inflatable orbital refueling

[natsirt721]

1 hour ago, farmerben said:

So you want 10 kg of shielding for 1 m2 of surface.  Is that just for the pilot's chair?  Or for all the fuel tanks as well?

So, that depends on the station I guess.  For a large station with access to resources, smaller inflatable bubbles might work. String a few dozen on a truss, and if one gets punctured, pump the fuel to a different one and bring the damaged one in for repairs or replacement or patch it in-situ. For anything smaller or less well equipped, yeah, shielding is a necessity. Fractals help a little bit if you don't have shielding, but repairing the damaged internal cells is nigh impossible, so eventually you need to replace the whole thing. I'm no expert on hypervelocity impacts but low density plastic shielding is not going to stop an impactor.

10 kg/m2 is probably high - again, no expert. You could probably use PE panels, if the station is so equipped to produce them, but a naked bag is not going to last.

The reason we don't see debris strikes as more of a hazard is that the ISS is pretty well armored. Most of the surface is either structural (trusses or pressurized areas) or redundant (radiator panels or solar arrays). Microdebris impacts against these have relatively little effect on the operational capacity of the vehicle. Against a pressurized plastic lung, the same debris could destroy it. You said yourself that PE tears rather than shattering - an impactor could create a tear which totally destroys the gas bag, like popping a balloon.

This storage solution is feasible under different conditions, but in the near term, I don't think it is practical.

[farmerben]

6 mil Polyethelene is about 0.1 kg/m2.  

Suppose we are willing to sacrifice up to 1000 kg of gas at 1 atm.  A cube 10 m across will have sides each 10 kg, and mass 60 kg of plastic.  

So we have a rubix cube and we lose one of the corner cubes.  The inner surfaces are still containing other bags of gas.  So we have ruined 30 kg of plastic and lost a ton of gas, but half the plastic is still doing a good job.  A center cube totally penetrated has 4/6 good surfaces, 20 kg of plastic damaged, the rest is still doing a job.  The damaged stuff is nearly all still there, so it could be repaired.  But the losses are acceptable without any repair.  For a rubix cube a triple cube puncture is only 1/9 of the total fuel supply, that is a comfortable safety margin.  Assuming all the inner surfaces are single layer rather than double, then a rubix cube with 10m segments, has almost equal the wet/dry mass ratio as the space shuttle tank.  The rubix cube is the simplest model, it can be improved.  But the real way to improve the wet/dry ratio is to increase size, trusting multiple tons of gas to single balloons.  

A rhombic dodecahedron has 12 sides and tiles space like a cube, but touches 12 others.  That means each surface is only 1/12 of the container value damaged, twice as good as cubes for soaking up damage without wasting plastic.

 

In LEO this type of thing might be the best debris sweep because of its sheer size.  But it could remain on station with thrust.  In the Van Allen belts, it is the best possible shield from proton radiation.  In deeper space impacts would be rare, but each impact would only take a small percentage of the fuel. 

Space is an abundant resource in space.  What abundance could humans create if we used all the resources that are abundant?

 

 

 

 

[Cheif Operations Director]

On ‎6‎/‎22‎/‎2019 at 3:43 PM, farmerben said:

The moon gets -173 C and it has thermal mass.  So a shade can make things colder.

-183 C is the boiling point of oxygen, just barely within reach.  But that is the freezing point of methane, which boils at -161 C.  So methane does not need insulation, only shade.  

Liquids are pretty tricky at low pressures though....  We only require liquids for transfer to other vehicles, and that requires a small compressor near the hardpoints.  

Nitrous oxide and ethane can exist as solid ice in LEO if they have enough shade.

I wonder if on certain bodies if you could mainulate an orbit so you are never exposed to heat. then you can be cooled constantly

[kerbiloid]

7 hours ago, Cheif Operations Director said:

I wonder if on certain bodies if you could mainulate an orbit so you are never exposed to heat. then you can be cooled constantly

https://en.wikipedia.org/wiki/Sun-synchronous_orbit

(Upd.
Maybe I misunderstood you a little. Probably you mean an eternal night.

Then the outer Lagrange point.

[natsirt721]

19 hours ago, farmerben said:

6 mil Polyethelene is about 0.1 kg/m2.  

Suppose we are willing to sacrifice up to 1000 kg of gas at 1 atm.  A cube 10 m across will have sides each 10 kg, and mass 60 kg of plastic.  

<snip>

At 1 atm LOX has a density around 1.14 t/m3, so each bag (10m edge cube) would actually hold 1140 tons. LH2 has a density around 0.07 t/m3, so each LH2 bag holds 70 tons.

Cubes are poor structures for storing things under pressure, because all of the stresses on the panels converge at the edges and vertices.  Spheres (and to a lesser extent cylinders) redistribute stress better across the surface of the structure, so you need less mass reinforcing the stress concentrations.  A cube also has more surface area per volume than a sphere, increasing overall mass for the same containment value.

After doing a little bit of research, I don't think that pure PE is the right material for the job. Consider that one side of the bag is exposed to a vacuum, which can lead to outgassing for many plastics. The other side is a cryofluid, and plastics typically become significantly more rigid as they cool. Either of these effects will severely compromise the structural integrity of the bag, even if it isn't hit by debris.

I don't think any cog-e is going to sign for 6 mm of plastic between 1100 tons of valuable propellant and the unforgiving vacuum of space.

[farmerben]

6 minutes ago, natsirt721 said:

At 1 atm LOX has a density around 1.14 t/m3, so each bag (10m edge cube) would actually hold 1140 tons. LH2 has a density around 0.07 t/m3, so each LH2 bag holds 70 tons.

Cubes are poor structures for storing things under pressure, because all of the stresses on the panels converge at the edges and vertices.  Spheres (and to a lesser extent cylinders) redistribute stress better across the surface of the structure, so you need less mass reinforcing the stress concentrations.  A cube also has more surface area per volume than a sphere, increasing overall mass for the same containment value.

After doing a little bit of research, I don't think that pure PE is the right material for the job. Consider that one side of the bag is exposed to a vacuum, which can lead to outgassing for many plastics. The other side is a cryofluid, and plastics typically become significantly more rigid as they cool. Either of these effects will severely compromise the structural integrity of the bag, even if it isn't hit by debris.

I don't think any cog-e is going to sign for 6 mm of plastic between 1100 tons of valuable propellant and the unforgiving vacuum of space.

Yes the liquids are about 1 ton/m3, and gasses are 1 kg/m3 approximately.  H2 gas is not a good canidate for this type of fuel storage, but methane is good.  

The bags need not tile 3-D space with a single type of polygon, that just makes the math simpler.  Honeycombs are good for tiling between two surfaces.

6/1000 inches is the thickness of large greenhouses on Earth,  that's 0.15 mm.  It can handle 20-60 psi.  It has a trace of extra carbon in it to resist UV degredation.  Weather balloons on Earth can be 20 times thinner.   https://www.sciencedirect.com/science/article/pii/S0273117702005276

Polyethylene remains flexible to -100 C, below that it becomes rigid so a rapid inflation or deflation could crack it.  Teflon remains flexible near absolute zero, and could be used for valves.  Compared to polyethylene every other plastic is more dense and has several disadvantages to offset each possible advantage, they might be justified as structure or shielding, but PE is the winner for large airtight films.

[natsirt721]

On 6/30/2019 at 3:59 PM, farmerben said:

Yes the liquids are about 1 ton/m3, and gasses are 1 kg/m3 approximately.  H2 gas is not a good canidate for this type of fuel storage, but methane is good.  

The bags need not tile 3-D space with a single type of polygon, that just makes the math simpler.  Honeycombs are good for tiling between two surfaces.

6/1000 inches is the thickness of large greenhouses on Earth,  that's 0.15 mm.  It can handle 20-60 psi.  It has a trace of extra carbon in it to resist UV degredation.  Weather balloons on Earth can be 20 times thinner.   https://www.sciencedirect.com/science/article/pii/S0273117702005276

Polyethylene remains flexible to -100 C, below that it becomes rigid so a rapid inflation or deflation could crack it.  Teflon remains flexible near absolute zero, and could be used for valves.  Compared to polyethylene every other plastic is more dense and has several disadvantages to offset each possible advantage, they might be justified as structure or shielding, but PE is the winner for large airtight films.

RIght, I forgot that a mil is a unit of measure. Well, that's even worse that I thought.

I completely agree that hydrogen is not a good candidate, neither as a gas nor a liquid due to its abysmal density across the range of feasible pressures and temperatures. 

Your cryofuels are going to be well below -100 C. Unless we're talking about storing them as a gas (presumably near STP), but I bet the decrease in density and larger volume requirements outweigh the mass savings from a cheaper material. Let's examine that real quick.

Say we want to store 100t of methane. At STP, methane has a density of 0.717 kg/m³. 100t would require a volume of ~140,000 m³, or a sphere with a radius of 32.2m and a surface area just greater than 13,000 m².  Surface stress on a sphere under pressure is (p*r) / (2*t).  Some quick googling puts the tensile strength of LDPE around 14 MPa, and HDPE around 30MPa, which (including a safety margin of 1.2, which is pretty conservative for space applications) gives a thickness of 0.139 m of LDPE, and 0.065 m for HDPE.  Density for plastics vary a lot by manufacturer, this source says 0.92 t/m³ for LDPE which seems high so let's call it 0.90 t/m³, and 0.95 t/m³ for HDPE. Given our surface area, the total container has a mass of 1625 metric tons for LDPE, and a better-but-still-awful 800 t for HDPE!

Holy crap, I expected a high mass, but that was truly horrible!  Now, we can bring that down a bit if we expect the bag to stretch slightly, but then the bag gets thinner and larger every time it gets deflated (which might not be ever, if you can keep enough propellant on-site). We can also lower the pressure of the gas, but gasses really like to expand, so I think the volume might increase faster than the lower pressure saves you from your thickness. Either way, its going to be a fuel-mass ratio of 1/7 for your tank, which is not really practical.

At -162 C, 1 atm, liquid methane has a density of 422 kg/m³, which is about 600 times more dense. Repeating the same same exercise as above gives us a total volume of 237 m³, sphere radius 3.9 m, surface area 191 m², wall thickness 0.016 m for LDPE and 0.0078 m for HDPE, total mass 3.41t for LDPE and 1.77t HDPE.

So yeah, storing gasses on-orbit is not worth it - the material definitely needs to hold up at cryogenic temperatures. You not only need a material that is flexible at low temperatures, but that doesn't outgass, doesn't degrade under full spectrum light, and can radiate away more heat than it absorbs at its operating temperature. That's a pretty tall order for plastics alone, and you said that PE already fails at sub -100 C temperatures.  Composite materials are probably going to be what you want, but they ain't gonna be cheap like PE.

The reason weather balloons and greenhouses (and sandwich bags) can get away with sub-millimeter amounts of plastic is because the pressure gradient (or gauge pressure of the inside fluid) is very small. In space, the pressure gradient is just whatever your contents are pressurized to, and 101 kPa is a lot of Pa. Consider this thought experiment: static water pressure increases at about 1 atm per 10 m of depth. If you take a plexiglass tank open on the top 1m by 1m by 10 m tall and fill it with water, the pressure on the bottom is about 2 atm (1 atm of atmosphere plus 1 atm of water). Now replace the bottom panel with greenhouse plastic and lift the column off the ground, so the pressure gradient is 1 atm (the pressure from the atmosphere cancles, and you get just the water). Does it survive (assume that the plastic is perfectly adhered to the tank)? It might, after all the total area is only 1m². How wide can you make it before the plastic fails? Does 10x10 m survive? That's only 100 m², not even enough for our liquid methane experiment. Our original column held 10 tons of water, this one holds 1000 tons - probably too much for the plastic to withstand.

Plastics are really only good for volume containment when the pressure requirements are low. For anything else, you need a higher strength than most commercial plastics can provide - that means metal alloys or composites.

Edited July 1, 2019 by natsirt721
forgot a zero

[kerbiloid]

If combine the inflatable gas storage with a space elevator, we can make a mooring mast for sats.

Spoiler

Also, it looks very Kerbal and would be great in KSP.

[farmerben]

Natsirt has some good arguments.  I'm fairly well talked out of the LEO inflatable refueling application.  It might have fuel applications in other locations like the asteroid belt, as well as non-fuel applications.

When large radius are involved polyethylene or any similar material requires additional structure.  Perhaps only a small fraction of the mass is needed for kevlar ripstop nets, etc.  But whatever we do the mass and cost go up.  

Also I quoted a figure of >20 psi for 6 mill polyethylene.  I think its closer to 0.5 psi per mil, or at most about 3 psi for 6 mil.  Which is pretty good for pure oxygen at 1/5 ATM.  We could probably grow plants with a low pressure mix of gasses.  I still really love the idea of low pressure bubbles protecting high pressure bubbles

[wumpus]

On 7/1/2019 at 7:40 PM, natsirt721 said:

At -162 C, 1 atm, liquid methane has a density of 422 kg/m³, which is about 600 times more dense. Repeating the same same exercise as above gives us a total volume of 237 m³, sphere radius 3.9 m, surface area 191 m², wall thickness 0.016 m for LDPE and 0.0078 m for HDPE, total mass 3.41t for LDPE and 1.77t HDPE.

[deletia]

The reason weather balloons and greenhouses (and sandwich bags) can get away with sub-millimeter amounts of plastic is because the pressure gradient (or gauge pressure of the inside fluid) is very small. In space, the pressure gradient is just whatever your contents are pressurized to, and 101 kPa is a lot of Pa. Consider this thought experiment: static water pressure increases at about 1 atm per 10 m of depth. If you take a plexiglass tank open on the top 1m by 1m by 10 m tall and fill it with water, the pressure on the bottom is about 2 atm (1 atm of atmosphere plus 1 atm of water). Now replace the bottom panel with greenhouse plastic and lift the column off the ground, so the pressure gradient is 1 atm (the pressure from the atmosphere cancles, and you get just the water). Does it survive (assume that the plastic is perfectly adhered to the tank)? It might, after all the total area is only 1m². How wide can you make it before the plastic fails? Does 10x10 m survive? That's only 100 m², not even enough for our liquid methane experiment. Our original column held 10 tons of water, this one holds 1000 tons - probably too much for the plastic to withstand.

Plastics are really only good for volume containment when the pressure requirements are low. For anything else, you need a higher strength than most commercial plastics can provide - that means metal alloys or composites.

While I admit that withstanding 1-2 bar isn't in the realm of sub-mm greenhouse film, I still think that 16mm or 7.8mm is still way too big.  And width is mostly irrelevant (except for the greater increase of potential weak spots, something presumably examined on the ground), just how deep the fluid is (pressurized air or water in our thought experiment).  The reason you need tensile strength is gravity, simply holding the material doesn't require that at all (think more of the sides [but not edges] of a container, they still have to be strong, but not that strong.  I'd suspect that a plastic trash bag could handle a lot of water as long as it was sitting on the ground).  Still, I'd really hate to have to design something that takes fuel from an inflated bag and then turns it back into cryogenicly cooled fuel inside a fuel tank.  Expect that process to be slow and tedious, mostly thanks to using a blackbody as a heatsink (maybe use semi-conductive mylar, and use the fuel as a blackbody?  Or at least as the "coolant", although this assumes that you will always leave a significant amount of fuel/oxidizer in the tank).

Either way, cooling the propellant down to cryogenic temperature will be slow.

[farmerben]

Actually I think converting from gas to liquid and back is pretty easy, if we are dealing with things like nitrous oxide and propane.  A compressor with a small high pressure tank can liquefy it, and then releasing gas into another tank will cool it.  

[natsirt721]

On 7/12/2019 at 1:14 PM, wumpus said:

The reason you need tensile strength is gravity, simply holding the material doesn't require that at all

The reason you need tensile strength is to overcome tensile forces. Whether those forces are because of gravity or due to pressure across a membrane doesn't matter.  This is why, in the thought experiment, the plastic can survive as long as the area is small - the total amount of force is small. When the area increases, even though the pressure is the same, the forces increase. At some point, the plastic will fail - not because of the pressure, but because the total force on the plastic is too great. The experiment may have been a little misleading, because I emphasized the weight of the water - the critical piece isn't the weight necessarily, it's the pressure * area on the plastic that causes the failure.  It doesn't matter that the pressure is caused by the weight of the fluid. In space, there's no weight, but there is fluid pressure. Fundamentally, it's the same problem, just with different geometry. The 2d plastic across the bottom of plexi column becomes the surface of a sphere, and the pressure gradient is from the vacuum of space (0 atm) to the internal pressure of the tank (1 atm, or whatever) instead of from the weight of the water.

On 7/12/2019 at 1:14 PM, wumpus said:

I'd suspect that a plastic trash bag could handle a lot of water as long as it was sitting on the ground).

Yes, I would too. Plastic is good for containing things when the pressure gradient is low. But in space, the pressure gradient is not low, so you need a stronger material.

Here's where the confusion lies.  Materials don't withstand fluid pressure, they withstand stresses. Structures withstand fluid pressure, by determining how the pressure is translated to stress, and where the concentrations of stress are. The '6mil plastic at 20-60psi' figure is confusing - fundamentally it doesn't make sense because of the above. It could be that you are quoting the tensile strength of the plastic, but 140 kPa is a pathetic strength.

Frankly, the cryostorage solution is a pretty good tank-fluid mass ratio - as long as your polymer can withstand cryotemperatures without being structurally compromised and you have a practical way to deal with debris strikes, it's not a bad solution for moderate amounts of propellant. A 2 cm thick bag 4 m in radius is certainly plausible.