MechJeb issue

[Gargamel]

I have a docked ship, a mothership and a lander. The lander is docked inverted to the mothership, meaning it's lander can and MJ radio box are also inverted. The MS's command pod and Radio box are oriented correctly. The engines on the lander are shut down (but having them there affects the Dv estimations), and the MS command pod is controlling the ship. I can change orbits (raise, lower, circularize) using the manuever node planner correctly, but anytime i try to match inclination or transfer to mun or minmus, it fires the engines the instant it aligns with the node. The ships fly just fine individually. Any suggestions on whats going on? I've had inverted docked ships before and never encountered this problem.

[AfailingHORSE]

Right click on the CM of the MS and make sure you are controlling it from there, then right click on the engines of the lander and turn them off.

[Link]

You're probably telling it to match inclination after 0s, which would be immediately. Instead, change it to match inclination at either the an or dn node. Doing it this way is also more fuel efficient

[Sirrobert]

Mechjeb takes all it's readings from the controlling part, not the Mechjeb part itself.

If it fires the instant it's alligned with the node, that means it is already past the time it should have started the burn, and is trying to start as soon as possible. Give it more time to prepare for the manouver.

[Gargamel]

Every other time I've used the manuever planner, it waits till Node T-(1/2 burn time) to fire the engines. Now it is firing minutes early. It is not firing to catch upto a node it missed, but for one coming up. Flying seperate, the two craft behave correctly. I have a hotkey group for the lander engines, they are turned off, and the MS CM is the controlling part. Doesnt work with the MJ part controlling either.

[Link]

Your engines firing earlier means you probably have a lower TWR compared to the other ships you have tried. Thus it's compensating for this by burning earlier. Bring up the deltaV stats and check to see if the stage TWR is under 1. For every bit under 1 that it is, means slightly more time to execute your maneuver.

EDIT: on second thought, you saying "every other time"......is it with this same ship? Ie. It functions correctly except for inclination change?

[Gargamel]

It seems to work fine for Circularizations and such, but inclination changes and transfers to Minmus are firing the instant it points at the node. Seperated, the ships fly fine. I've flown inverted docked ships before, and as long as I shut down the inverted ship, it has always worked fine, but not this time.

yes the TWR is quite low, so I am expecting long burn times. But, for instance, it is starting a 6 minute burn at t-18 minutes. I have either AN/DN, or just the node selected. I'm not using "in 60 secs" or "immediately" as when to fire.

I'm missing something fundamental in the build, not the execution (I believe). Any ideas as to what it (theoretically) could be?

If it comes to it, I'll post up a craft file.

[Link]

Well, you certainly know what you're doing, so perhaps we shall take this and start with the basic first question(surprised it hasn't been asked yet)......are you updated on everything? Is it JUST this craft doing it? What other mods are you using? Anddddd......a picture of the craft *may* help, tho the craft file would be ideal.

Edited June 6, 2014 by Link
short term memory loss

[SpeedDaemon]

If the disabled engine is messing up your delta-v in MJ, it might also be messing up it's other calculations... Try moving that engine to another stage, so that MJ doesn't consider it for the current maneuvers.

[Gargamel]

If the disabled engine is messing up your delta-v in MJ, it might also be messing up it's other calculations... Try moving that engine to another stage, so that MJ doesn't consider it for the current maneuvers.

Thats probably it!! The lander engine, disabled, are on the bottom stage, while the MS engines, enabled, are one stage above it!

I never considered that an issue because upon separation they sort themselves out correctly.

Standby for confirmation.

[Link]

If the disabled engine is messing up your delta-v in MJ, it might also be messing up it's other calculations... Try moving that engine to another stage, so that MJ doesn't consider it for the current maneuvers.

Its a very good possibility, tho the calculations are done correctly on a spaceplane im currently building, that involves jets and rocket engines all on the same stage, switchable through action groups. When I switch, so does the deltaV. So funny watching it go from 22000 to 800 with one click.

I'm intrigued to find out if this is your issue, as I was going to overlook it!

[Gargamel]

Yup! that did it! Thanks guys!