The ground and flight operations requirements for the reusable booster system (RBS) concept have not been sufficiently defined to allow assessment of the impact on the vehicle design. The Air Force baseline mission model includes eight RBS launches per year, using four RBS boosters on each coast (Cape Canaveral Air Force Station and Vandenberg Air Force Base). This mission model does not result in a rapid ground processing requirement for the reusable boosters. The committee believes that the RBS should accommodate future rapid-response space launch needs. As such, it would be desirable to design the RBS booster and its ground infrastructure for relatively rapid processing between missions. Clearly, this would result in a trade-off between the development cost and recurring operations cost, without any clear driving requirement. However, because the unpiloted reusable booster will need to include Autonomous Guidance and Control and Integrated Vehicle Health Management (IVHM) in order to reliably perform its in-flight mission, IVHM is available for use to help streamline booster ground processing.
Operability considerations will likely impose design requirements of the RBS and may lead to changes to standard launch vehicle ground operations. A brief discussion of these operability considerations follows.
• Booster ground processing for reuse. The reusable booster returns from a mission and lands horizontally at the launch base landing strip. It needs to be made safe, remaining fluids must be off-loaded, tanks purged, and items requiring maintenance need to be identified. It is very desirable that any fluid handling be nonhazardous so that Self Contained Atmospheric Protection Ensemble operations are avoided. IVHM are used to determine maintenance requirements without needing technician access for checkout. Once required maintenance items have been completed and checked by IVHM, the booster is mated to the integrated large expendable stage (LES) with its encapsulated payload. This can occur at the launchpad or in a separate integration facility. Rise-off disconnects attach fluid, electrical, and data interfaces to the booster and upper stage. Following final automated vehicle checkout, propellants are loaded and the vehicle is launched.
• Operability design considerations. Based on NASA’s very challenging space shuttle orbiter reuse experience, rapid ground processing can be achieved only when system operational functions drive vehicle design. It is vitally important to apply system margins early in the design process to enhance operability. A robust reusable vehicle design should trade weight/performance to achieve increased component reliability and dependability, which are keys to low-cost operations. Hardware replacement costs should be a small percentage of the recurring cost of flight.
• Specific considerations for reusable systems. Design considerations that are important to enable efficient booster reusability along with associated accessibility and maintenance goals are listed below.1
— Strive to isolate booster ground processing from dependence on facilities and ground support equipment by incorporating frequently used ground checkout equipment into the booster. Routine, scheduled turnaround ideally replenishes only consumables. Eliminate “flight readiness-style” booster certification for every flight. Provide aircraft-style vehicle-type certificate for repetitive flight operations.
— Booster subsystems should be independently power up-able and easily accessible when maintenance is required. This allows work to be performed in parallel with other systems and avoids processing conflicts.
— Avoid closed launch vehicle compartments, which require purges before personnel entry and complicate access and closeout requirements.
— When common propellants can be used, primary and secondary propulsion systems should be highly integrated, using common storage tanks, feed lines, etc. This reduces handling requirements for multiple propellants and reduces interfaces.
— Thermal protection systems should be robust and maintenance-free. Avoid thermal protection systems that are prone to in-flight degradation or that absorb moisture.
— Fewer rocket engines, preferably between two and four main engine, provide multiple operability benefits, such as reducing confined spaces which inherently require purges, reducing servicing and ground interfaces and reducing the number of systems for leak detection.
— IVHM can provide component and system health monitoring to identify those items requiring maintenance. It is critical that IVHM include the nonintrusive detection of fluid leakage and other techniques to reduce the large amount of unplanned maintenance that may be between flights. See Appendix E.
— Provide designs that limit the need for leak-test verification for fluids and gases in both static and dynamic applications by the use of all-welded systems whenever possible. Provide designs for electrical power and data transmission that reduce the use of cable connectors to minimize troubleshooting and repair.
— Where possible, use environmentally benign technologies. Avoid the use of hypergolics for auxiliary propulsion, main engine start, or power generation, if possible. If hypergolics must be used for the reaction control system, incorporate them as modular subsystems allowing for easy removal and replacement. Also avoid toxic freons and ammonia. Avoid the need for vehicle purges wherever possible.
— Avoid hydraulics for engine thrust vector control and to actuate aerodynamic control surfaces, landing gear, etc.; use electric actuators for these functions.
— A reusable flyback booster has a relatively short mission duration, permitting use of batteries rather than propellants for power generation.
— Implement a flight vehicle on-board system that provides its own power management, requiring only one vehicle-to-ground interface at each ground-facility station.
— Equipment should be mounted so that it is readily accessible. Avoid mounting equipment in closed compartments where access can result in collateral damage and unplanned work. Locate equipment on walls with external access or in open compartments that permit easy personnel access. Use aircraft-like access panels.
— Launchpad fluid, electrical, and communications interfaces should be implemented through rise-off disconnects located at the base of the booster and LES. If a mobile launch platform is used, its services should be connected to launchpad infrastructure with auto couplers.
— All subsystems and components should be qualified for the specified life of the reusable vehicle. Where this is not possible, and when removal and replacement over a reduced number of flights is required, these subsystems/components need to be readily accessible for ease of removal and replacement and post-installation checkout.
1 See also Huether, Spears, McCleskey, and Rhodes, “Space Shuttle to Reusable Launch Vehicle,” presented to the Thirty-Second Space Congress, Canaveral Council of Technical Societies, 1995; NASA, An Operational Assessment of Concepts and Technologies for Highly Reusable Space Transportation, Highly Reusable Space Transportation Study Integration Task Force, Operations, NASA Centers, November 1998.
RBS OPERABILITY REQUIREMENTS AND TECHNOLOGY DEVELOPMENT IMPLICATIONS
The committee believes that achieving RBS booster ground turn-around operability is more of a design-requirements issue than a technology-development issue. The problem with many of the past reusable rocket vehicle designs (either single stage to orbit or an upper stage) is that they were so performance driven that even if operability requirements had been imposed, they could not have been effectively implemented without seriously compromising vehicle flight performance. Therefore, if operability requirements were imposed, they were assigned a low priority. This is certainly what happened during the space shuttle orbiter development (see Appendix E). The RBS booster, because its design is not nearly as performance driven, offers the opportunity for operability requirements to be imposed on at least an equal footing with performance requirements.
The specific operability recommendations contained earlier in this appendix were generated by NASA as a result of their orbiter experience. In addition to imposing a top-level operability requirement (e.g., the RBS booster shall be capable of re-flight in XX hours with a YY person ground crew), the NASA-generated recommendations listed previously could be tailored for RBS and specified by the Air Force as detailed operability requirements for the RBS booster. Also, they could be imposed as being equally important as the flight performance requirements.
From a technology development standpoint, IVHM and its associated sensors is clearly the tall pole for implementing operability, which is discussed in Section 3.1.8. Most of NASA’s recommendations are well understood design approaches that require little technology development. They are usually not “least weight” design options, which is why they have not been implemented in current expendables or performance-driven reusables. Many of the design approaches recommended are similar to, or fairly standard to, those used on commercial and military aircraft. Adapting these to the RBS booster will require redesign to accommodate the harsh vibration and acoustic environment imposed by rocket engines and then validation by extensive qualification testing. This redesign/qualification effort could be considered technology development, but it is not considered an especially high technical risk. However, the committee believes there is significant cost risk associated with attempting to qualify revised designs currently used on aircraft to the very harsh rocket launch environment.