Can a jet engine benefit from nitrous oxide boost?

Question

I just read this wiki article again about the GM-1 boost system. This was used in Nazi planes during WW2. It injected Nitrous Oxide (N2O) into the supercharger of the piston engine. The idea was to get more oxidizer in the engine, allowing for higher fuel-injection and therefore higher engine power temporarily. This could serve to climb faster or take-off better or maintain high altitude.

My original interpretation of the GM-1 system and the related MW-50 system is that they're sorta like afterburners of their era. But that analogy is not perfect. Afterburners use the same jet fuel as used inside the engine, but sprayed outside the back. GM-1 used a different fuel, N2O (actually a monopropellant with more oxidizer built into the chemical than fuel), injected inside the engine.

What I want to know is, can N2O boost work in a jet engine by injecting directly into the combustion chamber (along with increased fuel injection)? Is that a good idea for military jet aircraft (compared to afterburners)? Is it even possible without overheating something?

I'm interested in turbojets and turbofans, and would even consider a centrifugal jet engine.

Answer 1

For the combustor, I say the answer is no, because what is limiting the fuel rate, isn't the amount of oxygen (gas turbine combustors run very lean), but the ability of the turbine materials to withstand a higher gas temperature entering the turbine. Hence, to temporarily increase engine thust, water injection into the combustor was used on early jet engines, to increase thrust during takeoff.

This sounds quite counter intuitive, but the explanation is based on thermodynamics. Naturally, the water evaporates into steam, which then gets heated to the temperature of the combustor exit temperature. This evaporation and heating process of the water and then steam, is drawn from the heat of the combustor, and so reduces the combustor temperature (for the same fuel flow). However, the engine does not need to add more fuel to make up for this cooling, at the same level of thrust, because the energy added to the steam is still available in the turbine, to be released as the combustion gas / steam mixture flows through the turbine, expands and cools. As it cools, it gives up this energy, which is effectively used to power the turbine. The remaining energy is then converted to thrust, as the flow expands through the final propelling nozzle. But now, because the turbine temperature is lower for the same thrust level, the fuel flow can be increased until the maximum turbine inlet temperature is reached, and so now providing more thrust.

So, rather than inject N2O into the main combustor, inject water.

For the afterburner (AB) however, the case is quite different, and I believe a performance benefit could well exist. AB combustion runs much closer to an equivalence ratio of 1 (about 0.96), i.e. burning almost 100% of the fuel level that can occur with complete combustion of all the available air. And unlike the main combustor, there are no real material temperature limits (cool bypass air exits along the inside of the AB duct surface). But, the question is, do you really want to do this, operationally? Is it better to use some of the planes tank capacity for some N2O, or just entirely for fuel? I suspect operationally it is better to be able to use the AB for longer, at a slightly lower thrust level, than shorter, for a higher thrust. I don't believe military aircraft use AB to out run missiles, so pure thrust isn't the only aim. But that's another (next) question....

Answer 2

Power can be augmented in turbojets through water injection or afterburning as the other answers explain. Each technique has its advantages and disadvantages but neither is well suited for more than a few minutes -- typically during takeoff, climb, and war emergency power application during combat.

Water injection works best at low levels where there is more available oxygen. Afterburners work at all altitudes but make inefficient use of fuel compared to the base turbojet (afterburning specific fuel consumption is about twice as much per pound of thrust). The only jet engines that I know combine both techniques are in the MiG-25 and -31.

There are at present no jet engines that advertise nitrous oxide injection but that doesn't mean it couldn't be useful in certain situations. The first decision is whether to inject liquid or gaseous nitrous: liquid is clearly better both because of denser storage and phase change charge cooling. This cooling effect is the only reason why you might use nitrous at takeoff, but since you'd be adding oxidizer there would still be temperature concerns. The better place to use nitrous would be at altitude for extra climb performance or speed dashing where the engine would have excess available fuel (up to sea level flows). In such instances nitrous injection would probably be preferable to afterburning because maximum thrust would be greater (nitrous can produce up to 70 percent more power compared to afterburning's 50 percent) AND it would produce that extra power without afterburning's gas-guzzling specific fuel consumption. That means not only going faster but also going further. So there's definitely a place for nitrous injection in jets.

But what would Putin do? H would add nitrous to the water injection and afterburners on his MiG-31s and MAYBE get an airplane that's finally faster than an SR-71.

Answer 3

Nitrous works well in a piston gas engine because the mixture is in a rather narrow range, and additional oxygen can be utilized with a mixture that gets enrichened, plus has added oxygen. In effect it creates the equivalent of an inlet boost, because it adds available oxygen.

The turbine engine runs much leaner, and has a wide mixture range which supports combustion. Since turbines normally have extra oxygen, they would not benefit from the addition of available oxygen. At least not the way that a piston engine with a enrichened fuel mixture would.

So the net effect of injecting nitrous into a jet engine would be very small, and at max power settings, it would still be small due to the inherent lean operation and associated oversupply of oxygen in the combustion chamber. In other words, adding oxygen when there is already plenty there provides little benefit.