Scratch Built 24mm Near Minimum Diameter Rocket

[Earache]

I’ve long wanted to build a minimum-diameter 24mm rocket to use the amazing 6-grain 24mm motors CTI makes. I was originally going to make one of Wildman’s 24mm Blackhawks, but I waited too long (too many other projects) and it went oop. When it looked like SEMROC was going to close, the germ of an idea had formed in my head. I had built several of their Ready-To-Build rockets (to fly to over 1600 feet on Aerotech’s 18mm D motors) and realized that their #9 tube just fit inside their #10 tube. A #9 tube is a minimum-diameter tube, but it wouldn’t be strong enough by itself to survive a 6-grain G motor. However, a rocket made of concentric #9 and #10 tubes glued together could probably survive. I ordered four each of their #9 and #10 tubes, a number of #9 and #10 balsa tube connectors (BTC-9 and BTC-10), whatever #10 nose cones they had left, and a set of tabbed basswood fins (balsa fins would likely not survive a G-motor flight). If I recall they were SLS Javelin fins (FLV-16). I also bought an Aero Pack 24mm motor retainer. I know it adds drag, but the rocket was going to be near-minimum-diameter, not truly minimum-diameter, and I love the convenience. The Rocksim altitude was over 6000 ft, so dual deploy and GPS tracking made a lot of sense. The collection of parts has sat in my build pile for a long time. In the meantime, PerfectFlite released the StratoLoggerCF (SLCF), and Eggtimer Rocketry released the Eggfinder. They would both fit inside a 24mm tube. Yes, I know that Featherweight’s Raven will fit as well, at three times the cost of an SLCF. I prefer to destroy my Ravens (2 so far) in more expensive rockets. I have both an SLCF and a Raven for redundancy in my 54mm minimum-diameter Transonic II, which also has 4 custom-made (wire EDM) thermistor probes and processing electronics. It has measured the temperature profile at Lucerne Valley up to 12,500 ft AGL many times and the plots are available on-line.

Anyway, back to the 24mm rocket: The Eggfinder and the SLCF seemed the right choices. I finally got some time away from other projects to play with the parts and think through construction. Both the Eggfinder and the SLCF would fit in the tube, but how would I secure them and how would I power them? A standard 9V battery doesn’t come anywhere close to fitting. At Eggtimer Rocketry’s suggestion (in the manual) I looked up 300 mAh 2S 7.4V LiPo batteries. Venom makes one that fits very nicely in a 24mm tube, and I could pick up matching JST connectors on Amazon for a small sum. There would not be room for a mounting board or an allthread. What I decided on was a forward balsa bulkhead with a screw eye followed by the battery, followed by another balsa bulkhead for the screw switch, then the SLCF, and a rear bulkhead with a screw eye.

My final challenge: How to feed power for the ejection charges through the bulkheads. I considered lots of options including very small eurostyle connectors. I couldn’t figure out how to attach them to the balsa. While playing around with the #4 standoffs for my other e-bays, I wondered if there were ones long enough to go through the bulkheads. A quick search on McMaster led me to the needed parts, 91115A519, 3/4” long hex standoffs (round ones would be hard to keep from rotating). I cut one of the balsa bulkheads in half and trimmed each half to just over 3/4 inches, drilled slightly undersized pilot holes, and press fit the standoffs using my vise. I then sanded one end to just expose the standoffs. I then added the screw eye, being sure to avoid contact with the standoffs, and put a thick layer of glue on the outward facing end of the bulkhead. Balsa is porous and would actually let the pressure and gases from the ejection charge leak into the e-bay without the glue layer. The pictures below show the balsa tube connector with the cutting line, the standoffs and the sized bulkheads, and the finished bulkheads with feedthroughs. Believe me, the feedthroughs are not coming out.

 

[Tonimus]

Just curious, what is the stability margin with your selected motor?

 

[Earache]

2 calibers.

 

[Earache]

Next was figuring out how to mount the Eggfinder and battery. I looked and realized that the antenna was shorter than my chosen nose cone. If I drilled a hole in the nose cone and angled it slightly toward the tip, I could insert the antenna and save the length from the rocket body. The final design was to mount the Eggfinder against the nose cone with the antenna protruding into the nose cone, follow the Eggfinder with a Dremeled-to-fit balsa bulkhead, and then another 2S LiPo. The payload bay would have a balsa bulkhead glued in place with a screw eye at the end.

I had several places where the different sections of the rocket would need to be joined: the nose cone to the payload bay, the payload bay to the forward body tube containing the main chute, the forward body tube to the electronics bay, and the electronics bay to the rear body tube with the motor. Because you can cut the #9 inner tube and the #10 outer tube to different lengths, I could make these joints any length I wanted. I settled on 1-1/4 inches (except for the nose cone shoulder where I chose the length of the shoulder).

 

[Earache]

I calculated and cut all of the tubes to length and test fit everything together from the nose cone to the Aero Pack motor retainer. Everything fit with no gaps. It was almost time for the most harrowing part of the entire process, gluing the inner and outer tubes together. There were two principal challenges: determining where to spread the glue and how to push the tubes together to avoid big glue blobs on the inside, and how to push the tubes together without the glue seizing before they were in the correct position. However, before I did that, I had to consider the fins and how to attach the shock cord in the bottom body tube with the motor mount.

The fins had tabs on them for through-the wall mounting. The tabs were just slightly longer than the body tube thickness. Should I have them go through the #10 tube and partway into the #9 tube? Should I have them go through the #10 tube and sand them down to just contact the #9 tube? How do I cut the slots in the outer tube and not get glue in them when I slide/glue the two tubes together? Just how hard is it to cut precise slots in a tube with an X-acto knife anyway? What I finally did was the following: I used the fin guides from several of the RTB SEMROC kits that I had already built to mark the locations on the outer tube. I have a nice piece of angle iron I use for marking fin lines. I then numbered each fin, held the fin centered on the fin line and traced the outline of the tabs. I then carefully cut out the slots with an X-acto knife. I used a slightly undersized dowel longer than the tube inside the tube to cut against. I supported the ends of the dowel and let the tube hang as I cut. I then glued the fins in place and used one of the SEMROC fin guides to get them pointed out straight while the glue dried. I then used a dowel with sandpaper taped on the outside to sand down the fin tabs until they were flush with the inner tube wall.

 

[Earache]

Since I don’t plan on using motor back-up on recovery, I chose to make one more balsa bulkhead with a screw eye for attaching a shock cord. I calculated where the bulkhead needed to be to give the longest 6-grain motor case about 1/2 inch clearance. I spread a layer of glue with a marked craft stick and used the motor casing to push the bulkhead into place in the #9 tube. I then glued the Aero Pack motor retainer onto the opposite end of the #9 tube with JB weld.

 

[dhbarr]

Neat! What motor are you planning for?

 

[Earache]

Neat! What motor are you planning for?

CTI's full designation is the Pro24 144G65-8A. It's the G65 6-grain 24mm motor. All of the other 6-grain have a thrust over 80N and require Level 1 or higher. I'm TRA Level 2 but I like to go out and fly by myself without an FAA waiver, and the G65 will let me do that.

 

[Earache]

It was time to face my fears and slide and glue the #9 tubes inside of the #10 tubes. Except for the bottom body tube/motor mount, I chose to put the tubes together without attaching anything else first so I could make a new one if things seized up. I started with the payload section. I pushed the nose cone into the end of the #10 tube and barely started the #9 in the opposite end. I then put a generous layer of glue on the #9 (things tend to seize up when the glue is either too thin or too dry) and pushed the #9 into the #10 until it hit the nose cone. I then wiped off the excess glue. The next was the electronics bay. Since both ends of the #9 tube protruded past the #10 I marked the #9 with lines indicating when to stop. I put the #9 into the #10 to the first stop line, spread a generous layer of glue on the outside of the #9 down to the second stop line and then pushed the two together until the first stop line protruded from the opposite end of the #10 and wiped away the excess glue.

Once these two had dried, I did the upper body tube. I placed the Payload section into one end of the #10 tube and started the #9 in the opposite end. I then put a generous layer of glue on the outside of the #9 and slid it into the #10 until it hit the payload section. I had to push the #9 tube inside past the end of the #10 tube. Fortunately, it didn’t seize up.

I was left with the most challenging, the bottom body tube with the fins on it. If this one seized up, I would have to spend a great deal of time and money to fix things. I started the #9 inside the #10, spread the glue and started the slide. It got harder and harder to push. With about 2 inches left it wouldn’t slide at all!! I really panicked. I then realized that I had been squeezing the end of the #10 with my hand. I relaxed my grip and the #9 tube slid the rest of the way in, all the way to the Aero Pack. Whew!

Fillets: It was time to think about adding fillets. Rocksim was reporting a peak speed of 700 mph, so the fillets would need to be reasonably strong. The usual model rocket practice of using white or wood glue for fillets didn’t seem strong enough. On the other hand, the high power rocket practice of epoxy fillets seemed excessive. When I had ordered some chopped glass fiber to make fillets on my ill-fated Mariah (but I did ground-test the ejection charge several times ), I had purchased some powdered phenolic. A roughly 50-50 mixture by volume of phenolic powder and wood glue, when shaped by the rounded end of a tongue depressor will dry to a fillet that looks and feels about right and can be sanded. I used blue tape that I had pressed against my forehead several times to make it less sticky to make borders for the fillets. If you don’t make the tape less sticky, or if you don’t pull it off the paper tubes immediately after pulling the fillets, it can pull up the top layer of paper from the tube and leave a mess. Here are photos of the resulting fillets.

Rocksim was telling me that my rocket was way overstable with the original fins. I did some exploring and found that cutting the fins to a 1-inch semi span would give me two calibers of stability and give me close to 1000 feet more altitude so I trimmed the fins to a one inch semi span.

 

[dhbarr]

CTI's full designation is the Pro24 144G65-8A. It's the G65 6-grain 24mm motor. All of the other 6-grain have a thrust over 80N and require Level 1 or higher. I'm TRA Level 2 but I like to go out and fly by myself without an FAA waiver, and the G65 will let me do that.

That's my second favorite motor! It'd be tied for #1 if they'd recert it in a no-haz configuration. They're pretty hard to come by right now, though :-(

 

[Earache]

That's my second favorite motor! It'd be tied for #1 if they'd recert it in a no-haz configuration. They're pretty hard to come by right now, though :-(

I picked up two of them at LDRS in June.

 

[Earache]

I thought about creating a launch tower for this rocket, but it seemed like a lot of work for a paper rocket. With G motors, the standard 1/8 inch, or even the 3/16 inch launch lugs seemed too small, so I used a pair of Estes 1/4 inch launch lugs. The Aero Pack stuck out from the body tube enough that I needed a little clearance. I cut the tips off of two craft sticks and sanded a taper on the rounded ends and glued them between the launch lugs and the lower body tube.

 

[Earache]

Attaching everything: I decided to use 100° #4 flathead screws to fasten the nose cone to the payload tube, secure the bulkheads in the electronics bay ends, secure the forward body tube to the e-bay, and secure the bulkhead with the switch to the electronics bay. I purchased a six-fluted countersink from McMaster along with a .089 drill bit for making tapping holes, a 4-40 tap, and both 4-40 1/4” 100° nylon and 4-40 1/4" 100° steel flat head screws. The nylon were for the nose cone where they would be near the Eggfinder antenna. I knew from experience that the holes would be easier to machine and cleaner if I waited until after I had painted the rocket.

I have tried most of the recommended methods for removing the spiral grooves and for sealing the balsa. My favorite is to use Rustoleum’s Filler Primer Spray after having sanded out any major flaws. Unfortunately, I can rarely remember where to buy the Filler Primer (Home Depot) and Rustoleum makes several other primers that don’t work as well. I scoured the local Walmart and picked up two cans of Rustoleum Sandable Primer. After three coats with vigorous sanding in between I could still see most of the spirals and balsa grain. When I’ve used the Filler Primer, everything is smooth after two coats. I’m not obsessive about appearance so I went with the three coats of primer and some visible lines. A coat of gloss white went over that for all but the nose cone. I painted the nose cone gold. I then played with a glass sheet, a ruler, blue tape, and X-acto knife to mask for three coats of Fluorescent Orange. Finding rockets on a huge dry lake bed works really well with fluorescent orange and white.

Once all of the paint was dry, I went into the machine shop at work (available for personal projects when not otherwise engaged) and used a mill to drill, countersink and tap three holes to attach the nose cone to the payload, two to attach the forward bulkhead to the e-bay, two more to attach the forward body tube to the e-bay and forward bulkhead, one in the e-bay to hold the switch bulkhead in place, and three more to attach the rear bulkhead to the e-bay. I also drilled one hole to access the screw switch. It will be covered with a small piece of tape for flight. I finished with three small holes for venting the e-bay and one each for venting the forward and rear body tube. I could probably have gotten by without the last two, but Rocksim was still predicting 6500 feet on a G65, and the vent holes should prevent premature ejection.

Balsa won’t hold threaded holes for long, and holes in paper tube are subject to wear, so I put gap-filling CA in each of the tapped holes and they re-tapped them after they had dried. I also put a touch of gap-filling CA on each of the countersunk holes. Super thin CA soaks the paper better but it tends to go where it’s not wanted, and the gap-filling works quite well.

Here is the payload section and nose cone with the Eggfinder, bulkhead, battery, and foam cushions.


Here is the upper body tube. I have really grown to like PML’s piston ejection system, so I created a piston for the upper body tube from a balsa tube connector cut in half. The trick is to sand it so it slides freely, but doesn’t have enough gap to trap the 200# Kevlar shock cord. The main chute will go here as well.


Here is the electronics bay with the forward bulkhead, the battery, the screw switch bulkhead, the StratoLoggerCF, the rear bulkhead, and some more foam spacers.


Here is the lower body tube with the six-grain motor case, and the Aero Pack motor retainer. At apogee I plan on just separating the rear body tube from the e-bay with no chute or streamer. With 10 feet of 200# Kevlar between them, they should fall at a reasonable-but-not-too-fast rate.


Finally, here are the sections assembled with all of the electronics inside,

and the completed rocket.

 

[Earache]

What’s left to do is ground test the ejection/recovery systems to determine the correct amount of powder, and go fly the thing. I plan on a shake down flight with an Aerotech F39-8T, which Rocksim predicts will go to just under 2600 feet with five calibers of stability margin. If everything is working I’ll go with a CTI F51, predicted to 3900 feet with 3.6 calibers of stability margin. If I recover everything in flying condition, it’s on to the CTI G65 with 2 calibers of stability. Rocksim says 6839 feet and 694 mph. Since it is a G motor and the motor-rocket combination is under 1500 g (actually 415 g) I don’t need an FAA waiver. I will probably sneak out to Lucerne Valley in the middle of some week when the conditions are predicted to be favorable and fly it by myself. I’ll be sure to post how it works.

 

[BuiltFromTrash]

Nice project! Poke around a little and you will find a few of these projects on the forum. I really like the paint job.

 

[Earache]

I tested the ejection charges for the rear (apogee) and front (main chute). I was going to test 0.15g, 0.20g, and 0.25g until I found the amount that ejected the relevant section with just enough force to reach the end of the kevlar shock cord. Rear turned out to be 0.20g and front was 0.15g. Here are links to the videos: Rear <[video]https://youtu.be/84-8L2ib5x4[/video]>, Front <[video]https://youtu.be/WZv_cYn51u4[/video]>. I still need to find time for the test flights.

 

[Earache]

I went out to Lucerne Valley on Thursday. The weather was perfect: clear with almost still air. I loaded an Aerotech F39-9T for the maiden flight. Rocksim was predicting 2600 feet. Here is the rocket on the pad . The dual deploy worked perfectly. At apogee the two parts separated and fell linked by 10 feet of kevlar. They never tangled or approached other. At 700 feet the main deployed. Here is a shot under the main chute . And here is a shot on the ground . The StratoLogger CF reported 1808 feet, about 800 feet short of the Rocksim prediction .

With the shakedown out of the way, I went straight to the CTI G65 with a Rocksim-predicted apogee of 6500 feet. Well, things didn't go as expected. The launch lugs ripped off and stayed on the launch rod , while the rocket did a spectacular skywriting session. The dual deploy still worked, but the chute fully deployed just as the bottom section touched down. Here is the complete rocket on the ground . When cleaning out the motor casing, I found that the liner on the G65 had burned through over a fairly large area . The casing seems OK other than a large char deposit that won't come off in spite of vigorous scrubbing. The SLCF report an apogee of 531 feet, but you can see it was a wild ride . Other than the launch lugs there was minimal damage. I will try some launch lugs that don't have such tight clearance on the rod, or perhaps just one launch lug strategically located. I'll let you know how it all works.

 

[SkidmarkMonster]

I love your project! I'm a sucker for MD rockets. I like how you made it so compact

 

[Nytrunner]

Is there room to fit some buttons on?

 

[Earache]

Is there room to fit some buttons on?

There could be. The double wall gives me a little to play with. I'm not sure buttons with screws would be stronger than glued-on launch lugs. I'm debating sanding off the paint and putting a single layer of fiberglass around both the body and the lugs. That violates the wood-glue and cardboard ethos a little, but I bet they'd stay on.

 

[boatgeek]

CTI's full designation is the Pro24 144G65-8A. It's the G65 6-grain 24mm motor. All of the other 6-grain have a thrust over 80N and require Level 1 or higher. I'm TRA Level 2 but I like to go out and fly by myself without an FAA waiver, and the G65 will let me do that.

FWIW, you won't need an FAA waiver for any of the 6G 24mm CTI motors. Though they are nominal HPR because of high thrust, FAA only cares about propellant and rocket weight. Those motors are ~66g propellant, way below the 125g threshold for high power, and I'm quite sure your rocket is under 3 lbs. So fire away with any 24mm motor that fits your field.

 

[Earache]

FWIW, you won't need an FAA waiver for any of the 6G 24mm CTI motors. Though they are nominal HPR because of high thrust, FAA only cares about propellant and rocket weight. Those motors are ~66g propellant, way below the 125g threshold for high power, and I'm quite sure your rocket is under 3 lbs. So fire away with any 24mm motor that fits your field.

Good to know. I've wondered about that section of the code. Now, I'll just need to find some in stock.