How to ensure rockets aligned perfectly?!?

[SneakyBastd]

I've been trying to make a skycrane out of a 720 fuel tank with 6 of those little orange thrusters mounted radially around the sides near the top edge of it. Lander and rover hanging underneath.

Unfortunately, no matter what I do, the rockets do not seem to align vertically. As soon as I apply thrust, the thing starts spinning/and or tipping, and becomes uncontrollable. Powered descent is simply impossible. It's driving me nuts.

I could go with another design, but surely this should work! I should be able to have perfectly aligned rockets...!??! This seems like such a simple thing, how can it go so wrong?

[Awaras]

Are you placing them one by one or by using symmetry?

[SneakyBastd]

Definitely with symmetry. I have tried simply placing them and trying that. I've tried rotating them slightly vertically with SHIFT to 'fix' the alignment. I've tried rotating the thrust slightly outwards. It always rotates my crane.

[Awaras]

Also, does it spin even if you use ASAS? Can you post some pics?

[Flixxbeatz]

Make sure that the center of mass is exactly on the center even if the tanks are losing mass during landing to keep it upright. Remember, it needs meticulous part placement for it to work out (even the slightest of CoM shifts can tip your spacecraft during landing approach).

[SneakyBastd]

Yeah it's got ASAS, but I can't post pics as I'm at work. I'll try later. Just figured someone somewhere must have tried this before, curious if it's a known issue.

[Temstar]

Put your engines at the bottom, the game currently cannot handle engine gimballing correctly for "pendulum rockets" and will steer the wrong way. If you have a lot of maxRot you can get away from this if you lock gimbal for all the R24-77 engines and rely on only reaction wheel for steering.

Basically any time you have vectored thrust engines above the CoM you're asking for trouble, as the game will always assume thrust comes from the bottom of the rocket and steer the engine gimbals accordingly.

[Jason Patterson]

Try to launch your vessel and once it starts tipping, hit F3. It's a fair bet that some part of the rover is blocking the thrust from one or more of the engines. If that is the case, you'll see a string of "X part was damaged by blah blah blah" lines.

[RoboRay]

Please post pictures when asking for help with a design. That makes it easy to identify the problem and the value of the responses you get will improve dramatically.

In your case, if you have gimbal-control motors above the Center of Mass (which is what it sounds like you are doing), you must disable their gimbals. Gimbal behavior doesn't reverse itself properly when mounted above the CoM, so your gimbals are steering your craft away from vertical while the ASAS thinks it's steering the gimbals toward vertical.

[astropapi1]

If you're landing a rover, check out if it's CoM is exactly in the center, and that the skycrane is directly above.

If you want inspiration, check out the rover with skycrane that comes with the vanilla game in the VAB.

[SneakyBastd]

Thankyou everyone! It was a pendulum rocket design as you identified, and that was causing my issues it would seem. Placing the thrusters below the CoG fixed the problem. Although it's not as cool to look at

I love this game, but omg, it's so frustrating when little things like this hold you back...

[Eric S]

Two things to be aware of on pendulum rockets. First, as has been pointed out, KSP doesn't know how to deal with vectored thrust from engines mounted above the CoM. If you mount them there, make sure you lock the gimbals. Second, contrary to what a lot of people think, pendulum rockets are not more stable in terms of always wanting to point up.