FORMULA 75 Build Thread

I thought it would be helpful to do a build thread on the Rocketry Warehouse Formula 75. Lots of these were sold during Rocketry Warehouse' Black Friday sale. This build thread can be used as a reference to build most other fiberglass kits too.

Last year I bought a Rocketry warehouse Formula 54 fiberglass kit and was very impressed with the price and quality. This year I bought a couple of Formula 75 kits during Rocketry Warehouse’s Black Friday Sale. This is an upscaled version of the Formula 54 kit (54mm) kit. This kit uses a 75mm airframe, hence the name. The Formula 75 is manufactured by Performance Rocketry for RW and is a 3” diameter, 4 foot tall, all-fiberglass kit with color impregnated parts. It is supplied with Performance Rocketry’s latest filament wound nosecone with integrated graphite tip. The kit weighs about 3.75 Lbs built and flys on G to J motors. It comes with a 38mm motor tube. It also comes with a nice, black vinyl decal. At a sale price of only $59.95, these kits were an absolute steal. Heck, the nosecone alone is worth the price! Even at the regular price at $89.99 is still a bargain in my opinion. Rocketry Warehouse currently has them on sale for $79.95. This would make an ideal level 1 rocket and could even be used for level 2 with a small J motor taking it to around 6000'.

One of the things that attracted me to these kits aside from the price, was the variety of colors they come in and that they do not require painting. Either fly them naked or gloss them up with wax or the like. You might even consider a clear coat of paint to make the colors pop. All the kits come with black G10 fins and black nosecone. The 7 fruit colors offered during the sale were Lemon, Lime, Tangerine, Cherry, Plum, Blue and Cranberry. The Blue and Cranberry seem to have been dropped after the sale ended. A conventional natural-colored kit is also available. I chose both Lime and Cranberry versions.

Here’s what you get:

• 3 Inch G12 Colored Fiberglass Airframe 30 inches long
• 38mm Motor Mount G12 Colored Fiberglass 10 Inches long
• 3 G10 Black Fiberglass 3/32" Thick Fins
• G10 Black Fiberglass Centering Rings
• 3 Inch Black Filament Wound 5:1 Von Karman Nosecone
• G10 Black Fiberglass Bulk Plate for Nosecone
• Black vinyl decal

Another interesting feature of the kit is the black filament wound nosecone. It is VERY strong and is supplied with a black 75mm coupler which is glued into the nosecone to form the shoulder. The coupler is a precision fit in both the nosecone and airframe tube (No sanding required) and offers a crisp shoulder unlike anything you can expect from the molded style nosecones. Its embedded graphite tip is nearly indistinguishable from the nosecone itself as it is centerless ground at the same time as the nosecone. The nosecone’s appearance somewhat resembles carbon fiber and it is quite handsome. The separate shoulder tube also offers some advantages as you will see later. The disadvantage? Like the G12 filament wound airframes, this nosecone is heavy at 15 oz. That said, the advantages of strength, no seams, no sanding, uniform finish and thick walls far outweigh this disadvantage. Did I mention NO SANDING REQUIRED?

Having recently become a fan of the Garmin DC40 GPS dog tracker in my larger projects, I planned to incorporate it into this smaller diameter build. More on this later.

I used West Systems 105/205 epoxy for the build and JB Weld for the areas where I required a strong heat resistant bond. I used Pro Line 4500 black epoxy for the external fin fillets. I began construction by washing all the parts in soapy water to remove any mould release. Then I roughed up the 38mm motor tube with 60 grit sandpaper so the epoxy would key into the surface for better adhesion. I elected to use an Aeropack 38mm motor retainer.

I began by using my trusty two foot long ½” dowel with 36 grit aluminum oxide sandpaper taped to one end to rough up the interior of the airframe where centering rings, fin fillets and rail button retaining nuts would be glued. The super coarse 36 grit makes this effortless. I had cut some of this sandpaper from a 1” x 42” sanding belt I bought for this purpose. I keep 36, 60 and 80 grit belts on hand for these kind of jobs. They are cheap, easy to store rolled up and have a very tough cloth backing.

Photo of the green kit by Gary T.

Next I marked a line on the airframe half way between two fin slots to use for mounting two Delrin rail buttons from Giant Leap. I like the countersunk head on these and they are very inexpensive. I used stainless steel 8-32 screws to install them. I drilled two holes into the airframe with a #29 drill and tapped the fiberglass with an 8-32 tap. After installing the rear button with a 5/8” long stainless steel flathead screw, I prepared an 8-32 nut with some 5 minute epoxy on one end and threaded it onto the portion of the screw that was exposed inside the airframe. I was careful not to get epoxy in the threads. To be safe I put a tiny amount of oil on the threads to act as a release agent just in case.

For the forward button, I partially screwed a temporary, extra long 8-32 machine screw into the airframe. I then stuck a nut to the end of a square dowel using Blu-Tack adhesive. I carefully applied some epoxy to one side of the nut and guided the dowel into the airframe until the nut was positioned under the end of the screw. Holding it steady, I screwed the long bolt into the nut and then pulled off the stick and Blu-Tack. I then backed the long screw out until the epoxy side of the nut was snug against the airframe. When cured, I unscrewed the temporary long screw out and mounted the rail button with a flat head stainless steel screw. Note that the long, temporary screw was oiled so as not to stick to any epoxy that may have contacted it.

Now I had to determine the position of the forward centering ring on the motor tube. I planned to have the front centering ring butt up against the forward edges of the fin tabs so the joint could be epoxied for added strength. This is typical construction. I also wanted the rear centering ring to be positioned 1/8” forward of the aft end of the airframe with no gap between the rear centering ring and the Aeropack retainer. Normally the aft centering ring is butted up against the rear of the fin tabs, but I decided I did not want it so far into the airframe for personal preference. So I tried something new to figure out the position of the front centering ring in relation to the motor tube. First I pushed the centering ring onto the motor tube. It was a relatively snug friction fit but it could be moved. I then wiggled the assembly into the rear of the airframe so that the centering ring was just visible near the front edges of the fin slots. I then used one of the fins to push the centering ring flush with the front edge of all three fin slots. The centering ring was quite snug in the airframe.

Next I slipped the rear centering ring onto the motor tube and shimmied it forward until it was just inside the aft end of the airframe by the 1/8” that I preferred. I then placed the Aeropack body onto the motor tube and checked the gap between it and the rear centering ring.

I repeatedly removed the Aeropack to twist the motor tube forward in the centering rings until there was finally no gap between the Aeropack and the rear centering ring. I verified that the front centering ring was still in position and used the extended lead of a mechanical pencil poked through the 3 fin slots to mark the motor tube to centering ring position. All a bit awkward, but it worked in the end.

After removing the centering rings and motor tube I used a rat tail file to make a groove for the Kevlar shock cord to pass through the front ring. Then I roughed up the centering rings with 60 grit paper.

Next I passed some ¼” tubular Kevlar shock cord through the front centering ring and positioned the ring on the motor tube. I tacked the end of the shock cord onto the motor tube with CA adhesive. Then I ran a bead of JB weld where the shock cord was to be attached.

Next I ran a bead of JB weld where the front centering ring was to be positioned. You can see the pencil marks on the motor tube which I was careful not to cover completely yet with JB weld.

Next I pushed the front centering ring into position on the motor tube and locked it into place with a few drops of CA adhesive on the forward side. Then I pulled the shock cord taught and worked the JB Weld into it. Next I dressed the JB Weld on the CR to motor tube joint and set it aside for the next day. I also marked an index line on the aft edge so I would know where the shock cord was in relation to the motor tube when it comes time to insert it into the airframe. (You don’t want to accidentally position the shock cord where a fin must be glued to the motor tube!)

While waiting for glue to set I began preparing the fins. First I took the front, lower edge of each fin tab to the disc sander to create a 45 dgree angle. I have found from past experience that the epoxy bead on the rear face of the forward centering ring fouls the front corner of the fin tab. Grinding the corner off the fin tab in that area aleviates the problem.

Next I wet sanded the exposed edges with increasingly finer sandpaper to bring them up to a nice black shine. I used 320, 400, 600, 800 and 1500 grit. It only takes a few seconds to do each edge so total time spent was about 15 minutes.

Next I ran a bead of West Systems 105/205 epoxy thickened with 406 coloidal silica inside the airframe for the front centering ring. I used a long stick to dab and spread the epoxy inside where I had previously sanded for the centering ring. You can use 5 or 10 minute epoxy here but I prefer the extra strength of the West Systems epoxy.

Noting the shock cord location mark on the rear end of the motor tube, I pushed the motor tube into the airframe part way and slid the rear centering ring on to center it in the airframe. I continued pushing until the forward centering ring was just past the forward edge of the fin slots. I then inserted a popsicle stick biased forward in one slot and pulled the motor tube backward until it touched. Now the epoxy on the forward CR needed time to set. One last check to see the shock cord was placed roughly half way between two fins.

After the epoxy had set overnight, I used the popsicle stick through the fin slots to eject the rear centering ring. The motor tube was now solidly centered and fin installation could begin.

I placed masking tape on the airframe spaced about 3/16” from the slots and roughed up the exposed fiberglass with 120 grit paper to accept epoxy fillets later.

I numbered and inserted each fin and taped them 3/16” above the airframe. I then sanded the fin tab up to the tape line with 120 grit paper to provide a grip for the external fillets.

Then I sandwiched all the fins together and cut notches every inch or so to add more surface area for the epoxy to grip.

Using un-thickened West Systems 105/205 epoxy, I buttered one fin slot with the tab of a zip tie so epoxy would be absorbed into the G12 fiberglass for a better fin to body joint. I then buttered the bottom of the fin tab and carefully inserted the fin through the slot to but up against the motor tube. Eyeballing it for trueness, I taped it to the airframe and continued on with the last two fins. Once they were all done, I measured between adjoining fin tips to insure all three dimensions were identical. This would confirm all fins were aqually spaced. I adjusted one fin to get it bang on then let them all set overnight.

Looks great! Can't wait to see it in person!

Now the fun part! I tend to build strong and use a lot of reinforcement in my fin cans. I like using hardwood dowels epoxied in as fillets on the inside of each joint. This makes a very strong, reinforced joint and actually weighs less than using an equivalent amount of epoxy alone. It also saves on epoxy. The resulting joints are actually strong enough to eliminate exterior fillets altogether but in this case I decided to use very small external fillets to transition the black fins to the colored airframe. I opted for Pro Line 4500 high temperature epoxy for the external fillets because of its black color and its ease of use for making fillets. It has excellent flow-out, sets up in under 15 minutes and has a glossy finish which needs no sanding. Oh yah, it's stronger than :no:

But wait, I'm getting ahead of myself here.

I chose 1/4" hardwood dowels and cut 12 of them to the same length as the fin tabs. Then mixing West Systems 105/205 epoxy with 406 coloidal silica to the consistency of runny peanut butter, I use a long stick to partially fill each joint line, and then buttered up a dowel. I carefully insert the dowel and press it into the epoxy with my long stick. I usually do two joints per session, orienting the rocket so the epoxy puddles into the crevices between dowels and fiberglass.

When finished, there is no way a fin will dislodge on impact. It will break first!

At this point I rough up the aft centering ring and set it aside.

Now for the external fillets. As mentioned previously, I chose Pro Line 4500 high temperature epoxy because of its black color and handling properties. This is the first time I tried this product. Its pot life was about 15 minutes at 70 degrees. It has great flow-out and leaves a wonderfully glossy surface. Because I was using this epoxy for cosmetic reasons to provide a black transition between the black fins and green airframe, I should have been more precise with the masking fore and aft of the fins. Next time I'll remove the masking tape sooner so that I get better flow-out at the edges AND I will use my 3M automotive masking tape for a cleaner edge. I may even consider doing only one fin at a time so I don't feel rushed. I did all three fins at once.

I spread a bit in each fin to airframe joint and dragged the end of a popsicle stick along to form the fillets. The tape was removed a few minutes later leaving glossy fillets which required no sanding whatever.

When all joints were cured, I put a bead of epoxy around the inner end of the airframe and JB weld around the motor tube. I then installed the centering ring until it was flush with the end of the airframe. I added some JB weld to the inside of the Aeropack retainer body and pushed it all the way onto the motor tube, thereby positioning the centering ring correctly in the airframe. After wiping any excess JB weld off the inside of the motor tube with a tissue soaked in isopropyl alcohol, I stood the rocket on the Aeropack retainer and let it set overnight.

Now for the nosecone. As mentioned these new fillament wound cones offer an opportunity to design an avionics bay into them more easily than the cast type of nosecones. The nosecone comes with a loose coupler that normally gets glued into the nosecone to form the shoulder. This procedure is addressed elswhere in the forums. In my particular instance I decided to install my Garmin DC-40 GPS transceiver into a foam liner inside the nosecone. To make it more accessable, I decided to secure the coupler tube to the nosecone with three 4-40 countersunk stainless steel flat head screws instead of glueing. The eyebolt and bulk plate would be glued into the end of the coupler conventionally.

First I ran some masking tape around the perimeter of the base of the nosecone about 1/2" from the edge. I measured the circumference, divided by three and made 3 marks on the tape for pilot holes. I installed the coupler and gave it a light tap to seat it and inserted it into the airframe to make sure it was on straight, Then I removed it from the airframe and drilled three 1/16" pilot holes through the nosecone and into the coupler. I then used a countersink drill to allow the flat head screws to sit flush in their holes. I made two index marks with a drill bit so the nosecone and coupler could always be realigned quickely. I removed the nosecone from the coupler and drilled out the 3 holes in the coupler to accept PEM nuts on the inside. These were compressed into place by use of a machine screw tightened from the outside.

Next I driled out a 1/4" hole in the bulk plate for the GPS antenna to pass through, secured the eye bolt and glued the bulk plate in with JB weld.

I then lined the cone with some foam, inserted the DC-40 followed by more foam. Then screwed it all together.

To follow, more nosecone shots and decal application.

Here are more shots of the nosecone configuration. First the GPS unit is turned on and placed inside the white foam followed by the blue tubular foam. The hole in the center of the blue tube prevents the ON-OFF switch from being depressed due to acceleration forces. Finally the coupler is installed, compressing the foam slightly and the 3 flat head screws are screwed in.

Once the nosecone was done, all that was left was to apply the decal.

*** FIN ***

Great build man. I like the in depth descriptions. Well done.

Nice build, and beautiful rocket, I need to learn how to make those nice of fillets

Going to build this kit next month, will have to steal a few of these ideas like the internal dowel reinforcements.

I plan on certifying level 2 later in the year, how do you feel about this rocket for L2 on a baby J with single deploy? Planning to use a 5 grain J, and build the rocket to about 4lbs final weight. Recovery is with a small skyangle.

New Ocean said:

I plan on certifying level 2 later in the year, how do you feel about this rocket for L2 on a baby J with single deploy? Planning to use a 5 grain J, and build the rocket to about 4lbs final weight. Recovery is with a small skyangle.

Click to expand...

I just removed the Garmin in Rocksim and I come out to 3.91 lbs as I built it. Loaded with a J285 CTI 5 grain motor it's launch weight is 5.25 lbs. That will bring you to about 4500 feet altitude. I am using a 45" Rocketry warehouse chute which would give you a 23 ft/second descent. If you use a smaller chute it will not drift as far but will obviosly contact the ground a bit harder. That said, this kit is STRONG.

For my level 2 I used dual deploy on a 4" rocket weighing 12 lbs to 3300' which gave me a much better chance of finding it. It also had a GPS tracker in it and it landed 1/2 mile away. I'm glad I had the tracker on this flight.

DavieRockets said:

For my level 2 I used dual deploy on a 4" rocket weighing 12 lbs to 3300' which gave me a much better chance of finding it. It also had a GPS tracker in it and it landed 1/2 mile away. I'm glad I had the tracker on this flight.

Click to expand...

I may consider a 36 inch addition to the airframe for J motors then.

I flew the Formula 75 for the first time yesterday. Great flier! I used a the Garmin DC 40 GPS tracker in the nosecone and glad I did. The 50" chute was way to big and the rocket drifted about a mile away. I jumped in the car with Astro 220 in hand and parked on a road next to a farmer's field. I was able to walk in on a dirt road another 320 yards and found the rocket thanks to the Garmin. Nice, long video HERE

Nice flight! One of the few descents I can watch without getting vertigo!

Thanks Jason.

Here's a pic of it on the pad with camera yesterday at Red Glare. Also the flight photo.

DavieRockets - thanks for the excellent build documentation. I referred to it several times during my build. I am using the Formula 75 for an L1 certification attempt on 4/4 on an H148 RMS with motor charge deployment. What size parachute do you use and how much black powder do you use in your RMS builds? My calculations show 1.3 - 1.4 grams. I know I need to ground test, but I am just looking for various points of reference. Thnx -

Hi Rob,

Nice to know that this old thread is helping people. I use a Rocketry Warehouse 45" parachute on my Formula 75 that weighs 68 Oz w/o motor. I have even flown it with a 36" in windy conditions, landing in a soft farmer's field. Aim for a decent rate of 20 to 25 FPS and you will be OK. I use Rocksim for my calcs but you can use a variety of other sources to figure this out. I fly this rocket with CTI motors and just use the BP quantity that comes with the reload and have never had an issue. Your calc of about 1.4 grams sounds OK. I tend to go 25% more than calculated especially with anything that uses shear pins and always ground test those designs. I think the saying goes "Blow it out or blow it up" and I want to be sure the nosecone comes off with enough force to pull out the laundry so to speak. Using motor deploy on a simple design like the Formula 75, there is less likelihood of a problem. It's also a very strong rocket. Good luck on your L1 attempt. The Formula 75 is a great choice.