Musings on 3FNC

I am always playing around with OR. I figure that, one day, I will actually figure it out. Well, my designs are often inspired by something that I have seen here or some existing kit.

Last night I git to thinking about Pemberton Technologies' 3FNC and decided to create my own version. I played around until I got the look that I was going for and then used tabs so that the templates would be sufficient to make patterns that would fit the contour of the PNC-55. The resulting plan looks a bit wonky but it suits my purpose when build time comes around.

The next task was the motor mount. Since this is nothing more than a nose cone with fins, I decided to eject the mount and just have to decide on how to keep it all together on the way down. Of course, motor selection was a part of this process which led to another idea:jaw:

I was just going down the list of 24mm motors without paying too much attention other than selecting single use and came across a sim that exceeded 1300 fps on an F72! Well, the motor was too long for the rocket as designed. I tweaked the design by adding a 2 in tube to accommodate the longer motors and it still simed at more than 1200 fps. I have some glass and epoxy laying around, so this one is going in the future plans file for sure.

The OR files are attached for your amusement. I'll post PDFs later if anyone is interested.

View attachment My_3FNC.ork

View attachment 3FNC_Mach.ork

Well bummer, I'm very interested in seeing your creation but the ork file won't open. You caught my interest, PDF's would be greatly appreciated.
An F72???? Dang, you are a wild man.

Suggestion for keeping the airframe attached to the motor mount. Drill through the nosecone a bit back from the tip and run a hefty dowel through both holes. Cut to size, epoxy in place, and tie your shock cord around the dowel.

Pem Tech said:

Well bummer, I'm very interested in seeing your creation but the ork file won't open. You caught my interest, PDF's would be greatly appreciated.
An F72???? Dang, you are a wild man.

Suggestion for keeping the airframe attached to the motor mount. Drill through the nosecone a bit back from the tip and run a hefty dowel through both holes. Cut to size, epoxy in place, and tie your shock cord around the dowel.

Click to expand...

Wow, I am honored that the Chief Head Imperial Llama Leader has chimed in!

The plan for this one would be rear eject but I have not decided if the mount would recover separately or not. I am not stuck on the idea of rear ejection but that is the best idea that I have at this time.

This is more of a design study at this point. I imagine that it will have a life much like my Fatter-By, where I will puzzle and ponder over it for a bit while I find solutions to various challenges in the concept.

This is where TRF comes in. Y'all have contributed quite a bit to my two previous scratchbuilds. The Fatter-By would not have made it without the contributions of TRF and Orange Gush (Gas-Dynamic Stabilization) was tweaked based on the input of the forum as well. I do have a bit of an itch to build silly ideas, (short, stubby supersonic rocket anyone?) so this one shouldn't be doomed to empty promises, I just have to work it in with the Pinewood Derby (we will be building at least 3 cars), baseball, Scout meetings, a new work schedule and my typical glacial build pace. The The low speed version will probably get built first and serve as a proof of concept.

Here are the PDFs for your amusement and critique.

View attachment My_3FNC_Mach.pdf

View attachment My_3FNC.pdf

Interesting design... I noticed that the main (larger) fin's tab is in direct conflict with the strake (smaller fin)'s fin tab.

Rear ejection would be pretty cool with this design. Estes Sizzler (2127) might be a good model to base the recovery on.

K'Tesh said:

Interesting design... I noticed that the main (larger) fin's tab is in direct conflict with the strake (smaller fin)'s fin tab.

Click to expand...

Some of the tab (the portion FWD of the leading edge) has to be removed from the actual template. As for the rest of the tab, on a low speed version I would only use the portion needed to match the contour of the rocket. I used the same technique on the GDS model since OR doesn't contour the fins to curved surfaces. I am pondering using TTW for the high speed version, so the tabs height may get tweaked on that one.

How do you plan to do rear ejection with TTW fins? I have a way in mind, but I'd like to hear yours.

I haven't really dove into that part at this point but, as currently designed, they would just be minimum depth. I would just sand them flush with the ID of the BT. I know that may not add much strength but the mach buster version would get glassed fins with FG fillets. Right now, there is as much of this floating around in my head as there is in OR so I would love to hear your idea to add to my options.

My notion (actually part of a larger notion for a modular construction standard that I've been mulling over on and off) is to use an intermediate tube in between the airframe and the motor mount. I call it a socket. The fin tabs are glued to the outside of the socket, and the motor mount goes inside it.

In your case, a BT-52 would work. The BT-50 motor mount would be a loose fit so you'd have to build it up with tape. Or, if I remember right, a heavy wall BT-50 would work if you sand a bit off. You can also make an 18 mm mount that can be swapped in when you want lower power.

If you want to make the socket tube yourself, you could make one that would take a mount for 29mm motors or one for 24mm or one for 18mm, field swapable.

That is an option that has crossed my mind and is the route that I will likely go if I can't figure out a lighter option. I have a silly idea that involves the minimum height TTW fins with FG going through the BT and wrapping the ID of the tube. I'll have to talk to my airframes guy to get some ideas on that one though. It is possible to do but the entire fin can would have to be done in one shot.