Small Change in Jet Engine Behaviour - Big Leap for Kerbalkind!

[DocMoriarty]

While working on my documentation about space planes in KSP and testing VTOL's I came upon something in the simulation of jet engines which is really bad in terms of flying VTOL's. It would be easy to change, yet not changing the overall behaviour of jet engines.

Some definitions for the coming formulas:

ST = Set Thrust (current thrust input set by user and shown on gauge)

CT = Current Thrust (current engines thrust value)

TL = Thrust Limiter Factor (current thrust limiter setting 0.0-1.0)

TF = Time Factor (constant depending on how long thrust changes are supposed to take and the simulation cycle duration)

MTC = Max Thrust Delta per Simulation Cycle

My guess is that the formula for CT changes per simulation cycle currently is as follows:

CT = CT + (ST*TL-CT)/TF

This causes big changes in thrust setting to take as long as small changes because it takes always the same amount of cycles regardless of the value of change in thrust (actually never gets to finish the change). To reproduce that just launch a jet engine, right click and watch what happens and how long it takes until thrust value is stable after a small or big change of thrust. Keep in mind that especially for small changes the change in thrust at the end is not visible due to 1 digit precision in thrust value when looking at the engines right-click window. This is ok for normal jet operation since tiny changes in speed don't really matter that much.

It's realistic that jet engines take a long time to get from idle RPM to max RPM and this is what the algorithm is supposed to simulate. But small changes should be much quicker and more accurate. Also a thrust limited jet engine should react much quicker. So the thrust delta should be calculated depending on an engine specific RPM/s (delta RPM per second) value.

A better and more realistic algorithm would therefore look like this:

CT = CT + (Sgn(ST*TL-CT)*Min(Abs(ST*TL-CT),MTC)))/TF

It simply makes acceleration linear using a max thrust delta per cycle to limit changes so that small changes are executed quickly and big changes take their time as usual. Also heavily thrust limited engines like the one in the Kopter example would INTENTIONALLY react a lot quicker which would make it a lot easier to fly the VTOL only models while the overall performance of jet engines would stay the same with their slow response to major input.

http://www.docmoriarty.com/ksp/mods/ssx_kopter_v_1_06.craft

And without a fix it's practically impossible to fly a model like the Dragonfly precise enough to actually pick something up with the claw.

http://www.docmoriarty.com/ksp/mods/ssx_dragonfly_v_1_01.craft

[thereaverofdarkness]

If this would make VTOLs both easier AND more realistic then I'm all for it! I'm okay with them being a little difficult but I feel like they're too much at current, considering how comparatively easy everything else is.

[chaos_forge]

Agreed. The most difficult thing about VTOLs is how long the jet engines take to respond to changes in thrust

[Kasuha]

Of course the real formula is likely more complex as it must take framerate into account. But that's not too important.

I guess this assumes drastical reduction of the TF constant, otherwise it would slow down large changes and leave small changes as they were. Apart of that, I think it would be useful. Today when I need a VTOL to increase thrust fast by small amount, I need to "overshoot" the throttle, then return it to desired position. It works somehow but it's very imprecise.

[Majorjim!]

You could also try clipping little rocket motors into the jets (they don't have to be clipped). Then map the intake cutoff to an action group. that will instantly cut the jets thrust.

Then action group the rockets to a separate key and use both. After some practice they complement each other perfectly.

Watch any 'Cupcake landers' videos. That's what he uses.

[Justin Kerbice]

So much for so few, air breathing/jet engines use:

engineAccelerationSpeed = 0.30

engineDecelerationSpeed = 0.50

(here is values for my own mk2 engine which is very slow to respond to throttle on purpose)

stock basic jet engine values are:

engineAccelerationSpeed = 0.12

engineDecelerationSpeed = 0.21

It really makes an engine to be slow or not, easier than your formula.

Feel free to change the part's values to whatever suits your needs, and even create a "VTOL trainer" jet engine, with very small values, just to get a rocket type jet engine.

[DocMoriarty]

Sure i can mod just about anything and could even write a mod that moves my VTOL and anything else by pure magic with unimaginable awesome graphical effects!

This post though is to show that with a little change in stock KSP it would improve the VTOL experience a lot without changing the overall performance of jet engines.

[Kegereneku]

Alternative : Suggest to make a different type of jet engine specifically meant for VTOL with a very low reaction-time.

Preferably something that can't be used at high-speed (or say with an higher fuel consumption)