Nuts & Volts - May 2007

NOTE

The maximum altitude that a near spacecraft will reach is influenced by the weight of the near spacecraft, the original volume of helium in the balloon, and variations in the balloon’s manufacture. So the numbers I’ve given you are just rough estimates. You can experiment with changing lift amounts and payload weights with a program called Lift Win. This program is available from the website of the Edge of Space Science (www. eoss.org) as a free download.

■ FIGURE 5

Track as a Windows application. Now, Troy Campbell has created an online version of the software that takes you through the entire process of making a flight prediction (see Figure 3).

Currently, you don’t have to enter information in the General section. In the future, you’ll want to enter this information because of

the planned enhancements to the application.

The Launch Site section asks for the latitude and longitude of the planned launch site. This information is used to map the mission’s predicted flight path and recovery zone. The elevation field is not very important because the effects of launch

■ FIGURE 6

elevation are usually swamped by inaccuracies in the predicted winds aloft, precision of the balloon’s helium fill, and manufacturing variations in the balloon. One exception may be when the balloon is launched from the top of a tall mountain.

The Flight Parameters section asks for the predicted ascent rate, descent rate, and burst altitude of the balloon. Here’s where you need the result from the Ascent Rate Calculator. In my experience, the descent rate is between 1,000 and 1,200 feet per minute. If the parachute is larger or the payload lighter, the descent speed is lower. You’ll get a better feel for the proper descent rate after you’ve flown a mission or two. As a rule of thumb, I expect to reach 50,000 feet with a 300 gram balloon, 80,000 feet with a 1,000 gram balloon, 85,000 feet with a 1,200 gram balloon, 93,000 feet with a 1,500 gram balloon, and over 100,000 with a 3,000 gram balloon (see the note in the sidebar).

There’s one data entry field and three buttons in the Data Source section. The Wx Station (weather station) field asks for the station ID for the weather station closest to your launch site. Since very few people know the three letter designation of their local weather station, there’s an option to look it up in the Station List link. You’ll find weather stations sorted by state and listed in alphabetic order. Next to each weather station is its three letter designation. I discovered that the Boise weather station has a station ID of BOI.