Mechjeb suicided my rocket!

[Tex_NL]

Pretty sure I know what happened.
You launched your rocket with the ascent guidance autopilot but forgot to check the altitude. The intended ascent guidance altitude was lower than your altitude before launch. As soon as you launched the ascent guidance assumed it had already finished the gravity turn and tried to circularize the orbit. Normally this would mean MechJeb points the rocket at the orbital prograde, warps to Ap and ignite the engine. Since you were already at your' highest point' it immediately flipped your rocket and ignited.
I can't imagine any other behaviour that could cause the crash. MechJeb did exactly what you told it to do. You can not hold MechJeb responsible for wonky results when you give it the wrong info or target.

PEBCAK and not a MechJeb error!

Edited March 4, 2016 by Tex_NL

[Loren Pechtel]

11 minutes ago, Tex_NL said:

Pretty sure I know what happened.
You launched your rocket with the ascent guidance autopilot but forgot to check the altitude. The intended ascent guidance altitude was lower than your altitude before launch. As soon as you launched the ascent guidance assumed it had already finished the gravity turn and tried to circularize the orbit. Normally this would mean MechJeb points the rocket at the orbital prograde, warps to Ap and ignite the engine. Since you were already at your' highest point' it immediately flipped your rocket and ignited.
I can't imagine any other behaviour that could cause the crash. MechJeb did exactly what you told it to do. You can not hold MechJeb responsible for wonky results when you give it the wrong info or target.

PEBCAK and not a MechJeb error!

I was launching to rendezvous  with my tanker.  How would I enter a negative altitude?

This does give me an idea, though--I recall someone else having Mechjeb fly their rocket into the planet because they were using ion engines and the burn time was so long that the burn started far around the orbit but Mechjeb aimed without considering this.

Now, it appears that Mechjeb is only looking at the best time to launch within the orbit and is not considering that the ground track can vary between orbits.  (Mechjeb's approach would be fine when aiming for something equatorial.  My tanker was in a polar orbit as I wanted to hit every biome and that would have cost a lot more in plane changes if my tanker was equatorial.)  Perhaps it was trying something like that--trying to match the tanker's orbit when it should have waited for it to rotate around instead.

Unfortunately I don't have a save (I waited for a better alignment and then took it up manually) and now I'm on a new game where I'm not to the point of building such craft yet.  When I get there I'll see if I can reproduce it, though--I want to do some experimenting with it and see if I can write something that will let it handle a "land here" command while sitting on the ground rather than in orbit.

[Tex_NL]

When was I talking about a negative altitude? I never did.
Minmus has many locations that are several kilometres above 0. The highest peak goes all the way up to 5750m. If you're landed on a spot that's 4 km high and set the ascent guidance for 3 km your rocket will immediately flip and crash.

[Loren Pechtel]

28 minutes ago, Tex_NL said:

When was I talking about a negative altitude? I never did.
Minmus has many locations that are several kilometres above 0. The highest peak goes all the way up to 5750m. If you're landed on a spot that's 4 km high and set the ascent guidance for 3 km your rocket will immediately flip and crash.

But my target was my tanker.  It should have taken the altitude of the tanker.  IIRC it was something like 100km.

[cicatrix]

3 hours ago, Loren Pechtel said:

This was a hopper, IIRC on it's third launch from Minmus after the last refueling.  The rocket flew properly on the previous flights.

Can you post pics of your hopper? I think this might not have anything to do with MJ. This may happen if you have asymmetric thrust and your center of mass shifts when you burn up your fuel. The lighter your craft is the greater the effect. Up to the point when your reaction wheels or RCS cannot handle it. Perhaps the picture of your hopper may help to see what's wrong.

[Loren Pechtel]

4 minutes ago, cicatrix said:

Can you post pics of your hopper? I think this might not have anything to do with MJ. This may happen if you have asymmetric thrust and your center of mass shifts when you burn up your fuel. The lighter your craft is the greater the effect. Up to the point when your reaction wheels or RCS cannot handle it. Perhaps the picture of your hopper may help to see what's wrong.

Sorry, I don't have a save around anymore.

When I did it manually, just press Z and zoom into the sky it flew perfectly.  Furthermore, it flipped without the engine even being lit--this was clearly commanded.

[GraviTykillz]

10 hours ago, artwhaley said:

but I want to start by reminding folks again that mechjeb is a volunteer project that has taken thousands of hours to put together..  for nothing but fun.  So... please remember to keep your tone appreciative of what it DOES do... and not accusatory.  You have no RIGHT to mechjeb.

I just was reading thru this thread..and Ive seen so many like it. so...i just want to thank the great ppl who are keeping this mod alive and tell them how much i appreciate their efforts keeping this mod alive. THANK YOU SARBIAN ..

 

 

 

[Tex_NL]

3 hours ago, Loren Pechtel said:

But my target was my tanker.  It should have taken the altitude of the tanker.  IIRC it was something like 100km.

MechJeb has never taken the altitude of the target automatically. You have to manually enter it.

[Tex_NL]

I just tested my theory. I landed a craft on a Minmus hill just over 2700m. I set the ascent guidance altitude to 2500m (200m lower than my current altitude!) and targeted my Minmus station. As it's supposed to do the ascent guidance waited correctly to launch into plain. But as soon as it was time to launch exactly as expected instead of launching it flipped over the craft, ignited the engine and ran it into the ground.

@Loren Pechtel @sarbian MechJeb did exactly what it was supposed to do. This was pure input error.

Edited March 6, 2016 by Tex_NL