Alternative oxidizers for combustion engines

[LethalDose]

As a heads up, this topic is going to span the game and real life. If the following idea isn't complete off the wall, I'll move it over to a development sugestion.

Jet engines suck up atmosphere, and use the molecular oxygen to oxidize jet fuel and produce thrust. Currently, only Kerbin and Laythe have molecular atmospheric oxygen, while Jool, Duna, and Eve have atmospheres without atmospheric oxygen and jet propulsion is not possible on these worlds.

However, there other compounds that can serve as oxidizers (diatomic halogens, oxygen containing compounds, maybe even sulfur), and IF an engine were designed to burn/oxidize fuel using these compounds, jet propulsion could be feasible on these planets.

So, to you, the viewers, how feasible is it to design a jet engine that uses an oxidizing agent other than O2? Have humans ever tried to develop these engines for atmospheric flight in alien atmospheres? What kind of fuel would be used? What kind of design challenges would have to be overcome?

I suspect engine part corrosion would be a MAJOR issue since these compounds tend to be very reactive.

[K^2]

Straight up jet propulsion would be pointless. A turbojet minus the atmospheric oxygen is just a rocket. A high bypass turbofan, however, can be way more fuel-efficient. If you want to build something like that in an atmosphere that does not have oxidizer, you would build your turbine to run completely on the on-board fuel and oxidizer, but you'd still have that drive a compressor fan that run on exterior atmo. Or, you can run a turboprop. Either way, it would be way more efficient at slow speeds.

As for being able to build a turbine that runs on an oxidizer you brought with you, yeah, it's definitely doable. Turbopumps on most conventional rockets already work almost like that. You'd just have the turbine run a compressor or a gearbox to the propeller.

[zxczxczbfg]

Well, it might not be practical, since atmospheres can be vastly different in composition. However, something like the SABRE engine on the Skylon, which use jet propulsion in atmosphere but switch over to on-board liquid oxygen outside the atmosphere, is much more weight-efficient because it only needs to carry fuel for the trip but only enough oxidizer for the portion of the trip outside the atmosphere. It's basically a jet engine that brings its own air supply.

Edit: RAPIER engines in 0.23! Woo!

Edited August 20, 2014 by zxczxczbfg

[K^2]

Something like Sabre, wouldn't do you a whole lot of good on, say, Mars. A turboprop with really large blades, kind of along the lines of V-22, should be able to fly very efficiently there, however. I think that's the sort of thing that OP is talking about.

[LethalDose]

Maybe I didn't make myself clear in the OP: I'm not talking about an engine (rocket or turbine) that brings it's own oxidizing agent like the SABRE.

I'm talking about an engine that substitutes a local atmospheric gas (e.g. chlorine) in place of oxygen. It may not be a literal form of combustion (no oxygen), but there are atmospheric gases that can be exothermically reacted with fuel, the gas expands, and is expelled causing thrust.

Obviously, you would need separate engines for each atmosphere, but we've only got a few planets with atmospheres, so you'd only need a few different designs.

[K^2]

Ah. In that case, no dice. Anything in atmo that can serve as an efficient reducing agent would already have reduced something. The only known exception is Earth, and the only reason we have free oxygen in atmosphere is because of plants.

[DaveofDefeat]

I think it would be more plausible if you switched it around and brought your own oxidizer, having the jet engine use native gasses, such as hydrogen or methane, maybe...

[Nuke]

you could probibly have a nuclear reactor provide the thermal energy neccisary for a turbine engine to function, without the need to actually burn anything.

[Bunsen]

you could probibly have a nuclear reactor provide the thermal energy neccisary for a turbine engine to function, without the need to actually burn anything.

That can even be done with decay heat, if the power requirements are low enough. There's been a design or two for radiothermal-powered jets capable of flying around Titan.

[koshelenkovv]

Jet engines suck up atmosphere, and use the molecular oxygen to oxidize jet fuel and produce thrust. Currently, only Kerbin and Laythe have molecular atmospheric oxygen, while Jool, Duna, and Eve have atmospheres without atmospheric oxygen and jet propulsion is not possible on these worlds.

Jet engine sucks atmoshpere in, compreses it and then heats it to high temperature. Hot gas expands out of engine and creates thrust. The greater gas is compressed before heating and the higher temperature difference before and after heating is, the more it will expand and more thrust will be produced.

To make working jet you need to compress and to heat working body and then expel it out of the engine in one direction.

So, you can bring your own oxidiser and burn fuel in it to heat atmospheric gases. Or, for gas giants, you can bring only oxidizer and burn atmoshepic methane in it to heat unburnt rest of methane, which will act as working body. Or you can refrain from usage of any fuel and oxidizer and make jet with nuclear heating like one in http://en.wikipedia.org/wiki/Project_Pluto.

[Nuke]

whats cool is if you have a methane atmosphere, instead of bringing fuel, you just bring oxygen instead. i really wish ksp had more varied atmospheric compositions so we could do this, as opposed to, has oxygen or not has oxygen.

[lajoswinkler]

whats cool is if you have a methane atmosphere, instead of bringing fuel, you just bring oxygen instead. i really wish ksp had more varied atmospheric compositions so we could do this, as opposed to, has oxygen or not has oxygen.

Well oxygen is the fuel. Both compounds are fuels in an engine which uses them.

Your suggestion is pretty good.

[Warhorse]

Engine part corrosion won't be an insurmountable problem. Some US ICBMs used red fuming nitric acid as an oxidizer in Real Life . As an FYI, the Kethane mod already contains a jet engine that can use non-oxidizing atmospheres.

[K^2]

This is common misunderstanding of how jet engine works.

[Otto cycle, Brayton cycle...]

Earth atmosphere have only 21% oxygen in it.

That last bit is all you needed to say. But yeah, good catch. Nitrogen in the atmo is going to make a big difference in favor of a turbojet.

Point stands, though. If you are flying through an inert atmosphere, you can get better efficiency by having completely isolated turbine and compressor stages than trying to feed fuel and oxidizer into combustion chamber of a conventional jet engine.

[Tommygun]

It probably would be even simpler to go with a propeller that's power by a simple high efficiency piston engine or electric motor.

[koshelenkovv]

That last bit is all you needed to say. But yeah, good catch. Nitrogen in the atmo is going to make a big difference in favor of a turbojet.

Point stands, though. If you are flying through an inert atmosphere, you can get better efficiency by having completely isolated turbine and compressor stages than trying to feed fuel and oxidizer into combustion chamber of a conventional jet engine.

In real turbojet only small fraction of this 21% of oxygen is used in combustion. Most of oxygen comes out of engine unburnt.

Now simpler and shorter explanation came into mind:

Turbojet uses ambient air as reaction mass, while rocket carries all of it's reaction mass along.

So even if you carry both fuel an oxidizer, using turbojet still is more effective than using rocket.

Edited December 7, 2013 by koshelenkovv

[LethalDose]

The point of the engine that I was proposing was to avoid bringing an oxidizer with the craft, using local atmospheric gasses instead. K^2's response made a good point: Any atmospheric oxidizers would have reacted with local compounds and been consumed.

I like the idea of jets and electric turbines powered by other energy sources, though, for atmospheric propulsion.

Anyway, yeah, K^2's point pretty much makes the idea dead.

[Razor235]

Remember that you don't need to rely on thrust entirely from fuel. 80% of a conventional turbofan's thrust comes solely from the fan, so you'd only need fuel to get the fan spinning and to maintain its speed.

[K^2]

In real turbojet only small fraction of this 21% of oxygen is used in combustion. Most of oxygen comes out of engine unburnt.

A turbojet is going to burn through about half of the available oxygen, and afterburner will burn through almost all of the rest. For example, Concorde's Olympus 593 Mk 610 pulls 186kg of air per second. In dry cruise, it will inject 4.7kg of Jet A per second into that. So out of the 42.8kg of O₂, 18.8kg will be consumed every second in the dry stage. That's nearly half. With afterburner engaged, fuel consumption more than doubles, taking care of the remaining oxygen.

The purpose of the turbojet is to operate at high speeds, and if your overall exhaust velocity is low, you start losing efficiency at high speeds. To increase exhaust velocity, you have to increase the fuel-to-air ratio. This means that even on a commercial airliner, which is all about efficiency, even the dry stage will consume a lot more than a "small fraction" in cruise setting. While punching through the transonic region, the engine will run almost stoichiometric ratio.

Running a turbojet very lean is just pointless. A lean mixture will have a much lower combustion temperature, reducing the overall efficiency of the turbine. You are much, much better off having a turbofan design and simply allow the air to bypass the combustion chamber. That allows you to maintain high temperatures and increase reaction mass. The only reason a turbojet typically runs a bit lean is because you need a lot more thrust punching through the transonic and because temperatures the turbine can take are limited. So it makes more sense, both from perspective of engine longevity and fuel efficiency, to have an afterburner which lets you run a stoichiometric mix when you need it, at a cost of a slightly reduced thermodynamic efficiency of both stages.

When you go to even higher speeds, even that provision no longer applies, and you are looking at engines that run close to stoichiometric mix even in cruise. But then, you typically aren't going to be looking at turbines at all, but rather a ram jet of some sort.

Now simpler and shorter explanation came into mind:

Turbojet uses ambient air as reaction mass, while rocket carries all of it's reaction mass along.

So even if you carry both fuel an oxidizer, using turbojet still is more effective than using rocket.

I'm pretty sure I covered that in the first post in the thread. Except that running a turboject by mixing ambient atmosphere with fuel and oxidizer is a really inefficient idea. You are far better off running a turbine on just the oxidizer and fuel, without mixing in ambient air, and then using turbine's output to spin a compressor or a prop that uses ambient atmo as reaction mass.

[DaveofDefeat]

O there is also electric jet engines, which could be used on a multitude of atmospheres provided you had the electricity.

[Idobox]

Nuke, there are places with methane atmosphere, like Titan. Some other oxidizers might fare better than oxygen if you don't care for cost or pollution, but the good ones are usually unstable and thus difficult to store.

Lajoswinkler, I've never heard oxygen being called fuel. Isn't that word reserved for the part that is oxidized? The English language doesn't appear to have the words ergol or comburant (according to Firefox's dictionary), so I'm not really sure.

[Kryten]

Lajoswinkler, I've never heard oxygen being called fuel. Isn't that word reserved for the part that is oxidized? The English language doesn't appear to have the words ergol or comburant (according to Firefox's dictionary), so I'm not really sure.

'Fuel' in english means two different things in rocketry, in different contexts. In chemical rockets it does indeed refer to the part of the propellant that's oxidised, but in rocketry in general 'fuel' can refer to the material used to deliver the energy to the reaction mass (what actually leaves the rocket and causes thrust). For example, in NERVA type nuclear-thermal rocket uranium is the fuel and hydrogen is the reaction mass. By this definition, both the oxidiser and fuel of a chemical rocket are 'fuel' and the reaction mass is the combustion products.

[lajoswinkler]

Nuke, there are places with methane atmosphere, like Titan. Some other oxidizers might fare better than oxygen if you don't care for cost or pollution, but the good ones are usually unstable and thus difficult to store.

Lajoswinkler, I've never heard oxygen being called fuel. Isn't that word reserved for the part that is oxidized? The English language doesn't appear to have the words ergol or comburant (according to Firefox's dictionary), so I'm not really sure.

From a chemical standpoint, oxygen is not more special than the stuff it's reacting with. It oxidizes the reducer, i.e. it rips its electrons away, but you can say the reducer shoves electrons down oxygen's throat. It's a redox reaction and nobody is a winner. The "fuel and oxidizer" paradigm is an antropocentric view because we live in an oxgen atmosphere where reducers readily burn.

Oxygen is tough to oxidize, I don't think even fluorine tackles it, but you can think about chlorine. It is an oxidizer. It will oxidize most elements, yet it can itself be oxidized using fluorine.

[Nuke]

Nuke, there are places with methane atmosphere, like Titan. Some other oxidizers might fare better than oxygen if you don't care for cost or pollution, but the good ones are usually unstable and thus difficult to store.

i was thinking a hypothetical primarily methane atmosphere, titan is a poor example as it only has 1.6% methane, not sure if that is enough to base an effective engine on. its ridiculously easy to fly on titan though, you probibly dont even need to burn fuel at all, simply use electric propulsion.

[K^2]

titan is a poor example as it only has 1.6% methane, not sure if that is enough to base an effective engine on.

Nope. Won't even burn at that concentration.

Edit: Because it actually just came up elsewhere, the LEL for Methane is 5%. So you have to have at least that much methane in atmosphere to allow for combustion.

Edited December 10, 2013 by K^2