NASA & SpaceX Moving Toward Refueling Demos

As SpaceX prepares for the coming Integrated Flight Test, IFT-4, they're still working toward their important role in the Artemis program, the version of Starship that will land on the moon, called the Human Landing System or HLS. While the target for orbital simulations is "next year", they've already begun some tests during IFT-3 to try to capture data on various aspects of the problem. 

NASA contracted first SpaceX and later Blue Origin to develop manned lunar landers. Both ships have been designed for refueling on orbit and the way Artemis has been created to establish a larger presence on the moon than the Apollo missions makes it essential they achieve refueling. 

Amit Kshatriya, who leads the "Moon to Mars" program within NASA's exploration division, outlined SpaceX's plan to do this in a meeting with a committee of the NASA Advisory Council on Friday. He said the Starship test program is gaining momentum, with the next test flight from SpaceX's Starbase launch site in South Texas expected by the end of May.

"Production is not the issue," Kshatriya said. "They're rolling cores out. The engines are flowing into the factory. That is not the issue. The issue is it is a significant development challenge to do what they’re trying to do ... We have to get on top of this propellant transfer problem. It is the right problem to try and solve. We're trying to build a blueprint for deep space exploration."

During IFT-3, it was said they were doing testing involving transferring liquid oxygen (LOX) inside the Starship; part of a demonstration funded by NASA. Kshatriya said that while engineers are still analyzing the results of the LOX transfer, the test on the March Starship flight "was successful by all accounts."

"That milestone is behind them," he said Friday. Now, SpaceX will move out with more Starship test flights. The next launch will try to check off a few more capabilities SpaceX didn't demonstrate on the March test flight.

SpaceX has released conceptual drawings that depict two Starships docked and connected belly to belly for refueling.  Image credit: SpaceX

Before Starship's first landing on the Moon with astronauts, NASA's $2.9 billion Human Landing System (HLS) contract with SpaceX includes ship-to-ship propellant transfer testing and an unpiloted landing of a full-scale Starship on the lunar surface.

But each Starship test flight builds on the prior mission. This means pretty much every Starship test flight over the next couple of years will have goals that feed into the first Artemis lunar landing. During these upcoming Starship test flights, engineers will measure the slosh of propellants inside the ship, along with tank pressures, and observe how the fluids respond to impulses from small thrusters. In microgravity, these small rocket jets provide "settling thrust" to guide the ship's liquid toward the outflow needed for refueling.

Engineers will also monitor the boil-off rates of the methane and liquid oxygen in space. Over time, cryogenic liquids transition to a gaseous state without insulation or other measures to prevent boil-off. SpaceX and NASA officials want to know how much of the propellant will be lost from boil-off to know how many refueling tankers they need to launch for a Starship lunar landing mission.

Refueling is something that everyone involved in long term planning about spaceflight and making the entire solar system more accessible has said is the big engineering problem that needs to be solved. NASA's Kshatriya is comfortable that SpaceX is making good progress with the engineering analysis. 

One of the reasons SpaceX is working hard to build a second launch pad in Boca Chica is to support multiple launches closely spaced in time. For the first full-scale refueling demonstration, still looking to be in '25, SpaceX has to launch two Starships within three or four weeks of one another. The first Starship will serve as the target vehicle in low-Earth orbit and will have an augmented power system with more battery capacity to survive in space long enough for the launch of the second Starship that will be the refueling tanker. In addition, both ships will also have thermal insulation and vacuum jacketing around their internal plumbing to limit boil-off. "Otherwise, the demo itself will not be achieved," Kshatriya said.

This is hard, but it's doable. Various engineering groups have been working on this big task in the background over the years. Computer models and flight data from numerous rockets show it is possible to control cryogenic boil-off, tank pressures, and propellant settling in space. It's just the big step, getting the cryogenic propellants from one spacecraft to another in orbit has never been attempted. 

"In my mind, all the technical issues associated with cryo transfer in space are solved," said George Sowers, former chief scientist at SpaceX rival United Launch Alliance and a longtime proponent of depoting propellants in space. "It’s just a matter of demonstrating it and fine-tuning the technology and the procedures. So, I think we’re on the cusp. I’m happy to see SpaceX taking the steps to make it work."

This chart from a NASA Artemis meeting outlines plans for SpaceX's ship-to-ship cryogenic transfer demonstration planned for 2025. Image credit NASA: Amit Kshatriya

My belief is that there will be lots of learning going on here, in the usual SpaceX mode of try - and if you fail, figure out what went wrong and fix it. Not by '25, but maybe by 2030 this will be as routine as a Falcon 9 booster landing - and you remember how insane that was thought to be just about a decade ago.