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I've got a very small probe rocket that I'm looking to launch to harvest a bunch of "Return science from X location" contracts- in this case, Kerbin orbit.

However, when I try to launch the probe, the rocket becomes uncontrollable in a strange way. First, the engine begins to shut down- the throttle is decreasing as if I were holding shift, though I'm not. Secondly, after the engine has shut down, the craft suddenly spins completely out of control, even though it was easily in control before. This starts to occur at alt 3km, speed about 330m/s. I've tried various combinations of the wings- it's a cheap vessel so I originally went with 8 basic fins, and I also tried 4 control surfaces, but it didn't make a lot of difference. Finally, the engine also randomly re-ignites after the craft starts spinning, which as you can imagine puts a terminal dampener on my aspirations to orbit this craft.

Why is my probe rocket's engine shutting down and then wreaking havoc with my stability?Probe rocket

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    I tried to rebuild your rocket and was able to get it into orbit without any problems just by using 4 instead of 2 winglets. I could not reproduce your problem with the engine shutting down either. Can you please post the .craft file? You find it in Kerbal Space Program\saves\[name of your savegame]\Ships\VAB\[name of your rocket].craft. You also seem to be using at least one mod. Maybe one of your mods is interfering? Please post a list of the mods you use.
    – Philipp
    Jul 26, 2015 at 22:13
  • Have you tried adding more fins for stabilization and taking 1 fuel engine off and test that? Also, if you don't have a SAS unit on that craft, try adding one and put on T + R before launch.
    – Coreus
    Jul 27, 2015 at 1:57
  • I found that I could effectively mitigate the problem simply by reducing the throttle to ~60% for the first few kilometers of atmosphere. As for mods, it's just MechJeb. That probe core already offers SAS- but it's way too low in the atmo for that to have much effect. Ultimately, it's the engine behaviour that I could not explain and was most damaging- the random spinning after that seems more problematic.
    – DeadMG
    Jul 27, 2015 at 18:58
  • ... also I didn't actually save the .craft, unfortunately, but I succeeded easily in getting to orbit with a close relative and some throttling down.
    – DeadMG
    Jul 27, 2015 at 19:02
  • "Just MechJeb"? You are using a mod with the purpose to take control of your rockets, and then you complain that control of your rockets is taken over?
    – Philipp
    Jul 29, 2015 at 10:29

3 Answers 3

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If you are using MechJeb, a likely cause would be if you checked off

Limit to terminal velocity

As your probe approaches terminal velocity MechJeb will cut your engines so you don't overspend fuel. A side effect of this is that you lose thrust and ultimately control as the lift from your fins comes into play.

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  • Wouldn't that be checked on?
    – DeadMG
    Jul 28, 2015 at 18:09
  • Well, whaddya know- it is checked on by default.
    – DeadMG
    Jul 28, 2015 at 18:12
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Given that the commenters couldn't reproduce the problem, and I effectively prevented it by simply throttling back for the first few kilometers of atmosphere, I'm inclined to believe that this is just some random glitch in the game.

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is it possible that you had your control point upside down? at launch you would see the brown pointing up instead of the blue on the nav ball. Mechjeb does not work when this is the case.

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  • Try not to ask further questions in your answers. When you reach 50 reputation, you can comment on all posts.
    – Timmy Jim
    Jan 28, 2017 at 16:55

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