Author Topic: NASA tests methane engine  (Read 7498 times)

Offline privateer

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Re: NASA tests methane engine
« Reply #20 on: 05/07/2007 02:43 PM »
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TyMoore - 5/5/2007  7:55 AM
I think the main advantage with methane is storability. Liquid methane's boil point is close to that of liquid oxygen, which is a lot 'wamer' than liquid hydrogen--so the boil off should be quite a bit less. Also, you can save weight on a booster by thinning the insulation over the fuel tank a bit from liquid hydrogen...

In this case, propane may be better:

___________ Methane Propane
Melting point -182.5   -187.6   (LOX boiling point is -182.95)
Boiling point -161.6   -42.09   (all temps are in Celsius)
Liquid density  0.42     0.58

See? Propane will also stay liquid at LOX boiling temp (with larger margin), but it will remain liquid in much wider range of temps. (Heh, -42 celsius is a normal ambient temperature in many areas on Mars!) Density is also notably higher. Two reasons why tanks can be lighter.

Isp of methane, ethane and propane (with LOX) is virtually the same IIUC.

Quote
As far as making methane on Mars--it's not as much balogny if there's plenty of water up there. Water can be electrolyzed into hydrogen and oxygen. The hydrogen can be reacted with carbon monoxide in a Fischer-Tropsch synthesis reaction to create methane. No problem really--you just need a power source to supply process heat.

Now this can be a problem for propane. How hard it will be to produce it instead of methane?

Offline Tom Ligon

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Re: NASA tests methane engine
« Reply #21 on: 05/07/2007 08:34 PM »
Don't you guys know?  We're going to stick out a few solar panels on Mars and use those to make fuel for the return trip.

The illustration of this I saw would have been about enough solar panels for an off-the-grid home, if you were stingy with power and had Earth sunlight.  I wondered how long they figured it would take to refuel the ship.

With the problem of reactor cooling being about the same on Luna or in space, and only slighly better on Mars, I wonder if the best use of the nuke might be to just be honest and go with nuclear propulsion.  If you have to carry the reactor anyway, find a way to use it that gives better Isp than chemical.  You still need reaction mass, but water or hydrogen get the job done.

On the moon, long-term some sort of launch rail may make the most sense from the standpoint of scarce reaction mass.

Offline BarryKirk

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Re: NASA tests methane engine
« Reply #22 on: 05/07/2007 09:11 PM »
For lunar stuff an EM railgun launch makes some sense.

1) No air friction.
2) Lower escape / orbital velocity so acceleration required is less.
3) Required Acceleration or rail length is still a little high for manned launch, unless, the EM rail
just provides an initial boost.

But, there may be a better possibility for lunar launch.

Lunar soil contains lots of Oxygen, Aluminum, and Iron.  There was some discussion a while back
of manufacturing solid rocket fuel on the lunar surface.  The ISP would be horrible, but it doesn't take much
to launch off of the moon.

Offline Tom Ligon

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Re: NASA tests methane engine
« Reply #23 on: 05/07/2007 09:46 PM »
The ISDC will have panel discussions and speakers on topics like this.  I may be on a couple.  About the only thing I can offer them is the possiblity of plenty of electric power if Dr. Bussard's reactor works.

That won't help a lunar base in the next 5 years.

I'm sure we'll use rocket fuel shipped up from Earth initially.  Luna is the only target without the option of aerobraking.  It is not a big problem to get off it, but it is as much problem to get down to it.

The result will be an accumulation of whatever it takes to get the fuel down.  At least tanks, probably landers.  The mass involved in getting that fuel to orbit and into TLI is no doubt worse.  At some point, the accumulated mass of those starts to make you wish you'd invested that in sending up something better.  Better sooner than later if you plan a continued presence.

Luna has a curious phenomenon in which dust is lofted electrostatically to great heights as the result of sunlight charging it.  Originally, this was very mysterious as it made it look as if the Moon had an atmosphere that it clearly did not.   This suggests to me that very fine lunar dust might be usable as reaction mass in an electrostatic motor of some type.


Offline BarryKirk

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Re: NASA tests methane engine
« Reply #24 on: 05/07/2007 09:57 PM »
Well, the ISP for that type of motor would be huge.  Again the problem is low thrust.  But with the moon
the thrust to weight ratio required is much lower than for earth.

I had no idea that lunar dust was present at altitude.  Thank you for that interesting tidbit of info.

Offline TyMoore

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Re: NASA tests methane engine
« Reply #25 on: 05/08/2007 01:34 AM »
Quote
privateer - 7/5/2007  7:43 AM

Quote
TyMoore - 5/5/2007  7:55 AM
I think the main advantage with methane is storability. Liquid methane's boil point is close to that of liquid oxygen, which is a lot 'wamer' than liquid hydrogen--so the boil off should be quite a bit less. Also, you can save weight on a booster by thinning the insulation over the fuel tank a bit from liquid hydrogen...

In this case, propane may be better:

___________ Methane Propane
Melting point -182.5   -187.6   (LOX boiling point is -182.95)
Boiling point -161.6   -42.09   (all temps are in Celsius)
Liquid density  0.42     0.58

See? Propane will also stay liquid at LOX boiling temp (with larger margin), but it will remain liquid in much wider range of temps. (Heh, -42 celsius is a normal ambient temperature in many areas on Mars!) Density is also notably higher. Two reasons why tanks can be lighter.

Isp of methane, ethane and propane (with LOX) is virtually the same IIUC.

Quote
As far as making methane on Mars--it's not as much balogny if there's plenty of water up there. Water can be electrolyzed into hydrogen and oxygen. The hydrogen can be reacted with carbon monoxide in a Fischer-Tropsch synthesis reaction to create methane. No problem really--you just need a power source to supply process heat.

Now this can be a problem for propane. How hard it will be to produce it instead of methane?

Propane is certainly possible. I am no chemical engineer, but what I do understand of Fischer-Tropsch Synthesis is that by tweaking the composition of the catalysts and the temperature and pressure of the chemical reactor, then virtually any hydrocarbon can be fairly easily synthesized given feedstocks of CO and H2, or even carbon and steam, or Methane and Oxygen...given an energy source and plenty of feedstock chemicals, it should be possible to synthesize heavier hydrocarbons for really useful things like polymers and synthetic lubricants and greases.

Propane could be useful as a propellant, however, I would imagine that methane may well be easier to synthesize and may thus be an easier and more useful hydrocarbon synthetic propellant system for initial sorties.

In regards to using solar panels to provide process energy--yes, certainly possible. However, the beautiful thing with a nuclear system is that it can provide process heat necessary to get the synthesis reactions going. This could be done with solar panels, but the heater loads would necessarily add more collection area to the panels--no way to get around that. Not only that, solar photovoltaic systems necessarily mean a cyclic synthesis--methane synthesis can only occur during daylight cycles, unless you want to figure out how to store that much energy--lead acid batteries are way out of there, unless you make them from Martian materials... A nuclear power source can provide electricity and process heat 24/7, and so a continuous synthesis approach can be used which is likely simpler and more efficient anyway.

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