Exceeding the Speed of Lexan

jadebox

Roger Smith
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I flew my "Tikva" on a K555 motor at the TTRA launch on Saturday. Tikva is a six-foot, four-inch diameter rocket with clear Lexan fins. It can carry two small video cameras in a camera bay just below the altimeter.

2008-07-09-20-55-55-079.JPG


For this flight, I installed just one camera - a new Panasonic video camera that's shock-resistant (more on that later). It's small, actually smaller than the little AIPTek-type cameras I've used in the past. It weighs a little more than the AIPTek ones because it has real glass lenses and optical zoom. It also appears to be built sturdier (more on that later).

I bought a K555 "Green Thunder" reload onsite from Jim of Gorilla Motor Works. I assembled it into my 54/1400 case. When I tried to install it in the rocket, it didn't fit into the Aeropack retainer I had installed.

I borrowed a battery-powered rotary tool from a fellow flyer. Just a few passes of a grinding bit around the thrust ring fixed that problem.

I carried the twenty or twenty-five pound rocket to the far launch pad and installed in on the rail. Then I turned on the camera and started to walk back to the LCO. Realizing I'd forgotten something, I returned to the rocket and armed the altimeter. That could have been a problem.

"3 .. 2 ... 1 ..."

The rocket motor chuffed. Then it chuffed again. And again. Finally, the finicky green motor roared into life and sent the rocket soaring skyward.

On the way up, someone shouted that the rocket had lost a fin. "No way," I thought. The rocket continued flying well - gently arcing into the wind. And I knew that those fins were screwed into 1x2s and weren't coming out without someone using a saw and hammer (more on that later).

At apogee, the drogue 'chute deployed, but the upper-section of the rocket separeated. While the booster section drifted quickly down under the small drogue 'chute, I watched the upper-section, with the electronics bay, main 'chute, and (I hoped!) the camera pod, free-fall for several seconds until it reach 700 feet when the main 'chute deployed. The rocket was whipped around violently, but appeared to stay together (more on that later).

Under the main, the upper-section of the rocket slowly fell to earth.

When "Pads are open" was called, I went to collect the booster section, while my wife, Bracha, trekked to get the upper-section. Back at our table set up behind our mini-van, I began to survey the damage.

The first thing I saw was that one of the fins was sheared in half. A fin didn't fall out, but one broke.

Next, I noticed that one of the camera shrouds was missing a panel. But, I was relieved that it was the other shroud - not the one over the section where I had installed the camera. So, I turned the rocket over to access the camera from the other side.

It wasn't there.

Did I mention that this was a brand new camera that cost much more than the ones I've flown before?

So, Bracha walked out to the area where the rocket was launched while a few kids and I started looking around where the main 'chute had deployed and the upper-section landed. After a while, the kids got bored and left, but "AstronMike" joined me for the search. It didn't take him too long to spot something shining on the field. It was the camera.

The camera was a little dirty and soot from the ejection charge coated the front of the camera. Although, it appeared intact, it was off. So, I popped open the battery cover to make sure the battery was in place. It was, but the SD memory card was sticking out as if it had been ejected. I pushed it back in.

I switched the camera off then to "playback" mode and, oh my, it actually started playing back the video from the flight.

tikva-onboard.jpg


The video makes it easy to see why the fin broke. A second into the flight the fins begin to flutter violently. But, other than that, the flight up was pretty good. When the drogue charge fired, the video gets blurry. Soot from the ejection charge coated the lens of the camera - something that never happened on previous flights.

The video ends when the main parachute deploys. The violent whipping of the rocket when the parachute opened probably ejected the memory card from it's slot and the camera from the camera bay. How the 4" tall camera fit through a 2" tall opening remains a mystery.

The video from the flight can be veiwed at:

https://www.payloadbay.com/video-7959.html

I'm planning to rebuild the booster by replacing the Lexan fins with plywood fins, reinforced with fiberglass.

Sunday, the day after the flight, I used a rotary tool to cut through the epoxy fillets along the booster's Lexan fins. I had slotted the tube from one end so that the fin can could slide into it. So, I was able to use a chisel to seperate the fiberglass body tube from the rear centering ring and gently pry the ends of the tube away from the centering ring and the fins. The fins were screwed into strips of wood that were glued to the motor tube and the centering rings. I was able to use the chisel, a hammer, and some strong language to separate the wood from the tube and rings. Then I was able to pull the fin assemblies out through the back end of the tube.

It shouldn't be too difficult now to make new fins and glue them to the motor tube. Then I'll clean up the body tube and make fiberglass fillets between the fins and body tube to help hold them in and cover up the slots.

-- Roger
 
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rstaff3

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Excellent account of the flight and a great video! I'd suspect Lexan fins of that size were awfully floppy.
 

AstronMike

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Roger,

It was truly an honor to *finally* meet you in person as I had been looking at your payloadbay.com photos since you started posting them. Always wanted to be able to fly where you were present to record some of my stuff, and thankfully this finally came to pass! So, all my postings on my 'unusual' methodology concerning gliders has now been recorded by the 'eenfumuss' Roger Smith himself under the new TTRA section of his site.

While I still am thankful to you for that Ellis G20 which made a pretty big glider turn into a dot in the sky and a long drive (not walk) to recover, I am glad we found your camera out there. Its an awful large site with lots of 'dense brown lumps' that look like, well, everything else.......

I really hope to see you more often at TTRA launches. If the wind cooperates, there might be something ridiculous I may attempt soon :eek:

Thank you again for you help and support, Roger!
 

jadebox

Roger Smith
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It was truly an honor to *finally* meet you in person as I had been looking at your payloadbay.com photos since you started posting them.

We enjoyed the launch and will try to make it to more TTRA launches in the future.

I was happy to finally see your large gliders in person. They are very impressive.

I recorded slow-motion video at the launch, including several of your flights. I'll try to get it edited and upload to the site soon.

-- Roger
 

cjl

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That's quite an impressive flutter video - glad you were able to find the camera to see the full extent of the flutter. I'm not too surprised about it though - those are pretty large lexan fins. What thickness were they?
 

jadebox

Roger Smith
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That's quite an impressive flutter video - glad you were able to find the camera to see the full extent of the flutter. I'm not too surprised about it though - those are pretty large lexan fins. What thickness were they?

I think they were 3/16" - so not very thick. I built the booster as a stable platform for my camera rocket. I wanted large fins mounted straight to avoid the rocket rolling as my previous camera rockets have done. The first part of the video shows that I was successful.

I used the Lexan mainly because I had it. I made the fins extend past the end of the motor tube in order to make it easier to use a simple jig to align the fins to make sure they were very straight.

Lexan has some benefits as fin material.

  • It's relatively inexpensive (especially when compared to G10).
  • It's easy to work with. I used a regular jigsaw to cut it.
  • It's (almost) indestructable. You can bend a sheet almost in half before it breaks. (But, watch out for "fin flutter!")
  • It looks cool.

The cons are that ...

  • It's very heavy. Thicker lexan might not have fluttered and broken. But, it would have made the rocket much heavier on the wrong end.
  • You can't really glue it to anything. (I bolted strips of wood to the root edges of the fins and glued those into the rocket).
  • It's easily scratched.

-- Roger
 

jadebox

Roger Smith
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Another lesson I learned on this flight - be careful of bulkheads in fiberglass tubes or couplers. The separation was due to the bulkhead at the bottom of the camera mount pulling out of the coupler. It was epoxied inside, but the joint didn't hold.

This is the second time I've had a failure like this. The previous time a bulkhead glued into the base of a fiberglass nosecone came out allowing the nosecone to separate.

For this rocket, I sanded down the edge of the bulkhead that fit into the nosecone so that it slid about a half-inch into the nosecone's shoulder. I glued it in using epoxy mixed with small pieces of fiberglass - building it up about 1/4" think around the inside of the tube.

But, I didn't think about the similar bulkhead at the base of the camera pod.

It was just epoxied into the fiberglass coupler and pulled out when the drogue deployed. I need to build a new camera pod and, when I do, I'll attach the bulkhead more securely.

-- Roger
 

SRBell

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Another lesson I learned on this flight - be careful of bulkheads in fiberglass tubes or couplers. The separation was due to the bulkhead at the bottom of the camera mount pulling out of the coupler. It was epoxied inside, but the joint didn't hold.

This is the second time I've had a failure like this. The previous time a bulkhead glued into the base of a fiberglass nosecone came out allowing the nosecone to separate.

For this rocket, I sanded down the edge of the bulkhead that fit into the nosecone so that it slid about a half-inch into the nosecone's shoulder. I glued it in using epoxy mixed with small pieces of fiberglass - building it up about 1/4" think around the inside of the tube.

But, I didn't think about the similar bulkhead at the base of the camera pod.

It was just epoxied into the fiberglass coupler and pulled out when the drogue deployed. I need to build a new camera pod and, when I do, I'll attach the bulkhead more securely.

-- Roger

Now I'm pretty new to rockets, but I have some experience with glueing bulkheads to FG from my background in RC jets. Before turbines we used nitro burning motors that created a lot of vibration. Pretty much anything other than glassing in the bulkhead would come loose. That is until I started using Hysol glue (loctite 9462). You'll need a gun and mixing nozzles to use it. Maybe even better is good old shoe goo. I've never had a FG joint fail using it and you don't need specialty guns and mixing nozzles.
 

bobkrech

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Great flutter video.

I'm thinking the fins could be 1/2 the span and 1/2 the height of the present fins and still have adequate stability. That would reduce the fin weight by 75%, and then doubling the present thickness after 75% area reduction would still reduce the weight by 50% and subtantially increase the stiffness and thus the flutter velocity.

G10 weighs ~37% more than Lexan(R), but good aircraft grade plywood weighs about 25% less than Lexan(R) so that another way to keep the weight down.

Bob
 

rockets4kids

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Did you do a simulation on this rocket? Those fins look *far* larger than they need to be. Smaller fins are going to be much less likely to flutter and snap.
 

niels popping

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I saw on your webpage you had some trouble with the camera,

I can come up with two solutions,

you can buy a small camcorder for less that 40$ than can run on its delivered memory for 4 hours,

or you can make a remote control switch that you apply to your camera, so you can turn it on on distance just minutes before launch.
also works with a simple timer,
you only need to connect it with the recorder button, if you don't like to modified your camera, you can make a coil with a piece of metal, that pushes the button,

I have made a remote control camera by phones ones,
I used two phones one to make a call, and one cheap one to receive,
when an incoming call is active, the display lights go's on, a photo-sensor detects the light, en turn on a switch, that switch turns my camera on.

190951_AB_00_FB.EPS.jpg
 

jadebox

Roger Smith
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Now I'm pretty new to rockets, but I have some experience with glueing bulkheads to FG from my background in RC jets. Before turbines we used nitro burning motors that created a lot of vibration. Pretty much anything other than glassing in the bulkhead would come loose. That is until I started using Hysol glue (loctite 9462). You'll need a gun and mixing nozzles to use it. Maybe even better is good old shoe goo. I've never had a FG joint fail using it and you don't need specialty guns and mixing nozzles.

What I did to fix the problem was to glue the bulkhead about an inch inside the coupler. That allowed a lot of room for a thick bead of epoxy around the edge of the bulkhead.

I still haven't rebuilt the booster for the Tikva. I've bought a length of Blue-Tube and sheets of G10 to use to rebuild it, but haven't gotten around to it yet.

-- Roger
 

jadebox

Roger Smith
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Great flutter video.

I'm thinking the fins could be 1/2 the span and 1/2 the height of the present fins and still have adequate stability. That would reduce the fin weight by 75%, and then doubling the present thickness after 75% area reduction would still reduce the weight by 50% and subtantially increase the stiffness and thus the flutter velocity.

G10 weighs ~37% more than Lexan(R), but good aircraft grade plywood weighs about 25% less than Lexan(R) so that another way to keep the weight down.

I made the fins oversize on purpose. My goal was to try to reduce the amount the rocket rolled so that the videos would be better. It seemed to work because the rocket rolled only a little ... at least until ... well, you know. :)

But, I'm sure that you're right. I overdid it a bit.

-- Roger
 

jadebox

Roger Smith
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you can buy a small camcorder for less that 40$ than can run on its delivered memory for 4 hours,

Yes .. I now have a few of the little cameras that can record for hours, so I haven't had the "camera quitting before the launch" problem in a while.

Your remote is pretty cool.

I rigged up a radio-control remote shutter release for my still camera so I can put it close to the pads for lift-off photos. It let me down when I tested it at the last launch. For some reason it didn't trigger the camera to get a shot of my rocket's launch even though I had tested it at that distance before. I'll have to figure out what went wrong before I try it again.

-- Roger
 

dixontj93060

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Enjoyed the video. Music made me want to put on a tie-dye shirt and headband.
 

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