Pulling rocket designs unstable?

[Derpist]

Hi guys I tiried to make a lander with it main landing engines on the fuel tank above the lander. I tried it on kerbin and as soon as any power was applied it flipped, I tried it on the mun anyway and the same problem occurs. I figured it was just a naff design, but i'm trying the NASA asteroid missions and the spacecraft happens to be a "Puller" and every time the power goes above 10% its flips out.

Tanks in advance

[Alshain]

They shouldn't be unstable, just not as efficient. It's more likely you have a balance issue in there somewhere. Pulling ships need to have perfect symmetry, not just in parts but fuel content too! Equal weight on either side of the craft and equal thrust on either side of the craft. Also the thrust vector needs to be the same, so if your using radial engines, make sure you didn't angle one more than the other (of course symmetry mode fixes that)

Possibilities include mis-aligned engines, misplaced fuel lines (one tank is draining faster than the other, causing a mass issue), and also obstruction of one or more of the engines (items placed below an engine that would fall in it's exaust cause the engine to produce no thrust)

Pulling designs are less fuel efficient than pushing but are necessary for segmented craft that have been docked together as docking ports tend to be flexible and bounce around when pushed.

Edited October 22, 2014 by Alshain

[Red Iron Crown]

Lock the gimbals of engines in puller designs. The engines aren't smart enough to realize they should vector thrust in the opposite direction when placed ahead of the center of mass, so their vectoring causes the ship to become less directionally stable rather than more. Use reaction wheels to control attitude in such designs.

Alshain, why would puller engines be less fuel efficient?

[Derpist]

Thanks for the quick replies, the landing engines were the radial nacelles, there were 4 symetrically placed, I didn't think about locking the gimbaling, I'll try that. Efficiency isn't a problemas I use mechjeb, could it be anything to do with Mechjeb? (haters gona hate)

[Derpist]

I cannot believe i hadnt thought of locking the gimbal, I chose my name wisely. I'll try a lander with the pulling engines tomorrow, thanks for the replies.

[KBluey]

Here's a trick to make the "puller" design work for asteroids--attach your fuel tanks to the asteroid, but have your puller engines attached to the tanks by a really flexible connector, like half a dozen FL-5(?) adapters (the little ones) stacked on top of each other, concertina-style. And attach at least a couple of the big SAS units to the engines. This will allow you to direct the engines independently of the fuel tanks and the asteroid, and keep the asteroid on track pretty easily.

[Derpist]

Turns out that locking the gimbal, rectifies the problem thanks guys.

[Nuke]

switch off your gimbals, ksp doesn't know to reverse the direction of gimballing on engines placed in front of the cg.

[bsalis]

I cannot believe i hadnt thought of locking the gimbal, ...

Not your fault, the gimbals should work correctly when the engine is in front of the CoM.

[worir4]

The reason that it flipped on kerbal might be becuase of the top heavy drag. I think if you tried it on a airless body itbmight work.