CoL is being unkind to me...

[Thorn_Ike]

Hi,

I am designing a shuttle for the Shuttle Challenge v5, and I've encountered a problem.

When I open my cargo bay, my center of lift has an arrow, and is where I want it to be:

But when I close the bay, the CoL is too far back, and has no more arrow:

\

This is a problem because with that CoL, I can't flare for a landing.

Help me?

[bewing]

Well, I don't like looking at "plane-style" aero forces in the VAB. They often just don't look right. So my suggestion would be: rename a second copy of the craft, reroot if needed, rip off the whole booster and throw it away, save the craft, transfer it into the SPH, make it horizontal, and then look at the CoM/CoL.

[swjr-swis]

51 minutes ago, bewing said:

I don't like looking at "plane-style" aero forces in the VAB. They often just don't look right.

Agreed. I do the same thing - load the 'plane' section separately in the SPH, and do any finetuning of fuel/payload balance and CoM/CoL positions in there. Once that is done, I assemble the launch vehicle in the VAB.

The CoL 'lift arrow' in particular often does very random things in the VAB.

[Guest]

Hear hear.

Also, make sure to give your wings a bit of pitch. If they're perfectly level, the CoL arrow gets confused too.

[Thorn_Ike]

8 hours ago, Brikoleur said:

Hear hear.

Also, make sure to give your wings a bit of pitch. If they're perfectly level, the CoL arrow gets confused too.

Worked like a charm. Thanks!

[AeroGav]

You may have heard this already but Space Shuttle lookalikes are about the hardest thing you can possibly build in KSP.

SSTOs and two stage to orbit spaceplanes are significantly easier.      You've got to deal with off axis thrust on the rocket stack without the benefit of FBW,  there is the fact that body lift is not modelled which pushes your CG back,  and the fact that your Space Shuttle main engines are twice as heavy and powerful as they should be, sending CG way aft.

I suggest designing the orbiter in SPH first (horizontally) then when you're satisfied it flies ok on its own,  rotate it vertical and export it to the VAB for mating to the booster stack.

In my shuttle designs i've tried using lighter engines to bring CG forward  a bit,  ideally in a cargo plane you want your CG to be in the middle of the cargo bay so it does not get affected by the vessel being laden or not, same with fuel (as much of it ahead CG as behind it)

3 aerosikes will give a better balance vs the vector,   but they do lack gimbal so you better have it on the booster itself to compensate for the poor off-axis thrust.      I suppose you could go with 3 Swivels and clip them inside a bit, would give an appearance not so different from the vectors while still being lighter.     But,  you may find yourself underpowered now to lift that external tank,   since the real one was full of LH2,  which is extremely light.   Most of the takeoff weight was actually in the relatively small SRBs

One tip I find helps with the Shuttles' aft CoM/CoL  - use the fine rotate tool to angle the forward strakes up slightly.

This provides the nose up force for the gliding approach.   Trying to do it solely with the elevons at the rear makes it glide like a brick, since these surfaces are very close to CoM thus have to push down on the tail very hard to overcome the strong nose down tendency that comes from CoK being even further aft than the CoM.    Generating all that downforce means those surfaces are deflected at a large angle, creating lots of drag, and all that downforce is actually counteracting lift frmo the wings anyway.