Large rockets don't want to hold together...

[keoki]

Ok, so I've been very successful with launching very tall 2.5 meter based rockets, pretty much as tall as you can build them in the VAB. As long as I don't over-power my ships with the boosters (which also means keeping them light) the snap-together construction in the VAB is working fine.

However I was doing some thinking on reducing the height of my stages for my standard workhorse, and I figured that this would add some additional capabilities. So I started unlocking some heavy rocketry, only to find that these larger parts are not reliably holding together. They frequently will bounce apart when dropped onto the launch pad. The assembly will have failed control connections intermittantly at different sections, so while it may land on the launch pad in perfect working order one time, the next 4 times, one or more stages will have a open connection and nothing below that stage works.

Now I've come up with a work around that works really well, but I don't see that anyone else is using that work around in their craft, so I'm wondering what other people are doing to bind their larger diameter rocketry sections together.

My work around is to add struts where they are easy to add because I have some angles to work with, and where I have straight sections with a trouble spot, I put some fins, with struts leading up and down from the leading and trailing tips of the fin, forming a rigging/brace structure that both stiffens and provides apparent compression for the rocket sections at the joints, resulting in perfect performance. This also allows for larger boosters to be used, and the resulting vehicle can be used to lift heavy loads.

But my question is, what do the people that are "doing it right" doing to get larger rocket sections to hold together? I mean just a single large engine, a short tank, and a small capsule rattle and shake like there is nothing holding them together at all, and I know they are "connected" just as well as my reliable 2.5 meter rockets that are as tall as the VAB.

To be fair, my tall rockets have bottom sections that are multiple 2.5 meter sections strutted together in clusters of 6, and large blocks of solid boosters strutted to the bottom, so it's not just a long pencil... And even those had problems when I apply too much force.

I was surprised my rigging system worked, for a long time I thought my collapses under high-power were from excessive thrust force, but now I see it was from insufficient section clamping force, and is actually lateral collapse, I can sustain much much greater engine loads (and support greater payloads) this way. And my end-to-end control path appears to be reliable during the "drop" to the launch pad. But my large diameter rockets look "unusual"...

Edited November 23, 2013 by keoki
got some good answers.

[Dogface]

They frequently will bounce apart when dropped onto the launch pad.

You should be using TT18-A Launch Stability Enhancer to hold the rocket steady during pre-launch.

http://wiki.kerbalspaceprogram.com/wiki/TT18-A_Launch_Stability_Enhancer

My work around is to add struts where they are easy to add because I have some angles to work with, and where I have straight sections with a trouble spot, I put some fins, with struts leading up and down from the leading and trailing tips of the fin, forming a rigging/brace structure that both stiffens and provides apparent compression for the rocket sections at the joints, resulting in perfect performance. This also allows for larger boosters to be used, and the resulting vehicle can be used to lift heavy loads.

Yup, this is essentially what I do. The use of fins is a good idea. It looks like theyre lighter than the structural stuff I am using.

Edited November 22, 2013 by Dogface

[Smidge204]

You really, really should use launch clamps, see post above.

You might also be overdoing your TWR, if your rockets collapse even after takeoff. Contrary to the memes, you do not need to be going 800m/s right off the launchpad. Any idea what your actual TWR is?

But if you really must, go ahead and reinforce everything with girders and struts. Place girders radially in 3x or 4x symmetry, and strut from the ends of those back to the rocket up and down to form a truss.

=Smidge=

[glibbo]

Yes, I have to agree with all the replies here, I too, and I suspect just about everyone else has built a long type of craft and experienced the same problems....... tanks, liquid or RCS and many other parts need reinforcement with structure / struts to overcome the problem. Maybe the clamping force could be raised in the programing? I don't know.....

[el_coyoto]

I agree with all that's been said before :

- Launching clamps

- Struts

- Too much thrust and/or drag ("pushing too hard against a wall made of air", ie do you have re-entry effects during the vertical ascent?)

I will only add that, if you don't like struts, you check this mod : Kerbal Joint Reinforcement. It tweaks the stiffness of parts and their connections, reducing (but not eliminating in some cases) the use of struts for bigger rockets.

[keoki]

It's not that I don't like them, it's that I don't see them in anyone's video clips. But the launch stability enhancer sounds like something I'm not paying any attention to. As for the girders VS wings, yes, girders would work, but at the cost of weight.

My main question was how do you build heavy in the first place, I have been successfully building light. But for launching craft capable of a return from a larger moon, the launch is going to be heavier than I had been making. Plus my science platform is getting crowded.

[Taki117]

WEll, I have a kerpollo launch vehicle that I used which was nothing more than 7 double stacked asparagus staged orage tanks with mainsails and a few struts. I have a pic if you would like. Also, you might not be seeing struts because they are either hidden (With part clipping) or they are using the invisible ones from B9 aerospace.

[Biker Joe]

I find adding a bit of aerodynamics like nose cones makes a big difference, that and struts, a whole lot of struts!

[Botch]

you can also use struts to stiffen radially connected parts by anchoring one end at the edge of a part and dragging it to the edge of the connected part. In some cases you won't get a good connection but with a little fiddling you can get the struct placed correctly. If you're using symmetry you can place struts all around your parts in one action.

[keoki]

Lots of good answers in here guys. Many of them I already had tried with varying success... Botch's suggestion was pretty much the first thing I tried, and I found it difficult to get struts to adhere to surfaces without enough of an angle to allow clearance for the strut. The strut seems to be the only part that forces clearance... Everything else will happlily go through walls... Walls can go around struts, if the struts are there first.... sometimes. And when you build sub-assemblies that use struts, sometimes struts will disappear and seem hard to replace...

Also, the struts weight doesn't appear to be as predicted by the part, they appear to be lighter, so perhaps the given weight is the maximum for the strut's longest length.

The struts on fins seem to be very useful, as the fins appear to have a side to side rigidity so you don't have to side strut your trusses if you don't want to, though you should if they were real. The fins seem to be stronger than they should be for what they are, I get girder strength for 1/6th the weight, and the part adds control surfaces during takeoff... though that is a mixed blessing as I find that too many fins has a cost in fuel, as does allowing the autopilot steer the ship while in the atmosphere (the autopilot can double the fuel it takes to reach orbit). Gimbals and just enough SAS to barely control the ship seems to be the right mix if you want to let the autopilot drive, though I suspect there are probably other ways to trim/damp the autopilot stick response.

I'm an engineer in real life, that is what makes this game so cool, it employs a lot of semi-realistic engineering. Sure, it is based on simplified models, but it is supposed to be fun! Now we just need a network version of the game so we can have joint missions... the only problem with joint missions is I guess you wouldn't be able to use the time warp... at least not until everyone agreed to the warp interval...

Thanks for the feedback guys!

Edited November 23, 2013 by keoki

[keoki]

Ok, so I looked up the link for the launch stability enhancer... I use those... In fact I've found ways to enhance those things holding power, you buttress them with additional units behind them. When you do that you can reduce the launch-pad drop quite a bit, and hold fairly heavy loads. I was hoping that was something else I wasn't using... oh well...

Kerbal Joint Reinforcement sounds like what I was looking for, but I was hoping for a feature that was part of the game, instead of a mod... I mean changing the laws of physics is fun and all, but it feels like a cheat... It sounds like the actual answer is that this "problem" is a feature of the difficulty level of the game, and it was my job to invent a way to overcome the problem. That last statement is the whole point of the whole game.

[Vanamonde]

If the size of the ship is getting out of hand, consider splitting it into sections, launching them separately, and assembling them in orbit through docking.