I was poking around at some of the other articles on that Lockheed page ,that I got sent a link to, and found some more interesting stuff. Once again, big caveat about hyperbole being used, but I still think that they make some really solid points. Particularly the section at the very bottom about Cryogenic Propellant Transfer. If you have the time, I’d seriously recommend poking through all these papers, particularly this one, and this one (which I’ll comment on later).
Here’s some thoughts, comments, and things I noticed on the technical side:
- This paper supports my point I’ve made here several times that once you have a good way to settle propellants, all of the other issues related to Zero-G propellant transfer go from being unknown tech-development projects, to straightforward engineering projects directly based on existing hardware and processes.
- I was wrong about the amount of settling force needed to make things manageable. I’d always used 0.001-0.01g as a conservative estimate. According to LM, they’ve flight proven that .0001g works, and they’ve flight demonstrated settling at as low as .00001g accelerations. That’s only about 10 micro-gees!
- Even without Zero-Boiloff techniques, it’s possible to keep the boiloff rate well below 1% per month with existing technology.
As I said, I’d seriously recommend reading the whole thing. The thing to remember, is that this is LOX/LH2 we’re talking about–pretty much the toughest commonly used rocket propellant combination to work with. I had never realized that while the Centaur was in orbit, that they have ducted vents being used to settle the propellants continuously, and that they have over 100 flights worth of experience with this stuff. As they point out, every Centaur flight demonstrates on-orbit propellant transfer–to the RL-10 engines. Including chill-down, propellant acquisition, propellant mass guaging, etc.
Now, the architecture they suggest is a “Vehicle to Vehicle” fueling architecture–basically it doesn’t include a propellant depot. Adding a depot would possibly require finding a non-propulsive settling route for the propellants, but would possibly simplify the remaining problems substantially. If the depot is manned, you could use remote piloting instead of fully Autonomous Rendezvou and Docking, you could add a small arm like the Canadarm Mini I suggested to allow you to berth modules instead of docking, which makes that even easier. And you could manually attach propellant transfer lines in a shirt-sleeve environment, which gets rid of the last complication.
There still is some work left to be done, but if anyone can honestly read these papers and not get the feeling that “this is a technology that is most of the way there, and not a long-shot bet at all”, I’d be surprised. In fact, I’m going to make a prediction. I’ll be so bold as to say that before a full CLV stack flies for the first time, there will have already been an on-orbit demonstration of cryogenic propellant transfer by a commercial company.
What do you guys think?