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  1. #1
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    98mm N5800 MD rocket for BALLS

    As I mentioned in a few other threads, I am currently building a 98mm MD rocket for BALLS, so I decided I'd make a build thread. I'm not going to post my full design yet, since it's still somewhat fluid as I'm building - especially with regards to the recovery system. However, I will post some pictures as I go. This design is expected to weigh in at less than 10 pounds with everything but motor (I'm actually shooting for 7, but I'm not convinced that will happen), has an overall length of 76.6 inches from the tip of the nose to the back of the nozzle. Unlike several of the N5800 designs proposed and under construction, I'm going for a metal-free build, in the hopes that I can make this work with all composite structures (just for a bit of added challenge). Depending on the sim you believe, as well as a few parameters that aren't quite nailed down yet, it's expected to go anywhere from 90,000 to upwards of 130,000 feet if it boosts straight and holds together, with a top speed of mach 4.2, so it should soundly take the N record if it actually holds together, as well as having a shot at the Carmack prize if it is still unclaimed as of BALLS this year.

    First off, the fins. These are relatively simple shapes, not swept terribly far back (sweeping back beyond the mach angle is impractical at this speed, so I'm instead going for a mild sweep with a razor sharp leading edge). I started with 0.44 inch thick high grade CF stock, which was custom made for me (and cut to the basic fin outline) by ACP composites. It was made in an autoclave using prepreg, and it was cured at 350F, with an anticipated Tg of around 400F, so it should be substantially more heat resistant than ordinary CF plate would be. It is also incredibly strong, incredibly heavy, and incredibly high drag, due to the 0.44 inch thickness.

    So, to somewhat mitigate that problem, I used a fellow clubmember's awesome fin beveling jig, involving a tile saw and a clever attachment to get a consistent angle. This was the result:
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    Sadly, despite the beautiful bevel, the fins were still fairly thick, which would result in an unacceptably high amount of drag:
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    So, after some further simulation and a bit of thought, I went after the fins with an orbital sander and some 40 grit sandpaper. After about an hour and a half of work, this is what the first fin looked like:
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    Here's a comparison showing before and after sanding:
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    If anyone's wondering, I sanded off almost 2 ounces of carbon per fin, and I highly, HIGHLY recommend a good dust mask and a dust collection system of some kind when sanding or cutting CF. The stuff gets everywhere, it itches, and I'm sure it's terrible for the lungs. I'm really pleased with the results though - I got a beautiful taper from root to tip, with a nice biconvex airfoil shape throughout the fin. I've done 2 (of 4) fins so far - the rocket will only use 3 fins, but I'm making an extra in case I screw something up. Tomorrow, I'm expecting to start work on the E-bay and finish the airfoil on the remaining two fins. Hopefully, the airframe tube will come in tomorrow as well, so I can start attaching the fins soon. I'm not expecting the nosecone until next week, so I won't be able to mock up the rocket until the middle of next week or so, but I'm hoping to have it mostly finished a couple of weeks before BALLS so I can give the high temp epoxy plenty of time to cure, as well as so I can get it a nice smooth paintjob. Currently, the plan is to use automotive header paint rated for 1500F, which should hopefully give the entire rocket an additional protective coat against the extreme temperatures expected during flight.

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  2. #2
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    With the thickness of the fin, are you planning on attaching them mechanically from the inside of the body up into the fin?

    Or are you going plain chemical bond?

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  3. #3
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    Chemical bond, possibly with T2T as well (that's still up in the air at this point, depending in part on how sturdy I feel that they are after the surface mounting). I'll be significantly scoring the bonding surfaces, and the fin root is quite thick (I didn't take much material off near the root, so at the root, it's pretty much just a biconvex airfoil with the thickest point at 0.44"), so there should be a lot of bond area for it to grab on to. My biggest concern is fin alignment - if aligned properly, the forces on the fin aren't bad, but with even a slight misalignment, they become enormous.
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  4. #4
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    Chris If the fin had been stock I would have suggested CNCing a curve onto the root edge. I will be doing that for my aluminum fins, and it should negate almost all chances of being misaligned.
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  5. #5
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    Those are awesome fins. Can you post a picture of the beveling jig?
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  6. #6
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    Quote Originally Posted by edwinshap1 View Post
    Chris If the fin had been stock I would have suggested CNCing a curve onto the root edge. I will be doing that for my aluminum fins, and it should negate almost all chances of being misaligned.
    Sadly, I am CNCless for the moment, hence the low-tech solutions.
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  7. #7
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    Quote Originally Posted by cwbullet View Post
    Those are awesome fins. Can you post a picture of the beveling jig?
    I'll ask the guy with the jig to take a picture, but he's on a business trip right now, so I don't know how long it will be until he will get back to me.
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  8. #8
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    Quote Originally Posted by cjl View Post
    So, to somewhat mitigate that problem, I used a fellow clubmember's awesome fin beveling jig, involving a tile saw and a clever attachment to get a consistent angle.

    Sadly, despite the beautiful bevel, the fins were still fairly thick, which would result in an unacceptably high amount of drag:

    So, after some further simulation and a bit of thought, I went after the fins with an orbital sander and some 40 grit sandpaper.

    If anyone's wondering, I sanded off almost 2 ounces of carbon per fin, and I highly, HIGHLY recommend a good dust mask and a dust collection system of some kind when sanding or cutting CF. The stuff gets everywhere, it itches, and I'm sure it's terrible for the lungs. I'm really pleased with the results though - I got a beautiful taper from root to tip, with a nice biconvex airfoil shape throughout the fin. I've done 2 (of 4) fins so far - the rocket will only use 3 fins, but I'm making an extra in case I screw something up.
    Nice looking fins, Chris. The radial taper should help you aerodynamically as well. Did I understand you did those by hand? How confident are you that they are symmetrical? And a matched set?
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  9. #9
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    Yes, they were done by hand, and I would say that they are symmetrical within about 0.01", possibly even better than that. The layers of carbon actually help a lot with both symmetry and matching one fin to another, since they effectively act as contour lines. I've basically been trying to match the carbon pattern with each fin that I do, and by doing so, they should be extremely close to a matched set. In addition, a slight camber or asymmetry doesn't cause very much side force, so I should be OK with any slight residual asymmetries (at least that's what I'm really hoping). I'm extremely confident that this rocket would be fine up to at least mach 3, based on both my own experience and observations of other mach 3 class rockets, but I'm doing a bit of guesswork here for mach 4 class stuff - not enough people have gone to this level of performance to really establish what is necessary to survive, so I'm going a bit on instinct and gut feeling, and a bit on engineering.

    That having been said, some rudimentary aeroelastic calculations indicate that 0.25" carbon plate would be sufficiently strong to withstand this flight, so mine (0.44" root, tapering to 0.2" at 80% chord, then tapering to 0.02" at the tip) should have a fairly enormous safety margin built in. Of course, I could always be proven wrong with the flight - part of the reason I'm trying this is to establish a baseline for what kind of construction is necessary to survive at these performance levels. I have some even more ambitious project ideas on the drawing board, but with nearly zero experience at this performance level, I'm still trying to establish just what is feasible and what is not.
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  10. #10
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    I would be wary of using T-T on this rocket. The fins are "precision" made to work a certain way, and you would have to sand/machine the carbon from the T-T to work. Plus the lamination quality of the T-T carbon is lower than that of the fin stock. So long as you get great adhesion to the body with a high strength epoxy (what epoxy will you be using?) you should not need to T-T.
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  11. #11
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    Quote Originally Posted by edwinshap1 View Post
    I would be wary of using T-T on this rocket. The fins are "precision" made to work a certain way, and you would have to sand/machine the carbon from the T-T to work. Plus the lamination quality of the T-T carbon is lower than that of the fin stock. So long as you get great adhesion to the body with a high strength epoxy (what epoxy will you be using?) you should not need to T-T.
    You just described the exact reason why I'm on the fence about it. On the one hand, it greatly increases the bond area, but on the other, as you said, the lamination will not be as good (although I do have access to vacuum bagging equipment, so it wouldn't be bad, but still...), and I would need to sand them again once they were attached to re-form the biconvex airfoil.

    Oh, and pretty much everything on the rocket that I am gluing that is exposed to the airstream is using Cotronics Duralco 4461. It isn't the strongest epoxy out there, but it isn't bad, and it cures at room temperature with a 500F Tg. I'm using even higher temperature products on certain key areas exposed to the greatest heating, such as fin leading edges and the nose tip, but I'm still trying out a couple of options for those, so I don't have it completely narrowed down yet.
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  12. #12
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    Very ambitious project, I wish you all the luck in achieving your goal. Question; since the airfoil of the fin was created by a jig, but fine tuned by an orbital sander, how critical will it be recreate what you have done with the first fin? I know that with an orbital sander, you will not recreate the shape exactly for each fin, so will this have any negative bearing on the flight that may give you any cause for concern?
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  13. #13
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    Nice work!

    I appreciate you recording your thought processes on finding the best solution. Very methodical. Good stuff.

    Best wishes on the project.

    Greg

  14. #14
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    What are you planning for the nosecone? A commercially available one?

    Will you have a tailcone?
    Several rocket motors burned this year.

  15. #15
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    I would not expect a composite rocket to hold together at those speeds & temperatures.
    I think you're looking at an expensive shred. Just my 2 cents.

  16. #16
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    Quote Originally Posted by tstone View Post
    I would not expect a composite rocket to hold together at those speeds & temperatures.
    I think you're looking at an expensive shred. Just my 2 cents.
    Based on what? An appropriately designed composite rocket will do just fine.
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  17. #17
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    Quote Originally Posted by patelldp View Post
    Based on what? An appropriately designed composite rocket will do just fine.
    Composite rockets have survived (just barely) a flight with a P6000 staged to an N4000. So it is possible. Is it the best choice? No metal is significantly better. But you cannot test the limits of composites without taking some risks.

  18. #18
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    Composite rockets can and do hold together at these speeds and stresses. USC launched a 6" rocket to mach 4 on composites by using a phenolic leading edge. It is definitely possible, but takes careful planning and a lot of practice link.

    Chris what nosecone will you be using, or will you be making your own?
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  19. #19
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    USC also used only normal laminating Aeropoxy. They melted the nosecone into the body tube during the flight, if I remember correctly.

    On a side note, the nosecone layup was something like an inch thick.
    Several rocket motors burned this year.

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    Quote Originally Posted by edwinshap1 View Post
    Composite rockets can and do hold together at these speeds and stresses. USC launched a 6" rocket to mach 4 on composites by using a phenolic leading edge. It is definitely possible, but takes careful planning and a lot of practice link.

    Chris what nosecone will you be using, or will you be making your own?
    The nosecone is a custom one being made for me by Shockwave Rocketry. It's a fiberglass, 6-1 VK, using the Cotronics 4461 500F epoxy.
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  21. #21
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    Quote Originally Posted by New Ocean View Post
    Composite rockets have survived (just barely) a flight with a P6000 staged to an N4000. So it is possible. Is it the best choice? No metal is significantly better. But you cannot test the limits of composites without taking some risks.
    Composites are substantially better than metal when used properly. It's easier to design a rocket for this kind of performance using metal, true, but there's absolutely no reason a properly designed composite rocket should have any trouble. I have no doubt at all that it is possible to design an all-composite rocket for this kind of performance and have it survive. I do have some doubt over whether my rocket will survive, since I'm making a few tradeoffs in the design to make it cheaper and easier to build, but all-composite should definitely be doable.

    As for heat resistance? As I already said, everything is using 500F epoxy, and I'm also evaluating a couple of options for the leading edges and nose tip which should hold up at upwards of 2000F. This will not be a standard composite rocket.

    Honestly, part of what is interesting to me about this project is that it is somewhat out of the realm of what amateurs have successfully done before. Since so few motors have the fast burn, high mass fraction, and high Isp of the N5800, it causes a rather unique flight condition, even relative to prior BALLS projects and the like. As a result, this is somewhat uncharted territory, so I definitely accept that a shred is a significant possibility. However, this is in a realm of aerodynamics where common sense often leads to the wrong answer, so I wouldn't expect people's intuition to mean a whole lot here. I'm trying to build this rocket with the best engineering principles that I can (in some cases limited by the tools and supplies that I have on hand, or the amount of time until BALLS), and hopefully that will be enough to allow it to survive.
    Last edited by cjl; 22nd August 2012 at 07:15 PM.
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  22. #22
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    Quote Originally Posted by cjl View Post
    The nosecone is a custom one being made for me by Shockwave Rocketry. It's a fiberglass, 6-1 VK, using the Cotronics 4461 500F epoxy.
    These are great nosecones. I'm using the same (but 54mm) for my Mach Madness flight in April. I did not order special construction though. I instead add a Kevlar mat laminate to the outside surface.
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    The other concern here is whether or not you can design a composite rocket that you can fly more than once. This is where we start to diverge from standard model rocketry... I may be mistaken, but does CJL plan to make a reusable rocket, or a single use record attempt?

    A rocket capable of these velocities is most likely NOT reusable without some attention.
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    I know that your materials are not standard, but will the construction be standard techniques? How about the airframe material?

    One concern I have is: you hope that any camber you accidentally generate won't cause the rocket to spin, but what if it does? How well centered is the mass in your rocket? Our recovery bay is fairly asymmetrical.


    We used a metal fincan at Mudd because of our far-from-standard "bare minimum" approach. We don't know what would happen to the leading edge of a composite layup sitting against the motor tube: will it peel back? Most everything else not touching the motor case is composite. The nosecone aside from the tip will be composite because of radio reasons. Our "airframe" is straight up Hawk Mountain coupler tube for convenience.


    The effects of the various approaches are interesting. You seem to be going for the "light and fast" route. Because we were worried about our ability to lay up fiberglass deep into a skinny nosecone mold (we have little experience with that), a significant proportion of our nosecone is the metal tip, adding a good deal of weight, so that our rocket is going to be around 12 pounds "empty" versus 8-10 pounds.

    However, it's all in the nose tip so our smaller fins are sufficient for stability. The resulting reduced drag should yield similar altitudes to your rocket, despite a lower top speed of about Mach 3.95 versus your calculated 4.2.
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    Quote Originally Posted by patelldp View Post
    The other concern here is whether or not you can design a composite rocket that you can fly more than once. This is where we start to diverge from standard model rocketry... I may be mistaken, but does CJL plan to make a reusable rocket, or a single use record attempt?

    A rocket capable of these velocities is most likely NOT reusable without some attention.
    Honestly, this is a single use rocket. I do not plan to fly it again, nor do I care if it is flyable again, so long as it survives the flight intact. It will of course have a recovery system designed to slow it to a reasonable speed for landing, but I don't actually care if it is flyable again. The amount of effort involved in coordinating a launch to this altitude, along with the cost of the propellant means that I'd rather design a different rocket for a new challenge rather than reuse this one if I do decide to fly again with this kind of performance.
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  26. #26
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    Quote Originally Posted by cjl View Post
    Composites are substantially better than metal when used properly.
    Better than steel?

    I completely understand your desire to use all composites, but they do impose some degree of risk that could be avoided if the overall goal was to prevent a shred. I think the overall goal here is more than that - to get performance and prove that composites are able to handle that performance.

    I would note that it has worked a few times in the past, such as the Kosdon Q motor with the shadow aero fins and nosecone, both composites with ablatives.
    Last edited by New Ocean; 23rd August 2012 at 07:50 PM.

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    Quote Originally Posted by CarVac View Post
    I know that your materials are not standard, but will the construction be standard techniques? How about the airframe material?

    One concern I have is: you hope that any camber you accidentally generate won't cause the rocket to spin, but what if it does? How well centered is the mass in your rocket? Our recovery bay is fairly asymmetrical.


    We used a metal fincan at Mudd because of our far-from-standard "bare minimum" approach. We don't know what would happen to the leading edge of a composite layup sitting against the motor tube: will it peel back? Most everything else not touching the motor case is composite. The nosecone aside from the tip will be composite because of radio reasons. Our "airframe" is straight up Hawk Mountain coupler tube for convenience.


    The effects of the various approaches are interesting. You seem to be going for the "light and fast" route. Because we were worried about our ability to lay up fiberglass deep into a skinny nosecone mold (we have little experience with that), a significant proportion of our nosecone is the metal tip, adding a good deal of weight, so that our rocket is going to be around 12 pounds "empty" versus 8-10 pounds.

    However, it's all in the nose tip so our smaller fins are sufficient for stability. The resulting reduced drag should yield similar altitudes to your rocket, despite a lower top speed of about Mach 3.95 versus your calculated 4.2.
    My recovery bay is quite symmetrical actually, as is my entire rocket. I'll show that part either later today or later this week, depending on how long it takes me to start building it. My priority right now is the external part of the rocket, since I want to give that epoxy and any coatings I give it as much time as possible to cure before paint (and I want the paint to have plenty of time to cure before flight too), so I'm working on the electronics and recovery whenever I don't have anything else to do. It is slowly starting to progress though...

    Overall, the mass in this rocket should be almost perfectly symmetric around the spin axis though, so if there is a bit of spin, it won't have a problem with any mass imbalances. I definitely agree about how interesting it is to see the various approaches though - it's fascinating to see how different people/groups are approaching this problem, and I'll be very interested to see which ones (if any) survive the flight.
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    Quote Originally Posted by New Ocean View Post
    Better than steel?

    I completely understand your desire to use all composites, but they do impose some degree of risk that could be avoided if the overall goal was to prevent a shred. But I think the overall goal here is more than that - to get performance and prove that composites are able to handle that performance.

    I would note that it has worked a few times in the past, such as the Kosdon Q motor with the shadow aero fins and nosecone, both composites with ablatives.
    Composites are absolutely better than steel, both in terms of heat resistance and strength to weight ratio. Of course, that's assuming you can get your hands on the correct composites. Sadly, I can't get my hands on any truly high-temperature composites (much as I would like a rocket made of RCC, it isn't going to happen for obvious reasons), so I'm using a combination of very high Tg epoxies and ceramic coatings to hopefully get a similar effect. Interestingly though, with high Tg epoxy, you can get composites that retain strength up to around 500F, which is pretty much as good as aluminum in terms of heat resistance.

    You're also correct that part of my goal is to get as much performance as possible while demonstrating that composites can handle that performance. I don't want to just make an all-metal rocket. I have no doubt that it could be done, but that's not really the point of this project.
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    Good luck with your attempt, Chris!

    It is really cool to read about all the excitement that these attempts are creating here on this forum.

    Jeroen

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    I actually contacted Scotglas manufacturing regarding a nosecone for another rocket, and eventually the discussion turned to my project. It turns out that polyester resins, which Scotglas uses for their cones, can have higher temperature resistance to epoxies; apparently they've made nosecones for hypersonic missiles and the like before.

    Just food for thought.

    Several rocket motors burned this year.

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