Methane as a primary rocket fuel

[wtrix]

I found myself wondering - why don't they use methane as a primary fuel in rockets instead of LH2? This far the talk has only been about methane usage in lunar landers, but why not in main rocket motors?

IMHO methane has some very serious advantages over LH2:
1. More density than LH2 in liquid form (500kg/m3 instead of 70kg/m3) enabling a lot smaller fuel tank and thus a lot smaller rocket
2. Low CO&CO2/high H2O emissions enabling fast exhaust speeds and thus almost as high Isp as in LH2 rockets
3. A lot higher boiling temperature, thus necessitating less insulation
4. Significantly slower outboiling during tank fill-up
5. Easier to handle
6. Some kerosene based motors shall be easily convertable to methane motors (Saturn 5 was petroleum fuelled for example)
7. Cheaper and abundant. In fact currently almost all of the H2 is made from natural gas, which is basically methane
8. Similar boiling temperature with LOX enables "stright trhough tank" plumbing conserving some weight on tubes and the insulation of those

Any liquid fuel rocket people here who can explain why LH2 and not CH4?

 

[wtrix]

Anyone?

 

[soyuztma]

I found myself wondering - why don't they use methane as a primary fuel in rockets instead of LH2? This far the talk has only been about methane usage in lunar landers, but why not in main rocket motors?

IMHO methane has some very serious advantages over LH2:
1. More density than LH2 in liquid form (500kg/m3 instead of 70kg/m3) enabling a lot smaller fuel tank and thus a lot smaller rocket
2. Low CO&CO2/high H2O emissions enabling fast exhaust speeds and thus almost as high Isp as in LH2 rockets
3. A lot higher boiling temperature, thus necessitating less insulation
4. Significantly slower outboiling during tank fill-up
5. Easier to handle
6. Some kerosene based motors shall be easily convertable to methane motors (Saturn 5 was petroleum fuelled for example)
7. Cheaper and abundant. In fact currently almost all of the H2 is made from natural gas, which is basically methane
8. Similar boiling temperature with LOX enables "stright trhough tank" plumbing conserving some weight on tubes and the insulation of those

Any liquid fuel rocket people here who can explain why LH2 and not CH4?

LH2/LO2 has a higher isp than CH4. So if you decide to use a cryogenic fuel it's tempting to use the cryogenic fuel with the highest isp. The price of the fuel is also insignificant in comparison to all the other costs. The difference in isp between CH4 and RP1 is also not very high so you have to trade the use of a fuel with a slightly higher isp against the complication of a cryogenic fuel (the density of RP-1 is also higher than CH4).

But CH4 seems to be gaining in popularity for the reasons you mentioned: NASA is interested in using it for the ascent module of the lunar lander, Armadillo Aerospace is testing with it and Taurus II will get an upper stage powered by methane.

 

[vulture4]

Methane is a bit more storable than LH2, and the cost of refrigeration might be minimized by insulating both the methane and LOX within a single insulating barrier. CH4 was proposed as a fuel for the large shuttles that would have carried materials for the Solar Power Satellites. One would have to work out the specifics for any particular application to see if it has an advantage, but I think it should be considered for any system that requires the energy of cryogenics but has to be storable or relatively dense.

 

[scottb50]

Methane is a bit more storable than LH2, and the cost of refrigeration might be minimized by insulating both the methane and LOX within a single insulating barrier. CH4 was proposed as a fuel for the large shuttles that would have carried materials for the Solar Power Satellites. One would have to work out the specifics for any particular application to see if it has an advantage, but I think it should be considered for any system that requires the energy of cryogenics but has to be storable or relatively dense.

Methane is the weakest Hydrocarbon molecule with the lowest ISP. Cryogenic storage would increase the total amount available but would probably not be worth the weight. Pressurized Propane would be better and probably need less weight for containment. That's the main reason Kerosene is used to begin with, containment is simple, low pressure, no cooling or insulation needed and higher ISP then either Methane or Propane.

Hydrogen needs more complicated storage and increased volume for storage but provides the best known ISP, Shuttle and Delta 4 take that into consideration.

 

[wtrix]

Well, I found a good article about the subject: http://www.la.dlr.de/ra/sart/publicatio ... 12prop.pdf

I still find myself wondering why is the ISP advantage so low. Methane contains double the hydrogen oer carbon atom relative to kerosene. That shall make the exhaust a lot faster. In fact, more close to hydrogen exhaust speeds than to RP-1 speeds. Why doesn't this hydrogen advantage materialize.

Nonetheless. I found another interesting subject, which is gelled liquid hydrogen: http://sbir.grc.nasa.gov/launch/GELLED.htm

Has there been any advances in this area as of lately? Why hasn't gelled hydrogen been materialized as a practical rocket fuel?
Otherwise anyone can satisfy he's curiosity about different propulsion methods here: http://www.hobbyspace.com/Links/LaunchPropulsion.html

 

[scottb50]

I still find myself wondering why is the ISP advantage so low. Methane contains double the hydrogen oer carbon atom relative to kerosene. That shall make the exhaust a lot faster. In fact, more close to hydrogen exhaust speeds than to RP-1 speeds. Why doesn't this hydrogen advantage materialize.

Methane is CH4 and Kerosenes C12H26 because of the Carbon to Carbon bonds the Hydrogen atoms are easier to strip from the molecule. One way to look at is Methane is odorless and will remain in the atmosphere for an extended period of time, kerosene is more volatile and evaporates fairly quickly.

 

[wtrix]

I still find myself wondering why is the ISP advantage so low. Methane contains double the hydrogen oer carbon atom relative to kerosene. That shall make the exhaust a lot faster. In fact, more close to hydrogen exhaust speeds than to RP-1 speeds. Why doesn't this hydrogen advantage materialize.

Methane is CH4 and Kerosenes C12H26 because of the Carbon to Carbon bonds the Hydrogen atoms are easier to strip from the molecule. One way to look at is Methane is odorless and will remain in the atmosphere for an extended period of time, kerosene is more volatile and evaporates fairly quickly.

That explains higher density, but methane has two times more hydrogen atoms per carbon atom compared to kerosene. The connection to evaporation is a bit foggy to me. In fact, methane already is a vapor (i.e. a gas) in room temperature. When kerosene evaporates, it tunrs in to a kerosene gas. Are you proposing that it decomposes? It doesn't AFAIK.

To my knowledge, Isp is very dependent on exhaust speed. Lower molecular mass exhaust gases generally give higher exhaust speeds. From 1 molecule of CH4 and 2 molecules of O2 you get 1 molecule of CO2 and two molecules of H20. If you burn kerosene, You'll get (very roughly) 1 molecule of CO2 and 1 molecule of H20. So the exhaust gases are a lot heavier. Am I mistaken?

 

[scottb50]

That explains higher density, but methane has two times more hydrogen atoms per carbon atom compared to Kerosene. The connection to evaporation is a bit foggy to me. In fact, methane already is a vapor (i.e. a gas). when kerosene evaporates, it tunrs in to a kerosene gas. Are you proposing that it decomposes? It doesn't AFAIK.[/quote]

What I said was it is harder to get the Hydrogen atoms away from the Carbon in Methane then it is in kerosene. The reason being the Carbon/Carbon bonds in the molecule require more energy to be maintained then the Hydrogen/Carbon bonds making kerosene more volatile then Methane.

 

[wtrix]

That explains higher density, but methane has two times more hydrogen atoms per carbon atom compared to Kerosene. The connection to evaporation is a bit foggy to me. In fact, methane already is a vapor (i.e. a gas). when kerosene evaporates, it tunrs in to a kerosene gas. Are you proposing that it decomposes? It doesn't AFAIK.

What I said was it is harder to get the Hydrogen atoms away from the Carbon in Methane then it is in kerosene. The reason being the Carbon/Carbon bonds in the molecule require more energy to be maintained then the Hydrogen/Carbon bonds making kerosene more volatile then Methane.

What you actually wanted to say is that a lot of energy released in burning methane goes in to strpping the methane molecule apart. It is not so with kerosene as kerosene molecule is more volatile and easier to decompose. Thus with kerosene there is more free energy released. Am i correct?

 

[scottb50]

What you actually wanted to say is that a lot of energy released in burning methane goes in to strpping the methane molecule apart. It is not so with kerosene as kerosene molecule is more volatile and easier to decompose. Thus with kerosene there is more free energy released. Am i correct?[/quote]

That and that a molecule of kerosene has a lot mor Hydrogen then a molecule of methane.

The advantage of using pure Hydrogen comes from not having to expend the energy to free the Hydrogen from the Carbon as well as the Carbon combining with Oxygen which reduces the energy of the reaction, hence the lower ISP. If you figure the added mass to contain Liquid Hydrogen as compared to kerosene the lower ISP is less of a factor and can be made up by longer burn time.

 

[propforce]

I found myself wondering - why don't they use methane as a primary fuel in rockets instead of LH2? This far the talk has only been about methane usage in lunar landers, but why not in main rocket motors?

IMHO methane has some very serious advantages over LH2:
1. More density than LH2 in liquid form (500kg/m3 instead of 70kg/m3) enabling a lot smaller fuel tank and thus a lot smaller rocket
2. Low CO&CO2/high H2O emissions enabling fast exhaust speeds and thus almost as high Isp as in LH2 rockets
3. A lot higher boiling temperature, thus necessitating less insulation
4. Significantly slower outboiling during tank fill-up
5. Easier to handle
6. Some kerosene based motors shall be easily convertable to methane motors (Saturn 5 was petroleum fuelled for example)
7. Cheaper and abundant. In fact currently almost all of the H2 is made from natural gas, which is basically methane
8. Similar boiling temperature with LOX enables "stright trhough tank" plumbing conserving some weight on tubes and the insulation of those

Any liquid fuel rocket people here who can explain why LH2 and not CH4?

LH2/LO2 has a higher isp than CH4. So if you decide to use a cryogenic fuel it's tempting to use the cryogenic fuel with the highest isp. The price of the fuel is also insignificant in comparison to all the other costs. The difference in isp between CH4 and RP1 is also not very high so you have to trade the use of a fuel with a slightly higher isp against the complication of a cryogenic fuel (the density of RP-1 is also higher than CH4).

But CH4 seems to be gaining in popularity for the reasons you mentioned: NASA is interested in using it for the ascent module of the lunar lander, Armadillo Aerospace is testing with it and Taurus II will get an upper stage powered by methane.

Methane is really not a desirable rocket fuel, it sits there in the middle of nowhere as far as optimum feul selection goes. NASA picked it as 'green' propellant, also thinking that it is easeir to make on Mars, but personally I think that is a wrong decision.

IF your're launching from earth, you might as well use RP-1 rather than methane. Methane gets a slightly higher Isp than RP-1, but RP-1 is twice as dense as methane therefore you'll have a ~50% size fuel tank than methane. A smaller vehicle is always less cost to build. If you need the high Isp for performance, then you might as well sticks with hydrogen.

If you want to produce methane on Mars, then you might as well go with propane. It's really not that much harder to make propane than methane, they have approx the same Isp, but propane can be cooled to lower temperature and is almost twice as dense as methane so you can pack twice as much propellant in a given tank volume, thereby having a better performing vehicle.

 

[propforce]

That explains higher density, but methane has two times more hydrogen atoms per carbon atom compared to kerosene. The connection to evaporation is a bit foggy to me. In fact, methane already is a vapor (i.e. a gas) in room temperature. When kerosene evaporates, it tunrs in to a kerosene gas. Are you proposing that it decomposes? It doesn't AFAIK.

To my knowledge, Isp is very dependent on exhaust speed. Lower molecular mass exhaust gases generally give higher exhaust speeds. From 1 molecule of CH4 and 2 molecules of O2 you get 1 molecule of CO2 and two molecules of H20. If you burn kerosene, You'll get (very roughly) 1 molecule of CO2 and 1 molecule of H20. So the exhaust gases are a lot heavier. Am I mistaken?

Methane does get higher theoretical Isp than RP-1. Here's a chart comparing theoretical Isp.

 

[EAK09]

I was wondering about this myself, but in particular in conjuction with materials reuse in mind. Do any of you know of any discussion on extracting methane from human waste in space? I don't know how their toilets work up there, but I suspect methane production has already been a wasted waste product for some time. If as little as 0.1% of all food mass wound up as methane, I figure that this could be enough for a 6-man ISS crew to generate to keep the station's orbit boosted. Any ideas?

 

[CalliArcale]

I'm thinking you probably wouldn't yield enough methane to be worth the bother.

As far as reboosting the ISS goes, you really need to use hypergolics for that. They're even more storable than methane and have great energy density. They're also pretty reliable. I think it'll be a while before we beat hypergolics for stationkeeping of large spacecraft. (For small spacecraft and even some reasonably hefty satellites, ion drives are starting to win out. Ion drives have vastly more ISP than any chemical engine, but extremely low thrust, which makes them useless for launch vehicles but handy for inexpensively tweaking the orbits of small satellites.)