Strategies for 3D Printing Advanced Hybrid Rocket Fuel Grains and Hybrid-Like Liquid Rocket Motors_Additive Innovation_Fuller

© 2016 The Aerospace Corporation

Strategies for 3D Printing Advanced Hybrid Rocket Fuel

Grains and Hybrid-Like Liquid Rocket Motors

Jerry Fuller,

Space Science Applications Laboratory

The Aerospace Corporation

October 19, 2016

[email protected]

2

Basics: Major Rocket Propulsion Systems

(A) Liquid - most capable, expensive – Saturn 5, Shuttle Main Engine

(B) Solid - cheap and simple, most common – Tactical missiles,

Shuttle SRM Boosters

(C) Hybrid - safe, moderate expense, low performance (thrust,

efficiency), rarely used (Virgin Galactic - SpaceShip 1, 2)

Rockets are categorized by the phase states of their propellants.

These states have a great deal to do with how a rocket is used.

Oxidizer/

Fuel

Mixture

(Solid)

Oxidizer

(Liquid)

Fuel

(Solid)

Valve

Oxidizer

Fuel

Turbo-

pumps

(A) (B) (C)

3

Hybrid Rocket Characteristics

Typical Selling Points for Hybrid Rockets

Advocates Like to say Hybrids:

Have Medium Performance (ideally near liquids)

Are Easily Shipped - “Responsive”

Can be throttled, re-lit like liquids

Simple, safe and inexpensive

The truth is…

We can’t burn them fast enough (low thrust)

They are not as efficient as they should be

Oxidizer

Valve

Fuel

4

Fuel Grain Production – The Fundamental Problem

A Simplified Explanation of Casting

• Real castings often involve multiple ports, and stiffening webs

• Mandrels must be simple shapes

– No undercuts

– Little surface area

Mold or

Liner

Mandrel

Propellant

Grain

Port

5

How We Generally Think of Hybrid Rocket Combustion

Similar illustrations appear throughout hybrid rocket literature

• Generally we assume gas flow parallel to the fuel surface

• We accept that convection will be weak

• Burn rate is limited by how fast gases can diffuse across the Flame

Pyrolysis by Radiation and Weak Convection

Fuel Grain (often HTPB Rubber)

FlameSheetHead

End

Radiation (weak)

“Blowing” Conduction (weak)

Oxidizer Nozzle

End

6

Traditional Techniques to Improve Performance

The techniques used to improve performance have problems too

Wagon Wheel

Low mass fraction

Challenging Fabrication

Chunks and splinters

Swirl Injection

Effect dies out

Slicing and Stacking

Complex

Chunks

Paraffin (wax) as High Regression Rate fuel

Mechanically weak

Ejects unburned fuel (lowering Isp)

Patent US5339625 A

Patent US6601380 B2

7

Printed Hybrid (and other) Rockets Major Advances Enabled by Additive Manufacturing

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Traditional versus Additive Manufactured Grain

Printing allows port shapes that dramatically increase performance

MJM Acrylic Fuel Grains

• Same

– Mass

– Dimensions

– Cross Section

• Helical shape forces

convection

• Creates 25% more

surface area

• More efficient

• Burns > Twice as Fast

AM fuel grains move from Radiation to Forced Convection

9

Printing Strategies

Several Ways to Take Advantage of AM

• Print the whole fuel grain

– Seamless, monolithic, strong

– ABS is as good as traditional HTPB, but…

• Friendlier Chemistry

– Stronger, Stiffer, Greener

– Recyclable scrap

• Print just the port, or port and shell

– Cast paraffin, HTPB or other fuels

– Use printed structures to manipulate oxidizer flow

• Turbulence – inducing features

• Swirl - inducing features

Printed ABS

Printed Paraffin

Printed

Turbulator/

Cast Paraffin

10

Printing Strategies (cont’d)


• Print multiple fuels at once

– MJM Acrylic/Paraffin

– FFF Multi-Head extruders and Mixing Extruders

• Print empty cells and fill with a solid or liquid fuel

– Extremely high regression rates are possible

– Very powerful fuels can be used

• Print empty containers and fill with cast composite

solid propellant

– Propellants like APCP can be cast as traditional

– May offer new grain shape opportunities to combine thrust

and efficiency

MJM Co-Printed

Acrylic/Paraffin

Printed Cell Structure

for Liquids

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Printing Strategies (cont’d)


• Print hollow fuel grain

• Use cast propellants

– Less expensive (currently)

– Print flow management features

• Swirl, turbulence…

– Include oxidizers to make new APCP motors

• ABS extrusion temperature too hot for Ammonium

Perchlorate

• May be able to have higher thrust, longer burn

Printed ABS/

Cast Paraffin

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Performance Characterization of Hybrid Rocket Fuel Grains with Complex Port Geometries Fabricated Using Rapid Prototyping Technology

D. Armold, E. Boyer, B. McKnight, J. D. DeSain, J. K. Fuller, K. K. Kuo, and T. Curtiss

International Journal of Energetic Materials and Chemical Propulsion, Vol.13, 2014

0.1

1.0

40 50 60 70 80 90 100

Acrylic Correlation

Straight-Port

ST-SW 1/8 tpi

ST-SW 1/4 tpi

ST-SW 1/2 tpi

Reg

ress

ion

Rate

(m

m/s

)

Oxidizer Mass Flux (kg/m2-s)

150

Hot-Fire Test Results – Acrylic MJM

Increased regression rate through printed features

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0.9

1.0

2.0

3.0

50 60 70 80 90 100

Paraffin CorrelationSPTRB 1/2 tpiST-SW 1/8tpi

ST-SW 1/4 tpiST-SW 1/2 tpiSwept HC 1/4 tpiLg Swept HC 1/4 tpi

Reg

ress

ion

Rate

(m

m/s

)


150

Hot-Fire Test Results – Paraffin MJM


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2.0

3.0

4.0

40 50 60 70 80 90 100

Paraffin CorrelationSP ParaffinSP Par80%/Al20%

1-V TRB Paraffin (1/4 tpi)3-V TRB Paraffin (1/4 tpi)1-V TRB Par80%/Al20% (1/4 tpi)

Reg

ress

ion

Rate

(m

m/s

)


1.4150

Hot-Fire Test Results (cont’d)


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Test Results- Effects of Pre-Heating

Testing of Hybird Rocket Fuel Grains at Elevated Temperatures with Swirl Patterns Fabricated Using Rapid Prototyping Technology

Brendan R. McKnight, Derrick Armold2, J. Eric Boyer3, and Kenneth K. Kuo4

John D. DeSain5, Jerome K. Fuller6, Brian B. Brady7, and Thomas J. Curtiss8

50th AIAA Joint Propulsion Conference

Significant Increase in Regression Rate without Drop in Isp

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

40 50 60 70 80 90 100 200

Paraffin CorrelationCast Paraffin SP 22CLg Swept HC 22C (1/4 tpi)Lg Swept HC 50C (1/4 tpi)Cast Paraffin SP 35CCast Paraffin SP 55C

Reg

ress

ion

Ra

te, m

m/s

Oxidizer Mass Flux, kg/(m^2-s)

AM enables faster burning, more efficient paraffin grains

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Combing Printed and Cast Propellants


Hollow AM grains with flow management features

AM hollow grains

• Provide Containment,

Structure

• Allow heating (energy

efficient, faster burn)

• Provide Flow Management

Features (Turbulators, etc.)

• Leave the port-positive in

place or melt it awayPour (cast)

Almost Any

Propellant

Print

Almost

Any Port

Shape

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Combing Printed and Cast Propellants (cont’d)



AM hollow grains accept propellants that

are hard to “print” like APCP (ammonium

perchlorate composite propellant):

• Ammonium Perchlorate and some

energetic additives degrade at FDM/FFF

temperatures

• Crosslinked, Thermoset, HTPB (if you

must)

• Mechanically weak propellants (paraffin,

etc.)

• Novel grain shapes realized through AM

could allow almost any thrust profile Some Traditional Port Cast

Propellant Shapes and their

Thrust Profiles

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0

50

100

150

200

250

300

350

400

450

0 2 4 6 8 10 12 14

ABS / Paraffin with Integrated 3-Fin Turbulator and GOX

Pre Tank Post Tank Chamber Temp

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Large Printers are Now Capable of Launch Vehicle Size

SEEMECNC’S “PARTDADDY”

PHOTO COURTESY OF SEEMECNC

• New, large format printers (industrial robots) eliminate

size constraints for some fuels (CF/ABS) - VERY large

prints are possible

• Large enclosed printers can print 154Kg (340lb) ABS

• Delta printers have stationary build platforms, enabling

very large prints

A LARGE FFF PRINTER BASED ON INDUSTRIAL ROBOTS

PHOTO COURTESY LOCKHEED MARTIN CORPORATION.

AN ENCLOSED CARTESIAN FFF PRINTER WITH A HEATED

BUILD VOLUME

PHOTO COURTESY OF COSINE ADDITIVE, INC.

Print size – no longer a limitation

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Large Printers are Now Capable of Launch Vehicle Size

2-SEAT CAR BEING PRINTED ON BAAM

PHOTO COURTESY OF CINCINNATI INC.

• Cincinnati Inc’s BAAM, billed as Largest AM

• Print envelope 20 x 7.75 x 6 feet

• Carbon Fiber filled ABS material

(probably capable of CF/Nylon)

Print size – no longer a limitation

BAAM PRINTER, PHOTO COURTESY OF CINCINNATI INC.

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Hybrid-Like Liquid Rocket MotorsFirst steps toward a (printed) liquid motor in hybrid form

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Liquid-Enhanced Hybrid and Hybrid-Like Liquid Motors

• Adding small amounts of liquid fuel

– Increases combustion temperature, further increasing regression rate for

the solid fuels

– Increases solid fuel surface area as liquids are consumed

• Adding large amounts of liquid fuel

– Moves us toward a simpler liquid motor

– Allows less expensive, more easily obtained fuels to be used

– Will need significant research

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Hybrid-Like Liquid MotorsRemoving Turbopumps = Removing a Major Cost

• Can Liquid Motors be made like

Hybrids?

• Expensive, highly refined

“Rocket fuels” like RP1 are

unnecessary

– RP-1 used to lubricate pumps

(typically no pumps on hybrids)

– Jet A (commonly available

kerosene)

– “Bunker Fuel” (Alkane lengths of

C= 12 – 70)

– Biofuels

Oxidizer

Fuel

Turbo-

pumps

Fuel

Management

Structures

Fuel

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Fuel Metering Strategies

• Tapered Port Wall

• Stacked Cones

• Spiral

• Enhanced Hybrid (small amounts of liquid)

C) Liquid-Enhanced

Hybrid

B) SpiralA) Stacked Cone

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Post Combustion Chamber Pressure

Analogous to thrust, increases with added kerosene

Adding kerosene significantly increases burn rate over solid ABS

(blowout)

27

Liquid Fuel Passive Metering Example:Triple Canted Helix + Tapered Port Wall

Liquid Fuel Pours

Into Port as Port

Wall Regresses

Thin Port Wall (top)

Burns-Through First

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Liquid Fuel Passive Metering Example (cont’d):Triple Canted Helix, Tapered Port Wall

Burned and Unburned Specimen

(50mm diameter)

Cut-away showing Canted Spiral

and Tapered Port Wall

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Post Combustion Chamber Pressure

Analogous to thrust, increases with added kerosene

(blowout)

30

Kerosene/SLA

Triple Canted Helix, Tapered Port Wall, ~45 g Kerosene

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Conclusion (and Caveats)

• Novel port shapes and design features are possible though AM

• Regression rate (Thrust) can be Doubled

• Specific Impulse (Efficiency) can also be increased

• Launch-vehicle–size fuel grains are within reach

• Inexpensive liquid motors may be possible

• A small amount of liquid fuel can enhance Hybrids

• Entirely new concepts like heated paraffin motors are enabled

• These things are true for SMALL motors

• Nobody knows how to passively control the flow of liquid fuel yet

• More work is needed, especially at larger scales

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Acknowledgements:

• The Aerospace Corporation

– John DeSain, Brian Brady, Andrea Hsu, Tom Curtis, Cody Shaw, Kevin Dorman

– The Aerospace Corporation’s Independent Research and Development

program.

• Pennsylvania State University

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Questions?

[email protected]

310-336-5027

mailto:[email protected]

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Strategies for 3D Printing Advanced Hybrid Rocket Fuel Grains and Hybrid-Like Liquid Rocket Motors_Additive Innovation_Fuller