Jet engines at higher atmospheric pressures

[Gordon Fecyk]

I'm not sure what tags apply here.

I'm tuning my Eve engine add-on. Unlike rockets where you can mess with the nozzle to deal with higher ambient pressure, jets have to draw and exhaust that higher pressure air. I do know that jet engine specific impulse doesn't change. What I don't know is if thrust would be better or worse with thicker air, though. And there's the consideration that I might not need as much speed in thicker air to get as much lift, so I wouldn't need as much thrust. Maybe I want a pressure-based governor.

I am trying out NASA's jet engine calculators and their EngineSim simulator but they seem designed starting with 101.3 kPa air and don't seem to have settings for higher ambient pressure.

Assume an older, simpler jet engine like the Junkers Jumo that powered the Me262 is in use, and assume an earth-like atmosphere but higher ambient pressure. What effect would higher pressures have on this engine? Assuming it doesn't come apart, or melt.

Intuitively I'm thinking this would give more thrust at speeds below Mach 0.5 at first, since it would pull and exhaust more air.

Edited July 15, 2017 by Gordon Fecyk
Changed link to current EngineSim edition

[Van Disaster]

You can see what happens in EngineSim by keeping the speed/altitude/temperature the same & varying the pressure - it might need the undergrad version to let you vary pressure, I'm not sure. Even that won't go above 137k Pa but you can see how the engine behaves with only varying ambient pressure. Generally yes, more thrust because of higher mass flow ( assuming you can provide enough fuel, I guess! ).

[Gordon Fecyk]

So far, more massive air does mean more thrust, at least below supersonic speeds, and also more fuel consumption to match.

The undergrad version appears to support pressures higher than 130 kPa, so I can use that. A user needs to add the site to Java's security exception list, but it otherwise appears to work in Java 8 on supported browsers for anyone else looking to try it out. Looking at how high the temperatures would get, the engines themselves would seem to be made of explodium. Interestingly, when the ambient air gets warmer, thrust decreases noticeably.

[MatterBeam]

I think it will have higher efficiency, since you are getting more air with a smaller inlet area (drag).
The Isp will be higher, since you are moving more air for little extra fuel, as is the case for turbofans. 
I'm not so sure about the thrust though. Your external atmosphere will be at a higher pressure, you'll need a lot of overpressure in the combustion chamber to generate positive thrust. 

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[Van Disaster]

20 hours ago, Gordon Fecyk said:

So far, more massive air does mean more thrust, at least below supersonic speeds, and also more fuel consumption to match.

The undergrad version appears to support pressures higher than 130 kPa, so I can use that. A user needs to add the site to Java's security exception list, but it otherwise appears to work in Java 8 on supported browsers for anyone else looking to try it out. Looking at how high the temperatures would get, the engines themselves would seem to be made of explodium. Interestingly, when the ambient air gets warmer, thrust decreases noticeably.

Warmer air is less dense, so yeap. There's a standalone version of the undergrad package too.

I haven't looked at a turbofan, that might be interesting.

 

[Gordon Fecyk]

7 hours ago, MatterBeam said:

I'm not so sure about the thrust though. Your external atmosphere will be at a higher pressure, you'll need a lot of overpressure in the combustion chamber to generate positive thrust.

Yes, this is similar to the nozzle problem that rocket engines have with higher ambient pressures. I'm glad you raised this.

I noticed that jet engines in KSP have only a single line in their atmosphereCurve for specific impulse. Generally jets are speed (velCurve) and pressure (atmCurve) restrained in the game, but this might not be realistic in thicker air. I wonder what happens if I start changing atmosphereCurve as well. Or maybe a combination of changing atmCurve for more thrust, but also changing atmosphereCurve for lower specific impulse at higher pressures, to represent the engine having to work harder to get the needed overpressure.

This is what I'm hoping EngineSimU will tell me when I have more time to play with it. Part of my challenge will be modeling the game engines in EngineSimU, and then playing with higher pressures and temperatures.

I think I have the basic real-world science answers now. I'll still watch this thread, but I'll take game-specific questions back to the add-on release thread.

I'm having trouble making the stand-alone edition work on my PC. This is a Java security thing and I'll deal with that, though. Java 8 does not like adding security exceptions for file:/// URLs, so it might be better just to use NASA's online version.

[Gordon Fecyk]

It turned out I didn't have to fuss too much about figuring out jet engine behaviour in higher pressures and densities. Advanced Jet Engines apparently calculates engine thrust on the fly, based on the environment, using NASA's EngineSim code directly. The engines do respond to higher pressures as expected, almost ripping themselves off the aircraft they're attached to when the air is thick enough.

[mikegarrison]

IRL, jet engines routinely experience what is effectively higher-than-ambient pressure, due to ram air effect. However, engines designed for really high speeds do have to take this into account, because the higher inlet temperature that goes along with it can be dangerous for their hot parts.

But yes, if you could get the higher pressures without the higher temperatures, the engines would no doubt produce higher thrusts. You would have to make sure the engine was designed for that level of thrust and pressure in order to operate safely.