Wire-testing issues, such as the gripping strains imposed on the wire, play a critical role in obtaining clean data. In a standard test frame fitted with flat wedge grips, the gripping action alone creates stresses on the wire specimen that cause the wire to fail at the grip location. A new test frame, which is outfitted with a vacuum chamber, negated the use of any conventional commercially available wire test fixtures, as only 7 in. (17.8 cm) existed between the grip faces. An innovative grip fixture was designed to test thin gauge wire for a variety of applications in an existing Instron test frame outfitted with a vacuum chamber

Burke, Christopher S.

NASA Technical Reports Server

Mechanics/Machinery
A plasma igniter has been developed
for initiating combustion in liquid-pro-
pellant rocket engines. The device pro-
pels a hot, dense plasma jet, consisting
of elemental fluorine and fluorine com-
pounds, into the combustion chamber
to ignite the cold propellant mixture.
The igniter consists of two coaxial,
cylindrical electrodes with a cylindrical
bar of solid Teflon plastic in the region
between them. The outer electrode is a
metal (stainless steel) tube; the inner
electrode is a metal pin (mild steel,
stainless steel, tungsten, or thoriated-
tungsten). The Teflon bar fits snugly
between the two electrodes and pro-
vides electrical insulation between
them. The Teflon bar may have either a
flat surface, or a concave, conical sur-
face at the open, down-stream end of
the igniter (the igniter face). The ig-
niter would be mounted on the com-
bustion chamber of the rocket engine,
either on the injector-plate at the up-
stream side of the engine, or on the
sidewalls of the chamber. It also might
sit behind a valve that would be opened
just prior to ignition, and closed just
after, in order to prevent the Teflon
from melting due to heating from the
combustion chamber.
The plasma jet deposits the energy re-
quired to initiate combustion, while
highly reactive fluorine and fluorine
compounds create free-radicals in the
flow-field to further promote rapid igni-
tion. The plasma jet is created and accel-
erated electrically, and the feedstock for
the plasma is maintained in a solid, inert
form, leading to a rugged, reliable and
compact design. The device should pro-
mote rapid and reliable ignition in
LOX/LCH4 engines in particular, and
in liquid propellant engines in general.
It could also be used in gas-turbine en-
gines where prompt and reliable restart
is critically important; for example, in
helicopter and jet aircraft engines.
This work was done by Adam Martin and
Richard Eskridge of Marshall Space Flight Cen-
ter. For further information, contact Sammy
Nabors, MSFC Commercialization Assistance
Lead, at sammy.a.nabors@nasa.gov. Refer to
MFS-32557-1.
Plasma Igniter for Reliable Ignition of Combustion in 
Rocket Engines 
Marshall Space Flight Center, Alabama 
Wire Test Grip Fixture
This fixture can be used in any thin-gauge wire testing.
John H. Glenn Research Center, Cleveland, Ohio
Wire-testing issues, such as the grip-
ping strains imposed on the wire, play a
critical role in obtaining clean data. In a
standard test frame fitted with flat wedge
grips, the gripping action alone creates
stresses on the wire specimen that cause
the wire to fail at the grip location. When
conventional wire grip fixtures are in-
stalled, the test span as well as the amount
of wire used increase dramatically due to
the large nature of the wire testing fix-
ture. A new test frame, which is outfitted
with a vacuum chamber, negated the use
of any conventional commercially avail-
able wire test fixtures, as only 7 in. (17.8
cm) existed between the grip faces.
An innovative grip fixture was de-
signed to test thin gauge wire for a vari-
ety of applications in an existing Instron
test frame outfitted with a vacuum cham-
ber. This unit was designed to adapt to a
predetermined test span constrained by
the vacuum chamber. The test frame was
fitted with flat grips so that no gripping
strain was induced into the brittle wire
specimen.
In order to accomplish the task of test-
ing small-diameter brittle wire, a very sim-
ple test fixture was designed. The first
task was to create a technique to relieve
the strains induced into the wire upon
gripping. This was accomplished with two
1.5 in. (38 mm) wire spools. These spools
were designed to relieve the gripping
strains by wrapping the wire around the
spools to grip the fixtures circumferen-
tially. On each spool, a small bolt was in-
stalled to attach the wire to the spool.
These bolts were placed 270° around the
diameter of the spool so the wire con-
tacted around the wheel. The concept
employed ensured that the strains in the
wire due to gripping would be reduced by
the smooth transition around the wheel.
A small groove was machined into the
spools to center the wire. 
In an effort to save test wire, as well as
simplify the installation of the test wire
to the spools, a locating rail was devised.
This rail established the span of the grip-
ping spools by pinning the spools to a
thin, flat plate. When assembled, the
wire is easily wrapped around and se-
cured to the spools. The wire is pre-
loaded slightly on the fixture to stay in
place. This unit is then installed into the
test frame. The leading edge of the rail
was designed to match up to the grips in-
stalled in the test frame.
The machine was placed in the test-
ready position. The loaded test rail was
installed up against the sides of the flat
test wedge grips, which, by design, estab-
lished the test wire centered in the test
frame. Pre-existing marks on the test
spools allow the operator to center the
fixture, top to bottom, prior to gripping.
The test spools were machined to a
width of 0.025 in. (0.64 mm), matching
that of a standard, flat test specimen.
When the flat wedge grips were closed,
the wedges griped the spools and estab-
lished the test span. After the grips seat,
the test rail can be removed, and the
wire is ready to test. If for some reason
the specimen needs to be removed from
the test frame, the installation rail can
be reinstalled on the pinned spools and
NASA Tech Briefs,  January 2011 17
https://ntrs.nasa.gov/search.jsp?R=20110003021 2019-08-30T14:19:45+00:00Z
the sample ungripped. This design uses
about 8-in. (20-cm) of wire per tests.
Multiple tests were conducted at both
room and elevated temperature with no
failures in the grip region. 
The novelty of this wire test fixture
lies in its simplicity. Its compact features
allow the user to install the fixture in a
test frame with little or no modifica-
tions. The self-alignment feature de-
signed into set-up places the wire speci-
men in perfect alignment with the test
frame. The loading spools, when
gripped, are in direct contact with the
test frame water-cooled wedge grips.
This helps to draw temperature away
from the fixture for ease of high-tem-
perature testing. 
This work was done by Christopher S. Burke
of Glenn Research Center. Further informa-
tion is contained in a TSP (see page 1).
Inquiries concerning rights for the commer-
cial use of this invention should be addressed
to NASA Glenn Research Center, Innovative
Partnerships Office, Attn: Steven Fedor, Mail
Stop 4–8, 21000 Brookpark Road, Cleveland,
Ohio 44135. Refer to LEW-18579-1.
One of the major technical
problems deep-space optical
communication (DSOC) sys-
tems need to solve is the isola-
tion of the optical terminal from
vibrations produced by the
spacecraft navigational control
system and by the moving parts
of onboard instruments. Even
under these vibration perturba-
tions, the DSOC transceivers
(telescopes) need to be pointed
l000’s of times more accurately
than an RF communication sys-
tem (parabolic antennas).
Mechanical resonators have
been extensively used to provide
vibration isolation for ground-
based, airborne, and space-
borne payloads. The effective-
ness of these isolation systems is
determined mainly by the ability
of designing a mechanical oscil-
lator with the lowest possible
resonant frequency. The Low-
Frequency Vibration Isolation
Platform (LFVIP), developed
during this effort, aims to re-
duce the resonant frequency of
the mechanical oscillators into
the sub-Hertz region in order to
maximize the passive isolation afforded
by the 40 dB/decade roll-off response of
the resonator. The LFVIP also provides
tip/tilt functionality for acquisition and
tracking of a beacon signal. An active
control system is used for platform posi-
tioning and for dampening of the me-
chanical oscillator.
The basic idea in the design of the iso-
lation platform is to use a passive isola-
tion strut with a ≈100-mHz resonance
frequency. This will extend the isolation
range to lower frequencies. The har-
monic oscillator is a second-order low-
pass filter for mechanical disturbances.
The resonance quality depends on the
dissipation mechanisms, which are
mainly hysteretic because of the low res-
onant frequency and the absence of any
viscous medium.
The LFVIP system is configured using
the well-established Stewart Platform,
which consists of a top platform con-
nected to a base with six extensible struts
(see figure). The struts are at-
tached to the base and to the
platform via universal joints,
which permit the extension and
contraction of the struts. The
struts’ ends are connected in
pairs to the base and to the plat-
form, forming an octahedron.
The six struts provide the vibra-
tion isolation due to the proper-
ties of mechanical oscillators
that behave as second-order low-
pass filters for frequencies above
the resonance. At high fre-
quency, the ideal second-order
low-pass filter response is
spoiled by the distributed mass
and the internal modes of mem-
brane and of the platform with
its payload.
The mechanical oscillator is
implemented using a particular
geometry of a stainless steel
membrane. This geometry pro-
vides a very soft mechanical
compliance along the axis or-
thogonal to the membrane and
about axes coplanar to the
membrane. It also allows the de-
sign of a membrane with suffi-
ciently stiff compliances on the
other remaining directions.
The proposed LFVIP has the potential
to yield a low-power, low-mass isolation
system for payloads requiring stable plat-
forms, such as imaging and free-space
optical communications.
This work was done by Gerardo G. Ortiz,
William H. Farr, and Virginio Sannibale of Cal-
tech for NASA’s Jet Propulsion Laboratory. For
more information, contact iaoffice@jpl.nasa.gov.
NPO-46862
A Sub-Hertz, Low-Frequency Vibration Isolation Platform 
This system can be used for vibration isolation in semiconductor manufacturing, for space-
based imaging systems, or fine pointing of free-space optical communication transceivers.
NASA’s Jet Propulsion Laboratory, Pasadena, California
A CAD rendering of the Low-Frequency Vibration Isolation Platform
with payload. Shown is the configuration of the six LFVIP struts (four
shown). One of the lower left struts clearly shows the steel membrane
with its topology designed to have a reasonably high stiffness along
the membrane plane.
18 NASA Tech Briefs, January 2011


Wire Test Grip Fixture

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