Scientists and engineers are continually working on new ways to carry expensive payloads into space. The X-34 is a small rocket designed to be launched by an aeroplanes. It is hoped that the X-34 will be able to minimize the cost of carrying satellites into orbit. The DC-XA was a new design for a single-stage-to-orbit vehicle. It made four successful flights before crashing. The Rotors is designed to work without the heavy technology needed to pump rocket fuel. Its rotor blades spin, literally throwing propellant into the combustion chamber.
RLVs as workhorse launch vehicles have yet to appear, however, for several reasons. The primary reason for the emphasis on expendable launch vehicles (ELVs) instead of RLVs has historically been the higher up-front development costs of RLV designs. In order to field fully reusable launch vehicles, reusable components and operations techniques must be developed along with the vehicle design. Although construction of RLVs has been possible, governments and commercial companies have been reluctant to provide the funding required to build RLVs that have higher initial costs, but that would reduce operating costs in the long-term. When the designs for the Space Shuttle fleet were first considered, fully-reusable concepts were introduced, and the original selected design was to have two reusable stages. Budgetary pressures, however, molded the vehicle into the partially-reusable system used today.
The materials and designs to construct TSTO RLVs have been available for 35 years, but innovations are still required to develop a SSTO vehicle that can transport payloads to orbit at low costs. Advances in propulsion and structural technology (such as new lightweight composite materials) have been made over the last few decades that are enabling the development of SSTO vehicles. SSTO vehicle technologies may be validated in the next few years with the testing of NASA’s X-33 and X-34 vehicles and with the development of commercial SSTO designs such as the Roton-C. Developments that will be demonstrated by the X-33 will include load-bearing fuel tanks and composite structures. Both the X-33 and Roton plan to use aerospike engines (the aerospike design has existed for decades it has yet to be fully flight tested).
Today there are several drivers that are pushing RLV development forward. The desire to reduce launch costs in the commercial and government markets is greater than ever. The growth in the number of proposed LEO satellite constellations for telecommunication applications has produced demand for low-cost launches, encouraging entrepreneurial aerospace companies to develop commercial RLVs to serve this market. RLV designs for space tourism applications have been seriously proposed in the last few years. The X PRIZESM competition is encouraging construction of passenger-carrying sub-orbital RLVs by over a dozen start-up companies by offering a $10 million prize to the first vehicle to demonstrate the capability to carry 3 people to a 100 km sub-orbital altitude and repeat the flight within 2 weeks.
Government programs are also a key source of RLV development. NASA’s current X-33 and X-34 RLV prototype and technology development programs grew out of a series of studies examining the next step following the Space Shuttle program. In 1985, NASA and the Department of Defense were directed by the President to devise a common plan to develop space transportation systems beyond the Space Shuttle. The resulting Space Transportation Architecture Study was focused on meeting civil and military launch needs, and endorsed examining air-breathing propulsion technologies, TSTO systems, and solid rocket boosters.