Advice wanted: Technic frame for 40-foot Star Destroyer

[agrof]

@allanp Think inside of the box!  Actually I like this approach very much, and I do think it is doable. Very good idea!

The whole structure divided into 1 meter sections, which have their on "feet", than connect them for exhibition purpose only.

[Kdapt-Preacher]

17 hours ago, Davidz90 said:

In the first one, pillars end on the lower spar and the bottom is parallel to the ground. In the second option, pillars end on the upper spar and the upper surface is parallel to the ground. Again, I couldn't resist putting main spars at an angle, but they are only connected with triangular framework so it should be no issue.

Generally, I agree that some real-life tests are definitely needed. At the very least, it would be very useful to test one full-length main spar. Or even 50% model of the spar, holding at least 25% of projected weight on the full one. 

All of the heavy bits and everything mounted on the midline, including the dorsal cityscape, the engines, and the ceiling of the ventral docking bay, run parallel to the ground (luckily!). There will definitely be beams running diagonally along the length of the ship like that, but I think they're more likely to just be effectively tension cables supporting a flat spine than major structural spars in themselves, just because there isn't really anything that would need to sit on them. Also, FYI, these diagrams that we've been using are actually a really bad drawing of the ship; they're from the original Essential Guide to Vehicles and Vessels, which was a.... reasonable..... attempt at an encyclopedia when it was written in 1996 but took a pretty casual approach to things like details and proportions. For example, that drawing has nine engines, because that's how many they claim the ship has, despite the fact that anyone watching the movie can see perfectly clearly that there are 13, and it shows the cityscape and bridge tower as several times larger and bulkier than they actually are (they were trying to convince people that the ship is only 8km long rather than 17.6, so they more than doubled the sizes of the features that could be compared to ISDs). When I've finished my measuring and 3D modeling (which will probably be next week some time as I've been busy IRL recently) I'll post the model here so everybody can play with a more accurate schematic.

17 hours ago, allanp said:

Another idea might be to take inspiration from how real life ships are built and not build beams, but instead to build boxes. These boxes would fit together like the modular buildings to make something much bigger, but in your case they could also come apart for transport and storage. The boxes would have to be seriously over built with a good way to hold them together. The sides of the boxes would line up to create beam like lengths that run the length and breadth of the model where you need them most. The unseen sides of the box wouldn't have to be completely filled in but could be made rigid with triangular trusses. Each box could also have its own removable stand, so that you're only having to work out how to support each individual box instead of the entire model. This would make transportation and subsequent assembly of the model much easier. If they all have their own stand you're literally just placing them together like a modular town. If you then find that a box can be fully supported by the surrounding boxes you can then remove the stand for that box. It's difficult without complex simulation software, or building and testing for real, to fully comprehend how such a large model will hold up if built for real. But with boxes you only have to worry about each box. This could allow for a fully Lego build without the need for superglue or a steel frame, that could be transported and set up easily....if you have access to a semi truck. The disadvantage is that, if each box is about 1 square foot, you would have several hundred if them, each probably the full height if the model (like chunks in Minecraft). So you couldn't ever make it any smaller. At least with a steel frame, or with brick built beams, it could be dismantled into much smaller piles of detailed panels and brick built beams/steel cross sections.

Another approach you could take is taking inspiration from cable stayed bridges or tower cranes, and extend the support legs up higher than the internal framework and use cables suspended from the top of the support legs to support the middle of long unsupported sections (especially the rear engine area that's probably gonna be over 6 feet of unsupported Lego). Parachord (or whatever it's called) may not be Lego, but it's much lighter, much cheaper, much stronger in tension than Lego and can be rolled up into a ball, it would save a lot of weight and Lego. But a foreseeable disadvantage here is that cranes and bridges are only strong when fully built and their upper faces orientated toward the sky. So you would have to take that into account while building and transporting.

I like the idea of building it in sections like that in principle, but I have no idea how I would put that into practice. I think the odds of this thing supporting itself are only middling even if it does have strong, solid beams running all the way through it, and without that I can't really imagine it standing upright. If every section had its own stand and there were vertical columns every meter or so then yeah, it would stand, but the bottom of the ship would have to be close to solid columns to such a degree that you could hardly see the hull. If they were every square foot, then I don't think you could even really build a lower hull; it would just be columns.

Honestly, I just don't think this thing is going to be practically transportable no matter what you do with it. I'm sure that there exists a method of building it in sections that could be disassembled, but there's no way around the fact that you would need more than one semi truck to haul the thing, and I would guess something like a week of work for multiple people setting it up and breaking it down, and that's if you can find somewhere to take it in the first place--I would guess that to house it appropriately you'd want something like 15,000 square feet of display space, which is a hell of an ask even for a major convention (and conventions would almost never be able to offer you that much lead time to set it up, too). Ultimately, given how many other difficulties there are in even thinking about transporting it, I don't think it makes sense to prioritize that as a design goal when it's not even certain that it can be built as a static model. If I wanted a practical model to carry around with me I wouldn't be building a 40-foot Star Destroyer.

The cable stayed bridge design is a good one, though. The top set of diagonal beams in the drawing I posted fill basically that role. There'll be a couple of feet vertically between the main spar level (probably level with the midline of the ship) and the 'roof' of the underside of the cityscape, so there should be plenty of room in there for those supports. I'm going to try making them with LEGO beams first, but I have way less objection to paracord than I do to steel, so if push comes to shove and it doesn't look like it'll hold I would consider reinforcing with that.

[Davidz90]

6 hours ago, Kdapt-Preacher said:

If every section had its own stand and there were vertical columns every meter or so then yeah, it would stand, but the bottom of the ship would have to be close to solid columns to such a degree that you could hardly see the hull

Yeah, there is some compromise between column density and column thickness. I'd say the sections could be two metre long, with columns on their ends (so that section connections are the most supported). Main spar could be single piece, with sections slided onto it.

I don't think that the columns need to be that thick - lego bricks can survive enormous amounts of pure compression. How about thinner columns (possibly h-beams themselves), cross-braced with heavy fishing line? That could be rather unobtrusive. Or, if you are not against using non-lego outside the ship, then steel columns (again with some cross-bracing) could be really thin. Third option is transparent polycarbonate tubes as columns. Overall, I think that there is little gain from making the columns any thicker than the main spar.

Or, if the most weight is near the top, maybe consider hanging it from ceiling with thin steel cables or heavy fishing lines? 

[Mechbuilds]

Tbh people underestimate how tough a technic chassis can actually be with correct geometry. All you need to do is make sure this thing is as hollow as possible. 
Do not use unnecessary parts. Try to make an extremely stiff chassis with as little parts as possible. 

[Erik Leppen]

There was a challenge on the Lego Masters TV show where the contestants had to build a 3 meter long bridge. It inspired me to think about "how I would do such a thing, if I had infinite time", and I started fiddling around in MLCad and eventually came up with this:

(only one side is shown). More pictures here if public: Huge bridge on Brickshelf Now I have to say it's not tested in real life, so I have no idea if it actually works, and most details of the actual build are missing (some parts overlap), but it was fun to design anyway. As you can see, I built the main beams from triangles, and then made a triangle of those beams. So it's triangles all the way down ;) in the hope that would be strong, even if some sides are only single parts. In fact I only used 3-4-5 triangles and multiples, to make things easy to interconnect. (The 16L axles with end holes are nice to use as the 15-side of a 9-12-15 triangle.)

[Zerobricks]

Indeed, as Erik said, use a lot of triangles, they are the strongest shape. Also using flat panels and frames as support beams would be a good idea just like the Zetors uses for the chassis. And having the support structure spread out will make it much more stiffer than a single dense beam.

[dr_spock]

On 2/1/2022 at 9:33 PM, Kdapt-Preacher said:

It's 200 of every part, as far as I know. Getting stuff directly from LEGO would be fantastic, but I think it would have to be through the LUGBulk program or something similar. Bricks & Pieces is unlikely to get me anywhere fast.

 

If you're ordering tens of thousand dollars worth of pieces, it would be worthwhile contact LEGO directly and work out a deal.

[Kdapt-Preacher]

Did an actual structural test. This beam design is roughly 10x10 studs and weighs about 0.5 lbs/foot. A 48-inch span of that beam loaded with 19 pounds of books at the center sagged approximately one inch. I think that's an encouraging result. Assuming a larger version of the beam had a similar Young's modulus (which I think it would?), that implies that the 40-stud version could support 100 lbs over a two-meter span and only sag about 2 mm. I'm playing seriously fast and loose with units here, so IDK whether that would really work out that way in practice, but I feel good for having done some kind of math. And that's completely unsupported; having some kind of cross-brace at the middle shouldn't be too hard and ought to improve things even more, since the amount of sag is (theoretically) proportionate to the cube of beam thickness.

[mdemerchant]

3 hours ago, Kdapt-Preacher said:

Did an actual structural test. This beam design is roughly 10x10 studs and weighs about 0.5 lbs/foot. A 48-inch span of that beam loaded with 19 pounds of books at the center sagged approximately one inch. I think that's an encouraging result. Assuming a larger version of the beam had a similar Young's modulus (which I think it would?), that implies that the 40-stud version could support 100 lbs over a two-meter span and only sag about 2 mm. I'm playing seriously fast and loose with units here, so IDK whether that would really work out that way in practice, but I feel good for having done some kind of math. And that's completely unsupported; having some kind of cross-brace at the middle shouldn't be too hard and ought to improve things even more, since the amount of sag is (theoretically) proportionate to the cube of beam thickness.

Not sure it will quite scale like that in practice because a truss behaves differently than a solid beam and a Lego truss very differently than an ideal theoretical truss.  But with the right design I still don't see why you couldn't make something to support 100lbs over a two meter span.  Many years ago when I was in school all the universities would have a competition to make bridges out of popsicle sticks, glue and dental floss.  For a 1m span bridge the top teams nationally regularly exceeded more than 1000kg load.  Lego pieces aren't quite as strong as popsicle sticks but 100lbs isn't that heavy and you don't really care what the structure weighs so you can put lots of pieces in parallel.

I am a lot more skeptical you could support that weight over that span with only a few mm of sag though.  With a truss I think it would be pretty hard because of the pin connections.  In a simple textbook analysis of a truss you assume the pins are perfectly rigid and deflection occurs as a result of stretching and compressing the beams.  But in a Lego truss you've got bendy plastic pins in oversize holes and actually the vast majority of your deflection will come from there.  Making it bigger doesn't help if you end up with the same number of pins.  By contrast, if you make a solid beam deeper, suddenly you have lots more material to resist the stresses so it gets stiffer and stronger non-linearly with depth.  That's why I would prefer a solid beam structure for this purpose, it seems like it will scale much better.  If you do make a really deep truss I think you are much better off to make it by tying lots of parallel small trusses together instead so you get more connections and less load/connection.

[Erik Leppen]

6 hours ago, mdemerchant said:

But in a Lego truss you've got bendy plastic pins in oversize holes and actually the vast majority of your deflection will come from there.  Making it bigger doesn't help if you end up with the same number of pins.  By contrast, if you make a solid beam deeper, suddenly you have lots more material to resist the stresses so it gets stiffer and stronger non-linearly with depth.  That's why I would prefer a solid beam structure for this purpose

If the top and bottom members of the truss are continuous beams, rather than joined sections, then you don't need solid material between them. The material wouldn't do much that a hollow structure wouldn't also do. That would only add a lot of weight, while the main goal is keep the top and bottom at the correct distance, so their tension/compression doesn't result in buckling. For optimal strength you want most of the material near the edges, that's why the I and H beams exist.

Compare this to a traffic barrier. These are thick rails on very thin posts, but this still works because there are very many posts, so the force required to bend the structure is the force to bend a post, multiplied by their huge number.

But yes, for the top and bottom I would definitely rely on stud connections rather than pin connections.

Also, I would assume axles are much stronger than pins. Especially if braced on both sides.

Edit: what I mean is this: 

The top and bottom act as full 2x6 stud (cross section) beams, which should have plenty of tensile/compressive strength, and the diagonals only serve to keep them from moving relative to each other, and, thus, from bending upwards and downwards. The pinned black beams may not be very strong, but there are a lot of them, so together they should do the job. (Again, the structure is not tested in real life. I just really enjoy designing stuff like this)

Edited February 26, 2022 by Erik Leppen

[2GodBDGlory]

Again, this is a really cool project. I'm not sure how much you follow the Technic news, @Kdapt-Preacher, but there are some large structural parts coming out this year that may be handy in this project. The first of these is a 3x19 frame in a couple sets, seen in this picture from mostlytechnic's revies of 42140:

and the second, while only a rumor at this point, is a 7x17 frame reportedly coming in the Liebherrr LR13000 this summer (or later if it's delayed, as has been rumored). It's probably not waiting on these parts, though, and I'm sure they'll be hard to get in the quantities you need for a long time!

[mdemerchant]

6 hours ago, Erik Leppen said:

If the top and bottom members of the truss are continuous beams, rather than joined sections, then you don't need solid material between them. The material wouldn't do much that a hollow structure wouldn't also do. That would only add a lot of weight, while the main goal is keep the top and bottom at the correct distance, so their tension/compression doesn't result in buckling. For optimal strength you want most of the material near the edges, that's why the I and H beams exist.

Yes, maybe solid beam structure wasn't the best choice of words.  I just meant a beam of some kind as opposed to a truss, an I section is certainly most efficient.  For a soft material like Lego with give in the joints it will be possible to make a stiffer beam than truss for the same overall dimensions.  Given the desired very low span to depth ratio here you would use I beams for this kind of structure 99% of the time in real construction.

Your truss design looks quite nice.  And we're definitely on the same page that if you're going to use a truss, you need lots of diagonal members to spread the load mainly to limit deflection.

[Davidz90]

5 hours ago, mdemerchant said:

I just meant a beam of some kind as opposed to a truss, an I section is certainly most efficient

Maybe I'm repeating myself a bit here, but I agree 100% with this. One cannot beat solid I-beam. Also, 16x16 plate is as big as Lego bricks go, I-beam with plate-based central section  would be pretty much indestructible. Unfortunately I don't have enough bricks to make a meaningful test.

Updated, simplified I-beam design:

ibeam by David_Z1, on Flickr

core is 2 layers of overlapping 16x16 plates. Flanges are 2x16 plates and 1x16 technic bricks. At some (possibly large) intervals, 2x4 plates (orange) stick out from flanges and support secondary beams (orange) that keep perpendicular bricks from popping out. One can also use longer perpendicular beams to connect two I-beams together side by side, forming a box spar.

Edited February 26, 2022 by Davidz90
added photo

[Kdapt-Preacher]

This is all helpful feedback, everybody. Like I said, I'm an ecologist and not an engineer. Ultimately I think I'm going to have to order a bunch of parts to test some of these designs; the advantage of the beam that I tested was that I could build it with what I had here to actually see what it would do, but I think y'all are right about trying to maximize stud connections.

On 2/26/2022 at 3:49 PM, Davidz90 said:

Maybe I'm repeating myself a bit here, but I agree 100% with this. One cannot beat solid I-beam. Also, 16x16 plate is as big as Lego bricks go, I-beam with plate-based central section  would be pretty much indestructible. Unfortunately I don't have enough bricks to make a meaningful test.

core is 2 layers of overlapping 16x16 plates. Flanges are 2x16 plates and 1x16 technic bricks. At some (possibly large) intervals, 2x4 plates (orange) stick out from flanges and support secondary beams (orange) that keep perpendicular bricks from popping out. One can also use longer perpendicular beams to connect two I-beams together side by side, forming a box spar.

This looks really good. My gut feeling agrees with yours, that a 16-stud-wide connection is going to have functionally zero horizontal flex before the point where the plates themselves just rip in half. And one advantage of that specific design is that this model is already going to use something like 5000 16x16 plates for the flat hull regions, so using those as the major component of the support beams as well would simplify the logistics of acquiring the pieces. It might only cost, say, $950K instead of $1M.

[Kdapt-Preacher]

So I've ripped the Executor model out of Battlefront 2015. I actually purchased the game just for this, so that tells you that I'm committed to this project to the value of at least the $4.99 that I spent on it. This will be my main reference for the proportions. There's no easy way to know whether this is shaped 100% exactly like the original studio model, especially since it is scaled to 19,000 rather than 17,600 meters, but it's certainly close, and since this is itself a canonical source I think I'm fully justified in following it. However, this model isn't anywhere near detailed enough to be useful as a reference for specific detailing (it's only seen in the far distance in the game, so it's much lower-res than even typical game assets), so the studio model will still be my primary source for greeblies and texture.

The main takeaway from this is that the exact measurements for the width here are slightly narrower than I'd estimated from the studio model, giving that bow point a slope of 4.40 rather than the 4.3 that I'd guessed. That's really convenient, because the 9-40-41 Pythagorean triple gives a slope of 4.44, which means that I can hugely simplify my life by basing the sides of the ship on that ratio and land within 1% of the correct width. The discrepancy will come to something like 4 studs over the 13-foot width of the model, which is ABSOLUTELY close enough even by my standards.

[Kdapt-Preacher]

Thought I should drop a note in this thread to say that I am indeed still working on it, and this is what I'm looking at right now: this is the exact shape of Executor's upper hull, with coordinates given in meters (scaled to the 19,000m length). Any robots in the audience will immediately notice that those points are not, in fact, coplanar! I.e., the hull panels aren't flat. That would, after all, make things too easy... they're close to flat, close enough that I don't think anybody would ever be able to tell the difference if I made them actually flat, but "close" in the context of a ship this big still means we're talking about them being off by a hundred meters or so, which is a hell of a rounding error. So what I've been thinking about for the last couple of days is exactly what I want to do about that, and whether it makes sense to try to replicate this exactly (mors ante compromissum...) and how much complexity that's going to add to the build (probably a lot).

[shroomzofdoom]

Make a 'unit' of something (example: a truss of lift arms and bricks), test it weigh it, and estimate the weight of the finished design. The issue I've had building with technic on macro-scale is the 15 unit liftarms. I think last time I measured it would take 28 15L liftarms to make about 3 lineal meters compared to roughly 21 3x19 frames. I'll bet the 3x19 frames would make this easier. It'll be another month before they're available and getting thousands of them maybe difficult. Lego engineers built a life sized working Bugatti, so don't let anyone tell you it's impossible. It's Lego, everything is possible. . If you look closely at this body you can see some cool techniques https://youtu.be/n-RtJOfFlZU