2.2 ZERO-PRESSURE BALLOONS (ZPB)
Zero-pressure balloons are so called because the atmospheric pressure inside the
balloon is the same as the atmospheric pressure outside the balloon, giving zeropressure at the point where the duct opens to the atmosphere.
With a zero-pressure balloon, additional layers of material are added outside
the shell layer to handle the higher gas pressure toward the top of the balloon.
These “caps” are added in a graduated manner to handle the increased loads; that
is, the caps are of different lengths down the gore with the maximum number of
caps at the apex of the balloon.
The zero-pressure balloon carries the scientific instrument to a density altitude
which is determined by the total mass of the system (suspended mass plus the balloon mass) divided by the fully inflated balloon volume. The balloon is only partially filled at the time of launch and expands to reach its full volume as the balloon
approaches its float altitude. NASA currently uses helium as the lifting gas.
The zero-pressure balloon has openings to the atmosphere, called vent ducts, to
release the excess gas known as free-lift, which provides the lifting force during
ascent. The balloon will continue to float at its equilibrium-density altitude until
there is a change in the radiation environment, such as occurs at sunrise or sunset
and upwelling Earth thermal flux. At sunset, the gas cools, the volume decreases,
and the balloon falls about ~9-15 km (~30,000-50,000 ft) to a lower equilibrium
altitude determined by the atmospheric temperature lapse rates and the radiation
environment. Altitude can be maintained by the reduction in total system mass
through release of ballast, which nominally amounts to about 8% per day. Thus
flights are limited by the total mass available as ballast. The duration of a flight is
typically 5-6 days in the mid-latitudes but much longer flights are possible at the
poles in the summer time when the Sun doesn’t set and cause the balloon to cool.
ZPB characteristics are:
• The balloon is inflated though sides.
• When the balloon reaches float altitude, excess helium vents out from ducts.
• The balloon uses “load tapes” to carry suspended load.
• The balloon shell/envelope is made of 0.8 mil (0.0008 in) single layer coextruded Linear Low-Density PolyEthylene (LLDPE) film.
• Requires diurnal ballast (6-8% of system mass) to stay floating overnight.
The flight missions are conducted from various U.S. and foreign sites throughout
the world. Over the past several years, the NASA Balloon Program has delivered
successful performance across the spectrum of balloon sizes, an accomplishment
that is unprecedented in ballooning given the large, heavy balloons that comprise
most of the program today. The program presently has five standard conventional
zero-pressure balloon designs ranging in volume from 30,000 m
3
(1.06 million ft
3
)
to 1.13 million m
3
(40 million ft
3
). The largest of these standard balloons will lift
in excess of 3,600 kg (8,000 lb) to an altitude of higher than 37 km (121,000 ft).
10 Stratospheric Balloon Descriptions
Zero-pressure balloons are so called because the atmospheric pressure inside the
balloon is the same as the atmospheric pressure outside the balloon, giving zeropressure at the point where the duct opens to the atmosphere.
With a zero-pressure balloon, additional layers of material are added outside
the shell layer to handle the higher gas pressure toward the top of the balloon.
These “caps” are added in a graduated manner to handle the increased loads; that
is, the caps are of different lengths down the gore with the maximum number of
caps at the apex of the balloon.
The zero-pressure balloon carries the scientific instrument to a density altitude
which is determined by the total mass of the system (suspended mass plus the balloon mass) divided by the fully inflated balloon volume. The balloon is only partially filled at the time of launch and expands to reach its full volume as the balloon
approaches its float altitude. NASA currently uses helium as the lifting gas.
The zero-pressure balloon has openings to the atmosphere, called vent ducts, to
release the excess gas known as free-lift, which provides the lifting force during
ascent. The balloon will continue to float at its equilibrium-density altitude until
there is a change in the radiation environment, such as occurs at sunrise or sunset
and upwelling Earth thermal flux. At sunset, the gas cools, the volume decreases,
and the balloon falls about ~9-15 km (~30,000-50,000 ft) to a lower equilibrium
altitude determined by the atmospheric temperature lapse rates and the radiation
environment. Altitude can be maintained by the reduction in total system mass
through release of ballast, which nominally amounts to about 8% per day. Thus
flights are limited by the total mass available as ballast. The duration of a flight is
typically 5-6 days in the mid-latitudes but much longer flights are possible at the
poles in the summer time when the Sun doesn’t set and cause the balloon to cool.
ZPB characteristics are:
• The balloon is inflated though sides.
• When the balloon reaches float altitude, excess helium vents out from ducts.
• The balloon uses “load tapes” to carry suspended load.
• The balloon shell/envelope is made of 0.8 mil (0.0008 in) single layer coextruded Linear Low-Density PolyEthylene (LLDPE) film.
• Requires diurnal ballast (6-8% of system mass) to stay floating overnight.
The flight missions are conducted from various U.S. and foreign sites throughout
the world. Over the past several years, the NASA Balloon Program has delivered
successful performance across the spectrum of balloon sizes, an accomplishment
that is unprecedented in ballooning given the large, heavy balloons that comprise
most of the program today. The program presently has five standard conventional
zero-pressure balloon designs ranging in volume from 30,000 m
3
(1.06 million ft
3
)
to 1.13 million m
3
(40 million ft
3
). The largest of these standard balloons will lift
in excess of 3,600 kg (8,000 lb) to an altitude of higher than 37 km (121,000 ft).
10 Stratospheric Balloon Descriptions