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.
Jonathan Goff
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Generally speaking, tanks are required to get the propellant there in the first place – so in theory, unless one uses insitu propellants, there is no shortage of tankage at the destination.
Inflatable tanks might be particularly interesting in space applications where tank pressure might be reduced to a fraction of an atmosphere – tank mass being roughly proportional to internal pressure. In the case of an orbital propellant depot or low acceleration mission, dropping tank pressure tenfold might enable a tenfold reduction in tank mass – this is not really possible with rigid tanks.
With regard to launch vehicle enabling technologies, inflatable tanks are I think up there with small reliable high performance rocket engines, matching positive displacement “turbopumpsâ€, and a good fast turnaround TPS system. Maybe in a few years I will be in a position to give inflatable tanks a go, although it is one of many such base development jobs that needs doing and I do hope someone else gets there first, (where is NACA when you need it).
Jon;
Did you ever see this article:
http://www.flightglobal.com/Articles/2005/12/20/Navigation/200/203670/Inflatable+launcher+more+than+hot+air.html
The tanks suggested ‘kevlar and aluminum coated mylar’ for LOX and RP-1. According to the article the skin is also Kevlar pressurized by helium with the tankage suspended in a ‘carbon-fibre’ space frame. The engine idea looks like an annular aerospike.
“This” version is designed as an expendable rocket, but there is thought to making a ‘reusable’ version also.
I’d thought that this might make a ‘good’ little small-sat launcher.
Randy
Another thought; Use non-cryogenic fuel/oxidizer with compatable flexible tankage.
Hmmm, :o)
Randy
The lightweight XCOR composite LOX tanks have a PTFE liner, I don’t know how flexible it is though. At the new, light, XCOR tank weights it’s not clear how big a win a sacklike tank would be. I’m gonna guess not so much as you might think, since you’ll still need thrust structure. Unless the bag is well pressurized with a mass of gas, it is not thrust structure.You may also need insulation.
From what I recall, the XCOR tank prototype used a PTFE matrix material – so as to avoid micro cracking. At a guess, rigid composite materials might get down to practical minimum wall weights as low as one kilogram per square metre, inflatable approaches might get down to a few tens of grams per square metre. The real advantage of inflatable tanks comes in scenarios that exploit his lower wall thickness capability. That is, smallish tanks at low pressures.
Randy,
While it would be easier to make inflatable tankage work if you stuck with just storable propellants, the whole point of having inflatable tankage is to enable you to use the propellants you want to use. For in-space transportation, I think you’re going to see more and more people switch to cryogenic combinations over time, as the technologies for storing and transferring cryogens in space becomes more mature. Storable propellants all have their issues too, and are lower performance to boot.
~Jon