When the Intuitive Machines Nova-C lunar lander lifts off on February 14, it will carry a new fuel gauge developed by NASA that can measure the cryogenic propellant in the thruster fuel tank using radio waves.
Measuring the amount of liquid in a tank is the easiest problem to solve on Earth. You can insert a dipstick directly into the fuel, or you can set up a simple device with a float and a pressure gauge marked E through F. But in space, because there is no gravity to pull the liquid to the bottom of the tank, the liquid floats and adheres to the sides of the tank due to surface tension. Engineers usually estimate how much propellant is left in the spacecraft by knowing the original payload mass and subtracting the propellant that has been used in the thruster. However, cryogenic fuel tends to boil over time and drain overboard, making the estimate somewhat "uncertain." This is a particular problem on long-range interstellar missions that can last for years.
To solve this problem, NASA has been testing a new method called the Radio Frequency Mass Gauge (RFMG) to estimate the amount of cryogenic liquid through an antenna installed in the tank. The antenna measures the interaction between the liquid and the natural electromagnetic resonances of the tank walls and compares it to a database. With appropriate calculations, the amount of fluid can be estimated within a few percent.
To date, RFMG has been tested on aircraft parabolic trajectories to produce instantaneous weightlessness, and on board the International Space Station (ISS). Now that it has been installed on the Nova-C lunar lander for field testing, NASA engineers can compare it to ground simulations and previous tests.
"Because gravity is so small, the fluid does not settle on the bottom of the propellant tank, but instead adheres to the walls of the tank, possibly anywhere within the tank," said Lauren Amien, associate program manager for the Cryogenic Fluid Management Portfolio at NASA's Glenn Research Center in Cleveland. "This makes it very challenging to know how much propellant is in the tank, which is important to maximizing mission duration and planning the amount of propellant needed for launch."