Stratospheric Solar Balloon Flight

It took three years and several false starts, but we finally got a solar balloon with a video camera on board to take off successfully.  Not only that, but the balloon reached an elevation of over 22,000 meters (72,000 feet), well into the stratosphere!  Then, it suffered what Elon Musk calls a “rapid unscheduled disassembly,” sending the payload into a 22 kilometre free fall that ended in the muddy banks of a cow pond.  Despite this, the photos, video, and GPS track were all recovered.  Here’s the flight video:

The balloon envelope consisted of a 19 foot diameter sphere of 0.31 mil clear plastic paint drop cloth.  Since the drop cloth comes in 12 x 400 foot sheets, we constructed the envelope from 5 gores that were 12 feet wide at the equator.  We attached the gores together using clear shipping tape and darkened the interior of the balloon with black paint pigment.  Each gore seam had a string attached to the bottom with black duct tape.  The opening at the bottom of the balloon is about 6 feet across, allowing two people to inflate it by hand (check out the full inflation and launch video).

The solar balloon in flight just after launch.  Image credit: Mary Lide Parker

The solar balloon in flight just after launch. Image credit: Mary Lide Parker

The payload consisted of a SPOT satellite tracker for recovery, an Arduino Uno with Adafruit High Altitude GPS Shield for trajectory determination, and a Raspberry Pi with camera module for video and stills.  The system was powered using a lithium battery pack meant for recharging cell phones (10 ampere hours).  All this was contained in a Tupperware box with a small hole to admit the camera lens.  We attached it to the balloon using four strings taped onto the box with white duct tape.  The four strings led to a fishing swivel to keep the payload from spinning too much.  We clipped it to the strings on the envelope using a black carabiner. Total payload weight was 800 grams (1.75 lbs).

payload

Payload with the lid off, showing flight instrumentation (left) and payload just before launch (right).

We kept an eye on the wind profiles for about 5 months, since most of the time winds in the troposphere blow out to sea.  Finally, we waterproofed the payload and decided to risk a flight even if some winds were going east.  On May 29, the winds in the troposphere were pretty low (max 10 m/s or so) and with varying azimuth.  The stratosphere had a steady breeze going west, so we figured if we made it that high, we’d head back over land.  However, we had a much faster ascent rate and reached a much higher altitude than we anticipated, so we ended up not flying very far from the launch site.  You can download the trajectory data in text format here or Google Earth KML here.

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Ground flight path (left), launch site is the origin. Altitude versus time (centre), local time was GMT – 4. Ascent rate versus time (right), local time was GMT – 4. I obtained the ascent rate by calculating a 1 minute moving average and dividing elevation by time.

The photos and video were very good quality considering that we were using the Raspberry Pi camera module (not the world’s most advanced camera).  The troposphere was pretty misty, and it seems like we even passed through a haze layer on the way up.  However, it could be that the lens fogged up temporarily.  Once we entered the stratosphere, the pictures are much better:

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The view from 22 kilometres in the sky.

You can watch a slide show of all the photos here, and all the video clips stitched together here.

Just as we approached neutral buoyancy, the payload unexpectedly separated from the envelope and fell back down to Earth.  We kept GPS tracking until about 18 kilometres elevation, and as far as I can tell the box was falling at about 320 km/hr (200 mph).  The Arduino kept track of time even after losing GPS fix, continuing to record until the moment of impact.  Thus, we know that the payload fell the remaining 18 km in about 12 minutes.  The impact speed was probably less than 100 km/hr (60 mph).  The Tupperware payload box was cracked, and everything except the SPOT tracker stopped working.  Had we landed 10 centimetres or so west, we would have splashed down in a pond.  Luckily, we hit the mud on the pond’s edge (and missed the cows that were in the area).

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The payload box at the impact site in Snow Camp, North Carolina.

We consider this flight mostly successful.  Our main objectives were to launch, recover, and extract data from an instrument package lifted with a solar balloon.  Furthermore, we wanted to inflate the solar balloon by hand.  Both of these objectives were reached. Our secondary goals were to fly until sunset, not land in the ocean, and make it to the stratosphere.  The flight was only about 2.5 hours, so we did not fly all day as we hoped.  However, we made it well into the stratosphere and were never in danger of ending up in the ocean.

We had a slightly tense launch when the payload got snagged on the eaves of a nearby building, but the balloon built up enough lift to detach itself in about 20 seconds.  The SPOT tracker did not record any positions during flight, so we did not know where the balloon was and were not even sure if the tracker was working.  In fact, it did not record positions until about an hour and a half after impact. The unexpected flight termination was upsetting as we did not anticipate having the payload detach from the envelope.  We assumed that either the envelope would rupture due to sun-induced heating at altitude or that it would deflate at sunset.  In either case, the payload would have had a large plastic streamer to slow it down to safe velocities.  Instead, it appears that the black duct tape we used to attach the payload strings to the envelope got too hot in the intense sunlight at 22 km.  This caused the payload to come loose from the balloon.  The lesson we learned from this is to never use dark coloured tape if there’s a chance the flight system will make it to extreme elevations.

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Jake III – The successful launch of a 20 ft solar balloon

Jake III Solar Balloon Launch!

Last Friday (May 18th, 2012), my coworkers and I successfully launched a 20 ft diameter solar balloon from Norwood, Massachusetts (42.173073,-71.213297)..  It had an Android cellphone on board with the free Instamapper app for tracking purposes.  It also had a group of 9 messages sealed in bottles, since I expected the balloon to land in the Atlantic Ocean.  These bottles were attached to the balloon using a caramel seal (a toilet paper tube filled with melted sugar), which should have dissolved when it hit the water, allowing the bottles to drift off into the current.

What is a solar balloon, you ask?  It is a hot air balloon heated by the sun.  That means it requires no fuel; the sun heats up the air inside the balloon, and the whole thing lifts off.  I will describe how I constructed Jake III later on in this blog post, for other good websites check here and here.  The design that inspired Jake III is here.

Below is a photo of Jake III heating up before launch:

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Launch sequence:

After I finished making Jake III, we had two weeks of cloudy, windy weather.  Go figure.  But we finally got a four day stretch of clear, calm weather.  My coworker Josh Simpson and I picked last Friday because the weather report said 0 mph winds at 6:00 AM.  We sent word out to our coworkers, and got quite a group together considering we were meeting at 5:30 AM.

After scoping out the winds (very gentle and from the southwest), we taped the Android right to the bottom of the balloon and laid it out on a couple of tarps.  I tossed charcoal powder inside the balloon to darken the clear plastic fabric, and Jason Chrzanowski fired up his leaf blower for inflation.

After inflating the balloon about halfway, we gave it about 20 minutes or so to heat up.  I was concerned because I have never actually launched a solar balloon before, but after a while it was clear that the air inside was heating up.  We added a bit more air and soon the balloon began to rise.

Jake III starts to become buoyant:

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Once the balloon started to rise, all the charcoal powder tumbled out.  This means that you should really add the charcoal powder prior to launch (we were hoping the leaf blower would distribute it but that didn’t pan out).  Still, part of the balloon was dark.

The balloon ascended to the end of the 30′ string I had attached to the payload, and we began to walk it out into the park.  After a couple of minutes more, the balloon had gained enough lift to pick up the payload.  I let it go and walked beside it for a while, and it slowly gained elevation.  It narrowly missed a light pole, and kept going up and up.

Here’s Jake III flying under its own power:

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We watched it drift south for a while, drift back north, and then finally catch some higher winds and take off to the southeast.  Our tracker was able to locate it until it got up to 9,000 ft, after which time it was above cell phone reception.  When we last heard from it, it was rising 300 ft per minute, moving at 18 mph to the southeast.  We expect that it caught the jet stream (80-100 mph that day) and was taken far out to sea, where it landed after sunset.  But we’ll never know for sure unless someone finds a bottle and writes us back.

Here’s Jake III in flight:

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How I built Jake III:

Cost:  around $50.

Materials:

9 x 400 ft roll of 0.31 mil clear painter’s drop cloth (get it at Home Depot for about $25)

Make sure it is 0.31 mil.  This is the thinnest plastic you can buy.  It is comparable to the fabrics used for scientific balloon launches.  Do not try to use trash bags-you will spend weeks ironing them together, and the balloon will be weaker anyway.

1 Jar of Artist’s Charcoal Powder (got it off of Amazon.com)

String

Sugar

Bottles

Duct Tape

Scotch Tape

Small plastic tubing

Clothes iron

We also used a smartphone for tracking, but it didn’t work as well as we’d hoped because it only tracked below 9,000 ft.  If you do opt for a smartphone, use the app I mention at the beginning of the post and make sure you use a phone on the website’s list.  Not every smartphone will work.  My coworker James Roehrig’s android was up for renewal, so he sacrificed his old phone to our balloon.  Obviously we got the phone for free, so I didn’t include it in the cost, but I believe you can get one that will work for around $50.

Construction of the Balloon Envelope:

Instead of using the tetroon designs that are common on the Internet, I made a spherical balloon by cutting gores from the plastic I bought.  You can use a gore pattern for a parachute (for example here); since a parachute is a half sphere.

If you are making a 20′ balloon, you’ll probably need help, and you’ll need a large space, like a gym (one gore for a 20′ balloon is around 33 ft long).  I recommend cutting each length of plastic from the drop cloth sheet and folding it twice; once lengthwise and once crosswise, then laying each folded sheet on top of each other.  Then draw the gore pattern on the top sheet and make a single cut.  I ended up with 7 gores that I could just unfold into their final shape using this method.

Next, you have to weld the gores together.  You can do this in a smaller space as long as you keep all your pieces of plastic organized.

I use a regular clothes iron to weld plastic.  You will need to practice and adjust the iron settings before the plastic will weld correctly.  Too hot, and the plastic will melt together and stick to the iron; plus the seam is very weak.  Too cold and it won’t stick at all.  Just right, and the two pieces of plastic will hold even when you pull pretty hard.  One piece of advice:  make sure you’re on a hard floor.  Carpet and ironing don’t mix.  Take the water out of the iron and use the iron edge rather than the middle.

A lot of people apparently use tape to attach the gores together.  This is probably faster but weighs a lot more. Also tape can make a huge mess if it comes undone and gets snarled in the rest of the balloon.

When you’ve ironed the final gores together, weld the two edges of the giant sheet of plastic you’ve made together, and now you have a truly massive bag-your balloon.  Try not to trap any air because it makes the balloon a lot harder to store.

Finally, cut a circular hole in one part of the balloon for the air intake.  I recommend doing this at one of the poles, where all the gores meet up.  This is a natural weak spot, so using it removes one place where the balloon could rip open.

I reinforce this opening by making a
loop of tubing, pulling the balloon fabric through, and folding it back on the outside of the balloon, then duct taping it on the outside and the inside.  This gives the opening enough strength to hold a substantial payload without ripping.

Here’s a picture of the opening (in the red duct tape).  The balloon is packed and ready to go!

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Making the payload:

A payload can be anything you want, but you absolutely need weight on the bottom to keep the balloon opening pointing down, so that the hot air is trapped in the bag above it.  For Jake III, I probably could have put a pound or two and it would have been fine.  I am not sure what the maximum payload I could have lifted is, but I expect I probably could have gotten around 10 lbs off the ground.  Of course, the bigger the payload, the lower the final altitude is.  Also, the FAA regulates payloads above 4 pounds, read the regs here and make sure you understand them.

However, since I was planning to launch it into the ocean, I thought it would be neat to have my coworkers and their children write messages in bottles (my wife and I contributed some too) and fly them on the balloon, so that when it landed they would drift off and be found someday.  To this end I developed a payload release mechanism that (I hope) did the trick.  I tied a string to a cardboard toilet paper tube for the balloon attachment, and then draped another string loosely inside the same tube for the bottle attachment.  I then filled the whole thing with melted sugar and let it harden.  My thought is that the water dissolves the sugar, and the bottles float off.  This is based on how underwater seismographs are deployed.  Of course, I won’t know if it worked or not unless at least two bottles are found unattached to each other!  Here’s a photo of the payload setup:

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Making the balloon dark:

I used artist’s charcoal powder to coat the inside of the plastic.  Jake III wasn’t too dark because I did it on launch day, but a prototype I did worked really well when I spent 10 minutes or so flapping the powder inside before I dumped it back out.  The prototype was 10′ in diameter and easily picked up a 2 pound bucket.

Launch day:

Obviously it has to be sunny, otherwise the balloon won’t heat up and fly.  But there also has to be absolutely no wind.  Wind ruins your day in a hurry.  First, it makes it harder to inflate the balloon because the wind just pushes the air out again.  Second, solar balloons take along time to climb out of tree range.  The more wind, the more chance the whole thing flies into a tree.

If leaves are rustling, there is too much wind.  Go back to sleep.

Also, you need a good inflation method.  A leaf blower works great, especially a gas powered backpack mounted one like we used.  Exhaust from vacuums work also, but the exhaust tends to be hot so your balloon might take off pretty quick.  Hair dryers are great for smaller solar balloons but it would take a very long time to inflate a Jake III sized balloon with one.  It took 10 or 15 minutes of continuous leaf blower action, and a leaf blower moves a lot of air even on idle (we didn’t crank it because I didn’t want to pop the balloon).

Make sure you are in a big open space, and that you are at the most upwind portion of it.  Even when you can’t feel any wind, the wind is going at about walking pace.  You’ll need the space for the thing to take off.

Inflate, wait for it to heat up, let it go, and enjoy the show!

Note: although these objects are legal when subject to the regs I linked to earlier, do realize that you are liable for injuries or destruction caused by something you launch.  Keep it away from airports.  I even notified the local airport so they could give pilots a heads up.  Also, secure the payload so that it doesn’t drop off from 20,000 ft and put a hole through someone’s car.  Finally, what goes up must come down; spare a thought to what the balloon might land on.  That’s why I chose the Atlantic Ocean.