Take the 2-minute tour ×
Space Exploration Stack Exchange is a question and answer site for spacecraft operators, scientists, engineers, and enthusiasts. It's 100% free, no registration required.

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?

share|improve this question
    
Discovered this book amazon.com/Ignition-informal-history-liquid-propellants/dp 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 at 13:08
    
Leaves out gel and hybrid fuels. –  Deer Hunter Aug 22 at 8:07

3 Answers 3

up vote 4 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.

share|improve this answer

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.

share|improve this answer

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.

share|improve this answer
1  
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 at 22:44
    
Interesting tech; for my purposes it's basically the same as LOX/kerosene. –  Russell Borogove Aug 21 at 22:48

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.