3.2 PARACHUTES
3.2.1 The Basic Design
Parachutes are employed on most scientific balloon flights to lower the scientific
payload upon termination of the ballooning phase of the flight. A parachute is also
used as a safety device on virtually every flight, even though the balloon system is
to be brought down intact by releasing gas from the envelope.
The basic design for this type of application consists of a woven textile or plastic canopy which is attached to the payload by means of suspension lines, also
called shroud lines. There are various forms of parachute canopy, but they are
usually axially symmetric about the vertical axis and can be made with or without
holes. For some special purposes the canopy is made of ribbons. A parachute with
its payload and ancillary equipment is termed a parachute system.
The suspension lines connect to the canopy at its outer periphery, or skirt. They
are usually fastened at the seams between gores. They may or may not continue up
over the canopy to the vent on the top (not all parachutes possess vents). The payload may be fastened directly to the lower end of the risers, but if there is a reason
to suspend it lower then extension lines may be used. A suspension point consisting of two or more cables is commonly utilized. If it is desirable that the payload
be free to turn independently of the parachute, a swivel may be placed in the suspension system somewhere below the risers.
Parachutes are commonly described by their nominal diameter. For example,
the nominal diameter of a flat circular canopy is the diameter of the material when
in its flat form. The effective diameter is the projected diameter in its inflated state.
The latter is a function of the parachute’s design and the load that it is carrying.
Packed parachutes are rarely used in scientific ballooning. Instead, the parachute is fully deployed (extended) at all times and inflation can start immediately
upon being separated from the balloon. The parachute is often 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 sufficiently strong to withstand any opening
shocks and carry the weight of the payload in every conceivable operating condition. It must also be capable of slowing the entire system to an acceptable vertical
velocity for landing. Other features may also be desirable, for example that the
parachute not swing or spin during descent.
Two proven types of parachute have been used for scientific ballooning. One has
a flat circular canopy with a vent in the center, and the other has a canopy in the
form of a cross with no vent. The flat circular canopy is known to open reliably and
rapidly, even at the high altitudes. It is commercially available in sizes up to 30 m
(~100 ft) in diameter. If necessary, two or more chutes can be used jointly. The
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
38 Balloon Elements
3.2.1 The Basic Design
Parachutes are employed on most scientific balloon flights to lower the scientific
payload upon termination of the ballooning phase of the flight. A parachute is also
used as a safety device on virtually every flight, even though the balloon system is
to be brought down intact by releasing gas from the envelope.
The basic design for this type of application consists of a woven textile or plastic canopy which is attached to the payload by means of suspension lines, also
called shroud lines. There are various forms of parachute canopy, but they are
usually axially symmetric about the vertical axis and can be made with or without
holes. For some special purposes the canopy is made of ribbons. A parachute with
its payload and ancillary equipment is termed a parachute system.
The suspension lines connect to the canopy at its outer periphery, or skirt. They
are usually fastened at the seams between gores. They may or may not continue up
over the canopy to the vent on the top (not all parachutes possess vents). The payload may be fastened directly to the lower end of the risers, but if there is a reason
to suspend it lower then extension lines may be used. A suspension point consisting of two or more cables is commonly utilized. If it is desirable that the payload
be free to turn independently of the parachute, a swivel may be placed in the suspension system somewhere below the risers.
Parachutes are commonly described by their nominal diameter. For example,
the nominal diameter of a flat circular canopy is the diameter of the material when
in its flat form. The effective diameter is the projected diameter in its inflated state.
The latter is a function of the parachute’s design and the load that it is carrying.
Packed parachutes are rarely used in scientific ballooning. Instead, the parachute is fully deployed (extended) at all times and inflation can start immediately
upon being separated from the balloon. The parachute is often 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 sufficiently strong to withstand any opening
shocks and carry the weight of the payload in every conceivable operating condition. It must also be capable of slowing the entire system to an acceptable vertical
velocity for landing. Other features may also be desirable, for example that the
parachute not swing or spin during descent.
Two proven types of parachute have been used for scientific ballooning. One has
a flat circular canopy with a vent in the center, and the other has a canopy in the
form of a cross with no vent. The flat circular canopy is known to open reliably and
rapidly, even at the high altitudes. It is commercially available in sizes up to 30 m
(~100 ft) in diameter. If necessary, two or more chutes can be used jointly. The
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
38 Balloon Elements