Item recovery mission--what went wrong??

[Loren Pechtel]

The objective was to recover a mobile processing lab from orbit around the Mun.  With considerable difficulty I chased it down and grabbed it--then the problems showed up.  Now I have something over -1000 m/s of dV according to Mechjeb.  Mechjeb does a horrible job of flying the rocket, when I try it with maneuver nodes I do even worse--burning retrograde is *increasing* my orbit!  Some experimenting shows me that burning anti-normal lowers my periapsis while also changing the plane of my orbit.  Fortunately I had enough fuel along, by the time I had the periapsis in the atmosphere I was also in a polar orbit.  (This burn was just after I left the SOI of Mun, it wasn't that expensive.)

Then to add insult to injury my decoupler failed to blow off the booster (since when is there a problem with misconnected items stacked vertically???) and I'm coming down with it on, not to mention the weight of the fuel in it.  Once I see I'm not shedding it I go to full throttle (the engine is small, I'm below 1 TWR) to burn off the fuel, enable the RCS and tumble around the best I can to spread the heat.  It works, I come out of the fire intact and fortunately I use Stage Recovery so there were chutes on that booster.  Thus I have the crazy picture of coming down with my engine burning while I have chutes deployed--it went dry about 200' up and shedding the fuel weight got me down to a safe landing speed.

[tsotha]

The maneuver node problem sounds like you selected "control from here" on your grabber and forget to change back to the capsule or probe core.  Is your grabber not pointing forward?

[Loren Pechtel]

31 minutes ago, tsotha said:

The maneuver node problem sounds like you selected "control from here" on your grabber and forget to change back to the capsule or probe core.  Is your grabber not pointing forward?

Well, I didn't realize I even needed to switch back but my grabber was on the nose and sitting right on the probe core that was flying it.  When I left the control from here on a docking port and didn't switch it back, nothing odd happened.

Edited February 26, 2016 by Loren Pechtel

[Blaarkies]

1 hour ago, Loren Pechtel said:

... Now I have something over -1000 m/s of dV according to Mechjeb.  Mechjeb does a horrible job of flying the rocket, when I try it with maneuver nodes I do even worse--burning retrograde is *increasing* my orbit!  Some experimenting shows me that burning anti-normal lowers my periapsis while also changing the plane of my orbit. ...

That sounds suspiciously like the control point switching automatically upon docking. Did you try right-clicking your command capsule(or probe core) and selecting "Control from here"?
Always check you navball vs actual craft orientation relative to the planet, if they don't agree with each other you had better investigate.

When you have an engine facing backwards, MJ will calculate that after a full burn, you will be going 1000m/s faster...in the oposite direction

[Loren Pechtel]

Just now, Blaarkies said:

That sounds suspiciously like the control point switching automatically upon docking. Did you try right-clicking your command capsule(or probe core) and selecting "Control from here"?
Always check you navball vs actual craft orientation relative to the planet, if they don't agree with each other you had better investigate.

When you have an engine facing backwards, MJ will calculate that after a full burn, you will be going 1000m/s faster...in the oposite direction

I think you just explained it.  When I grabbed the target the craft name changed to the name of the target.  I think my control point became the object I grabbed which was not aligned with my rocket.

[Snark]

10 hours ago, Loren Pechtel said:

I think you just explained it.  When I grabbed the target the craft name changed to the name of the target.  I think my control point became the object I grabbed which was not aligned with my rocket.

Yes, that'll do it.

KSP is a little arbitrary, when joining two craft, about deciding which one is "the" ship, which matters for things like ship name, control-from point, etc..  I've never taken the trouble to nail down exactly what its rules are for deciding (or even whether it's completely deterministic), but my very vague, anecdotal impression is that it tends to favor the more massive craft, possibly some and/or combination with preferring crewable parts.  If your grabber ship was unmanned and/or smaller than the 3.5-ton lab, that might explain it.

I really wish they had a rule that "if I connect two ships, and only one of them has any active engines, then keep control with whatever probe core or command pod is facing the same direction as the active engines."  It would save a lot of player confusion.

One way to tell that this has happened is when the name isn't what you expect.  Another is the visual signal that your navball doesn't line up with expected "world truth"-- i.e. you think you're pointing retrograde based on your navball, but when you look at your actual rocket and the planet, the orientation doesn't match what the navball is telling you.

This isn't only an issue with the grabber-- it happens when docking ships via docking ports, too.

Anyway, about the only solution I've found to this:  get in the habit of always, any time you connect two ships by docking and grabbing, choose "control from here" on the command pod or probe core that you care about.  Don't try to figure out whether you need to or not, just do it all the time.  Will save you grief. 

[Loren Pechtel]

38 minutes ago, Snark said:

Yes, that'll do it.

KSP is a little arbitrary, when joining two craft, about deciding which one is "the" ship, which matters for things like ship name, control-from point, etc..  I've never taken the trouble to nail down exactly what its rules are for deciding (or even whether it's completely deterministic), but my very vague, anecdotal impression is that it tends to favor the more massive craft, possibly some and/or combination with preferring crewable parts.  If your grabber ship was unmanned and/or smaller than the 3.5-ton lab, that might explain it.

I really wish they had a rule that "if I connect two ships, and only one of them has any active engines, then keep control with whatever probe core or command pod is facing the same direction as the active engines."  It would save a lot of player confusion.

One way to tell that this has happened is when the name isn't what you expect.  Another is the visual signal that your navball doesn't line up with expected "world truth"-- i.e. you think you're pointing retrograde based on your navball, but when you look at your actual rocket and the planet, the orientation doesn't match what the navball is telling you.

This isn't only an issue with the grabber-- it happens when docking ships via docking ports, too.

Anyway, about the only solution I've found to this:  get in the habit of always, any time you connect two ships by docking and grabbing, choose "control from here" on the command pod or probe core that you care about.  Don't try to figure out whether you need to or not, just do it all the time.  Will save you grief. 

Yeah, my ship was unmanned.  My best probe cores are now better pilots than my best Kerbels, the only reason to send up a Kerbel is to do science.  (Perhaps also engineering later on but I haven't reached that point.)

This game sure could use a list of gotchas to watch out for so we don't have to learn these things the hard way!

[HalcyonSon]

Careful with "control from here."  It can help with docking orientation, but depending on ship design will force you to re-learn your RCS controls.  I built a Mun Lander with the docking port on the side (because I really wasn't planning to need it) and a large parachute up top (whole vehicle return).  Needed to dock to refuel for biome hopping.  Since the docking port is "vertical" while the control pod is "horizontal," RCS controls were skewed.

Also - Alignment and Fuel Balance are very important.  Docked that Mun Lander to a refueling tanker and brought both back to Kerbin from Mun.  I remembered to shutdown the Lander engine so the Refueler was driving, and the ports were in the same plane, but the Lander port is off-center and its tanks were empty.  The first burn was backward and resulted in an awkward spin.  Had to fix "control from here" to a forward facing port, and transfer some fuel to the Lander tanks to balance the load.

[bewing]

Actually, you need to be doubly careful. There is a bug -- when you grab something, your "control from here" point can change every single time you switch focus to the craft. So check it every single time. Just setting it properly once is not good enough.

[Foxster]

A little tip...If you have a choice of "control from here" sites (claw, docking ports, capsules, probe core) that are all aligned and so all would work then choose the one closest to the Centre of Mass. Makes the craft more stable and less bendy for no obvious reason.