The HAPS (Hydrazine Auxiliary Propulsion System) is powered by three restartable, monopropellant hydrazine thrusters. The Reaction Motors LR99 engine was the first large, throttleable, restartable liquid-propellant rocket engine. The RS-27A main engine is neither restartable nor throttleable. This could be repeated, as NERVA was restartable. This could be repeated, as the NERVA engine was restartable. The engines are restartable in flight, and are contained within Kevlar armor shielding. The restartable Kestrel 2 could burn for up to a total of 418 seconds. If the system call cannot complete before the access, then it must be "restartable".