Technicians install a Recruit motor into the aft end of a Little Joe booster.
Motor Mount construction
With the external airframe essentially complete, our attention now turns to the motor mount assembly that we will insert into the airframe. The model is designed to fly on a cluster of four 24mm D12-3 motors, with a provision to add four 13mm A10-PT motors, although the A10's will not be used in Baikonur. Custom centering rings were laser cut by Bill Saindon at Balsa Machining Service to accomodate the engine tubes as well as the central tube and staging piston that will attach to the boilerplate capsule. (An aside about the centering rings: the laser cutting process leaves a nasty black residue on the edges of the rings, so make sure that any completed parts are well out of the way.) In addition to the engine tubes and cenering rings, we will also be using a coupler section that was created from a cut-down section of scrap tubing during airframe construction. Note that the hooks are sized for Estes E engines, and are a bit long; a vendor sent the wrong part, but we elected to make the parts sent work with the addition of an extra thrust block rather than go through the fuss of returning them.
The first step in the process is to cut four sections of 24mm tubing to a length of 8.5", then add thrust blocks and motor retention hooks to each section.
After the centering rings are glued to the ends of the coupler section, the 24mm and 13mm motor mount tubes are glued into place with a combination of CA and aliphatic resin glues. The center section of 24mm tubing will be used to align and secure the capsule, and a section of heavy elastic has been mounted to serve as a shock cord mount. There's also a section of plastic tubing in there that, well, let's not talk about that quite yet.
Our next step will be to build the recovery system "tray" in which the parachutes will rest. At ejection the tray will lift up and deploy the parachutes, eliminating the need for recovery wadding and speeding prep times. As noted on an earlier page, time is of the essence in FAI-style flying, and any feature that speeds prep and enhances reliabilty is a good thing.
For some reason, the completed motor mount assembly reminds me of the Mother Ship from the final scene of "Close Encounters of the Third Kind." Hmmm.....
The glue surface of the motor mount assembly is masked off with paper, then the aft portion of the mount is sprayed with gloss black. Most of this won't be visible once we attach the aft cover assembly, but the black paint will help to keep any of the mount from "peeking out." Once the paint is dry the mount assembly is glued into place inside the airframe tube using aliphatic resin adhesive.
The cruciform aft feature seen in the prototype photo at the top of this page is the next component to be fabricated. No consistent name for this assembly has popped up in any of the research to date, so for clarity's sake we'll just call it the "spider." We begin by fashioning a master for one of the Recruit motor mounts, then pull a mold from that piece with PolyTek mold material. Four identical parts are then made from the resulting mold. Next, the aft plate cover assembly is cut and detailed from .020" styrene sheet, and the cruciform spider frame is whipped up out of scrap .030" sheet.
The four Recruit motor mounts are then glued to the end of the cruciform frame, and a bunch of additional detail is added to the assembly. This has been one of the most enjoyable portions of the project so far; lacking any solid dimensional data for the spider, I was able to simply wing it!
Back to part 1: Capsule assembly...
Back to part 2: Escape Tower assembly...
Back to part 3: Transition assembly...
Back to part 4: Boilerplate assembly...
Back to Part 5: Fin assembly...
Back to Part 6: Airframe assembly...
Back to Part 7: Paint and Markings...
On to part 9: Final Integration...