Rocket Fuel Questions

[RuBisCO]

Ask your rocket fuel related question here.

Why have no engines been developed that operate on N2O4+Hydrazine? Hydrazine is easier to produce, has a higher density and a higher ISP then MMH or UMDH. All I could find was a random claim that MMH and UMDH make better coolants. N2O4 would make a great coolant (dissociating into NO2 would suck a lot of heat), if it was not so corrosive and decomposes at 150°C. Hydrazine will begin to decompose beyond 200°C but I can't find an exact number not decomposition temperatures for MMH and UMDH.

[sgt_flyer]

One of the issues i could find with hydrazine, in addition of it's instability, is it's melting point -

Pure Hydrazine has a melting point of 2°C, and a boiling point of 114°C

UDMH melting point is at -57°C and a boiling point of 64ºC

MMH has a melting point of -52°C and a boiling point of 94°C

On the other end, N2O4 has a melting point of -11.2°C, and a boiling point of 21.69°C

So between Hydrazine and N2O4, you only have 19°C of difference where both compounds are liquid - which would require a complex heating system and insulation, while MMH and UDMH can use the full range.

The hydrazine high melting point is one of the reasons the LEM used Aerozine 50 (50/50 mixture of hydrazine + UDMH - only to lower the melting point and being more stable - limiting the decomposition of the hydrazine, allowing it's use as a coolant)

http://en.m.wikipedia.org/wiki/Aerozine_50

On the other end, mmh / udmh are bad at being used as monopropellants (bad reactivity and thrust as monopropellants compared to hydrazine)

On the other end, there is ongoing research to replace all hydrazine based (hydrazine, mmh, udmh) fuels, because they are simply too toxic and require extra safety to be handled

Edited November 25, 2014 by sgt_flyer

[shynung]

The Titan rockets and ICBMs burned a 50/50 mix of hydrazine/UDMH with N2O4. They're out of service as of 2005, it seems.

[Bill Phil]

I think that's just it, NO2 isn't a very efficient reactant, so the advantage gained is lost. Not to mention Hydrazine is very hard to handle... Maybe not hard, but dangerous.

[sgt_flyer]

Here's a good site about the fuel/oxydizer 'base' sea level efficiencies

http://www.braeunig.us/space/propel.htm

Among hypergolics,

N2O4 + Hydrazine is one of the best oxydizer + fuel among hypergolic pairings.

The only hypergolic pairs better than n2o4/hydrazine use fluorine based oxydizers but let's just say that most fluorine based oxydizers are just waaay too dangerous for usage as a rocket propellant (mainly because the efficient ones can easily attack their storage tanks if precautions are not made, and most are hypergolic with a lot of stuff (some fluorine compounds can set concrete or water ice on fire...))

Edited November 26, 2014 by sgt_flyer

[Nibb31]

The Titan rockets and ICBMs burned a 50/50 mix of hydrazine/UDMH with N2O4. They're out of service as of 2005, it seems.

The reason Titan was retired is because it was way too expensive to operate. Hydrazine is nasty stuff, therefore it requires enormous precautions, which makes producing and handling it very expensive.

[K^2]

The Titan rockets and ICBMs burned a 50/50 mix of hydrazine/UDMH with N2O4. They're out of service as of 2005, it seems.

And mixes, as we all know, have much broader temperature ranges, which is probably one of the big reasons for that fuel mix.

[lajoswinkler]

Why have no engines been developed that operate on N2O4+Hydrazine?

There were.

[shynung]

And mixes, as we all know, have much broader temperature ranges, which is probably one of the big reasons for that fuel mix.

Makes sense. Titan rockets were originally nuclear missiles, which have to be kept ready for launch-at-will for years. One wouldn't have time to fill the tanks when the other guy's missiles were only a few minutes' distance from your location.

[Kerbart]

(...)The only hypergolic pairs better than N₂O₄/hydrazine use fluorine based oxydizers but let's just say that most fluorine based oxydizers are just waaay too dangerous for usage as a rocket propellant

One of the things I like about rocket fuel chemistry (I just finished Ignition! by John D. Clark) is how it is casually mentioned that certain chemicals are way too toxic/dangerous to handle and that's why they're using Hydrazine instead (which in itself is highly toxic and unstable). That tells you a lot about the alternatives!

[Laie]

The reason Titan was retired is because it was way too expensive to operate. Hydrazine is nasty stuff, therefore it requires enormous precautions, which makes producing and handling it very expensive.

I think most of the cost comes from running a liquid fuel rocket in the first place. These things are fragile, have lots of moving parts and are decidedly not maintenance-free. You need a lot of infrastructure and well-trained staff. Accidents are going to be nasty or fatal, so great effort has to be taken to make accidents as unlikely as possible: more training, time-comsuming procedures &c. All of that is the case for every sizable liquid-fuel rocket, even if it runs on alcohol and oxygen.

Nasty fuel surely affects the bottom line, but all things considered, it's just a few drips in the bucket.

[shynung]

That also applies to solid-fuel rockets. Sure, solid rocket boosters are simpler, but it won't do any less damage than its liquid-fueled counterpart if it explodes. Even more, the fuel in solid rockets are very similar to high explosives; both fuel and oxidizer are already mixed inside, so handling it with utmost care is a necessity, otherwise an explosion is very likely.

[Kulebron]

I was thinking why cryogenic equipment + liquid oxygen was preferred over H₂O₂. The conclusion was that it's very reactive, consumes the very tanks it's stored in, and in case of pad disaster can damage a lot of things, while oxygen just evaporates.

[Nibb31]

Nasty fuel surely affects the bottom line, but all things considered, it's just a few drips in the bucket.

Nope. As you say, rockets are expensive and always will be. However, handling hydrazine and N2O4 proved significantly more expensive than good old RP-1 and LOX, which is what pushed the USAF to start the EELV program instead. The USAF and NASA learned from several mishaps (and big craters) that handling nasty fuels required extra precaution.

Those precautions, when scaled up into huge volumes of nasty stuff, cost a lot. It was ground operations involving those propellants that made Titan prohibitively expensive, not the rockets themselves.

[Kryten]

The giant solids of the latter models weren't exactly cheap either.

[shynung]

True, they weren't. But they are far easier to handle; the engineers would simply assemble the payload (or the rest of the rocket), set it on the pad, and ignite it. And it works.

[morat242]

I was thinking why cryogenic equipment + liquid oxygen was preferred over H₂O₂. The conclusion was that it's very reactive, consumes the very tanks it's stored in, and in case of pad disaster can damage a lot of things, while oxygen just evaporates.

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.

[NERVAfan]

It would be great if someone could find a way to actually use ozone (which is unfortunately, incredibly dangerous) - it could give much better performance than H2/O2.

[morat242]

It would be great if someone could find a way to actually use ozone (which is unfortunately, incredibly dangerous) - it could give much better performance than H2/O2.

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.

[K^2]

Why the half-measures? Might as well go with Chlorine Triflouride.

[Guest]

That, or FOOF. Interestingly enough, the most efficient rocket in tested used lithium-fluorine combustion combined with hydrogen as reaction mass. Keeping all three in liquid form was a nightmare, though. This is because at some point, despite high combustion energy, you're starting to lose efficiency due to high molar mass of the exhaust. As such, the most efficient rocket would actually use the combustion energy to heat up hydrogen, not only reaction products. This kind of "chemical thermal" propulsion can also work with air, BTW (there were experiments with using this in ICBMs, but not much came of them), meaning you get free reaction mass when you need it the most.

[architeuthis]

Why the half-measures? Might as well go with Chlorine Triflouride.

Obligatory Charles Stross short story link.

[Streetwind]

Oh, that was a great read! Many thanks.

[SciMan]

No half measures, then you mention Chlorine Triflouride? FOOF?

I'll see your toxic oxidizers and raise you a Nuclear Salt Water Rocket. Might not exactly be "chemical" rocket fuel anymore, but...

That thing rides a CONTINUOUS NUCLEAR DETONATION. Basically an "always on" Orion drive.

Incredible ISP, AND great thrust, both at the same time... the problem is, how do you keep the fuel from self-detonating inside the fuel tanks?

Most theories for how to make the fuel tanks NOT explode end up with something looking like a pipe organ and an oil refinery had a baby.

Lots of parallel radiation shielded channels not larger than some inconveniently small diameter. Not the best recipe for a good mass fraction.

Now remember that nuclear salts can be things with cheerful names like Uranium and Thorium flourides, and you start to see why nobody's really considered making one even before taking in to account the difficulty of containing a continuous nuclear detonation. Get any on you and you're probably dead before the radiation can kill you, and if enough of it pools in one spot, you won't have time to worry about pesky things like your dosimeter or chemical exposure meters going off.

Here's my question about rocket propellants:

Why is methane being researched as a propellant, and not ethane, propane, or butane?

Edited November 28, 2014 by SciMan

[shynung]

Here's my question about rocket propellants:

Why is methane being researched as a propellant, and not ethane, propane, or butane?

It has the lowest molecular mass of all the hydrocarbons. For a given chamber temperature and pressure, less molecular mass means more specific impulse.

^{I just wish someone was brave enough to burn it with FLOX 70.}

Edited November 28, 2014 by shynung