Two years ago, a young woman watched a YouTube video of a father and son team launching a high-altitude balloon (HAB) into near space and was forever changed. With a newly-minted astrophysics PhD in hand from the University of Nevada, Las Vegas, Dr. Amanda Maxham soon established Flying Apple Space Technologies (FAST), a program that uses high-altitude ballooning to inspire students and laypeople to get involved in science, engineering, technology and mathematics (STEM). FAST uses latex weather balloons fitted with scientific packages for tracking, imaging and experimenting at their target altitudes. As for the “flying apple,” they simply explain: “Newton’s apple fell. Our apples fly.” Nice!
A recipient of the Nevada NASA Space Grant, FAST has had 17 launches in the past two years. Their first, on September 24, 2011, was an epic failure with their scientific payload crash-landing on the side of a mountain. It took two days for the team to locate and recover the payload. During another launch, the balloon lifted off with a non-functioning tracking beacon. Yet Dr. Maxham sees failures like these as opportunities for her students to identify scientific obstacles and creatively overcome them. It was one of these obstacles that brought the FAST team to our attention.
Mass flow meters in high-altitude ballooning
A common obstacle in HAB is determining the correct amount of lighter-than-air gas to add to the balloon. The amount of gas is critical to generate enough lift to achieve a target altitude before the balloon bursts. Because gases are compressible, volume is a function of atmospheric pressure (and therefore altitude) and temperature. Balloon volumes generally double with every 15,000 feet of altitude, so a balloon’s volume at 100,000 ft is roughly 128 times what its volume was when it was filled on the ground. Filling the balloon with too little gas does not generate enough lift; too much gas can cause the balloon to burst before reaching the target altitude, or to fail to become neutrally buoyant before bursting.
HAB teams commonly use weights on the ground to measure the lifting force of the gas within the balloon, but this method is inconvenient and inaccurate. Balloon filling must be stopped while taking weight measurements, and this must be done indoors to escape the constantly changing forces of shifting winds at the launch site. Stacking 1 kg and 0.5 kg weights on a weight stack while watching for signs of lift yielded the FAST team an accuracy of 0.5 kg at best. Dr. Maxham’s team needed a more accurate way to verify that the correct amount of lift was being achieved at launch. Knowing that calculating the amount of lift was simple if the amount of gas added to the balloon could be measured, the team began looking for a solution. When FAST’s Edward Giandomenico reached out to Alicat in August last year, we were excited to be able to provide them with their solution: We donated to the team a 1500-slpm portable mass flow meter with totalizer.
The FAST team can switch between helium and hydrogen gas at the press of a button, all the while maintaining measurement accuracy and doing so via battery power in the field.
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 measurement and totalization allows the FAST team to fill their balloons with a precise number of gas molecules on the ground so the lift and volume of the gas at altitude is just right. In fact, the team’s accuracy in measuring lift increased from 500 grams to just a few tenths of a gram! Because gaseous mass flow is standardized to a particular STP, this kind of measurement is fully independent of local surface weather conditions, including wind, temperature and barometric pressure. In this way, the FAST team can be certain that 250 standard cubic feet of helium on the ground will still be 250 standard cubic feet of helium at 120,000 feet, assuming a leak-free balloon.
Another challenge is that FAST sometimes fills their balloons with helium, and sometimes with hydrogen. Helium is safer (“Remember the Hindenberg!”) but currently much more expensive. Hydrogen is an inexpensive gas 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. It’s a good thing Alicat mass flow meters come pre-loaded with 30 gas and gas mix calibration curves. The FAST team can switch between helium and hydrogen gas properties data at the press of a button, all the while maintaining measurement accuracy and doing so via battery power in the field. Live mass flow totalization allows a team member to ensure that the target mass within the balloon is achieved without overshoot.
Stay tuned! We continue tomorrow with a look at FAST’s ambitious quest to cross the Atlantic.FAST’s next launch is Oct 5, 2013, weather permitting. You can follow its flight on launch day, so be sure to subscribe to FAST’s blog for updates and launch schedules.
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