January 15, 2011 Zero Pressure
This was our team's first zero pressure (ZP) balloon launch, and served as a learning opportunity for us. A ZP balloon is in principle a giant plastic bag with an opening at the bottom to allow for excess gas to spill out. Consequentially, there is zero pressure difference between the gas inside the balloon and the surrounding atmosphere, for as soon as there is the gases can equalize through the balloon opening. These balloons are used to fly at a constant altitude for several hours, and can travel significant distances.
On the ground only a fraction of the balloon is filled with a bubble of helium, or other lighter than air gas, which then floats to the top of the balloon. The rest of the balloon can fill with air, as long as the helium remains on top. As the balloon rises, the helium expands just as in a sounding balloon flight, only instead of expanding the balloon, the helium bubble forces the air in the balloon out, until the helium completely fills the entire balloon. At this point the balloon stops rising, for any additional expansion of helium simply causes helium to vent out of the bottom. Consequentially these balloons do not burst, as in sounding launches, and can float for incredible distances.
At night, temperatures drop, and the helium inside the balloon condenses. This causes the balloon to drift back down, and unless ballast is released it will land.
Our team launched this balloon from the UC San Diego campus, in the Warren engineering mall. The balloon was launched at 9:30 AM, and we tracked it online until it landed at 7 PM that same day. The balloon cruised at 45000 feet for the majority of the flight, and traveled approximately 350 miles. This balloon launch was featured in the UC San Diego Jacob's School of Engineering Blog.
Tracking equipment used:
ArgentData APRS Tracker HAM beacon:
Amateur radio (usually referred to as HAM radio) can be used to transmit data using HAM radio frequencies. The HAM radio tracker onboard the balloon gathers position information using GPS, then transmits that information through HAM radio frequencies using a HAM method called the Automatic Packet Reporting System (APRS). If there is a HAM ground station within range of the balloon, the ground station receives the balloon's APRS signal and automatically posts the information it receives online.
The APRS tracker we used for this launch is similar to the MicroTrak AIO Beacon, but at the time of launch it was not commercially available. This beacon was purchased from the owner of Argent Data Systems, who is currently developing a commercial version of this product. The ArgentData APRS tracker provides the following information, which was monitored live using google aprs:
SPOT GPS Messenger:
The SPOT GPS Messenger is a handheld GPS receiver and satellite messenger in one. It knows it's own position by using GPS, and then transmits it's position to a website (www.findmespot.com) using satellite communication. It can also be used to send brief pre-programmed messages, or to call for help in an emergency. It was designed for use by outdoor enthusiasts who may not always be within range of a cell tower, and allows them to keep in touch with friends and family while they are out of cell phone range. Because it does not require cell phone towers or HAM radio ground stations to communicate, the SPOT can be used to track our balloon in areas where other tracking systems cannot operate. The SPOT's "track progress" feature uploads position data to the SPOT website automatically every ten minutes. This feature is activated prior to launch, and has allowed us to locate balloons after all other tracking systems have failed.
One limitation to consider when using a SPOT is that the unit must be facing up in order to work. If the SPOT is strapped to a payload box and that box lands upside down, the SPOT will not be able to communicate with satellites. To overcome this, a gimbal system was constructed which would make sure the SPOT always faced skyward. The SPOT was hung under the inner gimbal, insuring that gravity would cause the SPOT to automatically orient correctly. The gimbal system was placed inside of a hamster ball to prevent any objects from jamming the gimbals. Please take a look at our SPOT ball video below (also available on YouTube here) for an example of the gimbal system in action. We have also posted SPOT Ball Construction Tips in our documentation section.
The SPOT ball
The SPOT ball gimbal system in action
Balloon Fill and Launch:
Prior to filling the balloon, all electronics onboard the balloon are turned on and tested. For this flight, this meant that both tracking systems were turned on, and checked to make sure they were transmitting position and data correctly. The payload box was then sealed, and firmly tied to the balloon.
Because this balloon was not intended to be recovered, the team did not need to tailor the launch location to land the balloon in a particular area. This allowed to team to pick the most convenient launch site: on campus at UC San Diego. The balloon was filled and released from the Warren engineering mall. This type of zero pressure balloon is made from a plastic material which feels a lot like a tough plastic bag, as opposed to the stretchy latex used for near-space balloons. The zero pressure balloons are filled slightly differently as a result: one person is responsible for holding the balloon 'bubble' while the helium gas in being released into the balloon. As more helium is released into the balloon, the team mate holding the bubble slowly walks towards the top of the balloon, releasing more of the balloon material on the way. This allows the balloon to be filled in a smooth, controlled manner, helping to reduce the risk of tearing the balloon.
Balloon fill in progress: person on the right filling Helium, while team mate
on the right allows the balloon to fill gradually.
Once the balloon has been filled, it is allowed to float while the team prepares for launch. It is best to keep this time to a minimal, in case a gust of wind pushes the balloon over (increasing the risk of damaging the balloon.) In the picture below, you can see the Helium has risen to the top of the balloon and formed a bubble. The payload box is hanging directly below the balloon, with the SPOT hanging below. The balloon is being held by the SPOT ball in this photo.
Balloon ready for launch
Prior to releasing the real thing, the team releases a helium-filled party balloon to see where it goes. If the wind pushes it too close to a tree or other obstacle, the team knows to move to a more favorable launch position. Once the position is finalized, the balloon is released and begins it's journey.
Just released, and on it's way up!
After launch, the team watched the balloon's progress in real time online. This allowed us to see where the balloon was, along with the temperature and pressure data being transmitted by the ArgentData APRS HAM tracker. It was fun to watch the balloon live online, and exciting to see that it reached a crusing altitude of 45000 feet. For comparison, most commercial airline flights occur at about 35000 feet. At 45000 feet, the blackness of space becomes visible and the curvature of the earth is noticeable.
The view from 45,000 ft. This picture was taken during a previous near-space balloon launch, and
is included here to provide an example of what kind of view our zero pressure balloon would have
captured if it had been transmitting images.
Balloon Flight Path:
The balloon's path is completely dictated by the winds, and cannot be steered from the ground. The wind strength and direction change with altitude, with high altitude winds often reaching speeds in excess of 100 knots (1 knot = 1.15 miles per hour). Predicted wind conditions are available online, and are reported for various altitudes. The wind conditions on the day of our launch (posted by NOAA at http://aviationweather.gov/) are shown below.
The wind conditions aloft on the day of launch. (Image obtained from http://aviationweather.gov/)
Balloon flight path: yellow path is the path generated by the ArgentData APRS HAM tracker (GPS position with altitude.) The blue balloons are theFlight Data:
position reports received by the SPOT GPS Messenger. The red line represents the overall balloon path, as determined from both tracking systems.
The balloon flew south immediately after launch, and then turned towards the east as it climbed. The balloon maintained an east-southeast track across the northern part of Mexico. It later made a turn to the south and descended into a remote desert area. The HAM APRS tracker provided data from the majority of the flight (shown as the yellow path in the picture above), but contact with the tracker was lost once it drifted beyond HAM radio ground stations. The SPOT GPS Messenger reported position near the launch, and resumed operation toward the end of the flight. Between the APRS tracker and the SPOT GPS, the entire flight path could be determined.
Because this balloon was not meant to be recovered, data had to be transmitted from the onboard ArgentData APRS HAM tracker to HAM ground stations. The HAM ground stations then uploaded the data to the internet automatically. However, our balloon eventually lost sight of HAM ground stations over Mexico. The data presented below corresponds to the portion of the balloon flight shown as the yellow path in the above flight path picture.
Altitude vs. time: from launch until loss of signal from ArgentData APRS tracker. Cruise altitude was approximately
45000 feet above sea level.
Air temperature vs. altitude. The temperature corresponds to the temperature inside the payload box compartment.