Blue Origin BE-4 liquid methane

[Cassel]

Why methane? Is it better than other fuels?

[sh1pman]

 

[satnet]

As with a lot of engineering endeavors there isn't one "best" option. You have a bunch of competing criteria and your trying to optimize for your highest priority ones while being acceptable in the lower priority ones. What is a high or low priority depends on what you're trying to accomplish. For rocket fuels methane is on a short list of pretty good in most criteria, though it isn't THE best in very many it just happens to be the second or third best in many of them. Depending on what a rocket is designed for, methane's broadly "good enough" status may win out or you may want hydrogen's lower molecular mass and higher energy and put up with its irritatingly low density and low boiling point. RP-1 is easier to handle being a liquid at room temperature, it has a long history in the industry, and is probably cheaper (I don't know for sure).

Methane has relatively low molecular mass, high energy, OK density, OK boiling point, and is clean burning. You can probably find a better fuel in each category, but most of them are noticeably worse in another. For deep space missions methane has the advantage of having a boiling point close to oxygen or OF₂ which means that you don't have to worry as much about the temperature of the tanks equalizing over time. It should also be possible to manufacture in-situ. For right now I'm guessing that the reduced risk of coking in the engine is what is pushing SpaceX and Blue Origin in this direction since they both want to re-use their engines, so a cleaner burning fuel is huge asset to them (though the deep space considerations are probably a significant factor given their long term plans).

I was curious what Ignition! had to say and it found this in the chapter on "Halogens and Politics and Deep Space":

"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!)"

 

[Scotius]

Excellent analysis satnet. I would also add ease of transporting, storing, producing etc. of methane on Earth. SpaceX openly states their intent of entering the business of long distance transport on Earth. I wouldn't be surprised if BO had similiar plans. Ubiquitous, cheap and storable fuel available pretty much in every place in the world is a huge boon for such enterprise.

[Hannu2]

3 hours ago, satnet said:

I was curious what Ignition! had to say and it found this in the chapter on "Halogens and Politics and Deep Space":

"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!)"

 

Everything was great but then some boring funkiller invented so called safety regulations and bye bye OF2.

[Xd the great]

24 minutes ago, Hannu2 said:

Everything was great but then some boring funkiller invented so called safety regulations and bye bye OF2.

Clf3 was also pretty dangerous, and is hypergolic with test engineers, as stated in Ignition!

Hydrazine and N2O4 too

[Spaceception]

22 hours ago, Cassel said:

Why methane? Is it better than other fuels?

SpaceX's big reason is that they're gunning for Mars. And Methane rockets is one of the best ways to go for ISRU production there. Whether or not you think they will achieve that goal doesn't matter. But they're planning ahead, which is important. And then Scott Manley's video summarizes everything else.

 

I just realized you were mostly talking about Blue Origin. Sorry, I just focused on the quote and not the title earlier.

Edited October 5, 2018 by Spaceception
Whoopsies, wrong topic-ish

[NSEP]

Liquid Methane doesn't have the by-product clogging problem that Kerosene has, doesn't have a density so low its a pain to work with unlike Liquid Hydrogen, and isn't a devilish venom like hypergolics, and all that, while maintaining and going even higher than the Isp of the legendary almighty RD-170/180/190 yet will probably never go further than RL-10 almighty.

Its good stuff you know.

Edited October 5, 2018 by NSEP

[Nightside]

Plus you can make it yourself at home. In the gaseous state at least.

[Scotius]

Grazing ruminants and termites produce prodigious amounts of methane. We got nothing on them

[Racescort666]

WARNING HEARSAY AHEAD: 

I don’t remember where I heard but I have been under the impression that BO was running the BE-4 off of LNG rather than ”Liquid Methane”. This might largely be semantics but straight commercial liquid natural gas (which is mostly methane) has various contaminates that might cause problems for rocket engines.

That being said, 1 DGE (diesel gallon equivalent) of LNG costs less than diesel and presumably RP-1 costs much more than diesel. For reference, a DGE is based on the energy of a gallon of commercial diesel fuel so per unit of energy, I would assume that LNG to be cheaper than RP-1. 

[sh1pman]

43 minutes ago, Racescort666 said:

That being said, 1 DGE (diesel gallon equivalent) of LNG costs less than diesel and presumably RP-1 costs much more than diesel. For reference, a DGE is based on the energy of a gallon of commercial diesel fuel so per unit of energy, I would assume that LNG to be cheaper than RP-1. 

I did the calculations, and LNG is VASTLY cheaper per unit of energy than any other hydrocarbon (or hydrogen) fuel. 50x cheaper than RP-1.

Edited October 6, 2018 by sh1pman
Woops, lost 2 zeroes :p

[Racescort666]

@sh1pman that’s what I figured. Surely whatever spec qualifies as “rocket grade liquid methane” is cheaper than RP-1 unless there is something we’ve missed.

[Bill Phil]

There are numerous advantages to liquid methane. Scott Manley explained it well. Lower combustion temperature and yet higher specific impulse. Less soot, and more

The only reason it isn't used often is because RP-1 was a lot easier to get back in the 50s since there was already a large industry for complex hydrocarbons. And if it is significantly cheaper than RP-1 that may be a big reason since BO and SpaceX are hoping to reduce launch price significantly. Eventually fuel price will be a significant factor, unlike the status quo, and so a cheaper, cooler burning, and higher performing fuel would be a good option.

8 hours ago, Nightside said:

Plus you can make it yourself at home. In the gaseous state at least.

Run it through the cold side of a Stirling heat pump...

[Cassel]

17 hours ago, Spaceception said:

SpaceX's big reason is that they're gunning for Mars. And Methane rockets is one of the best ways to go for ISRU production there. Whether or not you think they will achieve that goal doesn't matter. But they're planning ahead, which is important. And then Scott Manley's video summarizes everything else.

 

I just realized you were mostly talking about Blue Origin. Sorry, I just focused on the quote and not the title earlier.

Maybe I did not write it, but I meant why Blue Origin decided on methane-powered engines, while other manufacturers use different fuels. It seemed strange to me.

But now I know, thanks to everyone for the comprehensive answer.