Mass flow measurement in the quest for neutral buoyancy
Led by Dr. Amanda Maxham of the University of Nevada, Las Vegas, the FAST team has an ambitious goal: to 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. A significant challenge is that all latex balloons eventually burst due to degradation of the material by ultraviolet radiation, an effect that intensifies at high altitudes where the air is thin. To minimize daylight exposure of the balloon to UV rays, the team is shooting for a winter crossing, perhaps with a winter solstice launch. FAST must also precisely target an altitude whose winds will carry the balloon swiftly across the Atlantic before it bursts. This requires a precise knowledge of how much gas is inside the balloon.
The launch and flight of FAST-12 on August 4, 2013, was a landmark for the FAST team. This flight achieved a maximum altitude of 123,463 feet, landing the team on the Amateur Radio High-Altitude Ballooning altitude leader board for the first time. More importantly, the balloon achieved neutral buoyancy and maintained a consistent altitude for about 90 minutes until the balloon burst. Neutral buoyancy is critical to the team’s aspirations of crossing the Atlantic because the balloon needs to hover around a specific altitude in order to ride the fast-moving winds that blow at that altitude. On a larger scale, this goal is also a requirement for Google’s Project Loon, which is designed to maintain even balloon distribution by commanding the balloons to ride different directional winds at specific altitudes.
The key to attaining neutral buoyancy at a specific altitude is onboarding just enough gas to provide the required lift to ascend from the launch altitude, yet not so much that the balloon overshoots the target altitude. The balloon stops rising and becomes neutrally buoyant (if everything is done correctly) when the mass of the atmosphere displaced by the balloon and payload is equal to the mass of the gas inside the balloon plus the mass of the balloon and payload. 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, as Dr. Maxham calls it: just enough to get it to the desired altitude, but not too much so that the balloon can’t contain it when it gets there.
By totalizing the mass of gas within each balloon using the mass flow meter, the team is able to generate repeatable data during their test flights.
Dr. Maxham’s students use a complex formula to calculate the mass of gas required for each flight. The formula includes calculations using the ideal gas law, hooks law, the 1976 standard atmospheric model, the modulus of elasticity of the balloon and many other factors to determine just the right amount of gas to use. As if this calculation wasn’t difficult enough, 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, has even written an Android app to perform the necessary mass calculations while at the launch site.
By totalizing the mass of gas within each balloon using the mass flow meter, the team is able to generate repeatable data during their test flights. This expands the educational reach of the program, because not only can the students use the balloon to carry scientific experiments, but they can also do science on the balloon itself. And getting people engaged in science is what this is all about.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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