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How to deal with fuel tanks "Colliding" into each other during launch?

Sleipnir
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Sleipnir

Sleipnir

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Hello everybody,

this is kinda embarassing for me to ask since I have played for a while and have learned/know a great deal about this game, but.........How do you build rockets that don't break a apart during launch (Please spare me the "MOAR STRUTS" answer as I have tried that already)?

To clarify and expand: I know how to build rockets, I know the staging systems, the VAB buttons, the concept etc, most of my rockets.....in fact prolly 99% of my rockets all work perfectly. The only thing I am stuck on is when I want to build a stock heavyish lifter. My problem is that during launch in the lower atmosphere my fuel tanks that are connected to each other (Yes i made sure that they actually are) randomly decide to "collide" into each other (thats what the aftermath report says). I have tried using different tanks, more struts, specialised strut placements, less thrust, more thrust and a variety of other tips and tricks but nonetheless the problem persists, at random! I have build lifter rockets that fly fine the first try, but on the second one with no alterations made whatsoever one of my tanks likes to magically collide and go BOOM.

Since i am building a lifter capable of heaving 50T into LKO I need to use lots of fuel+engines. Also I have never had this problem using modded fuel tanks, they always seem to work fine.

How do you folks with Stock heavy lifters deal with this? Is this the work of the kraken?

Any help appreciated.

Dan

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Shrike42

Shrike42

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So, are they rotating about the connections horizontally (trying to go parallel to the ground) or vertically (swinging left/right parallel to the center tank) or are they tilting inwards/outwards?

Are the tanks colliding right after decoupling?

In any case, the answer IS more struts, but what works for me is putting the decouplers at the middle of the boosters, struts from the central tank out to the boosters near the top of the booster tank (with that placement order, central tank to outer tanks), and connecting the adjacent tanks with one or two strut(s).

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Uberick

Uberick

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Do you mean the fuel tanks that are stacked on top of one another are collapsing into eachother, or do you mean tanks that are adjacent to one another?

If they are adjacent, and you have connected them connected directly without using a separator of some kind, that might be causing the problem. . . I could see the tanks phasing into eachother if they're that close to start.

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White Rider

White Rider

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Don't just add struts between outer and inner tanks... Add struts between tanks in the same column.

Basically what you want to do is "stitch" them together with struts.

If you have for example a command pod at the top, then two fuel tanks and finally your engine; what you want to do is place struts on a x2 symmetry between the very top of the fuel tank above the engine and the very bottom of the fuel tank above that.

That should solve the problem.

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Immashift

Immashift

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In the cases where I've had fuel tanks collide into each other, both beside radially and on top of one another, it's been because one or two parts are bearing too much of the load.

For example, if I have a lander can on top of a rocket, and then a hundred ton lander on top of that, the can is gonna compress like tin foil when I light up the main stages. The answer is to strut to different parts of your vehicle so the load is more spread out. Also try designing stuff that doesn't hinge off one connection.

What I mean by that is that if you attach a tank radially to another, and then put another tank below the one you just attached, BOTH of them are anchoring their weight onto that one radial attachment, so instead try to spread the weight of your vehicle across as many of the components as you can.

Another problem can be if you cram a really powerful engine below a heavy vehicle. Like sticking a mainsail under something really heavy all piled on top and expecting it to be able to take the mainsail coming on at full thrust in a split second when you stage it. Throttle management is key there.

I dislike struts because of the silly high part count you can get if you go nuts with them, so I use them very sparingly. Most of my stuff, including my very heavy landers, are just put together in a way so that the 200+ tons of weight is spread out in a way such that each part bears a small amount of it.

Edited by Immashift
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Shrike42

Shrike42

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As I understand it, the physics calculations that occur at each step start with the command module, which I imagine is now "wherever the craft is initially supposed to be flown from". So, when you use struts, they are only effective if they're more 'closely connected' to that starting module as the physics engine checks for potential collisions/breakage.

If your fuel tanks get evaluated before the struts do, it's possible for struts to be ineffective despite looking correct. And I can only attest to what I know works on my rockets, as I haven't had my tanks explode from colliding with each other for quite some time.

To use struts correctly, you need to put the first side closest to the command module, then attach them to the part you want to secure.

Sadly, I think the Dev post explaining that was lost in the wipe.

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Sleipnir

Sleipnir

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Don't just add struts between outer and inner tanks... Add struts between tanks in the same column.

Basically what you want to do is "stitch" them together with struts.

If you have for example a command pod at the top, then two fuel tanks and finally your engine; what you want to do is place struts on a x2 symmetry between the very top of the fuel tank above the engine and the very bottom of the fuel tank above that.

That should solve the problem.

I have used that method as well, but it didn't seem to work. It appears that the problem isn't the tanks wobblying around and then colliding, it looks like they just smash into each other once the engines are activated.

Are the tanks radially attached or are they on top of one another? This is an important distinction.

Sorry my bad should have elaborated on that:

The problem is mostly occurring when STACKED on top of each other, but i have had it occur occasionally as well when radially attached to another tank as well.

@Shrike42: They collide into each other usually a few meters after launch, no decoupling at all. sometimes none of the tank stacks even appear to be moving in any direction and it still happens.

Also i do not have any craft file/ pictures. Once I encounter this and am unable to fix it I just scrap my design.......over and over. I take a pic nxt time.

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Sleipnir

Sleipnir

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Perhaps your TWR is too high? A TWR of 4 I have found, can lead to strange structural failures at full throttle.

My TWR for those heavy lifters is usually below 2, the first stage even below 1.5

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Immashift

Immashift

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Yeah if it's stack collapsing it sounds to me like your TWR is too high either when you're staging or taking off.

When you stage, try hitting X to set throttle to zero, then stage, then hold shift to get back up to power.

The sudden shock of engines coming on can be what collapses stacks.

If you're using Mechjeb, tell it to limit acceleration to like 20 or 40 ms squared. This also helps, and will save fuel since you're not trying to power through terminal velocity.

Back when I was using flimsy designs, I'd also do that at launch. I'd use the launch supports to raise the entire craft off the ground as far as possible, then hit space to launch with throttle at zero and immediately kick it up to full. This avoided the shock, but wasted some fuel. YMMV.

I just had this problem with a lander sitting on top of an interplanetary stage too. My solution was a couple struts from the lander's radial tanks to the part below the one the lander was attached to. This spread the load and all was good.

Edited by Immashift
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Sleipnir

Sleipnir

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Just to be sure, you saw the edited-in link to this other thread yes?

just saw that. very good idea!

@Immashift: I will try that as well, but the odd thing is this doesnt really happen right after I hit spacebar, the rocket usually flies up normally a few hundred meters (once even a few kilometers) and then the tanks collide at random.

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