A Level 2 Cert with Cardboard - Build Thread

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Not a shrubber
TRF Supporter
May 16, 2020
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Doerun, GA
Short version: L2 cert flight upcoming with a cardboard rocket.

I guarantee there are plenty of flyers out there who'd like to move up in the HPR world, but the cost of building a fiberglass rocket may be staggering to some of them. But it doesn't have to be that way...

An old flyer from the 90's is being resurrected here, made in standard LOC tube. I'm documenting it, partly to keep my ADHD self from falling too far behind on it. I'm also documenting it for those who are concerned that Level 2 flights require the toughest and most expensive products to be successful. There's not going to be anything cutting edge here, quite the opposite. Just a simple, 3FNC. It will be about as inexpensive as I can make it, and it won't be a fast and furious flight. But as a 3", 5-pound rocket on a small J-motor, it won't be especially low and slow, either. At least not for an east coast flight.

First, a little history on the original:
A 3"/38mm cardboard and plywood rocket, it was the first one I had "designed" myself and was my first scratch built. Made from a single length of LOC tubing. It was built quickly and with parts I had on hand simply to have something new for the next launch. I was bored with what I had. On hand items were one full length of 3" tube, a length of 38mm tube, a piece of 1/4" birch ply for fins and 3/8" hardware ply for centering rings. Oh, and one can of paint. White. Nosecone, recovery, etc was borrowed from another rocket.

Named* "Floyd", it turned out to be a predictable and consistent flyer. Easily handling the 38mm motors for which I had cases (Aerotech only), I had to get the 720Ns case for the "new" J350 to push it further. After a couple of those flights, Floyd became my first rocket with dual deployment, using a Black Sky AltAcc. From inception and throughout it's rocket life, Floyd was a learning tool for me to try new ideas on. It evolved, and eventually ended up with a 54mm MMT with an odd shock cord attachment, glassed airframe, G10 fins, and a mach flyer on J800T according to wRasp. That J800T flight was it's last, as I forgot to arm the altimeter. There was no electronic data to retrieve. However, there was quite a bit of mechanical data in the trash bag.

I only have two pictures of Floyd. One is the short first version in frumpy white that I didn't like in the least. It almost didn't get taken out in public. This look only for lasted a couple of flights, when it had proven itself worth keeping.
Floyd 2nd flight.png

The other is this fuzzy picture in flight, which a friend found in SOAR's website, and is the longer version of 38mm Floyd in black.

Floyd on J350 June 1998.jpg

After rebooting my flight status last year, it's time to get L2 certified again. Having gone through several bouts of indecision on which project/rocket to use, I came to the conclusion it would only be proper to rebuild Floyd. That had been my intention for the last 20 years anyway, so let it be written!

OK, so I've actually already started the build at this point and some of this has been posted elsewhere, but we'll see about catching up and continuing here on the process.

I started with vintage VCP printouts from the originals, which were used for measurements, along with info from the 1st picture above.

VCP Printouts.png

This was transferred into Open Rocket. After recreating the 2 originals shown, the longer version was modified a little and adapted as the new 2021 resurrection.

There are two main deviations from the original- I'm using a 54mm motor mount because the suppliers I was buying other parts from didn't have a full-length 38mm tube. And I'm using G10 fins because the plywood I had wasn't straight enough, and there really wasn't time to keep messing around with it. (I won't buy plywood for fins without laying eyes on it first, and there's no place around here that stocks it.)

So here's the preliminary OR image ready for construction:

Refloyd II preliminary.png

This is about 5" shorter the original, and that's because one of the body tubes was received with a shipper's ding right in the middle. Not a big deal, I only needed about 18", so I didn't bother getting another.

After estimating weights and lengths, etc, this looked about right, so it was time to make some parts...

*At the time, I had recently attended LDRS (XV?) in Orangeburg. I noticed the number of menacing rocket names, like "Fear This", "The Flying Pyramid of Death", and such. After a couple of days hearing that, I tired of it. So this rocket was given a name as if a member of the family, and I chose Floyd after "Floyd the Barber" because he was the opposite of a menacing, evil threat to humanity.
My goal is to L2 Cert on my Binder Design Velociraptor (伶盜龍) with cardboard (hardened with wood hardener). I've heard that BD kits have been able to handle some seriously large motors without being glassed.
This one doesn't go to eleven!

I'll be using 54/852 or 38/720 motors to try to keep it visible.

When are you planning to fly your Mac?
Good question. The next club launch is two weeks from today. I think I could pull it off by then. If not, definitely the December launch.
OK, first up are the fins. Cut to rough shape from a 12" x 12" piece of G10 from Hawk Mountain with a tungsten carbide blade. Cheapest WalMart jigsaw, and you can see the workbench. Fined them to size on belt/disc sander, nothing fancy happening here.

Fin layout.jpg

Fins 1.jpg

Fins 2.jpg
Fins 3 (2).jpg

Instead of using a belt sander, they could be cut very close to shape with a jigsaw and finished with hand files/sandpaper, jitterbug sander, whatever you have.

I typically just round over fins. Something made me do a little bevel on these. My brain said no, leave well enough alone, but my hands weren't listening and did it anyway. Again nothing fancy or measured, just simply done by hand and eyeball on the belt. Finished fins are shown here with the next step, centering rings:

Good question. The next club launch is two weeks from today. I think I could pull it off by then. If not, definitely the December launch.

Cool, good luck with it!

Our launch is in 3 weeks and if it isn't done, it'll be November of 2022. Because of that, I'm hoping to have it done in time to give it a low/slow maiden flight to check things out.
On to the centering rings. I'm using Midwest Products 3/8" 7-ply from Michael's. It was about $5 for a piece big enough to make 3 centering rings. I actually got two pieces because I wanted to experiment with a circle cutting jig I picked up with a used router. It cut decent circles, but was more trouble than my old jigsaw/hole saw methods and still had to be finished by hand. One day, I'll build a router table, but that ain't today!

Pattern drawn out, center "marked" with the hole saw, then rough-cut the OD with a jigsaw:


Fined in the OD with the belt/disc:


Rough-cut the ID with a hole saw:


Fined in the ID with a drum sander on a hand drill. Still nothing fancy here:


Centering rings done, test fit:



At this point, with three 3/8" rings and G10 fins, I'm at $15.00 in parts.
Next, cutting the tubes to length. I usually mark, then wrap the tube with 2"+ masking tape, using the parallel edges of the tape to keep it square, and cut along the edge of the tape with a fresh razor blade. No pics of this.

The booster tube was cut to 20", as the original was. As in the original, the remaining 14" is the lower payload section for the drogue, if I use one. I like the break point here. It leaves roughly 8" of single tube unsupported between the booster's coupler and the avionics bay above it.

The forward payload bay was cut to just over 14", from the second 3" tube that was received kinked from shipping. I wanted it 18", this is close enough. The other end of the kined tube yielded an additional 10" tube for other purposes. I'll paint it to match and it can be reserved for unforeseen events. I cut a switchband from the end of the kinked section, as it will be epoxied to a coupler and the light wrinkle won't matter.

Using a section of mailing tube that happens to perfectly match the LOC 3" tube ID, I made a tube sander based on this post, and used it to even up all the ends. I have 80 grit on one end of the sander and 220 on the other end. Cut, sand, cut sand...

Tube sander1.jpg

Tube Sander with coupler inserted.jpg

Tube fitting.jpg

With 2 sections of 3" LOC tube, the material costs are now at $32.58
As mentioned, the original rocket was built with a 38mm motor mount, because I already had the tube. I wanted to build this one the same way, and then build an additional 54mm booster later to exchange, but time/availability/shipping just all pointed to doing the 54 now. It will give me a better selection of motors to choose from anyway.

The mount was cut about 22", to the maximum motor length plus a little length for a Giant Leap ACME Hardpoint Anchor and a short thrust ring inside the tube to push the forward closure an inch or two back from the anchor. That's mainly about having the option to not use the aft thrust ring, and that will not affect the cert flight.

The weather was good enough to paint, so I took a couple of airframe pieces out for primer. After priming a payload section and the switchband, I realized that this rocket was essentially in kit form. So, all parts were gathered for a group photo, which was bombed by our lab:

Floyd kit with Bella.jpg

Adding in the motor tube, ACME anchor, ACME rail guides and couplers, we're now at $57.93 in materials.

That's the bulk of the cost because, like the original rocket, the nose cone and recovery items will be borrowed from other rockets.

I could have shaved a couple bucks off by buying a short piece of 3" tube for the forward payload bay, replacing the ACME hardpoint with an eyebolt or a section of shock cord epoxied to the motor tube, and using launch lugs/rail guides that I already have. But I like those ACME items and I wanted the extra bits of 3" tube as leftovers.

Next up, slotting the tube.
Looks good. Both my L1 and L2 cert flights were cardboard rockets. Right now I'm building a duplicate of my L2 rocket (38mm motor) with a 29mm motor mount. I think it will see more flights in our field (limitations).

One way to keep costs down is to use common components; nosecone, parachute and recovery, any electronics, etc.
My switchband cutter, this is cutting 54 mm bands for later. I used a block of acrylic instead of plywood for cutting the 3" tube because it was sturdy and the right thickness. It's very quick and easy cut bands this way.

switchband cutting.png

Speaking of tools... I spent an afternoon making a cool router jig (based on a TRF post) to cut slots in a 3" tube. As the afternoon wore on, I realized that I could manually cut the slots in less time than it would take to cut the last few holes in the jig. That had held me up for the afternoon, but wasn't wasted time, because I'll use it on a 3" fiberglass project down the road. After that off-track afternoon, I decided to write my ADHD-self some "kit" instructions to help stay pointed in the right direction. Of course I didn't finish the instructions... But what I got written did help a little bit.

While mulling over what type of slots I wanted, I went ahead and primed the other payload section and started painting them. Krylon gloss 2-in-1 black was my only paint choice, so I reluctantly bought it. I don't like the nozzle or the way it goes on. It ain't Mrs. Right, but it was Mrs. Right-Now.

_painted payload 1.jpg

I decided to slot the tube only for the fin tabs, rather than do full slots & build the can externally. I think the assembly order was bothering me. Instead of arguing with myself about it, I simply cut the slots to fit the fin tabs and will deal with assembly in steps ordered for that method.

_fin slot cut.jpg

With tubing this thick, I don't bother supporting it underneath. I just don't use much pressure, use a sharp (new) blade and only use the tip of the blade to start. Super-carefully and gentle on the first pass, and if it starts to go off course, turn the tube around and cut from the other end to correct it. I do this free hand, no straight edge, so that all of my attention is devoted to the where the blade tip is going.

After about 3 passes, it has started to cut pretty deep, and I reduce the blade angle so a longer portion of the blade is cutting. 5 passes is usually enough to start to cut through in spots, so I go ahead an insert the blade all the way through and slowly pull it down to the end of the slot.

Cut the ends last, they take very little effort.

_fin slot done.jpg

Test fit the fin after each slot

_test fit fin.jpg

After all slots were cut, I realized that I could assemble my rocket and see it complete again!

_Heres Floyd.jpg
With most parts cut, there is now a reasonable estimate of weights and dimensions and it's time to look at Open Rocket again.

When the original Floyd was flying in the previous millennium, I used wRasp to predict altitude, and around 5k' just sticks in my memory as typical prediction for baby J flights. I don't remember predicted speed, but it was spunky. I may have a printout somewhere.

(side note: IIRC, the only 38mm RMS J at that time was the J350W. It was designed with grains small enough (62.5g) to avoid the need for a Low Explosive User Permit and advertised as "Easy Access".)

Updating OR with current info and running sims for 38/720 and 54/852 motors gives these results:


Hmm... 🤨 Now it's time to watch the P's and Q's a bit with a cardboard rocket. Those sims are a little faster and higher than a gut-check approves for a cert flight. Most of those are over 600mph, pretty much 0.8 mach and up. Out of all those, only 3 are below 600mph, and two of those slow flights are well over 5000'.

I was expecting more like 500mph/5000' for a zippy, visible cert flight where my worries will be limited to properly functioning recovery devices, and keeping it on the field. If I could choose, I'd probably ask for 50 mph off the rod, 550mph max and 4000' altitude. I might even choose motor ejection for that flight.

With speed and altitude being borderline for my liking, let's look over how strong it appears to be. Will this airframe have to support much weight, and where will it be located? Here's the current setup:


The booster is going to be pretty stout with a full length motor tube, 3 centering rings and TTW fins. The forward CR will butt against a coupler epoxied to the airframe for mating with the lower (drogue) payload tube. Otherwise, it's fairly standard stuff. Everything is cut by hand, so nothing's very precisely fit, and no special attention devoted to construction for high speed or thrust.

Ahead of the booster's coupler is a 14" section of airframe. Then the AV bay, and another 14" section of airframe before the nosecone. So there are only two 8½" lengths of tube that are single-wall when assembled. I feel better now, but 600mph+ and 6k' are still beyond the target.

An Eggtimer Proton altimeter with batteries will go in the AV bay, and a Mini Transmitter with battery to add to the nosecone, along with the associated mounting hardware. That's a few more ounces in that location.

With the 38mm J-270 and adapter as shown, CG is plenty far ahead of CP. Other good-candidate motors stay in the same ballpark. Since CP/CG locations aren't close enough to need extra nose weight, perhaps it would be better to move the Mini transmitter and battery to the AV bay. The additional weight will still move the CG forward, just not as much, and prevent having to build another electronics bay into the nosecone. The AV bay can be made from an 8" coupler instead of 6", reducing the single-wall lengths to 7½". The switchband could also be offset to leave a little more coupler length on the aft side.

Of course, as things get painted and epoxied and screwed in, the overall weight will grow. Most construction, and hence additional weight, will occur in the booster and the AV bay. When it's all done, hopefully the CP/CG will remain pretty close to this sim and the flight will be a little slower and closer to the ground as a result.

That will be the new plan- 8" AV bay with all electronics located there. Re-sim after stuff is put in place. It's time to get building, and we'll see how it starts to shake out.
wow! an unglassed L3 rocket would have been cool to see!

I did my L3 on a cardboard unglassed rocket and it went mach 1.2 with no issues. With more and more fiberglass rockets coming into vogue, it is funny and sad to see the perception of cardboard rockets being not that strong. A well built, properly balanced wood and paper rocket can easily handle high thrust and mach conditions. This was common in the days prior to fiberglass. There is a guy whom I have seen at our launches, who flies his standard LOC EZI65 with 1/8th fins and no glass on a CTI long burn L motor. Wood and paper is a heck of a lot stronger than most people give it credit for.
I was expecting more like 500mph/5000' for a zippy, visible cert flight where my worries will be limited to properly functioning recovery devices, and keeping it on the field. If I could choose, I'd probably ask for 50 mph off the rod, 550mph max and 4000' altitude. I might even choose motor ejection for that flight.

Thats what I was curious about. My 6.5lbs (edited measurement units) LOC/Madcow 3" J350 sims at 94.2 velocity at off rod, and ~4250' in altitude. Thinking along the lines of J285 myself.
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Jumping in to armchair quarterback... Mach 0.8 is 614 mph. Before discounting the motors with a top speed of 600-650 mph, I'd take a look at how long it stays above 600 mph. Most of the times I've looked at that for my rockets; it's been around a second or two. That's not that much time in the grander scheme of things.

Also, with G10 fins the size you have, I wouldn't have any heartburn about passing Mach, though I understand not wanting to do that on a cert flight. Another option would be to put some ballast right near the recovery harness' anchor point in the fin can. That would slow you down a bit without mucking with stability too much. Speed off the rail might be a concern though.
Before discounting the motors with a top speed of 600-650 mph, I'd take a look at how long it stays above 600 mph.

Indeed, all of these motors have a fairly short burn time, and unpowered, the rocket wouldn't stay at top speed for long. Nothing discounted yet!

Don't sweat too much about getting transonic with cardboard.

No sweat here, under 600 mph is just my version of low and slow for a cert flight!

It's good to see all the comments in defense of cardboard flights!