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

[EzinX]

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

Also, your simple analysis is missing crucial details that would require research. Read the relevant chapter in Ignition! . I can dig it out, but simply put, you need to know the actual ratio of products produced in a real rocket engine, specific to the conditions actually encountered. You don't simply get H20 with a LH2/LOX rocket, nor do you only get CO2 and H20 with a methane/LOX rocket.

It's easier to just use the results of numbers others have already worked, like just saying "the ISP is 80% of the ISP with H2/LOX"

[Tarrow]

It's easier to just use the results of numbers others have already worked, like just saying "the ISP is 80% of the ISP with H2/LOX"

Ahhh that does indeed simplify it. So that's a 25% propellant mass increase required when going from LH2/LOX to meth/LOX to maintain a given power output?

[Kerbart]

From John D. Clark's ignition:

Johannes Winkler, however, picked up the idea, and working independently of the VfR, was able to fire a liquid oxygen-liquid methane motor before the end of 1930. This work led nowhere in particular, since, as methane has a performance only slightly superior to that of gasoline, and is much harder to handle, nobody could see any point to following it up.

[andrewas]

Lower power output, since power varies with thrust and with the square of the specific impulse. But 25% extra mass for the same total impulse.

[RuBisCO]

Both SpaceX and now ULA want to build a methane-lox fueled rocket system. Why? The advantages of methane above RP-1 (purified kerosene) is 4% higher ISP, cheaper, vaporizes, self-pressurizing and ability (theoretically) to work in a closed cycle. The disadvantage is that it is cryogenic and that it has nearly half the density of RP-1. The density disadvantage is frankly crippling for launching from the ground.

Propane would make a better alternative because it can be sub-cooled to LOX temperatures and have nearly RP-1 density, but with 2% greater ISP, it also vaporizes and could be used to self-pressurize and perhaps even operate in a closed cycle. Disadvantages is that it is heavier then air and without odor would be an extreme fire hazard if leaked.

Personally I just don't see the reason to leave RP-1, we have decades of experience and practice with it.

References:

http://settlement.arc.nasa.gov/Nowicki/SPBI1LF.HTM

[GreenWolf]

Two words.

Mars. ISRU.

Technically, it's five words, but that's besides the point.

[RuBisCO]

Two words.

Mars. ISRU.

Technically, it's five words, but that's besides the point.

What does that got to do with launching off Earth?

[Kryten]

Main advantage is coking is very limited, which is important for reuse. Cleaning out deposits would be very expensive, and missing some could be catastrophic.

[GreenWolf]

What does that got to do with launching off Earth?

Raptor is eventually planned to fly on the reusable Mars Colonial Transporter. If I were Elon Musk (which I'm pretty sure I'm not), I would try to take advantage of ISRU at Mars to reduce the necessary size of the MCT (which is already planned to be ginormous).

Also, what Kryten mentioned. If ULA actually want to do reusability, reducing coking would help.

[Kryten]

ULA are only going with BE-4 because it was already in development, similarly to how Delta IV is hydrolox because that's where most of the experience in the industry was post-shuttle. BO initiated it, and we know they are trying for full reusability.

[Bill Phil]

I think that a better question would be: Why not?

[RuBisCO]

Raptor is eventually planned to fly on the reusable Mars Colonial Transporter. If I were Elon Musk (which I'm pretty sure I'm not), I would try to take advantage of ISRU at Mars to reduce the necessary size of the MCT (which is already planned to be ginormous).

Also, what Kryten mentioned. If ULA actually want to do reusability, reducing coking would help.

Propane would reducing coking as well and still have density as good as RP-1.

I just don't see an engine as powerful as Raptor being used on Mars, just saying it is not likely to ever run off martian methane.

[Kibble]

Kerosene is a fossil fuel right? If so, eventually we'll want to switch to a different fuel that fills the same category but that is renewable.

[kerbiloid]

There was already a similar thread:

http://forum.kerbalspaceprogram.com/threads/113249-Liquid-methane-as-rocket-fuel-why-so-late-to-the-party

[RuBisCO]

There was already a similar thread:

http://forum.kerbalspaceprogram.com/threads/113249-Liquid-methane-as-rocket-fuel-why-so-late-to-the-party

I did a search initially and did not see that, my bad

[RuBisCO]

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

At least that's what I know...

Elon musk once said it cost half a million dollars to fuel up a Merlin rocket with RP-1, that is maybe 3-4 $ per kg tops. I calculate liquified natural gas cost $0.6 today, so roughly a savings of $0.4 M, even if the launch were to cost 4 million dollars, 1/15 its present cheapest SpaceX cost, that would be only 10% savings.

[magnemoe]

From John D. Clark's ignition:

Might be that he is wrong or simply use "slightly " too harsh, why else should SpaceX and other be interested. Back in 1950 it was of low interest anyway, main focus was non cryogenic fuel.

[Vanamonde]

Threads on the same subject have been merged.

[Bill Phil]

Elon musk once said it cost half a million dollars to fuel up a Merlin rocket with RP-1, that is maybe 3-4 $ per kg tops. I calculate liquified natural gas cost $0.6 today, so roughly a savings of $0.4 M, even if the launch were to cost 4 million dollars, 1/15 its present cheapest SpaceX cost, that would be only 10% savings.

LNG is more than methane... It needs to be refined, too. The cost savings aren't all that big.

Plus, no one chose to develop Methane rockets. They're only getting attention now thanks to LNG being more available.

[EzinX]

LNG is more than methane... It needs to be refined, too. The cost savings aren't all that big.

Plus, no one chose to develop Methane rockets. They're only getting attention now thanks to LNG being more available.

I read that straight LNG is fine for rockets. If you read Ignition, you'll see when he gets to describing the "standards" for RP-1 : they are some of the laxest standards for fuel imaginable. They simple restrict certain hydrocarbon chains that tend to foul up the rocket motors. The exact fuel composition is up to the manufacturer and can vary massively, otherwise.

Using straight LNG with a few dissolved gas impurities is not a big deal.

[A1Ch1]

Both SpaceX and now ULA want to build a methane-lox fueled rocket system. Why? The advantages of methane above RP-1 (purified kerosene) is 4% higher ISP, cheaper, vaporizes, self-pressurizing and ability (theoretically) to work in a closed cycle. The disadvantage is that it is cryogenic and that it has nearly half the density of RP-1. The density disadvantage is frankly crippling for launching from the ground.

Propane would make a better alternative because it can be sub-cooled to LOX temperatures and have nearly RP-1 density, but with 2% greater ISP, it also vaporizes and could be used to self-pressurize and perhaps even operate in a closed cycle. Disadvantages is that it is heavier then air and without odor would be an extreme fire hazard if leaked.

Personally I just don't see the reason to leave RP-1, we have decades of experience and practice with it.

References:

http://settlement.arc.nasa.gov/Nowicki/SPBI1LF.HTM

The optimum O/F ratio for kerosene/LOX is around 2.5. LNG/LOX O/F is 3.5. This somewhat offsets the lower density of LNG so the total volume is about 15% larger, not a dealbreaker. 4-5% Isp represents a similar increase from from going from gas generator to staged combustion.

[shynung]

I read that straight LNG is fine for rockets. If you read Ignition, you'll see when he gets to describing the "standards" for RP-1 : they are some of the laxest standards for fuel imaginable. They simple restrict certain hydrocarbon chains that tend to foul up the rocket motors. The exact fuel composition is up to the manufacturer and can vary massively, otherwise.

If the same book is to be believed, then JP-4's specifications are even more lax.

Justified only because gas turbines literally gulp anything that was pumped into the combustion chamber and still run reliable anyway. The ones installed inside M-1 Abrams tanks can be fueled with either JP, Jet-A, or Diesel fuel.

Natural gas reservoirs typically have very little components that easily gum up around engine parts (those that do usually gum up the reservoir instead). Heck, there are tuk-tuks burning straight LNG driving around my city nowadays, and they don't break down often.

So, yeah, rockets burning straight LNG should be relatively safe.

Edited June 2, 2015 by shynung

[Kryten]

Natural gas reservoirs typically have very little components that easily gum up around engine parts (those that do usually gum up the reservoir instead). Heck, there are tuk-tuks burning straight LNG driving around my city nowadays, and they don't break down often.

This doesn't follow. The whole point of having a separate RP-1 specification is that normal fuel blends have plenty of components that will polymerise or pyrolise in the harsh environment of the cooling channels in a regenerative nozzle-most aromatics, any sulphur content, anything unsaturated, et.c. None of these would produce major issues for most IC engines, particularly low-powered ones. The exact proportion of the components in RP-1 might not be exact, but it's almost pure alkanes.

[A1Ch1]

There's no such thing as standard LNG, contracts are for a certain energy/mass (or energy/volume). Suppliers add bulk if their LNG is too high energy. So not all LNG can be burned as-is, and rocket-grade LNG would have to be standardized. Regardless, LNG containing nearly pure CH4 is readily available using existing refining.

[DerekL1963]

There's no such thing as standard LNG, contracts are for a certain energy/mass (or energy/volume). Suppliers add bulk if their LNG is too high energy. So not all LNG can be burned as-is, and rocket-grade LNG would have to be standardized. Regardless, LNG containing nearly pure CH4 is readily available using existing refining.

This actually came up on the AROCKET list recently, and it turns out that "bog standard" commercial methane (the stuff you can call and delivered tomorrow) contains a number of nasties that can cause problems in the cooling channels.