Challenges in restart of pressure fed liquid upper stage

摘要

The advantage of liquid rocket engine is its multi start capability for optimizing the required injection velocity for the payload. Restarting of liquid stages in space flight enables injecting satellites in different orbits/inclination, payload improvement for mission specific elliptical orbit with greater argument of perigee. Restarting will also help in de-orbiting of spent stages and consuming the left out propellants in stages. This will help in reducing the risks of stage explosions and minimizing space debris. Liquid rocket thrusters have undergone restarts earlier in spacecrafts for orbit raising and 3 axis control as well as station keeping. For the first time in Indian launch vehicle program, restart of an upper pressure fed stage was experimented in two recent flights. The restart of engine calls for many challenges like firing the engine in heated condition, purging the engine after each hot burn and fixing the idle time between hot starts, mission planning and controllability of the stage to have a smooth restart operation in space etc. In addition to the above, solutions were arrived in propellant management for different fill fractions to settle them at the outlet of the tanks for smooth restart, ensuring gas free propellants at the outlet of the tank in zero gravity, adequate gas availability for commanding valves and regulators and benign thermal condition for the free flow of the propellants to the engine. Before attempting the restart in flight, ground hot test in simulated vacuum was successfully carried out with 2 restarts to demonstrate the restart capability of the upper stage engine. The temperature at the time of restart was predicted using mathematical models and the same was fine-tuned by taking engine skin temperature measured in earlier missions. Subsequently, restart was experimented in two missions and performance of engines during restart was normal and both the engines showed identical behaviour during transients and steady state. The demonstration trials carried out in flight has given sufficient confidence in taking up future restart mission requirements. The details of the restart during flight on upper stage engines and the challenges in stage propulsion system for restart and how it was overcome are elaborated in this paper.