Rocket Falling Apart After Clean Booster Separation at 10Km

[Captain_Brian]

I don't have a screenshot, but it is a pretty simple rocket so here is the layout:

[Lander]

[struts]

[With 4x thruster blocks]

[FAILURE]

[DESTROYED]

[DESTROYED]

x8 [Attached to Above Large x200-32 Tank, Separation For Boosters at 10km]

x8 [s1SRB-KD23k Large SFB, Attached to Radial Decouplers above, with proper Seperatron 1's for clean separation and struts for stability]

[iNTACT?]

For some reason as soon as I separate the boosters, the 2 fuel tanks right above the mainsail fall apart (they don't explode) and then the mainsail floats away from the rest of my vehicle.

I've tried taking the entire rocket apart and putting it back together, but it still does the same thing. At one point, instead of the fuel tanks falling apart, the mainsail "stretched" and then detached from the main body upon booster separation.

I've also tried reinforcing the fuel tanks and engine with girders and struts, but it didn't make a difference.

Any ideas?

Edited July 8, 2014 by Captain_Brian
Answered by NecroBones!

[michaelhester07]

Did the SRBs impact the mainsail engine when they separated (this is like 90% of my launch failures).

You can put the SRBs on the end of pylons (use the girders to make pylons) to help prevent them from hitting the mainsail when they detach. The other part is to make sure the SRBs are separated before you start your gravity turn or after the turn is complete as the boosters will drag into your ship if you're starting the turn when you release them.

[Laie]

That's these extra-long boosters, aren't they? It's common for them to collide with your vessel. Just look closely and/or check the F3 window, I guess that's what happens. In your case, they smashed the tank but spared the engine.

I found that a clean seperation is easy if you are still pointed straight up, ar almost straight (like, 85°+). It often works without further ado or -at worst- takes nothing more than one sepratron on top of every booster. However, if you're already at 60° by then, no seprator is strong enough: the boosters above your vessel will come falling down before you can get out of the danger zone. In that case, it will take a lot more effort (+trial-and-error) to effect a clean separation. I suggest you also look into winglets and/or parachutes rather than sepratrons alone.

At one point, instead of the fuel tanks falling apart, the mainsail "stretched" and then detached from the main body upon booster separation.

Or maybe a problem with recursive symmetry? In VAB, do any of your parts look a little strange? Is there a shine or glittering?

Edited July 7, 2014 by Laie
+symmetry

[Captain_Brian]

I'll have to try to check the F3 window when I get home later. I forgot what the hotkey was for that.

I did run it about 5 or 6 times and looked closely from every angle and looked like a completely clean separation before the gravity turn. The thing is that the mainsail is actually positioned at the 2/3rds point of the boosters, not even with them. When the boosters separate the Separatrons cause them to "flower" open and away very nicely.

What is really odd is that the fuel tanks don't explode, they just get destroyed. I've had tons of collisions in the past, and they're almost always accompanied by a bunch of explosions.

I checked for recursive symmetry as that has caused problems with a few designs in the past, but I couldn't find anything.

[Diazo]

Did you add struts to your center rocket to the SRBs beyond the decouplers?

The mainsail is powerful, it does not take much for its thrust to be sufficient to crush the fuel tanks it is attached to.

If you had struts between the two tanks above the mainsail and the SRBs, those struts would keep the connection between the tanks from compressing until separation when the struts disconnect. At this point the join between the two tanks goes from "at rest" to "compressed" and the full thrust of the mainsail is enough that this jolt exceeds the tanks strength and so they fail.

To test, cut throttle before separating the SRBs and then slowly throttle back up. If there is no failure, it is probably this.

If you still see a failure on those tanks when the SRBs separate with no thrust, it is a different issue.

D.

[Laie]

@Cpt_BRian: Your second post pretty much rules out all standard explanations. Would you please post your craft file later? I'd like to see that for myself.

[NecroBones]

How many Sepratrons are you using? The exhaust from those can destroy your fuel tanks. I find it's often best to angle the Sepratrons outward a bit, so their exhaust doesn't hit the tanks directly.

[Captain_Brian]

@NecroBones: I used 2 per booster, with 8 boosters that would be 16 hitting directly at the same tank so that could very well be the issue too. I'll have to try skipping activating them and seeing what happens.

@Laie: I'll be sure to post the file later! Though it seems like Diazo might be on the right track as to the cause. I really am surprised that a fuel tank can't withstand the force of an engine it was designed to connect to. :/ While I tried reinforcing the main fuel stack with girders and struts, I didn't try reinforcing the engine to it as well.

Did you add struts to your center rocket to the SRBs beyond the decouplers?

...

To test, cut throttle before separating the SRBs and then slowly throttle back up. If there is no failure, it is probably this.

If you still see a failure on those tanks when the SRBs separate with no thrust, it is a different issue.

D.

Ah, this is a good test to try! I'll let you know how it goes.

I did have struts on those two tanks to the SRBs, I actually had struts on all the tanks to the SRBs to be on the safe side.

[NecroBones]

@NecroBones: I used 2 per booster, with 8 boosters that would be 16 hitting directly at the same tank so that could very well be the issue too. I'll have to try skipping activating them and seeing what happens.

Let me know how that works out. I've seen it happen with as few as 4 of them. I've had this happen on many occasions, and after changing the angles so the exhaust didn't hit, the problem went away. And it was just how you described it, with the tanks vanishing, not large explosions.

[Alistone]

How fast is your ship going when it breaks apart? Due to expended fuel decreasing mass and an increasing T/W ratio I used to have large vessels break apart near 8-12km. Also, the flameout of your SRBs could cause a springback effect. Your ship is compressed between the boosters and the air resistance and when the boosters stop it elongates like a sprink which can cause damage. Either way, I suspect you can fix the problem by simply throttling back to 50% thrust or something just before your SRB burnout then slowly accelerate back to full throttle.

[Captain_Brian]

NecroBones is the winner!

The problem was with the Separatrons causing the fuel tank to be destroyed!

Here's a screenshot of the rocket, and the .ship file:

https://www.dropbox.com/s/u8f0292yo1nsg0x/LUNAR%20Mk1%28Broken%29.craft

Thanks everyone! Great advice all around and good things to keep in mind when designing a rocket!

BTW, anyone know how I change this to "Answered"? Thanks again!

Edited July 8, 2014 by Captain_Brian

[Kryxal]

I read a neat little trick somewhere about using separatrons ... basically, you place them low on the booster and pointing straight up, it pulls the booster down and rotates it clear.

[Blind Dead McJones]

I once saw a very Kerbal trick where someone placed the small SRBs on top of the booster rockets facing so the engine points at the central stack. Those rockets fired when the next stage started and the booster flew away. He did this instead of decouplers for some reason, but if you have those then it should be enough to get your large SRBs away if the ordinary sepratrons aren't working.

Mind, this approach was because he didn't use decouplers for some reason, and he had a video where the separation didn't work and the central fuel tank overheated and exploded.

[NecroBones]

Great! Yeah the sepratrons can really surprise you that way. I've done a variety of thing with them to get around this, including angling them vertically to just push the boosters down, and also placing them on diagonals so that the fire toward each other, more than at the center tank. All of these things work, as long as they don't hit the tank directly. As someone pointed out, you can place them vertically on the outside of the booster, and you will still get some rotation away from the ship, by virtue of the fact that they're firing off of the center of mass of the booster.

BTW, you can change it to "answered" by going back to the first post, editing it, and going to "advanced", and you should see an option there to do it.

Edited July 8, 2014 by NecroBones

[Captain_Brian]

Cool, I'll try that! Thanks again!

[Master Tao]

I've found a reliable position for sepratrons: nozzle up, vertically centered, and on the outside of the booster. This position pushes them down without rotating them so much that they collide with the central stack.