Pioneer Rocketplane planned in the late 1990's to produce the Pathfinder aerial propellant transfer spaceplane. Pathfinder was a two-seat fighter-bomber-sized aircraft powered by two turbofan engines and one kerosene/oxygen-burning RD-120 rocket engine. The Pathfinder aircraft was designed to take off with its turbofan engines, and climb to approximately 6,000 m where it would rendezvous with a tanker aircraft. The tanker would transfer 59 tonnes of liquid oxygen to the Pathfinder. After disconnecting from the tanker, the spaceplane ignited its rocket engine and climbed to an altitude of 110 km and a speed of Mach 15. Now on a suborbital trajectory outside the atmosphere, Pathfinder would open its payload bay doors, and release the payload with a liquid rocket upper stage. An expendable solid propellant upper stage could deliver a 2100 kg payload to a 300 km 30 degree orbit. Meanwhile Pathfinder would close its payload bay doors and re-enter the atmosphere. After slowing down to subsonic speeds, the turbofan engines would be restarted and the aircraft flown to a landing field. It was estimated that the price per launch could profitably be set as low as $7 million, 3 to 4 times lower than the price using the Taurus expendable launch vehicle.
As a piloted aircraft, Pathfinder could be tested incrementally. Unlike expendable vehicles, which must be flight tested all at once on the first attempt, the Pathfinder could work up to an orbital delivery flight incrementally, biting off small additional chunks of risk on succeeding flights. The aircraft would be fully tested before the rocket engine was ever ignited. Dry hook-ups with the tanker would be demonstrated before Lox is transferred. Tanking with liquid nitrogen would be done to qualify the mechanisms before Lox is loaded, and so on throughout the flight test program.
Whereas the regulatory environment for unmanned expendable vehicles was uncertain, the Pathfinder could be operated under FAA regulations. Pathfinder used conventional airbreathing engines for takeoff and landing. These engines allowed the aircraft to be operated normally for ferrying itself (unlike the Space Shuttle, which requires a carrier aircraft). The rocketplane could even be flown to a satellite manufacturer's facility for satellite pickup. Liquid oxygen was only loaded in the air, significantly reducing launch site hazards.
Every major component of the Pathfinder rocketplane (turbofans, rocket engine, avionics) was proven and in production or available through surplus. Apart from the liquid oxygen aerial propellant transfer, new technologies were not needed in order for Pathfinder to work.
By using conventional, robust equipment, aircraft design practices, and modern thermal protection materials, turnaround time was to be very short. The propellants were readily available, easily handled, non-toxic, and inexpensive. Most of Pathfinder's high-speed flight would take place above the sensible atmosphere; problems with sonic boom are avoided.
Several upper stages could be used. The large payload bay amply accommodated many possible payloads and upper stages. Liquid upper stages could lift up to 1800 kg into polar, sun-synchronous orbits. Because all of the required technology was in-hand, it would be possible to build the first Pathfinder and have it ready for launch operations within three years of full financing.
LEO Payload: 2,100 kg. to: 200 km Orbit. at: 0.0 degrees. Payload: 1,500 kg. to a: 800 km 97 degree sun-synchronous orbit trajectory. Liftoff Thrust: 85,000 kgf. Total Mass: 110,000 kg. Launch Price $: 7.00 million. in 1999 price dollars.