The PRIME project was the second part of the Spacecraft Technology and Advanced Re-entry Tests (START) program. It had the dual objective of testing advances in space hardware and further exploring the development of manned and unmanned lifting body vehicles. Four SV-5D vehicles were built by the Martin Co. to fulfill the objectives of the PRIME program. Outside the earth's atmosphere, the SV-5D was maneuvered by the release of high pressure nitrogen through jet thrusters. When the craft re-entered the atmosphere, its control system automatically switched to airplane-type flaps for pitch and roll control.
On a typical flight, the unmanned SV-5D was launched by an Atlas booster from Vandenberg Air Force Base, Calif. At the high point in its flight path, the Atlas pitched downward while its rocket continued accelerating it to speeds nearly as great as those of the spacecraft re-entering the earth's atmosphere. The lifting body's inertial guidance system directed it to a preselected recovery point. Three PRIME test flights were made, the first on Dec. 21, 1966, and the last on April 19, 1967. A scheduled fourth flight was cancelled due to successes of the previous tests.