Assembling a Vessel in Orbit - Mine are unstable

[Sublight]

I decided I wanted to build a vessel in space.  It was going to be fairly large.  I built it with the Large docking ports, but when I turned on the engines after 2 days of assembly and prep..... 

It started bending and flexing etc.  It was so unstable, I had to send undock all the parts, stabilize the command modules, and send a rescue ship to get all 3 kerbals that I was sending.  They were quite upset with me.

Im on the Xbox, so no mods.  Is there a better way to build a vessel in space that is stable?

[Signo]

Hard to say without a picture, however if you build a "puller" it is going to be more stable than a "pusher".

[WanderingKid]

In my series for 0.90.0, I built the Ares Helm, a ship similar to what you're discussing that was built in space.  Instead of relying on a single docking port to control for wobble, I used two docking ports per attachment.  I basically build the entire thing in the VAB with the docking ports setup, split it into pieces, and then shipped them to space.

It's arduous to get them perfectly lined up, but once you do it will cut down on your wobble tremendously.

If that's not your thing, then there's a few considerations.  First, do not put engines or control components on the hanging components.  You want to drag everything off the center component (with the engines) so that force stays relatively consistent.  Second, you want to turn, then stabilize, then light your engines.  You don't want to turn while under thrust.  Finally, try to make sure anything hanging off the center beam/components is much lighter than the center component.  You want your center component (with its reaction wheels and engines) to have the mass to enforce its domain.

[Vanamonde]

A large ship will flex. There's no way around that. To minimize it, visualize the ship as a building sitting on its stern, and add parts in a way that their "weight" will be supported. But don't be afraid of a degree of flexion, as KSP parts can handle it. Assemblages like this do just fine. 

 

[Norcalplanner]

Kerbal Joint Reinforcement is a mod which can also help a lot.  There are also mods like KIS/KAS which allow you to add struts after everything is docked together.

As said above, a picture would help.  What's the ship's TWR?

[Red Iron Crown]

1 hour ago, Norcalplanner said:

Kerbal Joint Reinforcement is a mod which can also help a lot.  There are also mods like KIS/KAS which allow you to add struts after everything is docked together.

Won't help in this case.

[Norcalplanner]

Just now, Red Iron Crown said:

Won't help in this case.

Nevermind  - just saw he's on Xbox.

Edited August 29, 2016 by Norcalplanner
I can't read

[Red Iron Crown]

Just now, Norcalplanner said:

Did I miss something?  Did he post a picture of his station somewhere?  Or are you saying that those mods won't work?

 

8 hours ago, Sublight said:

Im on the Xbox, so no mods.

 

[fourfa]

There's a hidden feature of current landing gear and landing legs - perhaps first described by a forum user and KSP streamer mod named Muddr.  These items currently install an invisible strut from the gear/leg to the most massive part in the craft (this was part of taming the early 1.1 build "dancing").  This feature can be abused to auto-strut between vanilla dockable modules.  Test it out on the launchpad, the difference is incredibly obvious and very, very powerful.

Slap a few landing gear on the outside of a module and see what happens.  You can even offset it completely inside another part and it will still work, but might explode sometimes.  No guarantees.

Edited August 29, 2016 by fourfa

[Sublight]

Thanks everyone!  Im sorry I have no pics.  The only way to get a pic is to take a pic with my phone, and they always suck.  Im on Xbox so no mods!

But thank you all for the advice.  Im surely going to try the landing stuts.  This is such a great game, and you guys do such a great job with helping newbs like me.  This is one thing that makes this game so fun.  Thank you everyone.

[capi3101]

Ancient wisdom from long ago (0.18 - when Docking Ports were introduced) was to use the Tricoupler on both bits you wanted to join up, and to fit them out with Clamp-O-Trons. The final connection will be very strong, and using three ports makes getting a proper alignment a relatively easy chore. 

Despite the age of the advice, it still holds up well in the modern KSP world. Tri-Adapters also work well, and if part count is a concern, try the Clamp-O-Tron Sr. if you have access to it.

[Sharpy]

@Capi: Tri-adapter is less flexible, with its 2.5m base. Tricoupler offers a very strong front, but the back still needs strutting.

Generally, multi-port IS an option, just the ports must be aligned perfectly in the construction (VAB) - e.g. create a subassembly that serves as your multi-docking element, and use it for both sides of the connection, or use strictly node-attached parts for creating the docking area.

 

[Sublight]

2 hours ago, Sharpy said:

@Capi: Tri-adapter is less flexible, with its 2.5m base. Tricoupler offers a very strong front, but the back still needs strutting.

Generally, multi-port IS an option, just the ports must be aligned perfectly in the construction (VAB) - e.g. create a subassembly that serves as your multi-docking element, and use it for both sides of the connection, or use strictly node-attached parts for creating the docking area.

 

It seems like the multi-port would be easy.  Just take a big fuel tank (or something standard) and place 3 small tanks around it in symmetry, and attach that to the vessel.  Then anytime I want to add to it just use the same tank and 3 more fuel tanks and since the sizes are exactly the same, they should meet up.

OR EVEN BETTER, take a little from both.  Use the Tri coupler on each of the 3 small tanks.  Giving 9 connection points.  It would have a high part count, but lots of connections are better right?

Edited August 29, 2016 by Sublight

[Kurld]

I've had the idea for a long time that you can't dock with more than one port on each side of a sub-vessel.  Is this no longer the case?

[capi3101]

18 hours ago, Kurld said:

I've had the idea for a long time that you can't dock with more than one port on each side of a sub-vessel.  Is this no longer the case?

Not in the VAB - if you try to attach two parts of the same rocket solely with docking ports, the game will only consider the one docked and you'll wind up with floppy rocket syndrome. In space, however, if you've got two docking ports that are the same size, oriented the right way and come close enough together, they'll stick.

[fourfa]

In the VAB use the offset gizmo to set docking ports very near each other.  When you go to launch, they will dock as soon as physics starts