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.
The tanks mass scales proportionally to its surface area,
So there I did "the math."
Quote from: DarkenedOne on 11/08/2012 02:19 am The tanks mass scales proportionally to its surface area, No, it doesn't. The hoop stress goes with radius. Therefore the wall thickness goes with radius. You assumed the thickness was constant above.
Quote from: Lee Jay on 11/08/2012 02:43 amQuote from: DarkenedOne on 11/08/2012 02:19 am The tanks mass scales proportionally to its surface area, No, it doesn't. The hoop stress goes with radius. Therefore the wall thickness goes with radius. You assumed the thickness was constant above.Using the info from this link that was given beforehttp://www.qtww.com/assets/u/129LTankBrochure.pdfWhat would the mass be using that material to make a tank the size of the shuttles ET? Lets see how it compares to the material used to make the SLWT.
Quote from: RocketmanUS on 11/08/2012 02:52 amQuote from: Lee Jay on 11/08/2012 02:43 amQuote from: DarkenedOne on 11/08/2012 02:19 am The tanks mass scales proportionally to its surface area, No, it doesn't. The hoop stress goes with radius. Therefore the wall thickness goes with radius. You assumed the thickness was constant above.Using the info from this link that was given beforehttp://www.qtww.com/assets/u/129LTankBrochure.pdfWhat would the mass be using that material to make a tank the size of the shuttles ET? Lets see how it compares to the material used to make the SLWT.Tank factor for this is:P*V/(m*g0)=(70 MPa*129 L)/(92 kg * 9.81 m/s^2)=10km, pretty typical, if a little conservative, for COPVTank factor for the SLWT hydrogen tank is:(230 kPa*1497 m^3)/(~19000kg*9.81 m/s^2)=1.8kmET has more demands on it though, and that includes its proportional share of interstage mass.
Have a look here:http://www.qtww.com/assets/u/129LTankBrochure.pdf
Quote from: DarkenedOne on 11/08/2012 02:36 amQuote 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. multiply tanks = greater mass, greater surface area for H2 leakage, added plumbing, and added cost.
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.
The trouble is building and deploying large tanks in space.
Another option would be to launch water and store it at a LEO depot.Only one type of tanker needed ( water ).Split the water into H2 and O2 then convert it into LH2 and LOX.
Quote from: RocketmanUS on 11/08/2012 05:21 amAnother option would be to launch water and store it at a LEO depot.Only one type of tanker needed ( water ).Split the water into H2 and O2 then convert it into LH2 and LOX.Only problem with that approach is Rocket engines burn fuel rich, so you will need more LH than would be generated by cracking water and using all the heavier O2. You either are mass inefficient and launch more O2 (locked in the H2O) or you launch extra LH.
Quote from: DarkenedOne on 11/08/2012 01:26 amI suppose it all comes down to the boil off. Also, you still haven't addressed the compressor.
I suppose it all comes down to the boil off.
A GH2 tank is going to mass at least 10 times the hydrogen it holds.
However the tanks volume would increase 1000 times, thus the hydrogen's mass would increase 1000 times. The ratio of the tank mass to hydrogen mass would then be 1.84:1.
Quote from: Lee Jay on 11/08/2012 02:43 amQuote from: DarkenedOne on 11/08/2012 02:19 am The tanks mass scales proportionally to its surface area, No, it doesn't. The hoop stress goes with radius. Therefore the wall thickness goes with radius. You assumed the thickness was constant above.Address the structure of the tank.
Quote from: DarkenedOne on 11/08/2012 02:19 amSo there I did "the math." OK, you did the maths but you did not do the physics.The force on the tank hull increases with the tank size which means the wall thickness (assuming constant pressure as you do) also grows linearly and all of a sudden your tank weight grows to the 3rd order like your volume does. No gain in making the tank bigger.Oops. Lee Jay beat me to it....
Guys, the mass ratio of a pressure vessel, to first order, doesn't depend on the scale. For compressed hydrogen, it's going to suck no matter what. Why is this conversation still going on?
Quote from: kevin-rf on 11/08/2012 01:12 pmQuote from: RocketmanUS on 11/08/2012 05:21 amAnother option would be to launch water and store it at a LEO depot.Only one type of tanker needed ( water ).Split the water into H2 and O2 then convert it into LH2 and LOX.Only problem with that approach is Rocket engines burn fuel rich, so you will need more LH than would be generated by cracking water and using all the heavier O2. You either are mass inefficient and launch more O2 (locked in the H2O) or you launch extra LH. What if you also had some liquid ammonia. You could electrochemically reform that into hydrogen and nitrogen. Now you've got more H2, a smaller amount of excess O2 and some excess N2 as well. The humans on board might enjoy that combination.
Quote from: Lee Jay on 11/08/2012 01:26 pmQuote from: kevin-rf on 11/08/2012 01:12 pmQuote from: RocketmanUS on 11/08/2012 05:21 amAnother option would be to launch water and store it at a LEO depot.Only one type of tanker needed ( water ).Split the water into H2 and O2 then convert it into LH2 and LOX.Only problem with that approach is Rocket engines burn fuel rich, so you will need more LH than would be generated by cracking water and using all the heavier O2. You either are mass inefficient and launch more O2 (locked in the H2O) or you launch extra LH. What if you also had some liquid ammonia. You could electrochemically reform that into hydrogen and nitrogen. Now you've got more H2, a smaller amount of excess O2 and some excess N2 as well. The humans on board might enjoy that combination.Have you people bothered to compute the energy it would take to drive the reactions you are talking about?
Quote from: JohnFornaro on 11/08/2012 02:10 pmQuote from: Lee Jay on 11/08/2012 02:43 amQuote from: DarkenedOne on 11/08/2012 02:19 am The tanks mass scales proportionally to its surface area, No, it doesn't. The hoop stress goes with radius. Therefore the wall thickness goes with radius. You assumed the thickness was constant above.Address the structure of the tank.The wall is the structure.
but the energy required to compress hydrogen [pales] in comparison to the energy required to break these chemical bonds.
Even if you have a compressed gas depot, you still need a cryocooling system to make LH2/LO2 for whatever rocket in question that takes delivery of the fuel.Q: How much energy does it take, practically, to liquify a kg of H2?
Quote from: DarkenedOne on 11/08/2012 09:39 pmbut the energy required to compress hydrogen [pales] in comparison to the energy required to break these chemical bonds. Goal post relocation alert.
Quote from: JohnFornaro on 11/09/2012 02:59 amQuote from: DarkenedOne on 11/08/2012 09:39 pmbut the energy required to compress hydrogen [pales] in comparison to the energy required to break these chemical bonds. Goal post relocation alert.Just pointing things into perspective. I ran the numbers, and I do not see the power for compression being a big issue, but I am not going to argue with Jim on the subject because he probably know more about it than I. However if people believe that the power to compress hydrogen would make a compressed hydrogen satellite unfeasible than they should not then start talking about methods of storing hydrogen that require even more power.
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. Now that is just for storage. For use the gases can be liquified again, and used in a traditional LH2/LO engine. Another possibility would be to keep it in gas form and just build rocket engines that run on the gas. Such a engine would not be as powerful, but it should in theory produce the same specific impulse.
Quote from: DarkenedOne on 11/07/2012 10:33 pmMuch 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. Now that is just for storage. For use the gases can be liquified again, and used in a traditional LH2/LO engine. Another possibility would be to keep it in gas form and just build rocket engines that run on the gas. Such a engine would not be as powerful, but it should in theory produce the same specific impulse. Actually, now that I think about it, this concept could actually be useful at a place like the Lunar surface in the context of an ISRU program. After all, after cracking dihydrogen monoxide, the resultant products are in gaseous form, and so have to be stored for at least a little while before they are converted to liquid form.If the tanks were made from Lunar steel, the mass wouldn't be a problem. But in the initial stages of a Lunar base, the capability to store gases might be so important so as to be worth it to import a tank or two from Earth. Thus, a useful tidbit of information would be the storage capability and volume of a 25 mT tank that could store gases at 2000 psi....