GPS Balloon Cutdown

This post goes over how to make a GPS based cutdown for your high altitude weather balloon. FAA regulations (FAR 101) require that unmanned balloons have two methods of flight termination. While balloon burst at high altitude is usually a very reliable way to end your flight, if there is any risk of the balloon reaching neutral buoyancy, such as with launches designed maximize elevation, a secondary cutdown is useful.

For our cutdown we used a ‘thermal knife’ made of 30 gauge nichrome wire (a type of wire also used in toasters).  To trigger the cutdown we used an Arduino Uno board and the “ultimate GPS” from Adafruit. The Arduino checks the GPS coordinates every 60 seconds, and if the GPS has crossed some preset line (for example, the balloon is 30 miles from an ocean or great lake) sends the cutdown signal. Alternatively, you could use a limit on time aloft to trigger the cutdown.  The Arduino cutdown signal triggers a transistor to switch on a reed relay, which completes a circuit with the nichrome wire and a battery pack. The reed relay is needed because the large amount of current that flows through the cutdown circuit loop would fry the transistor! A circuit diagram (created with Circuit Lab) is shown below:

Cutdown circuit diagram

Cutdown circuit diagram

And picture of the circuit soldered on perf board:

Cutdown Circuit

Cutdown Circuit

We used a battery pack with 4AAs to power the cutdown, which provides sufficient current to get the nichrome wire glowing red hot:

Glowing Hot Nichrome Wire

Glowing Hot Nichrome Wire

For balloon flight, the nichrome cutdown wire was coiled around the line between the balloon and the payload and sheathed in heat shrink tubing.  The setup is shown below:

cutdown_setupAs a first test of our cutdown mechanism, we launched a balloon with the Arduino set to trigger the cutdown at 20,000 m. Our blog post about the launch is here, and pictures from that cold snowy day are here. As show below the cutdown worked perfectly, terminating the ascent at 20,082 m:

Elevation recorded by GPS during flight.

Elevation recorded by GPS during flight.

The Arduino sketch that we made to do all of this is based on the Adafruit Ultimate GPS Library, and is available here.

A Hydrogen-Filled Weather Balloon Flight into Near Space

We describe a hydrogen-filled weather balloon launch in central North Carolina, and present a video, still images, and data from our GPS data logger.

Launch and Flight Video:

A view from our still camera aboard our weather balloon.

A view from our still camera aboard our weather balloon.

This flight was designated “Jake 7”, as it is our seventh tracked balloon launch attempt (5 successes, 2 failures so far).

Our goals: get video, test using hydrogen instead of helium for lift gas, try launching in close proximity to the ocean without losing the balloon, use a plastic rather than nylon parachute, and have a faster descent rate than on previous launches to cut down on flight distance.  We were also excited to get latitude, longitude, and altitude from launch to landing (our most recent flight before this one was deliberately cut down at 20,000 m, and that time the GPS only started tracking at 10,000 m).  We were not going for a spectacularly high altitude during this launch because the jet stream was blowing towards the Atlantic at 100 miles an hour.

The GPS flight data can be downloaded here:  jake7_flight_data.txt

Results at a glance:

-Hydrogen is great.  It’s cheap-currently 1/3 the price of helium and weighs less, meaning more lift.  I got a 200 cubic foot hydrogen tank for about $70, including 10 days of tank rental.  Hydrogen is also easy to find.  You can pick up a tank at a welding supply shop such as this one in North Carolina.

-Hydrogen is also very flammable and much more dangerous than helium.  Driving with a 200 cubic foot tank in the back seat is nerve wracking to say the least.  This may be a downside for the faint of heart.

-By overfilling our balloon we got a very fast ascent rate.  This cut down on our altitude (we made it to 79,000 ft, in contrast to 88,000 on Jake 2), but the upside is we didn’t lose our payload in the ocean.

-Got video and still images throughout the entire flight, but the payload was spinning very quickly.  The raw video makes one seasick and a lot of the stills are out of focus.  The edited video is the best I can do.  The spinning issue needs to be fixed in the future.

-The plastic parachute was too flimsy and tore apart during the descent, resulting in a pretty hard landing.  More on this later.

-The Arduino Uno and GPS shield recorded data through the entire flight.  Here’s an altitude versus time plot (see the raw data link above):

Ascent and descent of Jake 7.

Ascent and descent of Jake 7.

Equipment List:

a 600 gram weather balloon from Kaymont

a 200 cubic foot tank of hydrogen

one SPOT satellite tracker, so you know where the balloon went

one Arduino Uno flight computer with high altitude GPS data logger

two lunch boxes

tubing to move hydrogen from the tank to the balloon (1 inch inner diameter, if I recall right – measure the tank outlet)

plastic parachute (I will describe how I made it, but it failed! this is a “what not to do”)

Canon still camera, configured using CHDK to take pictures every 10 seconds

Kodak PlaySport video camera

zip ties

rubber bands

string

The Flight

I used a Python script I wrote to predict where the balloon would go based on the weather forecast.  Since we had a strong (~100 mph) jet stream, we found that there was a good chance of a water landing if the balloon ascended or descended slowly.  So we filled the balloon with a lot more gas than usual so it would rise quickly.

2013-03-30 10.17.31You can see two nice bright lunch boxes for the payload.  We had to use two because the SPOT GPS interferes with the Arduino GPS logger.  The launch site was near Saxapahaw, North Carolina.

After release, the payload swung back and forth violently.  Sometimes it was almost parallel to the ground.  This swinging motion was probably because the balloon was rising quickly through a fair amount of wind shear.  We watched the balloon disappear into a partly cloudy sky, and tracked it for about 20 minutes with the SPOT.  After that, it was above the SPOT maximum altitude, so we had to wait and hope it talked to us on the way down.

The black sky of near space.

The black sky of near space.

At high altitudes, the payload box spun rapidly.  This made for some nausea-inducing video!  The burst is audible, however, and the camera swings up briefly just after the pop.  You can see the expanding ball of plastic shreds in the YouTube video.  Pretty neat!

As the payload descended, the video camera swung upwards and pointed at the sky.  The still camera swung down and looked straight at the ground.  This is probably because the still camera was heavier, so the payload was off balance.

The still camera took excellent pictures until it went lens-first through a cloud.  All the pictures are foggy from then on.

Fourteen miles is a long way to fall!

Fourteen miles is a long way to fall!

The last photo before the cloud:

Moments before hitting the cloud.  The shadow of the payload is in the center of the halo.

Moments before hitting the cloud. The shadow of the payload is in the center of the halo.

The payload started falling at about 150 miles per hour.  As it descended, it encountered denser air and slowed to a fifth of its initial velocity.  I designed the parachute to slow everything down to 20 mph – but the thin plastic I used ripped during the descent, so the payload fell 10 mph faster than expected.  Everything survived just fine except for a crack in the Arduino case.  In addition, the lens cover no longer closes on the Canon camera.

We were very fortunate during the landing.  Had the balloon burst any later, the payload would have landed in a forest full of gigantic trees.  Instead, we found our lucky pink lunch boxes lying in a fallow cotton field in Selma, North Carolina –  about 50 feet from the forest margin.

Carrying the payload from the impact site.  The Canon is still taking pictures!

Carrying the payload from the impact site. The Canon is still taking pictures!

The moral of the story is: make a strong parachute.  I used the thinnest paint dropcloth I could find at Lowes (this material is what we use to make our giant solar balloons) to save on weight and also because I had it lying around already.  However, this plastic rips easily in the best of conditions.  Falling at 150 mph through thin, bitter cold air is certainly not a good place for it to be.  Next time, we’re going for the rip stop nylon parachute we used in our first weather balloon launch.

The take home message:

-Hydrogen is cheaper and lifts better.  We will be using it from now on, and thinking non-flammable thoughts as we do.

-Parachutes should not be made of plastic, and should be tested before deployment.  I think I will hold one out the window of the car at 20 or 30 mph.  If it can survive that, it will work (note that it falls 150 mph at 80,000 ft, but the air is thinner so the force is the same).

-A fast ascent seems to lead to bad image and video quality.  In any case, though, we need to think hard about how to make a stable camera platform.

We’ll leave you with a couple more cool plots – one showing ascent and descent rate, another showing the windspeed vs elevation, and a final one showing the flight track.  All of these plots were made using the data set included in this blog post.

The ascent and descent velocity.  The first jump is when we launched.  The switch to negative values is the descent.

The ascent and descent velocity. The first jump is when we launched. The switch to negative values is the descent.  Time should be in seconds, not hours.

This shows why you have to travel through the jet stream quickly if you want to avoid downwind locations.

This shows why you have to travel through the jet stream quickly if you are on the East Coast and the wind’s blowing east.

The balloon flew from the left to the right, starting at Saxapahaw and ending at Selma.  The gap is where the balloon burst.

The balloon flew from the left to the right, starting at Saxapahaw and ending at Selma. The balloon burst at the origin of the plot.