NASA wants to develop radio frequency (RF) gauging of propellant levels in launch vehicle cryogenic tanks for use in low-gravity environments on future exploration missions. An accurate, robust low-gravity quantity gauging system does not exist and is needed to ensure there is adequate propellant before beginning a human mission.
NASA’s planned Ares I crew launcher and Ares V cargo launcher would use the RF fuel gauging for their upper stages. The Ares I upper stage will place the Orion crew exploration vehicle into a suborbital trajectory.
The Ares V will use an upper stage that is also the Earth departure stage for a lunar mission, with the Lunar Surface Access Module as its payload.
Radio waves can be used to determine fuel quantities because, says NASA, “the resonant electromagnetic frequencies [in the RF spectrum range] of a tank…are modified by the presence of [the fuel].” A resonant electromagnetic frequency (EMF) is the response of the tank structure’s EMF to an external RF signal.
NASA has begun a programme to mature RF gauging to a technological readiness level of six, which means it must be tested in a relevant environment. That could require a sounding rocket flight that would expose the gauge and fuel to microgravity or experiments with test vehicles in orbit.