The Apollo portable life support system water-recirculation pump used for astronaut cooling is described. The problems associated with an early centrifugal pump and how these problems were overcome by the use of a new diaphragm pump are discussed. Performance comparisons of the two pump designs are given. Developmental problems and flight results with the diaphragm pump are discussed

Carson, M. A.

English

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LIQUID PUMP FOR ASTRONAUT COOLING
By Maurice A. Carson*
ABSTRACT
The Apollo portable life support s_stem water-recirculation pump used for astro-
naut cooling is described in this report. The problems associated with an early centrif-
ugal pump and how these problems were overcome by the use of a new diaphragm pump
are discussed. Performance comparisons of the two pump designs are given. Devel-
opmental problems and flight results with the diaphragm pump are discussed.
INTRODUCTION
Early in the Apollo Program, tests and engineering analyses revealed that gas
ventilation alone provided inadequate body cooling for the astronaut performing extra-
vehicular work. As a result, a liquid cooling system was incorporated into the extra-
vehicular mobility unit (EMU) to circulate water through a network of tubing sewn into
an undergarment worn by the astronaut. The water is cooled by a heat sink located
within the portable life support system (PLSS). A simplified schematic of the cooling
system is shown in figure 1. As indicated in figure 1, the cooling system is a com-
pletely closed loop. The rate of flow through the liquid cooling garment is constant,
and a comfortable fluid temperature is maintained by partial water bypass of the heat
exchanger.
REQUIREMENTS
Design requirements established for the cooling system pump were as follows.
1. Flow rate:
2. Pressure rise:
3. Inlet pressure:
4. Inlet temperature:
1.82 liters/min
0.37 kg/cm 2
1.31 kg/cm 2
289.15 ° K (16 ° C)
5. Input voltage: 16.8 V dc
*NASA Manned Spacecraft Center, Houston, Tex.
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https://ntrs.nasa.gov/search.jsp?R=19730010156 2020-03-17T08:20:42+00:00Z
6. Input power:
7. Weight:
8. Operating environment:
11.0 watts
1.1 kilograms (maximum)
0 to 1.013 × 105 N/m 2 (0 to 1.0 atmosphere)
In addition, the usual Apollo environmental, safety, reliability, and compactness re-
quirements were imposed.
Generally, commercially available pumps were unable to satisfy the requirements
for one or more of the following reasons.
1. The overall efficiency was too low for the 11.0-watt power limitation.
2. The envelope and weight exceeded established limits.
3. When subjected to design limit requirements, reliability was too low to be
qualifiable.
The conclusion was reached that the best method of meeting the established criteria was
to design a new pump for the stated requirements.
DESCRIPTION
Centrifugal Pump Design
The initial PLSS pump design featured a brushless direct current motor mated
to a centrifugal pump. As shown in figure 2, the pump impeller was an integral part
of the motor armature. This simple pumping concept proved in developmental tests to
be highly inefficient, requiring an average power input of 28 watts to achieve the spec-
ified flow rate and hydraulic head. It was estimated that "tuning" type design changes
to the basic unit would have a negligible effect on the overall power requirement.
Other problems associated with this pump that would have necessitated redesign
were in the following areas.
1. Bearings: The dry-film-lubricated bearings had an average lifespan of ap-
proximately 150 hours; the requirement was 500 hours and a target design life of
5000 hours. Replacement with a long-life bearing having the capability to accept the
high thrust and radial loads would have necessitated a housing redesign.
2. Corrosion: The primary pump materials, alodined aluminum and steel, had
significant corrosion problems. Elimination of this problem would have required a
materials-replacement effort.
3. Electronics: Developmental testing indicated that the electronics module was
unable to accept mission vibration and shock loads. A redesign would have required
denser packaging and potted electronics.
182
Considering these problem areas as a group, the principal reason for deciding on a
new pump was that of power. Solutions for the. other problems were readily available,
but it was considered impossible to approach the 11.0-watt input-power requirement
with a centrifugal pump.
Diaphragm Pump Design
The approach selected for a new pump was a double-acting, positive-displacement,
diaphragm-type unit that was under development at the time. The input power for this
development pump was within the specification requirements; further, isolation of the
motor from the water promised to eliminate bearing and corrosion problems. The
pump, shown in the cutaway view (fig. 3) and in the isometric projection (fig. 4), con-
sists of two diaphragms at the end of a walking beam that is supported by a torsion rod.
Inlet and outlet check valves are provided for each diaphragm chamber. Part of the
walking-beam structure is the armature of a two-position solenoid. A permanent mag-
net forms part of the motor frame, as shown in figure 3. When the pump is not ener-
gized, the armature rests against one leg of the permanent magnet, and two of the four
air gaps are closed. Energizing the coil weakens the magnetic flux across the closed
air gaps and strengthens the magnetic flux across the open air gaps. Energy stored in
the torsion rod accelerates the armature toward the center position when the magnetic
force at the closed air gaps is neutralized. Thus, the position of the bistable solenoid
armature changes with the electromagnetic field polarity established by the current
direction through the coil. This displacement of the walking beam causes one dia-
phragm chamber to discharge fluid while the other chamber takes in fluid. Upon re-
versal of the current, the opposite air gaps are closed and the diaphragms are displaced
in the opposite direction to deliver a pulse of flow through the check valves.
The integrated electronics control circuit consists of an amplifier section, which
directs current to the electromagnetic coil, and a timing-inverter section, which sets
the frequency with which the current is reversed through the coil. By suitably match-
ing the spring constant of the spring-mass system to the electronic timing section, the
frequency of the electromagnetic field reversals can be tuned to the natural resonance
of the spring-mass system. By using energy stored in the torsion bar to augment the
electromagnetic force on the walking beam, a significant pump input power reduction is
achieved. The physical envelope of this pump is shown in figure 5.
The significant problems that occurred during development of this pump and the
solutions of the problems are as follows.
1. Check valves: The valve seats, which were fabricated from a Teflon-based
material, tended to deform conically under pressure loading, causing leakage (thus
low pumping head). The problem was solved by changing the seat to 6061-T6 aluminum
for increased structural rigidity.
2. Diaphragms: Water leakage occurred in some units through the Dacron dia-
phragm as a result of cracks in the butyl rubber coating material. The problem was
traced to a manufacturing technique whereby a liquid butyl compound was applied to
both sides of the Dacron fabric followed by a single operation to mold the convolute con-
tour and cure the butyl. This combination form-and-cure operation had the potential
for causing the Dacron fabric to migrate to the inside radius of the diaphragm contour.
183
The problem was solved by performing 100 percent inspection under a 30x magnifica-
tion and rejecting the diaphragms that showed thin spots, tears, abrasions, or fibers
protruding through the elastomer.
3. Torsion bar: In an early test unit, a fracture of the torsion bar was attrib-
uted to residual tensile stresses in the bar caused by soldering at the ends of the bar
at the subassembly level. The problem was eliminated by redesigning to a pin and
clamp arrangement rather than soldering the end shoulders in place.
4. Electronics: During motor testing, it was discovered that incoming voltage
spikes of a microsecond duration were sufficient to burn out transistors in the inverter
module. These spikes could be generated by electromagnetic interferences on the line
or by transients during other PLSS switching functions. The problem was solved by
redesign of the PLSS electrical circuitry. The pump was isolated from other compo-
nents by an electromagnetic interference filter, and a high-speed switching diode was
placed across the spike-producing switch to capture spikes at the source.
All of these problems occurred during the ground-based test operations. The pump
operation on all Apollo flights has been without problems.
Performance
As noted previously, the first approach to liquid pumping in the PLSS was re-
jected, principally because of high input power requirements. With the design change
to the diaphragm pump, the specification input power requirement was met. An input
power decrease of 18.3 watts for an equivalent output was realized. Defining overall
efficiency as output work/input power, overall efficiency for the centrifugal and dia-
phragm pumps at design point conditions is 3.9 and 11.3 percent, respectively.
Considering that, on Apollo missions, three dual extravehicular activity periods
of 7 hours duration each per flight were anticipated and considering the fact that the
PLSS power supply is not spacecraft rechargeable, the power reduction achieved with
the diaphragm pump resulted in a considerable weight advantage for both the PLSS and
the lunar module. At a nominal battery weight of 110 W-hr/kg, the battery weight sav-
ing for each PLSS was 1.15 kilograms and for each lunar module was 6.9 kilograms.
This neglects any possible structural increases required by a heavier battery. Also,
the diaphragm pump was lighter and smaller than the centrifugal unit. Weight was re-
duced by 0.19 kilogram, and volume was decreased by 6 percent. The performance
differences between the units are summarized in table I.
CONCLUDING REMARKS
The diaphragm pump has been proven on several Apollo flights to be a highly reli-
able device for pumping water through the astronaut liquid cooling garment. By imple-
mentation of this unit, the power objectives established in the portable life support
system specifications were met. Performance within the power specification resulted
in a significant weight reduction for both the portable life support system and the lunar
module. The diaphragm pump also proved to be lighter and more compact than the
predecessor centrifugal pump.
184
DISCUSSION
J. H. Parks:
You stated that, essentially, the purer the water, the more corrosive its action.
Could you elaborate on that statement ?
Carson:
The pure water referred to meant deaerated water used in the portable life sup-
port system. Because deaerated water lacks dissolved oxygen, it is a reducing envi-
ronment for the natural passive oxide film which forms on metals and, thereby, causes
a greater corrosive attack on certain alloys.
185
TABLE I.- DESIGN POINT PERFORMANCE COMPARISON
BETWEEN CENTRIFUGAL AND DIAPHRAGM PUMPS
Characteristic
Voltage, V dc .........
Flow rate, liters/min ....
Pressure rise, kg/cm 2
Power consumption, W .....
Weight, kg ...........
Envelope, cm ..........
Overall efficiency, percent . .
Centrifugal pump
16.8 + 0.8
1.82
0.37
28
0.8
8.8 diameter by 9.3
3.9
Diaphragm pump
16.8 ± 0.8
1.82
0.37
9.7
O. 61
I0. i by 5.6 by 8.6
11.3
186
HEAT-SINK
SUBLIMATOR 7
DIVERTER
VALVE -_
MULTIPLE WATER
CONNECTOR
Figure 1. - The PLSS cooling-loop schematic.
ELECTRONICS
BEARINGS
ARMATURE
PU
BEARINGS
INLET
FILTER ATOR 1
Figure 2. - Centrifugal pump schematic.
187
i
CHECK VALVES
(4 PLACES)
MOTOR
OUTLET
PUMP m
ARMATURE --_
PUMP_
_f,--" ,,_1N LET
/_ PERMANENT
MAGNET
-- TORSION BAR
_'_ °_'7
,,,,,_jiii.._.; -MOTOR
RMANENT
MAGNET
Figure 3. - Diaphragm pump cutaway.
188
DIAPHRAGMS
(2 PLACES)
\
WALKING BEAM
TORSION ROD
CHECK VALVES
(4 PLACES)
/
MOTOR
ARMATURE
COIL, MAGNETS, AND
ELECTRONICS SECTION
Figure 4. - Diaphragm pump isometric projection.
Figure 5. - Diaphragm pump.
189



Liquid pump for astronaut cooling

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