A parachute consists of a woven textile or plastic canopy which is attached to the load by means of suspension lines, frequently called shroud lines. The canopy may have various forms, but is usually axially symmetric about the vertical axis, and may be made with or without holes in it. For some special purposes the canopy is made of ribbons.
Parachutes are used on most scientific balloon flights to lower the scientific payload upon te termination of the ballooning phase of the flight. A parachute is also used as a safety device of virtually every balloon flight even though the balloon system is to be brought down inttact by valving gas from it.
The figure at left schematically illustrates a descending parachute with attached load. Most features of parachutes used in scientific ballooning are shown, thought variations may be found. A parachute with its payload and ancilliary equipment will be termed a parachute system.
The Susponsion lines join the canopy at its outer periphery, or skirt. They are normally fastened to the skirt at the seams between gores, and they may or may not continue upward over the canopy to the vent on the top. Not all Parachutes have vents, but, although the theory of parachute behaviour is far from complete, its generally believed that parachutes having an axially symnetric canopy benefit from the vent.
The payload may be fastened directly to the lower end of the raisers, but if there is reason to suspend it lower, extension lines may be used. A prismodial suspension point consisting of two or more cables is commonly used. If it is desirable that the payload be free to turn independently of the parachute, a swivel may be placed in the suspension system anywhere below the raisers.
Parachutes are usually described by their nominal diameter, the diameter of a circle whose area is equal to the area of the drag-producing surface of the canopy. Thus the nominal diameter of a flat circular canopy is the diameter of the material in its flat form whereas the effective diameter is the projected diameter of the inflated parachute as shown in the figure at left. The latter is a function of the parachute's design and the load it's carrying.
Packed parachutes are used only ocasionally in scientific ballooning. Instead, the parachute is fully deployed (extended) at all times, and inflation can start immediatelly upon the separation from the balloon. The parachute often serves as a link in the suspension system between the balloon and the payload.
The most important factors in selecting a parachute for use in scientific ballooning are those which are directly concerned with safety. At the very least, a parachute must open reliably and be strong enough to withstand any opening shocks and carry the weight of the payload in all conceivable situations, it must also be capable of slowing the entire system to an acccptable vertical velocity at landing. Other features may also be desirable, e.g.,that the parachute not swing or spin during descent, but unless these can be provided without sacrificing the features which are essential to safety, they should not be considered.
Two types of parachutes have been used enough for scientific ballooning flights to be considered proven. One has a flat circular canopy with a vent in the center, the other has a canopy in the form of a cross with no vent. The flat circular eanopy is known to open reliably and rapidly even at the high altitudes where it is used in scientific ballooning. It is comercially available in sizes up to 30 meters in diameter, and two or more can be used together. Its terminal velocity can be predicted accurately when it is used within proper load limits. Its chief drawback is its tendency to oscillate more than is desirable during descent.
Parachutes with cross-shaped canopies have been used in scientific ballooning almost exclusively by the firm which manufactures them. Their use has been very uncommon in scientific balloon flights, aside from some time in the sixties. Althought they have perfomed well in all respects and they were less subject to oscillation than flat canopy parachutes, they never succeeded to imposse as an option.