Monday, February 20, 2012

Formula 75 build continues

Construction on the new kit continues apace. First, the upper centering ring was notched to allow the kevlar line to fit between it and motor tube. The kevlar was epoxied to the fiberglass motor tube, as was the centering ring. During this step, I selected JB weld for heat resistance. I do not know (and doubt) that this is significantly better than 30 minute epoxy even in this demanding location. 5 minute epoxy (used at times in this build) is rated for 200 degrees temporary. So I suspect that 30 minute epoxy would be fine. Only with the slimline retainer do I suspect 30 minute epoxy would be at risk. As previously stated, I don't really worry about appearances for internal parts, and the JB weld was just slapped on without masking tape. Most important is that coverage is complete and that all parts of the joint are covered on both sides. This was left for 24 hours to cure.

After filling the nosecone with expanding foam, the bulkhead plate was installed first with a thin layer of 30 minute epoxy, followed by a large (22.5 grams) pour of 5 minute epoxy. This potted the unit in place very securely, and also helped reinforce the edges of the nosecone to prevent damage if there is impact with the airframe. I enjoy using 5 minute epoxy when possible because it can be whipped up with air bubbles and remain fairly light, good for filling spaces like this. It does last long enough to settle flat after 30 seconds of mixing and a pour. The fins will be potted in place using more 5 minute epoxy after JB weld fillets. 5 minute epoxy is also quite cheap, and is less brittle than slower curing epoxies. This may mean weaker, but can also mean more impact resistant.


Urban wet-sanding may not look very professional, but it gets the job done. There is no safe way to dry sand in an apartment without a dedicated work-area. The results are better anyway, and cleanup is pretty simple. Fin slots and other contact areas were sanded for fit and better adhesion. These fin slots are very tight, and it looks like alignment will be almost automatic as a result.

After ca tacking the motor tube and upper centering ring into the airframe, the rear centering ring was installed as you see above. This ring will be removed after the fins are tacked in place, so that the fins can be reinforced fully. The nylon line (once a shock-cord from a rocket) has been tacked with ca so that the centering ring can be removed later. The alternative was to drill two small holes, and this strikes me as excessive. Once the ring is removed, it can be flipped over so the ugly side is facing in.

The fins were test-fit into the rocket, and taped off. The roots were all wet sanded (this time in the sink!) first with 400 grit, then 150 grit. The 400 grit was used to remove all residue of a sticky material that was found on the fins, possibly used to hold fiberglass sheets together while cutting. I found this material particularly hard to clean off. First it took razors, then scouring in hot water and soap, and finally the 400 grit when nothing else could finish the job. (Yes even alcohol was tried. No acetone or goo be gone on hand.) The 150 grit is to prepare the fins for epoxy. It should be plenty considering the amount of epoxy and foam about to be used.

The fins were then stacked, taped together, and placed on a visco-elastic foam sample block. I used a dremel and 1/8th inch bit to drill 8 holes along the root. These holes were at no time closer than 1/4th inch to each other or the edge of the fin, hopefully preventing any cracks or weakness in the fin structure. These holes are concentrated at the front of the fin and rear. In the front, 5 minute epoxy will pour through the holes (one fin at a time) and help form rivets. In the rear, epoxy foam will extrude through the holes also. Or that is the idea anyway. This may be overkill, but I intend to use a 28 inch skyangle 'chute with this 5 lb. rocket and strong fins are an absolute requirement. Furthermore, I do not plan on adding any significant fillets to the outside of the airframe because it will not be painted. Perhaps only a tiny strip, but nothing structural. Therefore, all fin strength will come from within the rocket.

Today I will attempt to fit and tack the fins in place, and perhaps even do the JB weld fillets. The rest will have to wait until after vacation. But the good news is, this rocket is more than half done! It will be ready to go by the first launch.

2 comments:

Bayourat Rocketry said...

Good luck keeping the fins on.

High Power Rocketry said...

Oh yes... With the amount of epoxy and foam in there, and the time it took to reinforce the fins internally, they had better last!

This is my strongest build yet and I expect these fins to be rock-solid at this point.