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Commercial Space Vehicle Emissions Modeling 25 3 Emissions Modeling Methodology The purpose of the commercial space vehicle emissions model is to enable users to produce emissions estimates that are relevant to environmental analyses. Figure 17 shows an overview of the emissions modeling methodology that was developed to achieve this goal. The inputs to the emissions model are an internal fleet database and user-defined operational data. The emissions model calculates the amount of propellant burned by a commercial space vehicle as well as the associated emissions. The results of the emissions model are expressed in the form of an emissions inventory, which enumerates the amounts of pollutants emitted by commercial space operations. Figure 17. Overview of the emissions modeling methodology. The following sections discuss the emissions model inputs, outputs, and calculations in more detail: ï Section 3.1 describes the internal fleet database, ï Section 3.2 describes the operational data that must be provided by the user, ï Section 3.3 presents the calculations performed by the emissions model, and ï Section 3.4 describes the propellant burn report and emissions inventory outputs. 3.1 Internal Fleet Database The internal fleet database contains the vehicle- and engine-specific data required by the emissions model. The database is comprised of three independent tables that describe the spacecraft, airframe, and engine parameters. As illustrated in Figure 18, the tables are relationally linked via unique airframe and engine identifiers, which connect each spacecraft to its associated airframe and engine parameters. The spacecraft is located in the database via its own unique identifier. The internal fleet database in the commercial space vehicle emissions model already includes numerous current commercial [46] and historical launch vehicles. Additionally, users can add new vehicles to the database to model emissions from emerging commercial space vehicles. The estimated propellant mass flow rates and nominal burn times for the first-stage rocket engines listed in Table 2 are included in the engine table of the internal fleet database. The propellant mass flow rate and nominal burn time are required by the commercial space vehicle emissions model to determine the amount of propellant burned. As discussed in Section 2.2, the mass flow rate can be estimated from either the sea level thrust and specific impulse or the total propellant mass and nominal burn time. The estimated mass flow rate in the fleet database is assumed to be constant over the entire burn time, which is typically a slightly conservative but reasonable assumption.
Commercial Space Vehicle Emissions Modeling 26 Emissions indices are the factors that relate the amount of propellant burned to the amount of pollutants emitted by a rocket engine. Thus, the emissions indices are required by the commercial space vehicle emissions model to produce the emissions inventory output. The emissions indices for each first-stage rocket engine are stored in the engine table of the internal fleet database. The methods for estimating the emissions indices are presented in Section 4. Figure 18. Structure of the internal fleet database. 3.2 User-Defined Operational Data The following user-defined operational inputs are required for the commercial space vehicle emissions model: ï Operation type (i.e., launch, landing, or static fire), ï Spacecraft, and ï Trajectory data. The user-specified spacecraft is needed to load the appropriate propellant mass flow rate, nominal burn time, and emissions indices from the fleet database. The trajectory is needed to calculate the amount of time the vehicle spends in each altitude band. Thus, the trajectory must include, at a minimum, the altitude as a function of time. Additionally, the propellant mass flow rate as a function of time can be included in the trajectory to override the average mass flow rate and nominal burn time from the fleet database. The commercial space vehicle emissions model includes several example altitude profiles based on historical trajectories. The user may select from these example altitude profiles if no mission-specific trajectories are available for the modeled commercial space operations. However, as discussed in Section 2.3, the trajectory depends on a large number of mission- and vehicle-specific parameters. Thus, the results of the commercial space vehicle emissions model will be most accurate if the user provides the actual altitude profile and time-resolved propellant mass flow rate for each operation.
Commercial Space Vehicle Emissions Modeling 27 3.3 Emissions Model Calculations The commercial space vehicle emissions model calculates the mass of propellant burned and the mass of each pollutant emitted by commercial space operations. The calculations are first performed at the most detailed level, and the detailed results are then aggregated to produce the propellant burn report and emissions inventory. At the most detailed level, the propellant mass burned by a single engine during an individual trajectory segment is calculated by �PropellantMass � = � Propellant Mass Flow Rate� à �SegmentDuration� (5) where the duration of the trajectory segment is the time between successive points in the user- specified altitude profile. Commercial space vehicle trajectories are often specified in one-second time steps, which provides high resolution for accurate emissions modeling. The propellant mass flow rate is either the constant mass flow rate from the fleet database or a time-varying mass flow rate specified by the user in the trajectory data. Next, the mass of each pollutant emitted by a single engine during an individual trajectory segment is calculated by �PollutantMass � = � Emissions Index �à � Propellant Mass � (6) where the propellant mass is the result from Eq. (5). The emissions indices are the factors that relate the amount of propellant burned to the amount of each pollutant emitted by the engine. The emissions indices are stored in the engine table of the fleet database for the specified rocket engine. Emissions indices are discussed in more detail in Section 4. Finally, the amount of propellant burned and the amount of pollutants emitted by a single engine during an individual trajectory segment can be aggregated over the number of engines, trajectory segments, operations, and vehicles to produce the propellant burn report and emissions inventory. 3.4 Emissions Inventory and Propellant Burn Report The final outputs of the commercial space vehicle emissions model are the propellant burn report and the emissions inventory. The propellant burn report provides the propellant mass burned by each type of engine on commercial space vehicles. Similarly, the emissions inventory enumerates the masses of the various pollutants emitted as a result of commercial space operations. The commercial space vehicle emissions model aggregates the results of Eqs. (5)â(6) to calculate the total amounts of propellant burned and pollutants emitted. Propellant burn reports and emissions inventories may be itemized by individual vehicles, operations, or altitude bands.
Commercial Space Vehicle Emissions Modeling 28 The reporting options and terminology for the propellant burn report and emissions inventory were adopted from AEDT. As in AEDT, the results may be grouped by the following options: ï The Operations Detail report lists the results by events (i.e., individual flights) at the trajectory segment level. This report is obtained by summing Eqs. (5)â(6) over the number of engines on the vehicle. ï The Operations Mode report lists the results by events and mode categories (e.g., altitude bands). This report is obtained by summing Eqs. (5)â(6) over the number of engines on the vehicle and the trajectory segments that comprise the defined mode category. ï The Operations Summary report lists the results by events. This report is obtained by summing Eqs. (5)â(6) over the number of engines on the vehicle and all trajectory segments during the event. ï The Operation Group Summary report lists the results by operation group and mode category. An operation group is a user-defined grouping of vehicles and events. This report is obtained by summing Eqs. (5)â(6) over the number of engines on the vehicle, the trajectory segments that comprise the defined mode, and the number of operations in the user-defined group. Mode categories are discussed in more detail with respect to AEDT integration in Section 6.1.4. Example emissions inventories and propellant burn reports for the Space Shuttle are presented in Section 6.4.
