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Light Show

At Aeronaut 2013 (where I flew my Comanche 3), Jim Green flew a great night launch rocket, his "Blinkasaurus Maximus". I hadn't built many night launch rockets (really only the Mosquito Swarm) so it seemed like time to expand my reportoire.

What really sparked this project was talking with a friend at work about his Burning Man project, a sound-controlled shirt using LEDs controlled by an Arduino. After that, some searching and experimentation settled on the basics of my rocket.

I decided to use addressable RGB pixel strips (NeoPixel strips on Adra Fruit), allowing complex light patterns controlled by an Arduino.

 

The Pictures

Here are the just-completed pictures on July 19, 2014. Normally one shows off a rocket with daylight pictures, but this rocket looks better at night.

   

Photographing this rocket is quite hard. The two photos above right are in the dusk (early and late) since at night the lights just wash out the picture. Even with the iPhone HDR setting, it's hard to get a good exposure. Then of course you have to imagine the rainbow is moving along the length of the rocket... (I tried to get video; here's my not-very-successful result.)

 

XPRS 2014

I had intended to fly the rocket at Aeronaut in August, but electrical problems had forced me to postpone until XPRS in September. The rocket flew Friday, September 12 on an AeroTech J415.

I powered up the light display and brought it out to the RSO table, which immediately attracted a crowd.

Above you can see the rocket on the scale as I fill out the flight card (picture by Peter Thoeny).

The flight looked good, until apogee when the recovery system failed to deploy. The rocket came in ballistic and smashed to pieces right on the range. (I found the electric match, which hadn't popped, so either continuity was lost or the R-DAS failed to fire it.)

The next morning, I dug out the rocket, buried past the nose cone, and picked up innumerable shards of the Acrylic outer body tube (picture by Peter Thoeny).

 

The Design

Like Jim's rocket, mine would have an Acrylic® body with LEDs inside. I decided on a 4" O.D. tube (1/8" wall) for the body and standard 3" phenolic tubing for the core struture. The LED strips would be mounted to the surface of the inner structure and the outer tube would slide over after preparation.

A big concern was power to drive the LED pixel strips; each meter draws about 800mA when drawing typical patterns (more if solid white). I wanted the lights to be able to last at least 40 minutes (pre-launch, on the pad, flight and recovery), so that meant that I would have to fly a lot of power.

The pixel strips are 1cm wide (0.4") wide, so the maximum number that could fit on the outside of a 3" tube is 25 (9.9" / 25.1cm outer circumference). I ended up settling on 20 strips due to power concerns (four batteries, driving five strips each) and to leave a little space between strips for wiring and access.

Speaking of batteries, their volume and mass were major considerations in the design. Four 5000mAh R/C car batteries (2 cell LiPo) make for freight in a smallish rocket and fitting them into the 3" inner core meant lenghthening this 4" rocket from the typical ratio. (At 87" overall, this rocket has a 22:1 length/diameter ratio.)

There was no room for dual deployment (and I couldn't break the main aiframe because of the LED strips), so the electronics bay had to go into the nose. After that long preamble, that here is the overall drawing.

I have to give a big thanks to the members of AERO-PAC, and in particular to Paul Campbell, for answering my stupid electronics questions.

 

Body Core

From the overall drawing you can see how much room was taken up by the battery sled to power all those LED strips. The battery sled was cut from 6mm baltic birch plywood, with parts that fit together like a puzzle.

Above you can see the parts being cut out on a ShopBot CNC router at my local TechShop. And below you can see the battery slide put together and bonded with epoxy.

The four battery packs fit tightly into the bays and are retained with large tie wraps, the Arduino is mounted using stand-offs through its pre-drilled holes and the 9v battery is also mounted with a smaller tie wrap. The central spine rod threads into a centering ring at the aft end of the 3" central section and has an eye nut on the top for the recovery harness.

The 3" core tube is 56" long, much longer than available lengths of phenolic tubing, so I had to join two sections of 36" tubing together. Normally one would do this using a coupler, but since I needed the inside to be clear for the entire length, I used a wrap of 6oz fiberglass to make an outer coupler.

Other than that, the main body is pretty simple: just a long 3" tube. At the aft end end, the fin can is built around the 54mm motor mount, which then slides into the core tube. (See below for more on the fin can.)

 

Above left you can see how the wiring for the LED strips goes through the forward centering ring/bulkhead on the motor mount tube so that it can feed out at the aft end of the 3" core tube. Four power wires (red/black) and one signal wire (green), plus fishing line through each hole in case I need to run more wires later.

Above right the fin can/MMT has been installed into the core tube and the wires brought out through slots on the outside. The small aluminum rectangles are plates for switches that control the power fed to the light strips (one per power feeder, for five strips).

Once the body construction was complete, the only painting was done; I sprayed flat black over the area which would be covered with LED pixel strips. Then it was time to start mounting the strips themselves.

The strips had an adhesive backing, so I used that to apply them longitudinally (20 strips, 18° apart). In the picture above, all twenty strips are on and wired up. Things things are bright; that's in a fully lighted room!

Above is the wiring for those first five strips (the 'A' bus on the first of the four LiPo packs). It's a bit messy, but I'll neaten it up once the whole thing is wired and working. As for the soldering, what can I say, I'm a software guy.

Above you can see how the main bridle attaches to the eyebolt at the front of the bay. The eyebolt actually turns within the sled so the aft end of the threaded rod can screw into the bulkhead at the forward end of the MMT.

 

Fin Can

I wanted the fins to be clear also, so they were cut from 3/16" polycarbonate sheet. That worked out well, but of course they could not be bonded to the tube, so I had to use aluminum angle to make rails they could bolt to.

The rails were bonded to the outside of the 54mm motor mount tube, and also the ends socket into slots cut into the centering rings. These centering rings were actually cut from ¾" plywood, much thicker than usual, so that I could mount threaded inserts into them for attaching the clear outer tube.

Above you can see how the aluminum rails ends slide into slots machined ¼" into the extra-thick centering rings. A pair of rails holds each fin in place. Below you can see the completed fin can built on the 24" motor mount tube.

The forward half of the motor mount assembly slides into the 3" core airframe tube. (See the overall drawing, or above for more on the body core.)

 

The final bit of construction was to slot the Acrylic outer tube for the fins. I did this using my trusty fin slotting router jig. And finally, in the above right photo, you can see the aft end of the rocket completed.

 

Nose And Electronics Bay

There was no room for dual deployment, so I decided to place the electronics bay into the nose. This bay does dual duty for lighting the nose and containing the recovery electroncs.

The nose cone shoulder didn't, of course, work with the outer Acrylic tubing, so I cut it off and built the electronics bay into a 3" coupler that would slide into the core phenolic airframe tube.

The top (forward end) of the bay inside the nose cone is actually an acrylic plate, which allows lights at the top of the sled to shine through and illuminate the nose. So in addition to an altimeter, there's also an Arduino Uno to drive a ring of LED pixels in this bay.

There was a lot to pack into this small bay: an R-DAS and an Arduino Uno, not to mention the 9v battery and all the various wiring.

 

Above you can see the bay with both sides populated. On the left is the altimieter side with the R-DAS and battery, plus the 5v regulator for the pixel ring on top. On the right is the Arduino side, and on the top you can see the ring of 16 LED pixels.

 

Animation

All that stuff above is just to create the canvas for the "light show" itself! Those 20 strips of 80 pixels create a grid of 1600 pixels that I had to paint in some interesting way. Of course, one can only see about half of them at a time when wrapped around the rocket, so that means a relatively simple moving design works best.

Plus the Arduinos, even the Mega, have a relatively limited amount of memory, so it's not realistic to use video as the source. What I ended up choosing was a rainbow that traveled up (down) the rocket body, which provides a colorful undulating design. (More complex patterns were too much for the Arduino to handle since addressing 1600 individual pixels takes time).

Despite its limitations, the Arduino is an easy platform to work with. Above you can see me with the computer connected to the rocket, tuning the timing of the animation and watching the results almost instantly.

 

Prepping the Rocket

Because I wanted the LED strips to be uninterrupted, it made for a difficult-to-prep rocket. Not only could I not easily do dual-deployment, but everything had to go in from the front end, meaning there was excess wire to manage.

Watch directly on YouTube.

Those of you would would like to see the details of prepping this rocket can suffer through it with me in the video above. It ends with the rocket lit up at night, so there is eventually a "light show," although the LEDs are so bright the effect isn't nearly as good on video as in person.