Vessel integrity in 0.21

[Klingon Admiral]

Does anyone know what's up with vessel integrity in version 0.21? It seems that rockets seem to be more likely to randomly break up during, or sometimes even before, launch, even if the decouplers are reinforced. One some other occesion radially mounted boosters (using the TT-70 decoupler) just randomly came off on launchpad, requiring far more stuts than in 0.20 to remain fixed. As one can imagine, all these random catastrophic mission failures are getting rather annoying (What are rockets good for if you cannot get a simple jumbo fuel tank into LKO?).

[stupid_chris]

Moved this to the gameplay questions forum

[Klingon Admiral]

Then there shall be picture(s):

The mission record of this launcher so far:

Mission 1: Sucess

Mission 2: Structural failure at decoupler designated "1" during early ascent

Mission 3: Structural failure at decoupler "1" after gravity turn

Mission 4: Structural failure at decoupler "2" during early ascent

Mission 5: Structural failure at decoupler "1" during early ascent

Mission 6: Sucess

Launches 1-4 were in my KSP main installation with mods, launches 5 and 6 in a vanilla installation. What really freaks me out about this contraption is that even in failure it is unreliable, with at least 3 different failure scenarios and the possibility of a sucessful mission, and I don't see anything that would explain the structural failures, as a more or less identical launch system worked absolutely fine in v0.20 and the acceleration is usually rather low.

[OptimumAmoeba]

I would say you need to add some winglets to give you more control authority in the atmosphere. I think the pod torque is putting to much stress on your decouplers.

I could be totally wrong but give it a try and see.

[dlmarti]

That ship needs struts.

[Jean Deaux]

I see struts, lot's of them. But I too am having LOTS of structural problems though if history has any bearing, it's usually because of the mod's I've added. The first thing the Dev's are going to ask you is if you've got any loaded. I have found that removing all the mods has resolved my structural issues in the past, but in turn the game becomes a LOT less fun to play without those mods.

[Klingon Admiral]

That ship needs struts.

Since when are 4 struts to reinforce a decoupler not enough? This payload isn't overly heavy (only 68ish tons with the propulsion stage), and I haven't ever seen a reinforced decoupler fail.

In comparision to my .20 crafts this thing is quite heavily strutted, with 4 struts connecting each radial mounted booster to the central stage (2 in .20) and a strut connecting each booster to the upper stage (wasn't needed in .20).

New fun phenomenon: Was looking if the gimbaling of the booster engines causes the decoupler to fail, while I was locking gimbaling on the launchpad the decoupler "1" randomly failed. And I just don't understand why.

[OptimumAmoeba]

I will say it again try some winglets, I built a very similar rocket over the weekend my first heavy lifter in 0.21, after having similar problems I realised I had too many reaction wheels and they where causing the ship to shake violently once I stripped them out things calmed down.

What I'm trying to say is that the torque wheels create a lot of stress if they are trying to work on their own especially with the thrust from the mainsails.

The only difference I can see with your rocket to mine is no winglets.

I came to the conclusion that winglets help in the atmosphere (we knew that anyway), RCS is now much more useful and less wasteful when your rocket is large in thin atmosphere / LKO and reaction wheels are really good once your craft is smaller.

I liked the new SAS at first just building small rockets to test it out then after my first big build I hated it now I think I've got my head around it I'm starting to love it again I think it's a big improvement over the last system.

[rkman]

I have a rocket of similar design, and i've found that depending on small variations in the upper stage and payload (resulting in small variations in total mass, length and 'flexibility' of the rocket), and depending on max throttle, it can either

- fly rock solid even with a minimum amount of struts (as few as 2 struts)

- unrecoverably veer off-course on takeoff

- collapse on the launchpad or early in the launch regardless of strutting

KSP works in mysterious ways.

Edited July 30, 2013 by rkman

[Geschosskopf]

I have a rocket of similar design, and i've found that depending on small variations in the upper stage and payload (resulting in small variations in total mass, length and 'flexibility' of the rocket), and depending on max throttle, it can either

- fly rock solid even with a minimum amount of struts (as few as 2 struts)

- unrecoverably veer off-course on takeoff

- collapse on the launchpad or early in the launch regardless of strutting

KSP works in mysterious ways.

I have similar adventures but they were also present in 0.20 so I don't see them as new.

Personally, I blame most of them on the Launchpad itself. The big grating under the rocket has issues. It's flexible and areas of it have clipping problems, allowing parts to sink partway through it. Neither effect is very consistent in either happening at all or in amount when it does, but both happen fairly often. I believe the inconsistency is due to rockets not spawning in exactly the same spot each launch, so they touch different parts of the grating, just a few pixels this way or that.

Anyway, your rocket spawns, then physics kicks in and it does a "drop test" onto the grating. It could be that the flexibility of the grating is intended to cushion this blow, but it doesn't bend uniformly. So, depending on luck of the draw with where the launch supports and/or rocket itself touch the grating, different parts of the rocket experience different amounts of cushioning during the "drop test". Also, some parts might actually sink through the grating a bit, amplifying the oscillations caused by the "drop test" or at least changing the direction of the net force of physics start experienced by the rocket.

And remember, physics start applies what is in effect an impact load. Parts that can withstand a given force if gradually applied can fail if that force is applied all at once. Depending on how the forces angle and oscillate from the dodgy Launchpad, you might get no damage, or certain parts (usually those in expected weak areas like separators, docking ports, and body diameter changes) could fail. Sometimes it's something vital and the whole rocket collapses on the pad. Sometimes it's just a strut or 2 that fails and you only find out when you try to do the gravity turn. And sometimes either nothing breaks at all or you have enough structural redundancy there that it doesn't matter.

So, if your rocket is breaking a lot and you don't think it should because it's essentially the same rocket you've launched 1000 times before no problem, try moving the launch supports around, so they don't touch the grating in the same place. Either move them to different parts of the rocket or change their angle of contact with the rocket. It could be that one of them is frequently touching one of the bad spots on the grating so moving it might give it a better footing.

[rkman]

I have similar adventures but they were also present in 0.20 so I don't see them as new.

Personally, I blame most of them on the Launchpad itself.

I won't say what you describe is a not issue. But i think it has more to do with the way the physics engine causes parts of the rocket to interact with one another.

I'd never thought a rocket with a 3 mainsail 1st stage could fly straight as an arrow without the engines being strutted to the fuel tanks and without struts on the decouplers (only 2 struts from the bottom of the 2nd stage to the top of the 1st stage), but that's what it did.

Then i reduced the amount of fuel in the 2nd stage (replaced the orange tank with the 3/4 large tank from KSPX)- and the thing wobbled all over the place, engines sliding under the tanks etc, 1st stage decoupler broke off the fuel tank maybe ten seconds into the launch. Reducing thrust to 90% seemed to fix it, though it was not as stable as the first one.

That's with mechjeb, procedural fairings and a few custom fuel tanks in the payload. I might try if i can reproduce it with an all stock rocket.