Esther's Awesome Reversible Rocket

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BDB

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My daughter had a cool idea that she doodled on the back of a music test last month. She was then mad because she had to wait until she got the music test back to show me. Here are two of her early-stage drawings.
reverse1.jpg reverse2.jpg
The big idea is to make a rocket that can fly in both directions. It will fly "forward" on a single 29 mm motor, and it will fly "backward" on a canted 4x24 mm cluster. I thought it was a cool concept, so I decided it was worth a shot. Here are the OpenRocket mock-ups. (Unfortunately, OR can't handle the canted motors, and the "reverse" simulation generates errors.)
Screen Shot 2020-03-28 at 12.52.42 PM.png Screen Shot 2020-03-28 at 12.53.27 PM.png
So this is the build thread. As a bonus, this build will also test the use of SLA-printed centering rings and bulkheads. It may end in disaster, but it will be fun to try.
 

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BDB

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One of the keys to the design is a BT60 that runs the length of the "aft" airframe tube. It will serve as an kind of chute cannon when the rocket is in reverse, and it will serve as a kind of motor mount (with an adapter) for when the rocket flies conventionally.
Screen Shot 2020-03-28 at 1.08.44 PM.png
So our first task was to make BT80-BT60 centering rings--not something that one could easily buy. We could have cut them from plywood, but my circle cutter is a death trap waiting to happen--not something I want for my 11 yr-old. So we opted for 3D printing. I have both a FDM printer (Ender 5) and an SLA printer (Anycubic Photon), so we opted to use them both.

Over the winter, we tested the strengths of several SLA resins in our garage. Siraya Blu with 10-20% Siraya Tenacious is ridiculously strong--nearly as strong as fiberglass in our ad hoc tests (kids standing on them). Esther drew the rings up in Tinkercad, and we opted to print the centering rings with the SLA. The added bonus of SLA prints is that 2-part epoxy is a good method for bonding SLA-printed parts (as opposed to PLA prints.)
CRs.jpg
Esther also designed a "nosecone" for when the rocket flys in reverse. We printed it on the Ender using eSUN PLA+. We may have screwed up the ID and the OD of the BT60, but after LOTS of sanding, it fits like a glove.
IMG_3405.jpg
After roughing up the surface where the rings would bond to the BT60, they were attached with 30 minute epoxy. The inner assembly/chute cannon is complete.
inner assembly.jpg
 

David Schwantz

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And you were thinking "rockets" instead of music??? GOOD JOB!!!!!!!!!!!!!
 
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kuririn

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Too cool. A Pushmi-Pullyu rocket.
(I was a Dr. Doolittle fan in my youth).
 
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shoebox9

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When in tractor mode, why do the motors need to be canted?
Not wanting to burn holes in your BT, but if this was for improved stability in the event of a failure to ignite, wouldn't an outwards cantor be better than an inwards facing cantor, so the thrustline was closer to pointing at the CG?

Why not just fly them straight?
 

Lugnut56

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Reading this makes me wish I had been into model rockets when my boys (now 36 & 29) were young.
 

neil_w

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When in tractor mode, why do the motors need to be canted?
Not wanting to burn holes in your BT, but if this was for improved stability in the event of a failure to ignite, wouldn't an outwards cantor be better than an inwards facing cantor, so the thrustline was closer to pointing at the CG?

Why not just fly them straight?
Based on the fin shape in the OR side view, the motors will indeed be canted outwards when flying in reverse. Given how far from the centerline of the rocket they are though, I'm not sure the canting will really make much of a difference, and it'll add some weird drag when the rocket is going "forwards".
 

BDB

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Esther's original idea was to not cant the motors. The canted motors were my idea. I was inspired by some of @lakeroadster's recent creations, and by my my canted booster for a LPR 2-stager (pic below). Like Neil said, the plan is for the motors' exhaust to point away from the rocket when flown in "reverse." My inclination is that canting adds some stability. That just seems to make sense, but I haven't ever ready anything definitive about that.

9AB8473D-8C46-4A7A-B9B1-1A6779DB7F99.jpeg

And I agree with @neil_w and @shoebox9 that this will likely cause some interesting aerodynamics in the "forward" direction, but this thing is just built for fun--not performance.
 
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KenECoyote

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Ooooh...so nice! Absolutely FANTASTIC out-of-the-box thinking and I truly love this! Your daughter is a genius! :):wos_love:

I actually experimented a bit with an upside down rocket and did have some small scale success; however I think (and I could definitely be wrong) the big problem here would be to keep the CG ahead of the CP...especially with the long tube pointing down...you'll likely need a LOT of nose weight and then that may push the limits of the motor. Still, DON'T let that stop ya...meant only to impart my own learnings and I do plan to continue the upside-down path one day. :headspinning:
 
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KenECoyote

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Ooooh...so nice! Absolutely FANTASTIC out-of-the-box thinking and I truly love this! Your daughter is a genius! :):wos_love:

I actually experimented a bit with an upside down rocket and did have some small scale success; however I think (and I could definitely be wrong) the big problem here would be to keep the CG ahead of the CP...especially with the long tube pointing down...you'll likely need a LOT of nose weight and then that may push the limits of the motor. Still, DON'T let that stop ya...meant only to impart my own learnings and I do plan to continue the upside-down path one day. :headspinning:
Here's a video of my "Whatside Down" rocket...I have to disclose though that there is a lot of hidden engineering going on, so it normally wouldn't work if built conventionally:
 

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OK, something tricky IS going on there! From the blurry video it appears there are fins at the 'top,' the rocket is clearly 'bottom' heavy, there is no apparent spin or ventrui/air intake gass pressure stabalization, no whining sound of an internal gyro, yet it still flys straight up! I'm officially impressed!

What am I missing?
 

BABAR

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Mucho coolo.

Suggestion.


Move the outboards to inboards. As is, if both outboards don’t light, or one is at the allowed 10% thrust variance, the rocket will cartwheel. Moving the tractors inboard mitigates this problem. You may need to cant the motors slightly so that the jets don’t cook the body tube
 
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shoebox9

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BABAR,

In the video demo he abandoned the original design and did something trickier.

Watch it carefully, though it is quite out of focus it appears to have a very small fin set at the top, none at the 'bottom' (as flown) and a startling amount of weight at the bottom (or at least there is after deployment).

It appears his daughter isn't the only genius, something very clever seemed to happen in that clip.
 

BABAR

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BABAR,

In the video demo he abandoned the original design and did something trickier.

Watch it carefully, though it is quite out of focus it appears to have a very small fin set at the top, none at the 'bottom' (as flown) and a startling amount of weight at the bottom (or at least there is after deployment).

It appears his daughter isn't the only genius, something very clever seemed to happen in that clip.
I am afraid I can’t make it out well enough on the video. I think it is fantastic to have an inventive creative outside the box rocket scientist in the family, and I am huge fan of odd rocs. I just don’t like outboard motors, either on standard rockets or on odd-rocs. I do a far number of clusters, and I do a lot of asymmetric clustering, but I try to keep the clustered motors as close to centerline as I can (I don’t use canting, and I find my rockets fly just fine with the inline alignment, but the extra motors are within 1/2 main body tube diameter of the centerline. For my clusters, even if they don’t all light, rocket either is stable even without all motors lit, or never gets off the rod.

Exception. I have seen some rockets with outboards with a huge main motor relative to small outboards. If the main doesn’t light, it probably won’t get off the pad. If the main DOES light, it so overpowers the outboards that it doesn’t matter of none or some or all of them light. The outboards in those rockets are mainly for show, and they are indeed cool.
 
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KenECoyote

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BABAR,

In the video demo he abandoned the original design and did something trickier.

Watch it carefully, though it is quite out of focus it appears to have a very small fin set at the top, none at the 'bottom' (as flown) and a startling amount of weight at the bottom (or at least there is after deployment).

It appears his daughter isn't the only genius, something very clever seemed to happen in that clip.
Hey Guys!

That's my video of one of my "RAIS" Ram Air Intake System rocket designs and the upside down one specifically. I wanted to show that I did try tests on upside down and also had exp with the challenges. I don't want to hijack this thread, so here's my original thread and I've also updated the thread with reveal pics of how I did it.

Post #236:
https://www.rocketryforum.com/threa...gn-ram-air-intake-stabilization.131572/page-8

Be different, think different, do different and enjoy the different!
 
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BDB

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Mucho coolo.

Suggestion.


Move the outboards to inboards. As is, if both outboards don’t light, or one is at the allowed 10% thrust variance, the rocket will cartwheel. Moving the tractors inboard mitigates this problem. You may need to cant the motors slightly so that the jets don’t cook the body tube
That’s a really good idea. And canted inboards would look really cool too—kind of like inlets for a jet engine. And I like the idea of designing the inboard canted motor mount.

But we’ve already cut the fins and started the rest of the assembly, so that may have to wait until version 2.0.
 

BDB

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@BABAR. The more I think about it, the more I like the canted inboard idea because we could probably duct the ejection charges of the inboards to pop the chute when flying in reverse. The current design requires electronic ejection. We may have to start version 2.0 next week so we can have a side-by-side comparison.
 

BABAR

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@Daddyisabar is the tractor guru. I would thing that if the motors are nearly all the way forward, they wouldn’t have to be canted very much, just enough to avoid the jets scorching the sides of the main body.
 

Daddyisabar

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On my backwards forwards design I kept the used 29mm SU motor or casing for nose weight and it looks cool as a nose cone tip. I just used a slight cant to get the damaging heat away from the tube and did a traditional forward eject on each canted motor. Keep it simple. The design was to carry it up as a traditional rocket and then flip it over to fly as a tractor. If you are flying it both ways you might consider interchangeable fin rail locks where you put the largest set on the BOTTOM where they should be. Then after some test flights put the big ones up front on the tractor configuration . You could even change there direction until you get to the tractor with the fins mounted and pointed forward. With all the weight up front you should be fine but your RSO will probably have had a major coronary by that time. MOTORS AND FINS UP FRONT RULE.
 
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BDB

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3D Printed Pods

Building has been slowed by a little stomach bug (thank goodness it wasn't Corona), but it has since resumed.

We fretted about the best way to attach the pods to the fin tips, and finally decided on using 6-32 machine screws. Esther and I designed the pods on TinkerCad and then printed them on the Photon. We used Industrial Blend Black resin because it is temperature-resistant, and I had some if it laying around. (Though we did break one pod by dropping down the stairs, so I'm nervous that it may be too brittle.)

If you are not familiar with resin printing, the part is drawn out of the liquid resin, and you have to use a ton of supports. Here's a few pics of the design and printing process. The pods need a little sanding and painting, but we are pretty happy with them

Screen Shot 2020-04-27 at 6.19.14 PM.png IMG_0090.jpg IMG_3444.jpg IMG_3446.jpg
 
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BDB

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Fins

Previously, I stacked four pieces of 1/8" plywood together and drilled two 1/8 holes through them. I drove some spare roofing nails through the holes to hold the stacked pieces together and then cut them into fins on the table saw. Unfortunately, I didn't take any pictures of the process, but you can see the two holes in the fin tab in the last picture in post #2 (above).

Esther used the holes on the pods to mark the two holes needed to attach the pods on a fin, and then we stacked them and held them together using roofing nails again. She then drilled 1/8" holes in the fin tips.

IMG_0084.jpg IMG_0086.jpg

Attaching the fins was rather uneventful, except for the fact that I did a lousy job cutting the fin slots. (That's what I get for eyeballing it while watching an Avengers movie with the kids on the couch.) But we have recently discovered that heavy fillets with Titebond Thick and Quick cover a multitude of mistakes quite well.

IMG_0091.jpg

I should also say that we love using this Fin Alignment Guide by gravatite on Thingiverse. Only one guide is typically necessary, but since the slots were a little wide, we decided to make sure the fins were "extra straight" by using two.
 
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