Duna mission structural failure

[mrmcp1]

Hi I was wondering if anyone could help me work out how to stop the the launcher for the drive section for my duna mission suffering a structural failure. About 2 and a half minutes into launch I usally siffer a structural failier between the top orange tank and the large RCS tank. I have strutted them togeather but it has not solved the proble. I was wondering if there is a solution other then the kerbal joint reinforce ment mod. Thanks in advance.

Download link: https://www.dropbox.com/s/nztokomhwoqkuy8/Duna%20drive%20launcher.craft

Hope you can help.

Edited February 24, 2014 by mrmcp1

[Geschosskopf]

You're on the right track with those girders sticking out the sides. But run the struts up as far as they'll go towards the nose of the rocket instead of just to the RCS tank.

[mrmcp1]

Thanks for the quick reply I will try that tomorrow but some of the struts on the girders go to about half way up the orange tank. I hope your idea will work. Thanks again

[Vanamonde]

SAS modules and RCS tanks tend to be somewhat fragile. How about moving the RCS tank higher up in the design, where it's not bearing as much of a load? Also, what's the G meter reading when the failure occurs? Throttling back some might help.

[Sheiss]

I had a ship that did the same thing. The quickest fix was to keep the speed below 100m/s until i got out of the inner atmosphere.

[Jouni]

Hi I was wondering if anyone could help me work out how to stop the the launcher for the drive section for my duna mission suffering a structural failure. About 2 and a half minutes into launch I usally siffer a structural failier between the top orange tank and the large RCS tank. I have strutted them togeather but it has not solved the proble. I was wondering if there is a solution other then the kerbal joint reinforce ment mod. Thanks in advance.

The problem is with all the small parts between the top orange tank in the launcher core and the orange tank in payload. With a large number of joints close to each other, that part of the rocket is quite flexible, and even the struts can't hold it together. The connection between the docking ports is the worst, as it's not even supposed to be rigid. Replace the bottom docking port with a decoupler, and the rocket should hold together much better. Moving the RCS tank to the top should also improve things.

As a general construction principle, use large parts at the bottom of the rocket, where the forces affecting the joints are strongest, and put small parts as high as possible. Engines should be connected to large fuel tanks, while smaller fuel tanks should be placed on the top of large fuel tanks.

Your launcher is also a bit underpowered, with TWR barely above 1 at the start of each stage. You can probably make it more efficient by replacing the Skippers in the last pair of boosters with Mainsails.

[Skorpychan]

The RCS tank is a weak link. Note how it's got smaller green spheres on than the tank. And it's lighter than the tank anyway.

For launch stability, mount heavy items further down the rocket.

[SRV Ron]

Your launcher is also a bit underpowered, with TWR barely above 1 at the start of each stage. You can probably make it more efficient by replacing the Skippers in the last pair of boosters with Mainsails.

I noticed a big boost in performance using Mainsail in the last asparagus pair on one of my fuel can to orbit designs.

[SRV Ron]

This is weird. There is all Skippers under the boosters with a Mainsail for the core. It barely lifts off the pad. Yet, if I place a pair of Mainsail under the third pair of boosters, there is insufficient power to launch period. Some kind of bug is at work here since the weight being launched would require all Mainsails.

I don't think the asparagus is set up properly, parts there should select in pairs. They are selecting in single mode. I may try playing with rebuilding this later.

Anyhow, besides the wobble of the short tanks and the bracing issues in the asparagus, the weak link is the docking clamp on the payload stage.

Makes for great explosions.

Edited February 23, 2014 by SRV Ron

[SRV Ron]

Rebuilt the booster to the specs but with Mainsails. Moved the upper parts and replace one docking clamp with decoupler. Needed to brace the 4 LV-N. Note, fuel will not flow to those LV-N from the orange tank. Disable gymboling on the asparagus ring. Launch successful but hard to keep on course. Some fine tuning of bracing will fix.

[Sovek]

This is weird. There is all Skippers under the boosters with a Mainsail for the core. It barely lifts off the pad. Yet, if I place a pair of Mainsail under the third pair of boosters, there is insufficient power to launch period. Some kind of bug is at work here since the weight being launched would require all Mainsails.

I don't think the asparagus is set up properly, parts there should select in pairs. They are selecting in single mode. I may try playing with rebuilding this later.

Anyhow, besides the wobble of the short tanks and the bracing issues in the asparagus, the weak link is the docking clamp on the payload stage.

Makes for great explosions.

Are you sure you didnt take off the skippers only to replace them with skippers? Those two do look quite similar to each other.

[SRV Ron]

Are you sure you didnt take off the skippers only to replace them with skippers? Those two do look quite similar to each other.

You can see the size difference. Anyhow, the performance is much better with all Mainsails throttled back 10% as you can see from post #10.

[mrmcp1]

Thank you all for all of your suggestions I will follow the advice and move the RCS tank and replace the docking port with a decoupler to make it more stable. I will also alter the launcher. Thanks for the help now to Duna...

[capi3101]

I would humbly suggest removing the RCS tank entirely - as noted, it's a weak structural link. If you still want that much RCS available for your mission, replace it with four RCS Cylinders instead, that way you still have the same amount of Monoprop available but it won't have as much of a bearing on the structural stability or your stack. Or follow the excellent advice that you've already received up to this point. You might also try throttling back as you get further into the atmosphere; definitely consider this if your gee meter is climbing out of the green zone, as structural failures have an increased risk of occurring once the gees get to be a bit much. If that solves the issue, you might want to take another look at your asparagus and see if you can revise it to follow '>Temstar's general guidelines (old but still very much valid).