Rocket Ship 27: An L3 Build Thread

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JLebow

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You make it look easy! I think I will stick with G10. [emoji23] Are you thinking of any particular launch this year? I’d love to see her in person.
The target launch is Oregon Rocketry's AP Showers in late April, in Brothers.

Looking good so far Joey. Hoping to see this beastie live.
Thanks, Rich. I'll be at as many Oregon Rocketry events as I can make and Sod Blasters, but I will have to adapt down in motor to stay under the waiver at the Sod Farm. Will you make it to NXRS this year?

Pardon a dumb question . . .

Does anyone wet-sand their composites to prevent large amounts of dust and airborne particles ?

Dave F.
I don't see why not. You will need wet rated sand paper. There will be quite a bit of water involved if you start wet at this stage. I usually sand dry until 400 grit, and then it becomes worth the hassle because the water keeps the dust from clogging the paper.

You might like the second article also, particularly about pulling tension while rolling the tube.

https://www.raketenmodellbau.org/repository/archive/167793?view=true

One trick that I never published was to use the tip of a chip brush (tap, tap, tap) on top of the peel ply. The pinholes vanish. Not sure this would work as well with two layers of peel ply.

Jim
Jim, thank you for volume 2. I like the detail and generous sharing of your techniques. I saw your flight at Balls last year. What an incredible project.
 

Ez2cDave

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I don't see why not. You will need wet rated sand paper. There will be quite a bit of water involved if you start wet at this stage. I usually sand dry until 400 grit, and then it becomes worth the hassle because the water keeps the dust from clogging the paper.
I am thinking more in terms of safety than clogged sandpaper. Those airborne particles can kill !

Dave F.
 

rharshberger

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The target launch is Oregon Rocketry's AP Showers in late April, in Brothers.



Thanks, Rich. I'll be at as many Oregon Rocketry events as I can make and Sod Blasters, but I will have to adapt down in motor to stay under the waiver at the Sod Farm. Will you make it to NXRS this year?
I will not be at NXRS this year, if extremely lucky the only event I will attend away from TCR is LDRS. Hopefully we pay the car off soon and get a travel trailer, that will open up rocketry destinations as the wife will actually enjoy it too.
 

JLebow

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Time to glue some fins onto the air frame.

I wrapped a sheet of paper around the rocket tube to serve as a guide to trim and square up the tube ends. I cut with a hack saw and cleaned up with a sanding block. I checked the finish against a spent DMS rocket motor.
10 trim the tube.jpg

Because the fin has quite a bit of thickness at the root, the root edge is not flat (in a plane). The fin sits on a cylinder, that falls away from where a traditional thin fin would sit. My math came out that the would be a 1/4" height difference between the thick and thin portion of the fin. I included this feature in the fin molds. See how the fin doesn't sit flat:
20 root not flat.jpg
The fin is a pretty good fit, but to get a great fit, I sanded the fin with some sand paper stuck to the mandrel.
30 sanding the root.jpg
I wanted to do something better than "eyeballing it" to make sure my fin is straight (pointing radially outward). So I used my height gauge to indicate the tip of the fin, relative to some setup blocks. Then I rotated the fin 180 degrees. If the fin is leaning to one side or the other, the tip of the fin will be offset to the left or right from the tip of the gauge:
40 vert jig.jpg 50 vert jig 2.jpg

So I sand the fin root, check the tilt of the fin, sand some more, and continue until the fins are straight and all the same height:
60 height gauge.jpg

I like the shape of this compound curve:
70 curved root.jpg

Then I made up some alignment jigs from foam core poster board and tacked the first fin in place with some rocketpoxy.
80 first fin in place.jpg
I don't know if it is required, but my fins are hollow, and my flight profile will have a 6 psi differential in air pressure at apogee vs at the pad. I decided to drill three tiny vent holes into the airframe, placed 4 inches above the motor thrust ring.
 
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mbeels

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Those fins look great, very methodical fitting. Is that an electric Porsche in the background?
 

JLebow

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After setting the first fin and letting the epoxy cure, I feel more confident in glue the second and third fin at the same time. The first fin gives a better anchor point to set the other two fins.
10 Glueing on fins.jpg
I really like how the fins look:
20 Side view.jpg 30 View from behind.jpg

Next up fillets. I'm using a small length of 1 inch diameter PVC pipe to set the radius of my fillet. Epoxy gets everywhere, and it doesn't wipe up very well. I mask off the area next to the fillets with masking tape.
60 taped.jpg
A trick I use that works better than the old eyeball method is to wrap sandpaper over the pvc pipe and lightly sand in the area of the fillet. This will mark the edge of the fillet that will be pulled using the same pvc tool.
40 Mark the fillets.jpg 50 Fillets marked.jpg
Now that the tape is in the exact correct location, I dropped 12 grams of black tinted rocketpoxy into the fillet zone. I wet the pvc pipe with isopropyl (rubbing, disinfecting) alcohol to reduce the tendency of the pipe to stick to the epoxy.
70 fillets.jpg
After setting for about 20 minutes, I pull the tape. If I forget to pull the tape in time, I am rewarded with an extra hour of sanding the little ridge that is about the thickness of the tape. If I remember to pull the tape in time, I use a gloved finger, wetting with isopropyl alcohol to smooth out this little ridge. Don't overdo it. The epoxy at this stage is easily overworked and difficult to do anything other than make it worse.
80 tape pulled.jpg
 

JLebow

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To make a coupler, I need a mandrel that is 3.8 mm smaller in diameter than the 76 mm airframe mandrel. This is not a standard dimension for rod or tube, so I take a little off with the lathe.
10 turning the mandrel.jpg
I made a couple of end caps, added some bolts, and made a stand out of scrap wood.20 laying up the coupler.jpg
In this picture, you can see that I'm also setting up to layup the sustainer.
40 layput.jpg
The gunk on the ends of the mandrel is a reminder to go easy on the epoxy near the ends of the mylar wrap. When it wicks under the mylar and gets to the mandrel, you get an extra challenge of trying to remove the part from the mandrel.
30 finished coupler.jpg
The coupler was a very nice slip fit with no slop in the airframe. Once I figured out that my glass and layup technique yield 0.224 mm per wrap, I can get very consistent part thickness. My builds are getting faster, and most of it is the reduction in sanding and remaking parts that weren't quite the right size.

The sustainer hasn't been sanded yet and it is a very close match in diameter to the carbon booster.
50 booster and sustainer.jpg
I also made a coupler for the nose cone. The process is the same as the first coupler, only shorter length, so I didn't document the process. The rocket stands at 7 feet, 6 inches.
70 with nose.jpg
If you are wondering why the sustainer and nose cone are fiberglass instead of carbon: the trackers in the ebay and nose need radio transparency.

The mass of the rocket so far is 1.817 kg (4 lbs). 990 g for the booster and fins, 827 g for 2 couplers, sustainter and nose cone.
 

Mugs914

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Fully agree with Tim!! This is really a fun thread to follow. Thanks for bringing us all along with ya.
 

Mugs914

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BTW - Is that a '75 911? Did you do the electric conversion?
 

JLebow

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BTW - Is that a '75 911? Did you do the electric conversion?
1983 and I did the conversion in 2011.

Why is it called Rocket Ship 27?
The Trailer Park Boys have a couple episodes with rocketry. One of their rockets is called Rocket Ship 27.

Time for an update. Here is some work on the ebay. For electronics I'm using Altus Metrum gear: TeleMetrum with GPS and EasyMini. With tracking, radio transparency of the ebay and airframe is an issue. That is the reason for the switch from carbon fiber to fiber glass. Also, it is generally accepted (but rarely demonstrated) that medal threaded rod can interfere with the radio link to the ground station. This ebay build will not use metal threaded rods to retain the end caps of the ebay. I will use the ebay sled in tension instead.

Here is a simple drawing of the concept:
01 ebay plan.jpg

The parts are cut from G10 sheet, 3/32" in thickness.
02 cut the parts on jig saw.jpg

These end caps are cleaned up on the lathe and filed to fit on the sled.
03 end cap fit up.jpg
One end cap is getting pem nuts for the screws that will hold the ebay together.
04 pressing pem nuts.jpg
I'm using 1/2" long 4-40 hex copper standoffs as electrical feedthroughs for my deploy charges.
05 electrical connections.jpg
The caps get glues to the sled. One end has the traditional stepped lip that sits on the circumference of the ebay tube. The other cap is smaller in diameter and sits just below the surface of the ebay. I'm using nylon M3 standoffs for mounting the electronics.
07 sled profile.jpg
06 gluing the sled.jpg
The caps do not depend solely on the fillet of epoxy. Extra tabs are glued to the sled tabs, to strengthen the tabs and to secure the caps in compression. In this picture, the sled can only slide down. The bottom cap prevents it from sliding up in the ebay tube.
08 end cap with tabs.jpg
To secure the sled, a third disk (with a stepped lip), is placed on top of assembly...
09 end cap with shackle.jpg
...and secured with 4, 6-32 machine screws. A shackle, swivel, and kevlar harness mount to the sled tab.

I read about the finger trap method of creating a loop end on braided kevlar elsewhere on the rocketry forum. It is a hug improvement over just sewing the end over, and hiding the raw edge with heat shrink as I have done in the past. I used a bamboo skewer to pull the raw end of the kevlar into the center of the kevlar braid, and sewed the loop closed with some 100 lbs test kevlar thread.
10 finger trap.jpg

To be continured.
 

JLebow

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The G10 is used for the sled has a tensile strength 35,000 -50,000 psi. Using the smaller number, the tensile strength is around 10,000 lbs. Just in case the sled should break, I added a length of kevlar running the length of the sled, to be imbedded into the sled tabs and hooked into the shackle attachment of the harness.

Here is a balsa spacer to keep the loop open during glue up.
01 emergency kevlar.jpg2 kevlar and tabs.jpg

Here is a detail of the deploy charge electrical feedthrough.
3 deploy charge feedthrough.jpg

I've made deploy cannons out of 1/4" pex tubing and they work well for my 38 mm airframe and 1.2 grams of BP. When I scaled up to 5/16" pex and 1.8 grams for larger rockets, the pex ruptures. These brass fittings should do the trick.
4 charge cannon.jpg
The electronics are mounted and wiring in progress. I made a 1.5 inch long tube to get extra length on the TeleMetrum. I can't move the board any further to the left because the booster is carbon fiber, and the GPS receiver antenna would be obscured:
6 telemetrum side.jpg
7 easy mini side.jpg
For battery retention, I made covers from three layers of 6 oz fiberglass. I made forms based on the size of the of the cells. The forms are covered in PVA.
8 battery coffin.jpg
The epoxy is allowed to cure.
9 battery retention.jpg
The part is de-molded.
10 battery retension.jpg
Here the cover is trimmed up and mounted to the sled with M3 nylon screws.
11 battery cover.jpg
 

JLebow

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I like screw switches a lot. You get a secure electrical connection, that can be access from outside the rocket with a phillips screw driver. Here is how I make my own from brass screws, nuts, and sheet stock.
1 homemade screw switches.jpg
I put a small amount of flux under the nut, with a half wrap of thin solder. The I hit it with the propane torch. The flux bubbles and then the solder wicks all the way around the nut. The wires are soldered on with a solder iron.

I layout where I want the holes using a wrap of paper. The marks are equally spaced along the circumference. The mounting locations of my screw switches are made of scrap G10 and are glued onto my sled. This is the critical part - getting the sled positioned into the ebay and airframe with the screw switch structure directly under the holes I'm about to drill.
2 Layout.jpg
This V block helps keep the drill bit aligned. The diameter of the bit is a slip fit for the screw of the screw switch.
3 drill jig.jpg

If everything when well, I have some holes fairly centered on my screw switch structure. I install the top plate of the switch on top of my G10 bracket and the plate with the nut on the bottom. I then apply some epoxy around the perimeter of the brass shims to secure everything in place.
5 final easymini.jpg4 final telemetrum.jpg

So when the sled is reinstalled into the ebay tube and slid into place into the rocket airframe, my screw switch is perfectly aligned. I will re-drill the holes above the screw heads, because these holes are doubling as my static pressure sampling ports for the barometric sensors on the altimeters.
6 good alignment.jpg
 

JLebow

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So, we obviously didn't have a launch in April. Our May launch was recently scrubbed as well. Nothing to do but continue building rockets, grow a quarantine beard, and wait until it is safe and responsible to participate in launch activities again.
Corona beard.jpg
She is starting to look like a real rocket.

Up next is the nose cone tracking bay. I started by making a cardboard template of the interior nose cone at the midline.
10 cardboard template.jpg

Next I cut out a section of G10. In the nose, I'm going with a Eggfinder mini. There is a chance that this flight will go out of range of the Eggfinder, but as backup tracker to the TeleMetrum, I'm OK with that. I get about 1/2 mile range once the rocket is on the ground. The tab on the lower right corner is the mounting location of my homemade screw switch. I'm going to start doing more of these tab and hole joints with my G10 work. They make really strong bonds that aren't completely dependent on the shear strength of the epoxy.
20 layout.jpg

I'm securing the battery with a fiberglass clam shell cover. The mounting bracket at the bottom is odd shaped, so that I can clamp it down to the bulkhead with the eyebolt and keep the G10 board centered in the nose cone. With a plain "L" bracket, there is interference with the eyebolt, and I lose space offsetting the electronics.
30 populated.jpg

This is the form I used to make the bracket. The sheet metal is screwed to a 2x4 to hold the otherwise floppy sheet metal form at 90 degrees. Each time I need to make a nose cone payload bay, I just cut a few inches from this stock I made.
40 right angle bracket.jpg

The bulkhead is 4 layers of laminated G10, leftover from an old kit that was upgraded to metal ebay end caps. Using the vise to drill and later tap mounting holes. I find the top dead center of the bulkhead by dropping the drill bit (unpowered) onto a piece of scrap G10 on top of the bulkhead. I slide the vise left or right until the G10 is perfectly horizontal as it is clamped between the bulkhead and the drill bit. I clamp the vise down. Now I just rotate the bulkhead (it centers itself in the groove at the bottom of the vise) to each of the 4 points marked with sharpie for perfectly aligned holes into the bulkhead. The sharpie marks are transferred to the bulkhead from holes that were first drilled in the coupler.
50 bulkhead.jpg

Here the bulkhead and electronics are mounted and secured with 4 flat head screws. The counter sunk screws do not interfere with the main airframe.
60 harness anchor eye.jpg

Here is one final look at the electronics, all wired up. The screw switch was placed by drilling the access hole on the coupler and the through hole on the screw switch mounting plate at the same time, while the electronics sled was mounted into the nose cone. Doing it this way is not too difficult with transparent fiberglass parts, but the layout can be done with careful layout on opaque parts too. The benefit is that the screw switch access is in perfect alignment with the screw. In the past, when drilling the access hole after the fact, I've had to drill the hole several times and end up with a mess and a giant hole in the rocket. I really like xt30 connectors. They are small, polarized, only mildly aggravating to disconnect, don't seem to fatigue the wire at the end of the housing (like micro JST), and the bullet style connection is more than capable for firing deploy charges.
70 wired up.jpg
 

JLebow

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Makes my level 3 project look like an E2X!
Thanks. There are many ways to enjoy the hobby, and the only criteria that matters is the enjoyment you derive from doing it. My favorite parts are the build season, hanging at the campfire during a launch weekend, and that two minute period between when your flight card is announced and motor ignition. If that thrill ever goes away (it won't) then the hobby will be over for me.

Bulkhead and Retainer
The Aeropack minimum diameter retain is a great option. The main drawbacks: it is fairly long (to provide a good glue up area) and inspecting or replacing the harness is difficult because it is not removable. I've seen on the forum modifications to make the aeropack removeable, but in the end I decided to make my own.

Starting with a 1/4" thick scrap plate of aluminum, I scribed my layout with dykem and fired up the band saw.
10 layout.jpg
An octogon is a much easier starting shape than a square for the lathe.
20 turning.jpg
I carved out three areas with offset turning, leaving behind three radial spokes. This lightens the mass of the part quite a bit, while retaining a lot of the strength. I made a rod that is tapered to a point on one side and cupped on the other. The rod is pressed between the work and the tailstock of the lathe. I can adjust the 4 jaw chuck until the dial indicator shows no wobble while rotating the chuck by hand. This ensures that my layout point is centered.
30 offset turning.jpg
In general, I have struggled with getting holes drilled into the correct locations. I blame my thick glasses, but whatever the reason, I lack the coordination to hold a hand drill vertical. As my collection of tools have grown over the years, I know have a drill press. That on it's own still isn't enough to align a hole. My solution is fixturing. If I align my tube alignment jig (homemade V block) so that the drill bit hits the bottom of the V, then my hole will be drilled at top dead center of the rocket tube. I used another clamp to set the position along the length of the rocket for the hole. Now I only need to control rotation to get a hole in the correct spot. I use a sheet of printer paper with marks dividing the circumference in 3 equal segments.
40 drilling holes.jpg
I slide the bulkhead into the airframe, and transfer the location of the holes with inkpen to the bulkhead.
60 bulkhead installed.jpg
After drilling the holes (using the drill press and drill vise), the holes must be tapped. Taps are hardened and brittle. The same lack of skill with a hand drill contributed to my share of broken taps. I have not had a broken tap since I made this tap jig. Tap lube is helpful, even in aluminum, given the gummy unknown alloy of my scrap stock and the small diameters of the holes I commonly tap for rocketery (#4 and #6 hardware).
50 tapping machine.jpg
I installed a ubolt for retaining the harness. There is not enough thickness in this design for an eyebolt to share the center tapped hole.
70 ubolt.jpg
80 ready to install.jpg
There is a rather long 3/8" threaded rod connecting the motor forward closure and bulkhead retainer. This is to keep my options open in the future and allow the possibility of someday mounting the RMS 75/7680 case.
90 installation.jpg
Here is one of three flat head screws securing the bulkhead, counter sunk to be flush with the exterior of the airframe.
100 countersink.jpg
 

JLebow

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I designed and made my parachutes. For the main, I'm going with a 65" diameter, toroid (iris, or pull down apex), and the drogue is a 16", 70% ellipse. The math isn't too bad. Once you graph the cross section, the length along the perimeter becomes the height of the gore. The X axis dimension is the radius of the chute, so the gore width is the circumference for the given radius, divided by the number of gores.
10 design.jpg
I read the dimensions directly from the laptop, as I layout the shape on a piece of masonite.
15 layout.jpg
I have an engineering ruler, and it has inches divided in 10ths. If only there was some other base 10 measurement system out there. Actually, I find a mm scale a bit too fine, and 0.1" is about ideal for this job.

Finally all of the youtube videos on ship building is paying off. Instead of "eyeballing" a (not) smooth line between my layout marks, I bent a stick of wood until it touched all of my marks and traced the smooth (fair) lines on my template.
20 layout.jpg

Next is to cut out the template, and sand it too the line. As a double check, I measure the length of each side of the template to ensure that they are equal. This is important later on when we get to the sewing. This is the finished main chute template.
30 template.jpg

For cloth, I'm using some rip stop nylon fabric from JoAnn. They always ask what you're are making, and I get a few funny looks. If you cut this with scissors, the material frays. Seeing $200 price tags for hot knives sends me packing for the shop.

I turned some copper to hold an exacto blade.
50 making hot knife.jpg
And fit into this 40 watt solder iron.
40 hot knife.jpg

I then ground down the sharp bevel on the blade, making it blunt, to let the heat do the cutting. This is rev 2, as 25 watts had a cutting duty cycle of 50% with pauses to let the blade re-heat, and the 40 watts lets you cut nylon all day long.
39 Hot knife.jpg
 

JLebow

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I get a drag coefficient of 2 to 2.3 on the larger toroid chutes, based on altimeter data (complete with wind perturbations and real-world conditions), and the designed cross section area of the chute. On the smaller drogue chutes (I've tested 10 and 18 inch diameters) they don't do any better than a spherical type, so I don't bother with the complexity on my drogue.

I cut my gores on top of a piece of window glass. It is a nice smooth surface the hot knife glides across. These wood templates are a step up from paper patterns, which tend to get smaller over time as I fail to trace the pattern exactly the same each time. The substantial templates are well worth the time investment.
10 drogue on glass.jpg
Here is my stack of gores for the drogue.
20 stack of cut gores.jpg
I don't have pictures of the sewing. I use a home grade sewing machine, in a zig-zag pattern. For thread, I'm using nylon upholstery thread. The black ribbon that forms a loop for the shroud lines is 1/4" grosgrain. It was recommended by a kitemaker, and in 4 years (50 flights) no failures with the parachutes (once they deploy). The knot I use to secure the shroud lines is called a Kreh Loop. I won't slip, or come undone without a bit of work. I have tried bowline, but they work loose (luckily I caught it before flight).
80 shroud line detail.jpg
Here is a screenshot as I "play/test" with the new chute in the living room.
100 drogue video capture.jpg
And the toroid is too big to fly indoors.
90 toroid chute.jpg
But not too big to fit in the airframe, wrapped in nomex.
65 main packed in upper airframe.jpg
What else is there to say about home made parachutes?
 
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JimJarvis50

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I finally got around to checking back on your build. Lots of good ideas and I love the fins. Two suggestions....

First, your charge cannons look very much like the high altitude deployment charges that I use. They work in a vacuum. However, they have a heck of a kick, and I mount them on a thick piece of plywood. I think they are probably OK on your G10, but I would suggest ground testing.

Second, you have some flat head screws holding your bulkhead in place. I like to make connections like that using a socket screw with the top of the screw flush to the surface of the air frame. The larger diameter of the socket head, relative to the shaft of the screw, makes the connection very strong. Will survive anything.

Jim
 

JLebow

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I finally got around to checking back on your build. Lots of good ideas and I love the fins. Two suggestions....

First, your charge cannons look very much like the high altitude deployment charges that I use. They work in a vacuum. However, they have a heck of a kick, and I mount them on a thick piece of plywood. I think they are probably OK on your G10, but I would suggest ground testing.
Jim, thanks for the suggestions. There is a good reason the cannons look familiar. I really enjoyed your engineering approach to testing deploy charges in a vacuum chamber, in the article you published in Rockets Magazine (June 2011).

I will add plywood between the base and G10 bulkhead. Ground testing is not optional in my opinion, either.

Second, you have some flat head screws holding your bulkhead in place. I like to make connections like that using a socket screw with the top of the screw flush to the surface of the air frame. The larger diameter of the socket head, relative to the shaft of the screw, makes the connection very strong. Will survive anything.

Jim
To clarify, are you describing the configuration I have drawn with the red socket head cap screw? My current configuration is the yellow flat head screw. The red bolt has a huge amount of material to resist shear. The yellow bolt is clamping the airframe to the bulkhead.
bulkhead.png


I really appreciate the feedback
 
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JimJarvis50

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To clarify, are you describing the configuration I have drawn with the red socket head cap screw. My current configuration is the yellow flat head screw. The red bolt has a huge amount of material to resist shear. The yellow bolt is clamping the airframe to the bulkhead.
Yes, that's correct. With four of them, the bulkhead gets pinned and can't move, even though they don't actually clamp the air frame. I use that approach on lots of things and have probably done it a hundred times. I've never had one pull out .... and I've tried. The holes can be drilled with common drill bits (i.e., no special sizes are needed). When I install them into plywood, I use a pilot tip drill bit, and then I have the centered location for drilling the screw hole.

Jim
 

kbRocket

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How are you going to protect the carbonater in engine #4 from aliens? I'd hate for an incident to mess up your cert attempt.
 

JLebow

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Obviously this is not a normal year in any respect. With a limited launch calendar, there won't be many chances for my son to come out and fly with me this year. So when he expressed interest in building and flying some rockets this year, I dropped my L3 project for a bit and headed into the shop with my 9 year old co-pilot. His interest in rockets is not full blown, and I have to really check my enthusiasm, listen to what he wants to do, and not push him when he is ready to take a break. I really don't understand not being crazy and totally sold out for rockets, but I do understand that if I push, he will quit altogether. Got to keep it fun, and let the fire build within him, on its own. This hour and half recovery in a corn field last year was almost too much.
Corn recovery.jpeg

My boy has an infatuation with the Estes Bertha series. We have baby, boosted and Super Big Bertha - everything but the original Big Bertha. He says that in 2 years, he wants to do his jr L1 on a 6 foot bertha. He may get the Polecat Madcow Big Bubba as a birthday present in a couple years. The rocket he is holding is one he designed himself. It is a two stage ring fin, painted to look like candy corn. He had a mini candybar as payload. It flew really good on a D12-0 to D12-6.
In the Shop.jpg

Here is Boosted Bertha on the pad, and being recovered in the sage. I think a C to C is a bit light in the booster, given the fin area and 10 mph winds that are perpetually blowing at the launch site. We decided to build a new booster fin can with a D. Even with a high arching flight, we recovered with no zipper.
Boosted on the pad.jpg Recovery.jpg

The view of Three Sisters Mountains are a great backdrop for rocket camping. Sunset isn't bad either.

3 Sisters.jpg Sunset.jpg

Now back to the L3 build... the scheduled launch is going to be the weekend of July 17, just a few days away!!!
 

JLebow

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Only a very special few love sanding. I'm not among them. I also find that if I try to build up my epoxy layers too quickly (thickly), that I get runs and greatly increase the time I have to spend doing the sanding. I have thought about building a rotisserie, with a slow gear motor to keep the rocket rotating like a slow roasted chicken while the epoxy cures. Maybe someday. I wanted to maintain the circular cross section shape of my airframe that can be distorted during the sanding process. This is what I came up with - and I will say there is a lot of room to refine this technique.

Rotating shaft (3/4" metal conduit) and frame.
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Something for the drill chuck to grab.
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6 wood boards glued up into two blocks (3+3), and held together with 4 lengths of threaded rod and wing nuts.
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6 small holes, and 1 large hole. The large hole diameter was opened on the lathe to equal the outer diameter of my rocket plus the thickness of two sheets of sandpaper. The center hole breaks into the circle of small holes.
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What we have here is a rocket OD cylinder hone.
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JLebow

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Here, the hone is clamped around the airframe. If the wing nuts are tightened too much, the hone will freeze up against the airframe. I found the best results were with two threaded rods on one side only and a gap on the other side. The flex in the wood provides a bit of variable spring force. The small holes provide a break in the sanding surface, allowing the dust to clear out.
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I made these tapered end caps to jam into the rocket, and these aluminum hubs to clamp onto the conduit.
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If you try this at home. Make a handle for the hone, and keep your hands away from the threaded rod ends. If the hone grabs the rocket, your hands need to be clear of any spinning metal parts. I put a zip tie over the drill trigger (on a fairly slow speed), and ran the hone up and down the airframe. A lot of dust was generated.

I did a lot of coarse sanding with the drill and hone setup, but I still did final sanding by hand. Jim Jarvis wrote an excellent document on how he builds his rockets, and one of the best tips is after coarse sanding, apply a very light coat of epoxy to your rocket, and wipe most of it off with a rag. This fills all of the coarse sanding scratches, greatly reducing the time spent hand sanding. It is important to avoid drips and runs, so use just enough epoxy to erase the sanding scratches.

With hand sanding, it is so easy to put a flat spot into a round rocket. I use a foam sanding block. I keep it aligned along the long axis of the rocket, and make a motion as if I'm following an imaginary candy cane stripe painted on the rocket. I switch directions, and the pattern left behind is a series of X's. Let's see if this gif I made works.
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Watching this in a continuous loop makes me anxious, because the last thing you want to do is sand too much, and cut into the carbon fiber (or glass fiber if you are working on fiberglass). If you break through the epoxy only layer, stop right away. If more sanding is needed, add a coat of epoxy, let it cure, and sand some more. With practice and patience and a lot of thin epoxy layers, it should be possible to avoid ever sanding into the fiber reinforcement.

I use automotive body filler (bondo), in very small amounts to fill pin holes, and to get my upper and lower airframe to line up better.
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But when I have rocketpoxy left over from another operation, I will use it as body filler. Here I am filling the seam of my nose cone halves.
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This is automotive rattle can primer (rustoleum or duplicolor primer work well for me).
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This is engine block orange and engine block clear from Duplicolor.
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