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Really nice build
2.6-inch 29mm MPR Scratch Build
- Thread starter Dane Ronnow
- Start date
Thanks! If we can get some cooler temps here in Las Vegas, I'll paint it. Or, if I get really restless, I'll just fly it with primer.
- Joined
- Jun 4, 2018
- Messages
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- Reaction score
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Nice build.... there are plenty of good HPR engines that could be used 
Agreed... if I ever decide to get an L1 cert.
Picking up where I left off
After a not-so-brief hiatus for doctor's appointments, tests, and other inconveniences associated with health problems, I'll pick up where I left off in July. Before I had set everything aside, I did manage to finish the fin fillets and apply another coat of gray filler primer, then sand everything with #320 paper.
I'm going to spray a final coat of white primer, sand with #1000, then shoot the top coats of black and white enamel.
Before that, though, I want to secure the altimeter in a dedicated bay.
Altimeter Bay
Ever since I purchased a FlightSketch Mini, I've been kicking around various methods of securing it. Initially, I thought I would just tether it to the nose cone and tuck it in with the parachute. But I didn't like the idea of the Mini dangling for the duration of the descent.
I finally settled on the idea of building a small bay that would fit into the base of the nose cone. Before I could start construction, I needed to know exactly how much space the Mini would occupy, oriented with the long axis of the board parallel to the long axis of the rocket.
Using the ribbed, upper portion of a sock, I stitched a small pouch that would give the board adequate padding, while allowing the barometer to breathe. Slightly compressed, the dimensions of the pouch with the Mini inside were .5 inch x 1 inch x 1.5 inch.
The material I'll be using for the bay is .063 thick sheet polystyrene (from Hobby Lobby).
Now I needed to decide which cement would be best for bonding it to an Apogee Components PNC-66A nose cone. (Apogee's description says the cone is styrene, but I've come to realize that styrene is a term that encompasses a number of different materials.)
I have CA glue (of course), as well as Tamiya Extra Thin Cement, and a tube of Testors 3501 (same stuff as the 'airplane glue' I used for models when I was a kid). The Tamiya and Testors will melt the polystyrene to some degree, which is what I want. The CA? Not sure, but I'll find out by testing.
I cut three small pieces of the sheet PS and glued them to a section of the nose cone that I would ultimately remove for the finished bay, and numbered them 1 (CA), 2 (Tamiya) and 3 (Testors 3501).
I let these dry overnight, then tested by slowly pushing the top of each strip sideways until it broke.
Numbers 1 and 2 broke at the glued joint, but left a small portion of the PS strip adhered to the cone. Number 3 actually started breaking well above the glued joint, and left a sizeable chunk adhered to the cone. This told me the Testors 3501 does a better job of welding these two plastics together.
I also tested the Testors on a sheet-to-sheet joint, since this represent the bay itself. That joint broke the same way as the Testors joint on the nose cone.
So Testors will be my go-to glue for this project.
After a not-so-brief hiatus for doctor's appointments, tests, and other inconveniences associated with health problems, I'll pick up where I left off in July. Before I had set everything aside, I did manage to finish the fin fillets and apply another coat of gray filler primer, then sand everything with #320 paper.
I'm going to spray a final coat of white primer, sand with #1000, then shoot the top coats of black and white enamel.
Before that, though, I want to secure the altimeter in a dedicated bay.
Altimeter Bay
Ever since I purchased a FlightSketch Mini, I've been kicking around various methods of securing it. Initially, I thought I would just tether it to the nose cone and tuck it in with the parachute. But I didn't like the idea of the Mini dangling for the duration of the descent.
I finally settled on the idea of building a small bay that would fit into the base of the nose cone. Before I could start construction, I needed to know exactly how much space the Mini would occupy, oriented with the long axis of the board parallel to the long axis of the rocket.
Using the ribbed, upper portion of a sock, I stitched a small pouch that would give the board adequate padding, while allowing the barometer to breathe. Slightly compressed, the dimensions of the pouch with the Mini inside were .5 inch x 1 inch x 1.5 inch.
The material I'll be using for the bay is .063 thick sheet polystyrene (from Hobby Lobby).
Now I needed to decide which cement would be best for bonding it to an Apogee Components PNC-66A nose cone. (Apogee's description says the cone is styrene, but I've come to realize that styrene is a term that encompasses a number of different materials.)
I have CA glue (of course), as well as Tamiya Extra Thin Cement, and a tube of Testors 3501 (same stuff as the 'airplane glue' I used for models when I was a kid). The Tamiya and Testors will melt the polystyrene to some degree, which is what I want. The CA? Not sure, but I'll find out by testing.
I cut three small pieces of the sheet PS and glued them to a section of the nose cone that I would ultimately remove for the finished bay, and numbered them 1 (CA), 2 (Tamiya) and 3 (Testors 3501).
I let these dry overnight, then tested by slowly pushing the top of each strip sideways until it broke.
Numbers 1 and 2 broke at the glued joint, but left a small portion of the PS strip adhered to the cone. Number 3 actually started breaking well above the glued joint, and left a sizeable chunk adhered to the cone. This told me the Testors 3501 does a better job of welding these two plastics together.
I also tested the Testors on a sheet-to-sheet joint, since this represent the bay itself. That joint broke the same way as the Testors joint on the nose cone.
So Testors will be my go-to glue for this project.
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Next, I cut pieces for the bay. I started by cutting with sharp tin snips, but the narrower strips of PS were curling. So I scored the PS with a straight edge and #2 X-Acto knife, then snapped the pieces apart and cleaned up the edges with #320 paper on a sanding block.
The cementing process was straightforward—run a thin bead along the edge of one piece and press it against the second piece, then hold with moderate pressure for 20 seconds. With the plastic melting a little almost immediately, there was no movement in the pieces once I let go, and it was easy to get straight, squared joints.
Once dried, I tested the fit of the Mini in its pouch.
Next, I positioned the bay on the base of the nose cone, then traced an outline. I cut the opening using a saw blade in a Dremel, being careful to leave the penciled line I traced (for reference), then sanded the edges with a nail file until the bay slipped in with almost no gap at all.
Then I fashioned a flange.
The cementing process was straightforward—run a thin bead along the edge of one piece and press it against the second piece, then hold with moderate pressure for 20 seconds. With the plastic melting a little almost immediately, there was no movement in the pieces once I let go, and it was easy to get straight, squared joints.
Once dried, I tested the fit of the Mini in its pouch.
Next, I positioned the bay on the base of the nose cone, then traced an outline. I cut the opening using a saw blade in a Dremel, being careful to leave the penciled line I traced (for reference), then sanded the edges with a nail file until the bay slipped in with almost no gap at all.
Then I fashioned a flange.
Altimeter Bay Latch
After cementing the bay into the nose cone, I took measurements for a latch I had in mind, then drilled pilot holes in the bay flange, and cut the latch out of sheet PS.
After cementing the bay into the nose cone, I took measurements for a latch I had in mind, then drilled pilot holes in the bay flange, and cut the latch out of sheet PS.
Altimeter Tether
Finally, I need a tether just long enough to reach from the top end of the bay, to the attachment point of the nose cone. Finished length will be about 5 inches, but I'll need about 15 to make this.
Using 250-pound braided Kevlar, I tied a surgeon's end loop on the end that will attach to the cone. I trimmed the excess cord, then finished the knot with a piece of transparent heat-shrink tubing.
Then I slipped another piece of HS tubing onto the Kevlar prior to tying off the other end on the Mini.
For the Mini end of the Kevlar, I want a knot that I allows me to make the loop smaller after the knot is tied. So I used a simple slip knot with two turns, then finished it with HS tubing.
All that remains now is the final painting—a coat of white primer, followed by the enamel top coats. And with that in mind, I'm trying to decide if I want to wait until it's painted, or just head down to the dry lake bed at Jean and light 'er up.
Decisions, decisions.
Finally, I need a tether just long enough to reach from the top end of the bay, to the attachment point of the nose cone. Finished length will be about 5 inches, but I'll need about 15 to make this.
Using 250-pound braided Kevlar, I tied a surgeon's end loop on the end that will attach to the cone. I trimmed the excess cord, then finished the knot with a piece of transparent heat-shrink tubing.
Then I slipped another piece of HS tubing onto the Kevlar prior to tying off the other end on the Mini.
For the Mini end of the Kevlar, I want a knot that I allows me to make the loop smaller after the knot is tied. So I used a simple slip knot with two turns, then finished it with HS tubing.
All that remains now is the final painting—a coat of white primer, followed by the enamel top coats. And with that in mind, I'm trying to decide if I want to wait until it's painted, or just head down to the dry lake bed at Jean and light 'er up.
Decisions, decisions.
I love the plastic work on your altimeter bay! Your attention to detail for this build is fantastic.
I was curious, I didn't see any holes in the airframe or the nose cone for pressure equalization during flight to ensure accurate barometric pressure readings. Otherwise the altimeter readings would only be reliable after ejection.
Attached is the best method I've found to determining hole size and placement.
I was curious, I didn't see any holes in the airframe or the nose cone for pressure equalization during flight to ensure accurate barometric pressure readings. Otherwise the altimeter readings would only be reliable after ejection.
Attached is the best method I've found to determining hole size and placement.
Thank you! I appreciate your feedback.
The air frame has three .125-inch pressure relief vents located 3.5 inches below the top of the body tube. This is where the parachute is, which sits just below the bottom of the nose cone. The nose cone has a .25-inch vent located next to the altimeter bay.
Ah, I see why I was confused. I use a separate set of holes for airframe pressure relief and sampling barometric readings for two reasons:
- I keep my avionics entirely isolated from the ejection gasses
- The pressure equalization requirements are different.
- For the airframe, you want the holes just large enough so the rocket components don't have an unplanned separation during ascent. Larger holes vent ejection gasses; going too large puts the planned separation in jeopardy.
- For the avionics, you want large enough holes to ensure equilibrium between the avionics bay and the atmosphere. This requires larger holes than is typically used for airframe vents.
I really like how clean those holes are! I get impatient and just use a drill.
@rocketlabdelta --
I'm not using a separate avionics bay, and the nose cone uses no shear pins, relying instead on a slightly tight fit in the body tube to keep it in place.
The nose cone sits directly above the parachute package, which sits on the forward plate of an ejection baffle, which is midway between the motor mount and the top of the tube.
The volume of the tube from the forward centering ring to the bottom of the nose cone (a distance of 19.5 inches) is 103.5 cu. in.
Regarding the attachment in your post (Static Port Holes.pdf), a cursory review of this paper left me with the same feeling of panic I experienced in high school algebra. I had to lie down, wishing I had a Xanax.
Seriously, though, after wading through the equations in the paper (none of which I could follow competently), I found a reference at the bottom of the paper that refers to a spread sheet on the author's web site (offwegorocketry.com). Using this spread sheet, I calculated a port size of 6/64 inch. (See Static Port snippet 1 below.)
Compare this to the spread sheet I used in calculating port size for my rocket. (See Static Port snippet 2 below.) Based on this spread sheet, I came up with a port diameter of 1/8 inch, which is 1/32 inch larger than is recommended in Static Port snippet 1.
I can only hope that a difference of 1/32 inch x three ports is not a fatal miscalculation. If it is, I could fill the existing ports (not sure how), and cut new ones. I just don't know if I need to.
I'm not using a separate avionics bay, and the nose cone uses no shear pins, relying instead on a slightly tight fit in the body tube to keep it in place.
The nose cone sits directly above the parachute package, which sits on the forward plate of an ejection baffle, which is midway between the motor mount and the top of the tube.
The volume of the tube from the forward centering ring to the bottom of the nose cone (a distance of 19.5 inches) is 103.5 cu. in.
Regarding the attachment in your post (Static Port Holes.pdf), a cursory review of this paper left me with the same feeling of panic I experienced in high school algebra. I had to lie down, wishing I had a Xanax.
Seriously, though, after wading through the equations in the paper (none of which I could follow competently), I found a reference at the bottom of the paper that refers to a spread sheet on the author's web site (offwegorocketry.com). Using this spread sheet, I calculated a port size of 6/64 inch. (See Static Port snippet 1 below.)
Compare this to the spread sheet I used in calculating port size for my rocket. (See Static Port snippet 2 below.) Based on this spread sheet, I came up with a port diameter of 1/8 inch, which is 1/32 inch larger than is recommended in Static Port snippet 1.
I can only hope that a difference of 1/32 inch x three ports is not a fatal miscalculation. If it is, I could fill the existing ports (not sure how), and cut new ones. I just don't know if I need to.
Sounds close enough to me! No need for rework.
What really matters is how the rocket actually functions. Some of that you can figure out with a ground test ahead of your first flight.
I don't have the means to do a black powder test. I'm going to do a simple blow test with a tube that fits tight in the motor tube. If I can't blow everything out, I'll figure out something else. (Like how to do a black powder test.)
Yes. Aerotech F67, G74 and G80. All of them are .7 grams BP. The vacuum cleaner is an interesting idea.
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I don't have either. I'll start with a blow test, which is easiest with what I've got. If that doesn't work, I've got a shop vac I can try. It seems to me, out of those two, the blow test would be better at producing a sudden burst of pressure.
Rex R
LV2
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It can be fun accumulating bp. I built a saucer for 24mm and flew it mostly on F12s w/o ejection charges, seemed like every one liked it And I didn't have to walk very far to retrieve it. the unused bp went into my stash(I flew a lot saucers)
the unused bp went into my stash(I flew a lot saucers)
Was reading this thread to see how the launch of your rocket went…? No problems concerning vent / sampling holes ?
Really like what you have done with this baffle. I typically stay away from this style as I've had burning embers get past the baffle and up into the recovery gear and burn the shock cord and parachute.
In the past I've cut a third plate. Basically made a duplicate of the upper late with the holes around the outside and used it as the top and bottom plates and used the plate with the small holes in the middle of the baffle.
I think your design has even less chance of embers getting past and it's probably a little lighter as well.
I haven't launched the rocket yet. I keep getting sidelined by health problems (doctor's appointments, tests, etc.) On the days that I'm not dealing with that, I'm trying to finish painting the rocket. I just put the last primer coat on a couple of days ago. Now I'm waiting for a day with no wind to start spraying the top coats.
Concerning the vents, I go back to a conversation with @rocketlabdelta (posts 39-43 above) about vent sizing. The calculator he posted suggested a size for my rocket of three 3/32" vents. The calculator I used suggested 1/8". In those posts, we kind of decided that a difference of 1/32" wouldn't matter much, if at all.
Following that exchange, I started thinking again about the volume of body tube that needs to be pressurized, and my feeling that single-use F and G motors are going to be marginal in their ability to eject everything with .7 grams of BP. And while I'm running through all this in my mind, I'm staring at the holes, and it hits me that these are WAY bigger than 3/32". I pull out a 3/32 drill bit and stick it through the vent, and see this:
While I'm cursing quietly, my mind goes back to the brass tubing I used to cut those holes (page 1, post 28, after the rail-button hole cuts), and I realized that 1/8" is the inside diameter, not the outside. A very stupid mistake on my part because I've known since I was a kid that when it comes to tubing—brass, copper, PVC, even garden hose—IT'S INSIDE DIAMETER!
I throw the micrometer on it, and yes, it is most definitely bigger than 1/8". Almost twice as big.
If I wasn't worried about the ejection charge being enough to pop the nose cone with enough force to pull the chute out, I would leave these holes as they are. But I am worried. So, I decided to patch the existing holes and cut new ones.
And while your (@Philip Tiberius D.) question wasn't about this specifically, I'm putting this in here because I was going to post it anyway. I just never got around to it. (I apologize if this turns into 10 minutes of your life you'll never get back.)
Patching a hole in a body tube wall that is only .04" thick, in such a way that any kind of a plug wouldn't simply pop out under the pressure of ejection, took some brainstorming. I finally settled on a method that's the same as a plug-and-patch tire repair—Elmer's Carpenters Glue is the plug, paper tape is the patch.
I used this paper tape from Staples because it sticks better than any tape I've ever used. (The box tape from U-Haul is a close second.)
But it needs a hard surface to stick to. So, I started by swabbing the inside of the body tube around the holes with CA glue to harden the surface.
Next, I had to figure out how to place three small squares of tape down inside the body tube where I couldn't reach with my fingers. I took a wooden stirring stick and soaked the end of it in water, slowly bent it to a slight angle, then left it to dry. Once the stick was ready, I rolled a piece of Scotch tape with the sticky side out, then trimmed it to 1/4" square and super-glued it to the stick. Touch that to the back side of a piece of patching tape, and voila; it worked perfectly.
After placing the pieces of tape, I burnished them with the rounded end of a 1/4" dowel that I use for applying epoxy in hard-to-reach places.
Then I applied a drop of glue to the exterior side.
I let that dry, sanded it flat, dressed it with a little Carpenter's Wood Filler, then sanded it again. This is the tube with old hole on the left, new hole on the right:
This may or may not hold. I won't know until I launch it. And I WILL throw a post up here when that happens.
Thank you. I haven't launched this rocket yet, so I don't know how well it will work. But I feel pretty good about it. I'll post a launch report on this same thread when I know for sure.
Enamel Top Coats
Finally got around to spraying the top coats. This is the paint scheme I had in mind—white and classic gray, gloss:
This is not modeling any specific rocket, NASA or USAF. Just my own invention with a military feel. I'll call it the Joint Research Project.
The rocket is close to 40 inches long, so I clamped a 1-inch dowel horizontally to a step ladder rung that would slide up the motor tube. That way I could stand back and spray side to side without changing the distance of spray to rocket, and still rotate the body tube during the painting process.
The white went on fine. Slight orange peel, but only slight. Then gray on top of that, which actually coated better. I did have two small problems—a small spot where the paint creeped underneath the Tamiya tape, and another where the paint chipped.
[Edit] I forgot to mention, I used Dupli-Color Plastic Adhesion Promoter on the polystyrene nose cone. I sanded the cone with 600 paper, wiped it down, then sprayed three light coats, waiting three minutes between coats. Five minutes after the last coat, I sprayed the gray top coats. Turned out great, very smooth, no orange peel.
Finally got around to spraying the top coats. This is the paint scheme I had in mind—white and classic gray, gloss:
This is not modeling any specific rocket, NASA or USAF. Just my own invention with a military feel. I'll call it the Joint Research Project.
The rocket is close to 40 inches long, so I clamped a 1-inch dowel horizontally to a step ladder rung that would slide up the motor tube. That way I could stand back and spray side to side without changing the distance of spray to rocket, and still rotate the body tube during the painting process.
The white went on fine. Slight orange peel, but only slight. Then gray on top of that, which actually coated better. I did have two small problems—a small spot where the paint creeped underneath the Tamiya tape, and another where the paint chipped.
[Edit] I forgot to mention, I used Dupli-Color Plastic Adhesion Promoter on the polystyrene nose cone. I sanded the cone with 600 paper, wiped it down, then sprayed three light coats, waiting three minutes between coats. Five minutes after the last coat, I sprayed the gray top coats. Turned out great, very smooth, no orange peel.
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After I apply the vinyl lettering and logos, I'll coat the entire rocket with acrylic floor polish to protect the paint from further chipping, and to seal the edges of the vinyl so it doesn't peel off in flight.
All in all, I'm pretty happy with the way it turned out.
All in all, I'm pretty happy with the way it turned out.
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Thanks!
Applying Vinyl
I created the text for the vinyl lettering and logos in InDesign, then converted everything from typeface to vector art. Signs.com in Salt Lake City output the vinyl for me.
After cutting the text and logos into individual strips, I marked positions on the body tube in pencil, then Windexed the tube where the vinyl would lay. Then I used the standard method for applying vinyl to a car window or door, which is basically using blue masking tape for a hinge.
After applying the hinge, I lifted the piece and carefully peeled back the carrier paper. The piece was lowered against the body tube, then flattened with a 2-inch rubber brayer. The application tape was then peeled away by lifting the tape hinge first, then slowly peeling it back. The completed strip was burnished again.
The vinyl released from both the application tape and carrier paper nicely, with no hangups.
I'm really pleased with the way the vinyl turned out. The black lettering compliments the gray quite nicely.
As I mentioned in the post above, I plan on coating the entire rocket with acrylic floor polish to seal the edges of the vinyl, and to protect the paint from wear and tear. I'll post again when that's done.
I created the text for the vinyl lettering and logos in InDesign, then converted everything from typeface to vector art. Signs.com in Salt Lake City output the vinyl for me.
After cutting the text and logos into individual strips, I marked positions on the body tube in pencil, then Windexed the tube where the vinyl would lay. Then I used the standard method for applying vinyl to a car window or door, which is basically using blue masking tape for a hinge.
After applying the hinge, I lifted the piece and carefully peeled back the carrier paper. The piece was lowered against the body tube, then flattened with a 2-inch rubber brayer. The application tape was then peeled away by lifting the tape hinge first, then slowly peeling it back. The completed strip was burnished again.
The vinyl released from both the application tape and carrier paper nicely, with no hangups.
I'm really pleased with the way the vinyl turned out. The black lettering compliments the gray quite nicely.
As I mentioned in the post above, I plan on coating the entire rocket with acrylic floor polish to seal the edges of the vinyl, and to protect the paint from wear and tear. I'll post again when that's done.
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Thanks, @hobie1dog!
