How to fix engine "colliding" with its OWN fuel tank?

[Dunbaratu]

When the ship breaks apart and the after-crash report shows the first failure as being an engine "colliding" with its OWN fuel tank, how on earth do you change the design to fix that? First off, "collide" is probably the wrong word to use since it implies that two things start off NOT touching and then touch each other through movement, and this is the exact opposite - this is two things that already are connected and the connection breaks and they become disconnected. That's sort of the exact opposite of a collision. Secondly, what on earth am I supposed to do about it? How do I make a connection stronger than simply attaching the engine directly to the tank? Struts don't work because there's no bits sticking out to strut to.

[Sir Nahme]

I'm guessing this happens on load on the launch pad, your rocket is too heavy for the engine to hold it up by itself.

If so, launch clamps are the best solution. If not, we could use pictures

[Raxacorico]

You can attach bits to strut the two items with. Or you might consider managing your thrust differently during liftoff. Adding full power all at once can subject your craft to terminal velocity drag and excessive thrust-related g-forces. Imagine squishing your poor rocket in an invisible vise made of rocket exhaust below it and the ambient air above it. You are right about the two parts disconnecting though... it might be nice if the crash report gave a more descriptive analysis of what went wrong. Maybe someday there'll be a mod to playback your crashes in slow-motion with keys on an animation slider and call-outs indicating where each structural failure event occurred.

[rhoark]

This can be a symptom of excessive thrust-to-weight ratio.

[rkman]

I had the same problem with two fuel tanks stacked one on top of the other.

During ascent according to the flight log (F3) they would collide with one another without first braking the link between the two. That's with TWR < 2.

Solution: struts.

[Semni]

The solution to all these problems is indeed, launchclamps, struts and if you are really desperate throttling back in the dangerous area of your acent. The Mainsail is especially prone to doing this because of it's incredible thrust.

[Dunbaratu]

My TWR is very low. Max throttle all the way still only has me going 100 m/s at an altitude of 5000. It *barely* moves up until it gets into the thinner air, and the "collision" usually happens under 10,000 feet before it gets to where I can do that.

The mainsail engine is used because I'm launching something heavy.

The problem is just how do I strut it? There's nothing sticking out, and if I add girders to make the strutting work, I end up breaking the bits then.

[maackey]

The problem is just how do I strut it? There's nothing sticking out, and if I add girders to make the strutting work, I end up breaking the bits then.

Create a ring of cubic struts (8 usually works fine) and that should give you the angle to attach a strut from it to the engine. Then you attach another strut from the cubic strut to the tank.

I've previously had the problem of the large ASAS module being crushed between my payload and fuel tank, so in that case, I made two rings of the small cubic girders, above and below the part, and had 3 struts a piece to connect the tank to the lower girder, the lower girder to the higher girder, and the higher girder to the payload decoupler.

[Semni]

Putting a lot of thrust onto a very heavy airframe generates a lot of stress in the joints between parts. Try looking at the path of force and adding a ring of struts to the collumn directly above your mainsail.

[Dunbaratu]

I've been trying now for quite a while and the chief problem seems to be that the game utterly refuses to ever attach ANYTHING to a mainsail engine. I can't attach struts, girders, I-beams, or anything at all. Whatever I try to attach to it just stays red and won't go "green". When I thought I was attaching something to the mainsail engine, I was actually attaching it to the very bottom of the fuel tank above it. It seems the yellow ring around the top of the mainsail engine is just simply NOT a clickable surface - at least that's how the VAB is acting.

[tavert]

You can't attach FROM an engine, but you can initiate the strut on other components (say the inner surface of the fuel tanks of a ring of stages around your core) and connect TO an engine.

[rkman]

The Mainsail is especially prone to doing this because of it's incredible thrust.

Putting a lot of thrust onto a very heavy airframe generates a lot of stress in the joints between parts.

Without struts my rocket was fine with mainsail + two SRBs at full throttle, it fell apart after dumping the burned out SRBs.

The rocket is nothing special, just a 10 ton, 2 man Mun lander (20tons to LKO).

I think it's not right that a rocket that because of its mass needs the thrust of a (one) mainsail, is not strong enough to withstand the thrust of that engine.

Interestingly, none of the stock example crafts have struts as reinforcement of connections between stacked parts.

It is to be expected that nubs become a little frustrated with building rockets, until they discover the black art of strut placement. Also it's a topic on which there are virtually no tutorials.

Edited May 26, 2013 by rkman

[CozmikR5]

When in doubt, strut. In fact this game could be renamed "Kerbal Space Tape Program".

[Mihara]

The reason the engine collides with it's own tank is that stresses cause this particular connection to break, after which, the engine continues forward due to inertia, while the rest of the rocket, being so much heavier and no longer having the thrust of the engine, remains relatively immobile. This happens for one of the two reasons: Your thrust to weight ratio is too high, so the upwards force from the engine is excessive, or the mass of the rocket directly above the engine is too high.

The obvious approach to fixing this is reinforcing this particular connection or introducing extra connection to relieve stress. You cannot start a strut from the engine, but you can start a strut from something else and end it on the engine, which will sort of do that.

The better approach is to redistribute the load somewhere else by making your rocket wider, though, i.e. growing stages horizontally rather than vertically. KSP rocket design is not quite like in our universe because we have much stronger structural connections, tanks with much less dry weight, and a very different set of aerodynamic laws. Not only kerbals can get away with shedding stages horizontally, but actually they have to prefer it if they want a worry-free launch.

[Dunbaratu]

Thanks for the advice on making a strut that ends on the engine instead of starting on it. That works. Even though it makes no sense why the editor makes you do it that way. I can't see any real difference between strutting from engine to tank versus from tank to engine other than just a weird bug in the user interface.

The reason the engine collides with it's own tank is that stresses cause this particular connection to break, after which, the engine continues forward due to inertia, while the rest of the rocket, being so much heavier and no longer having the thrust of the engine, remains relatively immobile.

Shouldn't the engine become just as "immobile" when the fuel tank disconnected? It's just as dead in the water as the tank is when it has no fuel.

This happens for one of the two reasons: Your thrust to weight ratio is too high, so the upwards force from the engine is excessive, or the mass of the rocket directly above the engine is too high.

None of that is properly described by the word "collide" though. Things that were already touching each other aren't colliding, they're pushing.

It's the difference between boxing and wrestling.

[Dunbaratu]

Without struts my rocket was fine with mainsail + two SRBs at full throttle, it fell apart after dumping the burned out SRBs.

The rocket is nothing special, just a 10 ton, 2 man Mun lander (20tons to LKO).

I think it's not right that a rocket that because of its mass needs the thrust of a (one) mainsail, is not strong enough to withstand the thrust of that engine.

I'm trying a larger payload. I knew it would need more reinforcement but the chief problem I was having was the inability to connect anything to an engine so strutting was impossible. Finding out that it works when the engine is the endpoint but not the startpoint of the strut made all the difference. It's not a thing anyone can be expected to learn by experience because it is a very weird non-intuative workaround for a finicky user interface problem rather than something you can work out by any logic.

[Mihara]

Thanks for the advice on making a strut that ends on the engine instead of starting on it. That works. Even though it makes no sense why the editor makes you do it that way. I can't see any real difference between strutting from engine to tank versus from tank to engine other than just a weird bug in the user interface.

That's simple. Engines forbid you to attach objects to their sides. (Numerous reasons to do this, one is that their collision meshes are more like cylinders, while their visible shapes are much more complicated.) A strut is an object that, like a fuel pipeline, is attached to the object where it starts, and this is how it is stored in the object tree. As far as I can see, the rest of the strut is illusory in terms of physics and only creates a 'structural link' with no object inbetween the start and end -- it affects force translation when objects suffer stresses but can't be collided with itself.

None of that is properly described by the word "collide" though. Things that were already touching each other aren't colliding, they're pushing.

That would be the correct way to refer to it, but the game engine can't tell. As far as I can tell, from the game engine viewpoint, the following happens:

1. The strength limit of a connection is exceeded and the connection breaks.
   
2. Two separate objects are created -- the engine, and the rest of the ship. At this moment, both are still intact and even the engine is still working, because it would only check for the fuel source availablility in the next physics frame. Both are assigned slightly different speeds and motion vectors due to their different mass, drag values, etc.
   
3. The engine typically ends up the faster moving one, so it collides into the heavier tank.
   

What actually happens is best described by the engine and the bottom end of the tank crumpling into each other.

[numerobis]

The log often misses interesting events. I occasionally have a spacecraft fall apart quite spectacularly and the log only mentions a couple collisions and an engine damaging some random part. It makes it a bit annoying to figure out what went wrong.

[Friaar]

Hello!

Just putt alot of struts and stuff! and in takeoff so dont have 100% thrust