Is Air Hogging Possible In Real Life?

[quasarrgames]

That's my question! Say you had a radial intake as well as an inline one. At low altitudes when the extra intake air isn't needed, the air from the radial intake could be directed along the side of the engine, cooling it, and then it could flow out the back of the aircraft. For higher altitudes when there was less air, the air from the intake could be redirected into the engine, adding to its air supply.

Is this a sound idea, or would it not work? I don't know much about IRL spaceplanes really, and i know that real jet engines behave a little differently than KSP jet engines (although i'm not sure what the implications of that are). This was just a random idea i came up with.

Edited July 29, 2014 by quasarrgames

[SargeRho]

I think the engine of the SR-71 pretty much does that.

Also the Harrier has intake flaps that open during VTOL to increase the ammount of air the engine gets.

[Eric S]

It isn't possible to the extent that it is in KSP because in reality, overlapping air intakes aren't going to bring in twice the air as a single intake, and the more the overlap, the closer to a single intake the effectiveness would become. That said, high altitude craft do tend to have huge and/or multiple air intakes, so the concept has its parallels in the real world, as SargeRho pointed out.

[Nuke]

i think this is the idea behind turbofan engines. most of the intake air doesn't even go into the combustion chamber, it just goes around it. it doesn't make your engine any faster but it pretty much halves fuel consumption.

[Whirligig Girl]

I don't think it would be infinite. there's a specific ideal ratio of oxygen to kerosene, so after you get to that limit, airhogging ceases to work.

[LaytheAerospace]

I suspect the added complexity (and weight) of the engine wouldn't really justify it. If you're looking for high altitude, high speed, air-breathing action, you're probably better off working on (SC)RAMJETs than trying to force more air into a turbojet with additional intakes. Or doing something like the previously mentioned SR-71 and directing more air into the same intake.

[sgt_flyer]

We don't need air hogging in real life we face another problem in real life - the intake air heating due to compression. - if the intake air is too hot, it 'll drastically reduce the engine's effectiveness. (Because the mixed fuel + air combustion can only reach a maximum temperature, and thus expand. the higher the intake temps are, the lower you can gain in exhaust gas expansion - less thrust.

That's the ramjet's limitation - as they need to slow down the intake air to subsonic speeds to achieve good compression, the heat from compression / ramming will be the limit. (+ high drag )

Scramjets try to limit this effect by using supersonic air intake instead of subsonic - but that comes with difficulties for mixing the fuel with the supersonic air and igniting it. However, using supersonic air (around mach2 in the combustion chamber) means less drag from the intakes, less compression heat - so they can achieve higher speeds than ramjets.

one of the problems of ramjets / scramjets though, is their bad TWR. (You need turbopumps to keep the fuel flowing - and you need to drive those pumps - which is difficult without severly increasing the plane's weight (as there's no moving part in a (sc)ramjet, you don't have a turbine to drive the pumps (pumps also needed for the fuselage's cooling systems at those speeds)

The last possibily to fly at high speeds, is Skylon's way : supercooling the air in the intakes - so you can use the air at a much wider range of speeds and temperatures. That required extremely effective and lightweight coolers - which is what reaction engines ltd managed to create

Edited July 30, 2014 by sgt_flyer

[quasarrgames]

Okay then! But what if you used this same system with ramjets or scramjets instead of turbojets? Would it still work?

[softweir]

Probably not. The first problem is that the air taken in by the second intake can't be used to cool the engine - and in any case it won't need it at low speed! The second problem is that at high speeds the primary intake on a scramjet gets WAY more air than it needs and may have to be reduced in area to limit it's input to what the engine can operate with.

[Yourself]

i think this is the idea behind turbofan engines. most of the intake air doesn't even go into the combustion chamber, it just goes around it. it doesn't make your engine any faster but it pretty much halves fuel consumption.

Mmm, not really. In fact, the most powerful jet engines ever created were high-bypass turbofans. They actually generate quite a bit of thrust and most of it is generated by the fan rather than the exhaust (which is what makes them high-bypass rather than low-bypass). This is kind of why they tend to be more fuel efficient, rather than trying to derive thrust directly from the combustion products exiting the back of the engine (like a rocket), the engine is instead designed to use more of the energy from the combustion to drive the fan which is producing the thrust.

A turbofan is really just a ducted fan that happens to be driven by a gas turbine. Hence the name.

The downside is that giant fan tends to be kind of heavy, so they're beaten in power-to-weight by low-bypass turbofans (which you'll see more often on aircraft that need to be maneuverable or need to operate at transonic or supersonic speeds).