Author Topic: Compressing hot hydrogen  (Read 1239 times)

Offline goran d

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Compressing hot hydrogen
« on: 07/26/2016 04:10 PM »
When hydrogen is heated to 3300K, we can achieve exhaust velocity of about 8.5km/s. However, what if we heat the hydrogen to 3300K, then quickly compress it until it reaches e.g. 10000K, and have it exit a nozzle fast enough that the walls don't melt. It will obviously have to operate in pulses. Another possibility is to shoot pieces of metal straight in space through a cannon with 10000K hydrogen. Will need to exit very fast, with huge acceleration. We can make the walls porous, they can excrete some liquid before the compression, so that when hydrogen gets heated up the liquid will evaporate instead of the walls.

Offline Jim

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Re: Compressing hot hydrogen
« Reply #1 on: 07/26/2016 05:11 PM »
However, what if we heat the hydrogen to 3300K, then quickly compress it until it reaches e.g. 10000K,

How is that done?

Offline goran d

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Re: Compressing hot hydrogen
« Reply #2 on: 07/26/2016 07:15 PM »
Mechanically e.g. by moving a piston

Offline Jim

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Re: Compressing hot hydrogen
« Reply #3 on: 07/26/2016 07:16 PM »
Not at those temps

Offline Jim Davis

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Re: Compressing hot hydrogen
« Reply #4 on: 07/26/2016 09:04 PM »
Mechanically e.g. by moving a piston

Where is the power to move the piston coming from?

Offline scienceguy

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Re: Compressing hot hydrogen
« Reply #5 on: 07/26/2016 09:14 PM »
Tungsten, the metal with the highest melting point, melts at 3422 C
e^(pi)i = -1

Offline Asteroza

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Re: Compressing hot hydrogen
« Reply #6 on: 07/26/2016 11:43 PM »
Is this a thought exercise on mechanical compression methods, because if heating is the aim, something like VASIMR's helicon plasma generating element coupled with magnetic nozzles keeping the hot plasma off the walls so they don't immediately melt seems "easier".

If you are getting hot enough to be plasma, then various electric/magnetic field acceleration propulsion methods seem more accessible.

Mechanical compression/heating at high temperature usually requires extensive cooling of components. Piston type compressors would likely have seal failure at such high temperatures. Axial compressors with thin blade structures would require some kind of film cooling of the blades, assuming that is even possible. There is an interesting rotary compressor called the Rampressor, which is basically a rotary ramjet inlet throat mounted on the rim of a wheel performing supersonic shock compression, usually with the important throat part made of ceramics, but even ceramics will not be having a good day at those temperatures. If your entire flowpath isn't made of high temperature ceramics, you're probably gonna have a bad day.

Offline KelvinZero

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Re: Compressing hot hydrogen
« Reply #7 on: 07/27/2016 12:06 AM »
If you are getting hot enough to be plasma, then various electric/magnetic field acceleration propulsion methods seem more accessible.
Maybe this is a related question: I have wondered if there are ways to do better than the basic relationship between temperature and ISP without going the "first convert to electricity" route..

For example if you had some plasma trapped in a magnetic field, couldn't you heat it up but somehow allow only the flukey fastest particles to escape? Or does this violate thermodynamics in some profound way like Maxwell's Demon? (edit: I don't think it does, because it is not a one way door selecting for velocity.. it is just a barrier that only energetic ions can cross, in either direction.)

(obviously you would run out of energy if you only had chemical.. I was thinking more in terms of a solar thermal rocket that does better than you would gather just from applying the sun's surface temperature to hydrogen to calculate the maximum theoretical ISP)
« Last Edit: 07/27/2016 02:56 AM by KelvinZero »

Offline A_M_Swallow

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Re: Compressing hot hydrogen
« Reply #8 on: 07/27/2016 02:46 AM »
If you are getting hot enough to be plasma, then various electric/magnetic field acceleration propulsion methods seem more accessible.
Maybe this is a related question: I have wondered if there are ways to do better than the basic relationship between temperature and ISP without going the "first convert to electricity" route..

For example if you had some plasma trapped in a magnetic field, couldn't you heat it up but somehow allow only the flukey fastest particles to escape? Or does this violate thermodynamics in some profound way like Maxwell's Demon?

(obviously you would run out of energy if you only had chemical.. I was thinking more in terms of a solar thermal rocket that does better than you would gather just from applying the sun's surface temperature to hydrogen to calculate the maximum theoretical ISP)

If you put some black carbon in the hydrogen plasma you may be able to flash heat it using concentrated sun light. You would have to keep it in some sort of magnetic bottle, possibly with a nozzle at one end.

Offline CameronD

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Re: Compressing hot hydrogen
« Reply #9 on: 07/27/2016 03:51 AM »
If you put some black carbon in the hydrogen plasma you may be able to flash heat it using concentrated sun light. You would have to keep it in some sort of magnetic bottle, possibly with a nozzle at one end.

One teeny problem with over-heated hydrogen is not only that it's tricky to contain, but exposure to even trace amounts of oxygen following LOC results in a rather large explosion.  e.g Fukushima
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline Donosauro

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Re: Compressing hot hydrogen
« Reply #10 on: 07/27/2016 04:05 AM »
Hydrogen is compressed to very high temperatures in light-gas guns. See, for example, http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/21/016/21016664.pdf, page 10.

"The code indicates that the gas is driven to temperatures approaching 10,000 K ...."

Edit: temeratures -> temperatures
« Last Edit: 07/28/2016 02:10 PM by Donosauro »

Offline A_M_Swallow

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Re: Compressing hot hydrogen
« Reply #11 on: 07/27/2016 06:36 AM »
If you put some black carbon in the hydrogen plasma you may be able to flash heat it using concentrated sun light. You would have to keep it in some sort of magnetic bottle, possibly with a nozzle at one end.

One teeny problem with over-heated hydrogen is not only that it's tricky to contain, but exposure to even trace amounts of oxygen following LOC results in a rather large explosion.  e.g Fukushima


We do not need the hydrogen containing only ionising and directing out the back. As for oxygen, fortunately space is a vacuum. This drive may not be suitable for operating in an atmosphere.

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