2.6-inch 29mm MPR Scratch Build

[Dane Ronnow]

Before jumping into the nuts and bolts of this build, I want to give a little background on the design. It started out with a cluster of three 24mm motors. Early sims in OpenRocket looked good with the Estes E9-8, and not too bad with the D12-6 and E12-6. Those were a little hot on deployment velocity and much faster off the launch rod, but the E9-8 was beautiful—low and slow.

But there was a problem. Through discussions on TRF I discovered the E9 was prone to failure, specifically CATOs. Other discussions pointed to the difficulty of getting three Estes igniters to light simultaneously, followed by a discussion of adding some type of pyrogen to the igniters. And my sims with the D and E12 were not working (I couldn't get away from high deployment velocities). Frustration was rearing its ugly head. Then, in another discussion (I forget what it was about), @neil_w suggested trying an F67.

It took me about three minutes to come to grips with the idea that flying this rocket was going to be a little more expensive than I had planned, and another minute to consider the idea of cost-to-benefit. Then I dumped the idea of black powder motors for this rocket and redesigned it with a 29mm motor mount for the F67, later adding the G74 and G80 motors to the flight configurations. As far as expense was concerned, I figured I would limit my launches to two or three per outing, as opposed to six or eight. Time will tell if that holds true.

Anyway, I would have stumbled into composite-motor rocketry sooner or later. neil_w helped me get there with a minimum of time wasted. (Thank you, sir.)

Presently, the basic design is as follows:

Diameter is 2.6" (BT-80), with two body tubes—14" upper and 13" lower—joined by a 4" ejection baffle. Three .125" balsa fins on the back end, and a 12" ogive nose cone (9" with 3" shoulder) on top. Motor mount is a 4.25" thick walled tube with .125" plywood centering rings, and an Aero Pack motor retainer. Two 2" cardboard launch lugs 9" apart.

Total length is 38.25". Weight without motors is 19.3 oz. Calibers of stability are 1.79 with the F67, 1.74 with the G74, and 1.52 with the G80.

For those who are interested, the ORK file is at the bottom of this post.

Okay, now to the nuts and bolts.

I started by cutting the body tubes to length. (Before I could do that, I needed to finish sewing the parachute—a 36" ellipsoidal—so I knew how much space it would occupy in the upper tube.) Nothing noteworthy about cutting the tubes, so no pics. Two 18" slotted BT-80s from Apogee. I cut 4" off the bottom of the upper tube, and 5" off the top of the lower tube.

Next up, the motor mount. (Note: I started assembling the motor mount and the ejection baffle before I decided to document this build. So I don't have pics of the unassembled components. But you'll get the drift.)

Motor Mount

This is an Apogee Components motor mount, with a thick-walled tube, 7" long, and two .125" plywood centering rings. I don't know if I got an odd one, or if all of Apogee's 29mm BT-80 motor mount kits are like this, but the CRs were extremely loose on the tube, with a 3/32" gap.

After lightly sanding the tube and marking the CR placements, I glued (with Elmer's All Purpose) two layers of 24 lb. copy paper .187" wide around the tube at each CR location. This gave me a much nicer fit.

After dry fitting the Aero Pack motor retainer with the tube and aft CR, I epoxied it in place with J-B Weld, followed by the aft CR. I let that dry 24 hours, then epoxied the forward CR (J-BW). After that cured, I trimmed the tube to 4.25", then applied fillets to both CRs, and coated the front side of the forward CR with a thin film of J-BW to give it a little heat protection against ejection charge gas.

The reason for the short tube (barely 1/8" protruding through the forward CR) is that it allows me to rid the lower body tube of the ejection charge cap by simply shaking the rocket.



TTW Fins

I wanted to test fit the fin tabs in the motor mount CRs, so the fins are next. I lightly block-sanded a piece of .125" balsa, then cut three fins using a template printed out of OR. I rounded the leading edge with #320 sandpaper.

I wanted to paper these, and had read a boatload of forum comments concerning that process. I was leaning toward 24 lb copy paper—just a touch heavier than standard copy paper—with Elmer's Glue All or TiteBond II, then thought about Spraymount. I ditched that idea because I've worked with it before and I didn't want to mess with overspray.

In the end I used Avery 5265 full-sheet label paper. (Come to think of it, I think @neil_w turned me onto that idea too.) I cut single pieces that would wrap around the fin's leading edge and extend down both sides to the root. Applying them to the fin was a little tricky—there is no repositioning this stuff once you lay it down. So, getting the fin's leading edge precisely centered on the center of the paper is paramount.

Once in place though, it's nice stuff. (The paper weight of 5265 is 50 lb., so there's a lot of stiffening going on.) I squeegeed the paper from the leading edge to the root, then pressed the fins just to make certain there would be no warpage from the paper's adhesive.

The tabs were left bare, as well as .187" at the root for the CR fillets. With the paper applied, tab corners were trimmed to allow for the fillets.

Next, I tested the fit of the fins and motor mount.



With the fins finished and fitted, I turned to the ejection baffle.

 

[Dane Ronnow]

Ejection Baffle

(Here again, I began assembly before I started taking pics.)

This is an Apogee BT-80 ejection baffle. I'm using a 5/32" eye bolt for shock cord attachment, rather than the oversized (for my use) eye screw that came with the kit. The eye bolt passes through the baffle plate and reinforcing disk, with a flat washer and nut on each side of the plate. The lower nut is secured with a little J-BW. A dab of J-BW is also applied to the eye to eliminate the gap in the wire.

I modified the design in such a way as to eliminate particle build-up in the body tubes. (I don't know why I dislike crap accumulating in the rocket, but I do. And I will expend a little extra effort to keep that from happening.)

The short tube (1" BT-55) on the aft baffle plate is a choke. It concentrates the flow of hot particles and directs them into the longer tube—the collector—(1.75" BT-60) on the forward baffle plate, where they hit a solid wall (the gas vents are around the perimeter of the plate). After a flight, the particles that remain in the baffle can be shaken out the top of the upper body tube. Any particles that drop back through the baffle to the bottom tube can be shaken out through the motor mount after the motor is removed, along with the ejection charge cap.

The choke and collector tubes are epoxied in place with J-B Weld for heat resistance. Because it is facing directly into the blast of the ejection charge, the back side of the aft baffle plate is coated with a thin film of J-BW.



After the J-BW is fully cured, the edges of the baffle plates and the outside of the tube coupler are masked. Then everything is painted with high-temp paint.



Speaking of high temp, I stepped out onto the back patio to spray paint, and was reminded why we do things like this early in the morning in Las Vegas. This was June 6th. I can't wait for July. Then August?!



These are the components ready for assembly:

 

[Dane Ronnow]

Next, I cut a .375" wide strip of tube coupler. This is glued to the forward end of the baffle tube (above the forward plate) to give the plate more resistance to the shock of parachute deployment. I know this is probably overkill, but being a frequent victim of Murphy's Law, I prefer to err on the side of caution. Besides, it adds just .04 oz. of weight. Elmer's Carpenter's Glue, maybe another .02. Extremely small price in altitude for a little extra confidence.

There are a number of ways to cut a cardboard tube so it has a clean, even edge. The easiest one for me is to wrap the tube I'm cutting with another tube that has an uncut end. (I used the piece left over from trimming the lower body tube.) I position the uncut end on my cutting mark, secure the outer tube to the inner tube with tape, then use an X-Acto blade to cut along the edge. I make the cut slowly, taking four or five passes to cut all the way through.



Next, I place the cut ring inside the baffle tube and mark the overlap. I cut about 1/16" short of the mark, otherwise, the ring will end up with a gap. After applying a thin bead of Elmer's CG to the baffle tube, I press the ring in place, lining it up with the top of the tube.



Now to assemble the rest of the components. The forward baffle plate will come in from the bottom. This maximizes the epoxy's adhesion between the plate, tube and ring. I used BSI 30-minute epoxy for this, as well as for the aft plate, which is epoxied in place after the forward plate has cured.



With regard to weight, this baffle as shown above, weighs just .45 oz more than Apogee's stock baffle (1.05 oz.). I needed a little extra weight forward of the CG, so this works nicely.

That's it for now. I'll have the next part of this post ready soon—epoxying the motor mount in the lower tube, and joining the upper and lower body tubes with the baffle. Stay tuned.

 

[Bruiser]

I've never seen that baffle design before. I like that you can shake out any particles. Cool design.

Neil got me started on the composite motors too. I resisted at first but then Hobby Lobby stopped carrying the Estes 29mm motors. The "seed" Neil had planted blossomed when that happened.

Build looks great,
-Bob

 

[Dane Ronnow]

I've never seen that baffle design before.

I'd say it's my design, but it's entirely possible that someone else is doing the same thing. It's simple, lightweight, and while I haven't put it to use yet, it appears that it would completely contain the particles without impeding the expansion of pressure at all.

Regarding @neil_w, he's chimed in on a number of my posts. Always with good info. And while I may not learn anything new, he usually gets me thinking. And in fairness to others, it's not just Neil. I've learned a ton of stuff on these boards, from a lot of different people. It's a great place to ask questions, or just read about what others are doing.

 

[Dane Ronnow]

Lower body tube reinforcement

I got ahead of myself when I said the motor mount and ejection baffle installation would be next. Before I do that, I'm going to epoxy a 4" tube coupler into the lower body tube ahead of the motor mount. The reason is that this body tube is thin-walled and very light, and the tube coupler will reinforce the lower BT at the point where all the energy from acceleration is focused. Furthermore, when the motor mount is installed and butted up against this coupler, it will reduce the stress of launch on the adhesion between the CRs and the BT wall.

I'm using 30-minute epoxy because Elmer's All Purpose shrinks when it dries, leaving the outside of the body tube dimpled. And Elmer's Carpenters Glue is known to seize when gluing couplers inside body tubes, locking the coupler before it's in place.

First, I'm going to spray the interior of the coupler with high temp paint. So I'll mask the outside of it to keep paint off the surface that will be epoxied.

Once the coupler is painted, I lay everything out in front of me and walk through the steps in my mind. (I don't want to get halfway into pushing the coupler inside with two fairly large beads of epoxy in the process of being smeared, only to remember a step that I missed.)



I've got a 7/32" dowel, 10" long, that I'll use to apply the epoxy inside the tube. The end is rounded, making it easy to roll the epoxy off the stick and onto the tube wall. And I've marked it with masking tape in two places to help me gauge the depth at which I want the beads applied—4.25" from the aft end of the BT, and another at 6".

Next, I mix the epoxy and hardener in a small cup on a scale. I place the empty cup on the scale, hit the tare button to zero the scale, then squirt in the epoxy, note the weight, and squirt in the same amount of hardener. Then mix it thoroughly with a stirring stick.

Now comes the hard part—seeing exactly how much epoxy is being laid down in a bead that is more than 4" from the end of the BT. The important thing is to keep the epoxy away from where the motor mount's front CR will come to rest.

With two beads inside the BT, I slide the coupler in, then push it into position with the motor mount, watching the fin slots as the CRs pass through. When the CRs are exactly at the ends of the slots, I know the coupler is in place. Then I pull the motor mount back out and let the epoxy cure.



Once this is cured, I'm ready to install the motor mount, and, after that's cured, the ejection baffle.

 

[Bruiser]

Just curious about the high temp paint you are using? I've been coating some parts with JB Weld but high temp paint sounds lighter...

-Bob

 

[Dane Ronnow]

@Bruiser -- This is the paint I used: https://www.amazon.com/dp/B0010O0C94/

The paint is lighter than J-B Weld when applied the same way you would spray any other kind of paint—two or three coats. With a half-dozen heavy coats, the paint would surely be as heavy, perhaps heavier, than J-BW. And for the sake of comparison, I'm thinking of J-BW spread very thin, like squeegee thin.

Here's my reasoning when it comes to high temp paint, J-BW, or anything else used to protect components from the ejection charge:

Consider first the two aspects of ejection—the flame, which is brief but intense, and the hot gas, which is not as intense, but lasts longer.

Any surface that is exposed to those is going to deteriorate with each successive ejection. How much, we don't know. (At least I don't.) So anything we apply—paint, J-BW, or anything else that is heat proof—will slow down that process of deterioration to some extent.

In my case, the bottom side of the aft baffle plate will take a direct hit from the flame, as well as be exposed to the gas. I used J-BW on that surface because it not only coats the surface like paint would, but seals it too. The paint won't seal like that unless it's sprayed very heavily in four or five coats. At that point, the paint would be as heavy, or heavier, than J-BW.

The walls of the tube and coupler are not taking the direct hit like the baffle plate, but are exposed to the brief flame and the gas. And because they are untreated cardboard, browning from the heat and flame over time, and eventually, perhaps, burn-through, will occur. Paint will slow that process. And truthfully, any paint will provide some protection. But the high temp stuff obviously will bear up under the heat and flame better.

FWIW, I considered using aluminum foil on the walls of the tube and coupler. If it stayed intact, it would be better protection (with shiney side exposed, not the dull side). But particles from ejection, though very small, are moving fast, and could punch small holes in the foil. Once those holes are present, subsequent bursts of pressure from ejection could tear the foil. If that happens, I've got the problem of pieces tearing loose and possibly blocking the vent holes in the baffle. (All this in an area that is inaccessible once everything is assembled, so repair is impossible.)

So, in the end, the question is how long will cardboard hold up under those extremes? Anything I can do to extend the life of the cardboard walls is worth it, as long as it isn't adding significant weight.

 

[icyclops]

On exposed baffle surfaces like this, I apply with a squeegee a very thin coat of JBweld and then place heavy duty aluminum foil over all internal baffle parts before assembling it. I have had actual burn through of thin JB weld on cardboard as the cardboard can give up behind the JB weld and then allow the gas to burst through the weakened areas after many uses. The heavy duty foil prevents this to a greater extent as it provides one more barrier. Do not use the cheap foil, use good Reynolds heavy duty ply foil or you might as well just use JB for the whole thing. Cheap foil will burn up over time as you already mentioned. Also, you can laminate 2 layers of foil for more protection...JB, foil, cure, then JB, foil, and cure. Adds more weight but the JB sticks to Al really well sandwiched like this.

I find that with even 5-6 coats of High temp paint the cardboard behind it still becomes weak and shows signs of stress and burn through after multiple uses, especially with bigger motors. Many of my baffles are removable so I can inspect them so this is how I know. And I can replace them when they become worn. Your design may not allow for this but something to think about for future building.

Good luck and nice job so far.

 

[Dane Ronnow]

@icyclops -- You make very good points. When it comes to J-BW or high temp paint, or a combination of the two, I think we're just buying a little time. Sooner or later, the cardboard weakens, and eventually fails.

Foil, as you mention, is a better solution. The problem I was having with it was how to apply it down inside a body tube. Still, I know it can be done, and your description of the process is easy to see. Definitely heavy duty foil. And I will keep this in mind for future builds.

Thanks for your input. It's a good solution to the problem of heat.

 

[Dane Ronnow]

Motor Mount Installation

Installing the motor mount is fairly straightforward, and basically the same process as with the tube coupler—two beads of 30-minute epoxy, then push the MM home.

Lining up the MM with the fin slots shows me where my epoxy needs to be—slightly behind both ends of the fin slots. With the rounded dowel I apply the first bead in between the tube slots, about a quarter-inch behind the forward end. Then I insert the MM halfway—two inches behind the first epoxy bead—and let it hang over the aft end of the BT. Then I apply the second epoxy bead along the sidewall, just inside the end of the tube, rolling the tube as I go. The MM will stay at the bottom of the tube as I go, away from the epoxy.

After the second bead is ready, I push the MM in until it butts up against the tube coupler. A glance through the fin slots shows me the CRs are positioned exactly at the ends of the slots.



With that, it's good to go. I let this cure for 24 hours.

Ejection Baffle Installation

Before the baffle is installed, I want to paint the inside of the body tube with high temp paint, from the lower tube coupler (which is already painted), up to 2" below the top of the tube. That top 2" is where the epoxy will be applied.

I cut a 2" wide strip of copy paper and tape it inside the BT, aligning the edges of both. Then I wrap the outside of the BT with any kind of paper that is coated stock (glossy), which is less absorbent than newspaper. Finally, I wrap the top edge of the coated paper over the end of the BT, and tape it down to the copy paper.



Then I shoot the paint, holding the body tube horizontally, so the paint can is upright. I let this dry for about 10 minutes, then remove the paper.



After the paint has dried another 30 minutes or so, I apply two thin beads of epoxy to that 2" area at the top of the BT, one bead .125" from the top, the other bead 1" below that.

After marking the center of the baffle—2" from the ends—I slip it into the BT, twisting it as I push it to the mark.

Next, I want to make sure the baffle isn't cockeyed at all. I let it sit for about 10 minutes so the epoxy is beginning to thicken, then carefully slip the upper body tube over the top of the baffle, being careful not to push the baffle deeper into the lower BT.

Two ways I know of to align the tubes: first, place a long straight edge along the tubes. This, of course, depends on the straight edge actually being straight. (Don't laugh. I've seen plenty that aren't.) Roll the tubes 90 degrees and place the straight edge again. Then another 90, and so on.



The second method is actually easier, provided you have an absolutely flat surface. Just roll the tubes (like you did with a cue on a pool table when you were using the house sticks). Any bend between the tubes will be obvious, as they will flop instead of rolling smoothly.

If the tubes flop, roll them slowly back and forth a few times with gentle pressure. Then give them a push, letting them roll by themselves. If they roll smoothly, you're good. If not, roll them again with your hands, applying a little pressure.

Using this method doesn't really depend on both tube ends being absolutely square where they meet. If they're not, and you're rolling the tubes to bring them into alignment, you'll notice a slight gap somewhere in the joint. Ignore this. Later, when the tubes are joined permanently, you can fill the gap with whatever you use to fill spirals. (If you fill spirals.)

Anyway, once I had the alignment right, I let the epoxy cure 24 hours.



My plan is to primer the fins before I attach them to the body tube. They're easier to block-sand without stressing the joint. Then I'll prime the body tubes, get the worst of the dings out. Not sure if I'm going to fill the spirals. I've got a nice paint scheme in mind, and it would look pretty sharp with a glass-smooth finish. We'll see.

 

[fyrwrxz]

Really nice, thoughtful build! One thing you may consider is conformal launch lugs or rail buttons. 1010 rail is considerably less flexy than a rod. Straight smoke and good chutes!

 

[Dane Ronnow]

@fyrwrxz - The conformal launch lugs are interesting. From a quick search, I'm seeing lugs for 2.12", 2.26" and 3" body tubes. I'd be doing a little sanding if I want a precise fit (my tube is 2.6").

How would you attach ABS plastic to cardboard? Adhesive strip? Epoxy? I've never been crazy about the idea of putting plastic against cardboard and expecting it to stay put through rugged use.

As far as rails are concerned, I would consider it if this were a heavier rocket. With the G80 loaded, this tips the scales at 24 oz. (I gotta say, retailers are proud of those rails. $85 for an eight footer? Really?)

 

[Dane Ronnow]

Comparing actual weight to weight in OpenRocket

With most of the assembly completed, I wanted to get an actual weight thus far, and see how it compares to OR. So I popped in the fins, slid the upper BT over the top of the baffle, and slipped in the nose cone.

My desktop scale reads 8.907 oz. I'll round that up to 8.91.



Actual weights of additional items:

36" nylon ellipsoidal parachute with twelve 54" shroud lines, quick link and swivel -- 3.6 oz
12" Nomex blanket -- .93 oz
8' of 250 lb Kevlar (shock cord) -- .10 oz
2' of 1/4" sport rubber (shock cord) -- .15 oz
FlightSketch altimeter -- .10 oz
2 - 2" cardboard launch lugs -- .06 oz

Total dry weight so far: 13.85 oz

The OR file has the dry weight sitting at 19.6, which includes a 5.5 oz mass object added to account for paint and adhesives, and .5 oz of nose weight added to adjust the caliber of stability. When I remove those (for the sake of comparison to the rocket in its present state), the weight is 13.6 oz.

The only other weight considerations left in the construction of this rocket—excluding primer and paint—are the epoxy to attach the upper BT to the baffle (maybe .05 oz), adhesives to attach the fins (Elmer's CG for fin root to body tube, around .10 oz, and J-B Weld for tab root to motor tube, maybe .10 oz), and the fin fillets (TiteBond Q&T, around 1.5 oz).

Adding those to the figure above gives me a total ballpark weight of 15.6 oz, which leaves 4 oz for paint. I'm good with that.

 

[Dane Ronnow]

BTW, I'm rapidly outgrowing my build space. The house I live in has no garage or basement, so I'm working in the corner of a bedroom. (You work with what you have.)

 

[Dane Ronnow]

Attaching upper body tube

Before I epoxy the upper body tube to the ejection baffle, I need to attach the lower shock cord. And because the eyebolt will be 9" down inside the BT when I'm finished, I have to figure out a way that I can replace this Kevlar when it becomes worn.

Solution: loop the shock cord through the eyebolt instead tying one end to it. That way, when I need to replace it, I can cut the cord at the top, tie one end of a new shock cord to an end of the old shock cord, then pull it through. Easy peasy.

I first loop enough shock cord through that I have ample length to tie a loop at the top, then tie the ends together in a square knot. Those ends are trimmed, and a small dab of Elmer's APG is applied to keep it together. Then a short piece of heat-shrink tubing is shrunk over it to keep the knot compact.

(That piece of HS tubing has to be slipped over the shock cord before the ends are tied.)

Then, another piece of HS tubing is applied over the base of the loop knot.



Finally, before I epoxy the upper BT in place, I slip a drinking straw over the shock cord so as I'm dropping it through the upper BT for placement on the ejection baffle, it doesn't drag along the wall of the tube where the epoxy is, totally messing it up.



After that, epoxy is applied to the first inch or so of the bottom end of the upper BT. Then I slip it over the baffle, twisting the tube as I seat it.

Then I roll the entire BT on a flat surface to align both tubes, and set it aside to cure.



One last addition is a .375" piece of tube coupler cemented into the bottom of the lower BT, butted up against the aft CR. This is a little added insurance against the forces of ejection on the CR/BT joints and the fin tabs, and to strengthen the thin wall at the bottom of the BT.) I let it cure for 30 minutes, then sanded the bottom edge flush with the BT and sealed everything with thin CA.



Next up is painting the BT and fins with primer, then sanding. Unfortunately, outside temps are running well over 100 degrees, and will be that way for a considerable time to come. If I get a cool morning, great. Otherwise, it might be November before I can spray any paint.

I'll let you know how it goes.

 

[AfterBurners]

Ejection Baffle

(Here again, I began assembly before I started taking pics.)

This is an Apogee BT-80 ejection baffle, with a 5/32" eye bolt for shock cord attachment, rather than the oversized (for my use) eye screw that came with the kit. The eyebolt passes through the baffle plate and reinforcing disk, with a flat washer and nut on each side of the plate. The lower nut is secured with a little J-BW. A dab of J-BW is also applied to the eye to eliminate the gap in the wire.

I modified the design in such a way as to eliminate particle build-up in the body tubes. (I don't know why I dislike crap accumulating in the rocket, but I do. And I will expend a little extra effort to keep that from happening.)

The short tube (1" BT-55) on the aft baffle plate is a choke. It concentrates the flow of hot particles and directs them into the longer tube—the collector—(1.75" BT-60) on the forward baffle plate, where they hit a solid wall (the gas vents are around the perimeter of the plate). After a flight, the particles that remain in the baffle can be shaken out the top of the upper body tube. Any particles that drop back through the baffle to the bottom tube can be shaken out through the motor mount after the motor is removed, along with the ejection charge cap.

The choke and collector tubes are epoxied in place with J-B Weld for heat resistance. Because it is facing directly into the blast of the ejection charge, the back side of the aft baffle plate is coated with a thin film of J-BW.

View attachment 468258 View attachment 468259 View attachment 468260 View attachment 468261 View attachment 468262

After the J-BW is fully cured, the edges of the baffle plates and the outside of the tube coupler are masked. Then everything is painted with high-temp paint.

View attachment 468263

Speaking of high temp, I stepped out onto the back patio to spray paint, and was reminded why we do things like this early in the morning in Las Vegas. This was June 6th. I can't wait for July. Then August?!

View attachment 468264

These are the components ready for assembly:

View attachment 468265 View attachment 468266

I'm not sure I understand the baffle design? If the hot gases from the ejection change blow through the baffle at the aft end (holes) which are confined the "choke" tube they will hit the back of the plate and not pass through. If they don't pass through how will the ejection charge deploy the chute? Isn't that why you have the upper cut outs in the top ring so the gases can pass through?

 

[Dane Ronnow]

@AfterBurners -- There's a half-inch of space between the two small tubes. So while the particles are contained, the gas flows freely. In this pic, the parts are placed as they will be inside the baffle when everything is closed up.

 

[Dane Ronnow]

One thing you may consider is conformal launch lugs or rail buttons. 1010 rail is considerably less flexy than a rod. Straight smoke and good chutes!

When you mentioned this earlier, I said I didn't think a 24 oz rocket with a G80 needed a rail. Now I'm rethinking this.

The rail is stiffer than a rod, for sure. And it gives me more options for attaching it to a launch pad. As far as rail buttons, I've already assembled the body tubes, so that bridge is burned. But I think these rail guides would work nicely (https://www.apogeerockets.com/Build...des/Conformal-Rail-Guides-for-2-6in-66mm-Tube).

Plus, I found a 96" 1010 rail at 80/20 Inc that ships for $29. That's a lot easier on the budget than the $85 I mentioned earlier.

[Edit] I was wrong about shipping on the above item. But Grainger has a 96" rail for $33. I can pick it up locally.

I would be splitting this rail into two 48" lengths, so it would fit in my car. The question I have concerns joining two sections of rail. Does the hardware keep both sections aligned with no edges inside the rail that might catch a rail guide?

Any info you could provide would be greatly appreciated. Thanks!

 

[Budro0]

When you mentioned this earlier, I said I didn't think a 24 oz rocket with a G80 needed a rail. Now I'm rethinking this.

The rail is stiffer than a rod, for sure. And It gives me more options for attaching it to a launch pad. As far as rail buttons, I've already assembled the body tubes, so that bridge is burned. But I think these rail guides would work nicely (https://www.apogeerockets.com/Build...des/Conformal-Rail-Guides-for-2-6in-66mm-Tube).

Plus, I found a 96" 1010 rail at 80/20 Inc that ships for $29. That's a lot easier on the budget than the $85 I mentioned earlier.

[Edit] I was wrong about shipping on the above item. But Grainger has a 96" rail for $33. I can pick it up locally.

I would be splitting this rail into two 48" lengths, so it would fit in my car. The question I have concerns joining two sections of rail. Does the hardware keep both sections aligned with no edges inside the rail that might catch a rail guide?

Any info you could provide would be greatly appreciated. Thanks!

I got two 48" sections and connectors from McMaster and connecting it on three sides keeps everything straight and clean. No issues launching mid power on the split rails.

 

[Dane Ronnow]

@Budro0 -- That answers my question. Thanks!

 

[Jay Rairigh]

@AfterBurners -- There's a half-inch of space between the two small tubes. So while the particles are contained, the gas flows freely. In this pic, the parts are placed as they will be inside the baffle when everything is closed up.

[/QUOTE]

I use the exact same baffle setup except I extend the tube on the right to go INTO the tube on the left to about a half inch or so from the forward baffle ring.
This way the hot gases must make two 90 degree turns before leaving the baffle.

 

[Dane Ronnow]

I use the exact same baffle setup

While I was coming up with the plan for that, and while building it, I was thinking, 'I can't be the only person who's making a baffle like this.' There probably aren't a lot of original ideas left...just good ones.

This way the hot gases must make two 90 degree turns before leaving the baffle.

I get dizzy just thinking about it. (Just kidding.) Good idea. And that's the beauty of other baffle designs—going up one tube, down another, then out the top. I just didn't like the idea of trapped particles.

 

[Jay Rairigh]

I get dizzy just thinking about it. (Just kidding.) Good idea. And that's the beauty of other baffle designs—going up one tube, down another, then out the top. I just didn't like the idea of trapped particles.

Any particles inside the baffle will easily shake out the holes in the forward end.
...nice build going.

 

[Dane Ronnow]

Primering body tube and fins

I caught a break this morning on the weather. The forecast last night said early temps would be around 90, and re-reading the label on the primer cans—Krylon gray filler and white sandable—I saw 95 was the top end for spraying.

By the time I prepped everything, it was 8 am and right at 90 outside. I figured I had about an hour to spray.

Last night I filled the joint between the BTs with Elmer's Wood Filler and sanded it smooth. I also knocked off the glassine with #320 paper. Then I watered down a little wood filler and painted it on the unpapered edges of the fins, let it dry for 10 minutes, then sanded it smooth.



I also repurposed an old mop handle into something I could insert in the motor tube to hold the body tube while spraying it. Wrapping it with blue tape brought the diameter up to a snug fit.



I waited until morning to mask everything, since blue tape loses its grip after a few hours. I masked the fin tabs, and about a quarter-inch up from the root where the fin fillet will be applied. I inserted the handle in the motor tube, and held it in place with the motor retainer mask. Then I taped off the fin slots (again, for the fin fillets), and stuffed a wad of paper towels into the top of the BT.



Finally, at the painting table, I fashioned a quickie clothesline out of a metal coat hanger for hanging the fins between coats. And I dug up a quick-clamp for clamping the BT to the table while it dried.

I'm spraying gray filler on the BT because of the spirals and tube joint. The papered fins don't need that, so I shot them with white sandable primer.



The worst part of spraying paint in the heat (for me, anyway), is wearing a long-sleeved shirt, vinyl gloves and a mask. After 45 minutes—two light coats and one medium coat on everything—it was good to get back indoors.

Anyway, the first application of primer is out of the way. Now for sanding.



Not sure when I'll get to the next coats. Forecast is calling for record highs, which, for Las Vegas, means 115 and up. On days like that, the overnight low never gets below 90.

 

[Dane Ronnow]

I was sanding the body tube outside when I noticed big, wet handprints where I was holding it. Danged heat! I put on a vinyl glove and finished sanding, then went back inside and drank a quart of water.



Anyway, I sanded the BT with #180 paper to take most of the filler off, then made a second pass with #320. The fins, which were primed with white sandable primer, were block-sanded with #320. Of the two, the gray filler sanded much cleaner, with little clogging of the paper. The white, not so much.



I'm not going for an award-winning finish, just smooth. After a close inspection of the spirals, and the joint between the upper and lower tubes, I decided I'm good with where things stand now.

I've still got to punch the vent holes in the upper tube and attach the rail guides. (Yes, I decided to go with a rail instead of a rod.) Then I'll attach the fins and form the fillets.

Then, when I get a morning with decent temps (90-ish), I'll shoot everything with the white sandable primer.

BTW, just got this photo from a friend of mine who lives in Lake Havasu City, AZ (about 100 miles south of Las Vegas). I don't feel so bad about 118. And if you're wondering who in their right mind would live in a place that gets this hot, it's people who don't have enough fuel to make it all the way to the sun.



More later.

 

[Dane Ronnow]

Rail Button Attachment

Before I get into this, a word about how I got here. With the original design, I was going to use 1/4" launch lugs and a 72" steel rod. And the design stayed that way through construction—motor mount installation, baffle assembly, and joining the upper and lower body tubes.

Then in a comment on this post, @fyrwrxz got me thinking about using a rail instead of a rod. The first thing I looked at were conformal rail guides that would attach with some kind of adhesive. But I was never comfortable with the idea of gluing these to a body tube, expecting them to stay put through however many flights this rocket might see.

Then I looked at attaching buttons with a single screw to the forward centering ring and the aft baffle plate, both of which are .125" plywood. Needless to say, that's not much for a screw to grab, and if it does, it would probably split the plywood. (This with a #4 flathead screw.)

That's when Mike at Binder Design suggested using well nuts with a 6-32 machine screw. And everything else just fell into place.

All that to say that no matter how much time I spent thinking about every aspect of this design from the very beginning, things were bound to change. The problem was not considering alternatives to launch lugs until after the body tube was closed up. Knowing what I know now, I would glue pieces of wood to the CR and baffle plate to anchor flange nuts (or whatever they're called), which would probably be a little more secure than well nuts.

Moving on...

Before I cut holes for the well nuts, I want to know exactly how these work—specifically, how much the neoprene spreads inside the tube when fully compressed so I can place one next to the forward centering ring, and the other one next to the aft baffle plate.

In both locations, the well nuts will go through the body tube wall where tube couplers are located. So I grabbed a scrap piece of BT-80 body tube and secured a coupler inside with tape. Then I marked a 5/16" hole and cut it with a X-Acto blade, by piercing the tubes just enough that the tip of the blade goes through both layers, moving around the marked circle about 1/16" per cut.



The well nut fits slightly snug, so there's no lateral slop. I'm using 6-32 black oxide machine screws, 3/4" long, to attach these. I tightened the screw, then looked inside the tube to see the neoprene flared to a diameter that is very close to the diameter of the nut flange, which is just slightly wider than the rail button.



Another thing I noticed is that fully tightened, the well nut/button assembly doesn't flatten the body tube at all. If there were no coupler inside, it probably would.

 

[Dane Ronnow]

Next, I marked the tube for the two cuts—one at the forward centering ring, the other at the aft baffle plate. After the cuts were made, I inserted the well nuts to check the fit.



Well nuts and buttons will be installed after the rocket is painted.

I like this setup for mounting rail buttons. They're sturdy, requiring quite a bit of pressure to get any movement in the button.

The only concern I have is that the aft well nut is located right next to the forward end of the motor tube, which means it will be exposed to the flame and heat of the ejection charge. The nice thing is that these are very easy to replace if necessary.

One last thing before I move on to attaching the fins is cutting the pressure relief vents in the top of the tube. I'm centering these in line with the fins, 3.5" below the top of the tube, which is 1/2" below the nose cone shoulder.

I'm using a piece of 1/8" brass tube to cut these holes. I first thought of chamfering the inside of one end using an X-Acto blade, kind of carving the brass. Then it hit me—I have a tool for chamfering the necks on rifle brass. That worked perfectly, giving me a sharp edge.

I placed the end of a 1" dowel inside the body tube to press the brass tube against, then twisted. It took about 20 twists, but resulted in a very clean cut.



Pressure relief vents and well nut holes were strengthened with thin CA.

Attaching the Fins

These are TTW fins, with a tab that fits snugly between forward and aft centering rings. To begin, I mixed a little J-B Weld, then waited about 30 minutes for it to thicken slightly. Then I ran a thin bead of Elmer's Wood Glue along the fin root ahead of the tab, and behind. I'm double gluing, so I insert the fin tab into the slot, press the fin firmly against the body tube, then pull it out.

Next, using a wood coffee stirring stick, I applied J-BW along the edge of the tab. Slightly thickened, the bead stands about 1/8" high from the edge, and stays the width of the tab so I can insert it through the slot without getting it on the body tube.

Then I ran a second, thin bead of Elmer's along the fin root. Finally, I dribbled a little Elmer's into the slot so it would run down the inner faces of the CRs.

I've already placed the fin alignment guide over the body tube just ahead of the fin location. Inserting the fin, I press it firmly against the body and motor tubes, then slide the guide down to capture the fin.



I waited 30 minutes to do the next fin, giving the Elmer's on the first fin time to set so there would be no movement when I pulled the guide up to allow placement of the second fin. Same process here, and for the third fin.



With all three fins in place, I let everything dry overnight, then pulled the alignment guide.



Next up, applying fillets.

 

[Dane Ronnow]

Fillets

Everything I know about fillets I learned on TRF, from a lot of different contributors. Up until now, every fillet I did was simply wiping the Elmer's glue along the fin/body tube joint with a fingertip. That was on small rockets—Estes B- and C-motor kits—with fins just glued to the body tube. No TTW.

So these are my first. The technique I'm using I learned from a video posted by @JimJarvis50. And the first thing I needed to do was figure a better way to score the fin and body tube, because my fillet tool—a frosting fondant ball (used for decorating cakes)—wasn't leaving a mark I could see.

I cut a narrow strip of paper and covered it with pencil graphite. Then, holding the strip along the fin/body tube joint with the graphite face down, I ran the fondant ball back and forth several times. This left a line—faint, but enough that I could see where to place the strips of masking tape.



BTW, a post by @mtnmanak in the same thread as Jim's video is where I learned about the fondant tool. And I must say, they're pretty slick. You want to wipe them on a wet paper towel (if you're using water-based glue) after each pass. But they're easy to handle, and you don't need to worry about the angle.

These fins were a little more difficult to form fillets on because the trailing edge is behind the end of the body tube. So the fillet just kind of lops off. I'll be doing a little cleanup work with sandpaper, but I was expecting that.

I used Titebond Quick and Thick. And I discovered the hard way that the 'Quick' in the name refers to what you have to be when you're using it. This stuff glazes fast. Like 30 seconds fast. I forgot to have a wet paper towel handy, and in the time it took me to grab one off the roll and run it under the faucet, the glue had a skin on it that I pulled off as soon as I hit it with the fondant ball.

I wet my finger and tried to shape it back in place, and just made a hash of it. So I grabbed a plastic squeegee with a square corner and dragged all the glue off the rocket, wiped it with a wet towel, then started over.

Fortunately, the next application went great. I started with a thin fillet, forcing it into the corner where the fin meets the body tube. Then a pass with the heavy bead. With the fondant ball I pulled from the middle of the fillet to the rear, wiped the ball, then pulled from the middle to the front. And I was surprised at how easy the glue is to work with . . . for the first thirty seconds. If you don't have the fillet shaped by then, you're toast.



Also, this stuff is thick. Stiff. It doesn't shift at all if you flip the tube to do the opposite fin, or turn the tube, say 20 degrees. I wouldn't stand the tube on end, of course, but you can do fillets one after another. After the first one, I scored and masked the fins in pairs, letting things sit maybe five minutes between applications of glue.

You want to pull the tape soon, too. As soon as you've wiped the fillet tool clean after the last pass. Any bubbles should be popped—I used the tip of an X-Acto blade—immediately, so they'll fill in. If you wait even three minutes, you'll be left with a dimple.

All in all, I was pleased with the result. It was a learning experience, for sure, especially redoing the first fillet. But I'm comfortable with the process now.

If you want to give the fondant balls a try, Amazon has them for $5.99:

Amazon.com: 4 x Fondant Cake Decorating Clay Flower Sugarcraft Ball Model DIY Tool 8 Sizes (KT0105) : Home & Kitchen

Buy 4 x Fondant Cake Decorating Clay Flower Sugarcraft Ball Model DIY Tool 8 Sizes (KT0105): Sculpting & Modeling Tools - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

(Hat tip to @mtnmanak for the suggestion.)



Once these are bone dry, and I get a morning with temps below 90 degrees, I'm going to shoot the whole rocket with white primer and see how these look.

More later. (And thanks for reading!)

 

[Dane Ronnow]

I was overly optimistic when I thought these fillets would be one-and-done. The Titebond Quick and Thick shrunk more than I thought it would, and I'm going to have to apply a second layer. (Maybe a third. We'll see.)

The image below are the fillets after drying 24 hours. They're clear, so you can't really see the shape.



Below, left to right, are the fondant ball I used to pull the fillets, and the diameter (.742"). Then a fondant ball that fits the radius of the dried fillets, and its diameter (.435"). So the wet fillet is roughly 3/8" radius, and cured, it's less than 1/4"—about a 40 percent loss.



Anyway, I'll rough sand these a little, then try this again.