Longitudinal Control And Footprint Analysis For A Reusable Military Launch Vehicle

In this paper, we will examine a configuration for a reusable military launch vehicle (RMLS) concept. This configuration allows for the vehicle to land in an inverted attitude. Such inverted landing improves the turnaround time of the vehicle by reducing the maintenance requirements of the vehicle's thermal protection system. An analysis is performed to examine the impacts by the configuration on stability, control, and footprint for an RMLS configuration.

The ability to compute the maximum area on the earth's surface (footprint) reachable by an autonomous air vehicle can be useful in planning for the vehicle's safe operations. The information can be important when the vehicle experiences subsystem failures causing it to be unable to maintain its nominal performance. In this paper, we present a method to calculate the footprint of a reusable launch vehicle that experiences a failure in one or more of its aero-control surfaces. During a control effector failure, the maximum attainable moments of the vehicle are reduced, which may decrease the range of conditions that the vehicle can maintain a trimmed condition. Additionally, the lift and drag characteristics of the vehicle can change when control effectors are moved to off-nominal positions to correct for moment imbalance caused by failures or damage. As a result, the footprint of the vehicle is reduced. A technique for calculating the available effectiveness of the aero-control surfaces is used in conjunction with a footprint generation algorithm to include the effects of rotational trim on the vehicle footprint.

The United States is embarked on a journey toward maturity as a spacefaring nation. One key step along the way is development of a reusable launch vehicle (RLV). The most recent National Space Transportation Policy (August 1994) assigned improvement and evolution of current expendable launch vehicles to the Department of Defense while National Aeronautical Space Administration (NASA) is responsible for working with industry on demonstrating RLV technology. The purpose of this study is to help ensure the US military, especially the USAF, is prepared to take advantage of RLVs should the NASA-led effort to develop an RLV demonstrator prove successful. The focus of this study is an explanation of how the US military could use RLVs, by describing and analyzing two concepts of operations. Four major conclusions resulted from the analysis. First, RLVs have military potential. They can perform a variety of missions including responsive spacelift, reconnaissance, and strike. However, the economic feasibility of using RLVs for earth-to-earth transportation is questionable. Second, design choices for an operational RLV will have effects on risk, cost, capability, and operations efficiency. Trade-offs will have to be made between NASA, commercial, and military requirements if all three parties are to use the same fleet of RLVs. Third, increased investment in propulsion technology development is warranted to ensure success. Fourth, the top priority for the RLV program, even from the military's perspective, should remain cheap and responsive access to space. The research led to three recommendations. First, the US military should become a more active participant in the RLV program to ensure its requirements are defined and incorporated. Second, America should not pursue development of operational RLVs before the technology is ready.

The key to opening the use of space to private enterprise and to broader public uses lies in reducing the cost of the transportation to space. More routine, affordable access to space will entail aircraft-like quick turnaround and reliable operations. Currently, the space Shuttle is the only reusable launch vehicle, and even parts of it are expendable while other parts require frequent and extensive refurbishment. NASA's highest priority new activity, the Reusable Launch Vehicle program, is directed toward developing technologies to enable a new generation of space launchers, perhaps but not necessarily with single stage to orbit capability. This book assesses whether the technology development, test and analysis programs in propulsion and materials-related technologies are properly constituted to provide the information required to support a December 1996 decision to build the X-33, a technology demonstrator vehicle; and suggest, as appropriate, necessary changes in these programs to ensure that they will support vehicle feasibility goals.

This thesis considers the problem of generating optimal entry trajectories for a reusable launch vehicle following a control surface failure. The thesis builds upon the work of Dr. David Doman, Dr. Michael Oppenheimer and Dr. Michael Bolender of the Air Vehicles Directorate, Air Force Research Lab Dayton Ohio. The primary focus of this work is to demonstrate the feasibility of inner loop reconfiguration and outer loop trajectory retargeting and replanning for the X-33 reusable launch vehicle (RLV) following the imposition of a control surface failure. The trajectory generation model employs path constraints generated by an AFRL trim deficiency algorithm coupled with an inner loop control allocator and aerodynamic database that captures the full 6-DOF vehicle aerodynamic effects while utilizing an outer loop 3-DOF model. The resulting optimal trajectory does not violate the trim deficiency constraints and provides additional margins for trajectories flown during failure conditions. The footprints generated by the thesis show that contemporary footprint analysis for vehicles experiencing control surface failures are overly optimistic when compared to those footprints that consider vehicle aerodynamic stability and realistic landable attitudes at the threshold of the landing runway. The results of the thesis also show the performance reductions resulting from decoupling the inner and outer loop and that trajectories can be generated to the landing runway without using a region of terminal area energy management.

The X-33 and X-34 technology demonstration programs currently under way by NASA may ultimately lead to a commercial reusable launch vehicle (RLV). The first flight test for the X-33 vehicle is planned for early 1999. With NASA fully committed to the development of this first-ever completely reusable vehicle, it is prudent for the Air Force to examine the potential benefits, requirements, and costs for a military transatmospheric vehicle (TAV). This vehicle would take advantage of technologies developed during RLV-related programs. Although similar in concept, a military TAV may differ significantly in design and in capability from an RLV produced to satisfy commercial market needs.

This study was undertaken in response to a request by the U.S. Air Force that the National Research Council (NRC) examine whether the technologies that underlie the concept of a hypersonic, air-launched, air-breathing, hydrocarbon-fueled missile with speeds up to Mach 81 can be demonstrated in time to be initially operational by 2015. To conduct the study, the NRC appointed the Committee on Review and Evaluation of the Air Force Hypersonic Technology Program, under the auspices of the Air Force Science and Technology Board.