Craft rotation

[Barefoot Friar]

What causes my large asparagus lifters to begin rotating laterally as I drop stages? It starts spinning like a bullet, and while most of the time that's not a major problem (it is annoying, but not destructive) this time it caused boosters to dislodge and hit the main ship, causing mass panic and chaos and ultimately an untimely ultimatum.

I have tried adding reaction wheels to each stage, and that has helped some, but this last disaster (32 dead) beats all I've ever seen. RCS, ASAS, reaction wheels, fins... I'm out of ideas.

I was almost there, too. Grrrr.

So what's causing it? How do I fix it?

Edited October 30, 2013 by Barefoot Friar

[blizzy78]

Please provide a picture.

[Barefoot Friar]

I don't have one; in the chaos of the moment I forgot to hit the button.

Shouldn't need one, surely? It's more of a general question than about a specific craft. All the asparagus launchers I devise spin, no matter what the payload is or how I do the bracing. It just happens that the last one was especially bad.

[DMagic]

I don't have one; in the chaos of the moment I forgot to hit the button.

Shouldn't need one, surely? It's more of a general question than about a specific craft. All the asparagus launchers I devise spin, no matter what the payload is or how I do the bracing. It just happens that the last one was especially bad.

They shouldn't spin unless something is wrong with your design (or maybe a bug), so a picture would help. There is nothing inherently unstable about asparagus designs so there is probably something about the way you are building them that causes this to happen. It could be a fuel flow problem, uneven placement of radial boosters, or something else.

[pebble_garden]

I have a suspicion your stability module is far above your craft's center of gravity, and it's imparting torque to the rocket as it tries to do its job. Try adding a stabilizer closer to the rocket's center of gravity. I could be wrong, but it's worth a try.

Maybe it's a conflict between the multiple stabilizers you mentioned?

Another thing to check is some hidden asymmetry in your configuration. Even just a strut out of place can do it.

Edited October 30, 2013 by pebble_garden

[Oggula]

Actually I get this all the time with my designs, I usually just learn to cope with it. The main influence I've noticed is how far out you place your asparagus boosters; try to keep them close to the centre of the ship in the VAB. Two other influences I've noticed are:

1) Struts. If you don't strut the bottom AND top of boosters directly to each other, they will wobble to correct pitch and yaw, and your spin will slowly increase.

2) Too many boosters. Sounds silly but if you place 4 boosters on an asparagus ring, it is less likely to spin than with 6.

I'd recommend you try a few of these things, but in order to achieve optimal performance your craft will have to spin a bit. Remember the auto SAS is awful with spin and you may have to correct it manually if it gets out of control.

[Black_Tomcat]

One reason why it starts to roll might be the fuel lines.

For every force there is an equal opposite force, and the fuel moving is a force.

I would try fixing it using winglets on the core stage, they should provide quite a bit of roll authority while your boosters are still attached.

[Barefoot Friar]

Okay, some experimentation and I'm left with only a few answers.

First, I think it's something aerodynamic, because I don't think it does it when out of the atmosphere. I'm not certain, because by then I've usually always dropped all the boosters. However, on occasion I will purposefully launch with much more dV than needed, and thus still have the last two boosters attached when making my circularization burn. It doesn't rotate once it leaves the atmosphere.

Second, I can't replicate the problem because when I took the ship apart to rebuild the core stages I messed something up and now I can't get the payload to stay put on ignition. I did take a few pictures, however: Payload (habitation module for my space station) and the old launcher before I reworked it. Since the refit didn't work, I didn't bother to get a pic of it. It was pretty similar, though.

I am going to pare it down to the primary stage + payload and work out from there. I'm going to see if I can fit control surfaces on the core, along with using the lower profile radial decouplers. I like having the extra room that the taller ones give, but I think it's messing me up.

Thanks for the help so far. I'll post again later today or tomorrow with any updates and breakthroughs.

[Starwhip]

If no amount of RCS, SAS, or winglets/fins can correct your spin, you might have a "clipping" part. This is when parts are placed too close together. It results in a glitch commonly referred to as "The Kraken", or more specifically, "Gyro Kraken". Try looking close-up at your rocket for parts such as these.

[Taki117]

I use lots of struts between the center tank and the boosters. I also put two fins at the bottom of each booster and each of the stages between my launch stage and the command pod.

[Barefoot Friar]

I think I've fixed the immediate problem.

First, I started over from scratch. That thing had more struts than a cage full of indignant peacocks. I already had a bunch on the habitation module, but I put several more and then attached it to the base rocket. I moved the ASAS to the middle of the primary rocket (between two -32 tanks) and then put on some winglets with 6x symmetry. I then built out from there, doing a 6x asparagus, strutting the whole thing from top to bottom, and then another 6x layer of asparagus boosters, with even more struts.

The initial flight ended 30 seconds in when one of the jettisoned 7th stage boosters nailed one of the 4th stage engines, thereby throwing the whole ship off kilter. It happened on the opposite side of the craft from my viewpoint, so I couldn't shut down an engine fast enough to save it. I reverted back to the VAB and considered putting some sepatrons on it, but decided to just let it go again. That's probably the first time I've ever had that problem, and I figured if it did happen again I could change something. So the second flight was uneventful and I reached orbit.

Well, except that in my circularization burn the ASAS separated and the engine, going at full throttle, rammed itself into everything else and exploded it, and then I couldn't get the engines on the habitation module to fire fast enough to get out of the way. It wouldn't have mattered anyway, since the explosion sheared off a crucial docking port.

Still, I'm calling this a success, since my initial question is answered. I'll fix the flag on the title. Thanks for the help!

[Taki117]

the ASAS module has a habit of having a weak link. if you put the cubic struts with 6x symmetry below it and another set above the joint and join them with struts that should help with the exploding problem.

[SRV Ron]

Spin gets caused by twisting of the asparagus stages. With proper bracing, it can be controlled or eliminated altogether. Note how this one is braced;

This micro design didn't need bracing since the radical decouplers are nearly as long as the fuel cans. However, it can swap ends if you try to turn too quickly.

A better look at the bracing on this four pair design using NovaPunch fuel cans.

It won't be the number of braces used but rather how they are placed to distribute the increasing stress and prevent twisting that can cause the unwanted spinning as fuel gets consumed.