Author Topic: Compressed hydrogen and oxygen instead of liquid for storage  (Read 21825 times)

Offline DarkenedOne

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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. 

Offline peter-b

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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. 

Firstly, recall that LH2 has such an awkwardly low density that it makes LH2 challenging to use as a first stage fuel because the tanks need to be so large.

A tank capable of coping with 700 bar will be much (much much) stronger than one the same size capable of handling ~80 bar (LH2 tank pressure, IIRC), with concomitant necessary increase in mass. If it needs to be much larger (due to lower density of compressed H2 than LH2) and much stronger, then you're looking at paying a very high mass penalty for launching the thing, surely? Also, gaseous hydrogen has a horrible tendency to diffuse through things.
Research Scientist (Sensors), Sharp Laboratories of Europe, UK

Offline Robotbeat

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The tank would be heavier than the hydrogen it holds.
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Offline DarkenedOne

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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. 

Offline Jim

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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. 

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.

Offline RocketmanUS

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As tank mass increases so would the amount of propellent needed for station keeping.

ACES depots concept are to make the depot cheap enough to be able to dispose of them. They don't have to last for long periods of time. This has an advantage to upgrade them over time as practical. Life span also has to do with how many time the tanks are designed to be refueled. Longer life span could mean greater tank mass and design, and the tanks no longer in common with the ACES upper stage for manufacturing ( higher over all costs ).

The amount of energy that might be needed to liquify and cool down GH2 to LH2, you might as well just add a recooler for the H2 boil off. However that GH2 is used for station keeping.

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. 

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.
As I was typing my post , Jim posted.

To add, greater size tanks also mean greater size in Sun shield too.

For LEO or EML1/2 depots , power will most likely be by solar panels.

Offline Lee Jay

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Have a look here:

http://www.qtww.com/assets/u/129LTankBrochure.pdf

Note the mass of the tank and the mass of hydrogen it stores, as well as the service life.
« Last Edit: 11/08/2012 01:14 am by Lee Jay »

Offline RocketmanUS

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Have a look here:

http://www.qtww.com/assets/u/129LTankBrochure.pdf

Note the mass of the tank and the mass of hydrogen it stores, as well as the service life.

What would it's mass be to store 52,881.61 cu ft?

Five years of life! How many refills is it rated for?

At 70MPa , what is the leak rate of the hydrogen per week, per month?

Offline DarkenedOne

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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. 
« Last Edit: 11/08/2012 01:27 am by DarkenedOne »

Offline Lee Jay

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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. 

You are assuming the leak rate from a compressed H2 tank is zero.  I don't think that's a good assumption.

Offline Jim

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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. 

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.

Offline DarkenedOne

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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. 

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. 

Offline kkattula

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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. 

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?

Offline DarkenedOne

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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. 

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.   

Offline Jim

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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.   

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?
« Last Edit: 11/08/2012 01:54 am by Jim »

Offline RocketmanUS

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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. 

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. 

Boil off and leakage are two different issues.

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. 

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.   
Compressor needed to make LH2 from the GH2 for the EDS.
And how big of a power system is that going to be? ( more added mass )

Offline Lee Jay

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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. 

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. 


If you need a long-term depot, you have to reduce the boiloff.  That can be done, if necessary (see JWST).

Offline DarkenedOne

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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. 

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?

You should do the math first before you ask someone else to, instead of using wild guesses. 

The tank mass to hydrogen mass depends on the size of the tank.  The tanks mass scales proportionally to its surface area, where as the mass of the hydrogen it contains scales proportionally to the tank's volume.  For a tank of any reasonable shape the volume of the tank will increase faster than the surface area. 

Lets take a look at the hydrogen tank Lee made a link to.   It has a tank mass to hydrogen mass ratio of about 18.4:1, and it is only 1.6 meters long.  Lets suppose the radius and diameter were increase by 10 times.  Then its surface area would increase by 100 times, and thus the weight of the tank would increase by 100 times.  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. 

So there I did "the math." 

Offline DarkenedOne

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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.   

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. 

Offline RocketmanUS

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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.   

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.

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