Almost Supersonic with a Bone Stock Alpha III and a D21 (Theoretical)

After the kids went to bed last night, I was goofing off in rocksim and noticed with a fully stock alpha III Model (yes one of the old school models) and an AT D21 motor you can almost reach supersonic speed. Rock Sim Showed max velocity > 730 mph with an apogee of >2300ft. I suspect with the solid one piece fin can and low cross sectional area the model could handle it, This might be a good candidate for an altitude record, only thing is would there be a chance that I'd ever see this thing again?

For kicks this got me wondering, what if i pull the motor mount and sanded the ID of the fin can a bit so I could fit a 24mm motor in there, so I loaded an AT-F39T and added 20g of ballast to the nose. Still stable and shows Max-v of 776.56 and altitude of >4700 ft. I believe under normal circumstances and temperature that would be considered > Mach 1.

While interesting on paper, would there be any realistic probability of ever seeing these models again? Has anyone recovered a model of this scale after that kind of flight?

Will you ever see it again? Probably not but who cares unless you’re using a reloadable casing. I’ve been thinking of building an MD 24mm using the disposable F44...

I would wager good money the thin balsa fins will flutter and disintegrate at those speeds without some careful engineering.

jnobels said:

Will you ever see it again? Probably not but who cares unless you’re using a reloadable casing. I’ve been thinking of building an MD 24mm using the disposable F44...

I would wager good money the thin balsa fins will flutter and disintegrate at those speeds without some careful engineering.

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Yeah thats why I was considering the alpha III as opposed to its predecessor the Alpha, as the Alpha III has a one piece injection molded fin can.

Balsa has a better chance than the thin plastic Alpha III fins.

Those injection molded fins/cans probably won’t fare much better...

But you won’t know until you try

jnobels said:

Those injection molded fins/cans probably won’t fare much better...

But you won’t know until you try

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Definitely tempting, there's a cat on eBay that broke open a bulk pack and is selling them for about 8 Bucks each:

https://www.ebay.com/itm/Estes-Flyi...e=STRK:MEBIDX:IT&_trksid=p2055119.m1438.l2649

about an hour's worth of assembly work, and as much mylar streamer, shock cord and tracking powder I can cram in there, seem's like a fun shot from the sod farm

Hmmmmmm....

I have an OpenRocket sim of the Alpha III which tracks well with actual measurements on A-C impulse motors (I've been flying one with a PerfectFlite FireFly stuffed in the nose cone). It doesn't predict quite that much velocity or altitude on a D21....but still a combination that would only be recovered on a sod farm-type site and with a nice bright reflective streamer for recovery.


I have no idea if the fin flutter models out there can cope with polystyrene fins that taper in thickness from root to tip as they do on the Alpha III but it would be an interesting thing to look at for this sort of insanity.

Personally I think if I were to try a D in an Alpha III I'd go slightly more conservative and do a D10.....

I attempted a D21 scratch built Alpha a few years back. Fins were made from very stiff .75 carbon plate. I had created a fin can with warm CA to harden the tube. Fins were set through the wall with Rocket Poxy and 10mm fillets. Everything looked great. I launched it from my small PVC tower and promply lost a fin. The fin and its root were intact but the flutter had torn the side of the tube off. Pretty dramatic considering I used similar construction on a G80 powered Machbuster that pushed to 1.71 with no problems. The Alpha III fin can may be the best bet. I did a couple low power Machbusters since with the D21. Getting them back has always been a problem. Fin shapes were drastically different.

I've routinely built balsa fin Alpha stock using only wood glue to fly with AT E15's at our club launches. The only modification was the use of a another tube that slides in the BT-50 tube to keep the ejection charge from bubble ling the outer tube. If the kids get the rocket back, I install a standard engine mount. Though it doesn't have the get up and go like the D21, it's a B-E-A-U-tiful flight from a distance.

Keith

I used to fly my Alpha III on E25s all the time. The fin flutter was audible.

DankMemes said:

While interesting on paper, would there be any realistic probability of ever seeing these models again? Has anyone recovered a model of this scale after that kind of flight?


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Yes - I have flown my supersonic aspire to ~4000ft and recovered successfully twice on a g80:

https://www.rocketryforum.com/showthread.php?142536-A-supersonic-Apogee-Aspire

Secret is: big streamer and an eggfinder mini gps in a lengthened nose.

For supersonic, double diamond fin profile with paper, CA and epoxy attach and epoxy putty fillets.

IIRC if your d21 bird weighs more than about 5g dry, you will not make it to the top end of the transonic region. Getting a mach-proving altimeter in there and back again would be quite the challenge.

I've launched two Estes Yankee's on D21s, the first one disappeared after a straight up flight, that had epoxy fillets with glassed fin roots. the second just had epoxy fillets and also flew well, I was able to track that one on the way down but it landed on the other side of some trees in tall grass and I was not able to find it. I don't actually think you need epoxy for a D21, there are a lot of G's but there is not much mass and not much drag on those models so the forces probably arn't as extreme as you think. I could be wrong though.

Oh my - a Yankee on a D21. That would disappear in a hurry for sure.

I agree - 60 Gs on something that weighs a gram or so (a balsa fin on a Yankee, for example) is still not a lot of force.

A while ago, I built a "super Wizard": 18mm min diameter, 1/8" plywood airfoiled fins, small epoxy fillets, streamer, slightly longer than a standard Wizard. It broke fins off on landing on nearly every flight by delaminating the body tube. I eventually put a D21 in it. It flew great but almost certainly subsonic, and I did see it on descent, very far away. You could probably have fit a Firefly in it and used that to compare sim altitude to actual and take a shot at max speed.

boatgeek said:

A while ago, I built a "super Wizard": 18mm min diameter, 1/8" plywood airfoiled fins, small epoxy fillets, streamer, slightly longer than a standard Wizard. It broke fins off on landing on nearly every flight by delaminating the body tube. I eventually put a D21 in it. It flew great but almost certainly subsonic, and I did see it on descent, very far away. You could probably have fit a Firefly in it and used that to compare sim altitude to actual and take a shot at max speed.

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I’m hoping that the JLA4 is as small as the mockups John has posted - that sucker would fit in a BT5 nosecone...

retortec said:

I attempted a D21 scratch built Alpha a few years back. Fins were made from very stiff .75 carbon plate. I had created a fin can with warm CA to harden the tube. Fins were set through the wall with Rocket Poxy and 10mm fillets. Everything looked great. I launched it from my small PVC tower and promply lost a fin. The fin and its root were intact but the flutter had torn the side of the tube off. Pretty dramatic considering I used similar construction on a G80 powered Machbuster that pushed to 1.71 with no problems. The Alpha III fin can may be the best bet. I did a couple low power Machbusters since with the D21. Getting them back has always been a problem. Fin shapes were drastically different.

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That looks sweet! Maybe a thicker fin and tapering the edges?

Here’s why I think an alpha III can endure the flight, after over 25 years an alpha III is still flying regularly in my fleet. Beat to hell but it’s still flightworty.

I flew a small quest on the D21. Left one plastic fin on the pad, the other 3 held long enough for it to go pretty much straight up. Performed exceptionally and lived up to the project name: "single use" Might have actually gotten it back except it came down in the swampy area next to the river...but someone actually had eyes on it!