Question about strut attachment?

[drtricky]

So this glowing pair of wings on my plane is attached to the glowing fuselage on the right of this photo with the Goalkeeper CIWS in it, but I have struts attached to other fuselages along the wing.

Will those struts connected to the wing and parts of the fuselage that the wing is not attached to contribute to the wing's joint strength? Based on what I read about struts in KSP, they wouldn't seem to.

Edited April 13, 2016 by drtricky

[KocLobster]

First of all, I'd like to suggest using the Kerbal Joint Reinforcement mod; I'm assuming you don't already. This is a great mod and I absolutely recommend it. I have found with this mod that I rarely even need to use struts for structural stability and strength. Only in certain cases do I find them necessary; I can usually eliminate their need with careful designing.

With that said and out of the way, I'll do my best to answer your question. To be honest I kind of had a hard time understanding exactly what it is that you're asking. The answer is yes, struts will increase the strength of the joints between your wings and that fuselage. However, if you choose to install KJR, you most likely can get away with not needing any of those struts to begin with.

[drtricky]

7 minutes ago, KocLobster said:

With that said and out of the way, I'll do my best to answer your question. To be honest I kind of had a hard time understanding exactly what it is that you're asking. The answer is yes, struts will increase the strength of the joints between your wings and that fuselage.

Got it.

Edited April 13, 2016 by drtricky

[Snark]

KJR's a popular mod and lots of folks like it... but just to present alternate point of view, I'd suggest not using KJR, at least not until you're totally at home with getting along without it.

Rant on the topic put in a spoiler block so as not to clutter things up too much for folks who aren't interested.

Spoiler

Why I think KJR is overused:

I view the wobbliness of stock parts as a feature, not a bug.  It's not just that I enjoy the technical challenge of building a craft that stands up against all sorts of engineering challenges (including wobbliness).  It's also that wobbles are the game's way of telling you that there's a problem with your design.

I routinely build all sorts of craft, including big 'uns that are hundreds of tons.  And they get along fine without KJR, no wobble.  It just takes some attention to design.  I do use struts, but only minimally; typically, one strut per radial booster is about the limit.  I don't fly planes a lot, but on those occasions when I fly big ones, a couple of struts to reinforce the wings generally does the trick.   (And I mean literally "a couple", not "a bunch").

If you just go straight to KJR, you deprive yourself of the chance to learn how to design craft that don't need it.  Including big ones.

Also, there's a potentially more insidious problem:  a craft that's unwieldy enough to wobble a lot without KJR help is also likely to run into other flight problems as well, and using KJR to gloss over those problems has the potential to be like taking an aspirin for that nagging pain in your chest-- it would be better to find out what the pain is and fix the root cause.  I've seen Gameplay Questions posts from people who build a huge unwieldy monstrosity, then use KJR to stiffen it up... and then find out that it can't fly because it's horribly unstable, or some similar problem associated with the unwieldiness of the design.  "Too wobbly" can also be a warning sign that you're making your craft a lot bigger than it actually needs to be.

More on this rant here:

 

 

Anyway, about the struts:  Yes, they will strengthen your ship... but the question is what kind of strength.  Why do you need them on those wings?  Is it because the wings are flexing up and down?

If that's the case... those struts are placed very poorly.  You're having to spam lots of them, and I bet you're still getting a lot of flex in the wings.

You need to think about lever arms and mechanical advantage.  You can get a lot more strength for far fewer struts (just one per wing, really) if you mount the struts properly.  You want both ends of the strut to be far away from the joint between wing and fuselage.  The fuselage end of the strut should be way up near the top of the fuselage, not right at the wingline; then it should slope down at around a 45 degree angle to connect to the wing a fair distance out from the fuselage.  Try that, it'll be a lot more effective

[drtricky]

36 minutes ago, Snark said:

Anyway, about the struts:  Yes, they will strengthen your ship... but the question is what kind of strength.  Why do you need them on those wings?  Is it because the wings are flexing up and down?

The airplane in the picture is designed to pull fighter-like maneuvers, needing to be able to withstand over 27 gees of force. Don't ask.

36 minutes ago, Snark said:

You need to think about lever arms and mechanical advantage.  You can get a lot more strength for far fewer struts (just one per wing, really) if you mount the struts properly.  You want both ends of the strut to be far away from the joint between wing and fuselage.  The fuselage end of the strut should be way up near the top of the fuselage, not right at the wingline; then it should slope down at around a 45 degree angle to connect to the wing a fair distance out from the fuselage.  Try that, it'll be a lot more effective

So struts take torque into account? I thought they simply increased joint strength between parts irregardless of how they're placed, as long as they were connected between two parts.

Edited April 13, 2016 by drtricky

[Snark]

1 hour ago, drtricky said:

So struts take torque into account? I thought they simply increased joint strength between parts irregardless of how they're placed, as long as they were connected between two parts.

Struts don't work like that at all.  It's simpler than that.  They don't do anything to the joint itself.  All they do is add another mechanical component that acts like a really stiff spring that wants to be the length that it is in the vehicle editor.  It applies its force at the two endpoints.  If you try to stretch it, it pulls back.  If you try to compress it, it pushes back.

Think about those really tall radio antenna towers you sometimes see, the ones that are so impossibly tall and skinny.  They don't fall over because they have guy wires attached.  Think about where the guy wires are.

Do they use a short little ten-centimeter wire that ties the very bottom of the antenna to the ground?  No, they don't, because that really wouldn't help stabilize the tower.  I suppose it would keep the tower from being picked up and carried away from a tornado, but it would do almost nothing to keep the tower from leaning over one way or the other.  Instead, one end of the wire is way up at the top of the antenna, and the other is on the ground way the heck far away from the antenna's base.

A strut is exactly like that.  (Well, except that a guy wire can only handle tension, not compression.  A strut handles both.)

So the way you want to place a strut for maximum effectiveness is this:  Figure out "which way is my ship going to flex".  And then place the strut in such a position that when the ship flexes, it will try to stretch or compress the strut by the longest distance possible.  This will give the strut a high mechanical advantage and will make it the strongest at resisting such flex.

Think about how you have the struts placed in the screenshot with your OP.  If that wing bends up and down by, say, ten degrees, how much will it stretch or compress those tiny struts right at the joint?  Practically not at all, maybe a few millimeters at most.  But if you had a three-meter long strut that has one end anchored at the top of the fuselage, and another end anchored far out on the wing away from the fuselage, how much would that strut's length have to change for the wing to bend up and down by ten degrees?  A lot.  So it'll be many times more effective than the struts you have.

My favorite way to make really stiff wings if I have to make a big plane is to put a Big-S tailfin on the thing.  Then I run a strut from the top tip of the tailfin to a point far out on the wing, towards the wingtip.  One strut like that to each wing makes them practically invulnerable to flexing, because the Big-S tailfin is so tall, it gives great leverage.  (Yes, it looks kinda dorky, but it's incredibly effective.)

[drtricky]

@Snark I'm impressed! I put 4 3 struts as long as possible on the wing of that fighter, and it was able to pull out of a 400 m/s dive at 2000 m, totaling over 28.5 gees! Never before had it been able to do that, especially with 10 tons of weaponry!

Edited April 13, 2016 by drtricky

[Snark]

26 minutes ago, drtricky said:

@Snark I'm impressed! I put 4 3 struts as long as possible on the wing of that fighter, and it was able to pull out of a 400 m/s dive at 2000 m, totaling over 28.5 gees! Never before had it been able to do that, especially with 10 tons of weaponry!

Ta dah!

Final score:  Engineering 1, forces of darkness 0.    Glad it worked out for you.

Going back to my earlier anti-KJR rant:  This is an example of exactly what I was talking about.  I contend that for a lot of cases that people are using KJR... they really don't need to.  It's about ship design.  If you get the hang of KJR-less design and still want to use it (e.g. you care about looks and want more freedom to design things how you want them to look, or whatever), then use it.  But seriously consider giving the stock physics a good honest try first, before running to KJR-- you can learn all kinds of stuff!

[Kerbart]

15 hours ago, Snark said:

Going back to my earlier anti-KJR rant:  This is an example of exactly what I was talking about.  I contend that for a lot of cases that people are using KJR... they really don't need to.  It's about ship design.  If you get the hang of KJR-less design and still want to use it (e.g. you care about looks and want more freedom to design things how you want them to look, or whatever), then use it.  But seriously consider giving the stock physics a good honest try first, before running to KJR-- you can learn all kinds of stuff!

@Snark, that was very insightful. I'm “handicapped” with an engineering degree, so seeing struts as a superior solution to “make the joints stronger” is a no-brainer for me. However, your post makes me (finally!) realize why so many people insist that the game “is broken” and that KJR “should be stock.” If you’ve spent many hours on the drawing board drawing out calculations on girder constructions (yes, I’m that old school, hahaha) then reinforcing a rocket with two or three pairs of struts in order to make it as stiff as a door nail is easy and intuitive. But at the same time one can place dozens of bad-placed struts and still end up with a wet noodle at launch, if you have no good idea of what it is that struts are actually doing.

[KocLobster]

17 hours ago, Snark said:

Ta dah!

Final score:  Engineering 1, forces of darkness 0.    Glad it worked out for you.

Going back to my earlier anti-KJR rant:  This is an example of exactly what I was talking about.  I contend that for a lot of cases that people are using KJR... they really don't need to.  It's about ship design.  If you get the hang of KJR-less design and still want to use it (e.g. you care about looks and want more freedom to design things how you want them to look, or whatever), then use it.  But seriously consider giving the stock physics a good honest try first, before running to KJR-- you can learn all kinds of stuff!

One of the main reasons you didn't mention that I use KJR is for part count reduction. I have a crappy laptop that can't run KSP very fast, so part count is important to me and I always strive for C&C (Cheap & Cheerful) rockets.

Edited April 14, 2016 by KocLobster

[Alshain]

24 minutes ago, KocLobster said:

One of the main reasons you didn't mention that I use KJR is for part count reduction. I have a crappy laptop that can't run KSP very fast, so part count is important to me and I always strive for C&C (Cheap & Cheerful) rockets.

That is fine and all but suggesting KJR as the first solution is never good.  If that is how you want to play the game, more power to you, but here on the questions and answers page the first solution should be a stock one.  The first thing you said was (paraphrased) "you should use KJR".  It's not just KJR either, MechJeb, DPAI, and all of those fall under the same problem, if you use it you don't learn to do it correctly.  Not that you shouldn't use it, you just should only use it after learning to do it correctly.

[Snark]

2 hours ago, KocLobster said:

One of the main reasons you didn't mention that I use KJR is for part count reduction. I have a crappy laptop that can't run KSP very fast, so part count is important to me and I always strive for C&C (Cheap & Cheerful) rockets.

I didn't mention that "reason" because I would contend that it isn't one.  KJR doesn't help with part reduction, at least not to any significant degree.

If you have a wobbly rocket (and are thinking "gosh, I should get KJR")... one of the possible causes of wobble is you have too many parts.  Without KJR, this forces you to be diligent in your engineering-- for example, figuring out how to reduce part count until it stops wobbling.  With KJR, it just magically freezes everything and you don't have any reason to reduce part count.  So in this regard, KJR can actually make your part-count problem worse, by encouraging/enabling rockets with too many parts.

The only way I can imagine that KJR would reduce your part count, at all, would be "now I don't need to use struts."  No struts means fewer parts, right?

...Except that if you have so many struts that they're significantly affecting your part count and slowing down your frame rate, you're using the struts wrong.  A well-designed ship does not need very many struts.  Specifically, if you're correctly using struts (i.e. they're well placed), you only need a tiny number of them to keep things rigid.

If I build a really big Mk3 airplane and the wings are flopping... I add two struts.  Two.  On a craft that probably has around a hundred parts.

If I build a really huge multi-hundred ton ship, it probably has 200 parts or more... and fewer than a dozen struts.  Maybe only a half-dozen or so.

Eliminating those struts would be a trivially small fraction of the part count, and I seriously doubt it would have any perceptible difference on your framerate.

In short:  that's not a reason to use KJR.

[Plusck]

5 hours ago, Alshain said:

That is fine and all but suggesting KJR as the first solution is never good.  If that is how you want to play the game, more power to you, but here on the questions and answers page the first solution should be a stock one.  The first thing you said was (paraphrased) "you should use KJR".  It's not just KJR either, MechJeb, DPAI, and all of those fall under the same problem, if you use it you don't learn to do it correctly.  Not that you shouldn't use it, you just should only use it after learning to do it correctly.

Have to agree with this.

I know that I could be criticised (by multi-mod players) for having an excessively pro-stock POV, but to me it makes perfect sense.

Every single mod was created by players who got good at doing what stock does, and then wanted more. And the first people to try and to adopt every single mod were all players who had worked their way through stock (as it existed at the time) and felt the need for what the modders provided.

So I use KER, because I was constantly overestimating my fuel needs and I got tired of plugging my ship mass and fuel levels into the rocket equation to find out whether I was going to make it home. However, my rocket equation spreadsheet was mine, all mine, and I feel that I have learned a huge amount and have a real sense of accomplishment with doing all that on my own. So I'd never tell a noob to install KER or MechJeb or whatever because that would ruin that possibility. And the same goes for every other mod: do it yourself, and if there is still something missing, then maybe a mod might be an idea.