FAR Aerodynamic Failure

[Taki117]

So, I'm attempting to build a space shuttle. I get everything set up in the SPH/VAB (using editor Extensions and RCS build Aid) and get a very nice replica going. It flies just fine until right before SRBSep, or around Mach .8. At this point the bottom of the Main fuel tank falls off. Sometimes it impacts the SRBs, other times it just falls straight off. I have tried reinforcing the joint with struts to no avail. Any help would be appreciated.

Notable Mods used: FAR, B9, Space Shuttle Main Engines, KSO, Tweakscale. (these mods have parts on the craft or are required because of it's construction, file available upon request)

[LethalDose]

I would guess it has to do with the angle of attack on the tank causing an aero failure. If that's the problem, the only two options that come to mind are flying it differently or changing the part CFG to increase the link strength.

Mach 0.8 at 2 km is pretty fast. Maybe dial back the thrust until you're higher?

Edited November 3, 2014 by LethalDose

[Alshain]

What does F3 say happened?

[Taki117]

What does F3 say happened?

Aerodynamic failure between tank segments.

Also, I have tried slowing down and my Main Engines don't have enough Thrust to keep me going. Failure occurs as slow as Mach .5

Edited November 3, 2014 by Taki117

[LethalDose]

Failure occurs as slow as Mach .5

At what altitude?

FAR shouldn't be causing failures like that at those speeds, unless the the link that holds the parts together is really weak or part is at a really bad angle (I've had this issue with some radially mounted hexcans on spaceplanes). I'm guessing it's the former of these. It's a bit cheaty, but I think modifying the part config to increase the link strength is probably your best bet, because that really shouldn't be happening.

Edited November 3, 2014 by LethalDose

[Taki117]

At what altitude?

FAR shouldn't be causing failures like that at those speeds, unless the the link that holds the parts together is really weak or part is at a really bad angle (I've had this issue with some radially mounted hexcans on spaceplanes). I'm guessing it's the former of these. It's a bit cheaty, but I think modifying the part config to increase the link strength is probably your best bet, because that really shouldn't be happening.

about 2km. I used Tweakscale to upscale the 1.25m SSME Tank to 3.5m, could that be the issue?

[Voculus]

You could also exempt that part from the drag causing its failure by editing its aero classification in the FAR menu. Just click the FAR button on the toolbar while at the KSC building screen.

[LethalDose]

about 2km. I used Tweakscale to upscale the 1.25m SSME Tank to 3.5m, could that be the issue?

Well, yeah, anytime you're using a mod, there's the possibility theres an issue. And here, you have a modded part being edited with another mod which is screwing with a third mod tells that part to interact with the atmosphere.

I have never used tweakscale, so that being said, I would guess that tweakscale is doing something to the part that causes FAR to see it as something that needs to get ripped off.

I'd suggest you move this over to the Modded Installs Support subforum so it gets the appropriate attention. Maybe someone over there has had a similar tweakscale-FAR interaction.

[Taki117]

Well, yeah, anytime you're using a mod, there's the possibility theres an issue. And here, you have a modded part being edited with another mod which is screwing with a third mod tells that part to interact with the atmosphere.

I have never used tweakscale, so that being said, I would guess that tweakscale is doing something to the part that causes FAR to see it as something that needs to get ripped off.

I'd suggest you move this over to the Modded Installs Support subforum so it gets the appropriate attention. Maybe someone over there has had a similar tweakscale-FAR interaction.

I think it's a design issue. The good news is It makes it to orbit with fuel left over, I have a feeling it's occuring simply because the tank is made up of multiple pieces and is not a single piece like the actual shuttle. Thank you all for the help.

[Hodo]

So, I'm attempting to build a space shuttle. I get everything set up in the SPH/VAB (using editor Extensions and RCS build Aid) and get a very nice replica going. It flies just fine until right before SRBSep, or around Mach .8. At this point the bottom of the Main fuel tank falls off. Sometimes it impacts the SRBs, other times it just falls straight off. I have tried reinforcing the joint with struts to no avail. Any help would be appreciated.

http://i.imgur.com/r0GQq4K.png

http://i.imgur.com/q11RDrI.png

http://i.imgur.com/KtGMiGZ.png

Notable Mods used: FAR, B9, Space Shuttle Main Engines, KSO, Tweakscale. (these mods have parts on the craft or are required because of it's construction, file available upon request)

Your problem is actually pretty simple. Your TWR is to high on launch. You are overspeeding the craft. According to your mj readout you have a TWR of 1.8:1 on take off which is to high. Ideally you want a 1.2-1.3:1 on launch. Dial back you power on the SRB rockets.