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Commercial Space Vehicle Emissions Modeling 80 8 Conclusions and Suggested Research The objective of ACRP project 02-85 was to develop a method for estimating the emissions from commercial space vehicle operations. This report describes the research activities conducted under ACRP project 02-85. The following sections conclude this report: ï Section 8.1 summarizes the results of this research, and ï Section 8.2 provides suggestions for future research. The primary products of this research are a software tool and accompanying user guide that enable practitioners to estimate the emissions from commercial space vehicle operations. These products and the research activities described in this report fulfilled the objectives of ACRP project 02-85. 8.1 Conclusions To fulfill the objectives of ACRP project 02-85, our team performed the following research activities: ï Conducted a literature review to identify relevant sources, ï Created an emissions modeling methodology for commercial space vehicles, ï Built a database of emissions indices for commercial space vehicles, ï Performed model validation of the commercial space vehicle emissions model, ï Proposed an AEDT integration plan, and ï Developed an emissions modeling tool and accompanying user guide. The key results of these research activities are summarized in the following sections. Literature Review Space vehicle emissions are produced by a series of complex chemical reactions inside the rocket engine and in the high-temperature exhaust plume: ï Primary emissions are the chemical species present at the nozzle exit plane, which are formed by reactions inside the rocket engine; ï Secondary emissions are the chemical species formed outside the rocket engine due to afterburning and other reactions in the high-temperature exhaust plume; and ï Final emissions are the chemical species that the vehicle ultimately emits into the atmosphere, which include contributions from both the primary and secondary emissions. A literature review was conducted to identify quantitative emissions data, engine performance data, and trajectory data relevant to commercial space vehicle emissions modeling. The results of the literature review are summarized below: ï The availability of high-quality, quantitative emissions data is extremely limited. ï The propellant mass flow rate, which is required for emissions modeling, can be estimated from engine performance data typically published by manufacturers. ï Trajectory data are not publicly available for most commercial space vehicles. Instead, trajectories from historical NASA missions can be used for emissions research projects. For environmental analyses, trajectories are typically provided by launch vehicle manufacturers.
Commercial Space Vehicle Emissions Modeling 81 Emissions Modeling Methodology The emissions modeling methodology was developed to enable users to estimate the emissions from commercial space vehicle operations. The inputs, calculations, and outputs are summarized below. The inputs to the emissions model are an internal fleet database and user-defined operational data. The fleet database contains the vehicle- and engine-specific data required by the emissions model. The user-specified operational data must include the altitude as a function of time and may optionally include the time-varying propellant mass flow rate. The commercial space vehicle emissions model calculates the mass of propellant burned and the mass of each pollutant emitted by commercial space vehicles. The calculations are first performed at the detailed trajectory segment level. The propellant mass burned during a trajectory segment is a product of the trajectory segment duration and the propellant mass flow rate. The mass of each pollutant emitted is a product of the propellant mass burned and the emissions index for the pollutant. The outputs of the commercial space vehicle emissions model are the propellant burn report and the emissions inventory. The propellant burn report summarizes the propellant mass burned by each type of engine on commercial space vehicles. The emissions inventory enumerates the masses of the various pollutants emitted into each atmospheric layer. Database of Emissions Indices The commercial space vehicle emissions model uses emissions indices to estimate the amount of pollutants emitted by space vehicles. The definition of emissions indices, methods for estimating the primary and final emissions indices, and the database of emissions indices are summarized below. Emissions indices are the factors that relate the amount of propellant burned to the amount of each pollutant emitted by a rocket engine. For space vehicles, the emissions indices are defined as the grams of pollutant emitted per kilograms of propellant consumed, where the propellant includes the fuel plus the oxidizer. The emissions index for a pollutant reports the outcome of a complex series of chemical reactions as a single number. The primary emissions indices describe the chemical species present at the nozzle exit plane. High- quality predictions of the primary emissions indices are not publicly available for most commercial space vehicles. If high-quality values are unavailable, the primary emissions indices are predicted using the computer program Chemical Equilibrium with Applications (CEA). The final emissions indices describe the chemical species that the space vehicle ultimately emits into the atmosphere. The final emissions include contributions from the primary and secondary emissions. Secondary emissions are formed in the high-temperature exhaust plume by complex processes that depend on the properties of the surrounding air. Prior results from the literature were leveraged to develop first-order estimates for the final emissions indices as functions of altitude. The final emissions species predicted by the commercial space vehicle emissions model are water vapor, carbon dioxide, carbon monoxide, alumina, chlorine species, nitrogen oxides, and black carbon.
Commercial Space Vehicle Emissions Modeling 82 A database of emissions indices was compiled for the first-stage rocket engines of current and in- development commercial space vehicles, including every orbital-class rocket that was built or launched in the United States in 2019. The primary emissions indices are stored in the internal fleet database, and the final emissions indices are calculated within the commercial space vehicle emissions model using the first-order estimates. Model Validation Although the commercial space vehicle emissions model is based on the best available data in the literature, it still contains uncertainty due to the scarcity of high-quality emissions data. The emissions model was validated using the Space Shuttle, which has the most complete and well-documented emissions data in the literature. The validation results for the primary emissions indices, final emissions indices, and emissions inventory are described below. The accuracy of the commercial space vehicle emissions model depends on the accuracy of the internal fleet database and user-specified operational data. The primary emissions indices are the parameters in the fleet database with the highest uncertainty. For most commercial space vehicles, the primary emissions indices are not publicly available and were calculated using CEA instead. CEA is based on simplifying assumptions that introduce uncertainty into the results. A comparison between the CEA predictions and published values for the Space Shuttle demonstrated that CEA produces reasonable estimates of the primary emissions indices. The first-order estimates for the final emissions indices are subject to even greater uncertainty than the primary emissions indices, particularly at high altitudes. However, the high-quality final emissions indices published in the literature were used to develop the first-order estimates. No independent computations or measurements are available to directly validate the final emissions indices for commercial space vehicles. Ultimately, the emissions inventory is the final result of the emissions model. The calculated emissions inventory was validated compared to published results for the Space Shuttle, which indirectly validated the estimates of the final emissions indices. The differences between the calculated emissions inventory and the published values for the Space Shuttle were reasonably small. The results of this validation effort demonstrate that the commercial space vehicle emissions model is reasonably accurate to analyze the environmental impacts of commercial space operations. AEDT Integration Plan The commercial space vehicle emissions model was developed with the intent to be integrated with AEDT. However, due to the differences between spacecraft and aircraft, some elements of the emissions modeling methodology are necessarily unique to space vehicles. An AEDT integration plan was developed to identify the necessary software and user interface modifications to integrate the commercial space vehicle emissions model with AEDT, as described below.
Commercial Space Vehicle Emissions Modeling 83 The existing AEDT user interface will accommodate most of the inputs for the commercial space vehicle emissions model without modifications. Relatively minor modifications will be required to enable users to add spaceports, select space vehicles from the fleet database, and specify detailed spacecraft trajectory data. Conceptual designs of these AEDT user interface modifications were presented in the integration plan. Back-end software modifications will also be required to integrate the commercial space vehicle emissions model with AEDT. Pseudocode for implementing the emissions modeling methodology was presented in the AEDT integration plan. Additionally, our team developed a spreadsheet emissions estimator tool to provide future programmers with an implementation example of the commercial space vehicle emissions model. The spreadsheet emissions estimator tool provides a full working example for future software verification and validation efforts. Emissions Modeling Tool To address the immediate need for a tool to estimate the emissions from commercial space vehicles, our team integrated the commercial space vehicle emissions model into RUMBLE, a launch vehicle acoustic simulation model developed by BRRC and expanded under ACRP project 02-66. The resulting software tool, RUMBLE 3.0, and its accompanying user guide are described below. RUMBLE 3.0 is the first software tool that enables practitioners to accurately model the noise and emissions produced by commercial space vehicles. RUMBLE 3.0 implements the rocket noise and emissions models via a user-friendly interface, which adopts a similar workflow as AEDT. As described in the user guide, users can create a study, add a spaceport, define operations, create scenarios, and calculate noise and emissions metrics. Our team also developed a more than 100-page user guide to accompany RUMBLE 3.0. The comprehensive user guide provides installation instructions for RUMBLE 3.0, instructions for interacting with the user interface, descriptions of the input and output file formats, tutorial exercises, and technical details of the noise and emissions models. RUMBLE 3.0 and its accompanying user guide are the primary research products of ACRP project 02-85 and are freely available to the public through the ACRP website. RUMBLE 3.0 is designed to be the standard tool for conducting environmental analyses of commercial space vehicles until the emissions model is fully implemented in AEDT.
Commercial Space Vehicle Emissions Modeling 84 8.2 Suggested Research The research activities and results described in the previous section fulfilled the objectives of ACRP project 02-85. The following future research efforts are suggested to improve and apply the commercial space vehicle emissions model developed under this project: ï Conduct future model validation using high-fidelity modeling and field measurements, ï Perform AEDT integration of the commercial space vehicle emissions model, and ï Explore applications of the commercial space vehicle emissions model. These suggested future research efforts are discussed in the following sections. Future Model Validation The availability of high-quality emissions data for commercial space vehicles is extremely limited. Thus, additional work is needed to fully validate the commercial space vehicle emissions model. The proposed future model validation plan is described below. High-fidelity computational models should be leveraged as the first step to validate the emissions estimates for commercial space vehicles across a range of altitudes. High-fidelity modeling is significantly more cost effective than measurements for obtaining results across a wide range of vehicles, propellants, and altitudes. However, even advanced computational models must be validated based on measurements of current commercial rocket engines. Since measurements can be complex and expensive, a high-level measurement campaign roadmap should be developed before any emissions measurements are conducted. The measurements 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. 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. The first ground-level emissions measurements should be conducted in association with scheduled rocket engine testing activities. Additionally, ground-based emissions measurements should be conducted at rocket launch sites. As the ground-level measurement techniques mature, measurements at altitude should be conducted to validate the changing chemical dynamics that occur in the exhaust plume at higher altitudes. Emissions measurements at altitude can be conducted using remote sensing and plume capture instrumentation installed on chase planes. Emissions measurements in the troposphere and lower stratosphere will provide extensive validation data for the altitude dependence of the emissions produced by commercial space vehicles.
Commercial Space Vehicle Emissions Modeling 85 AEDT Integration The commercial space vehicle emissions model was developed with the intent to be integrated with AEDT. However, future work is needed to perform the AEDT integration. Future programmers should leverage the conceptual user interface designs and pseudocode presented in the AEDT integration plan to perform the necessary user interface and software modifications. Future programmers should also refer to the spreadsheet emissions estimator tool, which provides an implementation example that can be used for software verification and validation. Following integration of the commercial space vehicle emissions model, AEDT will provide a comprehensive suite of software tools for assessing the environmental impacts of both aircraft and spacecraft. In the interim, RUMBLE 3.0 is provided as the standard tool for modeling the noise and emissions produced by commercial space vehicles. Applications of the Commercial Space Vehicle Emissions Model RUMBLE 3.0 is the first publically available tool that enables practitioners to accurately model the noise and emissions produced by commercial space vehicles. Thus, RUMBLE 3.0 is ready to be applied to study the environmental impacts of commercial space vehicle emissions. The following examples are potential applications of RUMBLE 3.0: ï Apply the noise and emissions results from RUMBLE 3.0 in the preparation of environmental documents for spaceport and launch vehicle licenses, ï Apply the detailed emissions outputs from RUMBLE 3.0 as inputs to dispersion modeling codes to assess local air quality and environmental impacts, and ï Apply RUMBLE 3.0 to analyze the global environmental impacts of commercial space operations and compare the results to commercial aviation. RUMBLE 3.0 enables practitioners to develop emissions inventories for commercial space operations more accurately and more easily than ever before. Previously, most environmental documents for commercial launch vehicles contained limited or no vehicle-specific emissions data. Additionally, several conflicting and highly uncertainty reports on the potential global environmental impacts of future commercial space operations have been published. RUMBLE 3.0 enables practitioners to generate more accurate and detailed emissions results for environmental documents and global environmental impact assessments. Improved environmental documents and impact assessments will be valuable to decision makers in determining commercial space policies and regulations. As an example application of RUMBLE 3.0, our team compared the estimated launch emissions produced by a SpaceX Falcon 9 rocket below the mixing height to the landing and takeoff cycle emissions produced by a Boeing 737-800 . In the most recent years for which data are available, the Falcon 9 was the most-launched rocket in the United States , and the Boeing 737-800 was the most common aircraft in the United States commercial aviation fleet . Figure 43 shows the results of the comparison between the Falcon 9 and Boeing 737-800. At takeoff, the Falcon 9 is heavier and produces significantly greater thrust than the Boeing 737-800. Thus, the Falcon 9 emits more carbon dioxide below the mixing height. Additionally, a significantly greater amount of NOx is
Commercial Space Vehicle Emissions Modeling 86 formed due to afterburning in the extremely high-temperature Falcon 9 exhaust plume than in the lower-temperature exhaust of the Boeing 737-800. The results shown in Figure 43 demonstrate one useful application of the commercial space vehicle emissions model. Numerous additional applications are possible with RUMBLE 3.0, which enables practitioners to easily and accurately model the noise and emissions produced by commercial space vehicles. RUMBLE 3.0 and its accompanying user guide are freely available to the public through the ACRP website. Figure 43. Comparison between the estimated Falcon 9 launch emissions below the mixing height and the Boeing 737-800 landing and takeoff cycle emissions [57, 125, 127].