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View Full Version : Are any more improvements in rocket propellant specific impulse possible?



wd40
2013-Nov-06, 06:16 PM
In terms of specific impulse, has the limit in chemical rocket fuel technology/chemical rocket engine design been reached in this list of fuels
http://en.wikipedia.org/wiki/Rocket_fuels
been reached or could there yet be a surprise research breakthrough with a new chemical fuel being developed/discovered that gives an improvement >15%?

Noclevername
2013-Nov-06, 06:48 PM
Since the more energetic the fuel the likelier it is to go boom, there is probably a long list of chemical reactions rejected as too dangerous, unstable, or risky. And no doubt some that would expensive or rare materials. So physically possible, but impractical.

cjl
2013-Nov-07, 12:08 AM
The highest Isp chemical propellant ever tested is around 20% better than anything I saw on that page, but it hasn't ever been used beyond rudimentary experimentation due to the hazards of handling it (if I remember right, it had a specific impulse around 542 seconds).

It was a tripropellant mixture consisting of liquid lithium, liquid fluorine, and liquid hydrogen.

ravens_cry
2013-Nov-07, 12:13 AM
Jinkies! And people get freaked out by nuclear rockets!

ravens_cry
2013-Nov-07, 02:12 AM
Yeah, I was reading about it on wikipedia. Awful stuff.

swampyankee
2013-Nov-07, 02:36 AM
Jinkies! And people get freaked out by nuclear rockets!


Lithium fluoride isn't that bad. Of course, HF will suck the calcium out of your bones and totally screw up internal electrolytic balances.

ravens_cry
2013-Nov-07, 04:04 AM
Yeah, I was reading about i . . . wow, it's deja vu all over again!:confused:

Solfe
2013-Nov-07, 04:07 AM
Yeah, I was reading about i . . . wow, it's deja vu all over again!:confused:

As long as someone else saw that, I'm ok.

Whew!

Noclevername
2013-Nov-07, 12:53 PM
There's also chemical reactions that burn so hot they'd melt the ignition chamber or more, unless it was unreasonably heavy.

cjl
2013-Nov-07, 11:38 PM
There's also chemical reactions that burn so hot they'd melt the ignition chamber or more, unless it was unreasonably heavy.

That's generally not the issue - pretty much every chemical rocket burns hot enough to melt the combustion chamber. This is solved either with ablative coatings, or with active cooling of the chamber walls, usually by running the cryogenic fuel or oxidizer through the nozzle and chamber walls prior to combustion.

ravens_cry
2013-Nov-08, 03:57 AM
As long as someone else saw that, I'm ok.

Whew!
See what? I kid, I kid!

Gullible Jones
2013-Nov-08, 01:37 PM
Hmm. Wouldn't specific impulse be improved by using a propellant with higher atomic mass?

Perhaps a three-propellant chemical rocket could work. Use a normal hydrogen-burning rocket, and then inject something heavy (xenon? mercury?) into the exhaust stream to improve the specific impulse.

(But of course I Am Not A Rocket Scientist...)

Noclevername
2013-Nov-08, 01:39 PM
Hmm. Wouldn't specific impulse be improved by using a propellant with higher atomic mass?

Perhaps a three-propellant chemical rocket could work. Use a normal hydrogen-burning rocket, and then inject something heavy (xenon? mercury?) into the exhaust stream to improve the specific impulse.

(But of course I Am Not A Rocket Scientist...)

I for one would not want mercury vapor being sprayed all over the sky.

Gullible Jones
2013-Nov-08, 02:03 PM
No indeed. :) You'd switch on the mercury injection only after achieving orbit. Space is where you'd want higher specific impulse anyway IIRC; thrust isn't as important (no gravity and no air resistance) but more specific impulse means more delta-V. I think.

cjl
2013-Nov-08, 06:21 PM
Hmm. Wouldn't specific impulse be improved by using a propellant with higher atomic mass?

Perhaps a three-propellant chemical rocket could work. Use a normal hydrogen-burning rocket, and then inject something heavy (xenon? mercury?) into the exhaust stream to improve the specific impulse.

(But of course I Am Not A Rocket Scientist...)

Actually, it's precisely the opposite. Specific impulse is directly proportional to exhaust velocity, and for a converging-diverging nozzle with a given geometry, exhaust speed is (and I'm making a lot of simplifications here, but bear with me...) approximately proportional to sound speed. Low atomic mass means a high sound speed, as does a high temperature. So, to get the highest exhaust velocity, you want a low atomic mass, and a high temperature. Hydrogen is very, very good at this, since it is both light, and prone to energetic reactions with oxidizers. In a typical LOX/LH2 engine, you want to run fuel rich for optimum specific impulse rather than stoichiometric for this reason - the extra hydrogen does make the temperature slightly cooler, but this is more than compensated for by the lighter mean molecular weight of the exhaust (caused by the excess hydrogen). In the propellant mentioned above (Li/F/H), the lithium/fluorine reaction is used to generate as much heat as possible, and the hydrogen is added to lower the mean molecular mass of the exhaust products to improve the specific impulse.

This is also why the nuclear rockets all use hydrogen as their reaction mass - the low molecular mass makes it ideal, and since the heating doesn't come from chemical reactions, the mean molecular weight of the exhaust for a nuclear rocket is substantially lower than for a chemical rocket (it's pretty much all H2), which is part of the reason nuclear rockets have such a high Isp - their chamber temperature isn't actually that different from the chamber temperature of a chemical rocket.

ravens_cry
2013-Nov-08, 07:03 PM
Things like NERVA are rather limited by the fact you don't want to melt your fuel rods. Here's a thought, though very power intensive, a nuclear lightbulb (http://en.wikipedia.org/wiki/Nuclear_lightbulb) that uses plasma windows (http://en.wikipedia.org/wiki/Plasma_window).