Much of the discussion regarding fuel storage in space involves liquid hydrogen and oxygen. What I have been wondering is why not simply store the hydrogen an oxygen in compressed form. Doing so completely eliminates the problem of long time storage as extremely low temperatures do not need to be maintained. Now at 700 bar the density of hydrogen at room temperature is only half that of its liquid state, so you would need bigger tanks.
Guys remember we are talking about a depot here. Its not going anywhere so it will only cost you on getting it up there. Once it is built hopefully we can expect a reasonable long lifespan of like 15-20 years, which is the typical lifespan of most satellites. Secondly that is 700 bar at room temperature. Assuming you use a sun shield you can probably maintain the tanks at a temperature at -100 C, which would decrease the pressure proportionately according to the ideal gas law. Lastly for comparison you have to factor in the insulation and boil off of cryogenic tanks.
Quote from: DarkenedOne on 11/07/2012 11:14 pmGuys remember we are talking about a depot here. Its not going anywhere so it will only cost you on getting it up there. Once it is built hopefully we can expect a reasonable long lifespan of like 15-20 years, which is the typical lifespan of most satellites. Secondly that is 700 bar at room temperature. Assuming you use a sun shield you can probably maintain the tanks at a temperature at -100 C, which would decrease the pressure proportionately according to the ideal gas law. Lastly for comparison you have to factor in the insulation and boil off of cryogenic tanks. and a compressors and power supplies for them on the depot, along with the extra mass of the tanks on the receiving vehicles makes this unfeasible.The extra insulation pales compared to the extra mass of high pressure tanks. Boil off will be used for station keeping and attitude control and can be used for power production.
Have a look here:http://www.qtww.com/assets/u/129LTankBrochure.pdfNote the mass of the tank and the mass of hydrogen it stores, as well as the service life.
I suppose it all comes down to the boil off. The problem is that many of you are assuming small storage times. For a big mission especially a Mars mission it could take several heavy lift launch vehicles to provide the propellant. Launching them would likely take several months.
Quote from: DarkenedOne on 11/08/2012 01:26 amI suppose it all comes down to the boil off. The problem is that many of you are assuming small storage times. For a big mission especially a Mars mission it could take several heavy lift launch vehicles to provide the propellant. Launching them would likely take several months. You are assuming the leak rate from a compressed H2 tank is zero. I don't think that's a good assumption.
Quote from: DarkenedOne on 11/08/2012 01:26 amI suppose it all comes down to the boil off. The problem is that many of you are assuming small storage times. For a big mission especially a Mars mission it could take several heavy lift launch vehicles to provide the propellant. Launching them would likely take several months. wrong, that is still "small storage times" Anyways, if you are talking that amount of H2, then high pressure storage is even less viable. The mass of the tanks is even worse. Also, you still haven't addressed the compressor.
Compressor is not needed. Assuming the hydrogen is shipped to the depot as a liquid which would make the most sense, then the liquid will pressurize the tank as it boils to reach equilibrium.
Quote from: Lee Jay on 11/08/2012 01:29 amQuote from: DarkenedOne on 11/08/2012 01:26 amI suppose it all comes down to the boil off. The problem is that many of you are assuming small storage times. For a big mission especially a Mars mission it could take several heavy lift launch vehicles to provide the propellant. Launching them would likely take several months. You are assuming the leak rate from a compressed H2 tank is zero. I don't think that's a good assumption.Well maybe not, but some estimate the boil off rates of liquid hydro to be as much as 3.8% per month. I do not think that the compressed H2 leakage is that high, but I could be wrong.
Quote from: Jim on 11/08/2012 01:44 amQuote from: DarkenedOne on 11/08/2012 01:26 amI suppose it all comes down to the boil off. The problem is that many of you are assuming small storage times. For a big mission especially a Mars mission it could take several heavy lift launch vehicles to provide the propellant. Launching them would likely take several months. wrong, that is still "small storage times" Anyways, if you are talking that amount of H2, then high pressure storage is even less viable. The mass of the tanks is even worse. Also, you still haven't addressed the compressor.Compressor is not needed. Assuming the hydrogen is shipped to the depot as a liquid which would make the most sense, then the liquid will pressurize the tank as it boils to reach equilibrium.
Quote from: DarkenedOne on 11/08/2012 01:26 amI suppose it all comes down to the boil off. The problem is that many of you are assuming small storage times. For a big mission especially a Mars mission it could take several heavy lift launch vehicles to provide the propellant. Launching them would likely take several months. Please. Do the math. A GH2 tank is going to mass at least 10 times the hydrogen it holds. A LH2 tank is going to mass about 1/10 of the hydrogen it holds. So if you need 500 tons of LH2 for e.g. a Mars mission, would you rather launch a 5000 ton depot, a massive liquifier plant, and 500 tons of GH2? Or a less than 100 ton depot, 1000 tons of LH2, and let half of it boil-off? Or a slightly heavier depot that can reduce boil-off to say 100 tons?
Quote from: DarkenedOne on 11/08/2012 01:49 amCompressor is not needed. Assuming the hydrogen is shipped to the depot as a liquid which would make the most sense, then the liquid will pressurize the tank as it boils to reach equilibrium. So, after the first shipment of H2 is in the depot at pressure, how does the LH2 in the second shipment get from the delivery vehicle into the depot?
Quote from: Jim on 11/08/2012 01:53 amQuote from: DarkenedOne on 11/08/2012 01:49 amCompressor is not needed. Assuming the hydrogen is shipped to the depot as a liquid which would make the most sense, then the liquid will pressurize the tank as it boils to reach equilibrium. So, after the first shipment of H2 is in the depot at pressure, how does the LH2 in the second shipment get from the delivery vehicle into the depot?Such a depot would need multiple tanks. Unless the tank was launched by a rocket substantially bigger than the ones that launch the delivery vehicles than you are pretty much going to have a one to one ratio. Remember the LH2 in the delivery vehicle will expand two times when it boils. One tank will be able to hold the contains of one only delivery vehicle. Of course there is no issue with a depot with multiple tanks.