and have a mass fraction higher than 0.88. During development of a SSTO, any small increase in vehicle weight directly offsets payload capability, requiring either a very strict weight reduction activity (costly) or vehicle up-scaling (even more costly). If the SSTO vehicle’s mass fraction falls below 0.88, its weight for delivery of a given payload to orbit grows asymptotically as the vehicle becomes unable to lift even itself into orbit. Today’s most efficient expendable upper stage (Centaur) has a mass fraction of 0.90. Making a stage larger helps, but adding thermal protection for reentry, aerodynamic surfaces for lift and control, landing gear, and life support for manned use makes meeting the 0.88 minimum mass fraction requirement extremely difficult. X-33 was to address and demonstrate the many technology issues associated with meeting this requirement.
Significant advances were made with the thermal protection system developed by B.F. Goodrich, which also served as the aerodynamic surface of the vehicle. The Rocketdyne XRS-2200 Linear Aerospike main engines were on target to become the next generation of liquid fueled propulsion systems. Both of the conformally shaped LH2 and LO2 propellant tanks were initially to be composite, but after engineering objections, the smaller LO2 tank was successfully manufactured with aluminum-lithium (Al-Li). Management insisted, however, that for X-33 to be a useful technology enabler, the multi-lobed LH2 tank had to be made of composites.
Development problems were encountered with the LH2 tank, along with Rocketdyne’s decision to use Narloy-Z (a heavy copper alloy), which drove changes to the flight control surfaces. These major issues (and other smaller ones) resulted in substantial cost increases and schedule delays. The composite hydrogen tank failed during testing, which resulted in program cancellation in 2001. X-33’s development cost a reported $1.5 billion over 5 years; the vehicle was approximately 40 percent complete at cancellation, and its test flight facilities at Edwards Air Force Base were ready. If the LH2 tanks had been manufactured of Al-Li instead of a composite as proposed by the X-33 engineering team, they would reportedly have been lighter than the composite tank and, therefore, successful. Unfortunately, all the successful new technology was laid to rest along with the death of the X-33, and the opportunity to gather useful flight information that would have been applicable to a reusable booster was lost.3
In addition to the three large programs discussed above, there have been and continue to be numerous smaller efforts in the United States and internationally aimed at development of reusable launch vehicles. Within the United States, these include the X-20 Dyna-Soar, X-34, Delta Clipper, Kistler K-1, DreamChaser, Prometheus, and Blue Origin New Sheppard. Internationally, these include the HOTOl, Skylon, Sanger, and Buran. A detailed investigation of these programs was beyond the statement of task for this committee.
A number of important differences exist between the three reusable launch vehicle programs described above as compared to the RBS concept, including the following.
• The three RLV programs described above were intended for manned operations, which imposes numerous subsystem requirements that add to the inert launch vehicle mass. The RBS concept is not intended to be human rated, so it will not be burdened by this additional inert mass requirement.
• The previous RLV programs involved reusable vehicles that were launched into space and needed to survive orbital reentry conditions. In the base of the RBS concept, the maximum Mach number of the reusable booster is between 3 and 7, so the requirements for a thermal protection system are greatly diminished.
• The space shuttle used a reaction-control system that operated with toxic propellants, which greatly complicated operations and significantly impacted turn-around time. As a new design, the RBS concept development can pursue use of non-toxic propellants for the attitude-control system, which would provide significant operability advantages.
• Most RLV programs were based on the use of hydrogen as the fuel. (An exception is the Kistler K1 project, which was based on the use of LO2/RP propellants.) The RBS concept is based on the use of a hydrocarbon fuel for the reusable booster. This higher density fuel will likely result in a more structurally and aerodynamically efficient vehicle configuration.
3 R. Launius, After Columbia: The Space Shuttle Program and the crisis in space access, Astropolitics 2:277-322, 2004.