Hold it Together Jeb! [Structural Integrity]

[SgìobairOg]

So I've been playing around with heavy lift because I have a design I want to assemble in space and I want to be somewhat realistic about how it gets up there. To that end I've been using a design made using the techniques from Temstar's Nova design. Since I've been having trouble getting anything I make to orbit with any fuel left I've been using MechJeb to get up and let me see what I should be doing. All of my latest design though have been getting to some altitude then slowly drifting off course falling back to earth and going up in a fantastic ball of flame.

The root cause is that the thrust is breaking my connections between stages. It's not shaking apart or twisting apart it seems like it is just pancaking the joints. As soon as that happens MechJeb loses control or I lose a stage. I've tried addign struts but it doesn't look like I can do that very well between sections. Are there stronger components or something else I can do?

Of course when I went to grab screenshots of this in action it worked without a problem....

Here's the Craft file if you want to be hands on. https://www.dropbox.com/s/9ij8u24v6h1tbwp/Rionnag%20Cluster%20Test.craft

Edited September 4, 2013 by SgìobairOg
Added Craft file.

[piiscool]

You can add struts on side decouplers that fall of with the stages. I've used those many times and they seem to work.

[Johnno]

Here's a demo picture of different strutting techniques. You'll see how everything is stiched together to stuff above it and below it with struts going vertically, then small cubic octostruts/girders stick out to allow diagonally attaching struts as well.

Other than that, keep an eye on your G-meter, try not to go faster than 200m/s until you're around 10km in altitude.

[capi3101]

Johnno's ninja'd me here. Basically, the kicker is to watch your gee meter and throttle back if it tries to climb out of the green zone. And MOAR STRUTS.

[TheDarkStar]

First, try not using MechJeb; it can cause strange problems.

Second, try lowering your TWR by adding less engines. Also, use more efficient engines for a smaller rocket.

Edit: And, as has been said, throttle down (which MechJeb doesn't know how to do).

[AlamoVampire]

actually mechjeb does know how to throttle down, it has 2 options, limit terminal velocity and a limit on acceleration in terms of m/s. But, as they say more struts, more struts more struts! also, check your efficiency and TWR.

thought I would put this image up, if you look, you will see the 2 limiters I mentioned.

Edited September 4, 2013 by AlamoVampire

[Galane]

A technique I learned on the forum for putting struts across stage connections is to put a ring of small hardpoints above and below, then connect hardpoint to hardpoint with the struts.

Something useful I've found with that is the hardpoints can be stuck to a coupler or separator then more struts from the hardpoints to the part above. The hardpoints will go with the coupler or separator so they're not keeping weight on the rest of the rocket. Depending on thrust and other factors, 8 around will keep things together but can still wobble.

I built a craft to put two full Rockomax 64 tanks into orbit and all I had to do to cure its wobble under thrust was to lock the gimbal on the Skipper engine in the center. Once I did that I no longer needed struts to hold the two tanks together across the battery pack, RGU and other parts between, though other parts that stage off have struts bracing the upper tank until it gets into the gravity turn.

That was my #1 nemesis for a while, things folding up, breaking off and major failures going into the turn. Once into the turn it seems to be much less violent.

Something you can do is turn on SAS and launch manually then enable the MechJeb ascent autopilot when you get to the altitude just before the gravity turn. MJ *always* goes full throttle at launch even when it wouldn't need to. Once it gets some altitude, or if your rocket is hitting terminal velocity, MechJeb has a softer hand on the throttle.

[Sirine]

the following parts missing:

Engineer7500

mumech.mechjeb1m

[alioth]

For very large rockets I find that Mechjeb (and to a lesser extent, SAS) has far too high gain for course correction and tends to start the whole craft oscillating. In some cases Mechjeb's oscillations become divergent and only stop when the rocket breaks up. I've found for the largest rockets it's always best to manually fly them. Manned ones I usually do from FPV (first person perspective) because the navball is bigger in the IVA views making it easier to precisely control the attitude of the rocket.

[BubbaWilkins]

I've found that larger craft need to be limited in acceleration between 15-25m. The sheer amount of weight of the craft combined with the full thrust of the engines is usually more than enough to exceed the limits of the connection nodes. I've also had to limit my larger craft to not do a gravity turn until after 25km of altitude due to the FAR's aerodynamic forces on my vehicles.

[KerbMav]

because the navball is bigger in the IVA views making it easier to precisely control the attitude of the rocket.

Unhappy with the UI size slider in settings?