FAST achieves high-altitude ballooning goals using portable mass flow meters

FAST achieves high-altitude ballooning goals using portable mass flow meters

In 2011, Dr. Amanda Maxham established Flying Apple Space Technologies (FAST), a program that uses high-altitude ballooning (HAB) to inspire involvement in science, engineering, technology, and mathematics.

FAST teams are encouraged to find creative solutions to the challenges associated with the launching of latex weather balloons into the atmosphere. Each balloon is fitted with scientific packages for tracking, imaging, and experimenting at target altitudes, and it is the team’s job to make sure it gets where it needs to go.

With over 17 launches in a two year span, the team has had all sorts of opportunities to exercise their scientific creativity – and one such venture led them to Alicat!

Newton’s apple fell. Our apples fly! – FAST team
Nevada from 100,000 feet

Southern Nevada from about 100,000 feet during the flight of FAST-4

High-altitude balloon images

A panorama of images taken by the FAST-7 high-altitude balloon in near space

Sending a balloon across the Atlantic

The FAST team had an ambitious goal: send a weather balloon across the Atlantic Ocean. This feat requires the balloon to both achieve and maintain a precise altitude long enough to ride the stratospheric winds from southern Nevada to Africa or Europe, a distance of approximately 6000 miles.

Challenge: Filling balloons with the right amount of gas in the field

A common obstacle in HAB is determining the correct amount of helium or hydrogen gas to add to the balloon. Due to the compressibility of gas, the balloon’s volume can expand to roughly 128x its original volume as atmospheric pressure decreases! It is therefore important to fill the balloon with just the right amount of gas, where it is full enough to generate lift but not so full that it bursts before reaching altitude.

Gas facts: Helium is safer, but is also much more expensive. Hydrogen is inexpensive and produces greater lift for a given volume. Each gas has different properties that affect its volumetric expansion as the balloon gains altitude, as well as its reaction to changing conditions at the launch site.

Previous solution: Weight stacks

It is common for HAB teams to measure the lifting force of the gas within the balloon by using weights. First, the balloon is filled with gas. It is then brought indoors for measurement, to minimize effects from external forces such as wind. Previously, the FAST team performed measurements by stacking 0.5 and 1 kg weights onto a weight stack while watching for signs of lift off.

Measuring in this way was effective to an extent, but left the team with an accuracy of 0.5 kg at best. This wasn’t satisfactory for Dr. Maxham and her team, so they sought a novel approach with higher accuracy.

New & improved solution: Portable mass flow meter with totalizer

FAST’s Edward Giandomenico reached out to Alicat looking for a way to measure helium and hydrogen flow into the balloon with high accuracy. Alicat provided the team with a 1500 SLPM portable mass flow meter with a totalizer.

The FAST team fills a balloon prior to launch.

The FAST team filling the balloon prior to the launch of FAST-6

Mass flow meter for high-altitude ballooning

Mass flow meter totalizing gas flow into the high-altitude balloon

The FAST team was able to overcome several of their challenges with this device:

  • They could now take measurements in the field, due to device portability and an 18 hour battery life.
  • Multiple pre-loaded gas calibrations made it easy to switch between flowing hydrogen and helium using on screen commands.
  • Mass flow totalization could be set up to prevent overfilling.
  • Balloon filling accuracy became much higher.

The team was able to use mass flow measurement and totalization to fill their balloons with a precise number of hydrogen or helium gas molecules, with over 5000x higher accuracy (from 0.5 kg to a few tenths of a gram). To Dr. Maxham’s knowledge, FAST is the only HAB group that uses a flow meter to monitor the total amount of gas that enters the balloon.

Mass flow facts: Because gaseous mass flow is standardized to a particular STP, this kind of measurement is fully independent of local weather conditions, including wind, temperature, and barometric pressure. Knowing that a balloon is filled with exactly 250 standard cubic feet of helium allows for accurate predictions of behavior even at 120,000 feet.

Result: FAST achieves neutral buoyancy!

The launch and flight of FAST-12 on August 4, 2013 was a landmark for the team, reaching a maximum altitude of 123,463 feet and landing the team on the Amateur Radio High-Altitude Ballooning leader board for the first time. One of the most remarkable feats the team accomplished was achieving neutral buoyancy. With fluctuating winds and long distance to travel, it can be difficult to make sure a balloon stays at the right altitude and doesn’t prematurely burst. The balloon was able to maintain consistent altitude for an entire 90 minutes before bursting – a huge success for the team.

High-altitude balloon during burst

High-altitude balloon during burst, viewed from up-facing camera

How did they do it?

The key to attaining neutral buoyancy at a specific altitude is to fill the balloon with just enough gas that it will ascend, yet not so much that it overshoots the target. Neutral buoyancy is achieved when the mass of atmosphere displaced by the balloon is equal to the mass of the balloon itself.

The primary variable that makes neutral buoyancy possible is the elasticity of the balloon and the resulting increase of pressure inside the balloon as compared to the atmospheric pressure. This pressure increase is more significant at high altitudes thereby allowing the balloon to become neutrally buoyant.

The trick is to give the balloon the “Goldilocks” amount of gas: just enough to get it to the desired altitude, but not so much that the balloon can’t contain it when it gets there. – Dr. Maxham
Eric Lujan calculates mass and lift

Eric uses his app to make mass and lift calculations in the field

Members of FAST use a complex formula to calculate the mass of gas required for each flight. The formula includes calculations from the ideal gas law, hooks law, the 1976 standard atmospheric model, and the modulus of elasticity of the balloon. To further complicate things, the gas pressure against the balloon changes with pressure and temperature, as does the modulus of elasticity of the latex. Furthermore, the ambient temperature outside the balloon changes in an odd way as the balloon rises through the atmosphere: Within the troposphere, temperature decreases as altitude increases, but once the balloon crosses into the stratosphere, temperature increases with altitude. One FAST student, 15-year-old Eric Lujan, actually created an app to perform the necessary mass calculations while at the launch site!

Conclusion

By totalizing the mass of gas within each balloon, the FAST team was able to get their balloons in the air and generate repeatable data during their test flights. The team successfully used science, technology, engineering, and mathematics to solve very complicated problems – all the while getting to mess with some super fun and cool balloons!