Stratospheric balloons are high-altitude balloons that are released and operated into the stratosphere ( region of the atmosphere (15 to 45 km in altitude). Stratospheric provide platform to test and advance space science for far less than the cost of a satellite (up to 40 times less). The original stratospheric balloons were flown by NASA in the 1950s, and the agency still uses them for science missions. The Canadian Space Agency uses stratospheric balloons to test and validate new technologies developed for long-duration space missions and to perform scientific experiments in a near-space environment.
Google, is working on a new technology to provide 5G internet speeds to the world. Project Loon is a network of balloons traveling on the edge of space, designed to connect people in rural and remote areas, help fill coverage gaps, and bring people back online after disasters, says Google. Google is using weather balloons operating as an airborne Wi-Fi provider under Project Loon, and plans to launch as a trial over Sri Lanka, Indonesia and possibly India. Project Loon balloons travel approximately 20 km above the Earth’s surface in the stratosphere. Project Loon, successfully deployed such balloons to provide mobile communications in the aftermath of Hurricane Maria in Puerto Rico.
Military also has been employing various platforms like Balloons, Aerostats, Airships, Satellites, and UAVs for communications and persistent, wide area, real time Intelligence, Surveillance, and Reconnaissance (ISR) of battle space.
Stratospheric balloons technology
Stratospheric balloons provide a platfom from which a variety of scientific research efforts may be conducted. Stratospheric balloons are typically made out of ultra-thin plastic filled with helium and can stretch into a gigantic upside-down “teardrop” shape more about the height of the Eiffel Tower. They are equipped with several gondolas suspended on the flight chain. The gondolas can carry science, astronomy, atmospheric chemistry, weather forecasting and technological demonstration payloads weighing up to 1.1 tons altogether.
Some balloons are bigger than a football field and able to lift payloads of 2 tonnes to altitudes of 40 km. They can reach altitudes of up to 42 km, holding their instrument packages aloft for several hours. Some balloons can even conduct long-duration flights, lasting days, weeks and even months. These balloons require no engine and no fuel and are fully recovered after each flight. It relies exclusively on natural forces: buoyancy for lift, winds for direction and gravity to descend.
Two kinds of balloons zero pressure and super pressure
There are two kinds of balloons the Open stratospheric balloons or zero pressure balloons. They are gas filled balloons (hydrogen or helium) having one or several openings which enable the balance between atmospheric pressure and the gas inside the balloon. They stay aloft for no more than a week. They can carry Payload up to 2700 kg at Flight altitude up to 45 000 meters.
These balloons are made from very thin polymeric films which provide an effective gas barrier between the lifting gas and the atmosphere that provides the buoyant force. The film needs to exhibit a number of properties including low permeability, high toughness at low temperatures, a wide range of sealing temperatures, and a relatively low weight per unit area. The structural design of a zero pressure balloon is such that transverse stresses are minimized and meridional loads are carried by high strength fibers sealed into the seams, and thereby structural analysis is relatively simple and only requires some minimal estimate of stress or strength.
Some science missions require longer duration flights in the stratosphere than can be accomplished by zero pressure balloons. This is accomplished with very light weight pressurized spheres which do not require the use of ballast to maintain altitude at night.
Superpressure balloons are sealed and their envelope is stable enough for long-duration flights upto 6 months. They can carry payoad of 50 kg at Flight altitude up to 20 000 meters
The materials are extremely thin, high modulus, high strength elastic films with adequate permeability for the life of the mission. Polyester films, such as Mylar, are extensively used for this application.
However, as the weight of the scientific instruments is increased, the volume of the sphere must be increased to produce the lift necessary for equilibrium. The resulting stresses in the film increase linearly with the radius of the sphere which places a very definite limit on the size of the balloon and the magnitude of the instrument weight.
Researchers are now designing new type of balloons which will provide the heavy lift capacity of zero pressure balloons and the flight duration of superpressure spheres. This is accomplished with a pumpkin-shaped envelope formed from a high quality polyethylene film and high strength braided cables which support the payload. This concept relies on the formation of lobes with a local radius of curvature to limit the stress in the circumferential, or hoop, direction. This local radius is independent of the balloon volume and does not result in the same limitation as the superpressure sphere.
Rough Stratospheric conditions
Depending on the mission, the platform may be required to satisfy a wide range of performance characteristics.
While in the stratosphere, balloons can encounter 150°C temperature swings, with temperatures reaching as low as -90°C ( at altitudes of 35 km (22 miles)). Extremely low temperatures mean electronic components like batteries are prone to fail
Winds in the stratosphere are stratified, which means they’re comprised of layers that travel in different directions and speeds. While one layer may cause the balloon to drift far from its target location, another nearby layer might allow the balloon to blow in the right direction. Depending on winds, a balloon can drift more than 200 km (125 miles) from the place it was released.
The higher the balloon travels, the more it expands – from 2 meters (6.5 ft) to up to 8 meters (26 ft) across – because air pressure decreases as the balloon climbs higher in the atmosphere. Eventually the balloon bursts. The latex or neoprene balloon is flexible and can expand a lot, but eventually it breaks – usually when the balloon reaches an altitude where air pressure is only small fraction (a few thousandths) of what is found at Earth’s surface.
Current balloons shift with the wind and can only stay in one area for a few days at a time. Winds in the stratosphere are stratified, and each layer of wind varies in speed and direction. Project Loon uses software algorithms to determine where its balloons need to go, then moves each one into a layer of wind blowing in the right direction. By moving with the wind, the balloons can be arranged to form one large communications network.
“By flying higher we hope to take advantage of a larger range of winds,” says ALTA project manager Alex Walan. ALTA will operate even higher than Loon at 75,000 to 90,000 feet (22,900 to 27,400 meters or 14 to 17 miles), where the winds are less predictable. That shouldn’t be a problem if the balloon can see exactly where the favorable winds are. But while machine learning and better data are improving navigation, the progress is gradual.
In theory it should be possible to find a wind blowing in any desired direction simply by changing altitude.
DARPA Laser radar can see wind 8.6 miles away and enables permanent balloons
As part of its Adaptable Lighter-Than-Air (ALTA) balloon program, DARPA is currently testing a wind sensor called Strat-OAWL, which stands for “stratospheric optical autocovariance wind lidar.” The idea is to use lasers to deduce the speed and direction of wind gusting far away from a stratospheric balloon and then make the necessary adjustments to stay in one spot
It does this by shining laser pulses in two directions. Some of that laser light then reflects off the air, returning to the sensor unit, which analyzes its wavelength. The wavelength of the reflected light is changed slightly depending on how fast the air it bounced back from is moving, a change known as doppler shift. Changes in the wavelength allow Strat-OAWL to determine the speed of the air that reflected the light, as well as the direction in which it’s moving.
Previous versions of OAWL flown in aircraft have measured winds more than 14 kilometers (8.6 miles) away with an accuracy of better than a meter per second. The main challenge with Strat-OAWL has been shrinking it to fit the space, weight, and power requirements of the ALTA balloons.
Unlike other wind sensors, OAWL looks in two directions at once, giving a better indication of wind speed and direction. “It’s like looking out with two eyes open instead of one,” says Sara Tucker, a lidar systems engineer at Ball Aerospace.
The stratospheric balloon can then adjust its altitude to wherever a wind is blowing in the direction it wants to move, ensuring it can remain in one area indefinitely.
Military aircraft have ceilings of 60,000 to 65,000 feet, so they could intercept Loon-type balloons. Because it will fly higher, ALTA will be a much trickier target. The balloon could provide secure communications and navigation or act as a mother ship for drones.
The ALTA balloon itself is made by Raven Aerostar, which also makes the Loon balloons. The firm’s general manager, Scott Wickersham, says this sort of technology gets us much closer to balloons that stay aloft indefinitely—and that will make all sort of applications possible.
In 2017, U.S. Navy Admiral Kurt Tidd noted during a geospatial intelligence symposium that the military believed stratospheric balloons could have some “interesting applications” if capable of remaining airborne for 180 days or longer.
World View sees strong interest in stratospheric balloons despite test incident
World View, a company offering stratospheric balloon flights for research payloads, sees a bright future ahead for a platform that it argues combines the best attributes of satellites and aircraft, despite a recent testing incident at its Arizona headquarters.
The company believes that its stratollites can loiter in the stratosphere for extended periods, providing persistence that aircraft cannot offer at costs much lower than satellites. Those flights have carried research payloads, including for NASA’s Flight Opportunities program, as well as commercial users, such as a summer 2017 flight that carried a chicken sandwich as publicity for a fast food restaurant chain.
One new application of World View’s balloons is in remote sensing. At the conference, the company released the first high-resolution images taken from its balloons, using off-the-shelf camera equipment. The images, taken from altitudes of more than 20 kilometers, have a resolution of 50 centimeters, but the company expects it can improve that, with better cameras and observing techniques, to as sharp as 10 to 15 centimeters.
Those balloon flights use helium, but at the conference Poynter said the company was looking to use hydrogen, which is much less expensive. “It is still on our radar. We have not done it yet, but we are very close to that,” she said.
On Dec. 19, World View conducted a test of a hydrogen-filled balloon at its Tucson, Arizona, headquarters. At the end of the test, though, the balloon ruptured. Video of the test obtained by local news media shows the balloon bursting and what appears to be flames, suggesting hydrogen in the balloon combusted.
We have used hydrogen in ground testing, but only helium in flight operations at Spaceport Tucson,” he said, referring to the pad adjacent to the company’s headquarters, near Tucson International Airport, used for balloon flights. “However, to date our use of hydrogen has been limited to ground testing when required by customers or test objectives.”
“As of now, we do not have any requirements or future plans for using hydrogen in ground testing or flight at the spaceport,” he added.