3 ms·
It's the high temperature and the lower exhaust molecular weight. Specific impulse (Isp) is how much force you get from each mass unit of propellant you exhaus
by ridgeguy 3y ago
It's the high temperature and the lower exhaust molecular weight.
Specific impulse (Isp) is how much force you get from each mass unit of propellant you exhaust. So an engine with Isp = 300 gives 300lbs force for every pound of exhaust gas mass.
IIRC, Isp scales as the inverse square root of exhaust molecular weight. So a pure (molecular) hydrogen exhaust would have a molecular weight of 2. A pure H2/O2 engine would have an exhaust of H2O (assume it's running at stochiometric H2/O2 ratio to simplify things), or an average molecular weight of 18.
For everything else being equal (temperature, pressure, etc.), the H2 Isp would be sqrt(18/2), about 3x the H2/O2 Isp. That's mainly why rocketeers would like nuclear engines, more oomf from a given mass of propellant. It's also (conceptually) simpler because you only need to handle one propellant vs. two or more for conventional liquid fuel combustion.
It's more complex, of course. If the reactor is really hot, it can partially dissociate H2 into atomic hydrogen, lowering the exhaust molecular weight still more. But dissociation is an energetically expensive process, so there's bound to be a tradeoff between energy consumed by dissociation vs. increased Isp. I've no clue how that would work out.
- lazide 3y agoThe real limiting issue tends to be materials in the reactor - at some point, every known material is a liquid or a gas and the reactor stops being an ongoing concern. Usually reactors run cooler by several orders of magnitude, as this is referred to as a ‘meltdown’ and people get snippy about the releases of radiation and expensive cleanup crews, etc. Space is more forgiving and has fewer HOA types, so they can go closer to the limits - but it’s still the same underlying issue.
- ridgeguy 3y agoYeah, the materials issues dominate design basics. They've looked at HfC and HfCN as nuclear engine materials (m.p. ~3950°C), pretty exotic. I feel like if designers would drop their effort to wring out the last increment of Isp from nuke engines and reduce temperatures to ~2000°C, it would give faster development. There are many materials that can live in 2000° hydrogen for >100K hours. There's an old Rand study that showed H2 would give an Isp >1100 at 1 atm chamber pressure (!), temp ~2000°C, exhaust pressure 10e-4 atm. Seems worth exploring.
- lazide 3y agoThat does sound more reasonable! Part of the issue I suspect why reasonable isn’t being considered is that I don’t expect anyone to have a reason to really use them right now regardless - all the nuclear propulsion designs have a high ‘fixed’ weight cost in the reactor, so would only make sense overall in a ‘hundreds of tons to Uranus ASAP’ type situation, where the overall weight for thrust can be lower due to the better fuel efficiency. So far we thankfully haven’t had a reason to really need to do that right now. And if we’re doing theoretical designs, why not make it ‘interesting’? On a side note, if we’re considering crazy ideas - my favorite is the one that makes Project Orion seem clean! https://en.m.wikipedia.org/wiki/Nuclear_salt-water_rocket https://en.m.wikipedia.org/wiki/Nuclear_salt-water_rocket Any rocket which calls for ‘salts of plutonium’ as fuel has to be cool.
- WalterBright 3y agoYou'd also not have to worry about the propellant exploding.
- marcosdumay 3y agoWell, much less than with a traditional rocket.
- WalterBright 3y agoHow is H going to explode out in space?
- marcosdumay 3y agoRockets don't work with H2 alone.
- WalterBright 3y agoH alone is the propellant in a nuclear rocket. H needs an oxidizer to explode. No oxidizer, no explosion.
- marcosdumay 3y agoSo, you are agreeing with me?