Is there a 20,000m ceiling to the tubojet with the new physics?

[davidparks21]

I'm trying out v1 (1.02) for the first time. My first attempt at a small SSTO using a turbo jet (and a couple ant engines) didn't go nearly as well as it has in past versions.

Right at about 20,000m the turbojet's thurst drops off very quickly.

I'm assuming this due to the new physics and is realistic and accurate. But I want to ask in case there is some trick I need to know about. Previously I could fly upward of 30,000m given a couple of intakes to each turbojet. This time I was able to get my speed up to just shy of 1000m/s at around 18,000m giving me nearly 300 in thrust. But as soon as I hit ~20,000m the thrust drop drops out, even though I'm still got my speed in the optimal range and seem to have plenty of excess air intake.

I see threads that talk about peak performance of the turbo jet at 1000m/s speed, and about more drag and such. But I didn't see any threads that explain why the turbojet loses thrust at 20,000m, given that I was near ideal speed and seemed to have plenty of air intake available (under 50% air intake usage).

If drag caused me more challenge, or thrust dropped out due to speed I would understand perfectly well, neither are the case though, so I need more explanation as to where apparent 20,000m ceiling comes from. I notice a new piece of tech called the "Engine Pre-cooler" (I don't have it yet so haven't tried) but wondered if it was more than just another air intake?

[cybersol]

Thrust from jet engines is affected by two spline curves post 1.0. The first curve changes the thrust based on the mach speed, and the second changes the thrust based on atmospheric pressure. What you are seeing is the steep part of the curve where the thrust goes to zero very rapidly after a certain altitude. For an even bigger example of the altitude effect, check out the basic jet engine.

[Foxster]

I haven't found a way around this - yet. Maybe something that will get tweaked in 1.0.3.

An SSTO to orbit profile is now a bit funky. I burn at 45° at max speed, ignoring terminal velocity, and at about 15km light the orbital engine(s) (Isp should be OK by now) and then keep going until the jets die. You should have a pretty good Ap at that point and you can finish the orbit off.

[Red Iron Crown]

I've had better results with the RAPIER than the turbojet in 1.0.x. It's just better at high/fast flight, and includes a rocket engine to boot. I only use turbojets on an SSTO if it needs a bit more thrust when lower and slower to get the RAPIERs up to their sweet spot.

Gone are the days of setting your Ap out of atmosphere on the other side of Kerbin with stock jets, more time on the rockets is definitely required than before.

[eddiew]

An SSTO to orbit profile is now a bit funky. I burn at 45° at max speed, ignoring terminal velocity, and at about 15km light the orbital engine(s) (Isp should be OK by now) and then keep going until the jets die. You should have a pretty good Ap at that point and you can finish the orbit off.

That's kind of low... Most of mine save the rockets until 20km (with turbojets) or 25km (with rapiers). Whatever works though I guess

I've had better results with the RAPIER than the turbojet in 1.0.x. It's just better at high/fast flight, and includes a rocket engine to boot. I only use turbojets on an SSTO if it needs a bit more thrust when lower and slower to get the RAPIERs up to their sweet spot.

Gone are the days of setting your Ap out of atmosphere on the other side of Kerbin with stock jets, more time on the rockets is definitely required than before.

I struggle to make designs with just rapiers - usually seem to need 1 turbojet to get past the mach barrier. But it can be 1 turbo among 4 rapiers, and it can be turned off once you're over 600m/s to save the air for the rapiers, which will be kicking out oodles of thrust by that point

It's possible to get an AP outside of the atmosphere, with a really steep ascent - but it's never turned out to be a good thing in any of my attempts, probably because you can't go enough outside the atmosphere to give a nerva time enough to circularize.

[Amram]

regarding turbojets in 1.0, I've got a stock aircraft that can do 21000 @ 1150ish sustained, but to get up there takes forever, very shallow climb beyond 20,000, and very slow accel beyond 1100 while up there.

That said, yeah, they killed air hogging and gave the rapier a niche by tying the output to altitude, add more intakes all you want, won't do anything unless the intakes your using are under-performing. You won't get much higher on jets I don't think, and good luck keeping the turbojet alive above 25000 on its own.

Did a quick bit of math in excel and found the following for the turbojet:

the following assumes that the engine shuts down regardless at 10% of "nominal thrust", that being the value in the cfg which is 180kN, so when it reaches 18kN, it shuts down. So I end it at 26600. I could be totally wrong on the shut down point, so take that with a grain of salt. Mind you, its pretty useless when it only yields 18kN anyways, so yeah.

velocity is 1100m/s, speeds are in percentage of maximum, which is 964.787kN at 1100m/s at sea level.

Because they are percentages, and based on the altitude data, they should hold for any speed, though if you are slower, then you can't get as high before the engine starves and shuts down.

Its rather approximate, because I'm not trying to deal with slope smoothing, rather, I just take the upper and lower value, and interpolate the result

A quick example process before the table:

2000m is a pressure of e^(-2000/5000) = 0.67032

from the turbojet cfg: 1=1, 0.5=0.6, so my upper atmos value is 1, lower is 0.5, and upper thrust is 1 and lower is 0.6

(atmo - lower_atmo)/(upper_atmo - lower_atmo) * (upper_thrust - lower_thrust) + lower_thrust

(0.67032 - 0.5)/(1 - 0.5) * (1 - 0.6) + 0.6

(0.17032)/(0.5) * (0.4) + 0.6

0.34064 * (0.4) + 0.6

0.136256 + 0.6

thrust factor = 0.736256

did the same for mach number, I assume mach in KSP is a flat 340m/s?

then it was just multiplying the two factors.

[table]

[tr][th]Altitude[/th][th]Thrust[/th][/tr]

[tr][th]0m[/th][th]100.0%[/th][/tr]

[tr][th]200[/th][th]96.879[/th][/tr]

[tr][th]400[/th][th]93.864[/th][/tr]

[tr][th]600[/th][th]90.968[/th][/tr]

[tr][th]800[/th][th]88.186[/th][/tr]

[tr][th]1000[/th][th]85.512[/th][/tr]

[tr][th]1200[/th][th]82.943[/th][/tr]

[tr][th]1400[/th][th]80.476[/th][/tr]

[tr][th]1600[/th][th]78.104[/th][/tr]

[tr][th]1800[/th][th]75.826[/th][/tr]

[tr][th]2000[/th][th]73.637[/th][/tr]

[tr][th]2200[/th][th]71.534[/th][/tr]

[tr][th]2400[/th][th]69.514[/th][/tr]

[tr][th]2600[/th][th]67.572[/th][/tr]

[tr][th]2800[/th][th]65.707[/th][/tr]

[tr][th]3000[/th][th]63.915[/th][/tr]

[tr][th]3200[/th][th]62.193[/th][/tr]

[tr][th]3400[/th][th]60.539[/th][/tr]

[tr][th]3600[/th][th]59.62[/th][/tr]

[tr][th]3800[/th][th]59.058[/th][/tr]

[tr][th]4000[/th][th]58.519[/th][/tr]

[tr][th]4200[/th][th]58.001[/th][/tr]

[tr][th]4400[/th][th]57.503[/th][/tr]

[tr][th]4600[/th][th]57.024[/th][/tr]

[tr][th]4800[/th][th]56.565[/th][/tr]

[tr][th]5000[/th][th]56.123[/th][/tr]

[tr][th]5200[/th][th]55.699[/th][/tr]

[tr][th]5400[/th][th]55.291[/th][/tr]

[tr][th]5600[/th][th]54.899[/th][/tr]

[tr][th]5800[/th][th]54.523[/th][/tr]

[tr][th]6000[/th][th]54.161[/th][/tr]

[tr][th]6200[/th][th]53.814[/th][/tr]

[tr][th]6400[/th][th]53.48[/th][/tr]

[tr][th]6600[/th][th]53.16[/th][/tr]

[tr][th]6800[/th][th]52.851[/th][/tr]

[tr][th]7000[/th][th]52.555[/th][/tr]

[tr][th]7200[/th][th]52.271[/th][/tr]

[tr][th]7400[/th][th]51.998[/th][/tr]

[tr][th]7600[/th][th]51.735[/th][/tr]

[tr][th]7800[/th][th]51.483[/th][/tr]

[tr][th]8000[/th][th]51.24[/th][/tr]

[tr][th]8200[/th][th]51.008[/th][/tr]

[tr][th]8400[/th][th]50.784[/th][/tr]

[tr][th]8600[/th][th]50.569[/th][/tr]

[tr][th]8800[/th][th]50.362[/th][/tr]

[tr][th]9000[/th][th]50.164[/th][/tr]

[tr][th]9200[/th][th]49.664[/th][/tr]

[tr][th]9400[/th][th]47.856[/th][/tr]

[tr][th]9600[/th][th]46.118[/th][/tr]

[tr][th]9800[/th][th]44.448[/th][/tr]

[tr][th]10000[/th][th]42.843[/th][/tr]

[tr][th]10200[/th][th]41.302[/th][/tr]

[tr][th]10400[/th][th]39.821[/th][/tr]

[tr][th]10600[/th][th]38.398[/th][/tr]

[tr][th]10800[/th][th]37.031[/th][/tr]

[tr][th]11000[/th][th]35.717[/th][/tr]

[tr][th]11200[/th][th]34.455[/th][/tr]

[tr][th]11400[/th][th]33.243[/th][/tr]

[tr][th]11600[/th][th]32.078[/th][/tr]

[tr][th]11800[/th][th]30.958[/th][/tr]

[tr][th]12000[/th][th]29.883[/th][/tr]

[tr][th]12200[/th][th]28.85[/th][/tr]

[tr][th]12400[/th][th]27.857[/th][/tr]

[tr][th]12600[/th][th]26.903[/th][/tr]

[tr][th]12800[/th][th]25.987[/th][/tr]

[tr][th]13000[/th][th]25.106[/th][/tr]

[tr][th]13200[/th][th]24.26[/th][/tr]

[tr][th]13400[/th][th]23.447[/th][/tr]

[tr][th]13600[/th][th]22.666[/th][/tr]

[tr][th]13800[/th][th]21.916[/th][/tr]

[tr][th]14000[/th][th]21.195[/th][/tr]

[tr][th]14200[/th][th]20.503[/th][/tr]

[tr][th]14400[/th][th]19.837[/th][/tr]

[tr][th]14600[/th][th]19.198[/th][/tr]

[tr][th]14800[/th][th]18.583[/th][/tr]

[tr][th]15000[/th][th]17.993[/th][/tr]

[tr][th]15200[/th][th]17.426[/th][/tr]

[tr][th]15400[/th][th]16.881[/th][/tr]

[tr][th]15600[/th][th]16.292[/th][/tr]

[tr][th]15800[/th][th]15.653[/th][/tr]

[tr][th]16000[/th][th]15.039[/th][/tr]

[tr][th]16200[/th][th]14.449[/th][/tr]

[tr][th]16400[/th][th]13.883[/th][/tr]

[tr][th]16600[/th][th]13.339[/th][/tr]

[tr][th]16800[/th][th]12.816[/th][/tr]

[tr][th]17000[/th][th]12.313[/th][/tr]

[tr][th]17200[/th][th]11.83[/th][/tr]

[tr][th]17400[/th][th]11.366[/th][/tr]

[tr][th]17600[/th][th]10.921[/th][/tr]

[tr][th]17800[/th][th]10.492[/th][/tr]

[tr][th]18000[/th][th]10.081[/th][/tr]

[tr][th]18200[/th][th]9.686[/th][/tr]

[tr][th]18400[/th][th]9.306[/th][/tr]

[tr][th]18600[/th][th]8.941[/th][/tr]

[tr][th]18800[/th][th]8.59[/th][/tr]

[tr][th]19000[/th][th]8.254[/th][/tr]

[tr][th]19200[/th][th]7.93[/th][/tr]

[tr][th]19400[/th][th]7.619[/th][/tr]

[tr][th]19600[/th][th]7.32[/th][/tr]

[tr][th]19800[/th][th]7.033[/th][/tr]

[tr][th]20000[/th][th]6.758[/th][/tr]

[tr][th]20200[/th][th]6.493[/th][/tr]

[tr][th]20400[/th][th]6.238[/th][/tr]

[tr][th]20600[/th][th]5.993[/th][/tr]

[tr][th]20800[/th][th]5.758[/th][/tr]

[tr][th]21000[/th][th]5.533[/th][/tr]

[tr][th]21200[/th][th]5.316[/th][/tr]

[tr][th]21400[/th][th]5.107[/th][/tr]

[tr][th]21600[/th][th]4.907[/th][/tr]

[tr][th]21800[/th][th]4.715[/th][/tr]

[tr][th]22000[/th][th]4.53[/th][/tr]

[tr][th]22200[/th][th]4.352[/th][/tr]

[tr][th]22400[/th][th]4.181[/th][/tr]

[tr][th]22600[/th][th]4.017[/th][/tr]

[tr][th]22800[/th][th]3.86[/th][/tr]

[tr][th]23000[/th][th]3.709[/th][/tr]

[tr][th]23200[/th][th]3.563[/th][/tr]

[tr][th]23400[/th][th]3.423[/th][/tr]

[tr][th]23600[/th][th]3.289[/th][/tr]

[tr][th]23800[/th][th]3.16[/th][/tr]

[tr][th]24000[/th][th]3.036[/th][/tr]

[tr][th]24200[/th][th]2.917[/th][/tr]

[tr][th]24400[/th][th]2.803[/th][/tr]

[tr][th]24600[/th][th]2.693[/th][/tr]

[tr][th]24800[/th][th]2.587[/th][/tr]

[tr][th]25000[/th][th]2.486[/th][/tr]

[tr][th]25200[/th][th]2.388[/th][/tr]

[tr][th]25400[/th][th]2.295[/th][/tr]

[tr][th]25600[/th][th]2.205[/th][/tr]

[tr][th]25800[/th][th]2.118[/th][/tr]

[tr][th]26000[/th][th]2.035[/th][/tr]

[tr][th]26200[/th][th]1.956[/th][/tr]

[tr][th]26400[/th][th]1.879[/th][/tr]

[tr][th]26600[/th][th]1.805[/th][/tr]

[/table]

[Red Iron Crown]

It's possible to get an AP outside of the atmosphere, with a really steep ascent - but it's never turned out to be a good thing in any of my attempts, probably because you can't go enough outside the atmosphere to give a nerva time enough to circularize.

Oh yes, that's why I specified "on the other side of Kerbin". I've found I'm able to get the Ap up to about 50km/1 minute before the jets run out of air, a low TWR nuclear stage is then sufficient to raise the Ap the rest of the way and circularization is only a very short burn.

Here's a multistage setup that uses that ascent profile, not an SSTO but the same principles apply:

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[KerikBalm]

I haven't found a way around this - yet. Maybe something that will get tweaked in 1.0.3.

An SSTO to orbit profile is now a bit funky. I burn at 45° at max speed, ignoring terminal velocity, and at about 15km light the orbital engine(s) (Isp should be OK by now) and then keep going until the jets die. You should have a pretty good Ap at that point and you can finish the orbit off.

I find its much better to climb at 30, then drop down to about 15 to break mach 1, then climb at 20 to get to as fast as you're going to get on jets.

This should get your apoapsis above 30km, where air resistance will start to become quite small, and you should be going 1,300 surface speed, and rockets should be able to accelerate you to orbital speed while pushing up your apoapsis.

I've had better results with the RAPIER than the turbojet in 1.0.x. It's just better at high/fast flight, and includes a rocket engine to boot. I only use turbojets on an SSTO if it needs a bit more thrust when lower and slower to get the RAPIERs up to their sweet spot.

Gone are the days of setting your Ap out of atmosphere on the other side of Kerbin with stock jets, more time on the rockets is definitely required than before.

Well, I'm doing the career grind now, and all my SSTOs use turboramjets, simply because rapiers are far down the tree and require another very expensive research facility upgrade. They do provide more thrust at mach 1, and I think a turbo-rapier mix could be fine.

And good riddance to those ridiculous days!

You forgot to mention having an apoapsis on the other side of the planet, well outside of the atmosphere, and a perapsis very close by, at abut whatever altitude you happen to be flying

120x50km "orbits" on jets alone.... ridiculous.

Oh yea, and those jets could get 40,000 Isp... wunderbar!

[NathanKell]

It's density (in atmospheres) not pressure, btw.

The sweet spot on the TRJ's curves are Mach 3 (max multiplier, 5.8) and density = 0.045 (curve) * 1.225 = 0.055 kg/m^3. (about 18-19km). Above that, you start to run out of air to combust, so if you keep climbing (and staying at Mach 3) eventually you'll have so little air density your jet will flame out. Ceiling depends on lift too of course, as well as drag: you want the minimum drag possible, which may mean more wings so you fly at a lower AoA.

[cybersol]

It's density (in atmospheres) not pressure, btw.

It's not related to KSP, but 1 atm = 1 atmosphere is actually a standard unit of pressure (force/area).

[Shrike99]

Oh yes, that's why I specified "on the other side of Kerbin". I've found I'm able to get the Ap up to about 50km/1 minute before the jets run out of air, a low TWR nuclear stage is then sufficient to raise the Ap the rest of the way and circularization is only a very short burn.

Here's a multistage setup that uses that ascent profile, not an SSTO but the same principles apply:

http://imgur.com/a/5LY1C

Might i ask if that was inspired by my entry into the stock payload fraction contest?

If so i am very happy to see someone expanding the idea, new techniques are good for everyone!

If not i will still incorporate some design features seen there,

I had considered using LV-Ns, but the new limitations on them had held me off

But rapiers!, why didn't i think of that?

For some reason i only though about their air-breathing thrust at sea level, which is lower than the turbo, but i forgot that having them means you don't need as much rocket engine mass.

Anyway, it is just nice seeing people all taking different approaches to getting to orbit.

[Red Iron Crown]

Might i ask if that was inspired by my entry into the stock payload fraction contest?

It's my entry in the same contest (I took inspiration from several entries as well as some of my own calculations). I had started it as all RAPIER but it was too slow climbing off the pad before the engines got to a better speed for them. Switching two of them for turbojets made for a better initial climb rate.

[Shrike99]

Well then, i will be making some designs inspired by that i think:wink: