A few years back, I remember seeing some info about some of von Braun’s old lunar lander ideas he had written about for Collier’s back in the day. One of the interesting ideas that he had proposed was using on-orbit assembled landers that would use room-temperature propellants stored in inflatable propellant tanks. I remembered thinking briefly about inflatable propellant tanks then kind of filing the idea away in the back of my brain.
The topic came up again recently on my blog in discussions about propellant settling and storage (including Ken’s “Murphy Straps” idea), and then again on the aRocket mailing list. I initially wasn’t too interested in the idea, since I was pretty sure that no realistic bag material would be able to handle cryogenic temperatures, thus being of no use for LOX. However, after reading a bit more, there’s at least some evidence that there are some thin plastic materials that are still tough enough at that temperature to form the inner layer of an inflatable tank–most notably Teflon or Mylar. It also turns out that there’s some evidence that Kevlar is fine as a reinforcing mesh down to fairly deep cryogenic temperatures. Mylar is used all the way down to LH2 temps in dewars and such, some with the Mylar directly holding the LH2 in. Mylar is also the main material in MLI insulation (with in some cases IIRC, a kevlar mesh holding the layers apart).
So it’s an interesting idea, that might just work. There’s a whole bunch of details that would need to be worked out–what exact grade of plastic for the inner liner, how thick it needs to be, if it should be multi-layer, what sort of mesh reinforcement, if it needs intermediate insulation layers, how to attach fittings at the ends, how to make sure it inflates correctly, etc, etc, etc. Unfortunately, it really doesn’t look like there’s been much serious research and development on the idea yet. There’ve been several ameteurs who’ve dabbled with it, but nobody who’s gone as far as to try building even a subscale prototype. However, this might be a perfect match for Bigelow Aerospace, or some company working with Bigelow.
The potential benefits of inflatable tankage are that they would:
- be potentially much lighter than metal tanks
- not be as limited by the diameter of the initial launch vehicle–ie you could possibly get a 10m diameter tank launched on a 3m diameter launch vehicle
- make propellant depots easier to put up, since you could very rapidly launch truly massive ammounts of storage space
- If combined with a good several layers of MLI provide for very low boiloff cryogenic storage
- Possibly allow for very large transfer stage tanks or propellant tankers
And the list goes on.
Anyhow, I just wanted to bring the idea up, hoping that it might get enough attention that somebody else could do some of the legwork on seeing how workable it is. I’d be interested in doing some research on it, once we have time available, but it’d also be cool to have someone else put the technology “on the shelf” for us.