Is there a way to edit gimbal in a VTOL tilting engine aircraft?

[salaminho]

It's been a while I’m designing some VTOL aircraft. I usually use tilting engines with the BG servos, the biggest challenge is attitude control. Using engines gimbal would help a lot here.

Gimballing is automatic and works just fine when the engines are pointing backwards on level flight, but when I tilt the engines downwards for VTOL flight the gimballing gets all messy and doesn’t work the way it should. I know it can work pointing down if I set control to point up, like If I click “control from here” in a dorsal docking port, but then I loose situational awareness and it get’s really hard to fly as yaw and roll control switches place.

Is there a way to manually edit gimballing to point the way I want?

[OHara]

1 hour ago, salaminho said:

the biggest challenge is attitude control. Using engines gimbal would help a lot here.

For my VTOLs, gimbal does not help with attitude control, because the gimballing engines tend to be close to the centre of mass.  Even when the engines are spread out, mine tend to be spread horizontally, at the same vertical height as the centre of mass, so that even if an engine rotates on its gimbal it still provides the same torque trying to turn the craft.  Threads on VTOLs (link link) tend to suggest other ways to control attitude, like RCS or the mod throttle-controlled avionics.

The more recent versions of KSP let you connect the 'thrust limit' of an engine to 'Axis Groups' including the steering axes. (The SAS control, however, only steers with the hardwired steering controls, not the ones you connect using the axis-group feature.)

That being said, if some engines gimbal opposite the way you want, you can set Pause Menu => Settings => Advanced Tweakables : Enabled
and then manually edit under the right-click menus for each engine to selectively disable whichever of the pitch/yaw/roll responses are not what you want.

[18Watt]

Not sure if this will help you or not-  Your flight controls may be working against you sometimes too.

I ran into this while trying to build a vertical-landing spaceship, I'll link that thread below.  The flight surfaces worked fine while going UP, but actually worked against me while the rocket was descending (going backwards) to land.  My solution was to deactivate the flight surfaces while going backwards.

@camacju had a better solution, which was to set the flight control surfaces authority limiter to -20, so they effectively work in reverse.

As @OHara mentioned, I imagine (a screenshot would help) that your engines are mounted too close to the COM to be very effective during gimbaling.  I do suggest turning off the pitch/roll/yaw gimbals one at a time to see what effect they have.  I'd start with the roll axis.

 

[salaminho]

Thanks for the reply you guys.

3 hours ago, OHara said:

gimbal does not help with attitude control

Gimbal does not help because the engines are not gimballing the right direction for vtol flight.

I’ve created my own set of gimbals using the servo motors to test if gimballing would work.

For yaw control, gimbaling works flawlessly. For pitch and roll it works, but the vectoring range in most of the engines wouldn’t give much torque, so using differential thrust is more effective.

Pitch: https://imgur.com/ugQpIyT

Yaw: https://imgur.com/EakFTHM

Roll: https://imgur.com/OaDuT7C (by the way, I don't know how to use the "insert image from URL" button )

 

This vtol aircraft is working very well, but there are still some points to improve:

1-      Setting target angle of the rotors and thrust limit to kal-1000 and hardwire it to control axis doesn’t work with SAS, only manual input.

2-      Differential thrust is less responsive when using airbreathing engines since they are slow to throttle up and down, so using gimballing could work better.

 

RCS and reaction wheels works fine for small and medium size ships, but are not effective in bigger craft, this is why I’m looking for alternatives. 

5 hours ago, 18Watt said:

As @OHara mentioned, I imagine (a screenshot would help) that your engines are mounted too close to the COM to be very effective during gimbaling.  I do suggest turning off the pitch/roll/yaw gimbals one at a time to see what effect they have.  I'd start with the roll axis.

I tried doing this, but there should be a way to not just turn them off, but to set where it should point when I hit an axis input. This seems to be impossible.

[18Watt]

1 hour ago, salaminho said:

there should be a way to not just turn them off, but to set where it should point when I hit an axis input

I agree.  Unfortunately I do not know how to accomplish this in stock KSP.  Good luck!

[salaminho]

Ok I've been searching for ways to do this, but it seems there is just no way to set gimbaling the way I want in stock KSP. 

So, I'm looking for other ways to control big VTOL aircraft, and this thing you said about surface control kind of gave me an idea, to use elevons to deflect exhaust from engines so I don’t need to throttle up and down that is too slow on air breathing engines. 

Using elevons on the exhaust is a lot faster and responsive, I’m pretty satisfied with the results. Here is the link if you want to try it. 

https://kerbalx.com/salaminho/ASP-480-A-Drogon-SSTO-VTOL-differential-thrust

 

https://imgur.com/a/QXVEiPY

On 11/24/2021 at 3:59 PM, 18Watt said:

@camacju had a better solution, which was to set the flight control surfaces authority limiter to -20, so they effectively work in reverse.

 

[18Watt]

5 hours ago, salaminho said:

Using elevons on the exhaust is a lot faster and responsive, I’m pretty satisfied with the results.

That’s a really interesting solution!  Gotta admit, I would not have thought of that, and am suprised it works.

I’m not certain how engine thrust is handled in KSP, but I suspect the control surfaces might not be actually redirecting the thrust, but rather simply blocking it when needed.  Either way, if it works, that’s awesome!

I gotta mention that for drag reduction a single Shock-Cone Intake can drive 3.5 RAPIERS (or 2 of them will drive 7 RAPIERS..).  In your design I don’t think that information is useful, just wanted to mention it.

I’m always impressed with players who can use the KAL controllers, as I still haven’t figured out how to use them.  Also, that’s an interesting way to have nuke engines in space, without the drag during atmospheric flight.  Impressive!

[salaminho]

Thanks a lot! 

6 hours ago, 18Watt said:

I’m not certain how engine thrust is handled in KSP, but I suspect the control surfaces might not be actually redirecting the thrust, but rather simply blocking it when needed.  Either way, if it works, that’s awesome!

Precisely!  The idea was to redirect thrust, but actually it just blocks it. Game simulates thrust like a vector from the center of nozzle, a line with a direction and length but zero thickness. If something gets in the way of this line within its length thrust gets blocked.  

So, I set deploy angle of the elevons to a 17º (+15º of positioning), it barely touches the vector so there is no effect on thrust, when angle is increased to 18º thrust gets completely blocked. 

The rapier engine has four nozzles, so four vectors, each elevon can block two vectors. I.g. rear mount has two engines each side, so eight vectors. When I want to pitch up, rear elevons get activated and they cut 2/8 vectors each side, there is an instantaneous 25% reduction in rear thrust that makes total center of thrust move ahead the center of mass and this create a backward torque that makes the craft pitch up.  

There is the same kind of setup for roll control, and a completely different setup for pitch and roll control when engines are in close cycle mode. Elevons can’t take the heat of rocket engines, but rockets can throttle up and down much faster. Yaw is provided by tilting left and right engines to opposite sides for air breathing and closed cycle mode. 

 

6 hours ago, 18Watt said:

I gotta mention that for drag reduction a single Shock-Cone Intake can drive 3.5 RAPIERS (or 2 of them will drive 7 RAPIERS..).  In your design I don’t think that information is useful, just wanted to mention it.

I chose one intake for each engine mostly for aesthetic and realistic-looking purposes, besides intakes act as drag reduction. But that was a good nugget of information anyway, thanks for mentioning. 

 

6 hours ago, 18Watt said:

I’m always impressed with players who can use the KAL controllers, as I still haven’t figured out how to use them.  Also, that’s an interesting way to have nuke engines in space, without the drag during atmospheric flight.  Impressive!

Necessity is the mother of all solutions, when you have a problem that can be solved using kal-1000 you will figure it out. Anyway, if I can help you with that fell free to hit me up.  Yeah, the idea of nuke engines I got from Obi-Wan Delta-7 starfighter + hyperdrive docking ring in Clone Wars. The tug can carry the ship between planets but stays in orbit as the aircraft go down to surface.