NASA’s potential Mars Ascent Vehicle is a daunting endeavor that is part of a larger Mars Sample Return effort with the ultimate goal of bringing samples from Mars back to Earth. It will involve a rocket with the samples taking off from Mars into space to be captured by another spacecraft before eventually returning to Earth.
Launching a rocket from Earth is one thing; doing the same from Mars is another. The first challenge: What propellant do you use? It is this problem that Timothée Pourpoint, associate professor of aeronautics and astronautics, has been working on with NASA’s Jet Propulsion Lab. The extremely low pressures on Mars’ surface (about one-hundredth that on earth) and low temperatures (-50°C) complicate any potential fuel candidates, Pourpoint says. “Conditions of low pressure and low temperature make ignition in general harder.”
The Goldilocks propellant
To further complicate matters, the ideal propellant candidate must satisfy a number of additional parameters, Pourpoint says. It must be hypergolic, which means the fuel constituents ignite spontaneously when they come into contact with each other without the need for an external ignition source, and the fuel needs to be solid, so it can be more stable for the time period (up to a year) it is expected to be on Mars.
“We also want something that doesn’t get brittle at low temperatures, so it has to have good mechanical properties at low temperatures,” Pourpoint says. The fuel-flow rate — the rate at which it is consumed over time — has to be optimal to achieve the right rocket thrust, and the right fuel has to take payload capacity into account as well. “We want the rocket to take off from Mars under a certain flight profile and acceleration. If it goes too fast, it might run out of fuel too quickly, so that’s a factor too,” he adds.
Above all, the propellant fuel must be reliable. “We’re really looking for a means of igniting the rocket very reliably at least three times during the mission,” Pourpoint says. After years of research, he and his team have arrived at sodium amide and potassium bis(trimethylsilyl)amide as potential solid hypergolic additives that might pass these stringent parameters.
What reliable looks like
To make sure everything works as predicted on Mars, Pourpoint says the propellants must be evaluated rigorously on Earth.
“You have to test for so many variables,” he says. “What if the temperature is actually a bit lower than the temperature that’s expected to be the norm? What if the injection profile doesn’t come out as well as we want? What if we have a sticky valve and it doesn’t open as fast as we want?”
Even if all these potential scenarios can be tested on the ground, Pourpoint says, “We can’t account for every [eventuality]. At some point you take a risk. There are also challenges with respect to time and money. That is, in part, why space exploration is so exciting.”
To test for various conditions and eventualities, you need to simulate Mars-like conditions on Earth. Pourpoint plans on testing the propellant in an altitude (vacuum) chamber that can simulate the pressure conditions on Mars. “We will check that the propellant ignites quickly, that it ignites well and that the chamber pressure rises in a nice and uniform fashion,” he says. “We will check that we get the predicted thrust under those conditions and make sure to repeat tests until we’re satisfied.”
In situ resource utilization
An alternate theory is the reliance on found materials to use as fuels. Pourpoint has suggested, for example, that we use the ice on the moon and make hydrogen and oxygen for the rocket to come back to Earth. “SpaceX is suggesting we do the exact same thing on Mars except with methane, using CO2 in the atmosphere,” he says.
Such ideas are not far fetched, Pourpoint says, and he sees the value in using locally sourced materials. “We have to learn to use the resources where we’re going to and learn to live off the land. The first explorers to America didn’t bring all their food for months; they made their food here. It’s exactly the same if we go to Mars — we need to learn how not to bring all our fuel to Mars or to the moon. We have to use the resources on the land.”
As for back on Earth and at Purdue, after racking impressive early success with the sodium amide and potassium bis(trimethylsilyl)amide combination, Pourpoint is hoping to test the candidate further and make it a contender at NASA’s JPL design review process in the fall.
“This is all one step toward a solution,” he says. “It’s getting the project one step closer toward reality.” With his research on exploring the right fuels for the right conditions, Pourpoint is paving the way for exciting explorations in new frontiers of space.