Long Range Ship - Problems

[StainlessSteelRat]

So I'm trying to build my first ship to leave the home system, go to another planet and return. I'm having issues with my design. Please give me the reasons this sucks. As you can see, at power, even with 2 of 3 engines off I have stability issues. Lander has fuel tank on it still, was going to use all of it before getting rid of it. This assumes I ever make it anywhere. Are my issues because i've attached parts with engines with docking ports? Oh yeah, I'm using stock parts only. Thanks.

Edited March 1, 2014 by KasperVld
fixed images

[Captain Sierra]

As a rule of thumb, standard docking ports should NEVER be used for anything that requires structural integrity (unless you want to multi-dock). They, as you have discovered, are terribly flimsy. All docking port are unusually flimsy compared to standard part connections, even when they are attached in the VAB.

You can cut down on the wobble a lot by using Sr. docking ports, the large, 2.5m ones. They won't wobble as much, though even they fail to remain rigid as you start knocking on extreme sized crafts (which you are not).

[ZeeWulf]

Here's an example of 99% stock (a couple RCS thrusters and some KAS fuel connections I'd forgotten to install in the VAB being the exception) ship to similar specs as yours. I'm using all large docking connectors, aside from the one connection to the lander module, which shakes wildly sometimes. (Again, I've recruited KAS to put struts in, because, damnit, this is gonna work) I'm sure if you made a lander using a Senior port, you'll be fine.

[Vanamonde]

Docking ports are stronger than a lot of people will tell you they are, but any mass that is only secured at one end has the potential to start swinging like a pendulum. And the more massive it is, the bigger a problem that will be. This is a ship I built a while back that is similar in size to yours, and also assembled with medium docking rings:

The core and side tank mounts are all one piece, and the tanks themselves are smaller. It experienced a bit of oscillation, but nothing bad enough to damage the ship or make steering difficult. The trick is to keep the size of the swinging masses to a minimum, and cross-brace them wherever you can.

Incidentally, that design evolved into this one,

which launches as one piece and is very stable structurally. The struts between the engine stacks keep the ship rigid.

[Red Iron Crown]

I've found that for large ships assembled in orbit it is better to have all your main propulsion in one module that is properly strutted. Empty the tanks to allow you to launch it, or better still, use fuel lines to use that fuel to power the lifter motors that you later jettison. Once safely in orbit, use multiple launches of smaller tankers to refuel it before docking with the rest of the ship.

[Xavven]

I'm inferring that you launched your design in several pieces to assemble in space. Vanamode's comments about pendulums is spot on. If you leave it as an assemble-in-space design, consider moving your two other orange tanks to dock radially with the center orange tank instead. Also, the TWR for the orange tanks + LV-Ns is probably pretty low! You might want more engine and less fuel. How much delta-v does that thing have?

[Aragosnat]

Do have to agree that the normal docking ports are not good for anything to large. And anything longer then the T-400 fuel tanks you should be using Sr. ports or multi-ports as the farther away from your CoM the more torque will be placed upon the object.

The other thing I noticed outside of needed better or more ports is. That is it is a line and might be helpful once you get your wobble fixed is to make the profile like a + as that would give you an even turning all around.

[Jouni]

In my experience, multiple thruster modules pushing a rigid core module is a good basic design for an interplanetary ship. If there are any stability issues, it usually comes from the payload, not from the thrusters.

This is my current example of the design:

This payload works well with it:

This, on the other hand, has serious design flaws:

[problemecium]

Also try moving the engines to the front so your ship is more like a train. Physicists will tell you that it makes no difference how far forward they are on a rigid body, but being held together by docking ports doesn't make your ship much of a rigid body. So. Train.

[KasperVld]

I see all these massive, massive ships here. My advice would be to start small. A nuclear engine with one of the long 1.25m tanks has thousands of m/s of dV. Saving weight will make things a lot easier!

[Goldham]

Definitely want to use senior docking clamps. Also make it more of a train with the engines at the front. You can also use Kerbal Attachment system as I think you can manually strut things in space with it.

[Taki117]

Another bit of advice, though it doesn't really answer your question: Plan your entire trip before ever putting anything into orbit. You can get to and Duna for a little over 2kms if you do it right. (By contrast getting to and from the Mun takes 2,060 m/s assuming you don't do an Apollo-style mission and start from LKO)