The McDonnell Douglas DC-X, better known as the Delta Clipper or Delta Clipper Experimental, was an unmanned prototype of a reusable single stage to orbit launch vehicle developed in conjunction with NASA and the DOD SDIO from 1991 to 1993.
According to Jerry Pournelle: "DC-X was conceived in my living room and sold to National Space Council Chairman Dan Quayle by General Graham, Max Hunter, and me." The DC-X was built as a one-third scale prototype of the envisioned orbital launch vehicle. Once testing with the DC-X was completed, the plan called for a second, larger prototype vehicle titled the DC-Y and eventually, a full-size orbital launch vehicle called the DC-1.
The DC-X was never designed to achieve orbital altitudes or velocity, but instead to demonstrate a radical new concept of vertical take off and landing. The rocket would take off vertically like standard rockets, but also land vertically with the nose up. This innovative design used attitude control thrusters and retro rockets to control the descent, allowing the craft to begin reentry nose-first, but then roll around and gently touch down on landing struts at its base. The craft could be refueled where it landed, and take off again from exactly the same position — a trait that allowed unprecedented turnaround times. Another focus of the DC-X project was minimized maintenance and ground support. To this end, the craft was highly automated and required only three people to man its control centre (two for flight operations and one for ground support).
The DC-X flew a series of flight tests successfully, with Apollo astronaut Pete Conrad at the ground-based controls for some flights. However, on the eighth flight in July 1995, a hard landing cracked the aeroshell.
The DC-X program was transferred from SDIO to NASA. The craft was rebuilt and upgraded with new fuel/oxygen tanks and an improved control system. The upgraded vehicle was called the DC-XA, renamed the Clipper Advanced/Clipper Graham, and resumed flight in 1996. The DC-XA reached a maximum altitude of 3140 m and set a world record of a 26-hour turnaround between launches of a reusable rocket. On the fourth DC-XA test flight, the craft flew correctly, but was destroyed on landing. During testing, one of the LOX tanks had been cracked. When a landing strut failed to extend due to a disconnected hydraulic line, the DC-XA fell over and the tank leaked. Normally the structural damage from such a fall would constitute only a setback, but the LOX from the leaking tank caught fire and severely burned the DC-XA, completely destroying it. Pete Conrad priced a new DC-X at 50 million dollars, and NASA decided not to rebuild the craft in light of the budget constraints. Rather, NASA focused development on the Lockheed Martin VentureStar which it felt answered some criticisms of the DC-X; specifically the requirement for astronauts to wear a Hazmat suit when egressing the vehicle after landing and the fact that many NASA engineers preferred the "clean" airplane-like landing of the VentureStar over the "dirty" landing of the DC-X.
The future of the DC-X
Several engineers who worked on the DC-X have since been hired by Blue Origin, and it is rumored that their vehicle is based on the DC-X design. Also, the DC-X provided inspiration for many elements of Armadillo Aerospace's, Masten Space Systems', and TGV's spacecraft designs.
Returning the DC-X design to NASA's active research portfolio has been considered for some time now. Some NASA engineers believe that the DC-X could provide a solution for a manned Mars lander. Some design changes proposed include using hydrogen for fuel with oxygen and adding a fifth leg for increased stability during and after landing. Recently NASA's Centennial Challenges program has announced a Suborbital Lunar Lander Analog Challenge which is a prize for the first team to build a VTVL rocket that is capable of lunar landing and take-off speeds.