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Cont: Musk, SpaceX and future of Tesla II

theprestige said:
That's all very well when the rocket is accelerating.

The question is what to do when there's no acceleration. How do you create and maintain the pressure differential?

Personally I think it's not a serious problem, but apparently some of us are convinced that it's a major showstopper.
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My response is that I have no idea. I wasn't aware it was a problem to begin with. I have no expertise in fluid dynamics. I have a rudimentary understanding of how pumps work on earth. But that's it on this one.
 
acbytesla said:
I didn't realize pumping a fluid or gas in a weightless environment was all that difficult. Seems a pump and a pressure differential would all that is required.

This is what Google AI has to say.

NASA uses Lead Zirconate Titanate (PZT) mass gaging system to measure the mass of fuel and its location within a tank. Fuel is transferred actively using pumps, and sensors are used to monitor the amount of fuel remaining, tank rigidity, and defects in the tank.
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I think that Musk reflexively eschews NASA over engineered solutions.
 
a_unique_person said:
The reason I wonder how solved it is, is because there is estimated number of launches to do the refueling is very vague. Maybe 10, maybe 20. That's a lot of launches and a lot of infrastructure to support one moon mission.
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Presumably the infrastructure would support more than one mission. My personal dream is an orbital shipyard, where increasingly larger vehicles are assembled to deliver increasingly larger probes to all the corners of our solar system.

If a single dumbass moon mission is enough to get this infrastructure started, that's fine with me.
 
Exactly. You don't build infrastructure for one mission. The long-term intent is for this to be a launching location for missions to Mars. There's quite a bit to go before we get anywhere near that, though.
 
acbytesla said:
I didn't realize pumping a fluid or gas in a weightless environment was all that difficult. Seems a pump and a pressure differential would all that is required.

This is what Google AI has to say.

NASA uses Lead Zirconate Titanate (PZT) mass gaging system to measure the mass of fuel and its location within a tank. Fuel is transferred actively using pumps, and sensors are used to monitor the amount of fuel remaining, tank rigidity, and defects in the tank.
Click to expand...
Maybe I'm hitting the wrong articles but as far as I can see PZT (a piezoelectric material) is still being trialled as a way of measuring fuel transfer and is still not being done at significant levels.
 
Wudang said:
Maybe I'm hitting the wrong articles but as far as I can see PZT (a piezoelectric material) is still being trialled as a way of measuring fuel transfer and is still not being done at significant levels.
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To be honest. This is probably just one of a thousand issues that need to be resolved for man to go to Mars. Personally, I think there are far more rewarding scientific challenges to tackle before we think of sending a human to Mars. A waste of money and a waste of minds is my thought on this.
 
It actually Is a problem, because fluids behave oddly in Zero G.
A big problem when it comes to transferring large volumes of liquid is Sloshing: hundreds of kilos of weight pushing against the side of an attached container could mess with your trajectory, or even damage it.
 
jeremyp said:
Now scale that up to a tank containing 100 tonnes of cryogenically cooled fuel in zero g.
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you could have the liquid in a bladder inside the tank, and pump a gas in the space between the tank walls and the bladder, thereby squeezing the fuel out like a toothpaste.
 
Fluid slosh is a semi-solved problem. It's solved in the sense that we can model it accurately in linearized free-body dynamics, the Swiss Army knife of spacecraft dynamics and control. It's unsolved only in the sense that the actual engineering to apply the control in any given vehicle design tends toward hard. You typically solve this with internal baffles that restrict fluid movement. This makes it hard to have any other sort of structure in the tank for any other purpose such as pumping.

The major problem with pumping liquids from tanks in zero gravity is getting the liquid to stay pressed up against the pump intake. You can pressurize the tank all you want with gas (including vaporized contents) but it doesn't matter if the liquid is just a spherical blob floating around inside the tank volume.

The Great Zaganza said:
you could have the liquid in a bladder inside the tank, and pump a gas in the space between the tank walls and the bladder, thereby squeezing the fuel out like a toothpaste.
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This is how that problem is routinely solved for small tanks when you can't rely on ullage. The engineering gets harder as the tank gets bigger. Elastomeric bladders start to take on slosh behavior the larger they get and the emptier the tank gets. Only now you don't get to solve the slosh problem with baffles. You have to tweak the bladder so that it collapses in a predictable, semirigid way. That makes the bladder harder to make and harder to keep sealed.
 
The Great Zaganza said:
you could have the liquid in a bladder inside the tank, and pump a gas in the space between the tank walls and the bladder, thereby squeezing the fuel out like a toothpaste.
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I think that is how the header tanks in Starship work, at least initially.

However, apart from the problem that JayUtah has talked about, you need to carry the gas to squash the bladder up with you and that reduces the amount of useful payload you can carry.
 
jeremyp said:
However, apart from the problem that JayUtah has talked about, you need to carry the gas to squash the bladder up with you and that reduces the amount of useful payload you can carry.
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That's true, but historically that hasn't been too much overhead. The Apollo ascent engine used supercritical helium to maintain propellant tank pressure. Storing the pressurant as a liquid reduces tanking and plumbing, which is the real mass penalty. The Apollo APS fuel pressurization system had a total filled mass of less than 10 kg. That's on a spacecraft with a dry mass of 2,000 kg.
 
The Great Zaganza said:
you could have the liquid in a bladder inside the tank, and pump a gas in the space between the tank walls and the bladder, thereby squeezing the fuel out like a toothpaste.
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What are you making the bladder out of that it remains nice and flexible at cryogenic temperatures? Don't forget this isn't just pumping liquids in 0 gee but highly pressurized cryogenic liquids.
 
ponderingturtle said:
What are you making the bladder out of that it remains nice and flexible at cryogenic temperatures? Don't forget this isn't just pumping liquids in 0 gee but highly pressurized cryogenic liquids.
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The temperature is the big problem. The Apollo LM propellants were liquid at room temperature and needed to stay at about that temperature. But they didn't use the bladder method. There are elastomers that are good down to cryogenic temperatures, but then they have other trade-offs such as cost and workability in other ways. You might want to use a bellows-like internal structure using aluminum sheeting that is joined with elastomeric joints. But that too is very hard to engineer to be leak-proof and will incur a substantial mass penalty.

The pressure is not the problem. If you have a pump, you don't need high pressure in the tank. You only need enough pressure to satisfy the minimum inlet pressure of the pump and avoid cavitation. Under steady-state operation, the pressure difference across the bladder boundary is zero. Under start-up or fallback conditions, the pressurant is only at inlet pressure above the interior bladder pressure. If you can figure out how to pump with negative inlet pressure, you don't need much pressure at all from the pressurant gas.
 
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