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Summary On June 15, 2011, the Air Force Space Command established a new vision, mission, and goals to ensure continued U.S. dominance in space and cyberspace mission areas. Subsequently, and in coordination with the Air Force Research Laboratory, the Space and Missile Systems Center, and the 14th and 24th Air Forces, the Air Force Space Command identified four long-term science and technology (S&T) challenges critical to meeting these goals. One of these challenges is to provide full-spectrum launch capability at dramatically lower cost, and a reusable booster system (RBS) has been proposed as an approach to meet this challenge. Current Air Force medium and heavy launch capability is provided by the Evolved Expendable Launch Vehicles (EELVs), Atlas V and Delta IV, which are manufactured and operated by the United Launch Alliance for the U.S. Air Force. Projected costs for these vehicles have grown recently owing to the completion of the Space Shuttle Program of the National Aeronautics and Space Administration (NASA), which meant sub contractor support costs had to be fully absorbed by the EELV program. The Air Force would like to control this cost growth by finding a more effective method of space launch while maintaining the outstanding launch reliability achieved by EELV. The RBS concept is an unmanned launch vehicle with an autonomous guidance and control system that consists of a reusable first stage, powered by a hydrocarbon-fueled, oxygen-rich, staged-combustion (ORSC) engine and an expendable second stage, powered by a liquid oxygen/liquid hydrogen engine. The reusable first stage operates in a return-to-launch-site (RTLS) mode, whereby the first-stage booster is recovered and readied for future launches. Several RTLS options have been explored by the Air Force. Its preferred option is to use a "rocketback" maneuver following second-stage separation. The rocketback maneuver, which is executed at high altitude (about 30 km), uses one of the first-stage engines to decelerate the downrange motion of the booster and provide sufficient velocity for it to glide back to the launch site for an unpowered landing. The Air Force Research Laboratory has been developing many of the technologies that underpin the RBS concept, including hydrocarbon booster and vehicle technologies. Its Pathfinder project is under way to demon- strate the critical aspects of the rocketback RTLS operation, including propellant management and vehicle control using a small-scale flight-test vehicle. The Air Force Space and Missile Systems Center has developed a baseline RBS concept and development plan, which includes a midscale reusable booster demonstrator, two RBS Y-vehicles, which are full-scale prototypes, and eight operational RBSs located at Vandenberg Air Force Base and Cape Canaveral Air Station. The Aerospace Corporation, working in conjunction with the Air Force Space and Missile Systems Center, performed an analysis 1

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2 REUSABLE BOOSTER SYSTEM--REVIEW AND ASSESSMENT of the projected cost for this RBS baseline concept that predicted significant life-cycle savings compared to the EELV system satisfying the same launch manifest. This cost study used industry standard models for estimating the cost of the RBS launch stages, independent cost estimates of the required propulsion systems and ground infrastructure, and model-based estimates of the operational costs. The Air Force Space Command asked the Aeronautics and Space Engineering Board of the National Research Council to conduct an independent review and assessment of the RBS concept prior to considering a continuation of RBS-related activities within the Air Force Research Laboratory portfolio and before initiating a more exten- sive RBS development program. The Committee for the Reusable Booster System: Review and Assessment was formed in response to that request and charged with reviewing and assessing the criteria and assumptions used in the current RBS plans, the cost model methodologies used to frame the RBS business case, and the technical maturity and development plans of key elements critical to RBS implementation. The committee consisted of experts not connected with current RBS activities who have significant expertise in launch vehicle design and operation, research and technology development and implementation, space system operations, and cost analysis. This committee solicited and received input on the Air Force launch requirements, the baseline RBS concept, cost models and assessment, and technology readiness. The committee also received input from industry associated with the RBS concept, industry independent of the RBS concept, and propulsion system providers. Based on the input received, its own analysis, and judgment based on committee expertise, the committee came to six major findings. Finding 1: Cost estimate uncertainties may significantly affect estimated RBS life-cycle costs. There are several important factors that lead to significant uncertainties associated with the RBS cost esti- mates developed to date. First, the vehicle development costs were estimated using NASA/Air Force Cost Model (NAFCOM), which is an industry standard model, and reasonable model inputs were used, but operability impacts on vehicle design are not captured using this model. Since NAFCOM is largely based on historical data and lacks relevant experience on reusable systems, there are large uncertainties associated with implementing the vehicle features necessary to ensure operability. Second, the cost projections are based on the "Americanization" of Russian hydrocarbon engine technol- ogy, but cost risks associated with development of an operable engine are difficult to capture. Given the limited experience of U.S. industry in developing oxygen-rich, staged combustion (ORSC) hydrocarbon engines, the cost uncertainties associated with the engine development may be significant. Third, the details underlying the infrastructure needs are unclear, so uncertainties exist in the costs associated with the infrastructure. Finally, the estimated operational costs assumed modest postflight inspection requirements, which assume successful development of an effective integrated vehicle health management (IVHM) system and little added cost for the mission assurance requirement. Finding 2: The RBS business case is incomplete because it does not adequately account for new entrant commercial providers of launch capabilities, the impacts of single-source providers, Air Force need for independent launch sources for meeting their assured-access-to-space requirement, and technical risk. The cost uncertainties associated with these factors do not allow a business case for RBS to be closed at the present time. In addition to the basic cost uncertainties associated with the RBS, three additional factors exist such that an assessment that the RBS business case is not complete and cannot be closed at the present time. First, the RBS business case does not account for new-entrant commercial providers, in that the business case is based on a com- parison of the RBS concept to an extrapolation of recent EELV costs. Given the significant number of commercial entities pursuing novel approaches to achieve launch capabilities, the future of space lift may look very different from those employed today. With global competition to reduce launch costs, it is anticipated that recent EELV costs may not be the proper baseline for cost comparisons.

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SUMMARY 3 Second, the RBS business case does not address the impacts of single-source providers. So, while the business case assumes that RBS captures the complete Air Force launch manifest and is developed with a single-source provider, the cost risks associated with using a single source are not adequately addressed. Put another way, the cost benefits associated with retaining competition in vehicle development were not included in the business case. Since the commercial launch market is rapidly changing and will be driven by cost considerations, not assessing the role of competition is a weakness of the current RBS business case. Third, the RBS business case assumed full capture of the Air Force launch manifest, but the Air Force maintains a requirement for independent launch capabilities to assure its mission needs. With this need for development and maintenance of a second launch system capability, the RBS business case model is overly optimistic in assuming it will capture the entire Air Force launch manifest. The end result of these factors is that the uncertainties associated with the RBS business case are sufficiently large that the business case cannot be considered to be closed at the present time. Finding 3: Reusability remains an option for achieving significant new full-spectrum launch capabilities at lower cost and greater launch flexibility. The Air Force Space Command has identified a long-term S&T objective for achieving full-spectrum launch capabilities at significantly reduced costs, and reusability remains an option for realizing this objective. A robust reusable system might have additional benefits beside reduced costs, including replenishment of satellites on demand; deployment of distributed constellations; rapid deployment; robust launch operations from multiple defendable launch sites; and operability by Air Force personnel. Finding 4: For RBS to significantly impact Air Force launch operations, it would have to be more respon- sive than current expendable launch systems. However, no requirement for RBS responsiveness has been identified that would drive technology development. The existing business case for RBS is built on satisfying the current EELV launch manifest with a launch- on-schedule assumption to operations. With this assumption and given the lack of an operability requirement for RBS, the technologies necessary to significantly enhance operability and reduce operating costs will not be given a high priority. It is in the development of design features and technologies and the resulting changes to Air Force operations that the true value of a reusable system lies. Finding 5: Technology areas have been identified in which continued applied research and advanced development are required before proceeding to large-scale development. These areas include reusable oxygen-rich, staged-combustion hydrocarbon-fueled engines, rocketback return-to-launch-site operation, vehicle health management systems, and adaptive guidance and control capabilities. Finding 6: Given the uncertainties in the business case and the yet-to-be mitigated technology risks, it is premature for Air Force Space Command to program significant investments associated with the development of a RBS capability. While the committee found that the RBS business case cannot be closed at this time, and it is premature to begin large-scale RBS development activities, the committee does strongly endorse the continued research and advanced technology development needed for future launch systems. The committee makes the following six recommendations. Recommendation 1: Launch responsiveness should be a major attribute of any reusable launch system. To address this perceived disconnect, the Air Force should establish specific responsiveness objectives independent of the Evolved Expendable Launch Vehicle launch-on-schedule requirements that can be used to drive technology development.

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4 REUSABLE BOOSTER SYSTEM--REVIEW AND ASSESSMENT The committee believes that responsiveness should be a major consideration when developing reusable launch systems and their supporting technologies. At the present time, no responsiveness requirements beyond a launch-on-schedule philosophy have been defined. Since these requirements can drive vehicle and technology needs, it is imperative that the Air Force define nominal responsiveness goals to provide a focus for research and development activities. Recommendation 2: Independent of any decision to proceed with RBS development, the Air Force should proceed with technology development in the following key areas: reusable oxygen-rich, staged- combustion hydrocarbon-fueled engines; rocketback return-to-launch-site operations; vehicle health management systems; and adaptive guidance and control systems. These technologies will have to be matured before they can support any future decision on RBS, and most of them will be also applicable to alternative launch system concepts. Continued development is needed in these four technology areas so they can be matured to the point where significant investments can be committed to the RBS programs. Investments in the four areas should continue independent of a decision to proceed with RBS development. Since the technologies have application beyond RBS, with the exception of rocketback RTLS, their maturation will benefit the Air Force independent of RBS in advanced rocket propulsion, system reliability, and vehicle autonomy. Recommendation 3: The Air Force Research Laboratory's (AFRL's) Pathfinder project is under way to demonstrate in flight, using a small-scale vehicle, the critical aspects of the return-to-launch-site maneuver. To increase chances for Pathfinder's success, AFRL should develop and fly more than one Pathfinder test vehicle design. In addition, competition among RBS concepts should be maintained as long as possible to obtain the best system for the next generation of space launch. The use of a rocketback maneuver for RTLS operations of an RBS has not yet been demonstrated, so this approach to reusability carries significant risk. Given these risks and the resulting parameter space for innovative solutions, the Pathfinder program should be executed in a manner wherein several vehicle designs are developed and flown. While this approach will increase costs in the near term, the long-term benefits of one day achieving a true high-performance solution to reusability will overwhelm this initial cost. Recommendation 4: The decision to proceed with the RBS development program should be based on the successful completion of the Pathfinder activities and on assurance that the technical risks associated with the reusable oxygen-rich, staged-combustion hydrocarbon-fueled engines, rocketback return-to-launch- site, vehicle health management systems, and adaptive guidance and control systems are adequately mitigated. Given the immaturity of the principal technologies and the inherent risks of the rocketback RTLS operation, the decision to proceed with RBS development should be tied to the successful completion of the Pathfinder program and suitable mitigation of the principal technical risks. The committee understands that this approach will delay the achievement of an RBS capability. However, delaying the decision to proceed with the RBS development has an additional benefit: It enables the business environment for emerging new-entrant commercial launch providers to become clearer. Recommendation 5: Following successful completion of the Pathfinder program, the Air Force should reevaluate the RBS business case, accounting for the following factors: new-entrant commercial launch providers; potential impacts of single-source providers; and Air Force need for independent launchers to satisfy assured-access-to-space requirements.

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SUMMARY 5 Recommendation 6: When constructing the RBS program, the decision points for proceeding from tech- nology development to demonstration to prototype to production for RBS should be based on quantita- tive assessments during the successful completion of the previous phase. These go/no-go decision points should be structured as on-ramps to subsequent phases with technical underpinnings that are sufficiently well understood to proceed. The decision points for proceeding from Pathfinder and hydrocarbon boost technology risk reduction to a mid-scale demonstrator and from the demonstrator to Y-vehicle prototypes should be considered as on-ramps. Given the costs associated with the development of a new space launch capability and the technical uncertain- ties surrounding its operational approach, it is prudent to construct any future RBS program so that the decision to proceed to a next phase is strongly tied to the successful completion of the previous phase. Today, the United States finds itself in the midst of a fundamental transition of space launch from a model wherein the government develops and controls the launch vehicles to a service-based model wherein industry develops launch vehicles and then sells services to both commercial and government organizations. Despite the uncertainties of this transition, the committee is aware of a large number of organizations that are developing capabilities using innovative designs, development, and operational approaches. The review and evaluation of the RBS concept within this transition period is inherently difficult, but the committee firmly believes that the future of U.S. space launch will be strong if the technology developments recommended in this study are coupled with innovative designs and approaches to achieve cost-effective and robust launch systems.