Nuts & Volts - March 2006

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to decrease as it approaches 100,000 feet. That surprises me and now I have to wonder how accurate the pressure and temperature equations are at extremely high altitudes. But we’ll run with these numbers anyway since we’ll see shortly that the weather balloon will never get that high. By the way, the spreadsheets I used to create these charts are available on the Nuts & Volts website ( www.nutsvolts.com ).

According to Kaymont (the company where I purchase my balloons), the maximum volume of a 3,000 gram balloon is 40,479 cubic feet. So our astronauts’ 3,000 gram balloon carrying a 1.5 pound payload (under Martian gravity) can expand 40,479/13,066, or three times in volume before bursting. According to my chart then, the balloon will reach an altitude of 48,000 feet before bursting.

That’s the same as my lowest altitude near space flight on Earth. The average altitude of my 54 flights (all on Earth unfortunately) is 84,350 feet. So it’s apparent that near space flights on Mars are going to be a lot lower than on Earth. This really isn’t too surprising though, since the surface pressure on Mars is about the same that we see at an altitude of 103,000 feet on Earth.

Let’s quickly test the accuracy of this chart and its conclusions. So as a test, I have created a spreadsheet for the Earth’s standard atmosphere and with data that was collected on one of my flights from August 2003. I used the temperature and pressure from both my flight and from the Standard Atmosphere to calculate the diameters of two balloons and developed the chart in Figure 5.

What strikes me the most about this chart is that the balloon volumes follow each other very closely until the balloon reaches 50,000 feet. Above that altitude, the standard atmosphere predicts the balloon will expand faster than my actual flight data indicates.

Why is this? Is the real balloon having difficulty expanding above 50,000 feet? The ascent rate for this flight is constant through the 50,000 foot transition, so the balloon can’t be experiencing difficulty in expanding in volume (if it did, the ascent rate would

slow down). Besides, the air pressure changes smoothly throughout the flight, with no abrupt transitions. But there is one thing special about 50,000 feet: the stratosphere begins there (the stratosphere is lower in the winter, but this was a summer flight).

In the troposphere, the air temperature decreases with altitude. But in the stratosphere, the presence of ozone makes the air warmer. There’s more solar ultraviolet radiation higher in the stratosphere since it’s the ozone that is removing it. Helium doesn’t stop any ultraviolet that I know of, so I suspect it’s transparent to ultraviolet. Also, the balloon is white and its color may be reflective to the sun’s increased ultravi-

olet radiation. No doubt there are errors in my temperature and pressure sensors. Perhaps my temperature sensor is less accurate at low air pressure. Out of these factors, which prevails?

This calls for an experiment (I love it when missions generate more questions than they answer). In the future, I’ll measure the temperature and pressure inside the balloon and compare it to conditions outside the balloon. To further resolve this issue (or perhaps to muddy it further), I’ll also record images of the balloon as it ascends. I can then calculate the true volume of the balloon from the diameter of the balloon images.