Management of cryogenic propellants in a full scale orbiting space vehicle

Some lunar and deep space missions require a space vehicle that can restart its en gines after a coast period in space.An orbiting space vehicle using cryogenic propel lants requires that the propellants be positioned in the tank to ensure gaseous venting and liquid feed to the engine f o r restart.The fourth and eighth Atlas-Centaur vehicles (AC-4 and AC-8) were launched to study cryogenic propellant management during coast.The AC-4 flight showed that propellant management cannot be defined solely by the Bond number (ratio of acceleration to surface tension forces).Energies imparted to the propellants at engine cutoff, during coast, and at engine restart must be considered.On the AC-4 flight, kinetic energies imparted to the liquid hydrogen at first engine cutoff caused liquid motion within the hydrogen tank resulting in subsequent venting of liquid hydrogen rather than gaseous hydrogen.The vented liquid impinged on the vehicle causing it eventually to tumble out of control.The AC-8 vehicle was modified to reduce and/or control the energies transmitted to the pro pellant.Energy dissipators were installed on the hydrogen tank pressurization line and boost pump return flow lines.A balanced thrust hydrogen vent system was installed to reduce vehicle disturbances.A slosh baffle was installed in the hydrogen tank and a thrust schedule (with thrust levels increased over AC-4) for propellant settling and re tention was established.These modifications reduced the total kinetic energy at first engine cutoff from 200 foot-pounds (270 J) on AC-4 to less than 50 foot-pounds (68 J) on AC-8.An additional 8400 foot-pounds (1140 J) of energy theoretically could have been transmitted to the liquid hydrogen on AC-4 during the coast and engine restart periods if the mission had been successful.This was reduced to approximately 20 foot-pounds (27 J) on AC-8.In addition to verifying propellant management, the AC-8 flight provided data on the thermal environment of the hydrogen tank during the coast period.The information ob tained on heat inputs to the tank was used to establish the capability of the hydrogen vent system.Also, a thermal survey was made of the hydrogen tank ullage; this survey pro vided information on temperature stratification of the ullage gas.The flight of AC-8 dem onstrated a practical method for control of a cryogenic propellant in a full-scale orbiting space vehicle.The method of propellant control established for the Centaur vehicle is ap plicable to any space vehicle having short-term storage of cryogenic propellants in space.