morat242

To quote Ignition!, once again: Probably if the US had metricized, we'd be using exhaust velocity (thankfully, we abandoned the WW2 German convention of "specific propellant consumption", the reciprocal of specific impulse). But that hasn't happened, and now we're probably stuck with Isp for historical reasons even if we do go metric.

The rule of thumb is about 50% unreacted for LH2, 5-20% for hydrocarbon. Efficiency goes down when the mass of the exhaust products goes up, so adding enough oxygen to burn all the hydrogen would cramp down on performance as well as melt/burn the engine. I would be really surprised if any rocket ever output hydrogen peroxide as an exhaust product. The reason it never became more than a niche propellant is its tendency to exothermically decompose, which is catalyzed by way, way too many common substances. At exhaust temperatures, there is no way it could stay together.

The numbers I used were for the follow-on proposal for a functional nuclear-powered bomber (since the NB-36H's reactor filled the bomb bay) with a liquid metal cooled reactor instead of pressurized water to save weight. The reactor was to be partially shielded, probably to reduce the difficulties in ground handling.

Dude, they tried it in a B-36. The 48 tons of shielding deemed necessary for an operational aircraft was *with* shadow shielding the crew cabin and using the aircraft structure and fuel tanks as additional protection. With the experimental aircraft, they used the water coolant tanks for the reactor as well (as the X-6 was to have been air-cooled). And that was with no payload and only a partly shielded reactor. To actually have a useful payload, you need a massively larger aircraft (shielding being roughly proportional to the square root of reactor power). Like, even without any intentions of going to space, you need an 747-8 sized aircraft at minimum. Even then, you'll be taking out most of the passengers. You're talking about shielding the runway and the airport buildings, that's great. How the heck you can ensure that everything within several miles of an airport is devoid of life, I don't know, but whatever. What happens when the aircraft needs maintenance? And in the end, we get, what, the ability to save on kerosene and liquid oxygen? If you could guarantee the massive space payload demand required to make this even vaguely feasible, you could easily justify a laser launch system...and then you could leave the nuclear reactors on the ground in full containment domes. Or not have them in the first place. What problem does this solve that there isn't an easier, cheaper, and safer solution for already?

Except that they were never able to lift enough shielding mass to produce an aircraft capable of carrying a useful payload. They only ever managed to shield the flight crew from the reactor, and only ever pulled the control rods when it was already in flight. Who wants to service an aircraft that is spraying neutrons at you? Who wants to live under a flightline with a lightly shielded reactor flying overhead every half hour? What would reactors at full takeoff power do to an airport?And note, GE's design for the X-6 powerplant was 5 tons of reactor, 48 tons of shielding, 9 tons of engine, and 20 more tons to actually hook everything up. And that was for a reactor that was A) not fully shielded and directly heating the air, leading to radioactive exhaust as well as the expected spray direct from the reactor. P&W's indirect cycle engine was never anywhere close to ready, and would have been substantially heavier if it had been.

The toxicity and handling issues of fluorine, only the performance is worse and it explodes. They tried dissolving it in oxygen, but the problem is that O2 boils at 90K and O3 boils at 161. So when you shut down your engine, the O2 boils off first leaving concentrated O3 in the fuel lines. Which explode.

Hydrogen peroxide also exothermically decomposes in the tank (which is catalyzed by many, many things). And, of course, as it warms, it decomposes faster, which heats it further, which accelerates decomposition, repeat until explosion. The classic book Ignition! references this as why, e.g. white fuming nitric acid was less of a problem; it decomposes, but not exothermically (so it doesn't self-accelerate). It also notes that fire experiments showed that nitric acid + UDMH isn't much of a problem, because they're so reactive that spills in quantity can't mix and then explode, they just flare momentarily and fly apart. On the other hand, H2O2 + jet fuel + spark equals a fuel-air bomb as the fire vaporizes the fuel and the peroxide and oxygen mix with it before detonating.For a tactical missile, you want storable propellant that's usable in a wide range of temperatures, which in the late 50s came to mean IRFNA + UDMH. For an ICBM that lives in a steam-heated hole, N2O4 that would be frozen in a severe winter offers better performance. But you can't use cryogenic propellants, and you especially can't use hydrogen that will jolly well leak through the tank walls. The US and USSR grudgingly accepted ballistic missiles that needed to be fueled right before launch, but only for the first (Atlas/Redstone/R-7) types. Nobody is going to stand around a battlefield pouring (toxic, corrosive, explosive) propellant into a rocket. And nobody wants the ICBMs to be warned that there's an incoming strike, but it'll be an hour before they're fueled to shoot back. So practical missiles needed storable propellants (although ICBMs didn't need a low freezing point). The really early stuff had the worst of both worlds, propellants that were awful to handle and ate the tanks so you couldn't store them in the rocket. And then, as nuclear warheads got lighter and solid rockets got better, missiles pretty much all switched over. Yes, liquids have higher performance, but solids are more reliable. If you need more oomph, use a bigger rocket. On the other hand, space launchers care very much about the performance (and frequently want to throttle the rocket), don't want to deal with really nasty chemicals, and don't really give a damn whether their propellant is cryogenic. Thus, hydrocarbons or liquid hydrogen burned with liquid oxygen. Until you get to thrusters in space, which again have to switch back to storable propellants, and hypergolic combinations like UDMH and nitric acid/N2O4 also make the rocket design a lot simpler and more reliable. The really exotic stuff, like fluorine and boron compounds, are just never going to make it. Yes, higher performance, but the military doesn't care (or want to deal with the handling problems) and if the space agencies were willing to deal with hydrogen fluoride everywhere, why not switch to a nuclear rocket? Higher performance, less hassle. Otherwise, LOX and RP-1 are cheap, just build a bigger rocket.