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Three inch CTI M2245 minimum diameter build
#1
Well I was going to post this on TRF but why not post it here. 

I was planning a build for Aeronaut, but need to get my butt in gear if I'm going to finish it in time.

I picked up a CTI 6XL gen 1 case when they were blowing them out during the Wildman Black Friday sale otherwise I probably wouldn't be building this rocket.

The fins are 1/8 inch carbon fiber.  I purchased the CF plate from Rocketry Warehouse.  It is the $75, 12 x 12" plate they sell.

I cut and match sanded the fins but wasn't up to the task of beveling so I sent them to Giant Leap.


I was planning to mount the fins to a 48" piece of fiber glass tube for the sustainer, but after dry fitting with motor and av-bay from another rocket, I think I need a couple more inches clearance between the motor and av-bay to fit the recovery harness..  So I decided to build the fin can separately and will cut a second tube to length for a total of 50 to 51 inches.

Fin guides are from Upscale CNC.  The fin slots are 1/8 inch and need to be opened up a bit so that is where I am now, sanding the slots.

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#2
(06-16-2015, 09:29 PM)MarkH Wrote: Well I was going to post this on TRF but why not post it here. 

I was planning a build for Aeronaut, but need to get my butt in gear if I'm going to finish it in time.

I picked up a CTI 6XL gen 1 case when they were blowing them out during the Wildman Black Friday sale otherwise I probably wouldn't be building this rocket.

The fins are 1/8 inch carbon fiber.  I purchased the CF plate from Rocketry Warehouse.  It is the $75, 12 x 12" plate they sell.

I cut and match sanded the fins but wasn't up to the task of beveling so I sent them to Giant Leap.


I was planning to mount the fins to a 48" piece of fiber glass tube for the sustainer, but after dry fitting with motor and av-bay from another rocket, I think I need a couple more inches clearance between the motor and av-bay to fit the recovery harness..  So I decided to build the fin can separately and will cut a second tube to length for a total of 50 to 51 inches.

Fin guides are from Upscale CNC.  The fin slots are 1/8 inch and need to be opened up a bit so that is where I am now, sanding the slots.

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Subscribed.

I recently looked at building a longer booster for a 54mm MD rocket (just to fit that Loki L-2050) and never considered doing a separate fin can as an alternative to sourcing or wrapping my own tube, good idea Mark.  How will you attach the rest of the booster to the fin can?

I am a big fan of Upscale CNC.  Nat is a great guy, has been very accommodating and he does very good work at a reasonable price.
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#3
I plan to mount a bulkhead inside the upper tube, just above the motor.  The motor will be retained by threading an eyebolt with a fender washer through this bulkhead to the motor forward closure.  The fin can and upper tube will slide over the motor. When the eyebolt is tightened down to the bulkhead and motor, the upper tube and fin can will be cinched together.
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#4
Giant Leap did a great job on those fin bevels.  I have two smaller diameter fin alignment guides I purchased from Upscale CNC to fit 38MM and some Semroc LT-134 tubing and I love them.  

Very well worth the money I paid for them.  

Pretty cool build, subscribed.
Steve Naquin
TRA# 677  L2
NAR# 85518 L2
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#5
Nat at upscale has made some cool stuff.


Bill Clune L3 | NAR#88583
Been a launch Director
Founded a Club
MARS Club|SRC|MDRA - Forever
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#6
(06-17-2015, 06:32 AM)MarkH Wrote: I plan to mount a bulkhead inside the upper tube, just above the motor.  The motor will be retained by threading an eyebolt with a fender washer through this bulkhead to the motor forward closure.  The fin can and upper tube will slide over the motor. When the eyebolt is tightened down to the bulkhead and motor, the upper tube and fin can will be cinched together.

Mark that is very creative, I look forward to seeing some pics when you get to that part.  Do you plan on doing a T2T lay up if so under vacuum?
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#7
(06-18-2015, 08:07 AM)Pitter Wrote:
(06-17-2015, 06:32 AM)MarkH Wrote: I plan to mount a bulkhead inside the upper tube, just above the motor.  The motor will be retained by threading an eyebolt with a fender washer through this bulkhead to the motor forward closure.  The fin can and upper tube will slide over the motor. When the eyebolt is tightened down to the bulkhead and motor, the upper tube and fin can will be cinched together.

Mark that is very creative, I look forward to seeing some pics when you get to that part.  Do you plan on doing a T2T lay up if so under vacuum?
Pitter, I haven't decided on fin reinforcement yet.  The motor will be a CTI M2245, and should hit about Mach 2.8 to Mach 3, depending on final weight.  The 1/8 " thick carbon fiber fins are thick enough that I don't think they need to be laminated for extra stiffness.  The more critical part is the fin to airframe joint.  I've heard some with more experience than me argue that I would be good with only large fillets.  Currently I think I may do 3 layers of uni directional carbon fiber over the fin joint with the top layer going about 1/3 to 1/2 of the way up the fin.  Yes I will be vacuum bagging it.
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#8
I bonded one fin today  using Aeropoxy adhesive epoxy.  I tried to put multiple fins in the jig, but it seemed to be a fight to get them all exactly in the right position without having a fin move slightly forward or aft.  I'm thinking I may just use JB weld for the fillets, given that I have the industrial size pack and I'm out of Proline 4500. 

I hung the rousetech motor out the back to balance the jig and keep it from tipping forward


The fins look good, but there is a little more asymmetry (from one side to the other) in the bevels than I would like for a flight that will be going this fast.  Three of the fins are within +/- 0.5 gram of 80 g, and the 4th fin weighs almost 83 grams.  Three grams is almost nothing so I think the weight difference is not too bad.


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#9
(06-22-2015, 09:51 PM)MarkH Wrote: I bonded one fin today  using Aeropoxy adhesive epoxy.  I tried to put multiple fins in the jig, but it seemed to be a fight to get them all exactly in the right position without having a fin move slightly forward or aft.  I'm thinking I may just use JB weld for the fillets, given that I have the industrial size pack and I'm out of Proline 4500. 

I hung the rousetech motor out the back to balance the jig and keep it from tipping forward


The fins look good, but there is a little more asymmetry (from one side to the other) in the bevels than I would like for a flight that will be going this fast.  Three of the fins are within +/- 0.5 gram of 80 g, and the 4th fin weighs almost 83 grams.  Three grams is almost nothing so I think the weight difference is not too bad.

Man that tapered aft tail cone closure looks very sexy!  Why 4 fins over 3 fins?
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#10
Pitter,

Four fins are for increased stability margin.  CP starts to shift forward above Mach 1.5 or so, and continues to move forward with increasing speed.  If a three finned rocket and four finned rocket have the same sized fins, the four finned rocket will have a greater stability margin.  A three finned rocket has to have larger fins than a four finned rocket  to have the same stability margin as the four finned rocket.  A larger fin produces more aerodynamic loading on the fin, so four smaller fins provides the same stability margin as three larger fins, with less stress on the fins.

Mark
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