My Skycrane-esque rocket tug makes my ship swerve like crazy

[Insanitic]

Hey guys, I'm hoping some of you have experience with a lander config like this:

When I try to fly this module, it rolls all over the place and I can't control it, even with SAS on. I made sure the CoM is centered so I'm not sure what's causing the uncontrollable roll.

Any help is appreciated.

[The Yellow Dart]

I'm not sure what happened to your picture but it is very, very small. Do you want to try again? I don't think you'll get much help until you do. Until then, make sure you didn't accidentally adjust the thrust limiter on one of your rockets (trust me it can happen) or lose a fuel line somewhere along the way. There is also the possibility of thrust getting blocked asymmetrically some how so make sure there is nothing anywhere near the thrust vectors for each of your engines.

[Higgs]

honestly, id recommend you add more SAS modules to the craft for one. 2. idk your power situation <batteries/rtg's> but, you may be running out of power faster than the ship can cope with, so, add more power generation either batteries or RTGs. 3. perhaps you may want more engines or possibly bigger ones in place of the 4 you have now. Those are just my suggestions of course. best of luck side note: i thought of this as I hit post lol. what about the vernor engines? have you tried using those? also, as suggested above, check thrust limiting AND fuel flows, something may be wonky. best of luck XD

[RainDreamer]

Try RSC build aid, see if anything is off angle and cause torque

[DeMatt]

The first thing I'd check is that you turned the engine gimbals on those engines off. SAS (and indeed all steering controls) does not understand how to gimbal engines when they're "above" or "ahead" of the center of mass.

[KerikBalm]

Ditto to what DeMatt said.

When engines are ahead of the center of mass, the game doesn't know to invert the controls (but it does do that for aerodynamic control surfaces... strange).

I'm guessing you mod the rescale factors in the .cfg files, or use tweakscale?

[Insanitic]

The first thing I'd check is that you turned the engine gimbals on those engines off. SAS (and indeed all steering controls) does not understand how to gimbal engines when they're "above" or "ahead" of the center of mass.

I tried your tip DeMatt, and the ship still involuntarily banks to a random direction without me doing anything after a few seconds of going straight up from the launchpad.

I revised my design a bit, placing a bigger launcher with the engines more out of the way of the module and my problem still persists.

I have RTG's and batteries inside the ASAS disc so I'm not running out of power.

Of course when I place a launcher on the bottom of the module, everything goes well and it flies straight when I don't do anything.

What I'm wondering is how did Scott Manley managed to keep his ship straight whenever he does a tug/pull vehicle, like in his Duna Express vehicle?

Edited November 30, 2014 by Insanitic

[Simes]

I think you might need more struts to stiffen the link between engines and body. Check that your docking ports are secured, as well. They like to move around during launches if they're not strapped down.

[rodion]

I think you might need more struts to stiffen the link between engines and body. Check that your docking ports are secured, as well. They like to move around during launches if they're not strapped down.

this is a good point. if the connection between the crane and module isn't sturdy enough it can induce oscillations around the linkage whenever you bank which completely ruins any control stability you may have had, especially if the payload is the more massive of the two parts (which looks to be the case).

[Insanitic]

this is a good point. if the connection between the crane and module isn't sturdy enough it can induce oscillations around the linkage whenever you bank which completely ruins any control stability you may have had, especially if the payload is the more massive of the two parts (which looks to be the case).

I tried this also and it still swerves without me doing anything.

I noticed that even the stock skycrane craft with the rover also banks uncontrollably also, so I think it has something to do with KSP, since I made sure my craft is a rigid body with all the struts I'm placing.

Thanks for all the help guys, but I gave up on my skycrane idea and just went with a mini lander on the bottom of the module. I don't how else I'm going to land this module on the Mun without it banking uncontrollably at any throttle level.

Edited November 30, 2014 by Insanitic

[wasmic]

Due to the stock gimbal module being broken, the engines will gimbal in the wrong direction. Since you used the engines with extra-large gimbal, your craft is extra screwed. My advice would be to lock the gimbal and rely on reaction wheels or RCS for control.

[RainDreamer]

Just a question though: were you testing on kerbin? Sometimes things may work out differently without atmosphere.

[Simes]

It is entirely possible to run stuff in a puller config:

This craft has a tug attached to the front and the rear section splits into three, each of which is landed separately by the tug on the Mun to drop off the components. I did not launch from Kerbin in a puller config, so I can't say how well that works, but towing from LKO to the Mun and landing on the Mun is definitely doable.

I did, however, have to turn off the gimbals, as others have mentioned. Even with the two big reaction wheels the ship was drifting off course otherwise.

Edited November 30, 2014 by Simes

[Insanitic]

It is entirely possible to run stuff in a puller config:

http://i.imgur.com/xn303yT.png

This craft has a tug attached to the front and the rear section splits into three, each of which is landed separately by the tug on the Mun to drop off the components. I did not launch from Kerbin in a puller config, so I can't say how well that works, but towing from LKO to the Mun and landing on the Mun is definitely doable.

I did, however, have to turn off the gimbals, as others have mentioned. Even with the two big reaction wheels the ship was drifting off course otherwise.

So how did you manage to make the craft stay on course? Because whenever I tried it, in both Kerbin and Space, it REALLY goes off course without me doing anything.

I've even turned off gimballing and strutted the whole ship and experimented without docking ports. But whenever an engine is in front of a module, it swerves after a few seconds.

Oh and I'm curious as to what mod that huge structure in the middle is from? And the huge ASAS discs?

[DeMatt]

The other thing to check is that all of the engines have clear firing directions - that is, their exhaust doesn't hit any part of the spacecraft. Engines that hit parts try to shove them away, but if they're part of the same craft, they can't be shoved - so the engine's thrust is negated.

If some of your radial engines are hitting other parts (and thus generate no thrust), and some aren't... that'll send your craft off-course too.

[Insanitic]

The other thing to check is that all of the engines have clear firing directions - that is, their exhaust doesn't hit any part of the spacecraft. Engines that hit parts try to shove them away, but if they're part of the same craft, they can't be shoved - so the engine's thrust is negated.

If some of your radial engines are hitting other parts (and thus generate no thrust), and some aren't... that'll send your craft off-course too.

Yeah I made sure that the engines were clear. I posted a picture of the radial engines extended out from the main ship, and it still induces a mysterious banking force.

[DeMatt]

Yeah I made sure that the engines were clear. I posted a picture of the radial engines extended out from the main ship, and it still induces a mysterious banking force.

Okay... can you post up the .craft file for the ship? The only other thing I can suggest is that testing in Kerbin's atmosphere is interfering, because those MKS units are not terribly aerodynamic.

[Insanitic]

Okay... can you post up the .craft file for the ship? The only other thing I can suggest is that testing in Kerbin's atmosphere is interfering, because those MKS units are not terribly aerodynamic.

It's not that either, I tried a tug in space and it also swerves. Check Simes post, he can confirm this too.

[Zylark]

Try this approach to getting base modules down without using a dedicated lander:

Works really well. Easy to launch, the fuel in those side mounted tanks is enough to get from LKO to Mun orbit and land. You land two modules at once, not just the one.

And when all is done, just discard the tanks and engines.

Saves a lot of work. No need to refuel a lander, no need for plenty of docking. Nice rigid construction, and the game do not get confused by having the thrust above (or in front of) the CoM.

[Red Iron Crown]

It really, really sounds like a gimbal issue. Are you 100% sure that all gimbals are disabled? IIRC, you have to disable them individually even if the engines are placed symmetrically.

[Insanitic]

It really, really sounds like a gimbal issue. Are you 100% sure that all gimbals are disabled? IIRC, you have to disable them individually even if the engines are placed symmetrically.

YES THEY ARE DISABLED. Sorry for the caps lock, but I made 2 or so posts saying that ALL the gimbals are disabled.

[Red Iron Crown]

YES THEY ARE DISABLED. Sorry for the caps lock, but I made 2 or so posts saying that ALL the gimbals are disabled.

Forgive me, your posts only mention turning off gimbals, but not how you did so. Given that your problem is so similar to how a puller ship behaves with unlocked gimbals, I thought that perhaps you might have been disabling one and assuming that symmetry would take care of the rest.

[Insanitic]

Try this approach to getting base modules down without using a dedicated lander:

http://i.imgur.com/2Gj7XqH.png

Works really well. Easy to launch, the fuel in those side mounted tanks is enough to get from LKO to Mun orbit and land. You land two modules at once, not just the one.

http://i.imgur.com/5e1TuC1.png

And when all is done, just discard the tanks and engines.

http://i.imgur.com/0Q9Irm5.png

Saves a lot of work. No need to refuel a lander, no need for plenty of docking. Nice rigid construction, and the game do not get confused by having the thrust above (or in front of) the CoM.

Thanks for this. This is a really good idea. I might have to modify the side rocket boosters to fit them into fairings.

[Insanitic]

Forgive me, your posts only mention turning off gimbals, but not how you did so. Given that your problem is so similar to how a puller ship behaves with unlocked gimbals, I thought that perhaps you might have been disabling one and assuming that symmetry would take care of the rest.

Apologies if I came off stingy. Just kinda frustrated with this weird problem, since I've tried everything already and it seems like I'm the only that has it.

[Simes]

So how did you manage to make the craft stay on course? Because whenever I tried it, in both Kerbin and Space, it REALLY goes off course without me doing anything.

I've even turned off gimballing and strutted the whole ship and experimented without docking ports. But whenever an engine is in front of a module, it swerves after a few seconds.

Oh and I'm curious as to what mod that huge structure in the middle is from? And the huge ASAS discs?

I didn't have any difficulty keeping things on course once I'd turned the gimbals off on all four engines, but I did have to do it individually on each one. However I did just remember a problem I had when prototyping the vessel, which was accidentally clipping one of the hexagonal fuel tanks (45t when full) entirely inside another one so that the CoM was decidedly out of place. I had been carefully placing things with symmetry so didn't check the CoM before launching and could only work out what the problem was after I dismantled the ship and found an extra fuel tank somewhere it shouldn't have been. You've been flipping and scaling stuff a fair bit, from the look of things, maybe you've got something extra clipped inside part of the craft. Do any parts flicker weirdly when you mouse over them in the VAB?

The framework's from Freight Transportation Technologies, another of Roverdude's mods. The ASAS discs are actually 3.75m discs from Modular Rocket Systems which have been tweakscaled to 5m. A bit of a cheat perhaps, but I'd spent a very long time on trying to get the ship working thanks to the above problem and so as soon as it was working OK I said "done!"