Houdini - A 3" Minimum Diameter Project for LDRS 34

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EeebeeE

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Last November I took advantage of the Rocketry Warehouse Black Friday sale and bought a DIY Kit. It is a 3" diameter "kit" with 30" airframe, 16" airframe, a coupler, 54mm motor tube, centering rings, a coupler and bulkheads to build an AV Bay, and a 6" x 30" sheet of G10 FG for the fins.

I started playing around with a design that could take a CTI 6-grain 54mm motor. Then I looked at the prices of those vs. the prices of a 3-grain 75mm motor. OK. 7,000 more feet for the same money. I have a 6-grain 54mm case ... AND a 3-grain 75mm case. Guess what case won?

This is "Houdini." Now you see it ... Now you don't. Houdini.jpg

Still working on the fin design. I know a lot of people who don't like raked fins, but they will be surface mounted, then covered tip-to-tip with 6 oz. FG. I might take some of the rake out. Components will include:
- Fil. Wound G12 airframe
- Fil. wound G12 4:1 aluminum tip nose cone
- 3/32 G10 fins covered with tip to tip FG
- Drogueless Apogee separation, 54" PML Main Chute
- AV Bay will include (I bet you will wonder how I fit all this into a 3" x 6" coupler tube):
- 2 x Perfect Flite Stratologger Altimeters
- 1 x Perfect Flite MT-4 Timer
- 2 720p HD Video Cameras
- Beeline tracker in the nose cone

Project will start in a week or so after I get the right epoxy for this. Anyone interested in seeing the build thread?
 
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Well then, I guess I will have to post the build. Still messing with the fin designs. Has to be something I can easily stack saw. I will hand saw them with my trusty miter saw set. I did build a fin alignment guide. Also need to build a jig to lay on the tip-to-tip FG.

This is the 3" airframe in the fin alignment jig. Based on a design by Ted Macklin, who deserves the credit for this elegantly simple design.

Fin Alignment Guide.jpg

This is the latest rocket design. Note the modified fins. shorter in length and no rake. What do you think?

Houdini 012015.jpg
 
Much better than original fins....should be way less prone to damage upon landing.

That jig is pretty slick, how large a diameter tube can you use in it?


By the way...with that fin design you really don't need T to T at all. I have a very similar rocket with 54 hole in 3in.tube [3/32 G-10 fins] Flies on anything i can put in it & never had any problems..... L-motors mach 1.6 ish, no sweat. If you do add more weight that means bigger motors. Then I built another one Minimum ...same fins...same thickness, surface mount, decent size fillets. Flew on 4-grain 75 18,600ft Mach 1.8......no issues.

Keep it light = fly on larger variety & smaller 38's= more fun & cheaper flights. Just something to think about.
 
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Here's the 75-54 or 38 adapter I made for the minimum version. Utilizing an Aero-Pac tailcone retainer....it acts like thrust plate.

Just a coupler with 2 CR's to hold 54 tube in place.

3/8 Kevlar glued to exposed motor tube [wrapped with tape just to look better..LOL] for recovery cord attachment when flying the 54's & 38's. I use eyebolt in closure for 75's

Held in place with bottom rail button.

Bottom 1 in of fin can is not painted. I wrap aluminum duct tape around that area and motor for retention when flying 75's & it will pull paint off anyways.

DSCN0862.jpg

DSCN0869.jpg

DSCN0873.jpg
 
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Nice adapter design Jim. You ought to get Tim to sell those. BTW those fins look familiar.
 
Much better than original fins....should be way less prone to damage upon landing.

That jig is pretty slick, how large a diameter tube can you use in it?


By the way...with that fin design you really don't need T to T at all. I have a very similar rocket with 54 hole in 3in.tube [3/32 G-10 fins] Flies on anything i can put in it & never had any problems..... L-motors mach 1.6 ish, no sweat.
QUOTE]

This is headed for plus Mach 2. Think they will hold. I ordered Apogee Components Rocket-Poxy for this. long cure, but 5,000 lb. shear.

I hear you on motor size and cost. It is going to have a bulkhead motor mount for a 3-grain CTI 75mm case. I am going to make a 54mm motor tube with a forward closure that will screw into that bulkplate so I can fly it on 54mm motors. Even a 2-grain 54mm motor will make it scoot. From there I can adapt down even to a 38mm.

The jig can hold up to a 3" airframe and as small as a 38mm.
 
Much better than original fins....should be way less prone to damage upon landing.

That jig is pretty slick, how large a diameter tube can you use in it?


By the way...with that fin design you really don't need T to T at all. I have a very similar rocket with 54 hole in 3in.tube [3/32 G-10 fins] Flies on anything i can put in it & never had any problems..... L-motors mach 1.6 ish, no sweat. If you do add more weight that means bigger motors. Then I built another one Minimum ...same fins...same thickness, surface mount, decent size fillets. Flew on 4-grain 75 18,600ft Mach 1.8......no issues.

Keep it light = fly on larger variety & smaller 38's= more fun & cheaper flights. Just something to think about.
This is headed for plus Mach 2. Think they will hold. I ordered Apogee Components Rocket-Poxy for this. long cure, but 5,000 lb. shear.

I hear you on motor size and cost. It is going to have a bulkhead motor mount for a 3-grain CTI 75mm case. I am going to make a 54mm motor tube with a forward closure that will screw into that bulkplate so I can fly it on 54mm motors. Even a 2-grain 54mm motor will make it scoot. From there I can adapt down even to a 38mm.

The jig can hold up to a 3" airframe and as small as a 38mm.
 
I am a little old school when it comes to cutting fins. I like to at least start the cuts with a handsaw because I get a nice straight groove. The fins will be 3.75" high and 6" long. Leading edge will come out at a 45 degree angle from the root. trailing edge will be angled only slightly.

First, using my hand miter saw, I cut 3 pieces of G10 3.75" x 6". These had to be cut individually. I cut these all the way through with the miter saw.
IMAG0431.jpg IMAG0432.jpg

Next I taped all three pieces together and marked them so that they could be stack-sawed (cutting all three fins at the same time).
IMAG0433.jpg

Then, using the miter saw I started stack-sawing enough to make a nice guide groove for the saber saw.
IMAG0434.jpg

Then using my saber saw with a hacksaw blade I finished the cut, then separated the fins.
IMAG0435.jpg IMAG0436.jpg

Not too bad. Next will be sanding leading and trailing edges.

I would still like to hear more discussion about whether I need to FG tip to tip. I am going to drill 1/16" holes in the airframe along where the fin roots should be to create epoxy rivets. I may also drill 1/16" holes about 1/16" from the root so that the fillets will also make epoxy rivets.
 
I actually started with an older kit: Formula 75.
Then, I enlarged the slots followed by gluing in the fins.

Just finished the CF ? FG layups on them:

Formula76CFbooster.jpg



Final decision of lamination is up to you and what you want to fly it on...


JD
 
One other note that might have been overlooked:

Have you checked the tube with a 75 / 76 mm motor casing?
On my tube it was really tight, even before I glued on the fins.
I had to take an engine hone to enlarge it enough to allow the casing to fit.


JD
 
I would still like to hear more discussion about whether I need to FG tip to tip.

If I were you, I would do CF, which will add both strength and stiffness. Your spans are pretty big for a 3in MD, and flutter increases exponentially with fin span. That plus you're using 3/32 G10 (a tad on the thin side).
You might not absolutely need any T2T reinforcement, but I'd do it anyway.

I like the design too!

Alex
 
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There is stability room to make the fins 1/2" shorter in length. As it is the fins extend 3.75" from the airframe and are 6" long. I can keep the length but only make the fins extend 3.25". That should reduce flutter. Have no choice on the G10. Stability at Mach is reduced, though.

This is how it would look.

Houdini 012515.jpg
 
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Cut the fins down 3/8". This should cut the flutter down a lot and it still is more than 1 ca. stable. Edges have been sanded. Made 5 passes with a belt sander and 100-grit, then followed up hand-sanding with 220 grit paper. I think they came out OK.

Completed Fins.jpg

This is the final design:
Houdini 012515a.jpg
Houdini 012515c.jpg
 

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Have you checked the mach shift stability with the new design?

Alex
 
As it closes in on Mach 2, stability drops below 1 ca. for a brief period. I might add a little nose weight to stave that off.
 
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Following up ... I created a custom simulation that shows stability in calibers, mach speed, and pressure drag to look at stability. This is part of the reason I have a little larger fins. The other is that the waiver at Potter, NY is 22,500'. OR tends to overestimate altitude, so if I sim to that altitude I should be fine. Just don't want to blow past it.

This is the stability vs. mach without nose weight. Stability drops to 0.75 ca. at motor burnout. A little lower than 1, but should be OK because it jumps right back up.
Stability vs Mach - No Nose Weight.jpg

This is the stability with 5 oz. in the nose, and it stays above 1 ca. the entire flight.
Stability vs Mach - 5 oz Nose Weight.jpg

According to these sims, if I want to keep stability at 1 ca. or better, the minimum nose weight needs to be 3.25 oz. That will change as I glue components together. I am using actual weights of components as I do the sims, although I added 1 oz. to the fin weight to accommodate the fillets. I will need to add more if I decide to go T to T.
 
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Gluing one of the fins in place. Also cut the forward airframe. This is the final length and you should get an idea of what the finished rocket will be like. A 3-grain CTI tube is in the back. 4-grain is just a little long. I could get a coupler tube and glue the 7" section of airframe cut from the payload area to the booster and that should give me the room, but there is no area near the Northeast with a 28,000' waiver.

How does it look?

First Glimpse.jpg
 
How much nose weight will it need?

Looks awful my short to fly on a 3 grain 75 / 76 mm motors

JD
 
man that thing is going to be packed tight! did you decide if you are going to tip to tip? I just ordered 1/8" G10 for my DYOK because all the flutter calculations I've done said the 3/32 would flutter around mach 1. I know people have had great success with 3/32 fins but the comfort was worth the $15 in G10 to me.
 
Debating it. Didn't think there would be that much flutter because the fins are 3-3/8" tall away from the airframe. But the chord is over 4" and that might cause some. They are only slightly taller than those of my Mad Cow Arcas, and the Arcas fins are 1/16" I've sent that into transonic regions 5 times. Although the Arcas fins are TTW and they are glued to the inside of the airframe with strips of FG. Although the flutter may have been the cause for the fillets to crack and some section of them to break off.

If these do flutter, at least under thrust they will punch through transonic in less than half a second. Coming out of Mach they will be transonic for about 2 seconds and that could be an issue.

I tend to pack my rockets fairly tight. No reason for wasted space.
 
Ran a fin flutter analysis and stress test using FinSim. With a 1/4" fillet, the rocket's VNE is 1,833 MPH. The simmed speed of Houdini is 1,588 ... 13% below that number. I think it is going to be OK without T to T. FinSim calculated the chord to be 3.67".
Fin Stress Analysis.jpg
 
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how did you determine your angle of attack? I'm still trying to fully understand how to use finsim correctly. if you don't want finsim discussions in your build thread I had started a topic in the software section asking about finsim...
 
That's a tough one. I used 1o degrees. However I just noticed I input the wrong fin thickness. Turns out I need to T-to-T this thing. I will use automotive FG epoxy which is very strong, relatively easy to work with, and has a higher temperature rating.
 
Ok, glad you came to the same conclusion as i did. I was trying to figure out what i was doing wrong.
 
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