How to fly across an ocean

Flying across the ocean is no small feat.  It takes the concerted efforts of dozens of people, working hard at lots of difficult problems, from modeling balloon volume and flight dynamics, to planning interactions with air traffic control.  The diagram above gives a little bit of an idea of the effort involved in getting across the ocean.  Any single block represents tens to many hundreds of man-hours worth of effort.

Components in Purple represent things which will actually be flying across the ocean.  This hardware and software must perform flawlessly at all times.  Components with a red heptagon represent significant software efforts.  The red square shows the components which lie on amazon EC2, spread across three instances, with a total cost of $100 a month (during flight season) to maintain.  Pink commands are sent using PubNub, a service without whose generosity our public page would not be possible.

All of these systems are in the critical path, and a failure of any single flight system will compromise science data.  Fortunately, we always have positive control of our craft, thanks to a dead-man cutdown, which operates entirely autonomously, and a 9602 modem which will respond with rough location coordinates even if all other flight systems have failed.  Our ground systems all have hot-backups, and can all be operated from anywhere on the Internet, so these systems are as redundant as they can be.

How to Build the World’s Lightest Quadrifilar Helix Antenna

After the scrub on the launch pad for flight attempt A, we went back to the books, to try and figure out what we could do to improve our odds the next time around. One of our biggest setbacks was the inability make an antenna suitable for our use: We needed an antenna tuned for 149 MHz, not needing a ground plane, weighing as little as possible.

After 4 tries, and some expensive test equipment, the end result was a Quadrifilar Helix antenna weighing only 80 grams!

Our ground test antenna was a 5/8ths wave whip antenna, which works well, but unfortunately needs a ground plane. Tests with both a quarter-wave dipole and a J-pole antenna were lackluster. Documentation from our satellite service provider implied that a quadrifilar helix antenna would provide the best coverage at all. While these antennas are pretty, their design and construction was voodoo magic at first.

Thanks to some design documentation here: http://jcoppens.com/ant/qfh/index.en.php and some help from the balloon community, we had some baselines for creating such an antenna. We still went through *quite* a few revisions.  We went through 3 revisions that didn’t work, and one which works pretty darn well!

Here are the antennas which didn’t work:

Continue reading

How to Build the World's Lightest Quadrifilar Helix Antenna

After the scrub on the launch pad for flight attempt A, we went back to the books, to try and figure out what we could do to improve our odds the next time around. One of our biggest setbacks was the inability make an antenna suitable for our use: We needed an antenna tuned for 149 MHz, not needing a ground plane, weighing as little as possible.

After 4 tries, and some expensive test equipment, the end result was a Quadrifilar Helix antenna weighing only 80 grams!

Our ground test antenna was a 5/8ths wave whip antenna, which works well, but unfortunately needs a ground plane. Tests with both a quarter-wave dipole and a J-pole antenna were lackluster. Documentation from our satellite service provider implied that a quadrifilar helix antenna would provide the best coverage at all. While these antennas are pretty, their design and construction was voodoo magic at first.

Thanks to some design documentation here: http://jcoppens.com/ant/qfh/index.en.php and some help from the balloon community, we had some baselines for creating such an antenna. We still went through *quite* a few revisions.  We went through 3 revisions that didn’t work, and one which works pretty darn well!

Here are the antennas which didn’t work:

Continue reading

How to Measure a Very Large Balloon

Hi everyone.  We’ve been working constantly at getting this Balloon off the ground.  Until now, though, we’ve had no way to directly measure how far “off the ground” we’d get. Fortunately, Brad and Dan spent their Friday night building just such a measurement device.  When you’re flying a constant altitude balloon like ours, the amount of helium it holds, combined with the weight of the robot pilot dangling off the balloon, directly determines the altitude that you will fly at. When your goal is to cross the Atlantic using the thin Jet Stream, you need to fly at a pretty precise altitude, or you’ll miss it entirely!

Unfortunately, our balloon manufacturer was unable to give us a precise volumetric measurement of the delivered envelopes. Unwilling to bet our sweat and tears on such a response means developing a method to measure the volume of air going into the balloon.

We came up with a few ideas that wouldn’t work:

  • Buying dry ice, letting it sublimate inside the balloon, and comparing the before and after weights. (It would take almost 110 lbs of dry ice to generate 1000 cu ft of gas! That’s expensive!)
  • Buying a canister of CO2, and measuring the pressure in the bottle before and after filling. (Compressed CO2 is as expensive as dry ice, and we don’t have accurate enough pressure gauges)
  • Filling the balloon with air, measuring the pressure inside the envelope, then adding a small quantity of dry ice, and measuring the pressure increase. (Requires a constant volume inside the balloon, which may rip its seams)
  • Making a “bicycle pump” mechanism, and adding measured quantities of air at a time (This would take FOREVER!)
  • Placing a temperature sensor and a heater in the air stream, to measure the heating effect in order to measure the air speed, which can tell you the volume that flowed through a tube over certain amount of time. (Would take too long to calibrate the temp sensor and heater.)
  • The last idea inspired us, though.  If you can measure the speed of the air you’re putting into the balloon, and measure the cross sectional area of the tube through which the air travels, you can measure the volume of air that’s moved through the tube over time!  It’s a simple equation:

    Air Speed (M/S) * Cross-Sectional Area (M^2) * Time (S) = Volume (M^3)

    How can you measure air speed?  Well anemometers work OK, but they’re big and bulky and would block our tube.  Airplanes measure their airspeed using a device called a “Pitot Tube.”  It looks like the image to the right.

    The Pitot Tube works by measuring the difference between the ambient air pressure and the ram-air pressure.  Air is forced into the opening of the pitot tube at high velocity, and the difference in pressure gives you the fluid velocity by the following relationship:

    Where (Pt – Ps) is the differential pressure, and Rho is the fluid density.

    That seems pretty simple, doesn’t it?  Well, we were able to pull it off with parts lying around the LVL1 hackerspace in one evening!

    We happened to have a long plastic conical nozzle, about 3 inches long, sold at auto parts stores as part of a miscellaneous kit of  vacuum hose adapters.  Looked pretty much like a pitot tube already!  It also happened to allow some tubing we had ( 1/6″ ID 1/8″OD) vinyl tubing to fit up right out the tip perfectly.

    We hot glued the tubing into the nozzle. and mounted it on a thin sheet of aluminum with a cable tie and hot glue.  This would allow us to position the pitot tube in the center of the airflow in the filling tube, with minimal disruption in the airflow itself.  Disrupting the airflow too much is BAD for pitot tube accuracy.  Streamline it, and the things around and behind it.  Make sure it points directly into the airstream.

    For measuring balloon volume, we don’t need helium, plain old air is fine.  So, we called upon our in-house air source: the high-power vacuum cleaner blower head from a shop vac.  It would make quick work of inflating the giant balloon.

    We embedded the aluminum strip, with pitot tube, parallel to the airflow in the hard plastic extension tube of our shop vac.  A simple slot was cut on each side using a dremel cutting wheel, and secured with hot glue.  (Use safety glasses!)

    The pitot gives us ram air pressure, which is only half of the answer – the other half comes from the static source.  Where do you measure that from?  Well, we think that it should be measured in the balloon, a ways away from the fast-rushing airstream coming in.  To do that, we the open end of another tube of the same type about 4 feet into the balloon, away from the direction we’d be aiming the blowing the air in.

    Now, with two tubes to give us air pressure from two different places, we just needed a differential pressure sensor to measure difference at the static location and the the pitot tube in the airstream.  Fortunately for us, a few months ago we had designed and built a sensor module, complete with a differential pressure sensor that perfectly covered the pressure range we needed to measure today.  We just pressed the static tube and the ram air tube onto the two ports of the sensor, and were ready to measure the pressure!

    We connected the sensor board up to an Arduino microcontroller, and wrote a quick program to read the pressure in kPa every half-second.  We connected the Arduino to a PC running a serial port data logger to save the numbers as a text file.

    The procedure to get volume would be: record the pressure every half second, inflate the balloon to full, paste the recorded pressure list data into a spreadsheet to convert pressure to airspeed, and then each airspeed * time to get volume of air each time period, and integrate that M^3 volume over time.  For our balloon, we ended up with 28.04 M^3, which is just about 990 cubic feet.  That’s a reasonable answer.  We think it’s probably right.

    One thing we know for sure, is it was easy, cheap and fast.  It should work with helium right on the launch pad, which *might* really help amateur ballooning launches, where it’s really hard to tell how much helium you’ve put in the balloon.  We’ll publish some more details on how to precisely replicate this in the future.

    Pictures follow:

    LVL1 and White Star Balloons

    Good News Everyone!Good news everyone!  LVL1 is going to deliver a box of used balloon gizmos to Europe!  What’s that you say?  Europe has had enough of our crap already?  I think not!

    We’ve officially started the LVL1 White Star Trans-Atlantic Balloon Service with one goal: To send a small robotic balloon across the Atlantic Ocean and land somewhere over there on dry land.  No amateur balloon has ever done this.  The farthest amateur flight so far sank just 200 miles short of Ireland – tantalizingly close, but no cigar.

    We will be attempting multiple flights to reach this goal, spaced randomly from December of this year to April of next year.  We’ll be doing a short, but high, flight test on October 9th.  This test is to exercise the launch team and raise funds for the 4 to 5 Trans-Atlantic flights we predict will be necessary to refine the system and make it across.  These jet-stream sailing balloons are actually quite complex, and require a few expensive parts.  Still though, each flight of the White Star will be cheaper than most airline seats across the pond!

    Come along to our first launch Saturday, Oct 9 at 10am (show up between 9am-10am) and see the first White Star Balloon take off, dubbed HighBall-1.  Launch location: EP Tom Sawyer State Park, RC Model Airplane Airfield.  All in LVL1 and the public are invited to participate or spectate as desired.  There will be lots to do, and lots to see over the next few months.    Check our website for more details: WhiteStarBalloon.com and follow WhiteStarBalloon on Twitter and FaceBook for updates.

    Now for those technical details that we all love!
    To give you an idea of where money would be going, every SpeedBall trans-atlantic flight will cost around $600-700 each:  $100 helium, $300 balloon, $100 telemetry system, $50 GPS, $~100 remaining payload electronics and hardware.

    These balloons are small in comparison to anything else that has ever crossed the ocean, but they aren’t small when you’re next to them.  They will be 30-50 feet tall, and be lifting a whopping 12 lbs of payload – a bowling ball’s weight!  We promise not to send a bowling ball to Europe with our name engraved on it though.  Most of that weight will be ballast to drop when the balloon starts to descend from jet-stream cruising altitude (35,000ft).  Software onboard will control the ballast based on GPS and barometric altitude measurements.

    We’ll be up there with the jetplanes, so we need to be extra careful that they don’t bump into us.  There will be a constant stream of position reports from us to the 5 North Atlantic Air Traffic Control Centers, in the same format they receive reports from the jet planes.  If we stop hearing position reports from the balloon, that can be dangerous for the jets.  To prevent the balloon from becoming lost, we’ll be sending the balloon a heartbeat ping every few minutes.  If we stop hearing the balloon, we’ll stop sending the heartbeats.   When it misses our heartbeat for more than a few beats, the balloon will rupture and come fluttering gently down from the sky.  Now that’s sexy safety!

    More tech info will be discovered upon visiting http://wiki.whitestarballoon.com !