Alternative oxidizers for combustion engines

[Alchemist]

One of the main things about jet engine is that its exhaust mostly consists of what goes into intake and only partially of the combustion products (which make almost 100% of rocket exhaust). What does that mean? You have much more working body for the same amount of energy giving you much more impulse. That's why jet engine has so high Isp not because it takes one of the components from air. So you can have efficient jet in fully neutral atmosphere (at about half of Isp on Earth), just it will take both components from the internal tanks, burn them and then mix the combustion products with intake air. And in atmospheres when one component is present you can switch back to one component mode.

Also check nuclear ramjet concept - it doesn't use combustion and will work in any atmosphere, it just uses heat from a nuclear reactor to heat up the air.

[Idobox]

Thanks for the clearing up on the word fuel.

I have trouble conceiving an oxidized state of oxygen, as you would need to strip electrons away from it. Maybe in high temperature environment.

About methane, the LEL doesn't mean anything if you don't have air at 1atm with 20% oxygen. There is methane, it is possible to burn it, but not as an open flame at the end of an oxygen hose. I don't feel like doing the math, but you can compress the gas and/or use metallic catalysts to increase the reaction rate. An other option would be to inject a lot of oxygen, which would be terribly wasteful.

By the way, the atmosphere contains only 1.4% methane, but is at 1.5atm, so the partial pressure is similar to 2.1% at 1atm.

[lajoswinkler]

Here's a compound of oxygen where oxygen is at +2 state, its highest oxidation state.

http://en.wikipedia.org/wiki/Oxygen_difluoride

[K^2]

About methane, the LEL doesn't mean anything if you don't have air at 1atm with 20% oxygen. [...]

By the way, the atmosphere contains only 1.4% methane, but is at 1.5atm, so the partial pressure is similar to 2.1% at 1atm.

Partial pressure doesn't mean anything. The question isn't just about concentration. It's about how much energy is produced in the reaction and how much stuff there is to soak that energy up. So only volume ratios matter.

And by volume, CH4 requires 3x the oxygen to burn. That means that at 5% in air it's already very lean. Adding more oxygen makes very little difference, because there isn't enough methane to combust with. Reaction rate is going to be improved in pure oxygen, but without lowering activation energy, it will make little difference to LEL. So 1.5% Methane won't burn even in pure oxygen. And you still have 98.5% of other stuff, mostly nitrogen, to deal with.

Catalysts are the only thing you listed that will make a difference. Organizing catalyzed combustion chambers, however, is rather tricky. I don't think you'd be able to run a jet engine on that, and even if you could, it wouldn't be worth the trouble. By this point, it's a lot cheaper to just bring both fuel and oxidizer to run the turbine.

[christok]

It's completely possible to run jet engines in an atmosphere without a strong oxidising agent--you just need to pick a stronger reducing agent. Monosilane has been proposed for jet engines on Mars and Venus since it can burn in CO2. It has also been proposed as a rocket fuel for lunar ISRO, since silica and water should be available.

The stuff is quite nasty, as you can imagine.

[Nuke]

Thanks for the clearing up on the word fuel.

By the way, the atmosphere contains only 1.4% methane, but is at 1.5atm, so the partial pressure is similar to 2.1% at 1atm.

looks like i plucked the wrong number from wikipedia, my bad.

[Idobox]

from wikipedia :

Temperature, pressure, and the concentration of the oxidizer also influences flammability limits. Higher temperature results in lower LFL and higher UFL, while greater pressure increases both values.

Pressure influences the LFL, or the LEL. You increase the quantity of stuff to soak up energy at the same rate as the available energy increases, but you also increase the reactivity.

If you compress the atmosphere adiabatically, you will increase both temperature and pressure, lowering the LEL. And jet engines usually have such a step.

Metallic catalysts are used in mono-propellant rockets, I don't see why it would be so hard to use them in combustion engines.