Docked fuel pods keep disconnecting

[troherg]

Hey guys. I just finished assembling my first Duna ship in science mode and am having a bit of trouble. The ship is below.

It is a simple stock ship with KAS/KIS added. There is the large Duna burn stage at the bottom, with an orbit insertion/maneuvering stage above it (orange tanks). The two landers will drop down to Ike and Duna respectively, and once I am done gathering science I will have it all transferred to the storage module below the command pod, which will detach from the maneuvering stage and head back to kerbin with the tug at the end. However, despite trying to stabalize the vertical fuel pods on the maneuvering stage with KAS struts, every time I accelerate they seem to break off.

Anybody have any ideas on how to prevent this? I do have a port on the front of the tug, so I could send up more struts for Bill to place on the tanks. The struts fixed the initial problem, which was the tanks would wobble and snap off. I think the mission would still work if I just dropped the tanks altogether, but after spending the time to get them up to the ship I would like to keep them.

[WanderingKid]

For some reason RCS tanks seems to be very weak for holding onto heavy things.  If your design was flipped over, where the light part was on the outside, you'd be in a better position.

I know a few possible ways to deal with it, but I wouldn't call any of them great.

One is to strut the orange tanks to the RCS tanks.  That will help with the snap off they're incurring.

Also, accelerate at say 1/5 to 1/4 the burn rate until those orange tanks are empty and the mass is less.  Annoying, but the lack of g-force should help keep them attached.  You will probably want to experiment to find out where your limit is.

[DerekL1963]

You also try a 'side saddle' mount kinda like I used on my Jool-5:



Look at the drive units, you could mount your tanks 'flipped' (on top of the sidesaddle tanks).  I'd use Sr. ports though, rather than Jr.

The 'sidesaddles' are structurally attached, not docked.  Here's how the core looked at launch.  (See the thread linked in my .sig for more views, which may help you understand how it all fit together.)

Edited January 17, 2017 by DerekL1963

[troherg]

Thanks for the responses! I did end up trying to redo the fuel pods, putting the RCS after the orange tank and then putting struts all over the place. It helped a bit, but I ended up just having to keep the throttle down to about 25% for the entire Duna burn. On a plus side, I put some poodle engines on the new pods to try to balance the forces on the pods when I accelerated, so once I dropped the transfer stage I was able to go full throttle in the Duna SOI! Thanks again!

Sorry for the picture angle, I had trouble getting good lighting and Duna/Ike both in the picture. Derek I am going to try side saddle next time, after launching the core for this set up I thought about docking in that way but the solar panels would have broken off :/.