Design question

[Scarecrow88]

Let me start by saying that I am a complete noob, so my experience of rocket design is nil. I have a station in orbit, using the stock station body, and thought it would be nice to add a fuel tank to re-fuel craft in orbit. Don't know why as I have only been to Mun and Minmus so far, but heck, why not. Originally I just stuck a jumbo tank on the nose of the capsule at the top of the launch vehicle, but even with struts this thing shakes like a strawberry jelly having a fit during launch. I was wondering if there is a more accepted way of attaching spare fuel tanks to a launch vehicle and in what configuration are they added to an orbiting station?

[Brofessional]

For adding parts to a station it's best to build a tug with a docking port on top, then stick the station part above that with docking ports on each end.

That way you can dock the end of the fuel tank to the station, then undock the tug and deorbit it.

Docking ports are more wobbly than decouplers, but detaching the tug via decoupler will nudge the station.

I recommend the quantum struts mod for building your station. The strut gun part allows you to strut docked ships together.

You can stick some girders on the side of your tug and use them as attachment points for struts, that way you can attach struts from the tug to the upper part of the fuel tank.

Edited May 19, 2013 by Brofessional

[Penguinhero]

follow his advice /|\

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[Zephram Kerman]

Well if it's only wobbling during launch, you could simply lock gimbal on some engines. That's especially handy if your launcher has mainsails.

[Scarecrow88]

Launcher does have mainsails so that's an interesting point to note. Advice with regard to docking port on each end of structure is pretty much what I've been doing, so it's nice to know I'm kinda on the right track. Just thought that with lots of wobbling, there might have been a more elegant solution that I hadn't thought of.

[Actzoltan]

The best way i have found to stop heavy payloads from shaking is to use ibeams used as a cage around your payload.

Connect the ibeams to your main engine stack and build up as needed to cage the payload.

Strut the ibeams to your payload then to your main engine stack then to the other ibeams.

Something like this My payload doesn't flinch when i launch

[katateochi]

How fast are you accelerating during launch? With big and potentially unstable craft I find its best to launch quite slowly to start with. Throttle back a bit and climb more gradually, at least while in the lower atmo. If you're pushing too hard its like the lower half wants to overtake the upper half so ASAS is working to compensate with gimble on the engines.

[Scarecrow88]

I might try the I-beam approach to try and put a jumbo tank on my K.I.S.S. (Kerbal International Space Station). At the moment I have used my tractor to attach RCS refuelling tanks and a Max 32 fuel tank, but the way I have it set it can't handle loads any longer than that sticking out the top of the LV.

[Vanamonde]

To prevent a ship with a lengthy payload from wobbling, build the launcher around the payload instead of under it. For example, this

delivers this.

That's a robotic fuel tanker which you could dock to your station as storage. If you'd like, you can download it here: http://forum.kerbalspaceprogram.com/showthread.php/25779-Orbital-refuelling-tanker

[Kerba Fett]

For launching larger loads I use struts, but move them away from the rocket. Here I've used XL girders coming from the orange tank under the lower ring. They brace to the lower stage and to the boosters that have already been released. They also brace to the station rim which is too far out to reach the orange tank with a strut. They're attached with 4 way symmetry and have diagonal struts between them to stiffen the whole structure. This puts the strut attachment far enough out to reach across multiple rocket segments instead of needing a ring of struts at each joint. Once the final stage separates all the bracing and struts go with it. This is an older picture. In later rockets I put the girders on the lower stage as well to stiffen the joint between the upper and lower stages and clear up some wobble at the lower end of the boosters.

I like the idea of the I-Beam cage though. I'll have to play around with it and see if I can get it to release without banging up the payload.

This is the station with all stages released and an orbital tug docked through a 2.5m CBM.

http://kerbalspaceprogram.com/common-berthing-mechanisms-2-5m-docking-ports-2/

The larger sizes of these seem to give a lot more stability when docking larger ships or station modules.