smallsat thermal mgmt paper bugby j

SSC07-X-11

Bugby 1 21st Annual AIAA/USU

Conference on Small Satellites

Multi-Evaporator Hybrid Two-Phase Loop Cooling System for Small Satellites

David C. Bugby

ATK Space

5050 Powder Mill Road, Beltsville, MD 20705

[email protected]

ABSTRACT

This paper describes a small satellite thermal control architecture based on the multi-evaporator hybrid loop heat

pipe (ME-HLHP), a two-phase loop cooling system that combines capillary pumped loop (CPL) and loop heat pipe

(LHP) functionalities. The cooling system incorporates multiple heat load sharing evaporators for cooling/heating

remote (or nearby) components, dual counter-flow freeze-tolerant condensers for reduced attitude dependence, and

miniaturized Teflon wick evaporators for minimum control power. With concomitant functionalities like heat load

sharing and thermal diode action, this system can better meet the needs of future extended eclipse/limited power

small satellite missions compared to the traditional "cold-biasing plus heater power" approach. Extensive ME-HLHP

ground testing has been performed to demonstrate the capabilities of the system for future small satellite missions.

This paper will review the design/testing of this and five additional ground-based ME-HLHP cooling systems.

INTRODUCTION

The traditional spacecraft thermal design approach of

sizing radiators for the "hot case" and using heater

power for the "cold case" will not suffice for future

smallsat missions with large temperature extremes and

stringent power/mass limitations. Thus, NASA initiated

the ST-8 research effort several years ago to develop a

new thermal management system (TMS) for small

satellites.1 One of the initial ST-8 studies, which is the

subject of this paper, resulted in the first-ever ground

test of a miniaturized multi-evaporator hybrid loop heat

pipe (ME-HLHP) cooling system.2 In the three years

since that study concluded, five additional ME-HLHP

based cooling systems have been successfully ground

tested. This paper will describe the design/testing of

each cooling system and outline an implementation of a

centralized ME-HLHP thermal bus for a small satellite.

BACKGROUND

The ST-8 program goal was a TMS that would enable

component placement flexibility, minimize power/

mass/volume, improve power resource efficiency, and

be scalable up/down from 150 kg, 200 W. The features

necessary to meet those objectives included a multi-

evaporator bus, heat load sharing (HLS), miniaturized

components, thermal diode action, multiple condensers,

set-point controllability, and high conductance. The

TMS that could best provide those features was a multi-

evaporator two-phase loop. The available two-phase

loop architectures included the capillary pumped loop

(CPL), loop heat pipe (LHP), and hybrid loop heat pipe

(HLHP). Although CPLs/LHPs had a TRL head start,

as several CPLs and numerous LHPs are now on-orbit,

the HLHP had features that made it the best choice.

Two-Phase Loop Architectures

Due to its remote reservoir design, as shown in Figure 1,

the HLHP architecture is more like that of a CPL than it

is like that of an LHP. Thus, the HLHP has excellent

temperature controllability/expandability. The LHP,

though, with its adjacent-to-evaporator reservoir design,

sacrifices controllability/expandability for robustness.

A secondary wick between the LHP evaporator and

reservoir allows it to manage high "back conduction"

heat leaks in high pumping metal wicks. CPLs must

depend on inlet liquid subcooling to manage back

conduction and thus are limited to low conductivity,

low pumping polymer wicks. The HLHP overcomes the

CPL wick limitation by using a sweepage evaporator to

create an auxiliary flow that sweeps vapor/heat from the

primary evaporator cores. The HLHP thus provides

CPL expandability/controllability and LHP robustness.

Figure 1: CPL, LHP, and HLHP Architectures

LIQUID

HEAT INPUT

CONDENSERSHUNT

SEC. EVAPRESERVOIR

4-Port Evaporator

HEAT REJECTION

VAPOR

SET-POINT

HEATERSWEEPAGE

HEATER

Sweepage FlowLIQUID

HEAT INPUT

CONDENSERSHUNT

SEC. EVAPRESERVOIR

4-Port Evaporator

HEAT REJECTION

VAPOR

SET-POINT

HEATERSWEEPAGE

HEATER

Sweepage Flow

CPL

LHP

HLHP

SSC07-X-11



Expanding to Multiple Evaporators

HLHPs and CPLs can be expanded in parallel virtually

without limit. Figure 2 illustrates the simple in parallel

expansion of a single-evaporator HLHP into a dual-

evaporator ME-HLHP. LHPs, however, cannot be

expanded without limit due to the geometric growth of

reservoir volume with the number of evaporators. Thus,

ME-LHPs are limited to 2-3 evaporators. The HLHP

can also be expanded (or plumbed) in series. Figure 3

illustrates the two methods for plumbing an ME-HLHP.

Series plumbing uses the liquid sweepage flow from an

upstream evaporator as the liquid inlet to the adjacent

downstream evaporator. The advantage of a series-

plumbed ME-HLHP is that the minimum sweepage

flow into any evaporator is the mass flow rate exiting

the sweepage evaporator, although the subcooling

nominally decreases downstream. The advantage of a

parallel-plumbed ME-HLHP is that the subcooling

entering all evaporators is equal, although the sweepage

mass flow rate may vary if the evaporator load varies.

HEAT INPUT

4-Port Evaporator

LIQUID

HEAT INPUT

CONDENSERSHUNT

SEC. EVAPRESERVOIR

4-Port Evaporator

HEAT REJECTION

VAPOR

SET-POINT

HEATERSWEEPAGE

HEATER

Sweepage Flow

HEAT INPUT

4-Port Evaporator

LIQUID

HEAT INPUT

CONDENSERSHUNT

SEC. EVAPRESERVOIR

4-Port Evaporator

HEAT REJECTION

VAPOR

SET-POINT

HEATERSWEEPAGE

HEATER

Sweepage Flow

Figure 2: Parallel Expansion: HLHP to ME-HLHP

SERIES

V

L L

TPSP

V

E1

SP SPTP TP

V

SPTP

L L

V

E3 E4

SE

E2

C

R

PARALLEL

V

L L

TPSP

V

E1

SP SPTP TP

V

SPTP

L L

V

E3 E4

SE

E2

C

R

Series

Plumbing

Parallel

Plumbing

Figure 3: ME-HLHP Parallel vs. Series Plumbing

SYSTEM DESIGN / TESTING

This portion of the paper outlines the design/testing of

the ST-8 ME-HLHP small satellite cooling system and

five additional ground test ME-HLHP cooling systems.

ST-8 ME-HLHP Small Satellite Cooling System

To meet the ST-8 goals, a quad-evaporator, parallel-

plumbed ME-HLHP was designed, fabricated, and

tested. Figure 4 illustrates the architecture. Key loop

features included: (a) sweepage evaporator for back

conduction management; (b) miniaturized Teflon wick

evaporators for low back conduction and control power;

(c) dual counter-flow condensers for multiple sinks

with freeze tolerance; (d) back-pressure regulator for

heat load sharing and vapor line clearing; (e) co-located

flow regulator for condenser switching; (f) cold-biased

heat exchanger (CBHX) for fine temperature control;

and (g) dual diode heat pipes for CBHX cold-biasing.

Figure 5 illustrates the test loop, which utilized a lab

chiller for condenser cooling and CBHX cold-biasing.

Figure 4: ST-8 ME-HLHP Architecture

Counter-FlowCondenser 2

SweepageSight Glasses

Liquid


EVAP 3(Heat Load Sharing)

Single-PhaseSweepage

Vapor

EVAP 1

Cold-BiasedHeat Exchanger(CBHX)

EVAP 2EVAP 4

SecondaryEvaporator

Chiller Lines

Reservoir Shunt

BPR

Subcooler

Subcooler

Reservoir

Two-PhaseSweepage


SweepageSight Glasses

Liquid


EVAP 3(Heat Load Sharing)

Single-PhaseSweepage

Vapor

EVAP 1

Cold-BiasedHeat Exchanger(CBHX)

EVAP 2EVAP 4

SecondaryEvaporator

Chiller Lines

Reservoir Shunt

BPR

Subcooler

Subcooler

Reservoir

Two-PhaseSweepage

Figure 5: ST-8 ME-HLHP Test Loop

With ammonia as the working fluid, a series of 21 tests

was carried out with the ST-8 ME-HLHP test loop

resulting in: (1) quad-evaporator transport of 8-280 W;

(2) single-evaporator transport of 2-100 W; (3) power

Sweepage

Heater

Sat. Temp.Heater

Thermal StorageUnit (TSU)

Thermal StorageUnit (TSU)

EVAP 3 (PTFE WICK )

EVAP 1 (PTFE WICK )

EVAP 4 (PTFE WICK )

EVAP 2 (PTFE WICK )

SecondaryEvaporator/Reservoir

Vap

or

Lin

e

Diode

HeatPipes

(provide cold-

biasing)

Liquid Side

Core Sweepage

Co-Located Flow Regulator(CLFR)

Vapor Side

Core Sweepage Liquid Line

Subcooler

Multiple Radiators

withCounter-Flow

Condensers

Back Pressure

Regulator (BPR)

Cold-Biased Heat Exchanger

(CBHX)

Sweepage

Evaporator and Reservoir

Thermal Storage

Unit (TSU)

Thermal Storage

Unit (TSU)

EVAP 3 (PTFE WICK )

EVAP 1 (PTFE WICK )

EVAP 4 (PTFE WICK )

EVAP 2 (PTFE WICK )


Vap

or

Lin

e

Diode

HeatPipes

(provide cold-

biasing)

Liquid Side

Core Sweepage


Vapor Side


Subcooler

Multiple Radiators

withCounter-Flow

Condensers

Back Pressure

Regulator (BPR)


(CBHX)

Sweepage


Thermal Storage

Unit (TSU)

Thermal Storage

Unit (TSU)

EVAP 3 (PTFE WICK )

EVAP 1 (PTFE WICK )

EVAP 4 (PTFE WICK )

EVAP 2 (PTFE WICK )


Vap

or

Lin

e

Diode

HeatPipes

(provide cold-

biasing)

Liquid Side

Core Sweepage


Vapor Side


Subcooler

Multiple Radiators

withCounter-Flow

Condensers

Back Pressure

Regulator (BPR)


(CBHX)

Sweepage


Thermal Storage

Unit (TSU)

Thermal Storage

Unit (TSU)

EVAP 3 (PTFE WICK )

EVAP 1 (PTFE WICK )

EVAP 4 (PTFE WICK )

EVAP 2 (PTFE WICK )


Vap

or

Lin

e

Diode

HeatPipes

(provide cold-

biasing)

Liquid Side

Core Sweepage


Vapor Side


Subcooler

Multiple Radiators

withCounter-Flow

Condensers

Back Pressure

Regulator (BPR)


(CBHX)

Sweepage


Thermal Storage

Unit (TSU)

Thermal Storage

Unit (TSU)

EVAP 3 (PTFE WICK )

EVAP 1 (PTFE WICK )

EVAP 4 (PTFE WICK )

EVAP 2 (PTFE WICK )


Vap

or

Lin

e

Diode

HeatPipes

(provide cold-

biasing)

Liquid Side

Core Sweepage


Vapor Side


Subcooler

Multiple Radiators

withCounter-Flow

Condensers

Back Pressure

Regulator (BPR)


(CBHX)

Sweepage


KEY FEATURES

for negligible back conduction and low control power

Sweepage evaporator/reservoir enables

core sweepage to manage back conduction

biased heat exchanger (CBHX) for

Back pressure regulator (BPR) for heat

load sharing, vapor line clearing

located flow regulator (CLFR) for switching between multiple radiators

flow condensers for freeze

tolerance (if no spot heating)

SSC07-X-11



cycling from 50-200 W; (4) maximum heat flux of 30

W/cm2; (5) conductance of 5-8 W/K per evaporator; (6)

heat load sharing greater than 95%; (7) successful

condenser switching; (8) freeze-tolerant condenser

(liquid exited at saturation); (9) temperature control of

+/- 0.25 K with a variable set-point; (10) rapid 30

minute start-up; (11) low secondary evaporator control

power of 4W; (12) loop isolation/diode action; and (13)

Teflon evaporator 233-353 K temperature cycling.

Selected test results are provided in Figures 6-10, which

respectively illustrate the power cycling, heat load

sharing, reservoir set-point control, heat transport limit,

and loop isolation test results.

10

15

20

25

30

35

40

18:30 19:00 19:30 20:00 20:30 21:00 21:30 22:00

Time [hh:mm]

Te

mp

era

ture

[C

]

0

50

100

150

200

250

300

He

at

Lo

ad

[W

]

Reservoir inlet (1) Reservoir (4) 2nd pump (6) BPR outlet (14)

FR outlet (43) Liquid line (45) E2 body (52) E3 body (57)

liquid sweepage line (68) vapor sweepage line (73) ambient (80) 2nd pump [W]

E2 Power [W] E3 power [W] Total Evap Power [W]

Figure 6: ST-8 Power Cycling

-5

0

5

10

15

20

25

30

35

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00

Time [hh:mm]

Te

mp

era

ture

[C

]

0

25

50

75

100

125

150

175

200

He

at

Lo

ad

[W

]



E3 body (57) E4 body (62) liquid sweepage line (68) vapor sweepage line (73)

ambient (80) Q-meter (82) Q-meter (83) 2nd pump [W]

E1 Power [W] E2 Power [W] E3 power [W] Total Evap Power [W]

Figure 7: ST-8 Heat Load Sharing

10

15

20

25

30

35

40

45

6:00 6:30 7:00 7:30 8:00 8:30

Time [hh:mm]

Te

mp

era

ture

[C

]

0

50

100

150

200

250

300

350

He

at

Lo

ad

[W

]



liquid sweepage line (68) vapor sweepage line (73) ambient (80) 2nd pump [W]

E2 Power [W] E3 power [W] Total Evap Power [W]

Reservoir

Primary Evaporators

FR outlet

Figure 8: ST-8 Reservoir Set-Point Control

10

20

30

40

50

60

15:00 15:30 16:00 16:30 17:00 17:30 18:00 18:30

Time [hh:mm]

Te

mp

era

ture

[C

]

0

100

200

300

400

500

He

at

Lo

ad

[W

]



vapor sweepage line (73) ambient (80) 2nd pump [W] E2 Power [W]

E3 power [W] Total Evap Power [W]

Figure 9: ST-8 Heat Transport Limit

-40

-30

-20

-10

0

10

20

30

40

50

7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00

Time [hh:mm]

Te

mp

era

ture

[C

]

0

25

50

75

100

125

150

175

200

225

He

at

Lo

ad

[W

]


Liquid line (45) E1 body (47) E2 body (52) E3 body (58)

E4 body (62) ambient (80) Cond #2 (25) Total Evap Power [W]

Figure 10: ST-8 Loop Isolation

ST-8 Status. One of the major accomplishments during

this 6-month ST-8 study was the development of a

miniaturized Teflon wick 4-port evaporator. Figure 11

illustrates this novel design, which features a 0.64 cm

outer diameter wick. Moreover, all 21 tests conducted

were highly successful. Despite this success, NASA did

not select the ME-HLHP small satellite cooling system

for the ST-8 flight experiment. A dual-evaporator ME-

LHP with TEC reservoir cold biasing was selected

instead.4 However, that selection elicits concerns such

as: (a) the risk of expanding beyond two evaporators;

and (b) the impact of TEC failure on loop temperature

controllability. Although these two issues are not

concerns for the ME-HLHP, flight verification still is.

Thus, a flight test to verify the ME-HLHP architecture

in zero-g, preferably with 4-6 evaporators, is needed

before future flight implementations are likely.

Figure 11: Miniaturized Teflon Wick Evaporator

SSC07-X-11



Additional ME-HLHP Cooling Systems

Five additional ME-HLHP cooling systems have been

successfully ground-tested since the conclusion of the

ST-8 study three years ago. These systems include the

following applications: (1) moderate flux laser; (2) high

flux laser; (3) large spacecraft; (4) rack electronics; and

(5) intermittent power instrument. The five applications

are discussed below. A summary table with key data on

each test system is provided later in the paper.

Moderate Flux Laser. The problem addressed in this

program was the cooling of a moderate flux (30 W/cm2)

laser crystal with a sub-ambient operating temperature.

The solution was an ammonia ME-HLHP with four

evaporators mounted in an Al heat sink and a liquid

cooled shield (LCS). The LCS is a self-cooling two-

phase loop plumbing feature originally developed to

enable the cryogenic CPL.3 Figure 12 illustrates the

architecture and Figure 13 illustrates the test loop. In

lab testing with a test heater, the cooling system met the

requirements for heat flux, operating temperature, and

heat sink uniformity. In testing at the customer site, the

cooling system was successfully integrated with a

working laser. Crystal waste heat was quantified by

turning the diode array off and applying power to a

Kapton heat sink heater and then matching the liquid

inlet temperature, creating an in-situ Q-meter.

Four Parallel Condenser Lines

Flow Regulator (4)

Liquid Header

Condenser Vapor Header

Back Pressure Regulator (BPR) Secondary Evaporator

Reservoir

Evaporator Mounting Plate(Four parallel four-port evaporators)

Liquid Cooled Shield (LCS)

Vapor Line

Figure 12: Laser Crystal Cooling Architecture

SecondaryEvaporator

Condenser

LCS

4 Primary Evaporators in Al Heat Sink

Vapor

Header

Reservoir

Flow

Reg.

Liquid

Header

Alum.

Shunt

SecondaryEvaporator

Condenser

LCS

4 Primary Evaporators in Al Heat Sink

Vapor

Header

Reservoir

Flow

Reg.

Liquid

Header

Alum.

Shunt

Figure 13: Laser Crystal Cooling Test Loop

High Flux Laser. The problem addressed during this

program was the cooling of multiple low profile heat

sources in a high power/high flux laser system. The

solution was a dual-evaporator ammonia ME-HLHP

with: (a) innovative inlet/outlet ports to enable the

cooling of multiple low profile heat sources; (b) series

plumbing; and (c) a mechanical pump (no sweepage

evaporator). Figure 14 illustrates the architecture and

Figure 15 illustrates the test loop. In lab testing with a

test heater, the system successfully operated in the

"flow-through" mode with a total heat load of 880 W on

the evaporators heated from one side. The system was

designed for two-sided heating of the evaporators (0.64

cm wick OD) and a total heat load of 1600 W. Other

key results are as follows: (1) system operated below

ambient due to high mass flow rate (sub-ambient

operation is possible even at 0 W due to the mechanical

pump); (2) target heat flux of 50 W/cm2 was achieved

with single-sided heat input (system was designed for

two-sided heat input); and (3) successful operation with

0-450 W and 0-224 W power cycling.

1

2

3

5

4

6

78

9

10

11

12

13

1415

1617

1

2

3

5

4

6

78

9

10

11

12

13

1415

1617

1. Mechanical Pump2. Filter3. Calorimeter4. Evaporator 1

5. Evaporator 26. Evap 1-2 Liquid Line7. Vapor Line

8. Condenser9. Reservoir 10. Sweepage Valve

11. Reservoir Chiller/Shunt12. Chiller Path 213. Chiller Path 214. Chiller Path 1

15. Chiller Path 116. DP Transducer17. Fill Tube

Figure 14: High Flux Laser Cooling Architecture

Figure 15: High Flux Laser Cooling Test Loop

Large Spacecraft. The cooling of very high power (up

to 100 kW) next-generation military spacecraft is the

problem addressed by the Dual-Use Science and

Technology (DUS&T) program, conducted jointly with

AFRL/PRP. The solution involves a mechanical pump

assisted ammonia ME-HLHP with the ability to

smoothly transition from capillary to mechanical

pumping. The program was conducted in two phases.

SSC07-X-11



During the first phase, a 3-evaporator 2 kW risk-

reduction test bed was designed, fabricated, and tested.

The results from the 2 kW system provided guidance to

design a 10 kW test bed for the second phase, which is

configured as follows: (1) 6 low impedance

evaporators; (2) series plumbing with parallel as an

option; (3) 4-port evaporators with 3-port evaporators

as a valving-enabled option; (4) mechanical pumping

with capillary-only pumping as an option; and (5) large

spacecraft features that include evaporator-condenser

separation of 5 m and evaporator elevation differences

of 3 m. Figures 16 and 17, respectively, show a block

diagram and a photo of the 10 kW test bed. Results for

the 10 kW system will be published shortly.

132”

V1

V18

V3

V11

V7

V12

Pressure Transducer (4)

V6

V5

Sight Glass (2)

V13 V16 V10

V15V9

V8V14

V4

V2

V17

Flowmeter Mechanical Pump

Filter

Subcooler

Condenser Plate (3)

E1SN010

E2SN006

E3SN012

E4SN011

E5SN008

E6SN007

Vap

or

Lin

e 93

”

Liquid Line 93”

Liquid Line 93”

Sweepage Line 93”

bonded saddle next to pump body)

Vapor Sweepage Line 93”

Sweepage Line 93”

V19

V20

Heater Blocks

Coolant Block

Figure 16: Schematic of 10 kW Test Bed

Figure 17: Photo of the 10 kW Test Bed

Rack Electronics. In conjunction with TA&T, this

Navy SBIR program addressed the problem of rack

electronics cooling on Navy ships/submarines. In

particular, since air-cooling is nearing its limits, and is

an acoustic noise source for submarines, an alternative

was sought. The solution was two-pronged: (1) box-

mounted water ME-HLHP cooling loops; and (2) rack-

mounted single-phase water-cooling loop with "wedge"

interfaces. Figure 18 illustrates the solution. Figure 19

illustrates the ME-HLHP test loop. Figure 20 illustrates

the wedge interface system. The results from this effort

are as follows: (1) 100 W heat load on each evaporator

(300 W total) with 5 W on the secondary evaporator;

(2) loop saturation temperature of 328 K; and (3) end-

to-end conductance was 9 W/K (305 W, 348 K heater,

313K chiller). Future work will involve installation and

testing of (derivative) single-evaporator LHPs with

ceramic wicks and air-cooled condensers. This alternate

approach will hasten system implementation given

infrastructure resistance to implementing rack cooling.

Figure 18: Rack Electronics Cooling Solution

Flat PlateFlat Plate

Figure 19: Water ME-HLHP

Figure 20: Wedge Condenser System

Elevation Control

Hinges/Flex Lines

3 Evaporators 1m

Below Condenser

3 Evaporators 2m

Above Condenser

Condenser

SERVERIN RACK

WEDGE

RECEIVER

WEDGE

WEDGE CONDENSER

SCREWTIGHTENED

TO FINALIZECONNECTION

WEDGE

MATES TORECEIVER

SSC07-X-11



Intermittent Power Instrument. This system involved

the cooling of an instrument with multiple distributed

heat sources, a 0-400 W variable load, and near-

ambient operation. The solution was a cascaded dual-

loop (instrument-side ME-HLHP, radiator-side HLHP)

system with many novel but necessary "thermal

toolbox" elements including the following: (1) liquid

cooled shield (LCS) for sub-ambient HLHP operation;

(2) thermoelectric cooler (TEC) for HLHP reservoir

cold-biasing; (3) ME-HLHP reservoir differential

thermal expansion thermal switch (DTE-TSW) shunt

for minimum control power; (4) ME-HLHP thermal

storage unit (TSU) condenser with a sub-ambient phase

change material (PCM); and (5) multiple HLHP

condensers for attitude independence. Figure 21

illustrates a system schematic. Figure 22 illustrates an

annotated Pro/E model of the system. Both loops used

ammonia as the working fluid. The cooling system was

able to very precisely control the temperature of a

distributed, intermittent power instrument.

LCS LINE "OUT"

CO

ND

EN

SE

R 1

TSU

EV

AP

CO

ND

EN

SE

R 2

CONDENSER

HEAT PIPES

EVAP2ND EVAP

TSW

RSVR

RSVR 2ND EVAP

STRAP

TECSTRAP

EV

AP

EV

AP

EV

AP

INS

TR

UM

EN

T

RA

DIA

TO

R 2

INSTRUMENT-SIDE

INSTR-RADINTERFACE

LCS

RA

DIA

TO

R 1

SOLDERED (LOC. FOR HP)

SW

EE

PA

GE

LIN

ES

VAPOR LINE

LIQHDR

LCS LINE "IN"

LIQUID LINE

LIQUID LINE

VAPOR LINE

SWEEPAGE LINES

SWPHDR

VAPHDR

HTR1 HTR2

HTR3

HTR4

RADIATOR-SIDE

HTR5HTR6

Figure 21: Dual-Loop Cooling System

Instrument-Side Valves to Enable 3 vs. 4 Port ME-HLHP

Sweepage Plumbing and Enhanced Reservoir Transient Operation(5X Regular Valves, 2X Check Valves [CV], Test Only)

Radiator-Side

Condenser #2

(lines on other side)

Instrument-Side Secondary Evaporator

Instrument-Side Primary Evaporator Plate (4X)

Instrument-Side Reservoir

Subcooler for C

ondenser #2

Radiator-Side

Condenser #1

(lines on other side)

Radiator-Side Reservoir,

Secondary Evaporator,TEC, and Strap

Instrument-SideTSU/Condenser

Radiator-SidePrimary Evaporator

Instrument-Side Thermal Switch (TSW)

Radiator-Side Flow Regulators (2X)(to maximize condenser utilization)

Radiator-Side

Liquid-Cooled Shield (LCS)

Instrument-Side Heat Pipes(to spread heat on TSU bottom)

Subcooler for C

ondenser #1

Instrument-Side Back Pressure Regulator (BPR)

6"

20"

6"

20"

Instrument-Side Valves to Enable Parallel or SeriesME-HLHP Flow Configurations (12 X, Test Only)

CV

CV

Instrument-Side Primary Evaporator (4X)

Patent US 6,889,754

Figure 22: Pro/E Model of Dual-Loop Cooling System

Summary of ME-HLHP Cooling Systems. A summary

table of the characteristics of the ST-8 ME-HLHP

cooling system and the five additional ME-HLHP

cooling systems is provided in Table 1. The table lists

the number of evaporators, wick OD, saddle width,

evaporator length, wick material, maximum loop heat

load, maximum evaporator heat load, maximum heat

flux, transport length, adverse elevation, evaporator

body material, working fluid, and advanced features.

Table 1: Summary of ME-HLHP Features

SYSTEM IMPLICATIONS

TMS Features/Rationale/Benefits

The TMS features necessary to meet ST-8 goals were

previously identified as: (1) multi-evaporator bus; (2)

heat load sharing; (3) miniaturized components; (4)

thermal diode action; (5) multiple condensers; (6) set-

point controllability; and (7) high conductance. Listed

below are reasons why the aforementioned features are

needed and the expected benefits for small satellites.

� Multi-evaporator buses can decouple the structural/

thermal design process, so that component placement

is more flexible, simplifying design and saving mass.

� Heat load sharing is needed to keep environmentally

exposed instruments warm when they are not turned

on, reducing the need for smallsat heater power.

� Miniaturized components are necessary to reduce

weight and expand smallsat packaging options.

� Thermal diode action is necessary enable smallsat

payloads to be isolated from extreme environments,

which expands the missions that can be undertaken.

� Multiple condensers (radiators) reduce a satellite's

need to adjust attitude for thermal control, resulting

in increased time for science.

� Set-point controllability reduces payload temperature

variations, which minimizes temperature cycling and

lengthens payload life.

� High conductance enables centralized component

configurations, which minimizes electrical harness

lengths, simplifies the structure, and saves mass.

TMS Implementation

Given the clear benefits listed above, how to implement

an ME-HLHP into a small satellite needs a brief

discussion. Consider first a traditional small satellite

configuration in which high power components are

placed on body-mounted radiators, space/earth viewing

instruments are mounted externally, and additional

components are mounted internally. Radiators are

sized/coated to handle the hot case environment and

heater power is used to keep components from getting

SSC07-X-11



too cold in the cold case. Figure 23 illustrates a layout

using the traditional thermal design approach. Figure 24

illustrates a centralized ME-HLHP using the Figure 23

architecture/components. In Figure 24, all components

except external viewing ones are coupled to a central

ME-HLHP bus, earth/space viewing components are

kept warm when OFF by HLS, and hot radiator soak-

back is small due to ME-HLHP diode action.

MLI Covers Non-Radiator Surfaces

High Power Components

Mounted On/Near Exterior(may need heater power when OFF)

Earth-Viewing ComponentsNeed Heater Power When OFF

Internal Components Coupled Conductively

and/or Radiatively to the Walls

Radiator 2

Sized/Coated

for Hot CaseIncluding

Hot Radiator 1

Space-Viewing Components

Need Heater Power When OFF

Radiator 1

Sized/Coatedfor Hot Case

IncludingHot Radiator 2

Figure 23. Traditional Small Satellite Design Layout

CBHX

All Components Except External Viewing Ones

Centrally Located Coupled to ME-HLHP Thermal Bus

Total MLI Coverage Except Protruding

Components, Lines, Standoffs

Radiator 1Sized/Coated for

Hot Case ... CanIgnore Impact ofHot Radiator 2

Subcooler 2

Radiator 2Sized/Coated for

Hot Case ... CanIgnore Impact ofHot Radiator 1

DHPto CBHX

Earth-Viewing ComponentsKept Warm When OFF by HLS

Space-Viewing ComponentsKept Warm When OFF by HLS

Subcooler 1 DHPto CBHX

Figure 24. Centralized ME-HLHP Satellite Design

CONCLUSIONS

This paper has described the design, fabrication, and

testing of a multi-evaporator thermal bus architecture

for small satellite thermal control. The basis for the

system is the multi-evaporator hybrid loop heat pipe

(ME-HLHP), a two-phase loop cooling system with

CPL/LHP underpinnings, but with key advantages over

each. This system was designed/built/tested as part of

the NASA ST-8 Phase A study from Jan-Jun 2004. At

that time, it was the first-ever ground test of a

miniaturized ME-HLHP cooling system. The design

and testing of five subsequent ME-HLHP based cooling

systems -- in the areas of laser, spacecraft, electronics,

and instrument cooling -- were also described. Although

the architecture has been clearly proven for ground

applications, to fully validate it for future smallsat

missions, an ME-HLHP flight experiment is needed.

ACKNOWLEDGMENTS

The author would like to gratefully acknowledge the

important contributions of the following individuals to

the work described herein: Matt Beres, Pete Cologer,

Jessica Kester, Dmitry Khrustalev, Steve Krein, Ed

Kroliczek, Chuck Stouffer, Dave Wolf, Kim Wrenn,

and James Yun.

REFERENCES

1. NASA New Millennium Program Space

Technology 8, NRA 03-OSS-02, February 2003.

2. Bugby, D., E. Kroliczek, and J. Yun, "Development

and Testing of a Miniaturized Multi-Evaporator

Hybrid Loop Heat Pipe," Space Technology

Applications International Forum (STAIF-05),

Albuquerque, NM, January, 2005.

3. Baumann, J., B. Cullimore, D. Bugby, and E.

Kroliczek, "Development of the Cryogenic

Capillary Pumped Loop," 33rd IECEC, IECEC-98-

I197, Colorado Springs, CO, August 1998.

4. Ku, J., L. Ottenstein, D. Douglas, M. Pauken, G.

Birur, "Miniature Loop Heat Pipe with Multiple

Evaporators for Thermal Control of Small

Spacecraft," Paper No. 183, 30th GOMACTech

Conference, Las Vegas, NV, April 2005.

smallsat thermal mgmt paper bugby j

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