Rocket Fuel Questions

[sgt_flyer]

Well, you also need to take into account fuel densities (which is 22% worse than RP-1 - which means larger (and heavier) fuel tanks, as you also need to insulate the fuel tank - you 'll end up having a worse mass fraction in the end.

http://settlement.arc.nasa.gov/Nowicki/SPBI1LF.HTM

Subcooled Propane at lox temperatures is still liquid, as nearly the same density as RP-1, and an ISP between RP-1 and methane. (And it can share the same insulation than the lox tank - no need to put insulation between the two)

Now, Methalox combustion is 'green' compared to the other alcanes. - which is becoming more and more of a concern (not necessarily for the space agencies themselves, but ecologists lobbying can be quite influent in decision making)

Finally, building and understanding methalox combustions in a rocket engine is of interest for the future ISRU technologies -

Methane is very easy to manufacture In Situ, compared to the other alcanes. (You only need water and carbon dioxyd to make methane + dioxygen)

[TythosEternal]

Obligatory Charles Stross short story link.

I hadn't seen this before; thanks for posting a link. Great yarn, and a very fun read!

[NERVAfan]

Why the half-measures? Might as well go with Chlorine Triflouride.

It was tried (in ground tests) but the Isp isn't that good. They tried pretty much every imaginable thing in the 50s and 60s.

[NERVAfan]

I'll see your toxic oxidizers and raise you a Nuclear Salt Water Rocket. Might not exactly be "chemical" rocket fuel anymore, but...

That thing rides a CONTINUOUS NUCLEAR DETONATION. Basically an "always on" Orion drive.

I love that concept.

Incredible ISP, AND great thrust, both at the same time... the problem is, how do you keep the fuel from self-detonating inside the fuel tanks?

Most theories for how to make the fuel tanks NOT explode end up with something looking like a pipe organ and an oil refinery had a baby.

Lots of parallel radiation shielded channels not larger than some inconveniently small diameter. Not the best recipe for a good mass fraction.

Sure, but with Isp ~20 times better than chemical rockets, you don't need much of a mass fraction to do anything in the solar system.

Totally unworkable to test on Earth, though. I'd try it on the Moon with Lunar thorium.

Here's my question about rocket propellants:

Why is methane being researched as a propellant, and not ethane, propane, or butane?

IIRC SpaceX is using it for their next engine (Raptor) because it's better specific impulse and easier to make on Mars (ISRU). I don't know why Firefly Space Systems picked it to work on.

[TythosEternal]

IIRC SpaceX is using it for their next engine (Raptor) because it's better specific impulse and easier to make on Mars (ISRU). I don't know why Firefly Space Systems picked it to work on.

Smaller rockets (Firefly is exclusively targeting the small-sat market) have more to gain from the higher density and lighter cryogenic requirements. You lose about 90 seconds of ISP compared to LH2, but in return you can store 90 degrees warmer (roughly the same as your LOX tanks) and density increases six-fold.

Additionally, LM burns clean, and is cheaply and readily available both here and in many other locations in the solar system (granted, it's not like your small-sat rockets will be lifting payloads from Titan anytime soon). While it's hardly a panacea, an argument can be made for LM as a good and practical compromise among the less-exotic chemical fuels.

That having been said, both SpaceX and Blue Origin (their LM-powered BE-4 was recently announced as the Atlas V replacement for the RD-180) are heavy-lifting main engines (multi-meganewton), so it's not exclusively the domain of small-sat lifters. It's still a good and efficient way to dump a LOT of mass really fast, but not exclusively so (though now that I think of it, you might have better design and safety margins during combustion staging cycles, making life a little easier and less risky for your now-more-reusable turbopumps). The logic in these cases is a little less clear-cut; I'd have to crunch some numbers and/or do some more research before trying to justify it.

Edited December 2, 2014 by TythosEternal
Clarifying use of term "staging"

[RuBisCO]

If SpaceX is going from an open cycle Kerosene+LOX to a full flow close cycle Methane+LOX that is going to be an improvement. If it was closed cycle Kerosene+LOX verse full flow close cycle Methane+LOX the advantage in ISP would not be great enough to warrant the extra tank mass.

My personal favorites Storable Propellants:

H2O2+Kerosene: Less ISP then N2O2+DMH/MMH/Hydrazine but cheaper and less toxic, not necessarily safer though

N2O4+B5H9: Highest ISP "storable" propellent, crazy dangerous, highly toxic, highly reactive, burst into flames in contact with air, etc, a better kerbal fuel there is none! And it makes a green flame!

[shynung]

N2O4+B5H9: Highest ISP "storable" propellent, crazy dangerous, highly toxic, highly reactive, burst into flames in contact with air, etc, a better kerbal fuel there is none! And it makes a green flame!

Won't work. Rocket motor tests have been done (there's an entire chapter devoted to borane in J.D. Clark's book, labeled 'Exotics'), and give a lower specific impulse than theoretical. Also, the exhaust includes B2O3, which is a solid anywhere below 1800 degrees C, which will the gunk the chamber, throat, nozzle, turbopump, and pretty much everything it touches.

[RuBisCO]

Won't work... that not a very kerbal thing to say. Certainly it is a grossly impratical, but a little pressure feed, raditively cooled engine could probably run on it.

[K^2]

And it makes a green flame!

GREEN FLAME!

But yeah, there is honestly no reason to invent anything new. If you need all the I_SP you can get, you are probably going to go with cryogenics anyhow. And if you are looking for something you can store, your considerations are probably closer to these of cost and safety, and then you can't do better than RP-1. Oxidizer, there are more options with, but again, it's hard to beat N2O4 for relative safety and ease of use.

Chemical exotics are just not worth consideration after nearly a century of rocketry. We ought to be looking strictly beyond chemical energy now.

[NERVAfan]

There's a whole book (out of print for a long time but now available on Kindle) called "The Green Flame" about the US boron fuels project in the 50s. I haven't read it yet though so I don't know how good it is.

[shynung]

Chemical exotics are just not worth consideration after nearly a century of rocketry. We ought to be looking strictly beyond chemical energy now.

Surely there'd always be a niche application for them that still works? One wouldn't use nuclear thermal rockets as RCS thrusters, for one.

[K^2]

Surely there'd always be a niche application for them that still works? One wouldn't use nuclear thermal rockets as RCS thrusters, for one.

I'm not saying we will not be using chemical fuels. I'm saying we will not be using any new chemical fuels.

[RuBisCO]

I'm not saying we will not be using chemical fuels. I'm saying we will not be using any new chemical fuels.

You sure about that?

http://en.wikipedia.org/wiki/Green_Propellant_Infusion_Mission

http://en.wikipedia.org/wiki/Nitrous_oxide_fuel_blend

And then there is the whole methane+lox thing, sure it not new chemical engineering, but large in-production engines using the stuff would be a new thing.

[K^2]

Ok, yes, there is still a lot we can do with monoprop that we haven't done before. I should have clarified that I'm talking about biprop in main stages, not maneuvering systems.

[Kryten]

SS2's nylon/NOx mix is a new bipropellant for a main stages, although whether it'll be successful remains to be seen.

[RuBisCO]

what ever I'm not a "prove people wrong" stickler.

Anyways nuclear engines (topic change): I've always felt alot more could have been done with them. rotating cores (cores held together by centrigual force rather then by structural strength) could have allow high thrust to weight and higher temperatures. One statement I've seen is that nuclear engines could run on just about any propellant, be it hydrogen, ammonia, water, N2, CO2, methane, etc, I'm pretty sure this is false, though hydrogen and ammonia have been tested those fuels are reductive and not as corrosive as water at exterme temperatures: >3000K mono-oxygen radical is a mean figher, and the latter three are just plan silly for their low ISP and chance of coking. Water would be nice though, all you need to do is mine for water, filter it and your good to go, if the nuclear engine was capable of a ISP above 400, the avantages of nuclear+water of LH2+LO2 would be phenominal! This study (http://www.neofuel.com/space98/) assumes much lower ISP and still comes out requireing ~60 times less materials to mine and process fuel if it is water and not LH2+LO2. My question is how high do you think water could go in a nuclear engine?

[K^2]

I_SP of an NTR is inversely proportional with the root of the mass of the propellant. So if you go from H2 to H20, your I_SP drops to a third. That's still a pretty decent I_SP for just water, though.

[RuBisCO]

Well I've seen 412 s claimed for 2520 K nuclear engine, LH2+LO2 is combustion temperature is 2985 K with a nominal ISP of 459, certainly if reactor temperatures above 3000 K could be achieve ISP even in the 500 could be possible with water, hydrogen would get an ISP above 1000. The problem is H2O super heated would come out the nozzle as not just water but also equilibriums of H2, O2 and even monoatomic hydrogen and oxygen ions and radicals, thus changing the ISP calculation. Can such temperatures be achieved is my question, I've seen it been proposed with hydrogen but water is far more corrosive then hydrogen at such extreme temperatures, that free proton-free oxygen equilibrium is a near ultimate solvent (I well I guess *H+ and *F- is the ultimate) I can't imagine anything that could be resistant against it.