I just tried to build a rather tall rocket which also uses several Quad-Couplers to add more engines, and as you can see in the screenshot below this thing is rather floppy and unstable:

enter image description here

It also doesn't fly far due to these problems. I tried adding struts, but at the critical points between stages I can't seem to add any as the Quad-Couplers are wider than the fuel tanks.

How can I build tall rockets that don't flop around so much and that are actually spaceworthy? I'm both interested in this specific case, but also on general guidelines for this.

  • 5
    I remember reading about a mod that fixes this problem. Unfortunately I don't remember the name. Also I can't believe you wrote this question without a single innuendo. Missed opportunity. – Lawton Nov 21 '13 at 19:12
  • From my extensive experience watching youtube videos and imgur slideshows shared by friends who play the game the KSP way of making a big rocket is to primarily build your stages out horizontally and not in a real world vertical stack. – Dan Neely Nov 21 '13 at 21:52
up vote 9 down vote accepted

Don't do this! It's bad, bad, bad!

Even by adding struts as per @char1es' answer, your rocket still isn't structurally sound. The reason for this, as I've said before in other answers, is that the topology of a KSP rocket is in the form of a tree. This means that only one of the stack decouplers is attached to the bottom quad coupler. The result is that the other three are left "floating", and have no problem clipping through the other parts of the rocket.

No, what you want to be doing is using a single rocket with similar thrust and specific impulse to the combined 4 rockets. For the second stage, I'd hazard a guess that the skipper is probably what you want, while for the upper stage you probably want an LV-909.

As an aside, if your rocket can't support its own weight on the launch pad, it's not going to survive launch. If there's significant flex without it even moving, you may have done something evil, and at the very least something immoral. There are usually better approaches than doing stuff that's evil and/or immoral.

  • So I can only use the Quad coupler in the first stage of my rocket? Anywhere else I run into this problem with stability. Well, looks like I have to go back to my previous designs without the couplers. – Mad Scientist Nov 21 '13 at 19:47
  • Not necessarily, but you do have to maintain the tree structure. There is a way around the problem by using docking ports, but the added weight and complexity is generally not worth it. It's also a tiny bit evil as well. – MBraedley Nov 21 '13 at 19:55
  • the quad de-coupler mid-stage will never work as you said because of the tree topology. i agree that a quad-decoupler mid rocket is results in 3 free-floating tanks. However my answer is for the more general case when you have a tank/engine stages. In my experience, when i have an excessively tall rocket, the external frame works quite well. – char1es Nov 21 '13 at 20:35
  • @Mad Scientist I've edited my response to reflect the general case. MBraedley is correct in that a quad-decoupler will always fail. – char1es Nov 21 '13 at 20:41
  • The quad-coupler will work midstage if the "lower" stages are radially attached. You could for instance connect a lower stage built from 4 FL-800 stacks connected radially to the middle stage. A hybrid solution I sometimes use is to have FL-800's on top of the solid boosters, connected by fuel lines to the "middle" stage. The idea is that the boosters don't need to provide the full trust on launch. – MSalters Feb 19 '15 at 12:17

This may not be the most aesthetically pleasing solution but this is what i find works best for tall "wobbly" rockets.

You need to build an external frame around the rocket using struts and i-beams.

  1. add horizontal i-beams at the center of your craft in radial pattern.
  2. next add struts from a lower part of the craft
  3. add struts from the i-beams to the "wobbly" part of your craft (top)

You can add as many levels of i-beams as you want, but keep in mind that it takes a certain amount of force to break the struts supporting your craft.

edit: this will not work for the quad-decoupler in the middle of the rocket, as mentioned by @MBraedley. However it works quite well for most excessively tall rockets.

This answer is for the more general case, however in your case, you cannot improve structural integrity because of the tree topology, as mentioned by @MBraedley.

      /\
     /__\
     |  |
    /|  |\
   / |  | \  <== Struts
  / /____\ \
 /  |    |  \
====|    |====  <== I-Beams
 \  |    |  /
  \ |    | /  <== Struts
   \|    |/
    |    |
    |____|
    ||||||
  • 1
    This is what I normally do for tall skinny sections that have a heavy payload. But I don't use ibeams, I use the girders as they're longer and lighter, and just add several struts. I also do this for large transfer stages that have a smaller payload, where the girders are placed on the transfer stage and strutted to the payload. – Dorian Dec 5 '13 at 13:10

little trick I figured out is to instead of using decouplers use docking ports. If you set them up right they attach immediately at launch. That way all four connections are made

Where the quad couplers are, I don't think you need those, if you want them you should at struts between the two couplers and some to the rockets.... Suggestion though!

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