StratoSpear: 38mm 3-stage for complex L record attempt

[Adrian A]

Now for the build thread, starting with the chute cannon. A 24mm FG tube JBWelded to a FG bulkhead. This will get a bolt through the center to hold it on to the end of the coupler that sticks out of the sustainer body. It's a little long for my sustainer main chute but I can trim it down later.

 

[Adrian A]

Prep for reinforcing the chute cannon:



There's 1 wrap of unidirectional carbon (looks like a coarse weave fabric), some more uni to wrap around the cylinder at the base where the charge will detonate, and a braided carbon sleeve to go over everything. I cut the little nylon threads holding the uni together for about an inch along the bottom so that I can splay the fibers out radially on the base. The blue is peel ply

In the past, when I have done these chute cannons, I have tried to use the 4 threaded rods from the Featherweight 38mm av-bay to hold the cannon onto the end of the sustainer. This caused to issues, one was that it was pretty challenging to connect the charges because the nuts were right up against the cannon, and those nuts also ate into the available space for reinforcement of the cannon attachment, causing the whole thing to be a bit fragile. Now with the cannon bolted down through the center, I can have structure all the way out to the outer diameter and it should be a lot stronger. The uni carbon is oriented to keep the cannon from breaking off if the sustainer body tumbles at touchdown, and the uni in the other direction is to reinforce the thin-walled FG tube to keep it from splitting when the charge first goes off and the piston is at the bottom. I may do another wrap at the other end to make that end stronger for the nosecone ejection piston attachment I have in mind.

I'm also getting ready to make the nosecone. I plan to make it carbon except at the base, where the GPS receiver and most of the LoRa transmitter antenna will be. Since the base has to slip precisely over the coupler, I normally splice in a section of premade airframe tube into the mold during the layup. Unfortunately, when I looked at the FG airframe tubing I have in hand, it was all a looser fit than I wanted, plus it's filament wound without fibers going in the ideal directions (0-90), so I decided to wrap my own section of FG tube over a 1.5" mandrel that I have from years ago.

The mandrel is covered with mylar that is sealed with scotch tape underneath. Looking back at it after the layup, it was a mistake to have only sealed the section that I did the layup on. We'll see tomorrow how bad of a mistake that was.

The layup was 2 wraps of a medium-weight 0-90 FG cloth, followed by the medium-weight braided sleeve to hold everything together. Below is an image of how I like to wet out the fabric, sandwiching it between two pieces of plastic and using a squeegee to get even resin content


I'm using epoxy from Soller Composites that I was impressed with on some previous builds. The mixture ratio is 18:100 by weight, so I printed out a cheat sheet for use with my scale. Squirt out the hardener first, and then tare the scale and squirt out the appropriate amount of resin.


I think I want to update the cheat sheet to include a third column for total weight so that I don't have to tare it and have time to forget the numbers.

I'm pretty out of practice with my vacuum bagging, but I think it it was adequate in the end. The chute cannon reinforcement bag came out better than the tube.

 

[Adrian A]

Prep for reinforcing the chute cannon:
View attachment 530517


There's 1 wrap of unidirectional carbon (looks like a coarse weave fabric), some more uni to wrap around the cylinder at the base where the charge will detonate, and a braided carbon sleeve to go over everything. I cut the little nylon threads holding the uni together for about an inch along the bottom so that I can splay the fibers out radially on the base. The blue is peel ply

In the past, when I have done these chute cannons, I have tried to use the 4 threaded rods from the Featherweight 38mm av-bay to hold the cannon onto the end of the sustainer. This caused to issues, one was that it was pretty challenging to connect the charges because the nuts were right up against the cannon, and those nuts also ate into the available space for reinforcement of the cannon attachment, causing the whole thing to be a bit fragile. Now with the cannon bolted down through the center, I can have structure all the way out to the outer diameter and it should be a lot stronger. The uni carbon is oriented to keep the cannon from breaking off if the sustainer body tumbles at touchdown, and the uni in the other direction is to reinforce the thin-walled FG tube to keep it from splitting when the charge first goes off and the piston is at the bottom. I may do another wrap at the other end to make that end stronger for the nosecone ejection piston attachment I have in mind.

I'm also getting ready to make the nosecone. I plan to make it carbon except at the base, where the GPS receiver and most of the LoRa transmitter antenna will be. Since the base has to slip precisely over the coupler, I normally splice in a section of premade airframe tube into the mold during the layup. Unfortunately, when I looked at the FG airframe tubing I have in hand, it was all a looser fit than I wanted, plus it's filament wound without fibers going in the ideal directions (0-90), so I decided to wrap my own section of FG tube over a 1.5" mandrel that I have from years ago.
View attachment 530515
The mandrel is covered with mylar that is sealed with scotch tape underneath. Looking back at it after the layup, it was a mistake to have only sealed the section that I did the layup on. We'll see tomorrow how bad of a mistake that was.

The layup was 2 wraps of a medium-weight 0-90 FG cloth, followed by the medium-weight braided sleeve to hold everything together. Below is an image of how I like to wet out the fabric, sandwiching it between two pieces of plastic and using a squeegee to get even resin content
View attachment 530514

I'm using epoxy from Soller Composites that I was impressed with on some previous builds. The mixture ratio is 18:100 by weight, so I printed out a cheat sheet for use with my scale. Squirt out the hardener first, and then tare the scale and squirt out the appropriate amount of resin.
View attachment 530516

I think I want to update the cheat sheet to include a third column for total weight so that I don't have to tare it and have time to forget the numbers.

I'm pretty out of practice with my vacuum bagging, but I think it it was adequate in the end. The chute cannon reinforcement bag came out better than the tube.

View attachment 530513

 

[Adrian A]

No problem getting the FG tube off of the mandrel, but it looks like only half of it is usable due to the mylar bunching up underneath at one end to make a nasty wrinkle, Half is all I need, though. The good news is that the ID is perfect, a nice snug fit on the end of the sustainer. I'll sand off the wrinkles on the good half and then have a new layer of FG between this part and the mold.

The chute cannon also came out a little wrinklier than I wanted, but it's fully usable as-is. It feels hard as steel and it rings at a frequency almost too high to hear. I don't think I need more reinforcement for it. It's 30 grams. I just need to trim the OD of the base and glue on a little reinforcing/centering bulkhead of a smaller diameter that fits inside the coupler.



The bolt and t-nut you see in the photo are what's going to hold the chute coupler onto the end of the sustainer, under the nosecone.

 

[3stoogesrocketry]

Update to the motor selection: Going for a Loki K1127 to a Loki K627 to a CTI J530 (the longest motor compatible with my 2nd stage and sustainer. If the flight goes off correctly, it will go ...ah... really high.

I'm guessing your sims start with a 1. I know the K627 is a must have for altitude .

 

[Adrian A]

I'm guessing your sims start with a 1. I know the K627 is a must have for altitude .

This is just an innocent little 38mm diameter rocket.

 

[mikec]

This is just an innocent little 38mm diameter rocket.

Good luck with the Class 3 committee paperwork there, Adrian.

 

[Adrian A]

Some work on the parachute:
My current chute packs nice and small, but it's really beaten up, and it's a little undersized for a sustainer that might land without burning its motor.




The way I like to do my chutes is to start with a flat disc of thin ripstop and make pleats or darts to convert it from a parasheet to a sorta-hemispherical parachute. This gives most drag per packed volume that I know of. The chute above will land 300 grams at 15 feet/second and it packs into about 2" of a 24mm tube. Based on this performance, I'm scaling it up from a 24" original disk for the yellow chute, to a 31" diameter disk in some new bright pink fabric that I bought for this:



If the sustainer lands with an unburned motor, with the old chute I would expect 37 feet/second descent rate, and 30 feet/second with the new one.

All that material does fit into the chute cannon, but just barely. I'm hoping that when I cut out the darts, that will make enough room for shroud lines, piston, and deployment charge. If not, then I'll need to start over with a little bit smaller size.



I calculated how much of the perimeter of the disk would need to be removed in order for the disk edge to be the open edge of a hemisphere, and divided that between the 6 shroud line attach locations. Then I freehanded a curve that would go tangent about halfway to the center of the chute. Next up, sewing the darts and the shroud lines. I'll see if I like the shape before cutting of the darts. Then hopefully it fits into the chute cannon. I guess I could also extend it a little with the construction of the nosecone ejection piston if I need to. Fingers crossed.

I have been debating with myself whether I want to order some fresh Cotronics epoxy for when I make the new nosecone. I have some from about 10 years ago but I'm not at all confident about the shelf life for that. The epoxy I have now can be cured at 200F, and maybe post-cured somewhat higher.

The sustainer I built about 10 years ago has a variety of finishes on the fins that I applied as a high-temperature surface test. Out of 4 finishes I tried, the one that came out the best on a Mach 2.5+ flight was the Cotronics 1200F stainless steel spray paint. I'm planning on refinishing the fins with that paint, and maybe the nosecone too. But first I wanted to see if it would affect the GPS reception, so I painted a spare FG tube yesterday and today I tested the reception inside the painted tube that was closed off at both ends with motor cases:



There was a little bit of variation in the GPS strength when I put it all together, but then it popped back to reception that was at least as good as the open-air version. So I know what the sustainer is going to look like now; dark silver nosecone and first part of the cylindrical section, black carbon midbody and silver fins. I should also probably give the 2nd stage fins the same treatment, because they will see Mach 1.8 at about 10,000 feet.

 

[Adrian A]

I'm happy with the shape and the seams, executed by my skilled quilter fiancée Karen, who also got her L1 cert earlier this summer. I may trim the lower edges straight, especially if I run out of volume. Currently, they have the radius of the original disk.

 

[Adrian A]

Some new circuit boards arrived today, including a variation on my standard 38mm av-bay passive bulkhead. This one fits inside the ID of the coupler, and it has a plated through-hole for a t-nut. I manually added the holes for the t-nut prongs and then glued it on with some JB-Weld. Also shown is the screw that bolts the chute cannon to this bulkhead. The bulkhead shown here will be recessed into the sustainer coupler by 1/4" or so. The electrical connections for the main (24mm chute cannon) apogee (18mm nosecone piston) are made under nuts against this bulkhead, with wires around the threaded rods that come up from the active bulkhead of the av-bay. I'll need to drill some small holes around the base of the chute cannon for the 2 charges and the tracker antenna. Using the 4th channel of the Blue Raven for the sustainer head-end ignition at the other end of the av-bay, and with the sustainer separation charge performed by the 2nd stage, I'll have the 3rd channel of the Blue Raven available here if I think it makes sense to have a backup charge in the nosecone piston. There won't be a lot of room up there, and I'm not sure that the backup charge wouldn't get ignited by the primary charge.

 

[Scrapmaster87]

View attachment 530885
I'm happy with the shape and the seams, executed by my skilled quilter fiancée Karen, who also got her L1 cert earlier this summer. I may trim the lower edges straight, especially if I run out of volume. Currently, they have the radius of the original disk.

I have a torroidal/pull-down pattern about that size if you want something a little more compact. Clean shape for a spherical though!

 

[Adrian A]

I have a torroidal/pull-down pattern about that size if you want something a little more compact. Clean shape for a spherical though!

My impression of the toroidal chutes is that the extra seams and risers would negate the packing advantage you would otherwise get. I'd like to be proven wrong, though.

The one I'm holding there, after the risers were attached, fits into a 4.5" length of 24mm tube.

 

[G_T]

The drag coefficient is a lot higher with a toroidal, and the lines don't have to be done the usual way. Particularly with a small chute, you could probably gather 4 to at most 6 lines worth in the middle of the chute, and pull down that center point with one line. The top hole is larger than a usual spill hole. The diameter of the chute should be about 0.7 to 0.75x the diameter of an elliptical chute. Tune the length of the center pull-down line to maximize drag.

The decreased diameter also means those outer shroud lines are shortened proportionally. That loss of outer line compensates very well for the addition of the top/center line, and you have only about half as much fabric. So it should come out more compact. I'm used to toroidal chutes packing notably smaller than the otherwise suitably sized elliptical would.

I use toroidal chutes for mains, of a couple different brands. I haven't made my own though.

Gerald

 

[Scrapmaster87]

The drag coefficient is a lot higher with a toroidal, and the lines don't have to be done the usual way. Particularly with a small chute, you could probably gather 4 to at most 6 lines worth in the middle of the chute, and pull down that center point with one line. The top hole is larger than a usual spill hole. The diameter of the chute should be about 0.7 to 0.75x the diameter of an elliptical chute. Tune the length of the center pull-down line to maximize drag.

The decreased diameter also means those outer shroud lines are shortened proportionally. That loss of outer line compensates very well for the addition of the top/center line, and you have only about half as much fabric. So it should come out more compact. I'm used to toroidal chutes packing notably smaller than the otherwise suitably sized elliptical would.

I use toroidal chutes for mains, of a couple different brands. I haven't made my own though.

Gerald

This

Even accounting for the seams, there is significantly less material in a torroidal than a eliptical of the same diameter. I did an experiment last year and made a 24" eliptical and a 24" torroidal. The result (under experiment conditions) was a chute that weighed 15% less and had 10% more drag!

 

[Adrian A]

The decreased diameter also means those outer shroud lines are shortened proportionally. That loss of outer line compensates very well for the addition of the top/center line, and you have only about half as much fabric. So it should come out more compact. I'm used to toroidal chutes packing notably smaller than the otherwise suitably sized elliptical would.

I use toroidal chutes for mains, of a couple different brands. I haven't made my own though.

Gerald

That's pretty persuasive.

This

Even accounting for the seams, there is significantly less material in a torroidal than a eliptical of the same diameter. I did an experiment last year and made a 24" eliptical and a 24" torroidal. The result (under experiment conditions) was a chute that weighed 15% less and had 10% more drag!

Nice experimental setup!

I have a torroidal/pull-down pattern about that size if you want something a little more compact. Clean shape for a spherical though!

I'm game if you want to post those.

Thanks.

 

[Scrapmaster87]

So I just cooked up a 20" chute pattern that should fit on standard printer paper. It's based on the test chute I showed previously, in particular it has a an extended skirt. I was gonna turn that option off, but I've actually flown this chute. I didn't want to give you something unproven!

If you were wondering, that 24" chute weighed 0.85oz. I used 1.1oz rip-stop and 50lb Kevlar shroud lines from Emma Kites.

 

[Adrian A]

So I just cooked up a 20" chute pattern that should fit on standard printer paper. It's based on the test chute I showed previously, in particular it has a an extended skirt. I was gonna turn that option off, but I've actually flown this chute. I didn't want to give you something unproven!

If you were wondering, that 24" chute weighed 0.85oz. I used 1.1oz rip-stop and 50lb Kevlar shroud lines from Emma Kites.

Thanks for the pattern! Since you flew it, do you have descent rate, mass and altitude data? I wonder how much it would be affected if the shroud lines were shortened somewhat.

 

[Reinhard]

Another source for patterns is Thomas Schmid's ChuteMaker:
https://chutemaker.lfence.de/The tiling feature currently only supports ISO paper formats, but Acrobat Reader can do that too, so you can use that instead if needed.

The source code is available here:
https://github.com/rckTom/ChuteMaker
Reinhard

 

[Adrian A]

I sewed the gores together on Sunday and today I will attach shroud lines. Sewing slippery silicone-coated ripstop can be quite a pain.

In other progress, I made an 18mm nosecone ejection piston. I started with an 18mm composite motor as a mandrel, and then wrapped it in mylar. There is a real trick to this in case anyone is interested in making their own tubes. 1. Wrap and cut the mylar with some overlap. 2. Use a razor blade to cut through both layers of mylar ("Carpet seam") 3. Put tape on one edge of the mylar with sticky side facing out. Thin, supple tape works best. 4. Pull the mylar tight-ish around the mandrel and press the edge down on the exposed tape. The mylar should be preloaded snug so that it won't wrinkle when the layup is compressed, but not so tight so that the mandrel doesn't move freely underneath. This usually takes me a couple of tries. It's easier with a small piece like this.

As I reminded myself recently, it's important for the mylar to be sealed past the ends of the mandrel, though the diameter out there isn't critical.

I did an initial layup with 2 different diameters of biaxial woven sleeve, wrapped tightly with peel ply. Here is the mandrel sliding out pretty easily after the layup cured:


Then I cut and squared off the tube, cut out a 24mm bulkhead and glued it on. After that cured, I added a bunch of reinforcement to contain the force of the BP, including uni carbon wrapped around the base, and 2 more layers of biaxial sleeve. The result is pretty bombproof I think, but still lighter than a metal tube. Here it is installed on the end of the chute cannon:


I'm probably going to drill out the propellant from this D engine and use it for the moving part of the piston. Underneath the FG portion you can see will be a Blue Raven, a Featherweight Tracker, 2 batteries, a magnetic switch, a screw switch, and a USB battery charger for the Raven. I have a separate build thread for the electronics over in the electronics section.

 

[Adrian A]

I have some parachute results. Below is the basic hemi chute I already made, and below that in the pink and red is the toroidal chute I sewed this week from the pattern from @Scrapmaster87.



The toroidal chute packs noticeably smaller into a 24mm tube than the hemi chute:


I have two 500g calibration weights for scale, and I used them to do a drag test. Both chutes have identical weights and harness:

View attachment IMG_9589.MOV

Going frame by frame, the pink hemi chute was a little later to inflate, so it starts out lower than the red/pink toroidal chute. But then the toroidal chute catches up a bit, though it's very close, so I think it's got a little less drag than the hemi. There was another drop test before this one where the hemi appeared from above to do a little better. Considering the packing size, the toroid is a little more efficient even considering the shroud lines and seams.

 

[Adrian A]

The av-bay is coming together. I had to go to the hardware store for a Dremel chuck that would work with the 1/32" drill bit for the antenna:



Later I'll do some LoRa signal strength testing to see if I need to insulate the antenna where it goes through the carbon fiber.

 

[Scrapmaster87]

I have some parachute results. Below is the basic hemi chute I already made, and below that in the pink and red is the toroidal chute I sewed this week from the pattern from @Scrapmaster87.

View attachment 532176

The toroidal chute packs noticeably smaller into a 24mm tube than the hemi chute:
View attachment 532177View attachment 532178

I have two 500g calibration weights for scale, and I used them to do a drag test. Both chutes have identical weights and harness:

View attachment 532175

Going frame by frame, the pink hemi chute was a little later to inflate, so it starts out lower than the red/pink toroidal chute. But then the toroidal chute catches up a bit, though it's very close, so I think it's got a little less drag than the hemi. There was another drop test before this one where the hemi appeared from above to do a little better. Considering the packing size, the toroid is a little more efficient even considering the shroud lines and seams.

Nice! Glad I got you a win at least on packed volume, I trust that translates to less weight as well? I've noticed the rapid opening too in flight with the chute I made for the experiment.

In the video it looks like the toroid was trying to fly away from the other chute. I'm thinking it would perform a little better in cleaner air?

 

[G_T]

I haven't watched the video, but something you might not realize is the degree of center pulldown on a toroidal makes a great deal of difference in the resulting drag. It is something you need to tune.

Gerald

 

[Adrian A]

I haven't watched the video, but something you might not realize is the degree of center pulldown on a toroidal makes a great deal of difference in the resulting drag. It is something you need to tune.

Gerald

I played with the center pull distance as I was putting loops on the shroud lines, and I picked a distance that seemed to inflate the chute into the intended shape when I pulled up on it. Is there a way to dial it in more precisely?

 

[G_T]

What I have done is very primitive but turned out to be very effective. I ran down a parking lot with the chute behind me, and had it rigged so I could vary the pull on the central line separately from the rest of the shroud lines. The effect was quite obvious. Note, this was a 5' chute I tested with, so the greater pull might make it more obvious. It is a simple test.

Essentially I put the outer lines on a ring (I used a small quick link) and ran the center line through the ring. So I could pull on both independently. Variable drag is readily achievable this way. Find the max point, and then fix all the lines. Or for anyone who wants to play with a variable drag chute, I've just given most of the info on how to do it.

Gerald

 

[Scrapmaster87]

My test set up was just a substantial floor fan pointed straight up. From there I placed a scale on the fan with the chute tied to a brick. Zero the scale with the fan off then you're ready to go (include the chute on the scale). Fire up the fan and the config that has the greatest "negative weight" should be the best. On my test set up I made a flow straightener out of shipping tubes and clamped a dowel to the ceiling to control spin and sway respectively. My chutes still spun with the straightener, but slow enough to get a good reading off the scale.

Note you'll be moving a lot of air in a small space if you do this indoors. You'll need to note the placement of the fan in the room for repeating the experiment.

One thing that really jumped out when I started testing misc small chutes was how unstable a parasheet was! The eliptical and toroidal chutes we're remarkably stable with hardly any sway.

 

[Scrapmaster87]

I really hope we're not causing you to come down with a case of analysis paralysis. After all, perfection is the enemy of better.

 

[Adrian A]

A little more progress today finishing up the chute cannon while taking a break from Blue Raven development:

I picked out the of Kevlar I want to use in this rocket, and drilled the smallest holes through which this harness will fit, and then used a Dremel sculpting bit to round out the inside edges to be gentle on the Kevlar. One end of the Kevlar will be connected to the nosecone and the nosecone ejection piston, and the other end will be connected to the main chute and main chute piston inside the chute cannon. That reminds me I need to notch out the top of the cannon for a place for that harness to get in under the nosecone ejection piston.

I also drilled out holes for the apogee and main deployment charge wires, and the hole at the bottom of the main chute piston for the main chute ematch to go in. The antenna is the 5th hole, outside of where the harness is looped through. I experimented with putting everything together with an ematch in the main chute cannon. It's challenging but doable to fish the wire through there. I learned how much length I need to leave in the wires for that short run. It looks like I have just enough volume available in between the chute cannon and the e-bay to stuff the extra length of wire after making the connections.


I'm planning for a test flight or two this weekend at the Tripoli Colorado launch, if it's not cancelled for fire restrictions etc. With all of this summer's Northern Colorado Rocketry launches cancelled so far due to fire risk, this will be one of the few times I can launch over 8k feet before Balls, within a 2 hour drive. If I make enough progress on the Blue Raven, tomorrow I may lay up a nosecone with my normal epoxy, so I re-learn how to do it, and so that I have a realistic nosecone to test with this weekend. The Cotronics 4461 resin I ordered isn't expected for another 2-3 weeks.

With a correctly-shaped nosecone I can make better estimates for the 3-stage altitude. Some sims are coming in under 100 kft, depending on how (un)-optimistic I am.

 

[Brian H.]

I'm planning for a test flight or two this weekend at the Tripoli Colorado launch, if it's not cancelled for fire restrictions etc. With all of this summer's Northern Colorado Rocketry launches cancelled so far due to fire risk, this will be one of the few times I can launch over 8k feet before Balls,

I've been following this build and look forward to seeing it in person. I will be there doing my L3 cert.

Just keep in mind our waiver is 8000'.

 

[Adrian A]

I've been following this build and look forward to seeing it in person. I will be there doing my L3 cert.

Just keep in mind our waiver is 8000'.

With call-ins to 14k, right?

Good luck on your L3.