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So I was chasing something else when this page popped into my field

I'm quite sure the page isn't exhaustive. What little content is there on the page though seems to indicate the below listed propellant are what have been used majorly over the last 6 decades, or so of space exploration.

  • RP1/LOX
  • Xenon
  • LH2/LOX
  • Argon
  • N2O4/MMH
  • Methane/LOX
  • LH2/LOX
  • APCP
  • HTPB
  • N2O4/UDMH
  • H2O2/Kerosene
  • Solid

The listed propellants are both launch systems, as well as ion/deep-space systems so it looks a little odd to read.

The propellants of choice, as per this page, are (in descending order)

  • LH2/LOX
  • RP1/LOX

The list above seems to indicate space-craft launch vehicle propellants have not evolved much over this period with. Is this impression correct?

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Discovered this book and so far, delighted, at what a wonderfully smooth reading it makes. I'm no expert, so my take could be wrong... IMO this book makes light work of the subject. – Everyone Aug 17 '14 at 13:08
Leaves out gel and hybrid fuels. – Deer Hunter Aug 22 '14 at 8:07
@Everyone: your link has expired, but this one (provided by Mark Adler) still works: – Jerard Puckett Oct 14 '14 at 13:48

5 Answers 5

up vote 8 down vote accepted

The performance of a rocket engine is limited mainly by chemistry: a given rocket fuel always has the same performance, as long as the engine is strong enough to contain the burning fuel. The stronger the engine, the higher the chamber pressure which increases performance, but that also runs into chemical limits (the strength of the chamber materials).
So to really improve performance, you'd need to switch to a different fuel. But there are only so many fuels that can be used. Your list contains most of them. There have been some experiments with a fluorine/lithium/LH2 mix:

The impracticality of this chemistry highlights why exotic propellants are not actually used: to make all three components liquids, the hydrogen must be kept below -252°C (just 21 K) and the lithium must be kept above 180°C (453 K). Lithium and fluorine are both extremely corrosive, lithium ignites on contact with air, fluorine ignites on contact with most fuels, including hydrogen. Fluorine and the hydrogen fluoride (HF) in the exhaust are very toxic, which makes working around the launch pad difficult, damages the environment, and makes getting a launch license that much more difficult. The rocket exhaust is also ionized, which would interfere with radio communication with the rocket. Finally, both lithium and fluorine are expensive and rare, enough to actually matter.

This shows the difficulties you can run into when researching better propellants: your propellant may have side effects that make it undesirable to use in practice.

The chemistry of combustion was pretty well known when the first rockets were developed, so the best chemicals available have been used from the start, and the scope for evolution was/is limited.
Instead, there has been a lot of research into other forms of propulsion. Nuclear thermal drives and ion drives have been developed, for example.

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LH2 has several nice properties that make it the propellant of choice.

One, it releases a lot of energy when combusted with oxygen; this is obviously one of the most important factors in a rocket fuel.

Two, the molecular weight of both the fuel and its combustion product is low, which yields high exhaust velocity, which is directly proportional to specific impulse, the fuel-efficiency of a rocket. Obviously hydrogen is going to be a winner here.

Three, the combustion product of LH2/LOX is mostly friendly, non-toxic water vapor; any unburned fuel or oxidizer is also relatively harmless.

On the down side, the low density of LH2 means that your fuel tanks are much bigger than they would be for kerosene/LOX, and you have to keep LH2 very cold; it's impractical to store it in the rocket's tanks for any substantial length of time.

If you can't afford the volume of LH2 (like in the already-enormous first stage of Saturn V) you go with RP-1 (kerosene)/LOX instead, which is almost as powerful and convenient; the combustion products include a lot of relatively-harmless CO & CO2 in addition to water vapor.

If you need to keep the rocket fueled for long periods of time - like in an ICBM that has to launch in a hurry - then LOX isn't practical, and you start having to get into room-temperature fuels like the hydrazine family of fuels with N2O4 oxidizer, which are toxic in both raw and combusted forms.

Everything else tends to fall short of LH2 or kerosene in safety, stability, convenience, performance, or all of the above.

So those are the factors for the majority of conventional launchers.

There's growing interest in methane/LOX for Mars return missions, since it's theoretically possible to produce methane for the return trip on the surface of Mars. The specific impulse is in the same ballpark as kerosene or a bit better.

If overall thrust isn't a factor, as for upper stages, ion engines using Xenon as reaction mass have vastly higher ISP but terribly low thrust; it takes them a long time to leave Earth's orbit to go anywhere.

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I'm not sure if we're supposed to limit ourselves to liquids alone, but LOX/Paraffin hybrids also come to mind that have equvivalent Isp of LOX/RP-1 or LOX/Kerosene (~ 340 s). They have yet to mature as system components, but with ability to simply optimize oxidizer path and/or mix with Aluminum powder to increase regression rate, they certainly can tick all the boxes, including high-thrust applications or long duration mission where boil-off rate of low molecular weight propellants could be a problem. – TildalWave Aug 21 '14 at 22:44
Interesting tech; for my purposes it's basically the same as LOX/kerosene. – Russell Borogove Aug 21 '14 at 22:48

For the most part, yes that impression is correct. In terms of chemical rockets, LH2 is still king in ISP. Non-Chemical rockets have improved to get better/different performance.

Solid fuels have progressed over time, with all sorts of different compounds being used.

Chemical propulsion, in terms of raw performance, and in terms of things that can be used, has mostly plateaued.

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I would like to learn more about the solid fuels that are currently being tested. – Stu Sep 30 '14 at 12:49

It's unfair to say that propellants haven't developed.

Until last decade Europe still used storeable propellants for it's Ariane 4 main stage. India, China and Russia (Khrunichev) continue to do to so, although Russia with Angara and China with Long March 5 and onward are moving towards LOX/Kerosene.

I would expect that the hypergolic storables will continue to be phased out because of their high toxicity and low performance. They will be replaced by "greener" propellants, as mechanisms to start the engine are developed further. Even for roll-control-thrusters I expect that in some years monoprop thrusters will be replaced by laser-ignited thrusters running on the same propellant as the main stage.

Also, there have been interesting developments into methane combustion, which offers a compromise between kerosene's high-density-low-performance and hydrogen's high-performance-low-density.

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There is currently a means to modify kerosene into a more energetic, more completely burning substance. Advanced Refining Concepts of Reno NV is producing a modified diesel fuel and heating oil using methane, a catalyst and a low temperature low pressure process. The molecules are modified so that their lengths are more uniform. The final product burns more easily and cleanly, releasing more energy than the standard petroleum products. Using this fuel for the kerosene portion of RP1 may make more energy available per volume of fuel.

Here is a link that describes the clearrefining process.

The material safety sheet says the specific gravity for GDiesel is .8158.

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What is the improvement in specific impulse relative to RP-1? Pat.7806947 B2 doesn't state this (although I can see that the fuel generates more particulate matter than usual). – Deer Hunter Jan 16 at 6:12
Interesting. Could you add a link to your source for this information? – Hobbes Jan 16 at 7:57

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