Building a rover with VTOL

[maddog59]

I have a rover with several spiders on rotation servos arranged along the sides. The servos are tied to custom axis controls so I can point them forward and back. The entire ship was built using symmetry and every time I check the CoM and CoT everything seems lined up. But when I try to take off it behaves as though there's another engine on the right side, and the thing rolls to the left.  I've added 2 large reaction wheels, and with even with SAS I can't keep it from rolling.

I've rebuilt it a couple of times, being careful to make sure there isn't a 'hidden' or invisible engine somewhere, and I've checked the CoT from all angles to make sure it's pointing straight down from the center, and is VERY close to the CoM. 

I'm completely stumped. I've used this configuration on other rovers without any trouble. Any suggestions on what I can check? I can share the ship file if someone wants to take a look at it and see if they can figure out what I've got wrong.  

Edited December 31, 2021 by maddog59

[Scarecrow71]

Images would be helpful.  Also, on what body is it spinnng?  Kerbin, one of the moons, another planet?

Do you have any other engines active?  Do you have the control point in the right direction (up, forward, reversed)?  Do you have the thrust on each engine the same?

Edited December 31, 2021 by Scarecrow71

[maddog59]

So of course, after stepping away from the problem for a while I seem to have figured it out. It had something to do with one or more of the parts being rotated. But to answer @Scarecrow71: I'm testing it on the Mun; no other engines are active; control point is forward; thrust on all engines is identical (100%). I re-built the beast from scratch and avoided rotations as much as possible.  

[magnemoe]

See two possible problems here, first is if the engine flame can hit the wheels, that will kill trust. Second is how well this is balanced as fuel get low, you also has an forward facing engine for some reason. 
Last why rotate the engines at all, if you rotate them you will just add more problems as I see it even if cool. 
Yes one strategy is to have an very efficient braking engine at the rear and then tilt and switch to lading engines to land. 
I would move the engines down between the wheels and set the upper tanks to empty first for lower center of gravity as an rover  

[maddog59]

@magnemoe Yes, those problems are what made it an interesting exercise. I was able to de-orbit this beast from 15Km to the Mun and it's performed fairly nicely once landed. Once landed I'm able to use the spiders (which are on servos) to lift off and land, then switching to the rear facing engine (not visible in the picture) I can get it across canyons and craters. The rear facing engine was also used for de-orbiting, but once I got within a couple hundred meters of the ground I switched to the spiders, leveled out, and landed it horizontally.  I use action groups to switch between sets of engines, and between pitch control and the servos I can get fairly good forward and backward motion for shorter hops.  

Once landed I empty the upper tanks into the lower ones manually, but that's because I forgot about setting fuel priorities. Thanks for the reminder, I'll have to give that try.  

The front facing engines are used for additional braking assist if I end up going to fast either on the surface or in flight.