After stage, top stage clips into bottom and blows up

[RoninFrog]

I built a massive lander for the moon which was attacked by a kraken while landing.  While landing, I deployed the bottom stage (14 Jumbo-64s and 7 mainsails) to eject the lander (2 X200-32s, mainsail, hitchiker, Mk1-3pod, and landing gear).

What I expected was that the stage would disconnect and the lander would fire the mainsail and land some distance away.  What happened was that the lander sank rapidly into the bottom stage, and when it was almost all the way in it registered that there was a collision.  My kerbal in the hitchiker went splat and the two in the command pod got launched at half the speed of light out into the outer void.  Is there a known solution to this bug?  I really want to land on the Mun with this lander, but if I have to eject the bottom stage early, I won't have enough fuel on reentry.

[Gargamel]

Did you throttle down to 0 before staging?  Some pics of your craft would help. 

And as I believe this is not an actual bug, but a design issue (or at best a random, non solvable kraken attack), I'm moving this thread over to Gameplay Questions. 

[Jestersage]

TL;DR: Check the root of your entire craft, and make sure the root is at the upper stage/payload.

One of the problem I discovered is the root of a craft can caused unexpected behavior upon stage and acceleration  Specifically, assume a craft have 2 stages, if a part in the the first stage serve as the root, without rerooting to a part in the uppermost stage, upon decoupling, a pogo effect on the supposed upper stage will be exhibited. This may or may not be related to MH engines plate.

In the case of my example, there is a reason I explicitly tell user to reroot once stack size is confirmed; otherwise, upon decoupling, the 4 terriers on second stage will launch itself into the fuel tank. The 2 solutions are either 1) Re-root or 2) Add struts from engine to fuel tanks.

Edited August 12, 2019 by Jestersage

[bewing]

I gotta agree with the others here. The only reason why your two stages would smush together like that is if your bottom stage is still under high thrust after you decouple. That's always a very bad idea in general.

X to kill thrust, space to decouple, Z to restore thrust to your lander only. It only takes a fraction of a second to do.

 

[RoninFrog]

Here is the craft.  I was unable to screenshot the glitch (but it did happen twice while attempting a landing).

https://ibb.co/7j8Rx34

[dire]

Have you tried decoupling the bottom stage in orbit rather than while landing? 

[Snark]

On 8/12/2019 at 11:09 AM, MunsterIII said:

I built a massive lander for the moon which was attacked by a kraken while landing.  While landing, I deployed the bottom stage (14 Jumbo-64s and 7 mainsails) to eject the lander (2 X200-32s, mainsail, hitchiker, Mk1-3pod, and landing gear).

Things to check:

• Is your bottom stage still under thrust (i.e. still has fuel in it, throttle set > 0)?
• Which part on your craft is the root part?  Specifically, is the root part on the lander, or on the bottom stage?

Also, a side note in passing, unrelated to your question.  Any particular reason you've got a Mainsail on your lander stage?  That seems like massive overkill for the Mun, unless your lander is well over 300 tons or something, which it doesn't look like in your screenshot.  Looks more like you're in the 40-50 ton range, in which case a Poodle would be plenty for the Mun.  The combination of saving several tons of engine weight and getting a significantly higher Isp would mean the lander would pack a fair bit more dV, which in turn would mean that the bottom stage wouldn't have to be as hefty.

[RoninFrog]

Link has the info: the bottom stage had barely any fuel left, the throttle was slightly above 0 (I tapped shift once).  Lander is root part.  Also, the main landing engine wasn't supposed to be a mainsail; it was supposed to be a poodle.  I just had a little extra fuel in the previous stage and decided not to waste it.

Edited August 16, 2019 by RoninFrog

[Snark]

20 minutes ago, MunsterIII said:

the throttle was slightly above 0

Have you tried cutting throttle before staging?  You can crank it back up again after staging; all you need to do is zero the throttle, stage, then add throttle again.

If the throttle is non-zero when you stage, then the staged-away boosters are going to keep firing (assuming there's some fuel left), which will accelerate them up into your ship; it's a collision hazard.  And with all those Mainsails, and if the fuel tanks of the lower stage are mostly empty (thus low mass for the stage), it may actually have a hefty acceleration once it's not trying to lug the mass of your heavy, fuel-laden lander.

[RoninFrog]

Yeah, cutting the throttle seemed to work.  I actually had two stages above that to land and the extra got me clear of the wreckage.  It was a bit tight though.

Edited August 16, 2019 by RoninFrog