Struts 101-level questions

[HoustonDave]

EDITED: format of question and added photos.

I've been working my way through career mode in .23 and have hit a wall on lifter size.

Questions:

-Do crossed/angled configurations help or matter? Is vertical displacement against the decoupler an issue? How would I counter that?

-Horizontal strut connections seem to prevent nose-in of tanks. Is that best?

-What about between side-mates to prevent left-right wobble?

-Should I do strut connections between every contiguous tank? MOAR STRUTS?!?

-Does having more than one strut between two tanks at different points make a difference?

-Does placement on a long tank make a difference? Top of 1 to top of 2? Top 1 to bottom 2?

I have a multistage asparagus for lifting heavy probes anywhere in the solar system or landers to nearby planets.

Top stage is probe/lander

Middle stage is a rockomax jumbo-64 with dual side nuke thrusters. High efficiency, slow burn.

Bottom (boost) stage is a center jumbo 64/mainsail with a spiral pattern of 8 jumbo 64/mainsails around it connected in asparagus configuration.

This *just* gets me into low kerbal orbit for the nukes to kick in.

My issue is, I can't seem to get structural stability. Random tanks fail at decouplers at random points during launch. I have experented with different strut locations and configurations, and can't seem to beat this thing.

I'm not warping during launch. I am adjusting thrust to prevent exceeding terminal velocity. It rarely happens AT launch, often halfway up.

Side note, I am getting a mild autorotation when I'm down to the last five booster tanks, which disappears after I drop the next 2-tank/engine pair

I've searched extensively for more info on strut use, and gotten very little. I've built 30 variations on this, two rebuilds from the ground up, and I'm still getting unreliable launches.

Any insight into strut mechanics / accuracy appreciated. And if it's not struts, suggestions on what I'm doing wrong?

Thanks all.

https://www.dropbox.com/s/hlya0z7h9trt0x9/Screen%20Shot%202014-02-15%20at%202.50.54%20PM.png

https://www.dropbox.com/sh/lmv3bs2vw54290j/vGqfhgAxpC/Photo%20Feb%2015%2C%203%2006%2044%20PM.png

https://www.dropbox.com/sh/lmv3bs2vw54290j/lrQAf-sC_J/Screen%20Shot%202014-02-15%20at%202.51.54%20PM.png

And the Craft File:

https://www.dropbox.com/sh/lmv3bs2vw54290j/VWr_dl5r6A/Duna%20Probe%20BOR9_3.craft

Edited February 19, 2014 by HoustonDave

[DonLorenzo]

A .craft or picture would really help a lot in giving you pointers. I suggest you post that when you get to your computer

[HoustonDave]

EDIT: Merged into first post in thread.

Edited February 18, 2014 by HoustonDave

[HoustonDave]

EDIT: Merged into first post in thread.

Edited February 18, 2014 by HoustonDave

[kBob]

Use cross bracing (so they form an X) or in general you want them to form triangles as much as possible, one side of the triangle will be what you attach the struts to (like the fuel tanks in your ship). Triangles are stronger--look at the launch stability tower note how all it's trusses form triangles.

Edit: Note how most of my struts form triangles while I didn't cross brace same difference note the cross bracing the launch tower. You have to whack this thing pretty hard to make parts fall off, they stay put in normal flight.

Edit: Edit: If you're wondering this is in the middle of a transform from an old design to a new design so it's got stuff on it it doesn't need and I was experimenting with making my own high strength landing legs.

Edited February 15, 2014 by kBob

[The Destroyer]

My tactic is using modular girder segments on the sides, then sturtting up/down

Eg

T= Tank

D= Decoupler

M= Modular girder

S= Strut

T

T

M

S

D

S

M

S

T

T

T

[Tex_NL]

Struts don't have mass or have any effect on aerodynamics. When in doubt add more struts!

I can think of only two minor side effects struts can possibly have: Aesthetics, they can severely impact the look of your craft and they add more pieces in the physics calculations, possibly reducing your FPS.

To improve joint rigidity without the need to 'over strut' try Kerbal Joint Reinforcement.

[Kerbart]

Four launch towers might also be a little bit on the low side. Those four towers carry the entire rocket. Most of your rocket is carried by those radial decouplers. Consider adding launch towers to the inner section of your launch vehicle as well.

When you go to launch, your entire vehicle is all of a sudden exposed to gravity (which does not exist inside the VAB). That puts a lot of stress on all parts which is why you see it "bouncing" on the launchpad and that is usually where things break (press F3 to check before staging). Supporting a larger part of your rocket should help with that.

Right now you're doing this:

--> 4 -- 3 -- 2 -- 1 -- 2 -- 3 -- 4 <-- with the numbers being stages and only the "4" stages being supported. The parts in the middle will sag.

Try to do this:

--> 4 -- 3 --> 2 -- 1 -- 2 <-- 3 -- 4 <-- where the "2" stages are supported as well. Less stress.

[HoustonDave]

Interesting. Why did you mount girders on the decouplers instead of mounting the tanks directly to the decouplers?

[HoustonDave]

My tactic is using modular girder segments on the sides, then sturtting up/down

Eg

T= Tank

D= Decoupler

M= Modular girder

S= Strut

T

T

M

S

D

S

M

S

T

T

T

Is this to provide additional "spread" of components so you can have clearer attach points for the struts?

[HoustonDave]

Struts don't have mass or have any effect on aerodynamics. When in doubt add more struts!

I can think of only two minor side effects struts can possibly have: Aesthetics, they can severely impact the look of your craft and they add more pieces in the physics calculations, possibly reducing your FPS.

To improve joint rigidity without the need to 'over strut' try Kerbal Joint Reinforcement.

Thank you. Much appreciated.

Edited February 21, 2014 by HoustonDave

[HoustonDave]

Four launch towers might also be a little bit on the low side. Those four towers carry the entire rocket. Most of your rocket is carried by those radial decouplers. Consider adding launch towers to the inner section of your launch vehicle as well.

Thank you. I'll try adding more launch towers. Is there a way to adjust the radial distance of the launch towers? I've tried adding some to connect with the upper stages, but I can't get the bottoms of those to clear the lower stages.

[Xavven]

I agree with the advice here, but thrust plates unlocked a whole new game for me. Allmhuran posted a good explanation of the structural problem and solution here:

I can go up to maybe a 75 ton payload with struts alone, but thrust plates allowed me to get up to 350 tons (my personal record so far).

Looking at your screenshots, you have a lot of sheering forces from the second layer of radial decouplers. I would redesign it so that either your second set of tanks go underneath/inline instead, or thrust plates will solve it hands down.

[HoustonDave]

Xavven, that is just incredible. I was not aware of the higher strength of those plate/girder combinations. Thank you.

[problemecium]

Because tiny cubic struts in between two humongous parts look ugly, I prefer to use a 2x2 plate in there. The protruding corners give me more places for struts / RCS blocks, and the lack of crossfeed allows me to micromanage my fuel flow.

[HoustonDave]

Xavven, I wanted to thank you again. I was able to create a basic thrust-plate and with only two onion-layers managed to get a probe safely down on Moho.

Then I went crazy and added a third layer and SRBs and I'm doing a low-altitude flyby probe of the Sun. Woohoo! Man you really bumped up the enjoyment with that timely advice.

Edited February 21, 2014 by HoustonDave

[Xavven]

Hey, I'm glad that worked out for you! Allmhuran truly deserves the credit for this one. He's on these forums and I sent him a personal thank-you when I found his post.