. However, the accuracy of standalone measurements is directly related to the sampling locations. Currently, the optimal sampling locations for launch vehicle emissions are not well understood. A measurement plan should be developed based on the remote sensing results to determine the optimal standalone instrument locations and the expected range of pollutant concentrations at each location. Remotely operated instruments positioned at key locations where rocket plume encounters are expected would enable a wide range of emissions species and concentration levels to be measured simultaneously. These high-sensitivity measurements will provide extensive validation data for the primary and secondary emissions produced by commercial space vehicles. Measurements at Altitude The secondary emissions produced by rocket engines and the relative environmental importance of each pollutant vary significantly with altitude. Thus, emissions measurements at higher altitudes are required to fully validate the commercial space vehicle emissions model. High-altitude measurements are typically complex and costly, and inexpensive alternatives are unavailable. However, the experience gained from prior studies will inform the planning of an effective measurement campaign at altitude:
Commercial Space Vehicle Emissions Modeling 57 ï The ground-based measurements of commercial space vehicle emissions described above will provide the expected emissions species and concentration levels for commercial rocket engines. ï The RISO rocket encounters [41, 42] provide a baseline for measurement logistics planning and sampling techniques for launch vehicle emissions at altitude. ï High-altitude aviation emissions measurements, such as the recent international NASA/DLR Multidisciplinary Airborne ExperimentâEmission and Climate Impact of alternative Fuel (NDMAX/ECLIF) campaign , provide experience with state-of-the-art sampling techniques that could be applied to commercial space vehicles. Emissions measurements at altitude can be conducted using remote sensing and plume capture instrumentation installed on chase planes. Chase planes have been used extensively in atmospheric and aviation emissions measurement campaigns to sample aircraft exhaust plumes, as shown in Figure 30. Instrumentation requirements for rocket plume encounters are similar to those for aviation emissions measurements. However, a chase plane can follow an aircraft in steady, level flight to sample its exhaust plume over a long duration and distance, whereas a chase plane must fly laterally through a rocket exhaust plume. Rocket plume intercepts are typically only a few seconds in duration, so instruments with rapid response times are necessary. Additionally, logistical and safety concerns for rocket encounters limit how soon a chase plane can intercept the exhaust plume after the rocket passes. The amount of time available for useful emissions measurements before the chemical species concentrations approach background levels varies depending upon the winds aloft. Figure 30. Photograph of the NASA DC-8 chase plane following the DLR A320 aircraft during the NDMAX/ECLIF measurement campaign.
Commercial Space Vehicle Emissions Modeling 58 The chase planes that are instrumented for aviation emissions measurements, such as NASAâs DC-8 and DLRâs Halo aircraft, are capable of flying in the troposphere and lower stratosphere. NASAâs WB-57 aircraft is the highest-altitude airborne emissions platform, with a ceiling of roughly 60,000 ft (20 km). Emissions measurements in these altitude bands will provide important validation data for the secondary emissions produced by commercial space vehicles. Measurements in the upper stratosphere and mesosphere are also of interest, but plume intercepts at these high altitudes are not supported by existing measurement platforms. However, validation data obtained in the troposphere and lower stratosphere would likely reduce the uncertainty in the emissions estimates in the upper stratosphere and mesosphere. The measurements at altitude should be targeted to validate the emissions estimates for the chemical species with the most significant environmental impacts and highest uncertainty in the commercial space vehicle emissions model. The following key chemical species should be measured as functions of altitude: ï The amount of NOx produced by different types of rocket engines, ï The mass, number, and size distribution of black carbon particles produced by different types of carbon-containing rocket propellants and different sizes of rocket engines, ï The size distribution of alumina particles produced by solid propellants, and ï The partitioning between the different chlorine species produced by solid propellants. In addition, CO2 and CO measurements for carbon-containing propellants are needed to reference the concentrations of trace emissions species to the amount of propellant consumed in order to calculate the emissions indices. Measurements of water vapor and ice content are also useful for aviation contrail measurements and may be of interest to further understand PM evolution in the rocket exhaust plume. Alternatively, local water concentrations can be reasonably derived from the propellant mass consumed. Such a calculation approach is needed in the mesosphere, where water vapor is likely to have the most significant environmental impacts, since the mesosphere is above the ceiling of any chase plane. Nevertheless, emissions measurements in the troposphere and lower stratosphere will provide extensive validation data for the altitude dependence of the secondary emissions produced by commercial space vehicles.