Jet and rocket combo at 20k results in decrease in jet thrust

[Boelie]

Hi all,

Im working on my first SSTO (yes, it is big, i have ambition) and the problem is that I can't get it up to the speed it needs to reach to break out of the orbit (some 1200 m/s if i recall correctly). The problem lies in the combination of both jet and rocket engines. With just the jet engines (6, from B9 mod) i can get it up to about 22k before the air runs out. So, at 21k i want to use my rocket engine (any) to get the speed up, this is where i see a huge drop of about 50% in thrust output by the jet engines, this starts the second i turn on my rocket engine. As you can see the stats are (very) similar when taking the screenshots, so theoretically speaking my jet engines should keep on going, does anyone have any idea if this is a mod related thing or that im doing something wrong? See images below.

Followed by a huge decrease 5 seconds later (dont mind the 300m alt difference, that is not it)

Thanks and cheerio!

[Boelie]

A small heads up, this seems to be related to the jet engines seen in the picture, the ones standard in ksp do not have this issue. But these original ones are a bit bugged, due to the fact that they need less air and generate more output then they should.

[Radam]

A small heads up, this seems to be related to the jet engines seen in the picture, the ones standard in ksp do not have this issue. But these original ones are a bit bugged, due to the fact that they need less air and generate more output then they should.

I have no idea about B9 jets, but stock ones generate thrust based on speed. Turbojet starts with 115kn at 0 and peaks at 225 at 1000ms, then drops. And they always use air as if they operated at 225kn, so they use more than they should.

[Alfondoo]

Try using the stock Turbofan engine.

From what little testing I've done with the B9 jets it seems that they have a lower operating ceiling than the stock jets, especially the gimballing ones you're using (lose all thrust at 22k). Or switch out your jet engines for Sabres?

[Boelie]

Im now using the advanced jet engines, they work well and i managed to get it into space after some tweaking. This sudden drop in thrust, as i mentioned, seems only to happen with that engine, though i havent tested it with the sabre ones. It has to be a bug, why else would the stock engines maintain their thrust upon enabling the rocket engine.

[stupid_chris]

Jet engines don't work like rocket engines, their thrust isn't constant. You can look up the cfg files and you'll see what's called a "thrust curve". Jet engines give different thrust according to how fast your ship is going, and past a certain speed, the thrust starts going down when you go faster.

[RoboRay]

For you first SSTO plane, build smaller. A smaller, lighter craft is exponentially easier to design and build. Once you've worked out the design compromises that make for an effective SSTO, then start scaling it up to bigger craft.

[Boelie]

Chris, that I know, realize and understand. Though that, if you look at the pictures, is not the case. I could change the thrust output at will by switching the rocket engine on and off, no other variable changed and only the toggling of the rocket engine did this. The turbojet engine has no issues as the one you see above, hence, by the power of elimination *looks to the skies*, it is focused at the engines shown above. I can post a million pictures showing exactly this, with exactly the same altitude, speed setting and intake air, just ignore the slight differences seen in the pictures.

Roboray, I got it into space

[Captain Sierra]

Those particular engines have poor performance at altitude. If you want B9 engines, the DF-S7 turbofan engines are much better at high altitude and will burn higher with more power delivery. They should give you more altitude capabilities and better power at altitude.

[stupid_chris]

Heh, sorry I couldn't load up the pictures, school internet is not picture happy much. But yeah hm. This seems to be an issue with the B9 mod, I haven't been using it much so I'm not sure what's up with that, but you might want to ask in the forum thread someone might know what's going on there. Maybe peeking at the engine's cfg file would learn us more.

[Radam]

You could try getting Logomatic (mod) and test these engines. Isp vs altitude and Power vs speed. At least stock engines work like that, isp only changes with altitude power only with surface speed... I did that for stock turbofan and resoults are interesting.