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Yesterday I watched the presentation by Gary Johnson (GWJohnson) at the Mars Society Convention. I attend virtually. I wasn't able to ask a question. Gary suggested there is no polymer that can withstand the cold of cryogenic propellant, so a bladder would not work. At cryogenic temperature the polymer bladder would crumble. This is not true.
I have mentioned on this forum many times that PCTFE can withstand the soft cryogenics.
PCTFE embrittlement -240°C
Densified LCH4 93°K = -180°C
LOX boiling -182.96°C, densified 120°R = -207°C (temperature SpaceX uses)
PCTFE is sold by Honeywell under the brand names Aclar and Clarus. The brand name Aclar is for packaging for pharmaceuticals (pills) because it's the most impermeable to moisture. Clarus is targeted at military and aerospace applications. A Japanese company makes various fluoropolymers under the brand name Neoflon, so Neoflon PCTFE. 3M used to sell PCTFE under the brand name Kel-F, but stopped making it in 1995.
Liquid hydrogen boils at -252.78°C at 1 atmosphere pressure. This means there is no polymer that can withstand the cold of liquid hydrogen. That's why liquid hydrogen is called a hard cryogen. However, both methane and oxygen and soft crogens; meaning they're liquid at slightly higher temperatures. PCTFE can be used as a bladder for both densified liquid oxygen and densified liquid methane. There is not need for ullage or spin or any other fancy kinetic system.
Ps. Gary also mentioned he doesn't know which hydrazine the Russians are using. The Russians have used UDMH (Unsymmetrical dimethylhydrazine) since the first Soyuz capsule in 1963, they still use it today.
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I am wondering if a composite tank built and tested by space x with a bladder inside would be a structural mass shift from current builds if it can support launch of more stages on top of it.
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I suppose it is cold of me to intrude, but my notion would be to spin the fluids, and push them with a stream of vapors.
The Depot may have the engines to compress and deploy a stream of gasses into propellant starship. Nozzles may push the gas stream into the tank, from one end, and also introduce a fluid spin. This may cause the gas bubbles in the fluid to centrifuge out towards the center and towards the nozzle end. The liquids should congregate away from that.
If you are not feeding an active rocket engine, it may not be too essential that gas bubbles are not included in the flow into the Depot tank. You would not induce cavitation on a Raptor Engine.
Fluid Spin Induction might also be used inside of the depot ship to then feed the Starship to go on an interworld mission. Again, as you are not running the Raptors during filling the ship, you are not having cavitation problems for the engines. I will grant that if you spin the propellants, you may then also induce spin in the collection of ships working with each other. It may not be too bad to tolerate.
Also, to do this you do not have to squirt propellants overboard to spin the ship or push it in a linear fashion.
This is intended to be helpful. Hopefully it will be seen that way.
Done.
Last edited by Void (2022-10-22 12:48:21)
Done.
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Robert,
That last part about not knowing what the Russians are using is a bit odd, if he actually said that. The Russians have used N2O4 and UDMH for a very long time, similar to how we've used N2O4 and MMH for a very long time. Now that this part is out of the way, let's focus on your proposal.
You think you can simply insert a bladder into the tank and have it push the cryogenic liquid through a propellant transfer line. Maybe the material itself can withstand cryogenic temperatures of the propellants, but there are other much more complicated issues to contend with.
1. All large cryo tanks have fill / drain and vent / purge valves built into them. These fill / drain and vent / purge valves must be duplicated in the bladder, or the bladder will rupture before the rocket ever leaves the launch pad. The bladder must be rigid enough to not tear or otherwise collapse when the propellant in the line between the two orifices is filled with LOX or LCH4, which tends to make it less flexible. These hose-like connections must exist, in order to squirt out the contents of the tank, and in practice to fill / drain the tank on the pad. Basically, you must have some sort of bladder-to-airframe interface, merely to fill / drain and vent / purge the bladder on the ground. This is not easy to do and adds a lot of weight at the size / volume we're talking about (100 tons or more of propellant).
2. Starship goes a step further and has a pair of header tanks built within the main propellant tanks, in order to prevent propellant slosh from disrupting the supply of LOX/LCH4 to the main engines during landing. If the engineers put those tanks in there, then it's because they had to, not because they thought it looked cool. Could a bladder eliminate the need for header tanks by doing a better job of limiting the movement of the propellant? Maybe, maybe not, but it's never been done in practice because it adds weight and cost and a host of new failure modes.
3. This third issue is a showstopper. You need at least semi-rigid anti-slosh baffles built into the bladder, because Starship's tank structure itself is no longer what actually contains the propellant, limits its movement within the tank during flight, and also provides structural reinforcement to the tank so that it doesn't collapse. You'd otherwise need a very oddly-shaped bladder that conforms to that internal reinforcement / anti-slosh structure (which must be completely free of sharp edges, unlike all real-world anti-slosh baffles) or it would be pressed into the reinforcement by the weight of the propellant until it tears this thin and flexible bladder. Adding an internal bladder would de-facto mean completely redesigning the propellant tank support structure to use external vs internal reinforcement. All internal tank welds have to be baby's butt smooth. That is doable, but only at great cost. Putting the reinforcement outside the tank would also drastically increase weight.
Assume for a moment that you could do all that. What happens when moisture on the ground freezes the bladder to the stainless steel?
How do you get it "unstuck" after it's in orbit?
It contains cryogenically cold propellant. Do you heat it up electrically to release it from the side of the rocket?
That turns at least some of your propellant into gas, which must then be purged away.
There's also a tertiary issue related to Starship's design, and that is heat sinking the thermal load from the TPS into the stainless steel propellant tanks during reentry, because the stainless can take the heat. Now that it's lined with rubber or plastic, you need a thicker / heavier TPS to keep the thermal load within the tolerable limits of the plastic, defeating the purpose behind using stainless to begin with.
The above cited explanations are the reasons why we're not using flexible propellant bladders, but it boils down to:
A. Added inert mass fraction, which reduces payload performance of the rocket (less propellant load delivered)
B. Greatly reduced acceptable temperature limits, which adds weight in the form of upper stage TPS (for a workable solution)
C. Added vehicle cost and complexity
D. Earth's gravity still does its thing in orbital micro-gravity conditions, so we don't need any of this added cost and complexity
Even if all the other technical problems were easily solvable, which they're not, the added weight problem from using fuel bladders is not solvable.
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