My rocket explodes shortly after launch

[Vojta]

Second stage of my rocket keeps exploding 9 seconds after launch. I used TweakScale and Procedural Fairing.

 

 

Reason: High G forces (very powerful thrust) were destroying the rocket.

Edited March 3, 2018 by Vojta
Problem solved

[Aegolius13]

As Scott Manley would say, "check yo' staging."  I can't really tell what goes with what, but there's a decent chance you have your second stage engine lighting up too early.  

Also possible your boosters are not attached  securely (could be too few or too many struts/autostruts) so that they start to wiggle and eventually impact into something).

[Gargamel]

99% of crashes like this are from staging issues, but....

Where is the strut connector located in the first collision report?  If you are using struts to hold the tips of the boosters in place, and the struts are attached to a fairing, they may not be actually doing anything.   The booster may be striking the tank and causing a RUD this way. 

[Zhetaan]

@Vojta:

First, welcome to the forum.  I'm sorry to hear about your rocket trouble, but from what you've shown so far, I have a few ideas.

Nine seconds is very specific, so it points to a problem with the design rather than with the piloting.  I would expect both engine exhaust damage and staging issues to destroy the rocket more quickly, but it is worth checking both of those.

However, I suspect that your main issue is a combination of too-flexible connections in your core stack and too much thrust.  If your rocket flexes a lot before you release the launch clamps, that may be a clue.  In that case, the boosters burn off fuel which decreases their mass, increases their thrust-to-weight ratio, and increases the severity of any oscillations to the point of rapid unplanned disassembly.  Since the burn rate is constant, the destruction is predictable; hence it always happens at nine seconds.

As to the thrust, I noticed that you reached a maximum gee force of 6.0 gees.  That may be because of the way the rocket was destroyed (blasting the root part away at high speed will often trick the accelerometer) but it also says that you reached a speed of 213 m/s and at least an altitude of 1,120 metres.

If you could do that in nine seconds, then I think you have too much rocket.  Extremely high thrust will also stress the connections between parts and try to break your rocket apart.  If the connection is even a little flexible, then the tip of the rocket (which, if well-designed, has most of the mass) will deviate from the thrust direction.  Add the distance to the engine and you get a lot of torque on the joint; add extreme thrust and you multiply that torque to the point of breakage.  Torque, for the lay physicist, is force times lever arm times the sine of the angle between them; in KSP, this is roughly related to thrust times rocket length times the sine of deflection.  Given that the last item in your crash report is a linkage failure between a fairing and the base ring, I suspect very much that this is your issue--torque-related destruction is about the only way to get a base-fairing linkage failure in Procedural Fairings.

I don't know how TweakScale deals with connection strength, but it's possible that when you take a small part and make it into a large part, it will not scale the strength enough, or it scales the mass or thrust too much, or what-have-you.  If nothing else works, try a scaled-down version of your rocket and see what that does.

Edited March 2, 2018 by Zhetaan

[Zeiss Ikon]

I'm going to join others in pointing to the Strut Connector colliding with the tank being the first failure.  Assuming the 7200 tank is the second stage tank/structure, I think one of your boosters is shifting inward, destroying the second stage tank, which then allows everything else to collapse in a fraction of a second (all those failures are listed at 9 seconds MET).  Checking your staging is a sensible thing in any failure analysis, but in this case, I suspect you have struts that weren't applied in symmetry (even though the SRBs presumably were), leading to at least one that didn't get connected correctly; your nine seconds is the point at which something (likely aero forces) adds up enough to let the decoupler flex the strut connector into the tank.

[peteletroll]

If you press F3 just after the explosion, you should see a flight report that can help you point out what happened exactly.

[Zeiss Ikon]

18 hours ago, peteletroll said:

If you press F3 just after the explosion, you should see a flight report that can help you point out what happened exactly.

The OP already posted the log window that this brings up.

[Vojta]

Nothing happens to the first stage or payload, only the second stage blows up completely. The rocket start wiggling more and more until it explodes. I have tried launching it multiple times and it's always the same : 2 parts of stage 2 collide (including the two fuel tanks) and it disappears instatly. I also tried launching the rocket without payload and it worked okay.

Edited March 2, 2018 by Vojta
forgot to add extra info

[Vojta]

On 1. 3. 2018 at 4:44 PM, Zhetaan said:

 

As to the thrust, I noticed that you reached a maximum gee force of 6.0 gees.  That may be because of the way the rocket was destroyed (blasting the root part away at high speed will often trick the accelerometer) but it also says that you reached a speed of 213 m/s and at least an altitude of 1,120 metres.

If you could do that in nine seconds, then I think you have too much rocket.  Extremely high thrust will also stress the connections between parts and try to break your rocket apart. 

Thank you. High thrust was the cause of explosion. Throttling down worked.

[Plusck]

What are the struts from the top of the boosters connected to?

It's very difficult to tell from your screenshot, but it looks like maybe you have four kerbodyne s3-7200 tanks surface-mounted and then both angled and offset in to the craft.

That six-way booster symmetry cannot hope to connect properly via struts to a 4-way symmetry group of (whatever it is). In addition, if (whatever it is) is actually a group of 4 s3-7200s, you are adding horrendous amounts of drag to your rocket (unless you're using an aero-changing mod).

So assuming that the above is more or less true, you really have to rebuild all of the struts. Since you're going from 6-way to 4-way symmetry, you probably don't have any option other than placing each strut individually.

You should maybe also rebuilt the radial decouplers for the boosters: if the struts are not helping (and apparently they are not) then that may be because the radial decouplers are not properly attached (i.e. offset out and therefore making a very weak link) . KSP does alter the strength of a radial connection depending on how deeply it is set; you cannot offset radial decouplers inwards without causing disasterous effects on staging, but it is easy to offset them outwards and weaken the structure without necessarily making staging them a disaster.

[Vojta]

The struts are attached to the panels covering the second stage engines.

The parts offset into the second stage are scaled up C7 adapters.

There are no radial decouplers. I tried building something that looks like a Proton rocket, which is using radially attached fuel tanks connected to the center fuel tank.

[Zeiss Ikon]

2 hours ago, Vojta said:

The struts are attached to the panels covering the second stage engines.

That part is also called a "shroud".  Fastening struts to it is completely useless (in fact, in my experience, the strut will either refuse to attach (looks like it's just the base end without the attachment and second end) or will auto-jump to the tank just above the shroud.  The former is most likely if you've had the strut on, then removed and replaced the booster/tank with the base end of the strut.  Reset those struts so they connect to the second stage tank and they'll do much more to stiffen your stage joint.

[Vojta]

14 hours ago, Zeiss Ikon said:

That part is also called a "shroud". 

I am not using the engine shroud (created by a decoupler), I used structural panels.