NETS 2015

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Presenting Author: Tom Hammel, Teledyne Energy Systems

Co-Authors: Russell Bennett, Teledyne Energy Systems

Robert Sievers, Teledyne Energy Systems

Bill Otting, Aerojet Rocketdyne

The eMMRTG – to Europa, Titan or Mars

Nuclear and Emerging Technology for Space (NETS) 2015

February 23-26, 2015

Abstract 5034

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RTG - History of Reliable Operation Previous RTG’s

• SNAP19 (40 W)

- Pioneer and Viking

• MHW RTG* (160 W)

- Voyager

• GPHS RTG* (300 W)

- Galileo, Cassini, Ulysses, New Horizons

MMRTG (110 W)

• Robust solution designed for broader range of missions

- Planetary surfaces and deep space

• Based on TAGS and PbTe materials, with mature

system architecture

• From concept to flight unit ready for fuel in 5 years

• 1st unit on MSL, 2nd unit getting prepared for Mars 2020

* Deep space only

Needed a multi-environment, low risk

technology for future missions

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Next Generation Opportunity • Recent TE programs have made substantial

improvements in TE material performance o MMRTG used production ready couple technology

• Couples have been fabricated and demonstrated o Demonstrated couple efficiency from 9 – 14%

• Upgraded TE’s could provide 30 – 70% boost in power o Lowest risk using Skutterudite (SKD) TE materials

with a modest temperature increase in enhanced MMRTG

o SKD plus La-Te/Zintl would be used in Advanced RTG

Production Couple

JPL Couple Technology

Couple

Module

Bar

Spring

Piston

Insulation

Module

Production Module

Tech Transfer Started

Benefits

• Reduces Pu238 fuel

production needs and

cost

• Mission mass savings

• Increases BOL and EOL

power (more science)

Couple change out

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The Transition from MMRTG to eMMRTG

1. Substitute Skutterudite T/E couples for PbTe/TAGS

2. Properly size these SKD couples to increase T/E hot junction

temperature from ~510°C to 600°C

3. Put emissive coating inside the isolation liner to limit end support

Min-k temperature increase. a. Current MMRTG liner has coating on exterior.

b. Coating is grit blast followed by intentional oxidation

4. Use aerogel insulation at least surrounding the SKD couples. This

replaces quartz fiber stuffing.

5. Perform all necessary analyses and tests to verify adequacy of

design in anticipated environments.

Couple change out

5.9 mm OD

4.7 mm OD

3.2 mm OD

3.4 mm OD

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MMRTG Description • GPHS supported by load bearing

insulation under end covers

• Liner isolates He evolving in

GPHS from TE modules • He vented through end cap gasket

• Modules are in an Argon cover gas

• TE couples are compressively loaded

by springs and pistons in module

bars

• Couples connected in electrical

series-parallel circuit for additional

redundancy

Couple

Module Bar

Spring

Piston

Cold Strap

Insulation Module

Couple

GPHS

Liner

Heat Dist

Block

TE Modules

Housing

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“P ” Type SKD

“P” Leg TAGS/PbSnTe

“N” Leg PbTe

MMRTG Couple

“Drop-in Replacement” Couple

• Better high temperature capability

• Non-segmented

• Cut to shape (not pressed)

• Better mechanical properties

• Smaller element cross section • Required to increase hot side

temperature

“N” Type SKD

eMMRTG Couple

T/E Couple Assembly Comparison

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eMMRTG Engineering Efforts

• 2013 - Identify power potential and associated hot junction temperature and

changes required to the MMRTG design

- Evaluate associated risks

- Risk burn-down plan

• 2014 - All work based on design selected during 2013 work

- Evaluate three different mission profiles

- Margin analyses • Determine acceptable hot junction temperature

• Evaluate areas of concern

- Update thermal model

- Evaluate loss of argon cover gas (vacuum case)

• Over-arching ultimate goals - Trade studies to define requirements

- Settle on SKD T/E couple for testing

- and acceptable eMMRTG system design

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Three Mission Profiles – Europa, Titan and Mars

• Mission durations - Europa 11 years

- Titan 12.2 years

- Mars 5.6 years

• Maximum fin root temperatures - Europa 210C for 7 days at 29.5 VDC load

- Titan 170C for 3 years at 29.5 VDC load

- Mars 205C during Mars summer at 32.8 VDC load

• Maximum Load Voltage - 32.8 VDC

- Consider max of 36 VDC (in SRD)

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Key Margin Analysis Parameters

Thermal Inventory

• Per DOE, range to use is 244 to 256 watts/GPHS. This

should cover a viable design for 20+ years.

• 256 W/GPHS – 2048 W/eMMRTG

• 244 W/GPHS – 1952 W/eMMRTG

Load Voltage and Fin Root Temperature

• Both specified in mission profiles

Thermoelectric Thermal Conductivity

• Uncertainty reduction critical

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Thermal Model for Europa Worst Case

Deg C

Time, Years 6.0014

BOL Qinv, W 2048

Qinv at time t, W 1953

V load, V 36

Tfr, avg (max), deg C 210

Temperature, deg C Max Avg Min

Heat source max 779 774 765

Bellows 575 407 239

Min-K 1400 (ME) --- 770 ---

Min-K 1400 (FE) --- 760 ---

Thj 630 619 612

Tcj 235 232 226

Module bar 217 213 207

Tfr, avg 215 210 ---

Europa - Hot

(VGA for 7-days)

Hot junction temperature gradient and

magnitude are key results

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Maximum Temperature Cases for 36 vdc Load

0

100

200

300

400

500

600

700

800

900

1000

0

20

40

60

80

100

120

140

160

180

200

0 2 4 6 8 10 12 14

Tem

pe

ratu

re (

De

g. C

)

Po

we

r o

utp

ut

(wat

ts)

Mission time (years)

Europa/Titan/Mars; BOL Q = 2048w, Max TFR, EL = 36vdc

Power output, Europa Power output, TitanPower output, Mars THJ, EuropaTHJ, Titan THJ, MarsTFR, Europa TFR, TitanTFR, Mars

Load voltage = 36vdc for entire mission

Euro

pa

Tita

n

Mar

s Max THJ values occur at these points in time

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Margin Analysis Summary

Reviewed all results for Europa, Titan and Mars, full

range of load voltages and fin root temperatures, min

and max thermal inventories and launch conditions

• Worst Case Hot Observations - Fin root temperature exceeding 200°C design point for MMRTG

housing, but should be ok

- Min-k 1400 in danger zone of 760-800°C • Testing planned to evaluate

- Gasket near limit of design operation of 200°C • Needs review

- THJ near 650°C • JPL testing

• Worst Case Cold Observations - Fin root temperature below 50°C tested point

- Heat source clad just below 750°C

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eMMRTG Power Sensitivity To Design THJ

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

540 550 560 570 580 590 600 610 620 630

BO

L e

MM

RTG

Po

we

r O

utp

ut

refe

ren

ced

to

60

0C

va

lue

(wat

ts)

BOL Hot Junction Temperature ° C

Power output

BOL; 2048 watts thermal inventory 34 VDC electrical load 200°C fin root temperature

Message: Every 10°C THJ

conservatism reduces power

by at least 5 watts at BOL

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Thermoelectric Thermal Conductivity

• eMMRTG system thermal efficiency approximately 90% 90%

heat goes through T/E elements

• ΔT is 600-200 = 400°C

• SKD k uncertainty at 3 is +/- 10%

• Max THJ uncertainty is ~+/- 40°C

Excellent vehicle to evaluate k and burn down this risk is

planned

• Efficiency test planned

• By properly running this test and first testing with Min-k (or

Microtherm) in place of the TE module, this is a high accuracy

efficiency test of the TE module k test of a module of TE

elements.

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Margins Discussion

• Thermal inventory of 2048 watts - What is likelihood of 8 GPHS blocks at 256 watts each?

- Need to cover the worst case

• Actual operating load voltage is important as it affects THJ

- Design too high – too conservative

- Design too low – high THJ values at high load voltages

- Shooting now for 34 vdc

• THJ axial variation from average – 18°C - Output of thermal analysis (axial variation in eMMRTG)

- Can be reduced by optimizing the HDB – 2015 work

• Extra margin value of 20°C - Basis for this? Too much?

• TE conductivity - Knowledge, measurement and control is crucial!

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Loss Of Argon Study • Purpose – what happens with loss of gas fill (argon in converter and

helium in heat source) due to event like micrometeoroid puncture?

• Accomplishments: - Assessed temperature increase of T/E hot junction

- Assessed short term power change

- Did not assess T/E degradation effects

• How this was done - Estimated insulation thermal conductivity in vacuum

- Attempted analysis of cold end hardware effects in vacuum • Found cold end hardware test data (from 1973!) in argon, helium and vacuum

• Good example of knowledge base retention

- Modified system performance model for the above

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Loss of Argon and Helium Gas Fill • Loss of heat source gas fill will have little impact on system

performance or TE life.

• Loss of converter gas fill primary effect is increased cold end

hardware temperature drop and resultant hot junction temperature

increase - Pioneer/Viking testing showed an increase in ΔT from 71°F to 157°F

when the environment was changed from argon to vacuum

• ΔT from THJ to HDB will increase significantly but this effect is

small on system performance because of the high thermal

efficiency and the reduced k of the insulations in vacuum

Button/piston design

Flat bottom piston design

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System Results with Vacuum

Parameter Normal Vac in HS Vac in Conv. Vacuum in both compartments

Extra hot side

ΔT (°C) 0 0 50 50 150 50

Extra Cold

side ΔT 0 n/a 0 0 0 33(1)

THJ (°C) 611 612 635 636 634 661

TCJ (°C) 223 223 247 248 247 280

TFR (°C) 200 200 200 200 200 200

QLOSS (w) 241 233 201 192 211 198

thermal (%) 88 89 90 91 90 90

T/E(%) 8.8 8.8 8.6 8.6 8.6 8.1

Power (w) 154 155 154 155 153 146

All results at BOL, Q = 2048w, EL=32 vdc

(1) Higher delta T to assess sensitivity. 33 based on early 1970’s testing of

another cold end hardware design more sensitive to effect of vacuum.

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Loss of Argon; Vacuum Case Summary

• Results shown at BOL – worst case and unlikely time for problem. - At 4 years, THJ results about 6°C lower

• Issue is micrometeoroid puncture

• What is life of system and resultant performance?

• Key is SKD hot junction temperature and life in vacuum

• Testing best way to assess this

• Test also recommended for cold end temperature drop verification

in vacuum

• Option to add weight for puncture armor, if puncture probability is

concern

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Planned 2015 eMMRTG Engineering Work

• Optimize the HDB – Heat Distribution Block to reduce axial

thermal gradients: thermal and structural evaluations

• Produce combined thermal/thermoelectric model to enable the

above

• Structural analysis to: - Evaluate a pre-stressed Min-k 1400 heat source support system

- Perform random vibe evaluation of eMMRTG with OHDB and SKD

couples

The authors would like to acknowledge the

support of DOE, JPL and NASA that made the

supporting studies possible.