# Designing Space Hardware is Hard

## Метаданные

- **Канал:** Breaking Taps
- **YouTube:** https://www.youtube.com/watch?v=kXa0vUy2Tlg

## Содержание

### [0:00](https://www.youtube.com/watch?v=kXa0vUy2Tlg) Segment 1 (00:00 - 05:00)

today we're making hinges and yes I know that doesn't sound very interesting but I promise it's a fascinating topic because these aren't any hinges these are hinges designed for space so you always hear in the space community that space is hard and I mean obviously it's hard everything's up there in orbit but like what are the details that make it hard and today we're going to look into that a little bit by designing what seems like a really simple device a hinge a thing that opens and Clos poses there's a surprising amount of complexity when designing a hinge because you have all these different considerations you have to keep in mind that you just don't deal with when you're opening a door here on Earth I designed four hinges and end up building three of them and each had kind of a different design philosophy and we'll talk through the pros and cons the different designs I came up with and which one I like the best heyo um future editing breaking Taps here I thought I would break in for a moment because I realized I spoke for like 40 minutes without really talking about what the hinge is for so in short you can imagine this hinge being used for like a solar panel deployment mechanism uh for example this is Alba orbitals unicorn pocket Cube It's like a small version of a cube set and you can see the solar panels deploy and each solar panel you know is connected to the next with a pair of hinges so that's kind of what we're doing here we're making little hinges for little solar panels on little satellites okay back to the video This is the first design that I came up with and it has a basic hinge shape there's an outer section and an intersection held together with a pin through the middle there is this torsion spring which allows the hinge to automatically open for deployment and then you can see there's a stop down here so that the two Hales only can open to 180° and then they'll naturally stop for this design I had two main criteria kind of top of mind I was concerned about ease of manufacturer and thermal coefficients and so to that end I designed this to be all out of a single material so this was supposed to be all aluminum both the inner the outer and the pin through the middle and by making all the materials the same there's no mismatch between the thermal coefficients of expansion when it heats up or cools down everything will kind of stretch or Shrink at the same rate the retention method was planned to be just a simple press fit and the way this works if you're not familiar is that one side in this case the outer leaves of the hinge have a diameter that is slightly smaller than the pin diameter so you can see there's kind of an interference here and then the intersection was slightly larger so it was free to rotate and then you literally take the pin and just kind of shove it through the hole and it will plastically deform some of the metal and basically get retained by being stuck inside of the hole so I thought this was very clever because it was all one material so The Thermals were fine the Press fit would have made assembly very easy you just align everything use like an arbor press and press it together now unfortunately when I started to share this around and talk to people that know things about space they pointed out a few major and fatal flaws the first is that press fits are just not a very reliable method for space during launch that really high vibration load has a tendency to work press fits out the other major fla is that all of the materials are the same material and now I know I just said that was a good thing cuz all the thermal coefficients are the same and that is true but the downside is that during launch we're going to have an aluminum inner half of the hinge an aluminum outer rubbing on each other violently due to the launch vibrations and there's a high likelihood that when this is folded up like that so and kind of stowed uh the inner and the outer halves will bounce against each other and rub and if this starts to rub together they might cold Weld and cold welding is a phenomenon where if you take two metals that are sufficiently similar and you push them hard against each other and like rub so that you can get through the native oxide layer on aluminum then in the vacuum of space the metals don't really know that they're separate parts anymore they just see like hey you're aluminum and you're aluminum so like let's be friends and they actually just weld together now I should note we'll talk about this more later there are ways to get around the cold welding issue with like Coatings and lubricants and stuff like that uh we'll talk about that in a bit okay so that was you know a fine first attempt but clearly sh in some inexperience with designing space Hardware so with the whole like

### [5:00](https://www.youtube.com/watch?v=kXa0vUy2Tlg&t=300s) Segment 2 (05:00 - 10:00)

cold welding vibration thing top of mind I decided to go with a ball bearing hinge for the next iteration cuz I was thinking about it I'm like okay well there's always going to be a problem with a pin because the pin will always be touching you know the two halves of the hinge so you've always have either galvanic corrosion issues if you got two different metals or um you've got cold welding issues if it's the same material so why don't we just skip that by using uh ball bearings to bridge the gap between the inner and the outer and then you can choose bearings that are like Silicon nitrite or some other ceramic and you have no cold welding and no galvanic corrosion issues so that's kind of what I was thinking when I designed this setup you can see it is a little more complicated than the first one uh there's these two torsion springs uh in an Ideal World this would be a single like combined double torsion spring uh I couldn't find one of those that fit my requirements so I ended up trying to make one myself and it kind of worked it kind of didn't work but let's hide these for now if we look at the hinge itself it's made of three components there's kind of a Outer assembly that bolts together there's this middle assembly that goes in between and if we hide one of it you can see there's a race in the middle for bearings so there's a bunch of little ball bearings that are on either side of this middle section the crosssection of this hinge the inner section has a Ras cut out and then two outer section have races cut out and they're held together such that the bearings are keeping everything separated so it's only bearing on metal contact and then you can see there's an additional feature that I added here uh in the back there's this kind of hook shaped looking thing this serves two purposes uh it's supposed to hold the torsion spring so this spring will loop around the back and that's the retention mechanism for the spring and it also serves as a latch so if we rotate this 180° around after the hinge is opened you can see that the little hook section here catches on this sheet metal component in the back and then basically forms a self-latching mechanism so once it opens it's unable to bounce backwards because it'll hit this sheet metal and latch itself open so that is this design uh it technically works so it assembled uh the torsion springs are not very good because I had to take two regular torsion springs and bend them together and it's a little janky so you kind of use your imagination here but in theory if torsion spring was a combined double I think it would probably work the tolerances are not great you can see there's a lot of slop between kind of the inner and outer sections so that would need to get tightened up for a more production worthy sort of hinge but the bones of it seem like it would work uh the big downside here is it just a lot bigger than I expected so I mean I know it's a small hinge but this is supposed to be designed for a cube set or a pocket Cube uh satellite and so every millimeter matters and this ended up being actually quite large and I wasn't expecting it to be so large there's also a design flaw which is kind of obvious in retrospect and I don't know why it didn't occur to me at the time but if you'll notice the assembly the bearings are being retained by these two bolts in the back right so they're bolting these top two components together but it's away from the center of rotation there's no bolt going through the exact center in between all the bearings and so you can imagine if you would take this hinge and kind of pull on it it's possible that those two plates might open up a little bit and allow the bearings to escape and then your hinge is completely come apart you could of course fix that by putting a bolt through the center of rotation but that there's not a lot of space in the center here and so even if you use like an m1. 5 or an M2 there's not really enough meat there so you'd have to make the race a little bit bigger or use smaller ball bearings and it kind of makes this whole thing even chunkier than it already is so if we iterate on the ball bearing concept and make something that looks a little more like a normal hinge meaning it has a hinge pin but still uses ball bearings you come up with a design that looks something like this design has three main pieces there's an inner hinge and outer hinge and then kind of like a

### [10:00](https://www.youtube.com/watch?v=kXa0vUy2Tlg&t=600s) Segment 3 (10:00 - 15:00)

retaining piece which helps bolt everything together and then importantly there's now this hinge pin the pin is what's known as a shoulder bolt and this is basically just a bolt so it's got threads at one end and you know a hex Drive of the other but it has a really long shoulder that's reasonably precise diameter and surface finish and you can use the shoulder bolt as like a rotation mechanism so the bearings will actually ride on that shoulder bolt we do a cross-section you can see that there are small bearing races cut into the inner and outer halves of the hinge this means that the barings will ride against the steel shoulder bolt as well as the two halves of the hinge and keep everything separated so we're still relying on the bearings to keep the metals away from each other so like Silicon nitride bearings with aluminum inner and outer hinge parts and a steel shoulder bolt and the reason we have this third component is because we need something to why did you do that H I don't remember why I did that oh was for assembly reasons right took me a minute to remember why I did it this way uh right so the reason we need a third component is purely for assembly or manufacturing purposes so we've got these races right and we need to get the ball bearings into the race before we assemble kind of the next part that sits on top of it so if you made a single monolithic outer section and a single monolithic intersection you can't put those together and also fill it with ball bearings and so the compromise I made was having a third component that included the last race and this allows you to assemble things kind of in order and then add this cap at the end and bolt everything together there are also these little pockets that are milled out basically just to reduce weight uh the material there doesn't contribute significantly to the strength of the hinge so we can just get rid of it and then over here there's a little pocket that's M out and that's mainly so that there's clearance for this bolt in case it sticks through a little bit assembly of this hinge is definitely more complicated than the first one but before we can even assemble we need to talk Coatings and lubricants uh I didn't bother with this on the last hinge but with this one we're going to try to be as faithful as possible so most Hardware that goes into space that moves uh has some amount of Coatings or lubricants or both added to them to help keep things you know sliding freely so in this case I'm first burnishing on a thin layer of tungsten disulfide this is a dry lubricant uh it's basically very thin platelets of what's known as a Vander wals material that likes to slide against itself it has a low coefficient of friction with itself this is also a reasonable defense against cold welding because you no longer have metal on metal contact you just have the dry lubricant running rubbing against itself so tungsten disulfide is used malum disulfide uh hexagonal boron nitride is also used I think sometimes graphite's even used uh there's a lot of different dry lubricants you can pick from they all have pros and cons so that's kind of the first layer of defense against friction and cold welding and galvanic corrosion uh but it's not a super robust defense so there are plenty of papers and a lot of hard one experience uh in the industry showing that under high vibration loads the dry lubricant can pretty easily get rubbed off so from talking to folks it's not uncommon to apply a vacuum rated space rated grease on top of your dry luant just to provide a secondary kind of insurance policy uh there's a little more prep we need to do before we can assemble the shoulder bolt that I'm using has a reasonable spec for diameter tolerance but it basically has no specification for surface roughness it's not bad but it's not great so to help with the surface roughness make it a little smoother more polished I'm going to hit mine with some 1200 grit sandpaper I'm also going to coat the pin with the tungsten disulfide and then finally I'll add a little bit of Loctite to the end of the thread and there's one final thing that we're going to do and this was a tip that I got from Adam The Machinist thanks Adam this was a really good tip these torsion springs are kind of crunchy like they're not they don't feel great when you're actuating them and so a tip that Adam told me is if you take some Diamond slurry or in this case silicon carbide uh polishing grit paste and you put it on a thin piece of brass and run the brass and the polishing

### [15:00](https://www.youtube.com/watch?v=kXa0vUy2Tlg&t=900s) Segment 4 (15:00 - 20:00)

paste through the center of the torsion spring you end up lapping the interface between the torsion coils to a much smoother profile and it just gives a much nicer smoother actuation you don't have this like gritty nature anymore so this was a really good tip I'm going to start doing this for all torsion springs that matter uh because it's really night and day difference between the feel of a polished and lapped spring versus one that you just get out the bag so with all that prep work out of the way we can finally start to assemble the hinge I'm going to stand up the shoulder bolt and the outer portion of the hinge on a piece of putty then we just very carefully start loading in the ball bearings these are 132s of an inch ball bearings which is 79 mm for the metric folks uh I'm using steel bearing just for testing but for the real thing you'd want to choose some kind of ceramic to avoid the before mentioned issues once those are loaded we slide on the insection of the hinge and the torsion spring and repeat the process with the top bearing race after that's been loaded with bearings finally we cap it with the Third Kind of additional extra component that screws into the shoulder bolt and that is the finished and assembled hinge now obviously that was a bit of work so this kind of gets a c minus as far as manufacturing and assembly goes it's pretty tedious to do and it feels like it'd be pretty easy to mess up as well if you're making say a dozen of these in one batch the hinge feel itself is not bad but it's not great it's a little crunchy still which is mainly I think a problem of the bearing races so normally ball bearing races are hardened steel bearings which we have but then the material they're running against is usually also hardened and in this case it's just aluminum so this is a major kind of down side to this particular setup is that the bearing race is a soft material and that can lead to like dimpling or denting where the bearing kind of pushes into the softer material and leaves a mark the other major design flaw of this hello it's me from the future um my camera stopped recording yesterday and I didn't notice it so everything from here and out I get to do again but that's fine there are two additional details that the Savvy viewer might have no that I haven't talked about yet and one's kind of important one is less so the less important item is galvanic corrosion potential concerns so the hinge is obviously made out of aluminum but the shoulder bolt is steel it's a stainless steel but it's still a steel so you've got two dissimilar metals that are touching each other and over time in the right environmental conditions this can lead to galvanic corrosion is the process when you have two dissimilar metals touching each other in the presence of an electrolyte uh it basically forms a battery and so one side acts as an anode and the other is a cathode and you have a chemical reaction between the two metals and you get corrosion or like rusting at the interface between those two metals the amount of corrosion depends on which metals you're using so different metals have different kind of galvanic corrosion potential further apart they are on this chart the more they will corrode each other now unfortunately if we look at the chart we can see that stainless steel in our case we're using an 188 shoulder bolt which is basically a 304 stainless is pretty distant from aluminum on the chart so there is a decent chance that if an electrolyte becomes present we might see galvanic corrosion between this shoulder bolt and our aluminum hinge this is something that you can control right so we can keep it in a dry and clean environment where there's no electrolyte and then there's no corrosion so corrosion only happens when you have an electrolyte between the two interfaces so if we keep everything dry and clean and humidity controlled like in a clean room or a well-controlled lab not an issue we can also potentially mitigate this with more Coatings so more dry lubricants or some type of PVD coating that's applied over it um we could even potentially anodize the aluminum and that will put a thicker oxide layer between the aluminum and the stainless there's pros and cons to different choices but it's something that you can mitigate with a little bit of planning unfortunately there's a bigger flaw to this design which I didn't think about at the time but potentially could be catastrophic to emission and it's again related to the materials that are used here the shoulder bolt is steel and the hinge is aluminum and now we have a thermal mismatch between the two materials the thermal coefficient of expansion for aluminum is about two times that of steel or stainless steel in our case meaning that as the temperature rises the aluminum part of this hinge will grow twice as quickly as the steel parts

### [20:00](https://www.youtube.com/watch?v=kXa0vUy2Tlg&t=1200s) Segment 5 (20:00 - 25:00)

so for example if the steel bolt grows 20 microns the aluminum is growing 40 microns and if we don't have our tolerances sufficient to compensate for that we could get into a situation where the inner or the outer half of the hinge expands enough that it pushes on the shoulder bolt and finds itself in a position that can't open once we get on orbit all right that's enough said about this particular hinge let's look at the final design which I think is the winner for a number of different reasons so this hinge from just looking at it doesn't look too dissimilar has the same general shape there's obviously a shoulder bolt kind of pin going through the middle we've got the same torsion spring the hinge outer and inner leaves look pretty much the same uh but there's a big difference in how it functions and that's namely there are no ball bearings instead there's this thin little spacer or bushing that everything's riding on so here's what the little spacers or bushings look like and those provide the interface between the hinge and the shoulder bolt and this solves a couple problems for us these bushings are made out of a Teflon and graphite filled Peak material this is an engineering polymer That's rated for space so has very low outgassing characteristics it handles extreme temperatures very well from low to high temperatures it doesn't get too brittle it's not quite as strong or stiff as aluminum but it's pretty close for a polymer and then this particular variety has both Teflon and graphite in it which means it's naturally kind of slippery it's a low friction material also it's just kind of weird like when you hit it against itself it sounds it doesn't sound like plastic it's a strange material to work with it's also crazy expensive this bar was 1T 12 in uh half inch diameter and it cost $40 so yeah you don't want to use this stuff if you don't need it in any case the reason we're using this bushing right it's a low friction material and it provides a direct buffer between the steel shoulder bolt and the aluminum outer hinge pieces so we don't have to worry about galvanic corrosion because the metals aren't touching each other we don't have to worry about cold welding because again the metals aren't touching each other there's no ball bearings to kind of worry about getting out of alignment or dimpling or denting the bearing race so we don't need to use hardened materials assembly should be a lot easier because again we're not dealing with those tiny little ball bearings and frankly it's just a better design like a ball bearing style rotary mechanism is good for when you're doing lots and lots of revolutions it's a low friction thing for high Revolution or high RPM applications but we don't need it for this application we're just deploying you know the solar panel once and then it stays open so we don't really need the benefits of a low friction ball bearing race when a simple pin spacer would work folks seemed to enjoy when I went through the tool paths and cam for the robot 5x's prototyping video so I figured I'd show you at least one of the parts here and how it was manufactured so the setup is pretty straightforward we've got a square piece of stock with the part kind of positioned near the top held in a vice first Main operation is a roughing tool path just to clear out the majority of the material then we come in with a finishing tool to take care of this outer Contour then we come around to the side of the part and drill deeper and tap this small hole this is an m1. 6 so it's a really tiny little hole and the reason we're doing it now as opposed to later is that I would like to drill and tap it while there's still a lot of material in the part so we haven't removed much material whereas if we did it later when we've removed this intersection there's more of a chance for this outer hinge bit to flex while we're tapping and M 1. 6s are already very fragile so I prefer not to break a tap in this hole so we do it now while everything's nice and stiff with the sensitive tap done we can come back in with another adaptive and this will clear out the stuff in the middle and some of the stuff down here below we'll do an adapter from the side to help clear out this little pocket and the through hole will bore it to size so it's an accurate size this isn't a critical feature because it's really just the counter sync for the bolt so it doesn't need to be super tight tolerance but when possible it's easy to bore stuff so go for it we'll do an Adaptive to clear out these Pockets uh these are just the lightweighting pockets uh just to remove some extra mass that's done with an A 1 mm

### [25:00](https://www.youtube.com/watch?v=kXa0vUy2Tlg&t=1500s) Segment 6 (25:00 - 30:00)

endmill we'll come in with a finishing endmill that's designed for making flat faces and we'll face off this top section here just for Aesthetics then we rotate around and do another adaptive where we're clearing out kind of the bulk of the material that's underneath the hinge and we're doing this because it will make some of the future operations easier if there's not Material down here so some of the Contours have kind of curved edges that are only accessible right this Contour here is challenging to hit with an endmill if there's material underneath the part and so this adaptive is clearing that out so that we can access these edges but you'll note that we waited a really long time to clear out this material because the part will be cons considerably weaker once we remove it now there's like this large overhang it'll want to bounce around so we don't clear this material out until the very last minute cuz it it's adding stiffness to the part for the other operations with that gone we can come in and do a quick Contour to clean up these edges rotate around and do this curved profile that I just talked about ditto on the other side take care of this curved profile and you'll note that these curved profiles are important they're here for Reason Not Just Aesthetics this side of the hinge is flat because it is used as a stop so you can see here this is the hard stop to keep the hinge from opening past 180 so this needs to be a flat on flat surface basically whereas the other side I guess this doesn't need to be curved it could be flat too but this one is Aesthetics it looks nicer we rotate through here and clean up this inside face and this inside face we'll rotate around the top and help clear out some of this material along this sidew wall and then we walk in with a smaller tool and clean up these Corners so the reason I did it this way so this is a 1/ 18 in endm so it's pretty big we don't have to really worry about it breaking so it removes the bulk of the material and then we come in with these small 1 mimer tool to do the final passes to clean up these internal fillets uh once most of the material is gone there's very little chance of it breaking now and it can do just kind of the final cleanup pass but I should note that these fillets were chosen let's see that's the diameter is 2 mm I know I'm coming in with a 1 millimeter tool so these fillets were chosen to be sufficiently large that we wouldn't really have a problem with a small tool so it could have been a smaller fillet then we run a bigger chance of having chatter or breakage in the corner so in this case it was chosen to be sufficiently large that we just don't even need to worry about it okay and now it's basically just clean up so we come over to these Pockets we do a contour to clean up the outer walls run a flat tool path to finish the flat surface at the bottom we clean out this slot and I didn't mention it before this slot is the slot is here for holding basically the legs of the torsion spring if you don't have it like on one of my design iterations the spring just kind of moves all over the place it's not really well constrained so putting a thin slot in is important to keep the torsion spring from moving around then we drill these two holes deur them and come in and tap them these are m2s and these will be the holes that you'd use to attach to like the solar panels or whatever your Deployable is then finally we do a big 5axis deur path to take care of all the edges that we can reach just to minimize how much deburring we have to do by hand later come in with a cut off wheel and slice it off and just like in the 5axis prototyping video you can see that I designed this part so that it has no real features on one of the sides this bottom side is completely flat and that was done so that we can come in with a cut off saw and just saw it right off so that we don't need a second operation so one and done on the five AIS machine I usually like to toss the part into a tumbler or like a magnetic pin deur let it run for 20 30 minutes and that just helps take care of any of the birds that maybe we didn't get on the machine or help clean up some service finishes in places where maybe it wasn't quite as good as I'd hoped and that basically just leaves us with a assembly this is very similar to the last

### [30:00](https://www.youtube.com/watch?v=kXa0vUy2Tlg&t=1800s) Segment 7 (30:00 - 35:00)

ballbearing hinge just a lot easier the torsion spring is lapped and clean to make it less crunchy we coat the various surfaces with tungsten disulfide and then a thin layer of vacuum grease and then we just insert the bushings into the inner hinge and thread the shoulder bolt through the Middle with the torsion spring in place and tighten it all up and that's it and I have to say the feel of this bushing hinge is much better than the ball bearings it's a lot smoother it's not crunchy it has kind of a nice feel to it when it opens and locks into place and frankly this is just a better design than all the ball bearings even though they're very clever designs I think uh the juice wasn't worth the squeeze we'll just put it that way it was way too much effort for not really a market Improvement and in some cases a little bit worse and something like this would probably be much more robust in a space sort of situation now it's not to say it's perfect there is one major flaw again and that's the thermal coefficient problem that I mentioned on the last hinge again we've got a steel shoulder bolt that's holding an aluminum structure together and so as the aluminum grows we might run into issues here the easiest way to deal with that I think is either put a little bit more tolerance between the shoulder bolt and the shoulder of the aluminum part so it leave some spacing right here so that it has space to grow I think that would be fine the next easiest thing to do would be add a little more space to this pocket here and put in like a WAV spring something that just provides a little bit of cushion and can take up the slack as things are growing that would be pretty straightforward the only real problem is that this is such a small size this diameter here is only 5 millimeters they don't really make wave Springs that small so we'd have to make our own wav spring but that would be relatively doable without too much hassle another thermal issue that we would want to just keep an eye on I don't think it's a problem based on some napkin math that I've run but we've got steel Peak aluminum kind of in a sandwich but we'd want to officially run the math and make sure there's enough spacing between the aluminum and the peak and the steel that they don't bind as they grow at different rates but otherwise I am pretty happy with this design I think it's a neat little hinge it's certainly fun to play with uh and you can imagine Mass manufacturing these you know making a dozen or two dozen uh without a whole lot of hassle whoever had to assemble them wouldn't hate their life like they would with the ball bearing design uh so I think this would be the winner um next steps for something like this would be to make a couple of them and then start subjecting them to functional tests so putting them on Heating and Cooling blocks to see how it reacts at 80° C or -20° C eventually you would also want to run vibration testing on this so close it up so that it's in its like stowed position and throw it on a vibration table and shake the B Jesus out of it and see if it opens if it actuates after you've shaken it a bunch and that will help tease out you know is your locktite sufficient to keep the shoulder bolt in place or does it work loose uh are the slots for the torsion spring the right size or does it pop loose under vibration loads uh is it strong enough maybe this is just too weak of a structure and it'll just break under high enough vibration holding a large solar panel uh well apologies for the different change of location uh my shop is a total disaster right now I'm working on a project that's kind of Taken over a bunch of stuff and there's a lot of things going on right now in the shop unrelated to projects just in my personal life so things are a bit chaotic and hectic right now which is why we're doing it in my office and it was a little less scripted than usual I found the exercise of trying to think through the ramifications of space and like what that does to a design to be really interesting and enlightening for me personally because again you always hear that space is hard and like it makes sense because you have to make rocket ships and you know like engines and all that stuff seems hard but it's the little things too that end up being difficult like making a hinge you know no one thinks a hinge is hard until you actually have to think about oh well how does a hinge survive and operate in space with all of these really unique constraints and then understanding that this level of design needs to go into every component of a spacecraft um and you can quickly understand why people say space is hard and it's frankly kind of amazing that

### [35:00](https://www.youtube.com/watch?v=kXa0vUy2Tlg&t=2100s) Segment 8 (35:00 - 35:00)

anything works at all in space so yep uh that's all I've got for you thanks for watching I hope this was interesting and I'll see yall next time

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*Источник: https://ekstraktznaniy.ru/video/42392*