Propulsion Systems Specifications

Kenneth M Rock
rockken@uts.cc.utexas.edu
August 1995

The propulsion group will have three main responsibilities: launch vehicle selection; main propulsion system design; and reaction control system design. Additionally, the group will need to design all supporting propulsion subsystems and integrate the propulsion system into the spacecraft.

Launch Vehicle

The initial launch will be made by a NASA provided vehicle (Delta II, LLV, Med-Lite, or other American made launch system). All costs and operational requirements associated with launch will be provided by NASA. The propulsion group is responsible for the selection of a launch vehicle to perform the mission. An existing launch system is to be used. It should be noted that the less expensive launch vehicles will be more attractive to NASA, and this should be considered in the design study. Launch vehicle selection will ultimately depend on size and mass of final spacecraft. The propulsion group will provide final orbital characteristics to the NGC group after launch vehicle selection.

Main Propulsion System

The spacecraft Main Propulsion System (MPS) is responsible for providing the primary DV's throughout mission. Delta-V requirements result from the NGC group's trajectory calculations, including a 10% error budget. Trajectory estimates from the preliminary design yielded the DV's tabulated in table 1.0. These values assume a Delta-II launch and a PAM-D upper stage (approx. 1.45 km/sec). Furthermore, the earth orbit plane change maneuver may not be required, depending on launch time selection.

Mission PHASEDelta-V's
a. Earth Orbit Plane Change Maneuver 0.69 km/sec
b. Trans-Lunar Injection 1.67 km/sec
c. Lunar Orbit Insertion 0.54 km/sec
d. De-Orbit Burn 0.012 km/sec
e. Lander Deceleration (to approx. 1000m) 1.7 km/sec
f. Landing Site Selection (approx. 200m x approx. 200m area) unknown
g. Final Lander Descent to MPS Cutoff unknown

Table 1.0


Conceptual design iterations have yielded several requirements for the main propulsion system which include: restartability, throttleability, thrust for sustained hover, reliability, and low cost. An existing propulsion system should be strongly considered for use in order to minimize technological risk and development costs. Slight modifications to existing systems could be considered with caution. For example gimbal actuators may be removed for design simplification and weight savings.

The preliminary spacecraft design utilized a liquid fuel/oxidizer engine, the Rocketdyne XLR-132, for the MPS. As of 1992, the XLR-132 was still in development, and as such, selection of this propulsion system involves significant technological risk. Therefore, examination an alternative MPS concept should be done.

The propulsion group will provide a theory of operation for the MPS and all subsystems. The MPS design shall include the following: engine selection, fuel and oxidizer tank sizing, fuel and oxidizer pressurization system design, and fuel and oxidizer delivery system design. The designs shall specify all system related hardware such as piping, valves, and regulating equipment, as well as control interfaces.

The propulsion group will provide all requirements which the MPS impose on the spacecraft as a whole, such as thermal, vibrational, and power requirements, to all dependent groups. Specifically, the propulsion group will provide MPS performance characteristics to the NGC group, and will coordinate with the structures group on mounting requirements and mass/volume structural integration.

Additionally, the propulsion group will provide exhaust gas characteristics to the Chemical group for analysis of the reaction between propulsion system products and the lunar soil.

Reaction Control System

The Reaction Control System's (RCS) main function is to provide, throughout the mission: (1) coarse attitude control, (2) momentum dumping for the fine attitude control system, and (3) minor trajectory DV's. The RCS shall be designed for three-axis control of both translation and rotation.

The propulsion group will be provided the predicted translational DV's and moment requirements by the Navigation, Guidance, and Control (NGC) group. This information will be in the form of the maximum single impulses and a total usage budget. This data forms a basis for fuel requirements, which in turn provides for tank sizing and tank pressurization system requirements.

The propulsion group will provide a theory of operation for the RCS and all subsystems. The RCS design shall include the following: thruster selection, thruster positioning, fuel tank sizing, fuel pressurization system design, and fuel delivery system design. The designs shall specify system related hardware such as piping, valves, and regulating equipment, as well as control system interfaces. The RCS design should incorporate reliable monopropellant thrusters for simplicity. Rotations should be accomplished by thruster couples whenever possible.

The propulsion group will provide all requirements which the RCS impose on the spacecraft as a whole, such as thermal, vibrational, and power requirements, to all respective groups. Specifically, the propulsion group will provide RCS performance characteristics to the NGC group, and will coordinate with the structures group on the placement of the thrusters, and a fuel mass budget.

Propulsion System Interfaces

Input Group Interface/Information Output Group

Launch Vehicle
StructuresInitial Spacecraft Mass Estimate

Launch Vehicle Payload Size ConstraintsStructures

Refined Spacecraft MassStructures

Final Stage Orbital TrajectoryNGC




Main Propulsion System/Vernier System
NGCLanding Phase Requirements
NGCTrajectory Design (Delta V)

Real Engine PerformanceNGC

Propellant Tank Volume/PressureStructures

Pressurization Tank Volume/PressureStructures

Propellant ConsumptionStructures

Power RequirementsPower

Computer InterfacesComputer

Thermal RequirementsThermal

Exhaust Products & Lunar Soil InteractionsChemical




Reaction Control System
NGCDelta-V Requirements
NGCMoment Requirements (thruster couples)

Real Thruster PerformanceNGC

Thruster PlacementStructures

Thruster Propellant ConsumptionStructures

Propellant Tank Volume/PressureStructures

Propellant Pressurization Tank Volume/PressureStructures

Power RequirementsPower

Computer InterfacesComputer

Thermal RequirementsThermal


Propulsion System References

Griffin, M. D., French, J. R. "Space Vehicle Design", AIAA Pub., Washington DC, 1991.

Kane, T. R., Likins, P. W., Levinson, D. A., "Spacecraft Dynamics", McGraw-Hill Book Co., New York, 1983.

Leondes, C.T. and Vance,R.W. "Lunar Missions and Exploration", John Wiley and Sons, Inc., New York, 1964.

McDonnell Douglas Commercial Delta, Inc., "Commercial Delta II Payload Planners Guide". December 1989.

Pisacane, V. L., Moore, R. C. "Fundamentals of Space Systems", Oxford University Press, New York, 1994.

Regeon, P., Chapman, R. J., "Clemintine Orbiter Spacecraft System Design", AAS/AIAA Spaceflight Mechanics Meeting 95-130, AAS Publications Office, San Diego, CA, 1995.

Wertz, J. R. "Spacecraft Mission Analysis and Design", 2nd ed., Kluwer Academic Publishers, Boston, 1993.


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