High-altitude ballooning

Beginning with the 18th century, ballooning has continually achieved higher altitudes. From Charles’s 3,000-metre (10,000-foot) ascent in 1783 to U.S. Army Air Corps Capt. Hawthorne C. Gray’s fatal ascent to 12,950 metres (42,470 feet) in 1927, the maximum altitude was only limited by the pilot’s need for oxygen. Lacking confidence in the ability to seal an aircraft hermetically, American aviation pioneer Wiley Post and others concentrated on individual pressure suits. Even as late as 1937, prominent aeronautical engineers publicly derided the concept of building airplane pressure cabins.

The Piccard invention of the stratosphere balloon (see above Balloons reach the stratosphere) opened up new heights for exploration. The first stratosphere flights were mounted to study cosmic rays, which are absorbed as they enter Earth’s atmosphere. Early high-altitude work with plastic balloons continued cosmic ray research, air sampling for detecting atomic explosions, photographic flights over foreign terrain, astronomical observations above the disturbances of the troposphere, and even aerodynamic testing of free-falling payloads. A balloon is the only stable platform for any type of observation above the range of airplanes and below the range of orbital spacecraft. It is also the only aircraft that does not affect its environment and the only device that can sit relatively motionless above heights obtainable by helicopters.

High-altitude plastic balloons are only partially inflated at launch to allow for gas expansion as the balloon climbs. This expansion is roughly tenfold for each 15,000 metres (50,000 feet) of altitude. When the balloon is at the designated altitude, the envelope takes on the natural shape—an inverted teardrop with the load attached at the bottom point. If the balloon were pressurized with no payload, the envelope would be spherical. There is a complete series of envelope shapes that depend on the ratio of payload-to-skin weight and internal pressure. Normally, the internal pressure is zero and the envelope weight is a high percentage of the total balloon weight. This gives a very fat balloon with an almost flat top. The included angle at the base can approach 45 degrees.

Some heavy-payload high-altitude balloons have been made of nylon laminated to polyester film, but most research balloons use very thin polyethylene. Some of these polyethylene films are less than a tenth of a millimetre thick. In order to carry a payload, the seams are reinforced with load-bearing tape.

In flight the payload is generally suspended from an open extended parachute connected at its apex to the base of the balloon. The parachute can be released by radio control. At the moment of release, the elasticity of the parachute snaps it open almost instantly. A radio-controlled gas valve can be used at the top of the balloon. Excess gas is vented by ducts high on the side of the balloon, with their openings at the base level. If the duct is placed high enough on the balloon, it is impossible for outside air to enter the balloon.

Contamination of the lifting gas by outside air will lower the possible ceiling of the balloon and, in the case of hydrogen, create a fire hazard. The design must assure that flow resistance in the duct does not create adverse back pressure, which would burst the balloon. The diameter of the duct, its length, and any possible kinks must be considered. The weight of the duct itself can pull it down enough to block the opening into the balloon in some cases. With the duct design, any entrained air from mishandling during inflation or minute leaks in the balloon below the base of the lifting gas will pool in the base of the balloon and lower the ceiling accordingly.

The natural shape gives a balloon very low skin stress under static conditions. At altitude the balloon is very stable and not generally subject to turbulence or dynamic loads. On the way up, the balloon is only partially inflated and has great flexibility to distort, relieving any stress.

The first manned stratosphere balloon used a spherical aluminum cabin. Following ones used magnesium alloys and spun aluminum. Current high-altitude pressure cabins are made with various composite materials. Internal pressure is maintained by onboard liquid oxygen supplies and air scrubbers to remove carbon dioxide, moisture, and other body products.

Several flights have been made with no cabin at all. Crews in open gondolas wear space suits similar to those that astronauts wear.

Notable balloon altitude records

Notable balloon altitude records are listed in the table.