More on Inflatable Tankage and Propellant Depots
[Note: I wanted to address some of the comments made to my posts last week in some new blog entries so they don't get lost in the pile.]
Pete Lynn, in comments to my previous post, suggested that the main benefit to Inflatable Tankage was in potentially much lighter tanks, however I'm still not convinced. Right now, state-of-the-art LOX tanks tend to weigh about 0.7% of the mass of their contents. Unfortunately, most launch vehicles are designed to launch much denser payloads. For instance, if you wanted to fly a Falcon IX with an empty LOX tank as a payload, you could launch a tank big enough to handle 1.3 million kg of LOX. That comes out to a tank that is 1048m^3. That's one huge tank. At the same OD as the Falcon IX, that'd be a 109 meter long tank! Even hammerheaded to twice the diameter, you're still talking about a tank that is over 27 meters long. Compare that to the current planned length of the Falcon IX up to the payload interface: about 35m. Trying to fly something like that would be very difficult, as it drags the center of pressure forward, but shoves the CG back--ie making the vehicle very unstable. With an inflatable tank though, it'd be relatively easy to pack that much tank into the standard payload fairing, even if the tank ended up being just as heavy as an aluminum tank of the same filled volume!
Another couple of random points worth mentioning:
I could go on, but I just wanted to make those points for now.
Pete Lynn, in comments to my previous post, suggested that the main benefit to Inflatable Tankage was in potentially much lighter tanks, however I'm still not convinced. Right now, state-of-the-art LOX tanks tend to weigh about 0.7% of the mass of their contents. Unfortunately, most launch vehicles are designed to launch much denser payloads. For instance, if you wanted to fly a Falcon IX with an empty LOX tank as a payload, you could launch a tank big enough to handle 1.3 million kg of LOX. That comes out to a tank that is 1048m^3. That's one huge tank. At the same OD as the Falcon IX, that'd be a 109 meter long tank! Even hammerheaded to twice the diameter, you're still talking about a tank that is over 27 meters long. Compare that to the current planned length of the Falcon IX up to the payload interface: about 35m. Trying to fly something like that would be very difficult, as it drags the center of pressure forward, but shoves the CG back--ie making the vehicle very unstable. With an inflatable tank though, it'd be relatively easy to pack that much tank into the standard payload fairing, even if the tank ended up being just as heavy as an aluminum tank of the same filled volume!
Another couple of random points worth mentioning:
- While you probably want some redundancy in tankage, you really want to keep the numbers low. Rocket plumbing is a pain in the neck as is, without having to install it on orbit. So what you really want is one or two (or maybe three) fixed tanks per propellant that are filled and emptied by tankers and propellant customers. The whole idea that physically swapping around smaller tanks is going to be easy seems naive to me.
- Once you've added thermal insulation, ripstop layers, and micrometeorite protection, inflatable structures really don't have much of a mass benefit over their metal counterparts. What they are is more robust, and easier to launch in a launcher with a smaller payload fairing.
- One of the biggest benefits of a propellant depot, particularly one with fairly decent sized tanks is that it completely decouples the end-user's vehicle from the suppliers' vehicles. For instance, with a buffer tank like that, you could reasonably use high flight-rate, small RLVs (in the 1000-3000lb payload class) to refuel the depot, while the depot refuels a large transfer stage (say in the 100,000lb propellant class). The large transfer stage only has to deal with a single docking event, so the odds of damage to it are much smaller than if it had to handle 30-100 docking events. The depot on the other hand, since it isn't moving can afford more robust and beefy docking interfaces, redundant docking ports, robotic arms to allow for berthing instead of docking, etc. That way even for large transfer stages, you don't have to have large launch vehicles to put them up. Which allows you to use vehicles that already exist instead of needing to make something custom for which you will end up paying a lot more.
I could go on, but I just wanted to make those points for now.
posted by Jon Goff at 7:42 AM
11 Comments:
Thanks for bringing this topic back up. I was meaning to reply to your previous post.
I think that the potential for inflatable tankage is probably greater than for any other configuration. In addition to the points you raised, I think a flexible tank would make alot of otherwise difficult tasks much easier. One example is propellant transfer. The tank could probably be made with straps which could be tightened to create a positive pressure thereby forcing fuel into a waiting tank. There would be no need for settling the tanks by spinning or gravity gradient.
(Hmm... just had a thought about the tank on a lunar transfer stage being made out of a flexible material. It could store much more volume and maybe even be able to generate sufficient pressure through mechanical constriction to feed the combustion chamber... or maybe not. It was just a thought.)
I wonder how much fuel the Progress vehicles have left when they leave the ISS? It may be interesting to set up a fuel depot leading or trailing the ISS and have the Progress dump its extra fuel there before it burns up in the atmosphere. Or for that matter, have a module where the Shuttle can dump its excess water (from it's fuel cells) before reentry.
Anyway, sorry for the stream of conscience comments. There's just so many interesting applications of inflatable structures. I've been pondering several since I read about Bigelow's patent on the inflatable satellite bus. I'll try to post an entry on Spaceflight Sandbox about it sometime this weekend.
I have not been doing well at explaining when inflatable tanks become advantageous. Used in a standard fashion, there is little advantage in them, benefits come from using them in non standard fashions. Simply replacing rigid tanks with inflatable tanks is like just replacing steel with carbon fiber. To do it properly and get the greatest benefit, one actually has to adapt the fundamental design to these new materials.
Jon: “Right now, state-of-the-art LOX tanks tend to weigh about 0.7% of the mass of their contents.”
I am not sure of the details of these particular tanks, but from what I can gather the relationship between tank mass and tank volume yields something of a bath shaped curve. At small scale wall thicknesses become impractically thin, at large scale acceleration induced pressure head becomes significant. Both cases leading to wall thicknesses above the ideal structural minimum.
By their capacity to practically utilise much thinner wall thicknesses, inflatable tanks can delay the onset of the small scale end of this bath shaped curve by perhaps an order of magnitude. Hence inflatable tanks side step this small scale constraint, they reduce the mass fraction disadvantages of small scale rocket vehicles, this is I think their primary advantage.
I can not remember the details off the top of my head, but the balloon tanks of Atlas?, have often been compared to the heavy tanks of Rusian designs. The latter being very robust, practical, but very heavy, the former requiring delicate care including continued pressurisation during handling. The inflatable approach, however, should allow one to get the best of both these worlds.
This is not about whether inflatable or rigid tanks are better, but about under what circumstances one or the other is better. No one really has a good grasp of the answer to this question just yet. It is something that requires more analysis.
Well, what led me to inflatable tanks was the question of how to deliver propellant to the tank to be filled. The issue of propellant not necessarily being in contact with the pumps used to deliver the fuel to the target was the driver. My first thought was to use surface tension and wicking action to bring the propellant to the pump. Then I remembered the bota bag my dad got for me when he was TDY in Spain. You want wine? You squeeze the bag.
So by squeezing an inflatable bag you eliminate the need for pumps at a refueling staton. What is needed though is electric motors that tighten the straps around the bag. When refilling, just let the straps go.
So the issue isn't really how big you can get the bag, but rather how many fixed-size-equivalent tanks can be launched in the the same volume.
I never really considered them for use on vehicles, since as soon as you have acceleration you have the means to deliver propellant to the pumps. Now could physbrain be right and the electric motors provide enough pressure to eliminate the need for heavy pumps? Maybe. That is an interesting idea, and I guess would be a function of the torque the motors would be able to exert when tightening the straps.
Another possibility would be to design the LEO fuel depot to take advantage of gravity gradients in LEO, perhaps through the shaping of the tanks. This seems a less viable, conceptally more difficult option to me though.
Personally, I like the Murphy Bags, and for most of the same reasons.
Inflatables can pack more volume (more of a factor than weight) than rigid on launch, and pressurization and pumping are pretty much solved.
Now, I hadn't really considered making a spaceship's fuel tanks into Murphy Bags, but it does make sense, for many of the same reasons. While pumping isn't as much of a problem while you're in burn, you have to get fuel to the engine to *start* a burn somehow, and all of the same benefits to the design apply.
Has anybody done any testing on this concept? It seems like a really good project for a college engineering team somewhere. If we're going to use something like this, we need to start crunching the designs.
I'm guessing one of the issues in using Murphy Bags for spacecraft is that from what I understand the delivery of a consistent volume of propellant to the combustion chamber is what's important. I guess to prevent pogo and sputtering issues. Jon is infinitely more qualified than I to comment on such matters, but my guess is that the infolding of the bag would create inconsistencies in the flow.
Maybe use them as spare tankage? To carry fuel from EML-1 to GEO for servicing runs? Store liberated SWIEs on the Moon?
I'm sure there's someone out there that has looked into this. I've never been able to come up with a genuinely original idea except maybe for my Whipple-dozer. It's a big giant Whipple shield in orbit in front of a space station. It wouldn't eliminate all debris flux, but a good bit to be sure. I'd call it a Whipplebumper, but that would imply that it's fixed to the station, which would be a bad idea. It could also be used to clear debris from and around strategic orbits.
If one uses a brailing system on the tank for actual pumping, then it will require all the power of a turbo pump, but in a much heavier low speed high force form, and the tank will need to be as strong as a high pressure feed tank system. Hence I do not much like this approach. Similarly constructed inflatable positive displacement pumps, on the other hand, are a far more interesting proposition. I am fairly certain one could match the performance of a turbo pump in this fashion, and in a much more convenient and far lower cost package. Such a pump would then allow one to use ridiculously low pressure inflatable tanks – very light weight. Tank weight is almost directly proportional to tank pressure and very low tank pressures is where inflatable approaches really start to shine.
Such collapsible tank approaches do strike me as interesting as a means of settling propellants or for shifting them from one tank to another, but the necessity of external power systems with many moving lines does scare me a little, I can not help but think there are much better ways of doing this. For example, a small collapsible tank that is used to start the rocket engine and initiate an initial acceleration so as to settle the main tanks. This might consist of a small inflatable tank within the cabin area where cabin air pressure will provide a constant one atmosphere of pumping pressure. This would also be able to empty the tank to almost the last drop.
Another general system that might be more appealing is to use a double layer inflatable tank. By very lightly pressurising the gap between them the internal tank can be fully collapsed and compressed by the much simpler actuation of a valve. This achieves most everything desired and the pressurant gas is fully recycled. This general approach just seems much easier.
Jon, here is a question for you; for the XA 1.0, assuming a pump fed engine with a very low pressure tank of no more than two or three atmospheres and ignoring insulation for now, what kind of real world LOX tank weight might you expect? Would it be close to 0.7% by mass?
Now what might you expect would be similarly possible for an inflatable tank in this specific circumstance? What would the respective tank wall thicknesses be? Tank wall masses per square metre?
Pete,
Well, I haven't done much design/analysis on the final XA-1.0 design, but an interim design we were looking at would have a LOX tank mass in the 1.6% range. That's trying to avoid really fancy manufacturing (like Friction Stir Welding, since we don't have a good source for that yet), exotic materials (like Li-Al alloys), with a tank pressure of 50psi, and a FOS over yeild of 2.0. If you drop the FOS down to the usual aerospace level of 1.3, and cut the tank pressure to 25psi, you could theoretically get the weight down to something like .56%, but you'd be hitting minimum gage issues with that small of a tank, so you likely would only be able to get it down to 1% or so (that's for a 48in spherical tank). If you were doing a full LV sized tank, you could get that .7% number without even trying to use funky materials or manufacturing.
I'm just not convinced though that once you've added all the internal slosh baffling, structural interfaces, flanging, etc that an inflatable tank would really end up saving you very much weight at all on our system. It's possible, and Pierce seems to be convinced that he could do it, but I'm still skeptical.
~Jon
OK, say we assume a similar 48 inch inflatable tank, although there is no real need to be spherical or not use multiple smaller tanks - perhaps enabling better load distribution. Teflon liner, (0.002 inch film), with a braided Spectra envelope, 50psi, FOS of 2 on real world braided Spectra figures, (~1.1Gpa at ~770 kg/m^3 as braided) and adding a little in for plumbing, reinforced load carrying points, baffles etcetera, I get around 0.5%. (~0.5kg for the liner, ~3kg for the Spectra envelope and ~2kg for extras.)
A lot of strength is lost in braiding, which there might be a lighter way around. For example a method now often used is to slit a high strength film, (Cuben fiber, etcetera), into thin strips and then weave them into a fabric. I expect this could be significantly stronger and it might also be easier to make odd shapes, add structural interfaces, reinforced holes, etcetera. Heat sealing plumbing fixtures to the bladder is very straight forward and similarly adding Teflon baffles should be relatively easy.
Halving the above weight again is quite possibly practical, although at that point more significant weight savings can probably be found in the rest of the vehicle via taking advantage of such inflatable tanks to reduce load paths, plumbing, aeroshell, etcetera. Inflatable tanks can give one more design options that with clever design can in turn lead to lighter vehicles. For example, external self supporting inflatable tanks that can be retracted when empty have some significant attractions with regard to making the core vehicle design very simple, compact and light.
A quick note; Two quick and simple constructions systems for inflatable tank envelopes would be sewing a grid work of braided Spectra line onto a nylon fabric shell, and cutting the tank directly out of Cuben fiber fabric, taping and sewing the seams. Neither of these two systems are ideal, but they would both be quick and achievable with little more than a domestic sewing machine. They also quickly allow a wide variety of shapes and for reinforcements to be easily added where desired.
A taped and sewn seam is never quite as strong as the base material, and the nylon fabric shell would probably weigh around 50 g/m^2, say 0.25 kg in the 48 inch spherical tank example. Both construction techniques might loose an extra 10-20% in effective strength beyond that already lost in adhering the notoriously slippery Spectra filaments to one another in the first place. All in all I would probably tend to favour the former approach, braided Spectra line is cheap, robust, readily available and easy to use, Cuben fibre far less so.
If it is the one I am thinking of, I looked at it some time back, I expect Jon is aware of it.
It is very much a conventional design with rigid components replaced with inflatable ones, it seems to make no effort to actually adapt the design to best use inflatable methods. For example, the inflatable tanks are spherical and supported by a rigid carbon fibre truss, neither of which are necessary or desirable with an appropriate inflatable design. Ideally one would use inflatable cylindrical tanks, the bottom of which were mounted as closely as possible to the engine – minimum load path.
While that design seems to miss the point of using inflatable methods and is designed with a rigid vehicle mentality, it does attempt to use inflatable structures and as such open the door to such new approaches. It also alludes to the incredibly low mass fractions that are potentially possible with the inflatable approach.
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