1. Characterizations of Additively Manufactured Propellants
May 15-17, 2018
Ralph Hsiao*, PSEMC; Dr. Lori Groven, SDSMT
Cleared for open publication
DoD Office of prepublication and security review 18-S-1233
12th CAD/PAD Technical Exchange Workshop

2. Introduction
Additive manufacturing (AM) of solid propellants provides many advantages over conventional/ cast propellants in both fabrication and performance
CAD/ PAD designers can’t readily adapt AM produced propellants into their designs until the ballistics, sensitivity, mechanical properties, and other characteristics of AM propellants are defined
PSEMC working with SDSMT converted its existing AP/HTPB propellant formula into AM manufacturable formula using rheology modifier
This is PSEMC internally funded project
Printed propellant
grain
PacSci EMC Proprietary Rights are included in the information disclosed herein. Information is furnished for evaluation purposes only and shall no be used, disclosed, or reproduced for any purposes without the expressed written consent of PacSci EMC.

3. Parametric comparisons
Results are used to determine
the usability of AM propellant
ease of transformation from cast to print
There is no change in mixing process (except for RM addition)
Manufacturing
process
Sensitivity
Linear
burn rate
Tensile
properties
Micro CT
Without RM/ Cast x
Not perform
With RM/ Cast
With RM/ Normal to print direction
Not perform; Test using ground samples
With RM/ Parallel to print direction
RM: Rheology modifier
PacSci EMC Proprietary Rights are included in the information disclosed herein. Information is furnished for evaluation purposes only and shall no be used, disclosed, or reproduced for any purposes without the expressed written consent of PacSci EMC.

4. Safety/ Sensitivity
Propellant with modifier has the very similar sensitivity as
propellant without modifier
MILSTD1751-A Safety and Performance Tests for the Qualification of Explosives (High explosives, propellant, Pyrotechnics)
Method name
BAM Friction
ARDEC
Electrostatic Discharge Sensitivity
BAM Impact Test
Method #
1024
1032
1015
Unit N J
Criteria
6/6 NO go
20/20 NO go
50% probability
Propellant w/o Modifier 60
0.25
5.7
Propellant w/ Modifier 80
5.4
PacSci EMC Proprietary Rights are included in the information disclosed herein. Information is furnished for evaluation purposes only and shall no be used, disclosed, or reproduced for any purposes without the expressed written consent of PacSci EMC.

5. Linear burn rate
This study is to evaluate the printing effect on linear burn rate vs. conventional casting method
HLR06046 AM effect on linear burn rate
2.00
Burn rate (in/sec)
SDSMT 1P SDSMT 2P SDSMT 3N
SDSMT 4N
SDSMT 5
SDSMT 6
Average linear burn rate
1.8
1.6
1.4
1.2
0.20
1000
inch/s 1
10000
0.8
Pressure (psi)
0.6
Average linear burn rate
inch/s
Parallel
Normal
Cast
1000 psi
0.713
0.595
0.5725
3000 psi
1.068
0.985
1.02
5000 psi
1.658
1.595
1.585
No statistical significance
0.4
0.2
1000 psi
3000 psi
5000 psi
Parallel
Normal
Cast
PacSci EMC Proprietary Rights are included in the information disclosed herein. Information is furnished for evaluation purposes only and shall no be used, disclosed, or reproduced for any purposes without the expressed written consent of PacSci EMC.

6. Burn rate/ Ballistic Implications
Same propellant burns faster at
printing (flowing) direction
This flow effect (flow induced burn rate variation) is more significant at lower combustion pressure
Printed propellants burn at flow direction has lower pressure exponent (0.53 vs. 0.63/0.65) than ones normal to flow/cast propellants
Higher burn rate and still has low pressure exponent is very favorable in most of rocket applications
Burn rate (in/s) at various combustion pressure
psi=
500
1000
3000
5000
Parallel
0.44
0.64
1.14
1.50
Normal
0.34
0.53
1.05
1.45
Cast
0.32
0.50
1.03
Burn rate (%) at various combustion pressure normalized to cast
psi=
500
1000
3000
5000
Parallel
137%
126%
110%
104%
Normal
106%
102%
100%
Cast
Combustion pressure (psi) to produce various burn rates
in/s=
0.50
1.00
1.50
Parallel
639
2339
5000
Normal
925
2772
5268
Cast
987
2846
5291
PacSci EMC Proprietary Rights are included in the information disclosed herein. Information is furnished for evaluation purposes only and shall no be used, disclosed, or reproduced for any purposes without the expressed written consent of PacSci EMC.

7. Strain% Stress (psi) Young's modulus (psi)
Tensile properties
Tensile testing
Print_Par allel
Print_No rmal
Cast_W/ Modifier
Cast_NO Modifier
Strain%
9.1
10.5
7.1
19.3
Stress (psi)
236
225
208
130
Young's modulus (psi)
3214
3154
3853
1108
NO statistical difference
Tensile properties
4000
3000
150
2500
2000
100
1500
500
0 0
Propellant type
Strain% Stress (psi) Young's modulus (psi)
Tensile testing conducted on cast propellants w/ and w/o rheology modifier, printed propellants pulled at parallel and normal to print direction
Modifier increase strength and decrease strain capability
Printing direction has no effect
on tensile properties
There is no difference between cast and printed propellant with modifier, except printing enhance its strain capability slightly
Results indicate curative ratio adjustment is necessary to maintain the same tensile properties for propellant using modifier
Strain at Max (%), Stress at Max (psi)
Young's modulus (psi)
PacSci EMC Proprietary Rights are included in the information disclosed herein. Information is furnished for evaluation purposes only and shall no be used, disclosed, or reproduced for any purposes without the expressed written consent of PacSci EMC.

8. Hardness/ Density Adding rheology modifier
has little effect on density
Printing reduce density slightly
Printed propellant cure
slightly harder
Density by Pycometer
Modifier
Cast/Print
Density (g/cm^3)
Yes/ M
Cast
1.580
Yes
Print
1.565 NO
1.593
Sample 1 2 3
Average
Hardness Shore A
Cast 79 80
Print 84 83
Hardness Shore D 24 22 23 25 26
PacSci EMC Proprietary Rights are included in the information disclosed herein. Information is furnished for evaluation purposes only and shall no be used, disclosed, or reproduced for any purposes without the expressed written consent of PacSci EMC.

9. Micro CT
Micro CT (computed tomography) scan is to understand the propellant matrix
One has to review the whole scan (video) to see the whole results
It appears printing samples have smaller/ more evenly distributed voids compared to cast ones
Cast
Printed
w/ modifier
Print/ scan parallel to print direction
w/o modifier
Print/ scan normal to print direction
PacSci EMC Proprietary Rights are included in the information disclosed herein. Information is furnished for evaluation purposes only and shall no be used, disclosed, or reproduced for any purposes without the expressed written consent of PacSci EMC.

10. Morphology by Micro CT-2
Micro CT scan results can not definitely differentiate the print direction effect on AP particles arrangement except for the voids size/ distribution
Print
Cast
PacSci EMC Proprietary Rights are included in the information disclosed herein. Information is furnished for evaluation purposes only and shall no be used, disclosed, or reproduced for any purposes without the expressed written consent of PacSci EMC.

11. Summary Convert existing propellant formula to “AM-able”
Same Sensitivity and safety properties
Similar density
Similar burn rate (with option being different by printing direction)
Lower strain capability due to Rheology Modifier and needs to be compensated by curative ratio adjustment
Micro CT results do not explicitly show the oxidizers particles orientations affected by printing. Typical flow induced burn rate variation is not supported to explain printing effects on linear burn rate
PacSci EMC Proprietary Rights are included in the information disclosed herein. Information is furnished for evaluation purposes only and shall no be used, disclosed, or reproduced for any purposes without the expressed written consent of PacSci EMC.

12. Conclusions
AM process has potentials eliminating the need for expensive/ difficult
formula like many high burn rate propellants
Ballistics calculations demonstrated the possibilities of producing different rocket/ thruster performances from the same propellant formula/ grain geometry by simply altering the printing direction of AM process
From certain perspectives, the true values for Additive Manufacturing is creating propellants of various burn rates from the same formulation/ mix
Advantage of lower pressure exponent (form 0.6 to 0.5) is very
significant for rockets of operation pressure less than 1000psi
PacSci EMC Proprietary Rights are included in the information disclosed herein. Information is furnished for evaluation purposes only and shall no be used, disclosed, or reproduced for any purposes without the expressed written consent of PacSci EMC.