Why did it take so long for methane to be used as a rocket propellant?

Question

SpaceX have put methane on the map as a rocket fuel, but they weren't the first to consider its use. The first experiments in building a rocket engine that uses methane date back to 2007.
Now methane isn't exactly an exotic fuel. It's widely in use in other applications and its combustion process is well understood.
So why did it take until 2007 for methane to be seriously considered as a rocket fuel?
Or am I wrong and have there been attempts to build a methane rocket in the past?

Answer 1

Or am I wrong and have there been attempts to build a methane rocket in the past?

Well, if there were, I figured that John D. Clark's famous book Ignition! (1972, free online copy) would be the place to find it. And indeed, the index at the end of the book has a convenient entry for "methane, usefulness of" pointing to pages 8 and 191. On page 8, Clark describes an early test of a LOX-methane rocket engine in Germany in 1930(!) thus:

"Oberth had originally wanted to use methane as fuel, but as it was hard to come by in Berlin, their first work was with gasoline and oxygen. 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."

So, if Clark is to be believed (and I personally have no reason to think otherwise), the main reason why methane has been mostly neglected as a rocket fuel after these early experiments is simply that, with its low boiling point, it's harder to store and handle than more traditional kerosene-based fuels.

---

FWIW, the reference to page 191 is much less useful, as that page belongs to the (by now somewhat dated) chapter 13, "What Happens Next", and merely mentions methane as a possible low-temperature bipropellant fuel for deep space probes. The use of methane and oxygen difluoride as space-storable bipropellants is also discussed in chapter 6, "Halogens and Politics and Deep Space", on pages 83 and 86, where Clark notes, summarizing a bunch of NASA studies from the 1960s:

"All the hydrocarbons were good fuels, but methane was in a class by itself as a coolant, transpiration or regenerative, besides having the best performance. The OF₂-methane combination is an extremely promising one. (It took a long time for Winkler's fuel of 1930 to come into its own!)"

So it seems that methane has been considered as a rocket fuel long before the 21st century, but mainly for deep-space applications where its high volatility is not such as major issue, and may even be considered a feature.

Finally, for completeness, page 146 of Clark's book contains a rather disturbing mention of LOX + liquid methane as a proposed monopropellant:

"If Tannenbaum's mixtures were bad, that proposed at a monopropellant conference in October 1957 by an optimist from Air Products, Inc., was enough to raise the hair on the head of anybody in the propellant business. He suggested that a mixture of liquid oxygen and liquid methane would be an extra high-energy monopropellant, and had even worked out the phase diagrams of the system.* How he avoided suicide (the first rule in handling liquid oxygen is that you never, never let it come in contact with a potential fuel) is an interesting question, particularly as JPL later demonstrated that you could make the mixture detonate merely by shining a bright light on it. Nevertheless, ten years later I read an article seriously proposing an oxygen-methane monopropellant! Apparently junior engineers are allergic to the history of their own business."

I think we can see why this one never took off.

Answer 2

The three main competitors for liquid fuel choices to date have been:

• Hypergolics - easiest to get started with
• Kerosene/LOX - Good thrust, low performance, but dense
• LH/LOX - Best performance, hardest to do

So if you were starting a new space program with a clean sheet design, LOX/LH is out of the question, too hard. Hypergolics are well understood, storable, great for missiles, horribly corrosive and dangerous.

Missiles, converted to launchers were often hypergolics or solids.

Kerosene/LOX is a good choice to start with, if hypergolics are not in the running. LOX is manageable, Kerosene is very easy to manage.

The gold standard of performance is LH/LOX, especially for upper stages where it matters the most. But liquid hydrogen is very fluffy, needing huge tanks, embrittles metals, leaks very easily, and is just plain hard to work with.

Most space programs seem to want to jump to LH/LOX when possible, since they are often optimized for performance, not cost. (India's GSLV, Japan's H-II, Space Shuttle).

SpaceX has entered with a different goal in mind, cost. I was going to say SpaceX entered this market, but in reality, it was not really a market until SpaceX entered it. It was more a series of national government prestige projects. This has changed the trade study results. Sure, LH/LOX would have better performance, but if that is not your only goal, then suddenly Methane starts to look really good.

SpaceX's primary goal of Mars, where Methane is potentially available for refueling (with appropriate infrastructure to convert CO2 and water to methane) makes it doubly attractive to them.

Answer 3

Methane, while a high ISP fuel that is relatively easy to work with, does have it's share of difficulties. The hardest thing about working with methane is it requires an ignition source, as methane and oxygen don't burn spontaneously. In fact, there is a paper that directly compares Methane and RP-1 as rocket fuel. The major findings include:

• The ISP of Methane is about 10s higher than RP-1.
• Motor mass is increased for Methane.
• Overall, the payload for similar rockets is almost identical.

So, while Methane has some advantages, it would require R&D, and end up being more expensive than RP-1.

Of course, additional research will help the case for Methane, and more importantly, Methane can be made or is easily available in many places in the Solar System, making it attractive for In-Situ Resource Utilization (ISRU). Aside from that, there isn't a huge advantage today of using Methane as a rocket fuel.

Answer 4

In the past, the options were the optimal high tech option (Hydrogen) or the commodity option (RP1, basically kerosene.)

Methane is certainly much easier to handle than hydrogen, with a boiling point slightly higher than oxygen, nowhere near the -259C (only 14K above absolute zero) of hydrogen. Its energy per unit liquid volume is better than hydrogen too.

One factor is that liquid methane is no longer an exotic fuel. Global trade in liquefied natural gas has approximately doubled every decade for the last few decades, as can be seen at https://en.wikipedia.org/wiki/Liquefied_natural_gas (50Mtpa in 1990, 130Mtpa in 2002, 246Mtpa in 2014.) As a result the skills in handling LNG have gone from being specialist to becoming commonplace, which will reduce development costs. Perhaps more importantly, liquid methane/LNG is no longer a specialist material: you can now order it as a regular fuel.

https://en.wikipedia.org/wiki/Liquid_rocket_propellant shows that CH4/oxygen has only about 3% better specific impulse than RP-1/oxygen, whereas there is a big jump from CH4/oxygen to H2/oxygen. (Both ethylene, and suprisingly, hydrazine, are slightly better than methane.) Therefore I think the other answers are correct in that the main reason for using CH4 is that it is reportedly available elsewhere in the solar system, and Musk would like to gain experience with it, in order to be able to refuel on other worlds.

Answer 5

Aside from being, essentially LNG, Methane burns Cleanly. Kerosene produces coking and polymerization within the engine. This does not matter if you use the engine only a few times. If you want to reuse it 40-50 times or more (like Spacex), you need to clean a Kerosene engine every few uses and this gets expensive if you have to dismantle it. Methane is far superior from a "reusability" perspective. LNG is also cheap these days, so RP-1s advantage there is pretty much gone.

Answer 6

One aspect of this issue that seems to be overlooked in most discussions of it I see is experience and technological advancement. When we were trying to get people to orbit and then another planet for the first time, our primary criteria was to do whatever we had to in order to increase our odds of success. We used H2/LOX because we wanted every bit of ISP we could, no matter what the cost. 50 years ago, we were doing what no one had ever done, and had far less technological prowess and tools to attack the problem with. We had to "brute force" and "guesstimate" much more than we do now.

Now getting to LEO is routine rather than a great experiment. Now we know that thrust/mass ratio is considerably more important in getting to LEO than we did back then. We know that lower Isp is worth it if we get a large enough improvement in thrust/mass ratio. We have analytical tools, materials science advances, and manufacturing methods that are far better than those of 50 years ago. We can now store CH4 and LOX at close to their freezing points much more economically. We can design and build simpler, more reliable full flow combustion engines. Raptor's thrust/mass ratio is going to be greater than 2x that of the Saturn V's F1 engine. It's probably going to be more than 3x more than that of the SSME. And it's going to be far more reusable. All those advantages more than make up for the lower ISP of CH4/LOX vs that of H2/LOX.

In short, we have more experience along with more materials and tools available to us to take maximum advantage of that experience with.