Don't Blink - 0 to supersonic in under 90 feet

JordanT said:

I was wondering the same thing. I didn't calibrate it before the flight. Probably should have, since I just got it back from Adrian who did a repair (I knocked the top off the buzzer somehow, and while checking it out Adrian said the baro sensor was giving anomalous readings so he replaced it, too.).

I was kind of hoping someone would notice that and have a nice, neat answer. I've been full of Christmas concerts since I got back, and haven't even had a chance to unpack the truck.

I did manage to break through a speed barrier this weekend for the launch. Unfortunately, it was a state trooper who verified the "flight" and gave me a special present to take home with me.

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That is a bummer! Since you're from Blacksburg, I assume it was along 81 where they tagged you?

Nope - 64. Actually, I'd rolled through 3-4 traps along the way; I'm a cruise control guy on the open road - always on the safe side of enforcement. I'd taken it off when I passed the exit for 29 because I though I was supposed to turn there, but the GPS said I had like 6 miles still before getting off. I was fiddling with it and not watching my speed as I was gaining on some slow poke in the right lane. I was just shifting into the left lane out of habit when I realized that the slow poke in the right lane was a trooper in a black Dodge Charger as he hit his lights. Oh well, $#!+ happens. He was nice enough, even helping me with the directions (he used to live up that way). I'd complain, but it was my own stupid fault for not paying attention.

Just in case. This spreadsheet may be of some help - https://ebookbrowse.com/138-finflutter-xls-d28945889

I have some 0.75oz glass that I use on balsa every now and then. I think something like that would help the body tube if it collapsed due to weight from the nose. Was this setup for mid body separation or nose blow? If it was set for nose blow - maybe a fixed nose would help offset the potential for collapse.

Looking at this - if a fin did not come off (I'm a little confused on this point). And if the nose did not push in - could tiny minor misalignments exist that caused the corkscrewing behavior?

All three fins came off. Well, I think they did - I found one and a lot of little bits of tube. I need to post picture of the recovered wreckage.

Actually, I had glued the nosecone shoulder in, but I must have forgotten to sand the tube as it didn't bond. I didn't sand off all of the old epoxy, so there was a bit of a taper to the shoulder, and the diameter difference between the NC base and shoulder was small to begin with. If I forced the nosecone in too hard, I could push it into the BT.

Anticipating the G-loading, I static tested a sample of the body tube - about 8" long, longer than the un-supported length of tube - with roughly 12lbs of force (225gs x 20g NC mass x 1.2FS). It was applied axially by balancing weights on the bare tube.

The body tube was jointed in the middle with a baffle. That was one of the weak spots as I wasn't really happy with the coupler fit. It was close, but I thought it was the weakest part of the assembly. Well, weakest before I really considered the nosecone shoulder issue. The BT "broke" cleanly just below the baffle, but other parts of the recovered tube were unspiralled.

Don't Blink Mk II, should it get built, will either have a single piece tube with the NC shoulder properly bonded in OR will have a single piece tube with a lower and upper section separated by a bulkhead and a "thrust ring" placed under the nose cone shoulder. The former is if I re-use the plastic nose cone; the latter is if I try to save 5g by switching to a balsa NC. Note that 5g represents a 15% weight savings on the pad weight excluding the engine itself. If I go with the plastic cone, I realize that during bonding I need to ensure that the NC centerline will be as close to coaxial with the tube as possible. I haven't finalized the details of how to recover it with rear ejection yet.

I am partly tempted to consider an electronics free version with the engine casing recovered via streamer and airframe via tumble/ballistic (remember - it would weigh less than 3/4 oz), but (1) I'd likely never find the airframe if the flight was successful and (2) it's a bit dodgy from a safety code perspective, if safe in actual practice due to the mass involved.

Oh, and I need a better/more accurate tower. I tried to make mine quickly, easy to store and able to launch an 18mm rocket as well as a 24. In reality, the guides are not perfectly parallel due to slop in the alignment holes. I'll glue them into place after aligning them before a next attempt and forego the ability to launch an 18mm.

JordanT said:

The body tube was jointed in the middle with a baffle. That was one of the weak spots as I wasn't really happy with the coupler fit. It was close, but I thought it was the weakest part of the assembly. Well, weakest before I really considered the nosecone shoulder issue. The BT "broke" cleanly just below the baffle, but other parts of the recovered tube were unspiralled.

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I had a baffle related failure on a 3lb 3" diameter cardboard tube fin flying an AT I435 - the rocket shredded during thrust. It appeared to come apart where the baffle bulged the body tube slightly. The baffle was slightly misaligned when gluded in and perhaps a touch too big. In any case the destruction was pretty savage - above that point. Below it was just fine.

JordanT said:

I am partly tempted to consider an electronics free version with the engine casing recovered via streamer and airframe via tumble/ballistic (remember - it would weigh less than 3/4 oz), but (1) I'd likely never find the airframe if the flight was successful and (2) it's a bit dodgy from a safety code perspective, if safe in actual practice due to the mass involved.

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Just go forward with that plan but tether the two together. The extra drag of the body will help slow down the rest. Of course you will lack any way to validate the speed altitude you reach.

Not to be a downer, and I would love to have you prove me wrong, but I just don't think you're ever going to get an Estes BT tubed rocket with balsa fins to survive through Mach at G forces over 200. If you want it to survive and be that light, I would suggest you try carbon fiber.

Have fun trying though, that was one of the spectacular flights that day.

Sorry to hear it shredded mate!

Handeman said:

Not to be a downer, and I would love to have you prove me wrong, but I just don't think you're ever going to get an Estes BT tubed rocket with balsa fins to survive through Mach at G forces over 200.

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Oh, now that's just egging me on...but why?

Handeman said:

Have fun trying though, that was one of the spectacular flights that day.

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Well, I may not make it, but as long as it's entertaining and safe (and I get my expensive parts back), then I guess it's all good!

Edit: On the rebuild, I'm considering a blind version without a logging altimeter. One of my BF orders included a 1 grain CTI case, so I may send up the blind version on an E75 on my local field to test basic flight worthiness (on something more than a B6!). A very quick OR sim puts it at mach 0.87, apogee around 1500'. This is one of those rare times when It would be nice to have a 24mm 2-grain Vmax load.

Finally got around to unloading the truck, including the tattered remains of DB-I. On inspection, the recovered fin was almost pristine, with 95+% of the root intact with the small wood glue fillet and the first layer of paper from the body tube.

Now, the failure at the fin may still have occurred when the coupler went off axis or into the nose buried itself in the tube, but the weak point was at the tube outer layer itself, not the adhesive joint.

A rebuild is possible this week, but I'm frying other fish at the moment so it's 2-3 birds (fish?) from the front of the line.

JordanT said:

Finally got around to unloading the truck, including the tattered remains of DB-I. On inspection, the recovered fin was almost pristine, with 95+% of the root intact with the small wood glue fillet and the first layer of paper from the body tube.

Now, the failure at the fin may still have occurred when the coupler went off axis or into the nose buried itself in the tube, but the weak point was at the tube outer layer itself, not the adhesive joint.

A rebuild is possible this week, but I'm frying other fish at the moment so it's 2-3 birds (fish?) from the front of the line.

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It could also have been fin flutter that tore the fin off, or the drag on the flat leading edge when going 600+ mph. Losing the fins may have been what caused the destruction. The fins coming off can cause enough disturbance that the whole thing shreds just because of the speed its going.

THE dreaded fin flutter has claimed many a rocket. It is worse when the fins extends past the bottoms of the body tube, but it can happen with all rockets / fins.

JordanT said:

the first layer of paper from the body tube

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Same thing afflicted the Project60K team, only with an N5800, beautiful aluminum fins, and a delaminated Performance Rocketry filament wound carbon-fiber tube.

CarVac said:

Same thing afflicted the Project60K team, only with an N5800, beautiful aluminum fins, and a delaminated Performance Rocketry filament wound carbon-fiber tube.

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Thank you.

I am seeing this problem more and more as we try to push the boundaries if some of these flights.

I would be excited to see someone try this with G10 convolute tubing. I have yet to see someone peel that up.

FWIW, I actually don't suspect flutter or straight drag. The rocket only made it to 500FPS, and the tube unspiralled pretty cleanly, which makes me think the airframe anomaly caused the fin loss rather hat he other way around.

Luckily, I've got some 1 grain E75s on order for late January, which should fly to 0.8-0.9 Mach for a better check out flight without going so high I might never find it. Still, I'll at least reinforce the fillets with paper (or the mysterium234).

Okay, I've installed some of the weird TP that fyrwrxz sent me on a sample tube with 1/8" balsa, along with a new piece of 3/16". I need to figure out a way to hold the assembly fast to be able to test the stiffness (I have an indicator dial I can estimate to about 0.0002" and some weights). Unfortunately, the surface of the skin material is very rough where is wasn't compressed during drying (i.e. on the tube) and it's fairly rough on the fin surface - much more than a sanded balsa fin. That means more filler (weight). I thought about vacuum bagging it to try and keep the material compressed, but realized that if I did that the white glue wouldn't dry.



Anyway, I was doin' math in my head trying to fall asleep last night when I realized that the goof I made when ordering tube from BMS may have been a blessing in disguise. Somehow I chose BT-50 coupler stock instead of body tube stock, so I've got 2-3 34" lengths of coupler. I also have a couple yards of 4 oz FG and a yard of 5.7oz CF. I started trying to figure out what it would take to sand down a nose cone, then roll up a single piece airframe and came up with a weight that wasn't terrible if I could justify just two wraps of cloth and kept the epoxy loading low. It adds a few grams, but if I change the deploy to be out the rear and figure out how to hold everything together I might still top 220gs. I should be able to vacuum bag it, I think (first time for everything, right?).

Opinions on just two wraps of CF or three of FG as a tube material? I can fill the void volume with low density foam to reduce the local buckling issues, leaving only the length of the altimeter effectively unbraced.