question about added masses

[Dave]

I suppose this should go in the Orbiter forum really, but this forum's been so watched that I
figured I'd hijack it for my own selfish purposes

My question is, when two ships dock in Orbiter, and then one of the ships engages engines, does
the mass of the other ship get added to the mass that has to be moved?  I ask this because I just
realized that when docked at ISS or Mir with the DG, and engage engines on the DG, the whole
station moves.  Is this just because the engines are so powerful that they can move that huge
mass, or is it a bug in Orbiter?

Dave

[mgka]

The physics model accounts for this. If the station is coded with a realistic mass, The force from the
engines is applied to all docked objects. If the docking port is off-centre, the structure starts to
rotate. If you set unlimited fuel, it is (theoretically, at least) possible to use the DG to push ISS to
Mars. Steering might be a problem, though.

Another method, attachments, uses a simplified physics model. If you have a satellite on the end
of the arm on the shuttle, you can still fly it normally, and there is no increased mass or misplaced
CG.

[DanSteph]

Yes, seem to be the programmer's responsabilities to add the
mass to the ship and the CG modification that is resulting.

If I recall good this is stated in the SDK dock but I'm not sure I didn't
worked with this part of the SDK yet.

Dan

[Luis Rodriguez]

Hey Dan... if you solve that question, you will be the father of Mothership Class in Orbiter....

My wish: Have DGIII (or newer version) in the cargo bay of a big Mothership heading to mars....

[mgka]

A new class shouldn't be necessary. If you don't want to deal with weird CG issues
(As an example, look at the Starfleet Museum ships. If you have a shuttle in the shuttlebay, the
KILLROT autopilot doesn't work. It leaves a tiny bit of residual rotation that makes docking a
horrible pain.), just use attachments.

In that case, all that's needed is an attachment MFD (like the docking MFD, but with support for
attachments.). If you want more realism, and have docked ships add mass, just have a huge
dummy fuel tank on the mothership. Find the mass of the attached ship, and add that much fuel to
the dummy tank. (in this description, a "Dummy" tank is one that's not connected to any engines.
You could use the Fuel MFD to transfer it to working tanks, but that's cheating. Anyway, you can
already edit the scenario, or set unlimited fuel. Adding another way to cheat with fuel is nothing
major.)

Because of the simplified attachment physics model, there are no weird CG rotation bugs.

Also, "motherships" have been done in Orbiter. The above mentioned Starfleet Museum ships (on
Avsim, if you want them. At least, I think that's where I got them.) have shuttlebays that can hold
a class 1 Starfleet shuttle. This is done with standard docking ports.

A third option is to use docking ports, and dragonfly-style code to compensate for the CG
disruption with RCS or gimballing. This would be the hardest to implement, though.

[canadave]

Say, I just downloaded the Starfleet Museum ships.  Am I missing something--how do I get the
shuttlecraft to dock?  Is there a "docking permission" button I'm supposed to be pushing?

--Dave

[ChristopherT]

 There are no opening shuttle doors on the Birth of the Federation ships.  It's a matter of using
Ctrl-I from the shuttle cockpit and looking up the IDS frequencies.  The two closely matched
frequencies are for the shuttlebay, the third is for the front of the ship.   Just fly the docking hud all
the way into the shuttlebay.  I never noticed any residual movement of the ships after the
shuttle is docked.

[mgka]

The residual movement's pretty tiny. Just enough to get really annoying when you try to dock the
larger ship to the spacedock, and almost too little to figure out what's going on. Try going into
some rotation, hitting KILLROT, then going to 100x time acceleration. That should be enough to
notice.