The NASA Balloon Program initiated a series of tests that included the extrusion process. They enlisted the support of balloon manufacturers who intensified
their efforts to produce the high quality film essential for a reliable, low-cost platform operating in the stratosphere. One manufacturer managed to make a consistently high quality film for this application. Its approach was to blend newly
developed resins such as linear low density polyethylene (LLDPE) with other
polymers to achieve the desired properties. The resulting StratoFilm-372 film
enjoyed a 100% successful flight record for over three years.
Another manufacture, Raven Aerostar, introduced the results of their research
as Astrofilm-E2. The effects of stress and temperature on the mechanical properties of their film were studied using two techniques. The first was based on uniaxial tensile measurements of pre-strained strips of the film measured as a function
of temperature. Data was analyzed in terms of two temperature coefficients (one a
stiffness coefficient and the other a strength coefficient) and then comparing the
areas under the stress-strain curves. The second technique investigated the effect
of stress and temperature on inflated cylinders of the same material. The changes
in the mechanical properties due to preconditioning in the biaxial stress state were
evaluated using the ball burst test. Preliminary findings indicated the material to
be stress and temperature path dependent. Changes in stiffness coefficient, draw
strength, and toughness were measured and analyzed.
As a result of both NASA’s and the manufacturers’ efforts, the switch was made
from LDPE to LLDPE with two resulting film candidates: Winzen Stratofilm
SF-372 and the Raven Astrofilm-E. NASA made several test balloons with each
and flew them. Based on the results of those flights, the mechanical properties of
all of the films were modeled and a statistical specification was developed which
all future film needed to satisfy. These changes virtually eliminated all catastrophic
balloon failures in the troposphere. As part of the fallout from this investigation,
NASA enjoyed multiple years of 100% success as well as increasing the balloon
suspended payload from 2,132 kg (4,700 lb) to 3,629 kg (8,000 lb).
Flight 561NT was launched at Fort Sumner, NM on September 30, 2006 using
a 0.8 million m
3
(39.57 million ft
3
) zero-pressure balloon made by Raven Aerostar.
This was a qualification test of the new Stratofilm-430 polyethylene film for the
NASA/CSBF zero-pressure balloons with a three-layer co-extruded film and the
same resins as for Stratofilm-420. The total film thickness was 10.2 μm (0.0004
in) for the shell and 13.2 μm (0.0005 in) for each of the two cap layers. Relative to
traditional zero-pressure balloon film, the Stratofilm-430 had higher strength and
ductility at normal surface temperatures and this improved the ability of the shell
to withstand dynamic launch loads. The main objective of the flight was to demonstrate the ability of the balloon to fly under normal flight conditions near the
maximum permissible launch stress index of 1,800 psi. The payload weight was
1,814 kg (4,033 lb) and the overall weight was 2,722 kg (6,000 lb). It was a
success.
3.1 Balloon Envelopes 37
their efforts to produce the high quality film essential for a reliable, low-cost platform operating in the stratosphere. One manufacturer managed to make a consistently high quality film for this application. Its approach was to blend newly
developed resins such as linear low density polyethylene (LLDPE) with other
polymers to achieve the desired properties. The resulting StratoFilm-372 film
enjoyed a 100% successful flight record for over three years.
Another manufacture, Raven Aerostar, introduced the results of their research
as Astrofilm-E2. The effects of stress and temperature on the mechanical properties of their film were studied using two techniques. The first was based on uniaxial tensile measurements of pre-strained strips of the film measured as a function
of temperature. Data was analyzed in terms of two temperature coefficients (one a
stiffness coefficient and the other a strength coefficient) and then comparing the
areas under the stress-strain curves. The second technique investigated the effect
of stress and temperature on inflated cylinders of the same material. The changes
in the mechanical properties due to preconditioning in the biaxial stress state were
evaluated using the ball burst test. Preliminary findings indicated the material to
be stress and temperature path dependent. Changes in stiffness coefficient, draw
strength, and toughness were measured and analyzed.
As a result of both NASA’s and the manufacturers’ efforts, the switch was made
from LDPE to LLDPE with two resulting film candidates: Winzen Stratofilm
SF-372 and the Raven Astrofilm-E. NASA made several test balloons with each
and flew them. Based on the results of those flights, the mechanical properties of
all of the films were modeled and a statistical specification was developed which
all future film needed to satisfy. These changes virtually eliminated all catastrophic
balloon failures in the troposphere. As part of the fallout from this investigation,
NASA enjoyed multiple years of 100% success as well as increasing the balloon
suspended payload from 2,132 kg (4,700 lb) to 3,629 kg (8,000 lb).
Flight 561NT was launched at Fort Sumner, NM on September 30, 2006 using
a 0.8 million m
3
(39.57 million ft
3
) zero-pressure balloon made by Raven Aerostar.
This was a qualification test of the new Stratofilm-430 polyethylene film for the
NASA/CSBF zero-pressure balloons with a three-layer co-extruded film and the
same resins as for Stratofilm-420. The total film thickness was 10.2 μm (0.0004
in) for the shell and 13.2 μm (0.0005 in) for each of the two cap layers. Relative to
traditional zero-pressure balloon film, the Stratofilm-430 had higher strength and
ductility at normal surface temperatures and this improved the ability of the shell
to withstand dynamic launch loads. The main objective of the flight was to demonstrate the ability of the balloon to fly under normal flight conditions near the
maximum permissible launch stress index of 1,800 psi. The payload weight was
1,814 kg (4,033 lb) and the overall weight was 2,722 kg (6,000 lb). It was a
success.
3.1 Balloon Envelopes 37