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A D V A N C E D
M A T E R I A L S
&
P R O C E S S E S |
S E P T E M B E R
2 0 1 5
2 1
L aunching objects into earth’s orbit
is enormously expensive, so every
pound removed saves significant
cost. When considering materials for the
James Webb Space Telescope (JWST), a
lengthy competition took place to deter-
mine the best choices and beryllium was
ultimately specified for the mirrors. This
article documents the developmental
history leading up to that decision. It took
decades of improvements to make Be—
in particular O-30 atomized powder/
hot isostatically pressed (HIP’d) compo-
nents—homogeneous, isotropic, polish-
able, thermally and dimensionally stable,
and above all, predictable.
BERYLLIUM PROPERTIES:
IDEAL FOR SPACE
The most obvious requirements
for space optics include low mass, high
stiffness, predictable contraction when
cooled to cryogenic temperatures, and
the ability to be polished to a highly
reflective surface. These characteris-
tics were recognized and discussed
as design parameters as early as 1966
by Barnes[1], while limitations of the
available vacuum hot pressed (VHP)
Be were also detailed. A decade later,
after numerous telescopes with Be mir-
rors were already in orbit, hot isostatic
pressing was advocated for Be optics
and an evaluation of HIP’d Be material
in comparison with VHP’d Be from the
same powder was published[2]. The new
material was found to be superior and
therefore meets the specifications for
infrared mirrors.
Low thermal expansion glass,
such as the ULE fused silica (Corning’s
Code 7972 Ultra Low Expansion Glass)
selected for the Hubble Space Tele-
scope primary mirror, meets several of
the material criteria very reliably—but
not as well as O-30 Be, which was devel-
oped specifically for satellite mirrors.
Table 1 compares the important prop-
erties of O-30 Be, ULE, aluminum, and
magnesium.
In order to minimize mass, using a
low density material is an obvious first
step: The density of ULE is slightly less
than that of Al. However, conventional
mirror materials such as ULE and Al are
much heavier than Be, which is 31%
lighter than Al and 18% lighter than
ULE. Magnesium is 6% lighter than
Be. Yet another factor is stiffness, or
modulus of elasticity, which is equally
important because it measures how
well a material resists deformation
under load. Beryllium is 400% stiffer
than both Al and ULE and 670% stiffer
than Mg. Specific stiffness—the ratio of
modulus to density—determines the
engineering efficiency of a material,
and it is clear that Be is more than five
times better than the other materials in
this regard.
Specific stiffness measures how
well a structure maintains its shape
in the face of forces such as gravity,
launch, or maneuvering g’s. After a
mirror is polished to its desired shape—
or optical figure—on earth at 1 g, it
changes figure in the 0-g orbital envi-
ronment. This “gravity release” causes
only a slight figure change, but with
very lightweight mirrors these changes,
measured in fractions of wavelengths
of light, can be enough to substantially
distort the image. Grinding and pol-
ishing the mirror on earth to what will
become the correct figure in space is
called null figuring, which is much eas-
ier with a material featuring a high spe-
cific stiffness because the distortions
are so much smaller. Surprisingly, steel,
Al, Ni alloys, and Ti alloys all have sim-
ilar ratios to Mg and ULE, while the
specific stiffness of Be surpasses all of
these, as shown in Table 1.
Historically, Be was not considered
an ideal optical material[1] . However,
due to its significant weight and stiff-
ness advantages, a concentrated devel-
opment effort worked to overcome its
limitations. During the 1980s, the Stra-
tegic Defense Initiative emphasized the
need for extremely lightweight, high
performance surveillance satellites. It
was during the “Star Wars” initiative
that the shortcomings of Be as a mirror
material were generally recognized and
possible solutions imagined. Designers
of satellite mirrors and structures con-
stantly thought about how to save even
a single pound. Beryllium affords these
designers the opportunity to poten-
tially save hundreds of pounds.
BERYLLIUM SPACE
TELESCOPE HISTORY
The reflecting mirrors that act as
the compound eyes of the James Webb
Space Telescope are among the most
precise and complex space optics ever
fabricated. Essential to the telescope’s
performance are the characteristics of
18 adjustable segments that make up
the 6.5-m primary reflector. Specifying
Be broke new ground in high perfor-
mance space optical materials develop-
ment, and was a significant departure
from the glass Hubble Space Telescope
primary mirror that preceded the JWST.
However, even during the 1970s, many
space telescopes featured Be optics.
These early mirrors cleared the path to
the improved polishability and stability
of the JWST.
TABLE 1 —ROOM TEMPERATURE PROPERTIES OF MIRROR MATERIALS
Property Symbol Units O-30 Be ULE 6061 Al Mg
Density ρ g/cm3 1.85 2.21 2.70 1.74
Modulus E GPa 303 68 68 45
Specific stiffness E/ ρ 106m2/s2 163 30 25 26
Coeff. of thermal expansion α 10-6/K 11.4 0.03 22.5 24.8
Thermal conductivity k W/m•K 208 1.3 167 156