7.74X Cherokee N (Is there a mechanical engineer in the house?)

[rharshberger]

Any further updates on this project?

 

[dixontj93060]

Well yes. All carbon fiber laminate applied and cut/sanded to size on the fins. Next step will be applying a couple layers of 6oz fiberglass to airframes and inside/out of couplers. Hope to do this over the Thanksgiving holiday.

 

[rharshberger]

I am putting together my L3 package currently and would like to reference this thread as the technique I will be using for my fins. The only change in construction for me will be endgrain balsa instead of honeycomb and my upscale of the Cherokee-D will be a 5.5".

 

[dixontj93060]

I am putting together my L3 package currently and would like to reference this thread as the technique I will be using for my fins. The only change in construction for me will be endgrain balsa instead of honeycomb and my upscale of the Cherokee-D will be a 5.5".

Sure. Feel free.

 

[dixontj93060]

How is this for a slow build... Have the two couplers and two of the three airframes glassed now. I'm OK though, planned first flight is at Airfest 23.

 

[rharshberger]

At least you are getting there Tim.

 

[OverTheTop]

Just spotted this thread. Sorry for the slow start.

I am using an isogrid structure on the Apache stage of my 1/2 scale Nike Apache.

Isogrid is a structure developed by NASA to produce very light structures that have relatively isotropic properties. It was used, among other things, on the Delta IV, Orion CST-100 (and Skylab and the SST main tanks IIRC) and various other items.
https://en.wikipedia.org/wiki/Isogrid

Here is my take on it:


Fins start in the build thread here: https://forum.ausrocketry.com/viewtopic.php?f=6&t=5019&start=31

I could tell you if it flew successfully, but we had 2.5 months of scrubs and delays due to weather, then the fire season kicked in. Waiting...

 

[dixontj93060]

Just spotted this thread. Sorry for the slow start.

I am using an isogrid structure on the Apache stage of my 1/2 scale Nike Apache.

Isogrid is a structure developed by NASA to produce very light structures that have relatively isotropic properties. It was used, among other things, on the Delta IV, Orion CST-100 (and Skylab and the SST main tanks IIRC) and various other items.
https://en.wikipedia.org/wiki/Isogrid

Here is my take on it:
View attachment 308070 View attachment 308071 View attachment 308072

Fins start in the build thread here: https://forum.ausrocketry.com/viewtopic.php?f=6&t=5019&start=31

I could tell you if it flew successfully, but we had 2.5 months of scrubs and delays due to weather, then the fire season kicked in. Waiting...

Impressive build!

 

[dixontj93060]

Since OverTheTop brought up structure, I thought I'd discuss the direction I'm leaning for the internal airframe spans. This harkens back to a few comments made by Kevin Trojanowski (Troj) in the past about not relying on the Sonotube for structural support. I get that now. Examining the application of the FG from the tube ends, you see the soak of the epoxy into the material, but beyond that the Sonotube is still that characteristic spongy cardboard. Even with a CA soak, I'd still be worried that edges wouldn't hold up as they mate, e.g., against the nosecone/shoulder seam. Also, with just the outside laminated, because of the large diameter of the structure, I'm still getting quite a bit of flex when pressure is applied laterally. My current thought is to build a monocoque structure within the airframe and coupler spans. A very readable article on this technique is related in this blog post: https://aerospaceengineeringblog.com/aircraft-structures/.

 

[dixontj93060]

One more item that may be of interest... This morning I was doing some research before I order some specialized parts from McMaster-Carr for this build. There are a number of options to do the recovery termination on the nosecone. The one I'm leaning toward is adding a bulkplate and/or CR attached with threaded inserts. Of course this has been done many times, but an example for a retainer is HERE. Problem is, I don't use threaded inserts much and this is a hefty nosecone, so was looking into matching potential gee force to holding force of an array of threaded inserts which far exceeds retainer requirements. Fortunately I found some good analysis from two fellow Purdue alumni given HERE to calculate threaded insert shear failures in wood substrate. Of course, we normally use plywood, so make sure to substitute wood laminate shear in their equations for our application.

 

[OverTheTop]

You may find this build interesting (full scale Patriot PAC-1). There are some techniques you might like:
https://forum.ausrocketry.com/viewtopic.php?f=10&t=5308&hilit=patriot&start=18

 

[dixontj93060]

Wow, I need to spend more time on that Aussie forum. I have a login (if I can remember it). Met Blake and a couple of others at LDRS a few years back. Fun guys and always great projects!

 

[OverTheTop]

Have you considered T-nuts? They don't pull through as easily as the friction-fit inserts. Work great in plywood.

Search Google for "t nut" (I can't post pics at the moment )

 

[dixontj93060]

Have you considered T-nuts? They don't pull through as easily as the friction-fit inserts. Work great in plywood.

Search Google for "t nut" (I can't post pics at the moment )

Sure, that's a better option and I'd love to use them, but... the structure at the aft end of the nosecone is already built complement of Dan at Python Rocketry (it's what started the whole project). I'll have to take a picture of the shoulder area, otherwise it's hard to understand.

 

[pythonrock]

Tim,
How about a u-bolt on ply bulkhead epoxied up in center tube. Use heavier back plate and thick ply with filet.
Can't seem to upload pic sorry

 

[dixontj93060]

Tim,
How about a u-bolt on ply bulkhead epoxied up in center tube. Use heavier back plate and thick ply with filet.
Can't seem to upload pic sorry

Dan, I went as far as considering burrowing into the center tube to get under the main CR (to somehow to get a nut under there) but I didn't consider filling the central tube. This is because I want to keep that as a removable closure so I have a compartment for my BRB GPS tracker. I actually think the larger plate with 12 or so #10 screws might be a better strength option than epoxy around a central 4" bulkplate. The larger plate also gives me room for two u-bolts so I can better distribute the any harness shock forces.

 

[mccordmw]

With these larger diameter, longer rockets, I'm worried about bending moments at the couplers. Are you doing anything special to secure the airframe around the couplers and around the payload bay?

I'm currently in my early L3 build stages with an 8" diameter model.

 

[dixontj93060]

With these larger diameter, longer rockets, I'm worried about bending moments at the couplers. Are you doing anything special to secure the airframe around the couplers and around the payload bay?

I'm currently in my early L3 build stages with an 8" diameter model.

The Sonotube will be little more than a skin. Strength and rigidity will come from the internal grid structure of both the airframes and couplers (see section 2, Semi-Monocoque, here: https://aerospaceengineeringblog.com/aircraft-structures/). The end of the grids will have a mating ring to provide seating for the couplers in each section.

 

[Cl(VII)]

How is this for a slow build... Have the two couplers and two of the three airframes glassed now. I'm OK though, planned first flight is at Airfest 23.

View attachment 307934

The Cherokee is one of my favorite rockets, and your upscale is coming along nicely. This makes me want to get my V2 done in time for Airfest even more as that will be the excuse for the trip.