Liquid methane as rocket fuel : why so late to the party?

[EzinX]

So, ok, there is this chart on wikipedia.

If you look, liquid natural gas (liquid methane) has better MJ/kilogram than kerosene but it is not as good as liquid hydrogen. It's in between.

Similarly, it has better MJ/liter than hydrogen, but worse than kerosene.

Similarly, it's harder to store than kerosene but easier than hydrogen. I'm unable to find specific numbers on what dry mass ratios look like, but I would expect intermediate numbers.

So, how come, way back when, they did this : http://www.reactionengines.co.uk/images/saturnv/Satirn-V-1024-cut.jpg

Instead of 2 fuels, why not use an intermediate fuel for the upper and lower stages, getting almost as good a performance, but being able to use the same kind of tanks and the same engines for both rocket stages.

I'm guessing I'm missing a subtlety here, and that it must in fact be worse than either fuel, and the only reason SpaceX wants to use it is because of Mars ISRU, and they want to reuse lower stages easily, and methane doesn't leave residue behind.

[Bill Phil]

Price. Methane LNG has only recently been used for applications.

At least that's what I know...

[UmbralRaptor]

Cryogenic, maybe half the density of kerosene, and apparently offers an Isp advantage of <5%. Also annoyingly, its liquid range does not match up with that of liquid oxygen.

Given the budget thrown around, the Saturn V could specialize -- kerolox for high thrust in atmo, then hydrolox for high Isp at altitude.

edit:

Price.

I'm not sure it's expensive enough to make a difference. Edited March 16, 2015 by UmbralRaptor

[magnemoe]

Cryogenic, maybe half the density of kerosene, and apparently offers an Isp advantage of <5%. Also annoyingly, its liquid range does not match up with that of liquid oxygen.

Given the budget thrown around, the Saturn V could specialize -- kerolox for high thrust in atmo, then hydrolox for high Isp at altitude.

edit:I'm not sure it's expensive enough to make a difference.

Yes the two liquid states don't overlap, LOX might freeze methane however that would require long time as the difference is only 20 degree so its far less a problem than hydrogen.

Might say that if you already handle LOX you can handle methane too however it will require its own cooling system.

Mostly an question about price versus performance, I say it might be better to use hydrogen for upper stage and only have to deal with LOX and hydrogen.

[Streetwind]

The price of RP-1 + liquid oxygen makes up less than 0.5% of the launch cost of a rocket (which, itself, is only a part of the total cost of the launch, and then there's also the cost of the payload...) Even if liquid methane was ten times as expensive as RP-1 is (and I assure you that it is not, RP-1 is super highly refined and certainly not common kerosene!), its price point would be irrelevant in the grand scale of things.

I can't tell you with certainty why it wasn't adopted earlier, but I can speculate that it was largely a question of practicality. RP-1/LOX not only worked just fine, but in fact did something very specific very well: it delivered high fuel density, allowing the designers to keep the rocket stage small and easy to build. Then, if you wanted a high Isp fuel and density was no longer important, you could go with LH2/LOX. It's the most effective (sane) chemical propellant mix that we have, even today.

Compared to that, liquid methane had no immediate key selling point. You had no reason to pick it over RP-1 if you needed a dense fuel, because it is much less dense. You had no reason to pick it over LH2 if you wanted performance, because its performance is much lower. And yes, it is much better storable long-term than LH2... but for deep space missions, you take hypergolics anyway, because they make the design of a restartable rocket engine a million times easier, and the resulting engine that much more reliable.

The problem with a propellant sitting somewhere in the middle is that it is a jack of all trades, master of none. It is good at many things but not the best at anything, so when rocket scientists looked to use only the best stuff to maximize their efficiency, it got passed over at every opportunity.

Fast forward a couple decades, and you get renewed interest in methane because nowadays, we have computer-aided engine design that allows us to understand the combustion process, and design specific behaviors for specific fuels. We have better materials to build rocket hulls and tanks. Thirty years ago the advantages of methane seemed not worth the extra effort you needed to go to to unlock it, but today we have minimized that effort. Additionally, the scientists today are looking at dfferent things than they did in the past, such as:

- Self-pressurizing tanks, which saves weight and complexity by removing the need to carry helium tanks and pumps and seals and valves and control electronics and software to maintain the pressure. Methane can do that, hypergolics or RP-1 cannot.

- The ability to refuel a rocket in situ, something that just doesn't work with most fuels due to the complexity of the refinement process. Methane on the other hand is super simple to produce, if you cannot just collect it from the environment outright.

- Cost reduction. Mixing a RP-1 first stage with hydrogen upper stages might be performing better, but it requires a more expensive rocket and a more expensive launchpad and a more expensive servicing and launch process. With just one fuel in all stages, and it being less cryogenic than liquid oxygen which is present anyway, you can radically simplify a lot of things, and that might be worth a small hit in performance.

- Special applications. SpaceX wants a higher Isp engine because that higher Isp is critical to avoid losing too much in the way of payload capabilities by saving fuel for a boostback and landing of the first stage. A conventional rocket gains a bit of mass to orbit from every extra second of Isp... but a rocket that needs to reserve some of its fuel for after decoupling gains many times more of an advantage - or more precisely, it sacrifices less in addition to gaining the same benefit that a conventional rocket would. So for SpaceX, the advantages of running on methane are larger than they are for conventional approaches.

To sum up, nowadays we have the know-how and the technology to get an actual advantage out of methane while mitigating its downsides, and we have gained a new appreciation of things only methane can do that designers decades ago weren't even in need of (or aware of). It's a different fuel for a different era.

[EzinX]

Yes the two liquid states don't overlap, LOX might freeze methane however that would require long time as the difference is only 20 degree so its far less a problem than hydrogen.

Since you have to have a LOX tank anyway, couldn't you do the same thing they do for storing liquid helium, and make the lox tank inside a larger tank that holds the liquid methane? This would let you make the lox tank walls much thinner, as the cold methane layer would act to keep the lox cold. You'd have recirculation or something to prevent any of the methane from actually freezing. A bigger cryogenic tank that essentially stores both components to fuel in the same assembly would require a lot less insulation.

[K^2]

Again, methane freezes solid in boiling oxygen.

[Bill Phil]

Wouldn't it have to be refined as much as RP-1 is? Are we using LNG here or synthesized methane?

[Pipcard]

- Cost reduction. Mixing a RP-1 first stage with hydrogen upper stages might be performing better, but it requires a more expensive rocket and a more expensive launchpad and a more expensive servicing and launch process. With just one fuel in all stages, and it being less cryogenic than liquid oxygen which is present anyway, you can radically simplify a lot of things, and that might be worth a small hit in performance.

- Special applications. SpaceX wants a higher Isp engine because that higher Isp is critical to avoid losing too much in the way of payload capabilities by saving fuel for a boostback and landing of the first stage. A conventional rocket gains a bit of mass to orbit from every extra second of Isp... but a rocket that needs to reserve some of its fuel for after decoupling gains many times more of an advantage - or more precisely, it sacrifices less in addition to gaining the same benefit that a conventional rocket would. So for SpaceX, the advantages of running on methane are larger than they are for conventional approaches.

Also, SpaceX doesn't want to use hydrolox due to it being hard to handle.

[PB666]

Also, SpaceX doesn't want to use hydrolox due to it being hard to handle.

Hydrogen can be used in a fuel cell as well as a propellant.

Liquified Natural Gas is not just methane, the gas needs to be distilled first to remove the contaminants. It will generally come by pipe which means you need two facilities on site.

1. methane refinery

2. LNG plant.

[EzinX]

Hydrogen can be used in a fuel cell as well as a propellant.

Liquified Natural Gas is not just methane, the gas needs to be distilled first to remove the contaminants. It will generally come by pipe which means you need two facilities on site.

1. methane refinery

2. LNG plant.

SpaceX can probably just order pure methane in bottles - NASA got it somehow. More expensive, but probably still a drop in the bucket compare to the other costs.

You can use methane in a fuel cell just as easily as hydrogen. Possibly more easily.

- - - Updated - - -

Again, methane freezes solid in boiling oxygen.

Correct. However, you could insulate the inner oxygen tank a little bit, and keep the methane in the outer tank always moving with something to stir it so it doesn't freeze against the sides.

[Rakaydos]

Correct. However, you could insulate the inner oxygen tank a little bit, and keep the methane in the outer tank always moving with something to stir it so it doesn't freeze against the sides.

At that point, perhaps you could just make the entire oxygen tank out of a thermocouple- you cool the O2 by heating the methane surrounding it, keeping both liquid.

[EzinX]

At that point, perhaps you could just make the entire oxygen tank out of a thermocouple- you cool the O2 by heating the methane surrounding it, keeping both liquid.

That's a clever solution. Lighter than my idea of stirring rods, also simpler. The tank wall itself would be acting like the thermocouple. You'd probably be able to stop running it at liftoff - all the vibration would probably prevent any significant amount of methane freezing.

Or, if you had sufficient power once in orbit, you could keep running it, as well as a cryocooler on the outer tank to keep the methane cold.

[Kryten]

All this involves adding a lot of dry mass-for a first stage at least, it would probably be much worse for performance than just insulating the bulkhead between tanks.

[Rakaydos]

All this involves adding a lot of dry mass-for a first stage at least, it would probably be much worse for performance than just insulating the bulkhead between tanks.

but it means stripping out most of the Oxygen tank's insulation, as there is less of a thermal differential between the oxygen and methane, then there is between oxygen and air temperature.

[EzinX]

All this involves adding a lot of dry mass-for a first stage at least, it would probably be much worse for performance than just insulating the bulkhead between tanks.

No. The thermocouple would be dual purpose, both structural and to reverse the normal direction of heat flow.

[Kryten]

No. The thermocouple would be dual purpose, both structural and to reverse the normal direction of heat flow.

Would still be considerably more than a normal tank-just the tank-in-tank design in general would add significant structural mass.

but it means stripping out most of the Oxygen tank's insulation, as there is less of a thermal differential between the oxygen and methane, then there is between oxygen and air temperature.

Booster LOX tanks have little or no insulation. Boiloff before launch is replaced, boiloff after is not significant.

[EzinX]

Would still be considerably more than a normal tank-just the tank-in-tank design in general would add significant structural mass.

Got a source for that? I am skeptical : with tank in tank, you have shorter plumbing runs (because the bottom of both tanks are at the bottom of the stage near the turbopumps), less stress across the walls of the inner oxygen tank (because it is being pressed inward from pressure in the methane tank - you would tune the pressures so the methane is at a higher pressure), and so on. Pretty sure fundamental physics make tank-in-tank smaller if the design is optimized to theoretical limits for available materials.

[Iskierka]

The inner tank has to physically exist but provides no structural support. In normal rocket design, tanks are stacked so that their outer walls can be made load-bearing - if you use a tank within a tank, the inner tank is suspended, and can't be used as a load-bearing structure, so effectively becomes deadweight while the exterior tank has to be extended to the same original length to provide the same tankage volume. The non-structural tank is therefore subject to minimums above what is structurally required by the rocket as a whole, which is effectively deadweight. In stacked tankage, non-structural tank interface that is heavier than desirable is limited to one diaphragm bulkhead - in this Russian-doll-esque design, it's the entire internal tank - or at least its outer cylindrical walls, a much greater area. The result is that they both have the same load-bearing structure, but one has more excess structure, making it heavier for no practical advantage.

The closest you'll get to having a rocket fuel tank inside another is seen in a fair few upper stages, where LOx and LH2 tanks share a single curved bulkhead, as opposed to having two bulkheads in contact, such as in the S-IC first stage.

[prophet_01]

Back on topic: I would imagine that another reason why methane is getting more interesting, is the presence of space stations. Life support systems can use the sabatier reaction to recycle co2 into o2 by using h2. While that is pretty efficient, it also produces methane (ch4) that nobody uses atm ( http://en.m.wikipedia.org/wiki/Sabatier_reaction#/search ). I'm mostly guessing here, since I don't know if you can store methane and o2 efficently enough, but I think it could be a good idea to use that methane as fuel instead dumping it.

Curently almost all maned flights are going to a space station anyway, so it might be efficient to design maned vehicles upper stages to use the excess methane.

[EzinX]

The inner tank has to physically exist but provides no structural support. In normal rocket design, tanks are stacked so that their outer walls can be made load-bearing - if you use a tank within a tank, the inner tank is suspended, and can't be used as a load-bearing structure,

You don't have to do it that way. You could extend the walls of the inner tank to the bottom, and actually use the inner tank as the primary load bearing structure. There could be all kinds of internal structural pieces, as well. I suspect it could be made lighter than 2 tanks still because of this sharing.

[Tarrow]

There could be all kinds of internal structural pieces, as well. I suspect it could be made lighter than 2 tanks still because of this sharing.

It's worth remembering that in something like the external fuel / oxidiser tank of the space shuttle the dry mass of the entire tank setup is a hair under 3.5% of the mass of the filled tank. You can make all the weight-saving designs you want for a fuel tank - it'll never achieve much more than a 3.5% weight saving.

Why hydrogen over methane?

Hydrogen burns at a ratio of 2:1 with oxygen (2H₂ + O₂ > H₂O).

Methane burns at a 1:2 ratio with oxygen (CH₄ + 2O₂ > 2H₂O + CO₂).

So methane requires four times as much oxygen to burn as hydrogen, for roughly the same energy output per unit mass of fuel that undergoes combustion.

Using a space shuttle external tank as an example (again), liquid oxygen already makes up the bulk of the mass (630 tonnes of the 760 tonne filled weight). The whole shuttle ready for launch weighs about 2,030 tonnes. You would need an additional 1,890 tonnes of oxidiser on board to fuel it on liquid methane. And lifting that is gonna be an issue (edit: completely messed up the maths on the weight change, reworking that out now - ignore this last bit)

Edited March 17, 2015 by Tarrow

[EzinX]

Using a space shuttle external tank as an example (again), liquid oxygen already makes up the bulk of the mass (630 tonnes of the 760 tonne filled weight). The whole shuttle ready for launch weighs about 2,030 tonnes. You would need an additional 1,890 tonnes of oxidiser on board to fuel it on liquid methane. And lifting that is gonna be an issue

Pretty sure your numbers are wrong. ISP takes into account all of this, and with liquid methane, it's not that much worse than liquid hydrogen. The reason why would take a whole reacting species analysis, I don't have a way to disprove your stoichiometric ratios off the top of my head.

In short, though, that extra mass of oxygen becomes a combustion product heading out the nozzle at high speed. It contributes to thrust.

The average velocity of the exhaust products is 3816 for LOX/H2 and 3034 for CH4/H2.

So the methane is 80% as good as H2.

Edited March 17, 2015 by EzinX

[Tarrow]

Pretty sure your numbers are wrong. ISP takes into account all of this, and with liquid methane, it's not that much worse than liquid hydrogen. The reason why would take a whole reacting species analysis, I don't have a way to disprove your stoichiometric ratios off the top of my head..

The oxidiser / fuel masses for a shuttle external tank are directly taken from NASA's website, 692 tonnes liquid oxygen, 105 tonnes liquid hydrogen, empty container weight 26 1/2 tonnes

(link here http://www.nasa.gov/pdf/63752main_ET_Overview_Wanda_print.pdf).

I do see your point regarding the ISP taking the difference in fuel / oxidiers ratios into account though, that's purely a thrust-per-unit-mass-propellant calculation. I'd overlooked that, my apologies. The stoich ratios look spot on, but I've not factored in the difference in fuel MWs to adjust the fuel / oxygen masses correctly. Serves me right for posting before coffee I'll rework it properly now and correct it

[Iskierka]

You don't have to do it that way. You could extend the walls of the inner tank to the bottom, and actually use the inner tank as the primary load bearing structure. There could be all kinds of internal structural pieces, as well. I suspect it could be made lighter than 2 tanks still because of this sharing.

This would still be heavier - if you try carry load through the inside of the tank, then that load has to be transferred outward to meet interstage fairings and similar, meaning a very heavy load-bearing bulkhead at the top of the tankage. It also could not carry a significant portion of the load, as the outer tank's walls are not far from their load-bearing requirements just by meeting containment and pressurisation requirements. Either way you have a heavy tank that should be load-bearing but is not, and you may have an additional load-bearing bulkhead that was not necessary. Stacked tankage results in minimum number of structures and all structures have minimums defined by the ascent loadings, not by some additional factor that increases mass beyond what should be required. Tank-in-tank sounds like some elegant solution to storing two cryogenic fuels at once, but it is not a good idea once its disadvantages are considered.