Making plans for StratoSpear again

[Adrian A]

The fins came out straight, as far as I can tell, but I discovered that my nosecone was crooked. When I put the motor in and rolled it on the countertop, the nosecone tip wobbled about 1/4".

The main problem that caused the misalignment was that I put the CF tube section in too far when I was re-molding the nosecone, and it pushed up onto a wrinkle that was sticking into the nosecone, jacking it sideways. It's a little subtle, but you can see the wrinkle in the fiberglass section at the bottom in this photo and where that pushed the aft CF section out of round:



I checked and confirmed that the location of that wrinkle corresponded to the direction that the nosecone was crooked.

I decided to take fairly drastic action and cut off the cylindrical section at the bottom. First I made an alignment tool by cutting out two wooden centering rings that could work with the hollow steel rod that fits over the harness attachment hardware near the tip of the cone. Those were press-fit into the carbon fiber airframe section that I would glue in so that in theory, the straightness of the rod would ensure that with one end of the rod centered in the tip, then the airframe section at the other end would be aligned.



After I dremmeled out the old overlapped section of CF tubing and sanded down the front, I got a nice 1/2" of tapered overlap. I did a test fit and chucked the hollow rod into my drill to make sure it looked like it would be correctly aligned.



Meanwhile, I had some fillets to apply. Mixing up 15 grams of my Cotronics 4461 used up an alarming fraction of my remaining resin. Yesterday I ordered more but it takes them a while for Cotronics to ship. So I decided to go with small fillets. I may make them larger later.
Here is the rocket body and fins after prep for the fillets:


I oven-cured the nosecone joint and the fillets, and when they came out, the nosecone straightness was much improved, basically at the limits of what I can evaluate and the looseness of the nosecone over the motor.

 

[Adrian A]

I flew the sustainer at the NCR August launch as a single stage, and described the last prep and the flight over in the high power subforum. It was an interesting flight that resulted in fin fluttering that cracked the fins about 0.5 seconds after motor burnout and more than doubled the Cd. I have the sustainer airframe rebuilt up to putting on the fin fillets, but I'm waiting for my next shipment of Cotronics epoxies and paint.

In the meantime, I'd better get going on the 2nd stage or I'll run out of time. I'm making the second stage airframe out of the same 1.500 ID, 1.570 OD Rockwest composites tubing that I made the sustainer. Again I'm using a RotoZip power tool with a flap sander to open up the ID enough to install the Loki motor, like I did on the sustainer.

This time I decided to try clamping the RotoZip in place and then putting the tube over it. Bad idea, the spinning rod was too unstable.



Next up was to clamp the tube and run the flap sander with the extension rod down the length of it.



I wasn't making a whole lot of progress, and I decided to substitute in a shorter rod, though this one was stainless, rather than high-strength steel. Yikes!



Honestly, this is all a bit marginal. I'm wearing leather gloves to help stabilize the rod. After running the RotoZip for a while, it got hot enough to blister the rubber coating. I'm letting it cool down now.

I have made some progress, and the ID is opened up enough for one of my Loki 38-1200 cases, which is allocated for the sustainer, but only partial insertion so far for my other two Loki cases, which are newer.

 

[Adrian A]

Enlarging the ID of the 2nd stage tube was a PITA, but yesterday I got it done. I switched to a 1 5/8" dia flap sanding wheel which I made softer and smaller by aggressively sanding some concrete, so that I could squeeze it into the tube. I did a bunch of cycles of running it until the RotoZip motor overheated, then cooling the tool down on a fan inside. Sometimes I would get 1/2" more motor insertion depth on each cycle, sometimes zero. Yesterday I switched it up to hand-sanding it while the tool was cooling down, using my 1 3/8" dia oak rod, with 60 git sanding paper wrapped around some paper towels that squeeze the sanding paper up against the inner wall of the tube. It was a good upper body workout. Finally, an ugly victory photo:



The hand-sanding tool is on the left. It ended up being at least as fast as the power tool. Now all but about an inch of the Loki 38-1200 case fits in, which is fine because 2.5"-3" will need to stick out the back anyway. The forward end of the tube is too small for the new cases, but it's the perfect fit for the back end of the sustainer's 38-1200 case. I'm really pleased with how solid that connection is going to be.

I also like how thin and light the tube is so far. It's only 1.57" OD, and about 120 grams so far, though I'm going to reinforce the front end with a biaxial sleeve where the sustainer motor is the coupler. I usually think of +/- 45 degree weave of a biaxial sleeve as being pretty non-optimal orientations for a tube under bending. But in this case the stress is going to come from the sustainer motor trying to deform the front end of the 2nd stage tube, and I can imagine that adding +/- 45 fibers to the existing 0/90 degree fiber orientation would be useful for that case.



Yesterday I sanded off the shrink wrap epoxy marks off of the outside, finalized the 2nd stage fin design, cut them from fin stock and sanded them to final shape. Then I glued on the first fin overnight with JB Weld. Today I cut the tube to the approximate length and the 2nd fin's JB weld is curing in my car parked in the driveway. Too bad this stage is too long to fit into my oven, but the car will be a close approximation once the sun gets a little higher.

I'm going to go as far as I can today with finishing the 2nd stage airframe, including adding the fillets and then doing a secondary layup of carbon over the fillets. I think the 2nd stage fins might not really need that reinforcement, but I'm going to do it anyway to make the stage more durable, and also to get practice doing the reinforcement layup for the sustainer. I'll use the 810 epoxy from Soller composites while I'm waiting for my Cotronics order to arrive, since the 2nd stage is only expected to see Mach 1.9. I placed the Cotronics order on Aug 1, but now it's not expected to arrive until Monday, about 4 weeks later.

 

[Adrian A]

Three fins glued on and the mass is up to 139 grams.

 

[Adrian A]

Now with fillets:

 

[The_Quacken]

Best of luck. I saw that flight in august and I was suprised by the result to be honest. I hope the next attemp goes well this rocket deserves a good flight

 

[watheyak]

Now with fillets:
View attachment 599881
View attachment 599882

This are some good looking fillets!

 

[OzHybrid]

You've still got a LOT of sanding to do.....

 

[Adrian A]

Yeah, it was straight and clean for the time it was visible. I was surprised when it only went to 17k.

 

[pugachu]

I thought you were going to wait and do the fillets with your high temp epoxy. Did the Cotronics stuff show up already? I am debating if I want to get a higher temp epoxy for my next 54mm rocket. the Cotronics stuff is kinda pricey. I've used Aeropoxy on my current 54mm carbon bird, and it hasn't shredded yet. I don't push my stuff as hard as you do, though my highest altitude is about 14K.

 

[Adrian A]

You've still got a LOT of sanding to do.....

Not until the secondary layup

I thought you were going to wait and do the fillets with your high temp epoxy. Did the Cotronics stuff show up already? I am debating if I want to get a higher temp epoxy for my next 54mm rocket. the Cotronics stuff is kinda pricey. I've used Aeropoxy on my current 54mm carbon bird, and it hasn't shredded yet. I don't push my stuff as hard as you do, though my highest altitude is about 14K.

This is the second stage, which will only see Mach 1.8. I’ll use Cotronics for the sustainer next week. It could get up to Mach 4.3.

 

[Adrian A]

Here's another photo of one of the fillets, which shows how I like to extend them in front of the fin and in back.


My motivation for that is to smooth out the cross-sectional area transitions a little to reduce wave drag. I've thought a little about trying to reduce the fillet radius a bit where the fin is tallest, but I haven't implemented that yet.

This morning I did the 2nd stage layups for fin reinforcement and the tube reinforcement at the front end. I took a bunch of photos this time to show the details of the layup and technique. The fin reinforcement is at least partly for practicing for the sustainer layup, and to get a feel for how much more CF I need to add.



Above is the plan and a template for the layup. The innermost triangles are unidirectional layers to help give the fin a more rounded profile. I was shocked recently when I was updating in RasAero, and found that increasing the fin thickness from 0.07 (which had flutter failure) to 0.100 took 10s of thousands of feet off of the predicted 3-stage altitude. I have been using the "rounded edge" option in RasAero. Out of curiosity, I switched to the bi-convex profile, and suddenly the predicted apogee altitude jumped up to 174,000 feet, even with the 0.100" thick fins.



I'm not planning to attempt a perfect biconvex profile, but I think having a smoothly varying profile would be a good idea. Along these lines, I looked at the unidirectional carbon I have available, it was 4.9 oz, 9 oz, and a much heavier thickness. I was planning to try the 9 oz, but decided to use 2 layers of the 4.9 oz, with the stepped profile. The smallest profile is in the inside, then another layer of 4.9oz that goes most of the way to the edge, and then I have a plain weave layer at +/- 45 degrees that covers the whole fin to maximize resistance to twist.

Skipping ahead over a few obvious steps, I reproduced the pattern in paper, then wet out the carbon inside of a clear garbage bag, and used spray adhesive to stick the templates to it for cutting out.

 

[OzHybrid]

If you can maintain the integrity of the sharp leading edge of a biconvex profile its a good idea. But carbon fibre turns into a shaggy dog once it starts to heat fail. I've not seen a cd for a shaggy dog at M4.3 but its not going to be good. At that point the shaggy dog becomes even more shaggy rapidly and your rocket slows down. There are plenty of shaggy dog fin pictures on the forum.
An aluminium fin might be better if you want that profile although it comes with its own set of challenges.

 

[Adrian A]

Agreed, the shaggy dog is to be avoided. Currently I’m debating between wrapping the leading edge with very thin stainless sheet metal stock, or just going with the well-known technique of forming the leading edge with neat Cotronics epoxy.

If one of those methods can make the leading edge survive, the fins will be thinner, stiffer, and much lighter than aluminum.

 

[OzHybrid]

Agreed, the shaggy dog is to be avoided. Currently I’m debating between wrapping the leading edge with very thin stainless sheet metal stock, or just going with the well-known technique of forming the leading edge with neat Cotronics epoxy.

If one of those methods can make the leading edge survive, the fins will be thinner, stiffer, and much lighter than aluminum.

Stainless would not be permitted under Tripoli rules for body construction. Not saying I think it's any worse than CF, but that seems to be the current ruling. And for a record flight it needs to be legal.

 

[Adrian A]

Here’s the stage in its vacuum bag:


I also added a biaxial sleeve to the front of the tube, starting at the front of the 2nd stage motor, to help with stiffness for this very long rocket.

 

[pugachu]

Agreed, the shaggy dog is to be avoided. Currently I’m debating between wrapping the leading edge with very thin stainless sheet metal stock, or just going with the well-known technique of forming the leading edge with neat Cotronics epoxy.

If one of those methods can make the leading edge survive, the fins will be thinner, stiffer, and much lighter than aluminum.

10+ years ago there was a 4" Goblin drag race on K motors at LDRS. Those of us who built with plywood fins were encouraged to wrap a piece of aluminum HVAC tape over the leading edge of the fin. It worked for me, but I was only going 1000mph, not mach 4+.

 

[Adrian A]

Stainless would not be permitted under Tripoli rules for body construction. Not saying I think it's any worse than CF, but that seems to be the current ruling. And for a record flight it needs to be legal.

Stainless foil bonded to carbon would not be frangible, which is the motivation for the rule. Also it wouldn’t really be structure, more like a surface treatment. And it’s obviously safer in a CATO than steel recovery hardware, which is totally legit. So IMO would be pretty silly for someone to raise it seriously as an issue. I suppose I could use Al foil instead but the CTE mismatch would be worse, so it would have a greater chance of delaminating which would make the rocket less safe than if stainless foil were used.

 

[watheyak]

I'm pretty sure Nic Lottering's N record rocket had stainless steel capped leading edges. They were beautiful.

 

[boatgeek]

Is it just the general prohibition on metal airframe parts? If so, my understanding was that TRA allowed metal parts where necessary. That’s obviously a judgement call, but I would think that fin cuffs would be a standard allowed practice for high speed flights.

 

[OzHybrid]

Stainless foil bonded to carbon would not be frangible, which is the motivation for the rule. Also it wouldn’t really be structure, more like a surface treatment. And it’s obviously safer in a CATO than steel recovery hardware, which is totally legit. So IMO would be pretty silly for someone to raise it seriously as an issue. I suppose I could use Al foil instead but the CTE mismatch would be worse, so it would have a greater chance of delaminating which would make the rocket less safe than if stainless foil were used.

Get a formal ruling on it from the records committee. Before the flight. Then we'll all know. Or don't bother and risk the record.
In the interim, here's the formal ruling. From the Tripoli website link.

https://s3.amazonaws.com/ClubExpressClubFiles/795696/documents/Metal_in_Rocket_Construction_v2.0_1848375092.pdf?AWSAccessKeyId=AKIA6MYUE6DNNNCCDT4J&Expires=1693023794&response-content-disposition=inline; filename=Metal_in_Rocket_Construction_v2.0.pdf&Signature=3Ycjz6aed5kVTm7owIa+zVZf5yA=

It clears up whether you can or cannot use stainless, there is no mention of the thickness, the answer is no. However, I do not believe it fully clarifies the use of metal

 

[OzHybrid]

I'm pretty sure Nic Lottering's N record rocket had stainless steel capped leading edges. They were beautiful.

I believe they were stainless. I've raised this before. Plugger has possession of the rocket. Photos of the fins were requested from Plugger, but never provided.

 

[Adrian A]

I'm happy with how the layup turned out:

It came out of the bag at 177 grams, and then after trimming and sanding it's down to 169 grams. The fins are satisfyingly stiff. I can put most of my weight on the rocket when it's laying on the ground before the fins noticeably flex, and I think they are even stiffer in torsion, which I've learned is the important axis for preventing flutter. They are 0.07 at the edges and 0.098 in the middle. If I ever use a 54mm M as a booster, I think these fins would hold up to Mach 3+ They do need some more work on the surface finish though, and normally next I would add a skim coat of thickened epoxy to sand down from here.

I have been experimenting a bit with wrapping the edge with stainless (bottom) and aluminum (top) foil:



The stainless foil is a lot springier, and seems like it would be more difficult to bond it down flat. The aluminum foil is more malleable and I feel more confident working with it. One edge is also also 0.4 g vs 1 g. I think I'll continue experimenting for the sustainer using the second stage and bond it down with another vacuum bag and do the skim coat at the same time

 

[Adrian A]

I used thickened epoxy to glue on the leading edge foils and coat the fincan. Then I used peel ply and put it into a vacuum bag to cure. It's now 172 grams:



After a little bit of cleanup sanding, I'm not worried any more about the leading edge foil delaminating. I'll need to be careful not to sand through it, though. When I do this on the sustainer, I think I will do another coat of thickened epoxy at this stage to help round out the fin profiles and blend in the trailing edge of the foil.

 

[pugachu]

Looks great, Adrian!

How thick of aluminum foil did you end up using?

 

[Adrian A]

Looks great, Adrian!

How thick of aluminum foil did you end up using?

3 mil

 

[Steve Shannon]

That looks great. If you’re experimenting with high temperature foils, copper is also considered a ductile material by Tripoli and withstands heat even better than aluminum.

 

[Adrian A]

Last week I decided to go for a boosted dart for the Saturday launch at Tripoli San Luis Valley (Alamosa), with my new 2nd stage and my new sustainer. I did a lot of rework and improvements along with just getting ready. For example, I flew the sustainer at Airfest as a single stage (covered in the high powered rocketry subforum), and after having a failed nosecone ejection at apogee because the apogee charge wires had a hard short when they got pinched by the nosecone ejection piston, the main chute backup fired but jammed the baseplate of the noseone ejection piston into the nose, trapping harness that also trapped the main chute in the nosecone tip. So this week in addition to rebuilding the sustainer airframe and fins, I lengthened the inner piston to fix both problems. I made a 2nd stage an-bay, I added stuff to my sustainer av-bay for airstarts. Etc Etc.


After pushing hard with 12+ hour days all week to get ready, I still wasn't quite done until I got out to the launch site on Saturday. I had a couple of good ground tests and a good radio and electronics check.
View attachment 1694282859228.mp4

View attachment 1694274710225.mp4
But there were still so many things that I had to get right, and I wasn't confident that I did them all correctly. On top of that, the landing site there is high altitude (7600') and rocky, with no way to up-size my chutes, so even if I did everything right, there was a decent chance for damage. So I decided to scrub the launch. It was 4 hours each way (but a scenic drive) just to do a couple of ground tests, but I'm happy to come back with 2 undamaged stages. Now I have time before BALLS to make a few more improvements and testing to make sure everything goes smoothly out at the playa.

 

[Adrian A]

Ok, time to fish or cut bait on my final sustainer surface finish. The sustainer sims to 4400 feet/second at 30,500 feet (3500 feet ASL), which is about Mach 4.4.

Currently it's carbon with a couple of thin coats of thickened epoxy. My previous attempt to wrap the leading edge with 3 mil aluminum foil, bonded on with Cotronics, failed completely in a Mach 2.5 test flight. I probably could have done a better job with the surface preparation and bond line thickness, but I doubt that would be enough to make it happy at Mach 4.4, so I'm abandoning that plan.

I bought some Cotronics 1000F Aluminum spray paint, and I have been trying it out today on example fins and tubes that are the same construction as my sustainer. The directions for the epoxy call for a post-cure schedule of 1 hour 250F and 1 hour 350F. I did that. Then the paint directions call for a 1 hour+ post-cure at 450F. With some trepidation, I did that too. The epoxy of the tube stopped ringing and the paint got a little soft (scrape-able with a fingernail) while they were hot, but otherwise didn't have a catastrophic failure. After they cooled, the parts were normal again and were not misshapen (the tube stayed round, for example). But now I'm wondering if the paint is worthwhile if it's just going to be soft enough to erode, anyway. I got this paint because a previous Cotronics stainless steel spraypaint (Duralco 6106SP) had the best surface finish after a Mach 2.5 test flight when I tested a variety of paints and finishes on my previous sustainer 10 years ago. Unfortunately, that spray paint does not appear to be available any longer.

 

[Adrian A]

After putting my test fin and tube in the toaster oven at 450F for a while longer, the paint doesn't seem so soft now. I guess you just need to really keep hitting it hard with the high temperatures.

It's interesting when I take those parts out though and set them on the counter when they are are that hot. Instead of a ringing noise like they usually make, the noise is more of a dull thud as if they were made out of hard rubber. The test fin still seems stiff, though.

After getting the sustainer body and fin can with the surface finish I want, I am reluctant to paint it. The stagnation temperature of the swept fin leading edges (after applying cosine of the 70 degree sweep angle) is supposed to be around 250F. The nosecone, though, I think can use all the help it can get. It's also uglier so I'm more interested in a cover-up.