The origins of the U.S. HAST Program date back to Project Sandpiper - a 1960s U.S. Air Force effort to improve high-altitude target drone performance by using with hybrid rocket propulsion in place of dangerous, liquid fueled systems. The first phase of Sandpiper modified existing AQM-37 Jayhawks with a new hybrid motor. The hybrid system used a solid fuel grain and a storable nitric-acid oxidizer called IRFNA — inhibited red fuming nitric acid. The goal was to keep the advantages of a compact, air-launched, high-speed target while exploring a propulsion system that offered improved handling, performance, and control. The trials were promising and lead to the HAST program. HAST ultimately led to the Teledyne Ryan Model 305 / XAQM-81A Firebolt, which looked similar to the Jayhawk but used the new hybrid propulsion approach. The Firebolt was designed to fly faster, higher, and farther than the earlier target drones. Despite successful testing, it never entered production, and only a small number (some sources say 9-12) of test vehicles were built.