Incline Plane Wrapped Helicly Around an Axis

I am one of those people that really enjoy a bad joke, bad puns, and a creative play on words. In fact, in my home brewing and model rocketry, sometimes I enjoy coming up with names for my creations (with some obscure reference or bad pun) as much as I enjoy drinking the beer or launching the rocket. Earlier this spring I was watching a re-run of the Big Bang Theory and heard a great geek joke (I’d heard it the first time I saw the episode, but had forgotten about it), and through some weird series of tangentially related thoughts, I decided that “Incline Plane Wrapped Helicly Around an Axis” would be a great name for a rocket. Yes, I have decided that I am going to attempt to build a “screw” rocket, hopefully a single fin (that increases in width toward the aft) wrapped helicly around the body tube creating enough spin so that the combination of fin and spin will keep it stable. At first, I just kept the idea in my head and periodically pondered what obstacles I may run into during the build. Whenever I was in Lowe’s or Menards I would look around the different sections to see what might help overcome some potential problems in the build.

One of the first obstacles that I thought of...if the rocket is going to spin, I need to have the launch rod run through the middle of it. Which means I’ll need to build a cluster so there’s someplace for the launch rod to pass through. Considering I hadn’t built or launched a clustered rocket before and that was the FIRST obstacle I came up with, I decided to take a stepwise approach to this project:

Phase 1: Build a standard, non-spinning, clustered rocket. I figure if I can’t get a good launch from a standard clustered rocket, I have no business trying to launch one that spins and has fewer than the generally considered minimum number of fins.

Phase 2: Build a clustered rocket with three fins wrapped helicly around the body tube to create spin. The goal here is to have three fins so it should be stable on its own, but to see if I can generate a balanced spin. If that launches and recovers successfully, I’ll go on to Phase 3.

Phase 3: Build a clustered rocket with two fins wrapped helicly around the body tube. The idea here is to make any adjustments to the cant in the fins that may be necessary based on observations in Phase 2, and to see if I can create stability with just two fins. It may take more than one try to get it to work, and it may not work at all. If it doesn't, I guess I stop there and be happy with what I learned.

Phase 4: If all goes well in Phase 3, I will build a single incline plane wrapped helicly around the body tube that will hopefully, with the combination of the fin and spin, be stable. Again, observations from Phase 3 will be used to make any adjustments when I move on to this final step.

Phase 1:

Phase 1 is complete and was, other than being eaten by a tree, successful. I built a three motor (18mm) cluster in a standard BT60 body tube with a balsa Bertha shaped nose cone. I used a Sunward cluster kit, which consists of three motor tubes, standard motor blocks, retainer clips, and a single centering “ring” with three holes. You simply glue the forward end of the tubes onto the ring in line with the holes. There is literally no room for them to be anything but pressed together in a tight triangle. Once that dried, I put glue inside the body tube, and a thin layer down the outside of each motor tube, and slid the mount in place.

I had some one inch wide strips of airfoiled balsa left over from my last Starbucks build, and decided to use them for fins. I didn’t trust them to have enough surface area, so I cut three sets of fins three different lengths. In essence, I ended up with what looks like fletching on an arrow.

Other than the clustered motor mount, this was a very standard build. My oldest daughter calls it her Katniss rocket. But that name just seems too current and obvious without any obscure reference or bad pun, so I had to come up with something else for a name. It has three motors, and three sets of three fins that look like feathers on an arrow. I named it Plumage a Trois.

The first and only launch (so far) was after a day with pretty stiff breezes. I had just finished painting it earlier in the day and was sitting out on the deck with my netbook looking up different techniques of wiring a three motor cluster when the wind started to die down a bit. The closer it got to sunset, the calmer it got. With about 30 minutes left before sunset, I decided that I would prep the Plumage just in case. Considering the small area I had to work with, I wanted the wind to be virtually non-existent and I decided to stick with A8-3 motors.

After the rocket was prepped, there was still a slight breeze, but I figured I would continue to set up for a potential launch. By the time I got the pad and controller set up out back, the wind was virtually non-existent with the occasional breeze picking up but quickly dying down. Plumage was put on the pad. I got my camera phone ready, turned the safety key to the on position, armed the controller to show I had continuity, and then paused to look up into the trees to make sure it was calm. From what I could see, it was a stable flight, but I was too far under the tree to see it all the way up. The parachute opened, and a breeze picked up, pushing the Plumage right back to me, and into the top of the tree above me.

About two weeks later, as a storm was rolling in, I went out to check on it and the rocket had separated from the nose and parachute, but was still stuck in some branches. I went back in to check the RADAR and figured I would head out right before the rain got here to check on it again. The wind had picked up to 30 mph, with stronger gusts, and when I went out, there were some insane golfers at the tee box. As I stood there waiting for them to finish so I could check the tree, Plumage tumbled out. Other than a couple of minor scratches, it was in perfect shape and with a replacement nose cone, is ready to be launched again. I was able to confirm that all three motors lit.

Phase 1 was a success.

Phase 2

My two main goals for Phase 2 are to make sure I can successfully run the launch rod through the center of the rocket without getting the parachute or shock cord all tangled up around it, and to see if I can generate a stable spin. Some other side items are developing a technique to mount the fins at an angle (which will require some curve in the root cord), seeing if I can get a workable model in Open Rocket that is representative of what I am building, and practicing successful ignition of a 4-motor cluster.

I am using standard BT80 body tube and a plastic PNC80 nose. I have a hollow aluminum 5/16” rod that fits snuggly in the space in the center of the four motor mounts that will serve as my launch lug.

I started out with the motor mount, taking four motor tubes and clips and putting each together as you normally would. I didn’t have any motor blocks, so I took my Dremel and a spent motor and cut my own. After getting the four mounts built and dry, I put them all into the centering rings using Tightbond wood glue. I wrapped and tied off some hanger wire around the mount for the recovery system (with epoxy and JB Weld to keep it from coming loose), and then cut a slot into the outside edge of the forward ring to give room for the wire to pass through.

Phase 2 Continued

So that I do not have any aluminum sticking out the front, I plan to cut it short of the forward end of the body tube, and not have it pass all the way through the nose (like I had originally considered). As such, I figure it would need additional support. I got to try my circle cutter for the first time, cutting out a third ring with a 5/16” hole in the center for the launch lug tube. I drilled additional holes around the center to allow the ejection gases to pass through and deploy the recovery system. I had to do a bit of sanding to get it to fit snuggly into the BT 80 body tube and I cut a notch in the edge to allow the recovery wire to pass through.

With the motor mount/centering rings dried, I installed the launch lug tube into assembly. I decided to use JB Weld since the tube is aluminum and connecting to four motor mounts which may generate some heat. I taped off the forward ends of the motor mounts to avoid accidentally getting JB Weld into the motor tubes. I slid the mount up onto the tube, so I would be sliding it down in place to avoid accidentally getting JB Weld inside the tube. I applied a liberal amount of JB Weld to the tube, and slid the mount down until the aft ends of the motor tubes were flush with the end of the aluminum tube. I then twisted the mount around the tube to help spread the JB Weld around some.

I marked the launch lug tube 7” from the aft end, applied some JB Weld at the mark, and then slid the third ring in place. I made sure there was a nice amount of Weld on both sides of the ring.

Phase 2 Continued

After doing some research to find an optimal angle for the combination of good spin and resistance (I researched the wind farm stuff) I decided on a 15 degree angle for the fins. Phase 3 and Phase 4 may need a larger angle, because the 15 degrees only went a little more than halfway around the body tube, but I'll wait and see what happens here before making that decision.

I cut out a strip of paper and marked 15 degree angles on it (both directions because I had not decided on the spin direction yet).

I wrapped it around the body tube so that it was straight, and then used it to make marks around the body tube and then I connected the dots to finish my fin guide lines. I figure I'll cut the fins into sections, wrap some sandpaper around a BT80 body tube, and then hold the fins at the correct angle and sand until the root matches the curve of the tube.

After I got the guide lines marked, I attached a shock cord to the recovery wire with a ball bearing swivel and installed the motor mount assembly into the body tube. It was too hot in the garage for anything else. Next up is cutting out the fins, which should be easy enough (long right triangles). Cutting them into smaller sections, sanding the root edges, and attaching them will likely take a while.

Interestingop:op:

After cutting out some balsa pieces for the fins the way I originally planned, it became clear that it was not going to look the way I wanted without a lot of extra sanding and shaping after being attached to the body tube. So I had to rethink it. I made a trip to Staples and bought some drafting tools which should work out quite nicely. Not only should it help get the shape better, but it should go much more quickly. I hope to have them cut and attached over the weekend.

I just happened upon this thread. If you want some inspiration, you can check out my own spin-stabilized rocket here: https://www.rocketryforum.com/showthread.php?32275-Spin-stabilized-conical-rocket-Giga-Drill-Breaker

It uses a pair of G motors with a 15 degree cant in order to provide the spin.

In particular, check out the recovery system.

You probably will not want to have more than 15 degrees of fin cant, though. If the fin wraps too tightly, then it will not allow easy airflow in the gaps. Also, since you are aiming for a long and skinny rocket, if it spins too rapidly it will be more sensitive to misalignment of the principal axes of inertia. That is one problem I foresee with your one fin plan: the products of inertia will be hard to get rid of.

I built my rocket to be symmetrical so that that would not be a problem.

CarVac said:

I just happened upon this thread. If you want some inspiration, you can check out my own spin-stabilized rocket here: https://www.rocketryforum.com/showthread.php?32275-Spin-stabilized-conical-rocket-Giga-Drill-Breaker

It uses a pair of G motors with a 15 degree cant in order to provide the spin.

In particular, check out the recovery system.

You probably will not want to have more than 15 degrees of fin cant, though. If the fin wraps too tightly, then it will not allow easy airflow in the gaps. Also, since you are aiming for a long and skinny rocket, if it spins too rapidly it will be more sensitive to misalignment of the principal axes of inertia. That is one problem I foresee with your one fin plan: the products of inertia will be hard to get rid of.

I built my rocket to be symmetrical so that that would not be a problem.

Click to expand...

Actually, your spinning cone was one of the threads I looked through while I was pondering this. I fully expect to run into all kinds of problems like you’ve mentioned. That’s part of why I’m trying, to see how much I can overcome and solve. My hope is that I’ll be successful all the way through the plan. I’ll be happy if I can get the two fin version to work.

I’ve made some progress, though I’ve had some disappointments in the process. I mentioned that I had an idea for making the fins go on easier with some drafting tools. The thought was to use one of those bendable curve tools, wrap it around the body tube along my alignment line, and use it as a guide for cutting out a single fin. The problem turned out to be that the fin needs to also twist as it goes around the body tube in order to stay perpendicular to it. I had hoped that by soaking the fins I could gently twist them to shape, but after running through the process fully in my mind, I don’t have the tools (nor could I think of an appropriate tool short of a mold of some kind, which would have to be made to order) to hold the shape. So I went back to my original plan, which was to cut it in sections.

I took the three “fins”, and cut them into segments. Then I took a spare BT80 tube and wrapped sandpaper around it, with marks above and below that match my fin alignment marks on the rocket. I held each fin segment at the appropriate angle and sanded until the root edge formed a curve to match the body tube. Then I simply glued it on, went to the next segment, and repeated. I was very happy with how the sanding worked, it was not difficult, and really took very little time. But I was disappointed in the gaps, or the differences in angles I ended up with between each segment.
I knew I was going to have gaps between the sections to deal with because of the need for the twist I mentioned above, but I was surprised at how much the angle/gap turned out to be. In trying to figure out how to cope with the gaps, I came up with a plan for the next phase rocket (I’ll discuss later), but for this version, I’m kind of stuck now. I have all segments of one of the three fins completely attached including fillets. I’m going to experiment a little on this before moving on to the other two, so I have plenty of room to work without risking damage to the others. I have two ideas on how to deal with the gaps, both of which should work, one will take some patience and luck to avoid breaking them, neither of which will be aesthetically pleasing. But this is simply a prototype, right?

The simple fix is to simply cut some card stock and lay it over each gap to both cover and smooth them out. The rough part will be making it look decent as the incline plane will not be smooth but will have a series of flat segments that are somewhat smoothed out. If I go this route, I might try to use lightweight spackle over the paper and segments to do a little shaping.

The more risky fix is to try and bend/twist the segments to make them align better. It’s balsa, so there’s already some flexibility there, and if I were to wet them (how to wet them without getting the body tube wet is another issue…wrap it in plastic like I am masking it for paint maybe), I might be able to get this to work. The thought is to take strips of wood (probably craft/popsicle stick for strength), and glue them to two adjacent segments across the gaps, holding them in place with clamps until good and dry. To be safe, I’d likely put one on each side. This would add weight, create a sort of ridge along the fins, could result in me simply breaking one or more segments, and might still be tough to create the twist keeping it perpendicular to the body tube. Not to mention, that the fins would then be under stress just sitting there, wanting to straighten back out, which would make hard landings even worse. And I’d still probably have some open space between the segments that would need papered. Given all the bad compared to the good, unless I really don’t like the papered version, I’ll probably not try this approach.

Before permanently attaching anything, I will do some dry fitting. I figure I can tape down some card stock to see what it looks like, and then if I really don’t like that, I can try to simply clamp on the support strips without glue for a few adjacent segments to see what that does. Hopefully, I can make the paper approach look decent.


As to the plan for Phase 3, instead of trying to make a solid fin from balsa and shaping it how I want, I will build a fin skeleton. I’ll cut out thin segments of appropriate length and glue them along the alignment lines with some spacing between to create the skeleton. I have on hand (and will pick up more) some very thin ply which I purchased to line the inside of the ring for this rocket, that is very easily curved. I’ll use this to “skin” the frames, which should give me a better shape. But that’s a bit further in the future.

I suggest using flexible posterboard as a base material for the fins, due to its flexibility, then fiberglass it all together for stiffness.

Hey man, it looks like the grain of your fins is going the wrong way. Have you glued them yet? Grain should be parallel with the leading edge. That would make the balsa easier to flex with the curve of your inclined plane also.

Chevis

Chevis said:

Hey man, it looks like the grain of your fins is going the wrong way. Have you glued them yet? Grain should be parallel with the leading edge. That would make the balsa easier to flex with the curve of your inclined plane also.

Chevis

Click to expand...

It's a bit of a conundrum. As a single piece fin tapering toward the nose, the grain is parallel to the leading edge. Individual cut sections are not. It's one of the learning things to carry over to the next phase. But yes, they are glued and I'll just have to do what I can with it now. Yet another reason for papering them.

I may be too late with my suggestion, which I regret. What I have done on a number of projects was to layer. So for your fins, you could make them out of 1/64" aircraft plywood (really flexible and bends easily without water)and layer it. You glue one layer on, after it is dry, you glue on another fin set on top of the original, etc. With glue, three layers will be close to 1/16" thick. Now you will have made your plywood fins out of plywood. You can do it as many times as you need to get your thickness desired and get quite a bit of strength. To get the thickness faster, put on fiberglass cloth between the layers. I would also make my joints in different places for each layer so that you never have two in the same spot. Also, if you study screw threads more, you will find applications where 2, 3, and 4 thread screws are used in industry. You could do the same and have two or three threads on your cluster screw. (Seems as if 4 threads would be appropriate for a cluster with four motors.)

Yep, paper will make it aaaall right. I'm watching with interest, like the ideas.

Chevis

I've made some progress. I tried to apply some bend/curve to the individual sections to make a solid/flat fin and while it worked well for the larger sections at the aft end, the shorter ones toward the nose wouldn't do what I wanted. So I went with the paper route. I cut standard card stock paper in sections and glued them to the front side (the side that will contact the wind) of the fin across the gaps. This left me with a solid surface, but it was stair-stepped and I really want something more flat and smooth. I grabbed some lightweight spackle and created the surface I wanted. I'm not as worried about the back side being flat, so I just used some spackle in the gaps to kind of fill those in, but left them stair-stepped to save on some weight.

It sat like that for a while, and then I went a little nuts last night and got the second and most of the third fin in place (still need to do the spackling). I purchased some joint tape and am going to try it instead of card stock to maybe make the spackle more resistant to coming loose. It should still be the same weight as the first fin.

I also checked the max lift off weights of the Estes 18mm motors, and I should be fine for B and C motors as long as I stay around 12oz or less (and I'm at 6.3 right now). I might be able to keep it light enough for even A motors, but what's the fun in that?

I figure I should start thinking of color and scheme soon. I'm thinking simply painting the fins one color and the body another contrasting color (yellow and blue for example) would be good as it could have a nice visual effect during flight. But I'm open to suggestions

Here are some pictures...

View from the nose (hole is for the launch rod)...

Side view showing the back of the fin

Side view showing the front of the fin

With the second and part of the third fins in place.

All fin segments are attached complete with fillets. I just need to paper and spackle them and it will be structurally complete (structurally sound may be another story).

Looking good! I can't wait to see how well this flies.

What a way cool project!!! Then I started thinking--before the wine--what if you built up the fins from strips of balsa, tip to tail ?? Looking forward to your fight report. If It goes well, do you mind if I try my method of constructiion? ----H

So am I correct in that, given the helical design, whether this works or not, you're screwed?

The above comment needs a "like" button.


Later!

--Coop

First coat of spackle was put on and sanded down, then I added a second layer to better refine the shape and fill in some gaps. I just have to let that dry well and then sand it smooth. It's sitting at 8.6 oz. right now without parachute or motors, so I'm in good shape with my max target weight.

hornet driver said:

What a way cool project!!! Then I started thinking--before the wine--what if you built up the fins from strips of balsa, tip to tail ?? Looking forward to your fight report. If It goes well, do you mind if I try my method of constructiion? ----H

Click to expand...

Thanks, and no I don't mind...even if it doesn't go well I don't mind if you try it. I could have saved a lot of weight with any of the fin construction ideas you folks have given, and that could make a difference in how well it does. Just one request...I've seen your work posted in here, including the Berserker...try no to make me look too bad by comparison.

MrGneissGuy said:

Thanks, and no I don't mind...even if it doesn't go well I don't mind if you try it. I could have saved a lot of weight with any of the fin construction ideas you folks have given, and that could make a difference in how well it does. Just one request...I've seen your work posted in here, including the Berserker...try no to make me look too bad by comparison.

Click to expand...

Thanks , I might give it a whirl. No, I won't make you look bad--besides I've seen your stuff--It's your brainchild anyway----H

What engine are you thinking for your first flight?

BABAR said:

So am I correct in that, given the helical design, whether this works or not, you're screwed?

Click to expand...

No, the rocket is

Great looking build and funny use of a wonderful spoof for the Geeks show! they generally have us laughing most of the half hour
I also Like the Off beat, strange and some times, misunderstood realm of Odd-Rocs.

I have both Machine Screwn nuts and washers, or Threaded Stud, nuts and washer flying models. Both are a hoot and most folks think I just have strange hardware laying on the prep table....til that see them on the pad both are full szie 3/8-16 x 6" creations with NO metal parts at all

While neither of these Odd-Rocs spin wildy as you creation will if you helix is attached well enough it should make for a very interesting and Photo worthy flight.

Can't wait to see the flight pics and resultes.