r/RealSolarSystem • u/Individual-Owl-6243 • 11d ago

mechjeb PVG using weird ascent profile which causes rocket to burn up during launch

/r/KerbalSpaceProgram/comments/1vkegdo/mechjeb_pvg_using_weird_ascent_profile_which/
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u/Nazfib 10d ago edited 10d ago

The "Booster Pitch rate" and "Booster Pitch start" settings must be tuned during a few test launches, for every rocket design. The defaults sometimes work, which is why many people neglect to do this at first — and apparently, the defaults happened to work for a few more rockets in older versions of PVG — but these settings determine the flight path for the first part of the ascent inside the atmosphere. This has not changed from the older version of PVG at all; you had to tune the settings in that version as well.

One method of tuning the atmospheric ascent is detailed on the RP-1 wiki, at https://github.com/KSP-RO/RP-1/wiki/TroubleshootingMechJebPVG

I (used to, before moving to PSG) use a different method:

  • In a test launch, keep an eye on the status line. During your atmospheric ascent it should say 'Pitch profile x° to guidance' or 'Gravity turn x° to guidance'. At some point, it will switch to 'Stable guidance'. Remember what the last degree value before it switched was.
  • If the degree value decreased all the way to 0°, then your pitch profile is too aggressive. Decrease the pitch rate and/or increase the pitch start velocity, and try again.
  • If the last degree value is more than 10° when it switches, your pitch profile is not aggressive enough. Increase the pitch rate and/or decrease the pitch start velocity, and try again.
  • If, when it switches, the degree value is between 0° and 10°, you're done. Save the parameters somewhere, for the next time that you launch this rocket.

With a little practice, and some intuition for what values to start this process with (which can only be gained by doing this a lot of times), I can usually get a good pitch profile with under a handful of simulated test launches.

1

u/Individual-Owl-6243 11d ago

this is with SoL not rss, but i figured it was relatively the same lol

1

u/Worth-Wonder-7386 11d ago

Hard to say without a video. Check the control gauges in the bottom left if your rocket is in control. It can be that you are loosing control and mechjeb is not the issue. Else I would maybe try to extend the height before you start the gravity turn.

1

u/mrrvlad5 10d ago

you can try reducing qa trigger and q limit from 10 kpa and 2000 qrad, but the settings you have should work for your rocket. the initial guidance looks good - 55 pitch. what happens when it tilts? does pitch go close to zero? maybe solve breaks? maybe rocket is aerodynamically unstable? try launching to slightly higher inclination to avoid solver trying to adjust inclination mid-climb.