Excluding Engines from Center of Thrust

[DuoDex]

So I was trying to build a VTOL in sandbox yesterday, and I couldn't get the downwards-only CoT over the CoM because I had three engines-two lifting, one pushing.

Is there any way to exclude the pushing engine from the CoT calculation so I can precisely orient my lifting engines for minimum-flippy-outy-ness?

Thanks!

Edited October 31, 2014 by DuoDex

[capi3101]

I don't know if this actually will work or not, but try temporarily turning the thrust limiter on the pushing engine to zero. Don't forget to reset it before you launch unless you want a nasty surprise...

Alternatively, you could try temporarily replacing it with a set of non-thrusting parts of equal mass. Three Engine Nacelle in series have the same mass as a Turbojet or RAPIER, two with a drogue chute have the same mass as a Basic Jet.

Edited October 31, 2014 by capi3101

[Stratzenblitz75]

If you're not against mods, Get RCS build aid (As the name suggests, it helps you balance RCS thrusters, but it also helps balance "regular" thrusters). It is one of the most helpful tools for building VTOLs, since it lets you precisely align the CoT of whatever engines you choose, with your CoM.

[Hodo]

What I did for a long time was use a near weightless object like a light or an antenna to mark the CoM of the craft. Then I would remove the engines that were on the back of the craft, and adjust my CoT point around that marker I placed.

It wasn't the best system but it worked like a charm. Now I don't even really need to have a marker, I can eyeball it most of the time now.

[Red Iron Crown]

I don't know if this actually will work or not, but try temporarily turning the thrust limiter on the pushing engine to zero. Don't forget to reset it before you launch unless you want a nasty surprise...

This works, it's the technique I use. And it does make for some amusing fails when I forget to tweak it back up.

How does one use RCS Build Aid for this? Do the engines have to be in separate stages or something?

[Hodo]

This works, it's the technique I use. And it does make for some amusing fails when I forget to tweak it back up.

How does one use RCS Build Aid for this? Do the engines have to be in separate stages or something?

For RCS Build Aid you just set which engines you want to display the thrust for.

[Stratzenblitz75]

How does one use RCS Build Aid for this? Do the engines have to be in separate stages or something?

RCS build aid will calculate the CoT only for the engines in the first stage. So, simply put your VTOL engines in the first stage (or in KSP terms, the bottom-most stage), and put the other engines somewhere else.

[capi3101]

I use RCS Build Aid myself for spaceplanes, though I've only been using it to determine how far my CoM will shift during flight; I didn't know about the whole first stage thrust thing. Going to have to try that out my own self.

[Red Iron Crown]

RCS build aid will calculate the CoT only for the engines in the first stage. So, simply put your VTOL engines in the first stage (or in KSP terms, the bottom-most stage), and put the other engines somewhere else.

That seems to do the trick, thanks.

[How2FoldSoup]

In addition to using RCS build aid, you can just turn the thrust limiter for the horizontal engines to 0 like capi said in the second post. Once you only the vertical engines with thrust, just balance the the COT with the COM. If it moves, use something like TAC fuel balancer to make sure it doesn't shift too much or design around it so the COM doesn't move.

[Ruedii]

I hate to state the obvious when everyone else recommends a mod: To exclude engines from the center of thrust just use the tweekables to turn their thrust to zero.