Titanium heat pipes are being developed to provide light weight, reliable heat rejection devices as an alternate radiator design for the Space Reactor Power System (SP-100). The radiator design includes 360 heat pipes, each of which is 5.2 m long and dissipates 3 kW of power at 775 K. The radiator heat pipes use potassium as the working fluid, have two screen arteries for fluid return, a roughened surface distributive wicking system, and a D shaped cross section container configuration. A prototype titanium heat pipe, 5.5 m long, was fabricated and tested in space simulating conditions. Results from startup and isothermal operation tests are presented. These results are also compared to theoretical performance predictions that were used to design the heat pipe initially

Ernst, D. M.

Girrens, S. P.

English

Titanium heat pipes are being developed to provide light weight, reliable heat rejection devices as an alternate radiator design for the Space Reactor Power System (SP-100). The radiator design includes 360 heat pipes, each of which is 5.2 m long and dissipates 3 kW of power at 775 K. The radiator heat pipes use potassium as the working fluid, have two screen arteries for fluid return, a roughened surface distributive wicking system, and a D-shaped cross-section container configuration. A prototype titanium heat pipe, 5.5-m long, has been fabricated and tested in space-simulating conditions. Results from startup and isothermal operation tests are presented. These results are also compared to theoretical performance predictions that were used to design the heat pipe initially

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Long titanium heat pipes for high-temperature space radiators

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TRLE: LONG TITANIUM HEAT PIPES FOR HIGH—TEMPERATURE SPACE RADIATORS
W.Mmsr S. P. Girrens
D. M. Ernst
suBm"
. ED TO. 17th Intersociety Energy Conversion Engineering Conference
August 8-13, 1982
Los Angeles, California
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Fig. 1. Multiple panel heat-pipe radiator design.
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ABSTRACT
,.
Titanium heat pipes are being developed to pro-
vide light weight, reliable heat rejection devices
as an alternate radiator design for the Space Reactor
Power System (SP-100). The radiator design includes
360 heat pipes, each of which is 5.2 m long and dis-
sipates 3 kW of power at 775 K. The radiator heat
pipes use pota ,.sium as the working fluid, have two
semen arteries for fluid return, a roughened surface
distributive wicking system, and a D-shaped cross-
section container configuration. A prototype titan-
ium heat pipe, 5.5-m tong, has been fabricated and
tested in space-simulating conditions. Results from
startup and isothermal operation tests are presented.
These results are also compared to theoretical per-
formance predictions thet were used to design the
beat pipe initially.
BACKGROUND
The SP-100 space nuclear power system is being
designed to produce 100 kW of electrical power f.r
seven years and to be fully compatible for safe
launch into lob:-Earth orbit by the Space Transporta-
tion System (STS). The power plant configuration
shown in Fig. 1 has four major subassemblies: an
enriched uranium-oxide fueled reactor cooled by 90
sodium-filled molybdenum heat pipes; a lithium-
hydride shield that protects other parts of the
space-craft from reactor radiation; silicon-
germanium thermoelectric converters that produce
electricity from the reactor supplied thermal energy;
and a titanium heat pipe radiator that rejects waste
heat from the converter system into space.
The conical-cylindrical shaped radiator is seg-
mented into thirty panels with each panel containing
twelve radiator heat pipes as shown in Fig. 2. To
Main system redundancy, each core heat-pipe thermo-
electric (TE) module package is in direct thermal
contact with four radiator heat pipes through the
TE module heat pipes. The exterior surface of the
radiator panels is flat with the remaining portion
of the individual heat-pipe containers being
D-shaped. The D-shape was selected because it
minimized radiator mass. Titanium was selected for
the radiator material because of its relatively low
density, ease vi fabrication, good impact resistance
and high-temperature capability. Table I lists size
and mass data for a titanium radiator satisfying the
100 kWlie power system design criteria. Details of
the radiator design are contained in Ref. 1.
HEAT PIPE DESIGN AND PREDICTED PERFORMANCE
The radiator heat pipes are designed to attain
minimum mass white transporting the prescribed heat
loads isothermally. The cross-sectional configura-
tion of the titanium heat pipes varies from the view
storm in the lower half of Fig. 3 at the smallest
conical section radius to the view shown in the upper
half of Fig. 3 at the cone/cylinder interface. The
cross-sectional area available for vapor flow is
RADIATOR
LONG TITANIUM HEAT PIPES FOR HIGH TEMPERATURE SPACE RADIATOR'
by
S. P. Girrens*
D. M. Ernst**
TE MODULE
MEAT PIPE
S^
 MEAT
WICK
TE MODULE
ARMORED SURFACE
CORE HEAT ►IPE--
- --RADIATOR
STRNGERD	 MEAT PIPE
Fig. 2. Individual radiator heat-pipe panel.
TABLE I
TITANIUM RADIATOR CHARACTERISTICS FOR
100 kWe POWER SYSTEM
Radiator power (kW) 1010
Total number of Ti/K heat pipes 360
Operating temperature (K) 775
Qm&x/heat pipe (kW) 2.9
Heat pipe vapor area (mm2 ) 582
Heat pipe length (m) 5.2
Radiator mass (kg) 384
^ ^	 •RJR-	 r fME[M •RTtM[f
Toe
-	 1(•T MI[
usr^e-• 	 rsceEEe •eTEe[f
f
^R•pt•TpR
HEAT PM
Fig 3. Portion of a radiator heat-pipe panel
segment.
constant along the gntire length of the heat pipe
and equal to 582 sac . The flat side radiates the
waste heat to space. The TE modules are mounted on
the flat side; both evaporation and condensation
occur on this surface. Current designs assume a TE
evaporator section 0.6 w long and a condenser 4.55 m
in length. The resulting evaporator input heat flux
is 24.4 M/cm2; the condenser heat flux is 1.8
H/cm2.
The fluid wicking system uses arteries to trans-
port potassium from the condenser to the evaporator
and a distributive wick to transport the liquid to
and from the arteries. Two single-layer 80-mesh
titanium-screen arteries supply the liquid return
mechanism. Design of the distributive wicking sys-
tem is concentrating on nonscreen wicking surfaces
that include steel shot-blasting, machined V-grooving,
and pyramid-shaped embossing. These types of fluid
distribution systems eliminate the need to insert and
attach a separate screen mesh to the inside of the
i rregularly-shaped heat pipe and reduce heat pipe
weight.
Performance predictions for heat pipes consist-
ing of evaporator and condenser sections have beer
calculated with the ald of a Los Alamos developed
computer code NTPIPE. z
 Calculations are based on a
laminar-flow mode using the two-dimensional Navier-
Stokes equations and include compressibility effects.
The hydrodynamic model of the heat pipe is essentially
a pressure balance on the recirculating flow of the
working fluid.
Computer calculations were made, utilizing this
model, to predict performance limits of the titanium/
potassium heat pipes. Figure 4 is a plot of the max-
imum powers obtainable for different artery sizes.
Since the screen pore size determines the capillary
force that supports the liquid-vapor pressure dif-
ference in the feat pipe, finer mesh screens allow
for higher performance. Presently, titanium screens
are available in 50- and 80-mesh weaves. For the cur-
rent heat pipe design, 80-mesh screen would provide a
heat transport performance margin of 1.9. Selecting
the artery radius at 2.3 mm would allow the heat pipe
to meintain its design power even if one of the two
arteries becomes inoperable. Figure 5 is a plot of
maximum heat pipe power vs evaporator exit tempera-
ture. As is evident in both Figs. 4 and 5, the design
heat-pipe power of 2.94 kW is well below the sonic
limit, thus indicating minimal axial-temperature var ft
iation. Also shown in Figs. 4 and 5 is that maximum
heat-pipe performance is confined by the wicking limit
(screen core size dependence limit). Figure -6-i-IU*- 	-- -
trates the predicted axial-temperature variation for
the radiator heat-pipe design. Calculations indicate
a 4 K temperature drop in the evaporator and a tea.
perature recovery of 3 K in the condenser section.
Note that at all locations in the condenser, the
operating temperature is above the radiator design
temperature of 775 K.
M
w ^^
r:s~
•	 rw
e 	 Pa m . zo n<
2
e
•s	 1	 is	 2	 is
•.ran fw•e low)
Fig. 4. Performance limits of radiator heat pipes
with various screen meshes vs artery size.
70
•f
40
35
s m
e
a
W KfH
!• Ip11
s
0MMMMMMMMMMin M
Fig. 5. Performance limits of radiator heat pipes
vs temperature of evaporator exit.
M
wpm MCA-M MO
.nu. eu..3. O ...
M
	
u-M OM
MWAS o MM
M
770
•
	
on MW a•• Nee f•ee euo
MAT •ME Loom OW4
Fig. 6. Predicted axial-t+zLperature variation
of radiator heat pipes.
'
Ypil	 PROTOTYPE FABRICATION AND TESTING
Fabrication of prototype titanium heat pipes pro-
ressed in three stages from 0.3-auto 2: n to 5.5-8.
wnits. This sequence served the dual purpose of
developing fabrication procedures while providing
heat pipes for evaluation. The design features of a
titanium heat pipe are shown in Fig. 7. Heat pipe
components include the curved half-shell 0.56-mm
thick, the flat top plate 0.56-mm thick, the two
50-mah-Ti screen wick arteries, and two end caps
one containing the fill tube. These parts are
fabricated separately and then welded together. The
material used for the body of the heat pipe was
grade 2 Ti; the top plate and half-shell were fabri-
cated from sheet stock, and the end caps were em-
chined from plate. The single-layer screen arteries
[N U.,,. tilt	 (iTe hW	
Mr[r-YI
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	 ( -YN lid
' 1 ram
	
ram
_ ..&T SIN ... T a 11.1
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sett w t T.1[ r
n 
	
H.
G.00.aI
r...41 a1H.. ITO	
ar..tlCa
as T•..
Fig. 7. Design features of a titanium heat pipe.
for the pipes were formed over a mandrel and then
spot welded to the flit top plate before welding the
flat and curved sections to etherin order to
central artery dimaimion. The cr_o- ^s-sectional area
available for vapor flo^ is 6.3 cua^ mile the
arteries are each 12 m: in cross section. The
distributive wicking surface on the interior of all
heat pipes was provided by steel shotblasting the
titanium sheet prior to forming. Using the fill tube
In one of the end caps, the heat pipe is evacuated
tad then enough potassi.mm is added by vacuum
distillation to fill the arteries and saturate the
distribution wick. The 5.5-m heat pipe was filled
with 260 g of potassium. After completion of the
potassium transfer, the fill tube is weld sealed and
the heat-pipe fabrication thereby finished.
Heat-pipe testing is performed under space-
simulhi.- n conditions -in a water-cooled vacuum chamber
at 10-) torr. Heat input to the evaporator was
provided by rf heatinng9 in the 5.5-m hzat pipe tests.
Figure 8 shows the 5.5-m heat pipe instrumented and
ready for assembly into the vacuum chamber.
Tests on the 5.5-m heat pipe were designed to
obtain low power start-up characteristics and achieve-
ment of design power transport at an operating tem-
perature of 775 K. The testing of high temperature
performance limits was • not possible due to the un-
coated condition of the heat pipe (that is, a high-
emissivity coating is required for radiative heat
rejection at these higher powers). Figure 9 illus-
trates the variation in axial temperature profiles as
power is increased during startup. The good correla-
tion between analytical prediction and start-up power
measurements in Fig. 10 indicates that the startup
capability of the heat pipe is very stable and pre-
dictable in the low power range. As is evident in
Fig. 11, data were obtained covering a wide operating
')k fr
}ty^^-j
rT-r'^	 its' ^^tV
IT
^Zi ..	 r
l^F rj ar .r. ^ . _ ^ ^	 •
'	
..	 t
	
`` s!^ . •
	 •..A
r_
Fig. 8. Instrumented Ti/K heat pipe before assembly into vacuum tr.•,ik.
Soo	 9541	 an	 Gso	 Too	 7" - Goo
IEMMATM (R)
Fig. 10. Measured start-up powers of 5.5-m Ti/K
heat pipe.
7000
2500
2000
i
an
1000
on
a
an GSo	 no	 Gso	 Too	 no	 an
Tu.auTURE (R)
Fig. 11. Measured operating powers of 5.5-m Ti/K
heat pipe.
	
S.S M TV II KAY It/E	 WICKING
	
• WASIM OPERATING
	 11Yt1
POWERS
	
SWIf	 ~
temperature range of 180 K. The maximum heat pipe
power attained was 1.86 kW at an average operating
temperature of 760 K. This data point, 63% of design
power, was a significant result since the heat pipe
reached a heat input dry-out limitation. This limi-
tation was a result of the shot-blasted distribution
wick's inability to distribute enough working fluid
for evaporation. This result thus indicates the ne-
cessity for wire screen supplementation in the evap-
orator section of the heat pipe to increase the fluid
distribution capability. A comparison between the
analytically predicted axial-temperature variation
and the measured profile is shown in Fig. 12. The
heat pipe was also operated successfully against
ssn
no
450
4"
sso
300
sso
no
so
010
so
0
n	 so	 3OOD 7000 4000 S000 GOOD
LOCATION (MR)
Fig. 9. Axial temperature profiles of 5.5-m Ti/K
heat pipe during start-up.
S.S - TI/R WAY Plot
	
MICIIIND
• KASURD STMT-40 	LIMIT
POWERS
SONIC
s^
Goo
aso
and
	
no	 .
no
IN
no
	
no	 j	 tt .1 11h IRaT /^
	
AN	 i	 10KR .^{ YM
. ^.sulra mrnATU[s
400
	
ASO	 !!P0010
	n 	 '
	
a	 on 20oo moo 4W soon soon
RAT Omti LOYOM (ffm)
Fig. 12. Comparison of analytical and measured
temperature profile at maximum operating
power.
gravity (pipe tilted so that condenser section is
lower than evaporator) thus demonstrating art.ery fill
and operat Qa.
SUMMARY
A dual artery titanium/potassium heat pipe, with
a D-shaped cross-section, very lightweight structure
and length of 5.5 m, has been constructed and tested
in support of an alternate design for the SP-100 ra-
diator subassembly. The successful design, fabrica-
tion, filling and testing of the 5.5-m heat pipe rep-
resents an important technological advance in the
development of very long, light-weight, high-
temperature heat pipes for space applications. Its
length of 5.5 m makes it the longest alkali metal
heat pipe ever tested. The fabrication and test of
this heat pipe include the following significant
accomplishments.
1. Successful completion of a major advance in
heat pipe design, incorporating several de-
sign and fabrication procedures (thin sheet
material. D-shaped cross-section and long
axial welds) not previously demonstrated.
2. Successful filling and wet-in procedures
developed for very long, thin-walled heat
pipe structures.
3. Stable start-up of very long arterial liquid-
metal heat pipes demonstrated.
4. 1.86 kW of heat transported at 760 K demon-
strating overall capability of the pipe.
5. Successful heat pipe operation against grav-
ity demonstrating artery fill and operation.
6. Data indicating an input heat dry-out limit
for the shot-blasted distribution wick show-
ing the necessity for wire screen supplemen-
tatiLn in the evaporator section.
7. Excellent correlation between analytical
predicted performance and actual test data.
Further experiments on a similar heat-pipe design
utilizing a wire sc^een for the evaporator distribu-
tion system and a hi gh emissivity coating in the con-
denser would allow testing of heat pipe performance
limits.
3000
2500
2000
ado
i'
loon
Goo
0
f •	
ACKNOWLEDGEMENTS
-- --- - — TM- work was performed jotntly for the Advanced
Nuclei, System and Projects. Division of the U.S.
Department of Energy and under subcontract 955935 for
the Jet Propulsion Laboratory, California Institute
of Technology, sponsored by the National Aeronautics
and Spice Administration under Contract NASA-100.
The authors express their gratitude to James Sanzi
who contributed substantially to the assembly and
operation - of test facilities.
REFERENCES
1. S. P. Girrens, •Design and Development of a
Titanium Heat-Pipe Space Radiator," Los Alamos
National Laboratory report LA-9251-MS, March 1982.
2. F. C. Prenger, Jr., *Heat Pipe Computer Program(HTPIPE), User's-Manual," Los Alamos Scientific
Laboratory report LA-8101-M, November 1979.
1


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