Go back a generation of development, and excepting the shuttle-derived systems, all liquid rockets used RP-1 (aka kerosene) for their first stage. Now it seems everybody and their dog wants to fuel their rockets with methane. What happened? [Eager Space] was eager to explain in recent video, which you’ll find embedded below.
At first glance, it’s a bit of a wash: the density and specific impulses of kerolox (kerosene-oxygen) and metholox (methane-oxygen) rockets are very similar. So there’s no immediate performance improvement or volumetric disadvantage, like you would see with hydrogen fuel. Instead it is a series of small factors that all add up to a meaningful design benefit when engineering the whole system.
Methane also has the advantage of being a gas when it warms up, and rocket engines tend to be warm. So the injectors don’t have to worry about atomizing a thick liquid, and mixing fuel and oxidizer inside the engine does tend to be easier. [Eager Space] calls RP-1 “a soup”, while methane’s simpler combustion chemistry makes the simulation of these engines quicker and easier as well.
There are other factors as well, like the fact that methane is much closer in temperature to LOX, and does cost quite a bit less than RP-1, but you’ll need to watch the whole video to see how they all stack up.
We write about rocketry fairly often on Hackaday, seeing projects with both liquid-fueled and solid-fueled engines. We’ve even highlighted at least one methalox rocket, way back in 2019. Our thanks to space-loving reader [Stephen Walters] for the tip. Building a rocket of your own? Let us know about it with the tip line.
so rocketry iz wut we about, yo! lol
“*, all liquid rockets used RP-1 (aka kerosene) for their first stage.*”
Have we forgotten the Delta IV and Heavy already? It’s only been a year.
And SLS and Shuttle? All of them terrible rockets that were super expensive because they used Hydrolox first stages.
SLS and Shuttle are pretty damn “shuttle-derived”.
SLS, Shuttle Leftover Systems
Feel free to disagree, but I was lumping the Delta IV in with “shuttle-derived” as far as engines go, since the RS-68 engine looks a lot like a simplified, disposable RS-25/SSME to my eyes.
I would have characterized it as “SSME-inspired”, but fair enough.
And the Ariane V and the H-II.
And that’s just the hydrogen burners. There’s a whole mess of hypergolic first stages that ran (or still run) on hydrazine (with optional methyls).
You mean to tell me Outer Space does not belong exclusively to Uncle Sam!? /s
(I neglected to clarify that the video was talking about US rockets. That’s actually a pretty big oversight on my part.)
Actually outer space does belong exclusively to Uncle Sam
Forgot the major bean counting concern: methane has no real penalties and it’s much cheaper. Bean counters are in most top management operations
You’ve written that as though using a cheaper fuel is some kind of failure. Why wouldn’t you use a cheaper fuel?
I think you read something into it which was not actually implied
The descriptive phrase “bean counting” is pejorative, and they’ve used it twice to reinforce it, there’s no way that isn’t a negative implication.
IIRC one of the major reasons SpaceX is going with CH-4 for Starship is that it’s easier to synthesize in situ with the resources available on Mars.
Not saying that I believe them to have a specific plan on how to get from A to B, but I remember reading that it was a motivating factor.
I’ve also heard that argument.
But I don’t trust the musk guy.
I’m not even sure whether that whole Mars thing of him is anything more then a publicity stunt. The guy loves publicity.
Reduced cost for comparable performance is an argument I can understand though.
The main thing is that methane was mid. Middle-of-the-road for everything, didn’t excel in anything.
RP-1/LOX was dense (this gives smaller rockets with more payload per rocket) and had flight heritage.
H2/LOX was the upper-stage fuel of choice because of its high specific impulse, and despite its low specific gravity hydrolox kept being used in the Shuttle, because the engines were being made and rocket engineers were in the habit of chasing specific impulse.
Nitrogen tetroxide/UMDH was the hypergolic, storable star once they figured out how to stabilise it so that it could sit in a rocket in a missile silo. (Nearly all USA rocket research, especially propellant chemistries, was done by the armed forces and its contractors.)
Propane/LOX was considered more promising than methane. It stayed liquid when chilled down to liquid oxygen temperatures (and so could use a common dome between the pressurised tanks, saving on dry mass), was denser than kerosene and had a higher specific impulse than it.
High-test peroxide/RP-1 was the hidden hope of SSTO designers, because it was storable, non-toxic, non-cryogenic and very dense. The prototype of New Shepard flew using it, as did the defunct Armadillo Aerospace. The thing holding it back is that in US rocketry, HTP is viewed with the same enthusaism as a dead rat on the porch.
Just about the only arena where methane does excel is in orbital propellant depots. If shaded from the heat of the Sun (and the Earth, because it turns out that’s a significant factor too) it just sits there.
And orbital propellant depots have not been welcome at NASA since the SLS was proposed, not until recently.
Then some madman decided to go to Mars.
Propane/LOX is the combination of the new European rocket Spectrum, which made its first launch attempt this year.
As for why peroxide is not popular, the book Ignition! also cited by other commenters explains it in quite a bit of detail. It turns out it’s not as storable as it first seems.
.. And of course the Black Arrow orbital launcher from the 60s/70s used HTP to put the Prospero satellite into orbit.
Which is because of its habit to break down spontaneously into steam and oxygen upon any sort of contamination, or just because it feels like it because water itself acts as a “contaminant” to catalyze the breakdown of hydrogen peroxide.
It’s “storable” in the sense that you can keep it around for a while, if you’re super careful about it. The higher the proof, the longer it keeps, but the greater the kaboom when things go wrong.
In case anyone missed it (like me), Ignition! is back in press since a few years, from Rutgers University Press. It has the Asimov foreword and everything.
It’s a solid and well-told history of the early development of liquid rocket propellants up to ca 1970 when the book was written. You also get a ton of insight on why choices were made. It should come as no surprise that oil companies heavily pushed sulfur as an additive, for example.
The chapter on peroxide is rather appropriately titled “Always a Bridesmaid, Never a Bride”. H2O2 has the unsettling habit of decomposing in contact with…almost everything, including dirt and careless rocket scientists. Even when stabilized and stored in polyethylene, Clark pointed out that there is a slow but steady “gloop….gloop…” of decomposition.
It’s initially popular with beginning amateurs until they find out how difficult it is to get 95% H2O2. Some decide to make their own by concentrating more-available 30% solution. Which turns most of their effort into producing the oxidizer. That goes south pretty quickly.
It also doesn’t just go “gloop-gloop”, it produces large quantities of heat. Past a certain threshold, you get runaway decomposition with the entire oxidizer tank turning into steam and hot oxygen. If it spills on flammable materials, it can cause spontaneous ignition. It will quickly cause both chemical and thermal burns. And you really don’t want to inhale mist or vapors from hot high-concentration peroxide. It’s not quite as horrifically dangerous as, say, dinitrogen tetroxide, but I think people get entirely the wrong idea about it from the dilute solutions used as an antiseptic.
And yeah, it seems mostly popular with people who aren’t actually doing rocketry or with startups/hobbyists who haven’t actually had to use it at any significant scale. As far as I’m aware, only one rocket using it has ever launched, the Black Arrow. That rocket only did one orbital launch, and by the time of that flight the program had already been canceled due to its cost.
That hydrogen peroxide in your medicine cabinet is probably not even the 3% alleged on the label. After 6 months or a year it’s almost all water, from just sitting there.
The risk of carbon buildup in the cooling channels.
Methane is cleaner.
Give me old fashioned paraffin and liquid oxygen. Methane is horrific as a greenhouse gas once it accidentally escapes which it always does.
Methane escapes everywhere all the time!
Braaaaaaap
One issue that could be serious is based on the fact that methane is miscible with LOX. An explosion on the pad could be far more destructive than RP-1 and LOX. I’ve seen calculations by actual rocket scientists that put the yield at up to 15 kt….a bit smaller than the Hiroshima weapon.
Sorry, did not point out that that yield was calculated for Starship.
Does that really matter? Rocket launches have pretty large exclusion areas. Aside from being a waste of a rocket it doesn’t really mean much. The weapon dropped on Hiroshima was devastating precisely because there wasn’t an exclusion area.
Hydralox also leaks on long missions, which makes refueling a tanker to go to Mars nearly impossible.
Does the hydrogen leak? Or do they lose pressure from helium leaks?
The main problem is that liquid hydrogen needs to be kept very cold to stay liquid, and any that does boil off will have to be vented. It is also very good at finding it’s way out of leaks, but boil-off is the main problem.
From the other side of the spectrum, you may find the: “The Most Dangerous Rocket Fuels Ever Tested” video from Scott Manley interesting. It also has some quotes from the Ignition! book.
Yep great fun. The infamous FOOF… a.k.a. the rocket fuel that screams its own name as it vaporizes you and the surrounding city block just because you looked at it funny
5th paragraph is missing a word in the first sentence: “We [write?] about rocketry fairly often …”
One reason for methane is that it is abundant in the moons of Jupiter and Saturn, so any exploration there would be able to refuel there rather than carrying enough fuel for a round trip which might not be practical…not easy though. Can also be synthesized on Mars from frozen CO2 and H2O, so SpaceX made the choice for good reason.