Sunday, May 5, 2024

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.


  1. Assuming(!) the Goobermint gets the hell out of SpaceX's way, they will Git-'er-Done, I am sure. They've thought this out and are moving at light speed (compared to the arthritic Old Space Companies) to accomplish things. Just takes time and money.

  2. How can we be a space-faring nation if we have to ship all the fuel up a steep gravity well?
    Until we're using nuclear engines and mining ice for propellant, this is all fun, but it's still a sideshow.
    If .gov dominates space as the have the past 60 years, will it be another 60 years of space stagnation?
    It WILL be over if the dems take the election and lawfare Musk.

    1. Let me know when YOU think the watermelons will tolerate nuclear engines!

    2. When Hell freezes over, Mark. They ain't that bright in the first place.

  3. Vacuum insulation? Like a SST coffee mug. Use very light gauge outer shell with a vacuum between maybe or some rendition of that to account for its a rocket body.
    The large diameter liquid nitrogen bottles on casters, welding supply houses lease, they hold liquid N for weeks with very minimal boil off. Essentially big stainless thermos bottles.

  4. We need an acronym to describe the SpaceX (and others) method of development, since everybody seems to have to spell out why it's the more effective way. May I suggest "try-fail-fix-try" or TFFT? This is more pronounceable than OODA.

    The other way is "model-analyze-yearn-build-execute" or MAYBE.

    1. MAYBE is a good acronym, but seems too much like the ones Congress comes up with where every bill has to be named with a catchy acronym - that's pretty much the opposite of what's in the bill.

      As for OODA loops, ever seen the Cajun phrase, "who dat?" OODA can be pronounced the same way just drop the "T" sound. Or "who duh"?

      "TFFT" or "Tift" is a good one. Maybe TFFTA with TA for ending with Try Again?

  5. I dunno. TFFTA is awfully close to Poofta...

  6. So, is there a technical reason that the propeller transfer testing needs to be done with full up SpaceX systems?

    It couldn't be done with smaller and cheaper falcon 9 class payloads?

    1. Probably, but that's not Musk's style. If you build test system that will fly on a Falcon, that doesn't get a Starship version built. If you do the testing on the latter, then at the end you have a system you can just tweak and use. Also, goals like this keep the pressure on to keep Starship development sprinting as hard as it can go.

      That's my five cents, anyway.

    2. YouTube is among the worst places to seek information because every video is either something you've never heard of is better than SpaceX - or it's failing and worse than SpaceX. It's gotten to where I rarely watch anyone new. I only watch the handful I'm comfortable with.

      That out of the way, there was a headline the other day that some small company was going to demonstrate refueling with much smaller rockets and beat SpaceX. Truth or clickbait?

    3. Probably clickbait. But I must repeat: demonstrating moving a little fuel with a small system doesn't prove you can do it with a big system. Since one of the biggest challenges is fuel slosh and settling, how can you demo that in anything but a tank of the size and configuration you will be ending up with?

      We haven't heard how the small-scale internal transfer went on the last flight. I'm reasonably confident that that test was more informative than anything on a small payload.

  7. "full up Starship"

  8. So the plan is to have clumsy and expensive rocket infrastructures as visualized in the 50s, to colonize Mars so we can have another supply of dirt-poor peasantry.

  9. Wonder if it is possible to connect both booster and starship together , one stage, fly it to orbit. With no engines in the ship section it would make a dandy gas station, or a space station. Bring up a plasma cutter for cutting openings and a tig welder for creating carious features for habitation. Have to bring up the creature comforts, and there is Falcon for that. Leave an engine for manuevering, the rest you get to mount to anything you like. Make it so ship and booster can be separated and cylindrical pre-fab pod slide in sections could be brought up, say added between the booster-ship separation point, no limit on how many you can add, they be plug and play, so its double duty, fuel and space station all in one. Potentially a bunch of pods could be launched in place of the normal upper cargo section of ship. Only need a cone head for atmospheric aerodynamics. Be like a big SpaceX Leggo set.