The mind is a dangerous place, especially when it stumbles upon ideas for new rockets to build. Recently I came up with the idea to make a high power scale model of the SpaceX Falcon 9 rocket, complete with nine motors packed into the first stage. (It was going to be a two-stage rocket, but the design proved to be a bit more difficult than originally planned, so one-stage it was.) I’ve build and flown the low power kit that SpaceX sells, and it’s a really fun kit, but I wanted something with a bit more oomph. I discovered that I could stuff nine motors into a 3″ airframe–eight 18mm motors in the outer web allowed for any motor up to 29mm as core motor, so I decided to throw in a core 29mm mount and turn the model into a high power rocket. Because of the shape of the payload fairing, I had to 3D print a nosecone and a transition unit.
The motor mount was by far the most challenging component. I started by drawing up the centering rings in AutoCAD and quickly found out how tightly the outer web of motors would be packed. I also had to design in tabs for hooks for motor retention, including a hook to retain the core high power motor. I also drew up the fins, which I planned to cut in 1/8″ acrylic in AutoCAD. All parts were cut using a laser cutter, which was a good thing because the margin between edges and interior features was very small and the centering rings were cut from 1/8″ plywood.
As seen to the left, the 18mm mounts were arranged in a polar pattern with 1/8″-by-1/8″ hook tabs positioned 45 degrees off the radial direction. Copious amounts of wood glue and epoxy were used to assemble the motor mount tubes into the center rings, though I tried to avoid getting any adhesive in the hook tabs. The acrylic fins were inserted between every other pair of 18mm mounts in a very satisfying “click.” It turns out the spacing between mounts was less than 1/8″, so the 18mm tubes all deformed a bit. A fit test of motors later showed this was miraculously not an issue and standard C motors can still be inserted and removed from their respective motor mounts. Copious amounts of epoxy were used to anchor the fins into the cluster. The entire motor mount assembly, which included three centering rings, four fins, nine motor mount tubes, and an eye bolt took a very compact form that slid easily into the slotted airframe.
The stage 1 airframe conveniently only needed to be 34″ in length to match the scale, so a standard LOC 3″ airframe was sufficient. Aside from the four fin slots for the motor mount, no modification to the stage 1 airframe was required. It was appropriately decorated with the SpaceX logo, which was an official decal I acquired in my time out west. I don’t think the scale of the lettering was perfect, but I did pay close attention to the position and spacing of the lettering so it would resemble an actual Falcon 9 paint job. Added to the other side were a pair of 1010 rail buttons (white) with 1/2″ standoffs. The rail buttons were anchored to the airframe with small, low-profile plywood squares and tee nuts inside the airframe,
To match the scale, the stage 2 airframe would be just 11″ in length, though the total component length would be about 4″ longer because of the coupler I made for it. To this point, I had been hoping to make an actual two-stage rocket, but the fins would simply have been too large and unfeasible, thanks in large part to the large payload fairing that would site right atop it. The coupler was capped with a plywood bulkhead and an eye bolt to tether it to the bright orange 36″ parachute and the rest of stage 1 with 20 ft of 1/4″ Kevlar that I salvaged from a previous crash. The stage 2 airframe was marked with the most reasonably-sized Falcon 9 detail I had acquired, which was certainly off-scale and a 2″ American flag decal I had a remarkably difficult time finding (and the quality wasn’t great…).
The payload fairing made use of a 6″ segment of LOC 3.9″ airframe tubing and a couple of 3D-printed parts. To date, I have printed the nosecone, which is just over 4″ in length, with a 2″ shoulder and 0.1″ wall thickness. It is geometrically modeled on the shape of the Falcon 9 payload fairing.
The transition section has not been printed yet since I was using school printers to do the job and the semester was just about over. The transition length will be 1″, but there will also be 2.5″ shoulders on each side and 0.1″ thick walls. Friction fitting has worked better than anticipated for the nosecone (It’s definitely not coming off.), so I expect the same to be the case for the transition. That said, neither glue nor rivets should be needed to complete the build, though I may go over the printed parts with a coat of white spray paint so it better matches the rest of the airframe.
Watch for more posts later this year when I print out the last piece and fly this model! The goal is to have it all flying by February.