Jet Engine Burn Out

[locustgate]

So what is the desired burn out for turbojet?

I finally got a plane to get out of the atmosphere. My turbo engines suffocated around 20k with 1 shock cone and 6 Shock intakes per engine and 6 for a rapier engine, is this good or can I increase the range with radial?

Do the air intakes have to be connected to the engines, similar to rocket fuel, or does it not matter, similar to monopropelent.

Edited January 5, 2015 by locustgate

[Pecan]

Easy answer first - air is treated like monopropellant and is automatically taken from anywhere so you don't need 'air lines' in the same way as fuel flows.

The altitude you can run your engines on depends on the speed and angle of attack, so how much air each intake is getting. Do it right and you can run the engines all the way to space (69km). Personally, I find that spamming 8 RAM-intakes per engine makes that the easy way to do things but whether it's worth doing is a different matter.

Have a look at this thread to decide which intake(s) to use: http://forum.kerbalspaceprogram.com/threads/106114-Stock-KSP-90-intake-comparo-for-SSTO-turbojets

However you do it the benefit of jets is fuel-efficiency, so you want to get as fast as possible using them. That, in turn, means getting as high as possible to reduce atmospheric drag. You won't be able to accelerate much above 30km but by then you should be at, or near, orbital velocity so the rest of 'up' comes automatically. The important thing is speed - if you get that physics will take you up. If you aim for altitude first you'll never get fast enough.

ETA: Also note that it is very important to throttle-back jets to keep them working. Whenever they (are due to) flame-out throttle-back a little bit and they will need less air to keep going. 70% thrust is way better than 100% deadweight. As you get higher and the air gets thinner you will need to go faster to keep them supplied but, at the same time, the drag gets lower so you can go faster. It's the balancing-act that makes jet-launched vehicles 'tricky' to fly (but it all comes together with practice, like most things).

Edited January 5, 2015 by Pecan

[locustgate]

Easy answer first - air is treated like monopropellant and is automatically taken from anywhere so you don't need 'air lines' in the same way as fuel flows.

The altitude you can run your engines on depends on the speed and angle of attack, so how much air each intake is getting. Do it right and you can run the engines all the way to space (69km). Personally, I find that spamming 8 RAM-intakes per engine makes that the easy way to do things but whether it's worth doing is a different matter.

Have a look at this thread to decide which intake(s) to use: http://forum.kerbalspaceprogram.com/threads/106114-Stock-KSP-90-intake-comparo-for-SSTO-turbojets

However you do it the benefit of jets is fuel-efficiency, so you want to get as fast as possible using them. That, in turn, means getting as high as possible to reduce atmospheric drag. You won't be able to accelerate much above 30km but by then you should be at, or near, orbital velocity so the rest of 'up' comes automatically. The important thing is speed - if you get that physics will take you up. If you aim for altitude first you'll never get fast enough.

ETA: Also note that it is very important to throttle-back jets to keep them working. Whenever they (are due to) flame-out throttle-back a little bit and they will need less air to keep going. 70% thrust is way better than 100% deadweight. As you get higher and the air gets thinner you will need to go faster to keep them supplied but, at the same time, the drag gets lower so you can go faster. It's the balancing-act that makes jet-launched vehicles 'tricky' to fly (but it all comes together with practice, like most things).

I was able to get up 1km/s, I actually started to get burn on the bottom part of the jet.

[Pecan]

I was able to get up 1km/s, I actually started to get burn on the bottom part of the jet.

Faster! 2km/s at 30km :-)

Yep, ok 1km/s at 20km, but then level-out to climb comparatively slowly. As your engines are thrusting mostly horizontally most of your acceleration will also be horizontal and as you get higher (air thinner, Terminal Velocity higher, drag lower) you should aim for something like another 100m/s horizontal velocity per 1km altitude; 1,100m/s at 21km, 1,200m/s at 22km, etc. to 2,000m/s at 30km - roughly. If your horizontal speed isn't keeping up with your vertical speed then you're climbing too fast and not getting the most from your jets - which means needing to use more rocket-power, which is less efficient. If you can accelerate faster than that horizontally then you can afford to ascend faster as well, but the idea is to try to get to/above orbital velocity (~2,200m/s) before you get so high you have to throttle-back the jets so much that they're not really helping any more.

Spamming intakes the way I do helps with this because you don't have to throttle-back so much or so early but if you fly a jet-powered vehicle properly (I must practice more!) you can get away with way less than the 8 I suggested; it all comes down to regulating your vertical speed to get the best horizontal acceleration.

Edited January 5, 2015 by Pecan

[tsotha]

Depends on what you're doing. If you're very patient you can get a light, high lift plane into a 150km/35km orbit purely on the jet engine. That's with one turbojet and two ram intakes. The key is the faster you go the more air goes into your intakes, even at higher altitudes - you can still get some thrust over 50km if you baby it. A plane like that works just fine for rescuing stranded kerbals and dumping out light satellites.

Of course you can't haul a full orange tank into orbit with one turbojet. Building large aircraft isn't a trivial undertaking, and making large aircraft with good performance usually involves stacked control surfaces and clipped intakes.