When the Intuitive Machines Nova-C lunar lander lifts off on February 14, it will be carrying a novel new fuel gauge developed by NASA that can measure cryogenic propellants in the thruster tanks by means of radio waves.
Measuring the amount of liquid in a tank on Earth is about as easy a problem to solve that there is. You can insert a dipstick or you can set up a simple mechanism with a float and a gauge marked E to F. In space, things are more than a bit complicated. With no gravity to pull the fluid to the bottom of the tank, it floats about and adheres to the tank sides due to surface tension.
Engineers can estimate how much propellant a spacecraft has left by knowing the original load mass and deducting how much has been used in the thrusters. However, cryogenic fuels tend to boil away and vent overboard over time, which makes estimations a bit 'iffy.' This is a particular problem on long-range interplanetary missions that can last for years.
To get around this, NASA has been testing a new method called a Radio Frequency Mass Gauge (RFMG) that estimates how much cryogenic fluid there is by using an antenna installed in the tank. This antenna measures how the fluid interacts with the natural electromagnetic resonances of the tank walls and compares this with a database. With the proper calculations, it's possible to estimate the amount of fluid within a few percentage points.
So far, the RFMG has been tested in aircraft flying on parabolic trajectories to create momentary periods of weightlessness and aboard the International Space Station (ISS). Now, it's been installed aboard the Nova-C lunar lander for a field test that NASA engineers can compare against ground simulations and the earlier tests.
"Because of the very small amount of gravity, fluid doesn’t settle to the bottom of propellant tanks but rather clings to the walls and could be anywhere inside," said Lauren Ameen, deputy manager for the Cryogenic Fluid Management Portfolio Project Office at NASA’s Glenn Research Center in Cleveland. "That makes it really challenging to understand how much propellant you have within your tank, which is really important to maximize your mission duration and plan how much you need to launch with."
Source: NASA