Walter HWK 109-500
The Walter HWK 109-500 was a liquid-fuelled rocket engine developed by Walter in Germany during the Second World War. Description. The 109-500 is a self-contained, modular monopropellant "Starthilfe" (take-off assist) engine in a pod, able to produce thrust for thirty seconds. After the fuel was expended, the pod was jettisoned and it returned to earth by parachute, with the parachute packed externally, onto the blunt forward end of the pod. The T-Stoff monopropellant, stored in the large spherical tank within the "Starthilfe" module's forward end, needed to react with  a catalyst to provide the boosting thrust for an aircraft on takeoff - this Z-Stoff sodium or calcium-based, alkaline permanganate-compound (in an aqueous solution) catalyst was provided in a small tank above the reaction chamber just forward of the exhaust nozzle, with compressed air from a network of five pressure tanks driving the monopropellant and catalyst together through the reaction chamber, which featured an internal, fixed helical "swirl baffle" to lengthen the period of time the monopropellant and catalyst were in contact for a more complete catalytic reaction within it, before the reacted T-Stoff exhaust exited the  nozzle. It entered service in 1942, and some 6,000 were built, by Heinkel. It was "used extensively on a wide range of aircraft", especially the potentially underpowered (when heavily laden with external ordnance) Jumo 004-engined Arado Ar 234B, with two of the units uniquely displayed as mounted operationally on the NASM's sole surviving, restored Ar 234B.