High Altitude Ballooning
From the eighteenth century on ballooning has continually achieved higher and higher altitudes. From Prof. Charles’ 10,000 foot ascent in 1783 through Coxwell and Glaisher’s historic claim of 36,000 feet in1862 to Hawthorne Gray’s fatal ascent to 44,000 feet in 1927 the maximum has been limited by the human body’s need for oxygen.
Lacking confidence in the ability to hermetically seal such a complicated device as an aircraft, Wiley Post and others concentrated on individual pressure suits. Even as late as 1937 prominent aeronautical engineers publicly derided the concept of pressure cabin use in airplane design.
The Piccard invention of the stratosphere balloon opened up unlimited heights for man’s exploration. The first stratosphere flights were mounted to study cosmic rays which were found to be absorbed as they entered the Earth’s atmosphere. The mysteries of the atom and the unleashing of atomic power were strong incentives for this research.
Early high altitude work with plastic balloons continued cosmic ray research and included air sampling for atomic explosion detection, photographic over-flight of foreign terrain, astronomical robotic telescope operation 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 orbital spacecraft. It is also the only aircraft that does not affect its environment and the only device that can sit relatively motionless above helicopter height.
High altitude plastic balloons are launched only partially inflated to allow for lifting gas expansion as the balloon climbs. This is roughly tenfold for each 50,000 foot of altitude as the density of the atmosphere has a logarithmic decline. When the balloon is at altitude it takes on the “Natural Shape”. That is an inverted teardrop shape with the load attached at the point. If it were pressurized with no payload it would be spherical. There is a complete series of shapes depending 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. 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, such as Dr. Schwartzchild’s “Stratoscope” have been made of nylon scrim laminated to polyester film, but most research balloons are very thin polyethylene. Some are less than one thousandth of an inch thick. In order to carry a payload, the seams are reinforced with load bearing tape. The manufacturing process involves heat sealing machines that weld the plastic with jets of hot air.
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 ducks exiting high up 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. Just 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 such stress. The “Natural Shape”, then, may not be suitable for fully inflated low altitude balloons and, indeed, modern hot air balloons have either the Piccard bulbous gore “Pumpkin” shape or excess material, forming flutes, in natural shape balloons.
The first manned stratosphere balloon used a spherical soft 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 on board 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. The crews have been in open cars wearing regular space suits, as if on EVA on the moon or in orbit.
by Don Piccard
©2005