High Rate Strain Testing of High-Strength

Graphite as a Simulant for Fine Weave Pierced Fabric (FWPF) Aeroshell Material

D.P. Kramer, S.I. Hill, J. Chumack, S.M. Goodrich,

C.D. Barklay, and C.E. Whiting

University of Dayton Research Institute

Nuclear and Emerging Technologies for Space - NETS

February 2015

University of Dayton Research Institute Shaping the technology of tomorrow

Explosion of Antares 3 rocket seconds after launch from NASA’s Wallops Flight Facility 10-28-14 at 6:22p.m.

Explosion of Antares 3 rocket seconds after launch from NASA’s Wallops Flight Facility 10-28-14 at 6:22p.m.

“Inadvertent Event”: Total destruction of the rocket/payload

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Nov. 26, 2011 Mars Science Laboratory (MSL) was launched from KSC and is the latest planetary mission to be powered by a RPS

RPS = Radioisotope Power System Successfully landed the rover

Curiosity on the Martian surface in August 2012

Curiosity powered by ~5kg of 238Pu - MMRTG (Multi-Mission

Radioisotope Thermoelectric Generator)

2nd “Curiosity” - 2020 launch Atlas V 541

- “5” - 5.4m Payload Fairing - “4” – Four Solid Rocket Boosters - “1” – One Engine on Upper Stage

Atlas V 541

MSL’s Atlas V 541 launch required ~2 times the total fuel load of an Antares 130

Antares 130 Height ~40m Launch Mass ~296,000kg Mass to LEO ~5,100kg Mass Fuel+LOX ~242,000kg

Atlas V 541 + 4 Solid Rocket Boosters Height ~62m Launch Mass ~540,000kg Mass to LEO ~17,400kg Mass Fuel+LOX ~284,000kg Solid Fuel (all 4) ~164,000kg Castor 30XL ~26,000kg Total Fuel Load ~474,000kg

Need to protect the Pu-238 fuel in case of an

“inadvertent event”

MSL - Mars Science Laboratory

Curiosity’s MMRTG 238PuO2 fuel is contained within 8 General Purpose Heat Source (GPHS) Modules

Curiosity on Mars with its RPS - MMRTG (~110We/~2000Wth)

Cut-away schematic of a MMRTG

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RPSs are very carefully designed to protect and contain the radioactive 238PuO2 fuel during normal use conditions and in case of inadvertent launch or re-entry scenarios

Design Attribute Safety

Oxide Fuel Chemical stability

Ceramic Pellet Minimizes particle fines/Solubility

Iridium Cladding High temperature ductility

Graphite Impact Shell Adds impact strength

GPHS Modules Aeroshell / Impact strength

Low Melting Housing Aids separation of individual GPHSs

GPHS Stack Release Separation of individual GPHSs

Examples of several of the safety related RPS design attributes

Current RPSs contain the ceramic fuel within GPHS modules which helps to protect it in accident scenarios

• Fuel is contained within GPHS modules that are designed to ablate during re-entry and provide some impact protection in case of inadvertent events: a) A launch incident or b) During an Earth gravitational assist: Cassini V-V-E(~25+km/s)-J

• GPHS modules are currently fabricated out of a ~3D C/C - FWPF - Fine Weave Pierced Fabric

FWPF (Fine Weave Pierced Fabric) is a “~3D” C/C material

Four 238PuO2 pellets are in a GPHS which yields ~250Wth

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FWPF (Fine Weave Pierced Fabric) is a specialized material so initial high rate strain tests employed EDM-3

EDM-3 - Manufactured by Poco Graphite (Decatur, TX) - Isotropic fine grained graphite - Very good machining characteristics

Some properties of EDM-3 Property Typical Value

Average Particle Size <5µm Flexural Strength 935 kg/cm2 (13,300 psi) Compressive Strength 1,273 kg/cm2 (18,100 psi) Hardness 73 Shore Electrical Resistivity 15.6 µΩm (615 µΩin)

Original EDM-3 test specimens were machined for a “shoulder” loaded fixture – My mistake

MTS Servo-hydraulic test system Shoulder mounted test specimen prior to test

Shoulder Fixture

Additional EDM-3 sheet material was obtained and machined into “grip” loaded test specimens Bottom and top of the sheet stock were ground flat prior to

fabrication of the test specimens Tensile bars were machined to ISO 8256 Type 3 dimensions

10 mm gage length and gage width Large surface area for loading the bar within the grips

High rate strain testing may help support the safety basis related to the launching of 238PuO2 fueled RPSs

Interest in high rate strain data in the range of <100 s-1 for inclusion into the safety models

Tensile specimens fabricated - EDM-3 - ISO 8256 Type 3

MTS servo-hydraulic test station (up to ~10m/s) Two different techniques employed for obtaining high

rate strain test data - DIC (Digital Image Correlation) A high speed camera system (100,000 frames/s)

- Strain gauges positioned on specimen Two back to back

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High rate strain test specimens were fabricated and strain gaged and/or “speckled” painted

With Two strain gages With “speckle” paint

As-machined EDM-3 ISO 8256 Type 3 tensile bars

Test system set-up and an example of a EDM-3 specimen prior to testing

MTS Servo-hydraulic test system EDM-3 specimen prior to test

Examples of a test specimen after the completion of a fast rate strain experiment

(Left) Specimen still in test station after a test (Right) Specimen after removal from the test station with both strain

gages still attached

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Data acquisition system allowed high speed data collection during a high rate strain experiment

400,000 capture sample rate per second

10” Total stroke ~0.01” across screen

Strain gage measurements obtained until failure

Load cell profile

Stress - strain curves obtained on two EDM-3 ISO 8256 Type 3 tensile specimens STL #1 and STL #2

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Summary of some of the preliminary fast rate strain experiments on EDM-3 specimens

Specimen Ultimate Failure Stress

Strain at

Failure

Modulus

Machine Rate

Measured Strain Rate

Comments

STL #1 57.2 MPa (8,300 psi)

1.02% 9997 MPa (1.45 Msi)

9.804 m/s (386 in/s)

59.4/s

Single strain gage as one failed

STL #2 58.2 MPa

(8,440 psi) 0.79% 13,100 MPa

(1.90 Msi) 9.931 m/s (391 in/s)

50.6/s Back-to-back strain gages averaged to compensate for potential bending during test

STL #3 51.4 MPa

(7,460 psi) 8.661 m/s

(341 in/s) Set-up specimen

not strain gaged

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Lessons learned/recommendations for future high rate strain experiments on graphite specimens Test System - Capable of obtaining

fast rate strain data on test specimens Material –

- EDM-3 useful as a simulant for FWPF

Specimen Geometry – - ISO 8256 Type 3 test specimens provided good gripping area

Strain Measurements – - Back-to-back strain gages are recommended - Camera system may not be

optimum for this specimen material

ISO 8256 Type 3

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Acknowledgements

This research was conducted under U.S. Department of Energy contract DE-NE0000422

The technical support of Mr. Dirk Cairns-Gallimore and Mr. Ryan Bechtel of U.S. DOE Office of Space & Defense Power Systems is greatly appreciated