Prime Credit - USAF

The Prime (Precision Recovery Including Maneuvering Entry) project was the second part of the USAF START program. The purpose of START was to develop and demonstrate the technology for maneuvering lifting-body re-entry vehicles with a cross range of up to 1100 km. Planned applications included ICBM warheads, reconnaissance satellite film recovery capsules, and manned spacecraft. The X-23 Prime was a subscale re-entry test vehicle of the SV-5D lifting body configuration planned for the X-24A manned aerodynamic test aircraft. In three suborbital launches the robot vehicle demonstrated the full planned cross range maneuver, and the problems involved in using spray-on ablative heat shields.

Lifting bodies were originally conceived for use as maneuverable ICBM re-entry vehicles to defeat Soviet anti-ballistic missile systems, and as high cross-range film-return capsule for reconnaissance satellites that would improve quick response by providing more film recovery opportunities. NACA's Ames laboratory was the place where much early research was done, resulting in the M2b, M2F2, and M2F3 series of flight demonstration vehicles. NACA Langley had its differing HL-10 design, and the USAF developed the Aerospace Corporation A3 shape for its own specific cross-range requirement. In November 1960 Martin was contracted to study use of the Ames M1 shape for recovery of film capsules from the USAF Samos satellite. Instead it concluded that the Aerospace A3 configuration was superior, and developed a stretched version of the A3 as the A3-4 or SV-5. Tests of the SV-5 under the PRIME project were to include demonstration of 1145 km cross-range maneuvers. After the cancellation of DynaSoar, the remaining Asset subscale re-entry tests of that program were combined with PRIME, resulting in Air Force Program 680A, called START, and consisting of subprojects Asset, Prime, and Pilot (subsonic piloted lifting-body tests).

The Prime X-23A lifting body weighed 400 kg and was built primarily of 2014-T6 titanium alloy, with some structures built of beryllium, steel, or aluminum, as required. On a typical flight, the unmanned SV-5D was launched by an Atlas booster from Vandenberg AFB, California. At the high point in its flight path, the Atlas pitched downward while its rocket continued accelerating it to orbital re-entry velocity. The lifting body separated from the booster, and its inertial guidance system directed it to a pre-selected recovery point. 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. The shape was expected to demonstrate a hypersonic L/D of 1.0. Two types of ablative coating protected the vehicle during re-entry. Silicon-nylon fibers within an ablator applied to a silicon honeycomb base protected the main body of the shape. Thickness varied from 2 to 7 cm depending on the expected heating rates. The nose cap was protected with a carbon-phenolic resin. The lifting portion of the re-entry would be terminated at Mach 2.0 with deployment of a drogue ballute, followed at subsonic speeds by a 14-m diameter recovery parachute. Air-snatch of the re-entry vehicle under its parachute was to be accomplished by a JC-130B aircraft over the ocean.

After the successful third mission, the rest of the project was cancelled, and the two remaining unflown X-23A's were sent to the USAF Museum at Wright Patterson Air Force Base. After the cancellation of Dynasoar, the Air Force pursued further development of manned spaceplanes under the START Project.

Length: 2.10 m (6.80 ft). Maximum Diameter: 1.20 m (3.90 ft). Span: 1.20 m (3.90 ft). Mass: 405 kg (892 lb). Structure Mass: 78 kg (171 lb). Heat Shield Mass: 110 kg (240 lb). RCS Coarse No x Thrust: Cold gas (nitrogen). Electrical System: Batteries. Associated Launch Vehicle: SLV-3 Atlas.

• 1966 December 21 - Prime 1 - Launch Site: Vandenberg. Launch Vehicle: SLV-3 Atlas. Apogee: 1,500 km (900 mi).

  The first test of the X-23A SV-5D lifting body re-entry shape. It was a zero cross-range suborbital flight, with recovery 6935 km downrange. The ballute deployed at 30.440 m, followed by the main parachute at 13,700 m, and the vehicle was descending within 275 m of the target point. Nevertheless the air-snatch was unsuccessful, and the vehicle sank. However 90% of the planned telemetry was successfully transmitted by radio.

• 1967 April 20 - Prime 3 - Launch Site: Vandenberg. Launch Vehicle: SLV-3 Atlas. Apogee: 1,500 km (900 mi).

  The full design 1145 km cross range was demonstrated, and the X-23A SV-5D lifting body vehicle was successfully snatched at 3700 m altitude, 8 km from the target point. With this success the rest of the project was cancelled, and the two remaining unflown X-22A's were sent to the USAF Museum at Wright Patterson Air Force Base.

• 1967 March 5 - Prime 2 - Launch Site: Vandenberg. Launch Vehicle: SLV-3 Atlas. Apogee: 1,500 km (900 mi).

  The X-23A SV-5D lifting body vehicle demonstrated a 1055 km cross-range manoeuvre, but again air snatch failed.

• Gatland, Kenneth, Manned Spacecraft, Macmillan, New York, 1968. ISBN: 0025428209. First of a marvelous series of pocket-size hardbacks covering spacecraft and rocketry. More at amazon.com...

• Jenkins, Dennis R,, Space Shuttle: The History of the National Space Transportation System : The First 100 Missions, Third edition, Voyageur Press, 2001. ISBN: 0963397451. Excellent - the most comprehensive account of the design, development, and flights of the space shuttle.Takes the reader from the maze of designs during the first shuttle competition to future plans. More at amazon.com...

• Miller, Jay,, The X-Planes, Aerofax, Arlington, Texas, 1988. ISBN: 1857801091. Tremendous account of the entire X- experimental series of US aircraft, including the rocket planes X-1, X-15, and X-20. Another outstanding Jay Miller book. More at amazon.com...

• Houchin II, Roy and Smith, Terry, Quest, "X-20 (7 articles)", 1994, Volume 3, Issue 4, page 4.

• Peebles, Curtis, Spaceflight, "The Origins of the US Space Shuttle - 1", 1979, Volume 21, page 435.