sources it is credited to Harley Davidson, the motorcycle manufacturer, which appears to be due to a sequence of company buyouts. The engine uses "storable" liquid propellants, in contrast to cryogenic propellants like liquid oxygen and liquid hydrogen that have to be loaded up just before launch lest they evaporate away. The problem is that the storable propellants in common use are corrosive, highly toxic, and "hypergolic", meaning that the propellants spontaneously ignite when mixed together. This tends to make them troublesome to deal with. AQM-81A Firebolt As a result, in the late 1960s the Air Force investigated an alternate propulsion scheme for the AQM-37 under project "Sandpiper". The program involved fitting a few AQM-37As with "hybrid" engines that used solid fuel with storable nitric acid oxidizer. The tests were judged promising, and so the Air Force went on to establish a "High Altitude Supersonic Target (HAST)" program in the 1970s. HAST suffered various difficulties, and it wasn't until 1979 that a contract was awarded to Teledyne Ryan for the Model 305 / AQM-81A Firebolt. The first Firebolt flew on 13 June 1983, launched from an Eglin AFB, Florida, F-4D Phantom II. The new target looked very much like the AQM-37, but had the hybrid rocket engine. The flight test program was completed, but then the HAST effort stalled completely, and the AQM-81A never went into production. Variants Model 1019Designated AQM-37A by the United States Military. Model 1072United Kingdom variant. Short StilettoThe Beech Model 1072 modified by Shorts for UK use. Model 1088Italian variant. Model 1094French variant. Model 1100Fitted with a two-stage recovery parachute for the US Army. Model 1101Fitted with a two-stage recovery parachute for the US Army. Model 1102(AQM-37A)Fitted non recoverable version for the US Army. XKD2B-1Prototype targets. KD2B-1US Navy designation before the joint designation scheme was introduced. AQM-37ADesignation of the KD2B-1 after the joint designation system was introduced. AQM-37BNot officially used after orders from the US Navy were changed to the AQM-37C. AQM-37CUS Navy AQM-37DUpgraded electrical systems and avionics for higher reliability. Teledyne Ryan AQM-81A FireboltDeveloped for the USAF from the AQM-37 by introducing solid rocket fuel with liquid oxidiser (RFNA), for improved safety and performance. Beechcraft submissions were rejected as too expensive, so development contracts were awarded to Teledyne-Ryan Teledyne Ryan AQM-81B A US Navy version not pursued. Q-12US Air Force designation for the KD2B before the joint designation scheme was introduced. Surviving aircraft AQM-37 on