A tubular strut with an integral mechanism for adjusting its thermal expansivity and length was developed to fulfill the stringent thermal stability requirements anticipated for the metering truss in the Large Space Telescope. Its features may be advantageously applied to a general variety of structures and precision mechanisms where dimensional control of component elements in a dynamic thermal environment is required. Detail, design, fabrication, and test of a developmental strut are discussed

Nelson, P. T.

English

NASA Technical Reports Server

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5. A STRUT WITH INFINITELY ADJUSTABLE -'
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THERMAL EXPANSIVITY AND LENCTH
: By Paul T. Nelson
•; TRW Systems Group
, SUMMARY
_%_ _ :--_ _ l....-_'a tubular strutwith an integral mechanism
for adjustingitsthermal expansivity and length. The stimulus for itsdevelop-
•_xentderived from the stringentthermal stabilityrequirements anticipatedfor
:_._metering truss in the Large Space Telescope (LST). Itsapplicationis not
_xmited to the LST or even to spacecraft structures;itsfeatures may be advan-
tageously applied to a general variety of structures and precision mechanisme
there dimensional control of component elements in a dynamic thermal envi-
ronment is required. Detail design, fabrication,and test of a developmental
struthave been completed.
INTRODUCT!Oi_
A significantand ever increasing category of spacecraft structures has
to be substantiallyinert to th_ dynamic thermal envl,onment on orbit. The
metering structure for the Large Space Telescope (LST) fallsin thiscategory.
Since itis a precision optical structure, itsallowabh distortiondue to tem-
perature changes is near zero. A prime candidate for the metering structure
is a cylindricaltruss consisting of several circumferential rings interconnected
by tubular strutmembers. Figure i illustratesthis concept and shows how a
temperature gradient change along the axis can _ause dimensional changes in
the rings and the struts. However, ifthe expansivity relationshipof ring to
strutis properly proportioned, as shown in the inset, the truss length will
nominally remain constant. This is because the ring expansion is proportioned
to compensate for strut expansion and is called athermalization. A severe
constraint results on control o£ the expansivities(_R, @S) of the rings _nd
struts, however.
A straightforward means of controllingthe required ratio of _R/O S is to
fabricate the circular rings to an acceptable expansivit7 range and then&djust,
to suit, the expansivityof their interfacingstruts. Strut expansivity can be
precisely adjusted by the simple tuning principledescribed in this paper.
ORIGINALPA0]]
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PRBC_DINGPAGE BLANK NOT _ 59
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1976012084-064
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/EXPANDED
..TRUSS
,/
/ ' k'Ir,IG_,
EXPANDED STRUT J L(STRUT)=',s_Ts _-_'- _) A "
", /- ',2_ r ,.%L(RING)= URAT R _ STRUTS
° j_4 , .
" "" --- F
'"<.....i ",,! ,
' !.,,,..__. .... CO--STAr.i ....... RINO
LI_NGTH
Figure i. - Effect of temperature gradient change
on an athermalized truss.
Graphite/epoxy is a prime material candidate for the LST metering truss.
I: is uniquely suited :or this application because of its low coefficient of thermal
expansion (CTE), high thermal conductivity, low specific beat, high specific
stiffness, and high specific strength. Even with these outstanding material
properties, when used in the LST metering truss and enclosed in multilayer
insulation, tests indicate that variations i:, its CTE are intolerable. In fact,
conventional design approaches using any common constru.*ional materials
; available today, even with the most sophisticated fabricatio., methods, _re
' expected to be inadequate for the critical structure in the LST because of its
stringent thermal stability requirements. The expansivity tolerance band--due
to variations in constituent materials and processing_ cannot be held small
enough.
Figure Z shows a developmental
structural member that is reprcsen-
_ ._- tative of a strut for a three-ba£ meter-b
'--_,_,,,-_.L-: ll_g truss of LST size. The tubular
__' _ portioc of the strut is made of graph-
--_- , _ • ire epoxy w._th longitudinal graphite
'.:.¢.!-:_- .
.'.T,';_ . ._ fiber orientatieu and Style i8t glass
...._', : , _ cloth on inside '.nd outside di_ -tri-
, .;_, ." cal surfaces. This con.qtruct. , yields
: "-._._.,'_.,'., .- ., . a relatively low positive _.TE, but,
_,e_._ .;.... .-_._- ,.:_.. . because of variations in materials
:'. ': ,__,-_' .... ,. and processing, it does not yield ade-
..' i]__/,;,.,,. ' quate repeatabi, ity i'" "roduction :or
• ,. *- ._.... _.-- .. a means
•_I_"" _':L_::# . athermalization wi_' for
_:.'_/F_r'"_'_"_._,.'__..:):_ "tuning." The end f ,gs are metal-
_---. " :'"'"-_ lic, nonferrornagnetic, andincorpor- :
ate a mechanism which provides
adjustment capabilityto "fine tune"
Figure2, - Graphlte.l_po,_yathermal both the pin-to-pin length and "Effec-
strut with tunable end fittings, live CTE" of the strut, It is thisF
60
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1976012084-065
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adjustment feature that compensates for the inadequacy of conventional struco
tural design practice for athermat structures and is the main topic of this
paper.
SYMBOLS
L length
AI. change in length due to temperature change
_T change in temperature
a coefficient of thermal expansion
Subscripts:
A1 _iumlnum
eff effective
G graphite / epoxy
R ring
S strut
Ti titanium
tot total
ATHERMAL STRUT ,HANISM
The key to achieving precision control of thermal expansivity in the strut
is the mechanism illustrated in Figure 3. The primary load path for a typical
tension or compression load is from the graphite-epoxy tube through the bonded
titanium collar, into the threaded aluminum sleeve, and finally through the
titanium eyebolt. The sleeve and eyebolt locknuts serve to retain the adjust-
ment after it has been made and also to eliminate backlash in the mechanism.
The eyebolt locknut also serves as a linear bearing for the eyebolt.
To visualize the operating principle of the mechanism, consider a uni-
form temperature rise in the entire system. We are primarily interested in
the axial change in length induced by a change in temperature (AT). In this
particular system, the coefficient of thermal expansion of each component will
expand in direct proportion to its CTE and effective length, and thereby each
component influences the overall length of the strut. The key to controlling
the thermal expansivity of the strut system is the relatively high reverse
6t
1976012084-066
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expansion of the aluminum sleeve which compensates for the aggregate positive
expansion of all the other components. The strut also bar infinitely variable
length adjustment within the litnits of thread length. This is achieved by left-
hand threads on one end of the complete strut and right-hand threads on the
other; thus the strut length can be fine tuned in turnbuckle fashion and locked
in place with the two jam nu+_.
1figure 3, - _, Jew of sectional tunable end fitting.
If tile procedure is followed of first adjusting the length to a nominal
dimension and then ddjusting thermal expansion, a further fine length adjust-
ment will have a trivial effect on the calibrated expansivity of the strut system.
You will not : that the change in length (AL) of the system represented in
Figure 3 _.ue to a miform AT is the aggregate response of the components in
the load path. I_: the concert reported here, the expansion of all components
except the alur_,inurr_ sleeve tepds to increase the length of the strut. The
precise incr_ase in length (AL) of the complete strut system, the aggregate
response of 2._e _ystem, is as follows:
AT. = (aaG + baT i - CaA1 + daTi) AT S
The effective CTE of the system (as shown in _igure 3) is expressed by the
following:
a_ G + b_Ti - C=Al + d_Ti
_eff : L
62
1976012084-067
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From the above equation it is apparent that the cffective ezpansivity of
a strut is a function of: (1) the geometry (dimensions a, b, c, d, and L) and
(Z) the coefficients of thermal expansion (aG, aTi, and aAl) of the constituent
materials. The effective coefficient of thermal expansion of the system can
be made equal to "zero" for a given selection of m; erials by using a geometry
in which the numerator of the above equation equates to zero. Since the "c"
dimension can be adjusted about its nominal value, the effective CTE of the
strut "system" can be adjusted accordingly, limited only by the adjusting
range of the threads. The overall length "L" is unaltered by the CTE adjust-
ment (changing "c') because the thread series on the sleeve and eyebolt are
identical; i. e. , the thread pitches are the same.
FABRICATION
The graphite/epoxy tube is a six-ply unidirectional laminate of Modmor
Type 1/5208 material. It was laid up on an aluminum alloy mandrel with a
f}. 30-mm taper over its length. A lamina of Style i81 glass cloth/5Z08 was
l lid up on both the inside and outside walls of the tube, The prepegged t_ular
laminate was vacuum bagged and cured for 3 hours at i07oc at 0.5 x 10 °
newton/m Z autoclave pressure. This description is of the part that was tested
and is not intended to imply a choice for the LST metering truss. The adjust°
able end-fitting components were made of stock aluminum and titamum alloys
and were machined in a conventional manner. I_n extra assembly was made
for sectioning (as shown in Figure 3) to more graphically illustrate the
mechanism.
STRUT SYSTEM TESTS
Test Procedure
Tests performed on the strut assembly shovn in Figure Z included the
full range of expansivity adjustment of both end fittings. The primary purpose
of the tests was to verify the effectiveness of the thermal expansion tuning
concept.
The test setup is shown on a 3.7-meter granite surface table with the
test article inside a specially built thermal chamber (as illustrated in Figure 4).
The operating principle of the experiment is as follows. A quartz spacer
reacts externally against a heavy weight cast iron anchor at one end of the
thermal chamber, and another spacer with the strut in between bears against
a laser retroreflector at the opposite end. A steel coil spring maintains about
45 newtons force against the retroreflector throughout the tests assuring con-
tinuing compliance of the retroreflector with test specimen dilation. The
retroreflector base is free to move with the free end of the test article and
quartz spacer.
1976012084-068
0
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Figure 4. - Dilatometer for testing thermal
expansivity of a long strut.
The externally positioned re-
mote linear interferometer is the
basic reference from which distance
to the laser retroreflector is mea o
,,_.," sured. The laser head and laser dis-
play were located in a convenient post-
' tion for viewing. Figure 5 shows the
instrumented st_t with 15.24-cm
• quartz spacer rods at each end.
Three thermocouples were mounted on
the graphite/epoxy tube _ one in the
center, and one 15.24 cm from each
,:: end. A thermocouple was installed
i
7 cm from the outboard end of each
. quartz spacer. The reaction bar shown
" in Figure 6 assures a positive force
Figure 5. - Instrumentea strut _._ainst the anchor through the coil
_-_ with quartz spacers sp_:ng, the retroreflector, and quartz
at each end.
spacers, as well as the test article;
it is held in place by the heavy cast
iron angle plates.
o4
1976012084-069
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Figure 7 shows the laser retro-
reflector setup. Its polished base
....** was well lubricated with silicon oil to
assure virtually friction-free moticn.
r_ The thermal chamber was heated by
electrical resistance heaters and
" ,._ wrapped with fiberglass and aluminum
• '*:_ foil insulation. Insulation was par-
tially stripped in the center because
of a minor temperature imbalance
in the test specimen. Stripping en-
hanced local cooling, providing a
_-_,_.::;_--t-:_ rr_o r e even tempe ratur e di st ribution
"<._--=._ in the tube.
"_. _-_.-:a_ Vee-shaped support clips in
..... the chamber held the tube specimen
in the center of the chamber. End
caps supported the quartz spacers
(as shown in Figure _) and allowed
the spacers to move freely with test
" : '._ specimen and spacer dilation.
c.
, - , The validity of this test method
was verified by a preliminary experi-
ment in which an alternate method was
Figure 6. - View of dilatometer also evaluated. Th_ _ retroreflector
showing retroreflector spring and remote linear interferometer in
reactio_ bar. the alternate method were mounted at
Figure 7. ],aser retroreflector Figure 8. - End caps removed
setup, from chamber to sh_,w support :
of quartz spacers.
65
1976012084-070
4[ opposite ends of the test specimen which was not anchored at one end. Atmos-
pheric turbulence in the environment surrounding the specimen, however,
caused an unacceptable wavering in the laser display readout. On the othcr
hand, an experiment directed toward a test as finally executed resulted in a
very stable readout (within ±0. 025 micron), when the granite table was locally
heated in the most critical area beyond tolerance to human touch. During this
latter experiment, an aluminum tube was installed for protection of the laser
beam to preclude atmospheric disturbance. When it was ascertained that any
error due to table heating was negligible, the cor_cept of an anchor at one end
and movable retroreflector was verified. The linear interferometer could
then be located close to the retroreflector (as shown in Figure 6) without the
need for a protective tube for the laser beam. This latter approach appeared
to be better and was accordingly selected. The experiment also confirmed
that the effect of air turbutence to the laser beam over a short distance was
negligible.
The strut assembly was installed in the test chamber with the quartz
spacers and instrumentation. Thermocouples were installed on the graphite/
epoxy tube (three places), on both end fittings, and on the quartz spacers. The
length of the strut was adjusted by turning the eyebolts so that the distance
between the quartz spacers (overall length) was i98 cm. This i98-cm length
was nominally maintained for all of the tests. Each test run was started with
the specimen relatively near room temperature. Temperature recordings
were made for each thermocouple location and the laser display was reset to
zero. With this pretest activity complete, the chamber was heated to a peak
temperature and then allowed to level out and cool until all test specimen
thermocouples and also quartz spacer thermocouples were stabilized. Tem-
peratures were again recorded for end-of-rm_ condition.
Test Results
Three end-fitting adjustment positions were tested: (_) sleeve full in;
(2) sleeve at mid-position; and (3) sleeve full out. These positions correspond
to minimum, median, and maximum effective CTE adjustments, respectively.
The average temperature of the strut and quartz spacers at the beginning and
end of the run are listed in Table i. Temperature data and the corresponding
change in length (AL) were taken when all thermocouples on the strut were
within Z. 8°C. Furthermore, both quartz spacers were within 2. 8°C.
The average te..aperature differences (AT's) for the strut and spacers are
also shown in Table i. The effective strut coefficient of thermal expansion (a S )
is shown in the extreme right column. It is computed from the following:
ALto t - 2LQ_TQaQ
a S = LsAT S
In this case the length of each spacer (LQ) is i5. 24 cm and overall length of
the strut (L, S) is i98 cm.
66
1976012084-071
Table i. - Temperature and expansion data from strut system test.
Average Temperature Average Temperature
Test at Start of Test (oc) at End of Test {°C) _!_ &Ts ATt°t " r_°C} '
Configuration Quartz Strut Quartz Strut (°_l (°C) (cmx t0 "_ (pp
',' Spacer System Spacer System
Aluminum sleeve 23. i 23. i 63.6 66. 0 40. 4 42. 9 iS. 4 0.10
turned in line
with flats
r
Aluminum sleeve Z2.8 Z2.8 83.8 86. 6 61.0 63.8 59. 0 0. 39
at m/d-position
Aluminum sleeve 2t. 9 22.8 80. 6 83. _ 58. 7 60. 7 93.0 0. 6Q
turned to maximum
outward position
0.7 r / A total number of i 5 turns0.+ : comprised the full range of adjust-
o / ment. Therefore, the sensitivity0.s o of adjustment is as follows:
g -0.4 '
o., o.++.o.,oP-' = O. 039 ppm/°C/turn/ -
0.2 /
0.1 i--- Linearity of the adjustment is showno by the curve in Figure 9.
, -$ 0 I0 15
,_n,ON_ suEt_<No.c_TURNS)
Figure 9. - Effective CTE versus
position of aluminum sleeve.
CONCLUDING REM_ RKS
The strutwas "tuned" from 0.10 to 0.69 ppm/°C. Adjustment of expan-
3ivityis substantiallylinear in the adjustment range. A much wider range of
expansion could be readily demonstrated by approl:riatematerial and geometry
selection. Clearty the adjustment range could be made to pass through zero
and intonegative values. Viabilityof this TRW tuning concept is proven and
can be applied "in-principle" to a wide range of thermally inert structuralor
: mechanical systems.
67
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1976012084-072


A strut with infinitely adjustable thermal expansivity and length

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