Why is SpaceX considering Methane as fuel for their next engine, the Raptor?

Question

Currently, SpaceX has developed the Merlin 1 family (1B (Falcon 1), 1C (Falcon 9 v1.0), 1D (Falcon 9 v1.1/F9-R/Falcon Heavy), vacuum versions and sealevel versions) which are LOX/RP1 based. (75-205Klbs thrust)

They developed (and abandoned) the Kestrel engine used on the second stage of the Falcon 1, also LOX/RP1. (6.9Klbs thrust)

They developed the Draco and SuperDraco engines using hypergolics (mono-methyl hydrazine fuel and nitrogen tetroxide oxidizer). (90 lbs and 15K lbs thrust)

The next engine on their list is the Raptor, which is expected to be Methane (CH4) and LOX based. (660Klbs thrust)

Do we know whey they chose methane over hydrogen? Performance wise, LH2 is usually the go to propellant.

Answer 1

Methane has the benefit of being easier to store than hydrogen. Mostly passive cooling can suffice to keep it cryogenic, whereas hydrogen needs active cooling, and will still vent over time. Which makes Methane much closer to 'storable' than hydrogen can be. This would make it useful for deep space missions, with long mission durations.

Methane is less bulky than hydrogen. Which means tankage is smaller for the same mission. (The Shuttle external tank is mostly hydrogen tanks with a small oxygen tank (at the top?)).

Methane should be simpler to use in an engine due to its higher density than hydrogen, less needs to be pumped by volume.

Methane is potentially manufacturable on Mars. With imported Hydrogen (or native water), CO2 (Carbon dioxide) can be converted to CH4 reasonably straight forwardly.

There are ideas for ISRU (In-Situ Resource Utilization) and demonstrations on Mars. (Robert Zubrin's model is launch the return vehicle that uses ISRU to fill its fuel tanks, and do not launch the manned mission until the return vehicle is fully fueled and ready to go. Then you launch the manned mission, along with a second return vehicle, that uses ISRU over the duration of the surface mission to fuel itself).

SpaceX is focused on developing re-usability technology for their rocket lines. Traditional rocket-grade kerosene produces residue (a process known as "coking") when it burns. Methane fuel burns cleaner so there is no residue build-up which means engines can be re-used more times without refurbishment.

Answer 2

Methane (CH₄) and RP-1 are roughly equivalent in realizable performance. As previously mentioned by other posters, CH₄ has slightly higher impulse – about 370 s in vacuum vs the 360 s – at the same chamber pressure of 7 MPa. But, this is counterbalanced by its lower bulk density of about 830 kg/m³ vs about 1030 kg/m³. Bulk Density is the density of the combined Fuel and Oxidizer load in their appropriate ratios. Even though Methane is "only" 430 kg/m³ it is burned with 3.5 parts oxygen compared to 2.1 parts for RP-1, hence a CH4 rocket will be carrying more oxygen and less fuel by weight. Oxygen is pretty dense at a little over 1140 kg/m³ which is denser in fact than RP-1 (about 810 kg/m³). If we assume that chamber pressures and engine cycle efficiency will be equal, RP-1 outperforms CH₄ simply because a 20% larger tank will impose weigh penalties that slightly outweigh the 3% increase in specific impulse. However, the RP-1 advantage is contingent upon operating at an equal chamber pressure which may not be the case. And, Methane (CH₄) has additional advantages that are applicable in specific scenarios.

The reasons CH₄ is a front runner for SpaceX's Raptor can probably be attributed to four factors:

1. Methane does not coke (polymerize) at the operating temperatures of a rocket engine – it's coking point is roughly twice as high. This makes it easier to make an engine reusable and re-usability is a key SpaceX objective.

2. Because Methane does not coke, it is also easier to implement a full-flow stage combustion (FFSC) cycle where all the fuel and oxidizer flow goes through the pre-burner. Compared to contemporary Russian partial flow stage combustion engines higher chamber pressures are attainable resulting in a total impulse advantage of about 30 seconds, or 9%. This eliminates the performance deficiency of CH₄ compared to RP-1.

3. If SpaceX intends to use the same fuel in all the stages, CH₄ can be considered a better upper stage fuel and a worse lift-off fuel, even without enabling higher working pressures. This is because upper stages are typically 1/8th to 1/10th the size of the 1st stage, and here impulse is more important than density. Using Methane with the aforementioned FFSC cycle means that SpaceX can potentially get equivalent 1st stage performance and better upper stage performance.

4. Even though it is, IMHO, somewhat dubious that early Mars mission will use in-situ fuel production. If that ever becomes an applicable practice, Methane can be produced from water and CO₂ while RP-1 cannot.

Other than that, there is the non-factor that somewhat favor Methane, such as regular grade Natural Gas being good enough and not having to highly refine the fuel from regular kerosene to RP-1 to achieve low coking characteristics and consistent densities. I say it is a non-factor, because fuel cost is such a negligible part of launch costs that it really doesn't matter if fuel cost a few times more or less. Fuel is typically only about 0.3% of the cost of flying a rocket to orbit, so fuel cost really doesn't matter – Not even when you compare highly expensive propellant combos like Hydrazine/Tetroxide to the relatively cheap Kerosene/Oxygen.

Answer 3

Logistically, methane can be easier to work with than hydrogen. Methane's boiling point is about 110K, compared to hydrogen's 20K. This means that both fuel and oxidizer lines can be purged with gaseous nitrogen. Liquid hydrogen lines can only be purged with helium, as hydrogen's boiling point is below the melting point of other inert gases.

Answer 4

Another disadvantage to hydrogen is that it requires advanced metallurgy to prevent hydrogen embrittlement, where more common alloys tend to become prone to fracture and fatigue in high hydrogen environments.

Answer 5

Methane would allow for around 380 second specific impulse (~3.8km/s exhaust velocity), depending on the chamber pressure, expansion ratio, and other design parameters for the engine, while LH2/LOX engines have demonstrated ~450 second specific impulse (~4.5km/s exhaust velocity).

Despite this lower efficiency though, methane has a couple of major advantages. It is significantly higher density as a liquid than LH2 (0.42 g/cc, vs 0.07 for liquid hydrogen), so it requires far less tank volume and smaller piping for the same mass of propellant. It also doesn't need to be stored as cold as liquid hydrogen, which reduces the insulation and cooling requirements.

SpaceX has traditionally favored dense, easy to handle propellant (LOX/RP1) and simple engine designs (the Merlin is a simple gas generator cycle, rather than the more efficient [but more complex] staged combustion design used by most other modern rockets). As such, it makes sense that they would go for the easier to handle and simpler solution of a methane rocket rather than the high performance but difficult and complicated liquid hydrogen motor, so long as the methane provides them with the performance that they require (which will remain to be seen).

Answer 6

A good question. In pre-EELV studies, NASA and the U.S. Air Force looked at LOX/methane. EELV resulted in the LOX/kerosene Atlas V and the LOX/hydrogen Delta IV.

At the 4th International Conference on Launcher Technology in 2002, Burkhardt et al. compared a reusable LOX/kerosene launch vehicle using the RD-180 type engine from the Atlas V with a LOX/methane vehicle using a possible engine of the same efficient staged combustion cycle:

The LOX/methane engine had about 3% higher specific impulse but that advantage was outweighed by the lower density of the liquid methane compared to kerosene.

LOX/kerosene was slightly better performing overall in terms of payload and expected to be lower cost to build and operate, the same result as the pre-EELV studies.

The reason LOX/hydrogen is comparable to or better than LOX/kerosene is that the specific impulse is much higher overcoming the even lower density problem. For the Space Shuttle the main engines operated from ground to orbit so the higher specific impulse of hydrogen at higher altitude was the reason for its use.

For a first stage that only operates to low altitud followed by a LOX/hydrogen second stage as in the Atlas and Delta, kerosene has comparable payload performance and may be lower cost because of vehicle size. For the Delta IV another advantage is commonality with the upper stage propellants.

Methane is not currently supplied at launch sites so a major facility investment would be needed.

Lack of long experience with operation is another negative for methane.

If the Raptor were to be used in space as in a Mars mission then the fact that both LOX and liquid methane are relatively easy to store in space compared to hydrogen or kerosene would be an advantage.

References:

• Comparative Study of Kerosene and Methane Propellant Engines for Reusable Liquid Booster Stages. Holger Burkhardt, Martin Sippel, Armin Herbertz, Josef Klevanski. 4th International Conference on Launcher Technology "Space Launcher Liquid Propulsion" 3-6 December 2002 – Liège (Belgium).

Answer 7

SpaceX is not shooting at the moon, SpaceX is shooting at Mars. Logistically, I’m not sure there are viable choices besides methane / LOX and Hydrazine / Tetroxide. The return shot requires fuel stored for an unknown time, which means the default conditions are chilly on Mars. RP-1 is a rock hard solid requiring complex heating to liquify it and LH2 is high pressure H2 requiring complex cooling to liquify it. Most fuels are rock hard solids. Performance is important but secondary. Hydrazine and Nitrogen Tetroxide are what I had expected, with perfect handling and storage properties. Methane and LOX, however, are both materiels with an abundant supply of people experienced in handling them, so they can be handled and stored, it’s just harder than hydrazine and nitrogen tetroxide.

Why not LH2 is obvious, the question is why not hydrazine and nitrogen tetroxide. If you were an astronaut, what fuel would you trust to have waiting for you on arrival without leaks so you can come home? To hell with the trip there, get me back and I’ll know we’re ready to go.

Answer 8

Excellent question. But twinned to that is also the question of the thrust level which is more than three times greater than the current Merlin engine used in the second stage of the Falcon 9.

I admit this is pure speculation. But based on prior SpaceX practices with the Merlin engine, I believe the Raptor might be intended for use in larger launch vehicles than the Falcon 9. Perhaps as a common engine used in the lower and upper stages of a upgraded Falcon Heavy, where the Raptor replaces the Merlin engines.

But I think the most likely possibility is a major change in the Falcon 9 design (and maybe the Falcon Heavy too) spurred by recent successes with the Grasshopper vertical landing test vehicle.

The Raptor would be a good engine for an oversized upper stage for the Falcon 9 or Falcon Heavy. By transferring more of the effort to reach orbit to the upper stage, the first stage has greater performance margin and also stages at a lower altitude and speed making powered recovery of the first stage to the launch site easier. Recovery and reuse of the 1st stage would save SpaceX a lot of money (particularly for the Falcon Heavy) and permit lower pricing.

Answer 9

This article: http://www.nasaspaceflight.com/2014/03/spacex-advances-drive-mars-rocket-raptor-power/

also notes that methane/LOX engines do not suffer coking buildup as LOX/RP1 engines do, and can be run less oxygen-rich, which is easier on the pumps.

Answer 10

One factor nobody else mentioned yet is cost. SpaceX is a for-profit company, so cost matters a lot. Methane has become a lot cheaper recently: The price of natural gas declined quite a lot due to technological advances in production (i.e. fracking). This made methane the cheapest rocket fuel. As of 2001, NASA was paying $0.98/gallon for liquid hydrogen, which equates to about \$16/MMBTU, which is much more expensive than LNG nowadays.