# 38mm MD Design Opinions

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#### RAHagen

##### Well-Known Member
Hey Everyone! After all this time I'm finally jumping in and building a 38mm MD. For a while there, It seemed like everyone was building one of these and being the self proclaimed hipster/oblivious person I am I've decided to start one after that hype has dropped off some. So without further adieu here is my design for a 38mm MD. I've had this design bouncing around in my hard drive for at least a year now and I'm finally getting close to building it. Before I started though I wanted to run it by you guys first. Nothing super special but I'm exited to start building it!

Airframe: I'm planning to use the cardboard-composite method that I've used quite a bit and found to be very reliable. This time my plan is to make the tube out of a 38mm motor mount tube with two wraps of carbon fiber.

Fins: These will be made from 1/16" Carbon fiber plate. I'll be making these from scratch as well. I used the same thickness on my recent 29mm MD and I believe the same thickness should work. This is an area where my skill is quite limited so any help is greatly appreciated.

E-bay and Recovery: Due to the airframe being carbon fiber I'll be putting the electronics in the nose cone. For tracking and dual-deploy I'll be using my telemetrum flight computer with a cable cutter for drogue and main. One thing's for sure, I'll have much more space to "stretch out" so to speak in this E-bay than in my 29mm MD.

Motor Retention: friction fit

Planned Motors: The rocket is designed around Loki's 38/1200 case, so it'll be able to launch on pretty much anything.

Predicted Altitude: My current design is simmed to reach an altitude of around 12700 feet with the Loki 38mm K

View attachment minimum diameter blue tube V.3.rkt
So what do you guys think? Questions? Comments? Concerns?

P.S. Oh! And a name! I'm about the worst you can get when it comes to naming rockets. If you need proof, go look at the first few posts on my "Querty" thread.

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#### REK

Great design,

I'll be subscribed

op:

#### Cl(VII)

##### Chris Bender, Lab Rat
I would move the aft fins forward about 1-1.5 cal for a few reasons:

1) protect them on landing
2) CP versus CG isn't going to be a problem because anything over about 3 grains will start placing weight in front of the CP
3) Since you are friction fitting it would be nice (and some RSOs may require) a wrap of aluminum tape also, and this would be way easier with some uninterupted airframe at the aft end.
4) This is hearsay, but it sounds correct. You will reduce the overall drag by allowing the air disturbed by the fins to return to laminar flow before the aft end...or something like that, basically: smooth air=good, turbulent air=bad.

Also, fin thickness sounds on the minimal end to me, but I do not possess good mind-fin-sim. Someone more experienced will have to weigh in.

I very much like you use of CC for DD. This is a good plan, and I wish I had done mine that way.

#### RAHagen

##### Well-Known Member
SO something like this? I'd be uncomfortable moving the fins any further forward than .5". That leaves the the worst case stability margin at .41 calibers.
View attachment minimum diameter blue tube V.3.rkt

And thanks, I think I got that from Conman's MD thread or something. Really saves space. BTW how'd you MD fly? I don't think I was there to see it.

#### T34zac

##### Well-Known Member
Everything looks good to me so far although the fin thickness may be too think for a 38mm K. I wouldn't quite go 1/8" but 3/32" should suffice.

#### RAHagen

##### Well-Known Member
Everything looks good to me so far although the fin thickness may be too think for a 38mm K. I wouldn't quite go 1/8" but 3/32" should suffice.
Thanks Tzac, yeah I was thinking that 1/8" would be a little too much too. I agree though with Chris that 1/16" is probably pushing it so I'll probably go jump through the hoops to get fin sim on my computer. The problem is that my CF was a gift from a friend and he didn't know what thickness it was so all I know is that a six sheet layup cured in a press is just over 1/16". I was thinking I'd probably do a 7-8 sheet layup so it'd hopefully be around 3/16"

I also started thinking about ways to make this rocket a little more than just a MD. What If I inserted flex sensors into the fins and ran the data to a recording device. I could overlay the data with my flight stats from my Telemetrum and see where the fins produce the most flutter. What do you guys think? I'm pretty sure someone has done it before but I think it might be a way to make the build a little more interesting.

#### REK

Thanks Tzac, yeah I was thinking that 1/8" would be a little too much too. I agree though with Chris that 1/16" is probably pushing it so I'll probably go jump through the hoops to get fin sim on my computer. The problem is that my CF was a gift from a friend and he didn't know what thickness it was so all I know is that a six sheet layup cured in a press is just over 1/16". I was thinking I'd probably do a 7-8 sheet layup so it'd hopefully be around 3/16"

I also started thinking about ways to make this rocket a little more than just a MD. What If I inserted flex sensors into the fins and ran the data to a recording device. I could overlay the data with my flight stats from my Telemetrum and see where the fins produce the most flutter. What do you guys think? I'm pretty sure someone has done it before but I think it might be a way to make the build a little more interesting.
You probably have 7oz fabric. 8 layers should get you to 0.1" thickness.

Having a sensor would be nice to determine such effects. I say give it a shot.

#### RAHagen

##### Well-Known Member
You probably have 7oz fabric. 8 layers should get you to 0.1" thickness.

Having a sensor would be nice to determine such effects. I say give it a shot.
Thanks! Yeah I think ill be doing that. Just to make this rocket a little more unique.

#### kcobbva

Wow subscribed. looking forward to see your progression!

#### RAHagen

##### Well-Known Member
Go to harborfreight and spend $10 for a pair of dial calipers to measure the thickness of your cloths and composites. http://www.harborfreight.com/6-in-digital-caliper-with-sae-and-metric-fractional-readings-68304.html http://www.harborfreight.com/6-inch-digital-caliper-47257.html These go on sale every month or so for$10 so if they are not on sale now, wait a few weeks.

Bob
Thanks! I have a digital caliper but I wasn't sure if measuring the cloth straight like that would give an accurate reading.

#### cls

##### Well-Known Member
what about options for tracking it, to recover it? doesn't matter how high it went if we can't hear the beeps of the altimeter.

#### RAHagen

##### Well-Known Member
what about options for tracking it, to recover it? doesn't matter how high it went if we can't hear the beeps of the altimeter.
I believe I posted originally that the av-bay will be equipped with a Telemetrum flight computer that will transmit flight/location data, and control the rockets parachute deployment.