Uncommanded throttle down

[KerBlammo]

I spent a lot time in KSP this past weekend. At some point designs that previously flew with no problems started having issues with un-commanded throttling. I am using MechJeb on the crafts but I am not using it during the ascent. I engage SAS and set the throttle to 100% before launch.

Craft 1 has liquid fuel main stage and SRBs. During the ascent the main engine operates normally at first and then just throttles down. I can hold the shift key and throttles back up but then immediately throttles back down again. I press shift again and the same thing happens, after that it just stays down. At staging the throttle comes back up again and remains at 100%.

Craft 2 has LF main stage and LF boosters. This one just sits on the launch pad with the engines sputtering (rapidly oscillating between 0 and 50% throttle) after 30 sec or so it starts operating normally and lifts off. (Note this design previously flew properly and has a TWR of over 1 so its not a question of TWR <1). During the subsequent ascent it seems to run normally.

For each of the above the issue happens the same way every time I try to launch.

Edited to add: Upper stages all seem to fly normally.

I tried making a quick test example from scratch today (2 stage rocket with a Mainsail and 4 large stock SRBs (Kickbacks iirc) as the first stage) and that flew as expected.

I replaced the first stage on Craft 1 above and it still had the throttle down problem, but it manifested differently than before. This time it even throttles down after SRB staging and then spontaneously throttles back up.

Has anyone experienced anything like this?

Around the time this issue started I installed the NavyFish docking indicator but I don't think it happened right after that but I can't be certain. Removing that mod didn't fix anything (and I doubt it had anything to do with it but who knows).

Edited August 11, 2015 by KerBlammo

[Phil deCube]

Check Mechjeb's Ascent Guidance tab. You may not be using it, but it could have the Terminal Velocity button selected (I don't remember the exact name). This will cause throttling down while in the atmosphere, but not in space.

Edit: "Limit to terminal velocity" it's called. Untick it.

[KerBlammo]

Yes! That was it!

I only use the ascent guidance for Mun lift off (so the ascent stage gets on the correct orbital inclination) and must have inadvertently clicked it. It never occurred to me that a setting would have an effect if the feature was not active. Now I know.

Thank you very much!

My test craft this evening worked ok because I didn't put a MechJeb on it...

[Phil deCube]

Yes! That was it!

I only know because it had me confused at one time too.

[Pecan]

"Limit To Terminal Velocity" is one of the settings in MJ's 'Utilities' section, along with smooth throttle, prevent jet flameouts, etc. These are always active when selected and will cut-in whenever relevant, even if you haven't got any other MJ windows open or functions activated.

Having said that ... terminal velocity is incredibly high in 1.0.4, to the exten that it is hardly ever a limiting factor. If you are having problems with it - say on the launchpad!? - I'd think there has to be something else going on. "LF" main stage & boosters, not "LFO"? Could be air-breathers not having enough intakes for full throttle but not enough thrust to lift at low throttle?

[Wemb]

"Limit To Terminal Velocity" is one of the settings in MJ's 'Utilities' section, along with smooth throttle, prevent jet flameouts, etc. These are always active when selected and will cut-in whenever relevant, even if you haven't got any other MJ windows open or functions activated.

I must admit I've not worked out though, why this seems to kick in if the AoA starts to get high during ascent, e.g. if the rocket starts to tumble than invariably MJ will throttle back the engines, usually till my rocket is upside down, and then it goes back to full thrust...

Wemb

[Kryxal]

That's simple. If AoA gets high, induced drag gets high (probably) and terminal velocity drops. It's like flying a plane straight and level, then you roll nearly 90 degrees and pull up HARD ... you're going to shed speed fast though you're likely not gaining or losing much altitude.

[Wemb]

That's simple. If AoA gets high, induced drag gets high (probably) and terminal velocity drops. It's like flying a plane straight and level, then you roll nearly 90 degrees and pull up HARD ... you're going to shed speed fast though you're likely not gaining or losing much altitude.

Oh, of course- I'm used to thinking of TV as a function of just the altitude cause, obviously, my rocket isn't changing shape - till it's AoA changes and then it is..

Thanks.

Wemb

[KerBlammo]

"LF" main stage & boosters, not "LFO"?

Yes I meant LFO as in liquid fuel and oxygen (not to be confused with a Low Frequency Oscillator )

Not sure why it was limiting the rocket on the pad, but unchecking that box let it fly normally.

[HelloMaxxo]

hi

 

[CrashyMcCrashFace]

Yea, the MechJeb tools (sometimes, always, often, can, all-of-the-above) stay active when you close the window. Intuitively, closing the window seems like it should also turn off the feature.

Edited June 13, 2018 by CrashyMcCrashFace

[Xd the great]

Those sound like you have enabled the limiting acceleration thingy.

For the sake of being kerbal style, i usually set it to 9999m per second.

Probably not the docking mod. They work fine on my computer.

[Snark]

Note that this thread is from three years ago.  Everyone who had a problem has presumably long since found an answer, and in any case the discussion is now moot because the software involved has gone through updates since then.

Locking the thread to prevent further confusion.  If anyone has a question about a current problem, feel free to start a new thread.