Base jet engine performance off the intake

[wizzlebippi]

Right now in KSP, jet engine performance is completely arbitrary. The turbofan and RAPIER engines mysteriously reach peak thrust at 1000m/s (about mach 3 at sea level ISA) and produce only half that at 0m/s. No jet engine in existance does that! The SR-71 could go Mach 3+, but with 50% efficiency at takeoff, it would struggle just to get off the ground.

What I propose is basing thrust off intake "efficiency". For example, the radial and circular intakes are only efficient up to 500-600m/s where the ram air intake is worthless at low speeds, but really comes alive at 400-500m/s and allows you to go as fast as you want. This would at least better represent reality where fighter jets get plenty of thrust on the ground and at high speed due to variable geometry intakes and nozzles.

If you really want to get technical with it, any mechanical jet engine shouldn't allow you to go faster than about 1200-1500m/s. This is because the compressor can't eat air moving at faster than 0.3 Mach. Beyond that, you could add a ramjet engine for additional speed, though I don't think it's very necessary for KSP.

[Dodgey]

Minor point. You do know that jet airliners cruise at around mach 0.8 ish right?

And in your last paragraph when you say engine don't you mean intake?

[pa1983]

Well 80% off the thrust from the SR-71 blackbird was generated by the afterburners at mach 3 due to the ram air effect in combination with the bypass air around the engines core. Only 20% of the thrust came from the jet engines. So efficiency is relative to what you mean, efficiency like more power and higher speeds or better fuel consumption. But sure I think the SR-71 blackbird was more efficient in all regards at mach 3.2 then any lower speed. But it still needed to go fast enough to generate enough trust to go even faster Chicken in the egg problem.

[wizzlebippi]

@Dodgey. I meant exactly what I said. The intake diameter increases to slow air down before it hits the fan and air doesn't necessarily take a direct path into the core on a modern multi-stage engine, slowing it further. Also, many airliners on trans oceanic flights cruise at closer to 0.9 Mach.

I also meant engine. A ramjet engine has no moving parts because air is compressed aerodynamically.

@pa1983. Yes, the SR-71 did rely on afterburners, but not to the extent you're thinking. They were used on takeoff for extra thrust, though the aircraft had 65,000lbs of thrust without them, and for passing Mach 1. Beyond that, they burned too much fuel for continuous operation.

Here's a picture of how air moved through the SR-71's engines.

http://en.wikipedia.org/wiki/File:SR71_J58_Engine_Airflow_Patterns.svg

[pa1983]

Yea I have seen that picture before, read the wiki page and have some old SR-71 dockumentarys around here some where. But it clearly sees that the bypass air is used by the afterburner at mach 3+ to generate most of the thrusts. I know engineers in interviews describe the engines workings almost identically to the wiki page.

[wizzlebippi]

I obviously screwed up this morning when I posted. Yes, when at high enough speeds the turbojet core of the SR-71's engines was near useless and the afterburner served as the flameholder for the ramjet. I stand corrected.

But my point still stands. There is no reason why a turbofan jet engine would only put out 50% of it's rated thrust at takeoff and hit max thrust at a speed that mechanical jet engines largely become useless at. If you're going to schedule thrust off speed, there should be a reason for it and intake selection makes sense.

[Nemrav]

You know, jet engines already have performance issues when intake air is low (high atmosphere) and that the advanced jet engine gets much higher speeds higher up....

But yes jet engines aren't too realistic right now are they..

[wizzlebippi]

I'm not going for pure realism, just realistic tendencies. If ksp is going to be used to educate people on aircraft and spaceflight, parts in the game need to behave in a realistic manner. For example, how thrust scales with altitude in ksp isn't great, but it's understandable. The engine only needs so much air and the extra intakes just add drag. In the real world you can't just strap extra intakes to your aircraft to go higher, and you do loose thrust with altitude just because ambient pressure decreases.

This is one aspect of jet engines in ksp that has no bearing on reality, and I'd like to see it fixed.

[pa1983]

Yea well I brought up the KSP jet engine affair before and I never got the devs attention the last 4-5 updates. I hardly play KSP any more. Spent a few days in 0.23 when it came out but thats about it. Im not doing spaceplanes any more either. Pretty much exploited the game to its fullest and Im just waiting for a total over hull on aerodynamics and engines. Mods dont realy do it for me either since they are still limited by KSP to a big extent.

The new sabers is a good addition but intakes needs work. I have written about it in many posts before. Theres no difficulty from a programmers view to make intakes realistic and stop intake abuse that I can see.

I dont see why any more suggestions on jets and intake will make any difference. Devs will get around to it when and if they want to and thats about it.