SpaceX... Y U NO USE BETTER PROPELLANTS!?

[Naten]

Have you noticed that SpaceX is still using ye olden Rp-1+LOX mixture for their Merlin engines, and the Kestrel engine?

Why? Why do they do that? I know it provides slightly more thrust, but if they still use it for the Merlin Vacuum, then that's probably not the reason why?

Why? WHY? Anyone, anyone at all know why they're still using it?

[Cepheus]

For lack of better explanation, the well-trodden path is the easier one. Hydrogen is incredibly light, and requires massive fuel tanks.

[Naten]

For lack of better explanation, the well-trodden path is the easier one. Hydrogen is incredibly light, and requires massive fuel tanks.

Oh, yes. But at least the Merlin Vacuum, that should use it so it can be more efficient in space.

Have you heard of a modern rocket that uses RP-1 not made by SpaceX?

I think there are some, but those are rare, I am sure.

Still, what propellant do you think they should use? Please take the poll. I want to see what everyone thinks.

[Albert VDS]

Because it's cheap?

[Cepheus]

Have you heard of a modern rocket that uses RP-1 not made by SpaceX?

Copenhagen suborbitals is using lox/alcohol (the oldest of all the fuel combinations) for their HEAT1600. If it works, and it's cheap, why not use it?

[Naten]

Oh, yeah, alcohol, like the V-2.

[Fuzzy Dunlop]

Firstly SpaceX use Kerosene in the upper stage because they have experience with it in the lower stage.

Kerolox engines are completely different from hydrolox engines, so SpaceX would have to spend huge amounts of time and money developing another powerplant. They would also have to pay for the expense of maintaining two different production lines. Liquid hydrogen also requires a lot of extra pad infrastructure that SpaceX would have to build and maintain.

Liquid hydrogen has some disadvantages despite its high ISP. It's low tempurature and density requires huge insulated tanks, which can be more expensive to manufacture.

Also the Zenit rocket is fairly modern and that uses Kerolox on all stages.

[OdinYggd]

Because it's cheap?

This.

Although RP1-LOX is less efficient from an energy standpoint, from an economics standpoint it is much more cost effective than cryogenic fuels.

RP-1 is effectively just a highly refined Kerosene, differing only in additives and purity from JP-1 jet fuel and K-1 heating kerosene.

It also is a lot easier to handle, making the required fuel tank simpler and cheaper to construct.

By the time all is said and done, it would cost more to use the more efficient fuel.

Sound familiar? Although diesel engines are far more fuel efficient, they cost more to make, and use a more expensive fuel. So everyone uses gasoline/petrol instead.

Edited August 12, 2013 by OdinYggd

[Accelerando]

Oh, yes. But at least the Merlin Vacuum, that should use it so it can be more efficient in space.

Have you heard of a modern rocket that uses RP-1 not made by SpaceX?

I think there are some, but those are rare, I am sure.

Soyuz rockets are fueled by RP-1, as well as the first stage of the Atlas 3 and the Common Core Booster meant for use on the Atlas V, and perhaps others I have not found yet. There are a number of Russian rockets that use even worse fuels (usually some mixture of hydrazine as the fuel) than RP-1 by pure specific impulse considerations, but which are chosen rather for ease of storage and energy density, although they are often extremely toxic.

[K^2]

One of the bigger reasons is that they are flying relatively small rockets. The smaller the stage, the heavier the tank has to be to keep cryogenic hydrogen as fraction of fuel carried. For something like Dragon, it makes no sense to fly with LH2 because extra weight of the tanks will offset any benefit from higher I_SP.

[karolus10]

Every fuel has it's pros and cons, kerosene is more dense and don't need cryogenic tanks, making first stage lighter and simpler which is important factor If You want to land ditched first stage.

I guess that fuel cross-feed (Unlike our Yellow pipes, it's quite tricky thing IRL) for falcon Heavy Is easier to design with using semi-cryogenic propellant.

Anyway, Spacex chosen kerosene-lox, because hardware using it was cheaper to design and use, making whole investment safer... who knows if they not develop H2-LOX engine for upper stage later.

About Tri-propellants - I wouldn't want to be closer than 1000 meters from this stuff without SCAPE suit.

[Kryten]

Have you heard of a modern rocket that uses RP-1 not made by SpaceX?

I'm having difficulty thinking of rockets that don't, except ones which use even less efficient hypergolics or solids. Delta IV is currently the only all-hydrolox rocket in existence.

[rryy]

Have you heard of a modern rocket that uses RP-1 not made by SpaceX?

Maybe the Soyuz?

Also, SpaceX is making an engine that burns liquid methane and oxygen as an upper stage engine.

[Razor235]

And there's also the rubber and nitrous oxide hybrid (meaning using 2 or more fuels in different states, in this case solid rubber and N2O in either liquid or gas form, my sources didn't specify which) engine used by Virgin Galactic, SpaceDev, et al. The advantages in this case are that it isn't nearly as explosive as RP-1/H2 and O2 engines, and it also doesn't require refrigeration if oxidizer is kept in gaseous form.

[Bunsen]

It's not just that the propellants themselves are cheap -- propellant is such a small fraction of the total launch cost that they could spend ten times what they do and hardly affect the price of a launch (with the pricing I could most easily find, a $5*10⁷ launch burns about $2*10⁵ worth of propellant, maybe $4*10⁵ now with petroleum and energy price increases). Using old, established technology means most of the expensive, time consuming mistakes have already been made, and you can focus your engineering effort on making things light, reliable, and easy to manufacture instead of trying to figure out how to make it all work in the first place. For a young company selling its first product line, that's a much smarter thing to do than to gamble on breaking new technological ground. Conjuring new technology out of thin air is awesome work for the scientists and engineers, but it's absolute hell on budgets and schedules -- and companies live and die by budgets and schedules. Once they build a steady revenue stream, then you can expect to see more effort go toward higher-risk projects (like that methane-oxygen engine).

Fuzzy Dunlop's points are also quite important. Production lines are expensive to establish and support. The benefits of a high-energy upper stage have to be weighed against not just the mass of larger, insulated tanks and the inconveniences inherent in working with liquid hydrogen, but also the costs of cultivating expertise in another technology, designing a new piece of hardware from scratch, and running a separate manufacturing line.

[BFGfreak]

Scott Manley made a video about rocket fuel and the pro's and cons of each are laid out.

The reason why SpaceX uses the fuel it uses is because it's cheap, the rockets that burn them are easy to build, and it's generally one of the best fuels with the fewest drawbacks.

[Naten]

Whoops, forgot hydrazine.

[Apotheosist]

SpaceX also use Kerosene in the later stages because they can use the same engine throughout the vehicle. With slight modifications on the vacuum version of course. But having common engines vastly increases the reliability of the vacuum engine and thus the whole rocket over time. If you think about it, the engine on the falcon 9 second stage has been flown about 9x5=45 (I think it's been 5 flight of falcon 9?) times as well as the times it was in the second stage alone (5 times). That makes for an engine that has been flown 50 times. This must surely give lots of confidence in the reliability of the second stage.

But the ultimate fuel for SpaceX would be methane, which as Musk said, is the cheapest fuel in terms of cost per joule, and it can even be made on mars. SpaceX is working towards methane propulsion, so all is looking well