As propellant is drained from its tank, something must fill the vacated
ullage space to maintain pressure inside the tanks. This is for two reasons: first, rocket engines require a minimum inlet pressure to prevent
cavitation in their turbopumps, and second, rockets usually require that their tanks be pressurized for structural strength. In autogenous pressurization, a small amount of propellant is heated until it turns to gas. That gas is then fed back into the liquid propellant tank it was sourced from. This helps keep the liquid propellant at the required pressure necessary to feed a rocket's engines. This is achieved through gas generators in a rocket's
engine systems: tapped off from a
gas generator; fed through a
heat exchanger; or via electric heaters. Autogenous pressurization was already in use in the
Titan booster by 1968 and had been tested with the
RL10 engine, demonstrating its suitability for
upper stage engines. Traditionally, tank pressurization has been provided by a high pressure inert gas such as
helium or
nitrogen. Autogenous pressurization has been described as both less and more complex than using helium or nitrogen but it does provide significant advantages. The first is for long-term spaceflight and
interplanetary missions such as going to and landing on
Mars. Removing
inert gases from usage allows engine firing in a non-pumping mode. The same vaporized gases can be used for
mono- or bi-propellant
attitude control. The reuse of onboard oxidizer and fuel also reduces the contamination of combustibles by inert gases. Thus, autogenous pressurization is suited for booster engines which will operate under constant acceleration in a single direction, but is difficult to use when there are multiple engine burns separated by zero-g maneuvers. The
RS-25 engines used autogenous pressurization to maintain fuel pressure in the
Space Shuttle external tank. == References ==