4" Airframe x 75mm Motor Mount Scratch Build Thread

[GaryDean]

Now to the E-Bay itself, starting with the end plates. Picture attached shows my lay-out for the components on the end plates. Marked hole locations for the charge cannisters, drilled and cleaned holes, sanded contact surfaces, applied silicone adhesive to the faces of the charge cannisters, and installed with supplied bolts and locking nuts. Sanded the end plate surfaces for the terminal blocks along with the bottom surface of the terminal blocks, mixed some J-B Weld, and installed the terminal blocks. See 5th picture.
Now to wire the end plates. The nosecone end of the E-bay has Nylon Insert Lock Nuts on the threaded rods, along with 1/4" x 20 Nuts on the inside of the end plate, fixing the threaded rod in place, with blue threadlock on the inside nuts. So, this end is FIXED, and non-accessible. So, the wires to the NC end plate are igniter wire, nothing special.
The Aft end of the E-Bay has Wing Nuts to hold the plate against the coupler, which are removable, and also access to the E-Bay through this end. Therefore, has used the locking connector wires on this end, to easily allow the necessary access. In the Finished End Plates picture attached, so can see the use of the connector wires. Orange is for the Primary altimeter drogue / streamer ejection charge. Red (EMERGENCY) is for the Secondary altimeter altimeter drogue / streamer ejection charge. I always put a small knot in front of the terminal blocks to help prevent pull-through of the wires. Also put a small amount of silicone adhesive on the knotted wires to help keep them in place, and to also help in prevent ejection charge gases from entering the E-bay.

 

[GaryDean]

Now to the MissileWorks Sled. With the threaded rods attached to the nosecone end, I slide on the MissileWorks Sled, orientated with the rotary switch end towards the nosecone end plate. I put 1/4" nuts on first, then the sled, then 1 more 1/4" nut, locking the sled in the approximate position I expect, based on where the Switch Band holes will be positioned.

Now attach the altimeters, with provided screws, along with the rotary switches. For the rotary switches, I prefer to install them with the screwdriver ON / OFF slot orientated in line with the rocket. That way I "know" when they are OFF or ON.
Put the battery connectors on the 9V batteries. I usually tie a knot onto the red wire to prevent the stripped ends of the POS / NEG terminals from touching during the following procedures. Install the 9V batteries and cover plates with the provided screws.
Set up my soldering station to warm-up and started wiring the rotary switches.

- First, I connect the positive (+) battery wire into the marked location (+) on the altimeter, running the wire through the available guide holes.
- Second, I run the negative (-) battery wire to one of the 110 volt terminals on the rotary switch. I just fold the stripped end in place at this time.
- Third, I run a white igniter wire from the other 110 volt terminal on the rotary switch, to the marked location (-) on the altimeter.
- Note: All wire are cut to the shortest length necessary to reach their connection points!
- Forth, I solder the wires on the rotary switches.

2nd picture below shows the wiring between batteries, switches, and altimeters.
Now, attach the end plate Ejection Charge wires to the altimeters.

- For the Nosecone wires I cut the wires to the shortest length necessary, wrapping the wires around the threaded rod to provide some security.
- For the Aft End wires (orange and red connector wires) I obviously leave additional length (approx. 6 inches) to be able to put the end plate far enough away from the E-Bay to gain access to the connectors.

With wiring completed, it's time to check out the operation. I attached jumper wires at all terminal block connections to represent igniters. See attached picture. Now I checked the operation of the altimeters and the build quality by running both altimeters through the start-up sequence. See attached video. Both start-ups were perfect. Disconnect the jumper wires.

The Secondary Altimeter (Back-Up) must be programmed to delay the Drogue / Streamer ejection charge (DEPLOY MODE 2), and also to specifically to delay that ejection for 1 second (DROG DELAY 1). I attached my MissileWorks RRC2L LCD module to the Secondary Altimeter and ran through the programming sequence to make the appropriate changes, as shown in pictures attached.

That completes the E-Bay. I then assembled it and cut the threaded rods to the correct length with a metal cut-off wheel on my rotary tool.

 

[GaryDean]

Now would be a good time to complete the Tether Platform, by sewing on the tether. My design intent here is to have the tether hang just outside of the front of the Airframe, with the heavy shrink wrap in contact with the edges of the airframe. I am using the 2,500 lb Kevlar, along with 100 lb kevlar thread, my Quick Stitcher, the 1,500 lb swivel, and shrink wrap. This is a very heavy shrink wrap with adhesive on the inside diameter.
Started my effort by completing the connection on one of the welded eye-bolts and put the shrink wrap in place. Then assembled all other parts of the tether, confirmed the size (length), and started stitching the connection on the other eye-bolt. After a good portion of stitching I confirmed the size again, and when satisfied, cut off the unnecessary length of Kevlar. The 2nd picture below shows my stitching going towards the eye-bolt, which will then run back to the start point. It should be adequately strong. After completing that 2nd eye-bolt, I stretched out the tether, positioned the swivel in the absolute end, used small clamps to hold the kevlar firmly in position, and then ran a short series of stitches under the area of the shrink wrap just below the swivel. Then pulled out my propane torch and "shrunk" the shrink wrap.
Final stage was to sand the other diameter of the platform and clean with alcohol. Then, used sandpaper on the end of a wooden dowel to sand the ID of the airframe tube. Then it with alcohol. The location was easy to identify because of the Set Ring positioned previously.
Mixed a quantity of Bob Smith Industries slow cure 30 minute epoxy, applied with the wooden dowel to the airframe, and literally dropped the platform into place. Final picture (below) confirms my design intent of the tether contacting the airframe at the center of the shrink wrap.

 

[GaryDean]

Now to start work on some of the exterior design. I need some alignment marks for the E-Bay and Nosecone. Pulled out my aluminum straight-edge and scribed pencil lines, on the opposite side of the airframe from the rail button, exactly between the fins, through the E-Bay area and up through the Payload Tube / Nosecone interface.

Used gaffers tape to hold the tubes securely together and cut alignment marks at both locations using my rotary tool and Dremel Diamond Wheel. See attached pictures for the E-Bay area and Nosecone. I will later fill these slots with white paint to be easily visible.

The Rotary Switch holes need to be positioned. Used a pencil to draw a line running through the center of the Switch Band, in the area of the alignment marks. Measured the distance between the centerline of the switches on the MissileWorks Sled, and transferred that distance (Important: it is a horizontal distance!) onto the pencil line, centered correctly. Also marked and scribed locations for the shear pin (Aft end) and Retaining Bolt (Payload end). See attached picture.

Drilled the E-Bay Switch holes with a 15/64” diameter drill. This size was used because it works successfully on my previous builds with these rotary switches, and corresponds perfectly with my dedicated launch pad screwdriver! Note: I tried not to drill these holes at a 90 degree angle with airframe, but, rather straight down towards where the center of the switch face would exist. That effort is not really necessary, as it’s a simple matter to use a round file to accomplish the same, or to fix the hole when the drilling goes askew. After drilling the holes I assembled the E-bay to determine whether the location of the Sled needed adjustment for the switches to line up with the holes. Yes, about 1/8" of adjustment was necessary. The individually loosened the 4 nuts holding the sled in position, and put some blue threadlocker on the threaded rod, then tightened the nut. Attached picture shows the final results. This picture is misleading because of the angle. If you look straight down the holes, the switch is nicely in the center. Of course, it did take some corrective action with my round file to make that happen!

Sharp eyes might have noticed that I have removed all the blue tape from the E-Bay which was used as shims for fitting the aft airframe and payload section. The blue tape gave a dark “blotchy” appearance to the red fiberglass from the outside. Not desirable aesthetics to me. Removed the blue tape and re-shimmed the interfaces with scotch tape. I always use scotch tape on the surface as it is slides well against the fiberglass. The blue tape was just a filler so-to-speak.

 

[GaryDean]

Now to work on the Payload retention bolts. Rather than using the typical plastic push-pull rivets, I will use four (4) 4-40 Stainless Steel bolts / nuts with rounded heads. These steel bolts should provide solid retention for the upper section, while being easily removable for E-Bay access. Also adding epoxy dams / mounds on the ID of the coupler tube to provide a flat base for the stainless steel nuts, which will be epoxied in place. With this design, additional torque can be applied to the bolts. I have PEM nuts, but they are designed for flat stock, so I have a concern about curvature of the coupler, and whether they would permanently stay in place.

I am using four (4) 2-56 plastic shear pins at the separation point between the Aft Airframe and E-Bay. Again, using epoxy dams / mounds on the ID of the coupler. This epoxy is only to provide additional support to the shear pin, and prevent any flex of the plastic shear pin, prior to shearing during operation.

So, to get on with it, first mark the location for the retention bolt and shear pin holes. Using the location of the hole locations established in the last post, I wrapped my marked paper guide around the airframe and scribed locations for these bolts and pins. Mix up some J-B Weld, and, using these markings, I applied a small mound of mixed J-B Weld on the ID of the coupler, directly below the marks. Used a bright light to help establish the exact locations! As the epoxy became stiffer, I used a 1” wide flat wooden stick, dipped in alcohol to “flatten” the mounds. After the epoxy fully cured I used a metal file to establish a very flat surfaces on these mounds. See attached pictures.

In the next step I drilled and tapped the holes for the retention bolts, followed by inserting plastic 4-40 bolts, and then the stainless steel nuts. Using the plastic bolts gives me some safety factor for the next step of applying epoxy to retain the nuts in place. Mixed up some J-B Weld and applied it to 3 sides of the steel nuts from the open end of the coupler closest to the nuts. Then, got the backside with a longer wooden stick, being careful to not put epoxy on the plastic bolt.

After the J-B Weld cured, removed the bolts and re-tapped the holes to clean out any debris, and most importantly, was to establish a smooth thread guide between the airframe, coupler, and nut. I need these bolts to be easy to install and remove. Installed the steel bolts and snugged them down tightly in preparation for the next step.

Drilled and tapped the holes for the plastic shear pins. Then, installed the intended shear pins to confirm function. Everything came out quite nicely! Attached pictures to show the procedures, and a short video to show the final result for the retention bolts and shear pins.

 

[GaryDean]

Final work needed on the E-Bay is Static Port Holes. Used the sizing calculator on Off We Go Rocketry and it recommended three (3) 3/32” diameter holes. Hmm, nothing specifically wrong with that, but I went with four (4) 1/8” diameter holes. They are spaced in something of “X” pattern to be sure they are not across from each other. My only concern is that the barometers on the altimeters are located about 2 inches towards the aft end of the rocket, from the center of the switch band, where my static port holes are. So, to me, it’s like the barometers are in a “cave”, away from the holes. I will try to pay attention to any apparent delay in ejection charge activation in early launches. If an unacceptable level of delay occurs, might drill a couple of small holes towards the barometer end of the E-Bay.

I tried to improve the situation with the static port holes by carefully chamfering the fiberglass on the ID of the holes with a round grinding tool. This should allow more efficient movement of air within the E-bay.

Last but not least are the four (4) shear pins for the payload section / nosecone interface. Because it’s the same process as the Aft Airframe it’s not necessary to redo all the same words. Just;

• Identify the locations
• Apply epoxy mounds within the ID of the nosecone coupler directly beneath the marked locations
• “Flatten” the mounds with wooden stick, dipped in alcohol, and metal file the mounds flat
• Drill and tap the holes
• Insert shear pins to confirm function

Because I did the first 3 steps listed above at the same time as the previous post, this segment of work went very quickly! All I had to do was the final 2 steps. That’s what I just completed, with the attached pictures for reference.

 

[GaryDean]

First order of business this morning is to drill vent holes in the aft airframe and payload sections. I chose locations that were next to the charge cannisters to be sure of no blockage during flight. Used a 1/8" drill for the holes and used a round shaper to clean-up the internal fiberglass from the holes.

 

[GaryDean]

With some of the free time I had yesterday while epoxy cured, I stitched the shock cords for the drogue (streamer) and main parachutes. Used the Kevlar outlined in my original Bill of Materials (first post), the 100 lb Kevlar thread, shrink wrap, chute protectors, and swivel (main) to complete those items. The connection points for the streamer and main parachute are located 1/3 of the distance on the shock cord, from the Aft E-Bay connection point and the Nosecone connection point, respectively. Pictures attached. Now able to update my RocSim file with actual component weights, which are;
- Aft section (picture attached) = 2,747.26 grams
- Thrust Plate + Screws = 74.07 grams
- 75mm Motor Retainer + Screws = 128.49 grams
- Drogue (streamer) Recovery Items = 300.80 grams
- Main Parachute Recovery Items = 413.69 grams
- E-Bay Assembly = 740.25 grams
- Nosecone + Bulkhead + Mount Plate = 790.00 grams
- Eggfinder Transmitter + Mounting Tube = 93.15 grams
- Payload Tube + 4 Retaining Bolts + 4 Shear Pins = 556.45 grams
Total = 5,851.71 grams (12.9 lbs)

Used the information above to update my RocSim file (attached). Some of these numbers are likely to be revised slightly later, after completion of any future sanding and painting. But with these numbers my RocSim file is certainly accurate enough to give me confidence in evaluating the performance of any motor and launch condition.

Finally, used this opportunity to assemble the rocket (less dog barf), and check the CG. RocSim shows a dry (no motor, adaptor, motor casing, ejection charge) rocket CG of over 10 calipers. My actual measurement by hanging on a string shows 9.3 calipers. Both well above minimum requirements, but of course without any of the serious hardware. I always confirm CG ahead of the launch pad.

 

[GaryDean]

Meanwhile, further plans for today include a complete sanding of the exterior, including the nosecone. Started with 100 grit (picture attached), to be followed with 220, 400, 600 (wet), 1000 (wet), 1500 (wet), and finally 2000 grit, wet-sanded. Should result in a smooth surface for aerodynamics. Have a color (Rustoleum Metallic Pure Gold) selected for the fins area, nosecone, and switch band. Need something somewhat reflective for the Nosecone as the Florida sun is very unforgiving with polished black fiberglass, which results in very high internal temperatures while the rocket sits on the launch pad, frying the tracking device and battery inside.

 

[GaryDean]

Put in a couple of hours to complete the airframe sanding. For painting, sanded the fins section, Switch Band, and Nosecone, cleaned with alcohol, and masked with blue tape prior to applying the paint. Applied 4 light coats a few minutes apart, as recommended. I knew that I would not be painting those surfaces again, and was curious about the appearance. Allowed the paint to dry for 8 hours, removed the blue tape, and assembled the rocket to review the paint scheme. See attached picture of assembled rocket. I had an immediate and strong impression of ...... BORING! That is not acceptable for me!
Toyed with some ideas of black, more gold, triangles, chevrons, and even leaving it the way it is. Did finally make a decision, masked areas as necessary, broke out a new can of Gloss Black, along with another can of the Rustoleum Pure Gold. Sprayed some paint and giving it 48 hours to dry. The last picture shows the Aft airframe and Payload section painted in the new idea. It's probably really hard if not impossible to see the intended design on the Payload section. Also, it might appear that the Aft Section was being held hostage during the painting session. It's just that I used what was immediately available for masking to save time. No time to make any further paint revisions this week. I plan to launch it on Saturday, weather permitting. AND, assuming our motor vendor has something reasonable available.
Next update on Friday, to reveal the results of the latest painting, along with assembly of the thrust plate and motor retainer.

 

[GaryDean]

Now for the final assembly ahead of our launch plans tomorrow. Sanded 0.015" off the backside of the Thrust Plate, helping to ensure that all thrust loads are directed through the Airframe. Attached Thrust Plate with three (3) stainless steel screws. Used a straight-edge to confirm that no burrs stick "up" from the screwdriver tightening process. Assembed the 75mm Motor Retainer to the Thrust Plate using the provided 12 screws. No locktite, threadlocker, etc. was used on these screws. I will be checking them after each launch. Slid my CTI 75mm 4-Grain Case into the rocket and attached the Motor Retainer Cap, to complete the Aft End Assembly. See picture attached.
Gave the paint it's 48 hours, and peeled off all the masked areas. Identified the CP location from my RocSim file, and applied a marker onto the airframe assembly. It is no coincidence that the CP location just happens to be right at the aft edge of the black area. That way I always know where the CP is, even after my marker flys off. Assembled the rocket as shown in the final picture below. My intent on the upper section was to create an impression of the gold running down the airframe. Seems OK to me, not great. I think something more needs to happen on the Aft Section red fiberglass. Thinking of maybe 4 black, spiraling lines between the black stripes, 4 black spiraling lines that criss-cross, or even a number of black diamonds between the black stripes. LOTS of thinking before any implementing. Will paint some paper black and try out the ideas before putting anything more onto the rocket.
Hope to launch tomorrow with a lower thrust motor like J760WT (54mm 3-grain to approx. 3,500 feet) or K630BS (54mm 4-grain to approx. 5,000 feet) to keep the rocket visible during the acceleration and coast phases, apogee event, and behavior of the booster, nosecone, and streamer during the descent. Motor selection is also based on the fact that our motor vendor does not have any of the 75mm 2-grain or 3-grain motors I wanted. They will be ordered. Fingers crossed.
I am especially interested in whether the streamer provides adequate drag to keep the 4-finned booster from "floating" above the nosecone or streamer. And lastly, the performance of main parachute ejection at 600 feet. Will post any pictures that become available.

 

[pugachu]

Good luck on your flight!

 

[GaryDean]

Oh darn, if I knew the rain would come in, I would have launched this rocket as my first of the day, rather than my Wildman 5" Goblin. It did not happen today, hopefully on June 17th. I will post the details when it gets in the air.

 

[rfjustin]

Oh darn, if I knew the rain would come in, I would have launched this rocket as my first of the day, rather than my Wildman 5" Goblin. It did not happen today, hopefully on June 17th. I will post the details when it gets in the air.

Did this ever fly?

 

[GaryDean]

Did this ever fly?

Ah, ..... yes. On 6-17-2023 on a CTI J760WT to 3,124 feet, and then on 7-15-2023 on a CTI K630BS to 5,054 feet. My buddies tried for video as I focused on flight performance (launch, flight, drogue deployment, descent, main deployment, etc.). Excellent flights, but ........ NO useful video. Dang. But, yesterday (8-19-2023) corrected that video issue. Launched on a CTI L990BS with an excellent flight to 7,183 feet. I added weight at the Cg to keep the altitude down due to concern with wind and tracker unreliability. Attached video shows the launch. Will be at Airfest with L motors with increased Total Impulse, with a goal of 11 - 12K altitude. Also hoping for a video which provides a better presentation for this rocket! After L3 (different rocket) I can really push the limits, IF I am able to acquire the CTI motors I want..........