Poorman's Space Program

Paul Verhage

What you can Expect at your BalloonSat Launch

Days before the launch, software will use winds aloft predictions to calculate where the near spacecraft will recover. A more accurate prediction is possible if made just before launch or even during the chase of the balloon.

It takes anywhere from 30 minutes to an hour to fill a balloon and test the near spacecraft modules. Therefore, arrive at the launch site at least an hour before launch. Don't be surprised if this means you're arriving to the launch site in the dark. Usually the winds are at their lowest in the morning and low wind speeds make filling and launching the weather balloon easier. A typical near spacecraft ascent rate is between 210 to 365 meters (700 to 1200 feet) per minute, with an average around 300 meters (1000 feet) per minute. Typically the ascent rate is a little faster for the first 6 to 12 km (20,000 to 40,000 feet) and then it levels off to a slower but still constant ascent rate for the rest of the flight.

Figure 1. A near space system shortly after launch.

After launching the balloon, you'll probably go on the chase with the amateur radio operators. They usually carry a laptop in their chase vehicle that displays position reports from the near spacecraft's TNC. With the laptop, you'll be able to see where the near spacecraft is located with a software mapping program. As the near spacecraft drifts across the sky, its position updates on the map. Along with the position will be its speed, altitude, and heading.

If possible, you'll stop at a gas station before the balloon bursts. There you can get food and drinks while waiting for the balloon to burst. If the sky is clear, you should be able to see the balloon and detect its burst with the unaided eye. The balloon appears as a star in the sky (if you know where to look). At burst, the “star” fades out over about one second. Expect the time from launch to burst to be about 90 minutes.

Initially after balloon burst, the near spacecraft descends very rapidly. Depending on burst altitude, the initial descent speed can be in excess of 160 km per hour (100 mph) or 2.4 km (8,000 feet) per minute. During descent, the near spacecraft experiences a lot of shaking and bouncing. Therefore, it's important to pack the modules in Styrofoam peanuts or in some way tie the avionics in place. As the near spacecraft gets lower, its rate of descent slows. If the terrain is pretty flat and the roads well laid out, you have a good chance of seeing the near spacecraft land. Perhaps you can be one of the lucky few who have caught a near spacecraft as it lands. The touchdown speed will be around 16 km per hour (10 mph). So be careful it you try to catch it. Expect the descent TO take an hour.

Some Near Space Charts

Here are some Excel charts created from past near space flights. Your BalloonSat will be capable of developing the same types of chart by itself or with the help of APRS data.

Figure 2. Climb rate during a near spacecraft.

In Fig. 2 you can see that the near spacecraft's ascent speed varies a little, but that overall, it's pretty constant. There was a small decrease in the ascent speed 25 minutes into the mission. After balloon burst, the near spacecraft began descending at high speed. As the air density increased at lower altitudes, the parachute became more effective at slowing down the near spacecraft. A chart like this contains APRS position reports transmitted by the near spacecraft.

Figure 3. Acceleration during a near spacecraft flight.

In Fig. 3 you can see that the amount of acceleration experienced by the near spacecraft varied. The amount of variation, or jerk, is greatest early in the flight. At 35 minutes into the flight there's a short burst of increased jerking before it settles down to a lower level. This transition occurs at the same time the balloon's ascent speed decreases. Therefore, when the balloon slowed down, the amount of jerking around also decreased. For the first 20 or so minutes of the descent, the near spacecraft tried to free fall and enter into a zero-gee state. However, tumbling prevented any extended zero-gee state. As landing approached, the near spacecraft experienced a 50% reduction in the “gravity it felt” due to its falling under the parachute. This chart comes from the data collected by an Onset Computer Pendant G Logger.

Figure 4. Acceleration during a near spacecraft flight.

You've probably noticed that when you drive up a mountain, the air temperature gets lower. During a near space mission this is seen to the extreme. The air temperature decreases because the near spacecraft is climbing away from the lower atmosphere's primary source of heat, the ground. Once the near spacecraft enters the stratosphere, the air temperature begins climbing again. That's because the stratosphere's primary source of heat is where the ozone layer absorbs ultraviolet radiation. As the near spacecraft climbs higher, it's getting closer to the sun and to a greater flux of ultraviolet radiation. In the winter, the boundary between the troposphere and stratosphere (the tropopause) gets colder and lower in altitude. A typical summer temperature and altitude is -60 degrees and 15 km (50,000 feet). In winter, expect to see something like -90 degrees and 12 km (40,000 feet). The actual numbers vary by latitude and are colder and lower closer to the poles. A simple weather station can create a temperature chart like the one in Fig. 4. Alternatively, a Hobo data logger and temperature sensor can generate the same chart.

Figure 5. Barometric pressure during a near spacecraft flight.

Air pressure results from gravity pulling on the earth's atmosphere. This means air pressure is simply the weight of the air above us. Therefore, it's not surprising that as a balloon climbs higher, there's less air pressure. As a rule of thumb, air pressure decreases by 50% every 5.5 km (18,000 feet) change in elevation and by 90% every 15 km (50,000 feet) change in elevation. The pressure data in Fig. 5 came from the pressure sensor on a weather station. Onset makes a pressure sensor data logger that's also a good source of pressure data. The altitude information came from the GPS receiver onboard the near spacecraft. By relating GPS time and altitude, the altitude of each pressure measurement is determined.

Figure 6. Relative humidity during a near spacecraft flight.

Humidity is the measure of the amount of water vapor dissolved in the air. When given in units of grams of water in kilograms of air, the measurement is absolute humidity. Air at a given temperature can only hold so much water vapor. The actual amount of water vapor in the air compared to the maximum amount of water the air can hold is called the relative humidity. The unit of relative humidity is the percent. At a relative humidity of 100%, the actual amount of water dissolved in the air equals to the maximum amount of water vapor that the air can contain. Typically, clouds form where the relative humidity approaches 100%. In Fig. 6 the spike indicates that there may have been clouds at an altitude of 2.4 km (8,000 feet). Relative humidity data for this chart comes from data collected by a weather station. Like the atmospheric pressure and temperature data, the altitude came from combining GPS data with weather station data.

Figure 7. Wind during a near spacecraft flight.

As the altitude changes, the atmosphere can move in different speeds and directions. The data plotted in Fig. 7 illustrate the change in wind speed. Usually the highest speed winds occur at 12 km (40,000 feet). This is the altitude of the jet stream and here winds can exceed 160 km per hour (100 mph). The wind data in Fig. 7 came from GPS data. APRS records from the near spacecraft will include altitude and wind speed.

Figure 8. Cosmic ray count during a near spacecraft flight.

Cosmic rays are energetic subatomic particles entering our atmosphere from space (and therefore they're not really rays, which are electromagnetic in nature). There are several sources for this radiation and they include the sun and supernova explosions. Subatomic particles from space enter the atmosphere and collide with atoms and molecules. Each collision produces a shower of subatomic particles called secondary showers. Secondary cosmic rays can still possess enough energy to produce more secondary particles when they collide with other molecules in the atmosphere. However, atoms and molecules in the air eventually stop or absorb most secondary cosmic rays. That means at the earth's surface, the blanket of air overhead shields us from most of this radiation. Still, cosmic rays are a significant source of day to day radiation. As a balloon climbs higher, there is less air above it to shield the cosmic rays. Above an altitude of 18.9 km (62,000 feet), the number of secondary cosmic rays detected decreases as more primary cosmic rays are detected. The data in Fig. 8 were collected by a simple Geiger counter. The altitude data came from the GPS receiver.

Now you know what and where near space is. Next, it's time to look at how to construct your BalloonSat.

Onwards and Upwards,
Your Near Space Guide.

Previous articles by Paul Verhage in this series:

Poorman's Space Program Introduction

Federal Regulations Regarding Near Space Flights

What and Where is Near Space