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1 American Institute of Aeronautics and Astronautics

Development of Composite Solid Propellants Based on Dicyclopentadiene Binder

Stephen Bluestone1, Stephen D. Heister2, and Steven F. Son3 Purdue University, West Lafayette, IN 47906

Benjamin L. Austin, Jr.4 IN Space LLC, West Lafayette, IN 47906

Composite solid propellants have been formulated using polymerized dicyclopentadiene (DCPD) as a binder. When the DCPD monomer is polymerized it produces an extremely tough plastic with excellent tensile strength and fracture toughness. Studies were conducted with solids loadings in the 80-85% range and a number of plasticizers are evaluated for improvement of ultimate strain capability. When compared to the industry standard, hydroxyl-terminated polybutadiene (HTPB), it was found that DCPD composite propellant had nearly 1.5 times the stress capacity while still exhibiting over 77% of the strain capacity of HTPB-based composite propellant. The aluminized DCPD-based composite propellant also exhibits a burning rate nearly twice that of HTPB. These factors combine make dicyclopentadiene-based composite propellant a potentially attractive alternative for a number of missions demanding composite solid propellants.

Nomenclature

a St. Robert’s Law pre-factor constant AP ammonium perchlorate CTPB carboxyl-terminated polybutadiene DCPD Dicyclopentadiene DPM Dual Planetary Mixer HTPB hydroxyl-terminated polybutadiene JATO Jet Assisted Take Off LVDT Linear Variable Differential Transformer n St. Robert’s Law power constant P pressure (psi) PBAN polybutadiene acrylonitirile pDCPD poly-dicyclopentadiene r burn rate (in/s) ROMP Ring Opening Metathesis Polymerization

I. Introduction

For many years the field of composite solid propellants has been dominated by reliable binders such as hydroxyl-terminated polybutadiene (HTPB) and polybutadiene acrylonitrile (PBAN). These binders owe their success in the industry to years of research and experimentation beginning with John Parsons and his asphalt/potassium perchlorate JATO rockets.1 Seizing the potential of composite propellants other researchers went on to experiment

1 Graduate Student, School of Aeronautics & Astronautics, 701 West Stadium Avenue, AIAA Member 2 Professor, School of Aeronautics & Astronautics, 701 West Stadium Avenue, AIAA Associate Fellow 3 Associate Professor, School of Mechanical Engineering, 585 Purdue Mall, AIAA Member 4 President, 3495 Kent Avenue Suite G100, AIAA Senior Member

46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit25 - 28 July 2010, Nashville, TN

AIAA 2010-6589

Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.



with butadienes and other hydrocarbon-based compounds that improved either the motors performance and/or mechanical properties. Being hydrocarbons themselves these binders made excellent fuel additions to composite propellants. Hydrocarbon binders continued to improve over the years from Parsons’ asphalt binder to carboxyl-terminated polybutadiene (CTPB), followed soon after by HTPB, and PBAN.

Another advantage of hydrocarbon-based composite solid propellant binders is the potential for mass production. Before its use as a binder for solid propellants HTPB was used as caulk in bathrooms along with other commercial applications.2 HTPB would eventually replace CTPB due to its improved performance and mechanical properties. The benefit of using mass produced materials is the inherent low cost of manufacturing the material. It is partly this low cost, along with other attributes to be discussed later, that led to the investigation of dicyclopentadiene (DCPD) as a binder for composite solid propellants.

II. DCPD

Dicyclopentadiene (DCPD) is a hydrocarbon (C10H12) that when polymerized exhibits excellent impact strength and toughness. Two versions of the DCPD molecular structure are shown in Figure 1. The polymerized compound, poly-dicyclopentadiene (pDCPD), is an extremely rigid plastic that has been used for many years in car bumpers, snowmobile hoods, and various automotive and marine applications. In its native state DCPD exists as a hard wax-like milky-white solid at room temperature. When heated above 90⁰F (32⁰C) the monomer will melt into a low viscosity clear liquid similar to water at room temperature. This low viscosity is potentially beneficial as it should allow for the inclusion of a high amount of solid energetic materials. However, in processing, DCPD’s low viscosity would prove troublesome as solid particles tended to settle out if the mixture was not constantly stirred.

Figure 1. Two Common Depictions of DCPD Molecule3,4

In order to form a proper binder for composite propellant it is necessary to polymerize

DCPD after the addition of energetic solids. Dicyclopentadiene polymerizes to poly-DCPD through a process known as Ring Opening Metathesis Polymerization (ROMP) in which either one or both of the ring structures open to form a cross-linked polymer (see Figure 2). This process has been studied at length by a number of researchers and will not be discussed in detail in this paper. There are a number of catalysts which can be used for the ROMP of DCPD including the classical tungsten catalyst WCl6/(C2H5)2AlCl, Schrock’s molybdenum alkylidene Mo(CH-CMe2Ph)(N-2,6-C6H3-i-Pr2)(OCMe(CF3)2)2, and Grubbs’ 1st Generation catalyst (Cl)2(PCy3)2Ru=CHPh (where Cy is cyclohexyl).3-6 It is also common in industry to use less



costly co-catalyst systems which combine in two or more streams. However, these co-catalyst systems can produce unwanted reactions aside from the ROMP reaction.4

Figure 2. Grubbs' 1st Generation Catalyst ROMP of DCPD.3

In a previous phase of this research Grubbs’ 1st Generation catalyst was selected as the

catalyst for polymerization of DCPD. Grubbs’ 1st Generation catalyst is very effective at polymerizing DCPD with high monomer to polymer conversion rates as well as forming highly cross-linked polymer networks. These highly cross-linked polymer networks are what gives pDCPD its strength, a property sought during experimentation.

Without any inhibitors the reaction of DCPD and Grubbs’ 1st Generation catalyst occurs very quickly, reaching a gel point in less than thirty seconds. This is highly problematic as composite propellant processing requires the addition of a large amount of solid materials while the binder is still mixable. Depending on the mixing process being implemented this can potentially take several hours. By making use of a small amount of inhibitor, or perhaps a less effective catalyst also, the reaction can be slowed from a matter of seconds to several hours.

III. DCPD Composite Propellant

To create a composite solid propellant it is necessary to suspend large quantities, generally over 80% by weight, of oxidizer and metal particles in the binder. For much of this study the goal was to determine the best method for creation of a DCPD-based composite solid propellant. Therefore many mixes focus on a mixture of 80 wt-% solids and 20 wt-% DCPD/plasticizer, henceforth referred to as the “baseline” mixture. The composition of the baseline mixture used in the majority of mixes is 20 wt-% DCPD, 15 wt-% 15 micron aluminum powder, 48.75 wt-% 20 micron “fine” ammonium perchlorate (AP), 16.25 wt-% 200 micron “coarse” AP powder, and a very small amount of inhibitor and catalyst. The amount of plasticizer used is selected per mix and the amount added will be subtracted from the total



amount of DCPD. Over the course of this phase of research approximately 30 mixtures of fully loaded DCPD composite propellant were performed.

IV. Mixing

As use of DCPD as a binder in composite solid propellants is in its beginnings the process of mixing in energetic solids is still under development. Working with a previously undeveloped binder in composite solid propellants requires in-depth study of the binder’s rheology, monomer structure, polymer formation, polymerization characteristics, cure techniques, and aging behavior.

With many common binders, HTPB or PBAN, long mixing cycles can take several hours to ensure homogenous dispersion of solid ingredients. Due to the nature of the interaction between DCPD and Grubbs 1st Generation catalyst a different approach is taken to mixing propellant. The comparatively short pot life of a DCPD-based composite propellant, between thirty minutes and two hours, requires a reworking of standard procedures for producing composite propellants. Also, the low viscosity of DCPD monomer (similar to water) makes a high solids loading easier but adds some difficultly as solid particles can settle out early in the mix if not continuously stirred.

In the initial work done at Purdue University’s Maurice J. Zucrow Laboratory researchers used a Ross Dual Planetary Mixer with a 1-quart capacity. This mixer featured a heated water jacket that was highly desirable as DCPD needs to remain above 90⁰F (32⁰C) to remain liquid. A pair of either high sheer or low sheer blades can be attached to suit mixing requirements. Although this mixer was adequate for

initial experimentation it requires long mixing cycles that were undesirable with DCPD’s rapid polymerization process.

Purdue University’s Maurice J. Zucrow Laboratories recently purchased a Resodyn™ LabRam resonant mixer with a 1-lbm capacity (see Figure 3). This mixer uses high frequency oscillations automatically calibrated to the resonant frequency of the ingredients to mix the contents of the 1-pint container. Using the Resodyn™ mixer allows for significantly shorter mixing cycles through faster blending of the propellant ingredients. This is a benefit for DCPD propellant processing as once the catalyst has been introduced there is a very limited window of time before polymerization of the monomer makes mixing impossible. However, the switch from a standard bladed planetary mixer to a resonant mixer required the development of new mixing procedures to achieve high-quality propellant mixes.

Figure 3. Resodyn™ LabRam Mixer in Temperature Conditioning Box.



A drawback of the Resodyn™ mixer compared to the Ross DPM is the absence of a heated water jacket. As it is necessary to keep DCPD above 90⁰F (32⁰C) to prevent freezing of the binder a temperature conditioning box was constructed to surround the Resodyn™ mixer. This polycarbonate structure is able to keep the environment surrounding the mixing container heated above the freezing point of DCPD. Temperature within the box is monitored through the use of a thermocouple attached to the side of the mixing container. Heated air is then blown into the box to maintain the desired temperature.

V. Mechanical Evaluation

When a customer is selecting a composite solid propellant to meet their requirements one of the main interests besides the ballistic performance is the structural properties. Therefore it is necessary to understand the mechanical properties of the propellant created. Although there are a number of tests that would need to be performed in order to thoroughly evaluate a viscoelastic material, a basic tension test was chosen for rapid characterization of the mechanical properties of DCPD composite propellant. In order to assess the potential tensile stress and strain capacity of a DCPD-based composite solid propellant a test apparatus was constructed (see Figure 4).

The machine shown in Figure 4 is a pneumatic tension test machine capable of delivering a number of strain rates with which to load specimens of composite propellant. This tension test machine is used to perform tensile tests of propellant specimens commonly referred to as “dog bones” due to their shape (Figure 5). The dog bones are placed under tensile stress in the testing apparatus and are pulled until failure. By making use of an in-line tension load cell and a Linear Variable Differential Transformer (LVDT) it is possible to rapidly determine a propellant’s tensile stress and strain capabilities.

The composite propellant tension samples created are shaped according to JANNAF specifications7 for mechanical testing of composite propellant. In order to form propellant into these shapes the propellant can be milled, die cut, or cast into the specified shape.8 Due to the hardness of pDCPD, it was cast into shape rather than cut or milled after curing. Therefore, molds were created to cast the propellant into the JANNAF-specified shape.

Figure 4. Purdue University Maurice J. Zucrow Laboratory Tension Test Machine.



Figure 5. DCPD Dog bone for Structural Evaluation.

During previous experimentation at Purdue University it was found that DCPD had similar tensile stress capacity than an identically-loaded HTPB-based composite propellant. However, the DCPD was not able to handle nearly the same level of strain as the HTPB propellant; the DCPD propellant had less than a fifth of the strain capacity. This was not the case in the most recent phase of research as DCPD composite propellant proved to be significantly stronger than identically-loaded HTPB composite propellant.

All samples of baseline DCPD composite propellant, and identically-loaded HTPB composite propellant, were subjected to the same strain rate to ensure consistent testing. Analyzing the readings from the LVDT and load cell allows for the creation of the stress-strain curves for DCPD and HTPB composite propellant seen in Figure 6. The averages and standard deviations for the baseline composite propellant mechanical tests can be found in Table 1

Figure 6. Stress-Strain Curve for Baseline DCPD and HTPB Composite Propellant.



Table 1. DCPD and HTPB Composite Propellant Results of Mechanical Evaluation.

Maximum

Stress (psi)

Peak Strain at Max Stress

(%-strain)

Initial Modulus

(psi)

Age at Testing (days)

DCPD Average 559.97 2.35 46,170 21.40 DCPD Std Dev 54.75 0.54 10,640 11.65 HTPB Average 371.35 3.03 17,000 21 HTPB Std Dev 11.29 0.24 280 0

As seen in Figure 6 and Table 1 DCPD composite propellant exhibits significantly higher tensile strength than identically loaded HTPB composite propellant. Averaging the numerous baseline DCPD composite propellant mechanical tests shows that it is 1.5 times stronger than identically-loaded HTPB composite propellant while still enduring 77% of the peak strain at maximum stress. It is worthwhile to note that both of the DCPD and HTPB composite propellant samples tested failed in a single location (one crack), usually in the neck section of the mechanical specimen.

Additional tests were performed with DCPD baseline composite propellant using a higher solids loadings, 85 wt-% energetic solid particles, followed the same curves seen for DCPD composite propellant shown in Figure 6 with, as expected, reduced stress-strain capacity (14% reduction in average stress capacity, 33% reduction in average strain capacity). For evaluation of burn rate modifiers iron oxide was added in small quantities to the DCPD baseline composite propellant and found to have no effect on mechanical properties. Other experiments were performed with alteration of the cure cycle and inhibitor loading producing a batch of DCPD baseline composite propellant with an average maximum tensile strength of over 1800 psi (over 300 lbf).

VI. Plasticizer Evaluation

In this phase of research a series of plasticizers were investigated to see if the mechanical properties of DCPD composite propellant could be tailored. Although a number of plasticizers exist for DCPD, only three were investigated during this phase of research. Initially these plasticizers were investigated by mixing them with DCPD gumstock (DCPD monomer, inhibitor, and catalyst only) to make sure they were soluble and to test their effects on pDCPD mechanical properties. By observing the effects of the plasticizers on pDCPD it was determined that the three plasticizers could increase the flexibility of pDCPD in varying degrees in ratios of 10-to-1, 5-to-1, and a very high 2-to-1 DCPD-to-plasticizer. The next experiments would evaluate the effects of these plasticizers on fully-loaded DCPD-based composite propellant.

During experiments with gumstocks Plasticizer #1 was shown to significantly increase the flexibility of pDCPD plastic and to maintain the flexibility over many months of aging. Plasticizer #1 was then incorporated into DCPD composite propellant in the ratios specified above. Both the 10-to-1 and 5-to-1 ratios of DCPD-to-plasticizer were able to alter the mechanical properties of DCPD composite propellant but the high 2-to-1 ratio produced an extremely soft propellant that crumbled when removed from the molds.



When subjected to tensile stress the DCPD composite propellant samples with Plasticizer #1 tended to crack in multiple locations before failure. This gave the propellant samples a lower peak strain at maximum stress than pure DCPD composite propellant but a significantly increased failure strain (between 12-15 %-strain). The results of the DCPD composite propellant with Plasticizer #1 can be seen in Table 2.

Table 2. Results from DCPD Propellant with Plasticizer #1 Mechanical Tests.

Plasticizer Ratio

Maximum Stress (psi)

Peak Strain at Max Stress (%-strain)

Initial Modulus

(psi) Average

5-to-1 163.94 1.00 21,070

Std Dev 72.12 0.16 6,800 Average

10-to-1 292.91 0.73 38,550

Std Dev 30.22 0.18 6,860

Initial studies of gumstocks with Plasticizer #2 showed that the plasticizer could effectively increase the flexibility of DCPD polymer. Despite the samples lack of flexibility as compared with Plasticizer #1 gumstocks, they showed promise for evaluation as DCPD composite propellant plasticizers. The behavior of Plasticizer #2 in the DCPD propellant turned out to be very interesting when compared with Plasticizer #1. Even though the composite propellant had undergone significant strain, greater than 5 %-strain, no macro fissures (visible cracks) had developed. It is highly possible that micro fissures developed at this time potentially giving rise to the discoloration seen during experimentation. The comparative lack of fissures seen in the DCPD composite propellant with Plasticizer #2 is important as cracks in the propellant grain can lead to failures during propellant combustion. Results of the DCPD composite propellants with Plasticizer #2 are shown in Table 3.


Plasticizer

Ratio Maximum Stress

(psi) Peak Strain at Max Stress

(%-strain)

Initial Modulus

(psi) Average

5-to-1 156.54 1.08 12,450

Std Dev 9.49 0.10 3,040 Average

2-to-1 51.16 0.66 2,250

Std Dev 0.22 0.27 210

Plasticizer #3 was investigated due to its chemical similarity to DCPD, which would make it more likely to be soluble in DCPD. In addition, like DCPD, Plasticizer #3 is a hydrocarbon molecule that could aid in the combustion process during polymer breakdown. When combined with DCPD in fully loaded propellant Plasticizer #3 reduced the stress capacity of the propellant the most compared with the two other plasticizers.




Plasticizer

Ratio Maximum Stress

(psi) Peak Strain at Max Stress

(%-strain)

Initial Modulus

(psi) Average

5-to-1 129.55 1.10 8,000

Std Dev 2.00 0.12 - Average

2-to-1 35.68 1.45 3,650

Std Dev 3.89 0.22 1,060

Though only three DCPD plasticizers have been evaluated at three plasticizer levels, the initial investigation has shown that indeed DCPD composite propellants can be tailored by the addition of plasticizer materials. Although optimum loadings most likely exist for the plasticizers tested, the “optimum” value will depend on a system’s requirements for mechanical performance. The conclusion to draw from these tests is that the mechanical properties of DCPD-based composite propellant can be altered to increase the usefulness of the new binder in the composite propellant industry.

Table 5. Comparison of Mechanical Evaluation of Baseline and Plasticized DCPD Composite Propellants.

Propellant/Plasticizer Type Avg. Maximum

Stress (psi) Avg. Peak Strain at

Max Stress Baseline DCPD 559.97 2.35

Plasticizer #1 5-to-1 163.94 1.01 Plasticizer #2 5-to-1 156.54 1.08 Plasticizer #3 5-to-1 129.55 1.10

VII. Performance Evaluation

One of the most prominent questions when evaluating a new binder for composite solid propellants is how the binder will affect performance. Some applications will require a propellant to have a high burning rate for high thrust motors or boosters. Others will need low burning rates for sustainer motors or delay devices.

A low cost and quick way to evaluate the burn rate of solid propellant is with a strand burn test in a pressure vessel. The strands measure approximately 0.25” x 0.25” square, 0.75” tall, and can be seen in Figure 7. These strands are secured to a steel bolt head and coated with an inhibitor on all sides except the top 0.25” x 0.25” surface. They

are then placed inside a windowed Crawford Bomb and ignited using a nickel-chromium

wire at various pressures. Using visual tracking software it is possible to determine the burning rate of the propellant strands.

Figure 7. DCPD Propellant Strands for Burn Rate Evaluation.



One of the main goals of performing strand burns of composite solid propellant is to determine the constants for the St. Robert’s Law (Equation 1) of burn rate as a function of pressure.

nr aP= (1)

Here r is the burn rate (in/s), P is the pressure (psi), a is the pre-factor constant, and n is the power constant. By measuring the burning rate at different pressures it is possible to determine the value of the constants by fitting a line to the data.

As the baseline mixture of DCPD composite propellant is of the most interest, its burn rate will be considered first. From examination of literature the products of DCPD decomposition will be a series of smaller hydrocarbons. These smaller hydrocarbon groups should provide excellent additional fuel for the combustion zone where the AP and aluminum powders are combusting. Propellant samples were combusted at a number of pressures in an attempt to provide the constants needed for St. Robert’s Law.

Figure 8. Baseline DCPD Propellant Strand Burns (Designations are Batch Numbers).

As can be seen in Figure 8 there can be quite a bit a scatter between different batches and even within the same batch of baseline propellant. (Note: All batches shown are of similar composition.) Some of this scatter is attributed to error in assigning the burning surface during analysis. Other factors such as voids, poor casting, or poor ignition could also be causes of some of the scatter. In addition, strands are not the most optimal method for determining burn rate. Small ballistic motors can more accurately determine the burn rate of a propellant. These will be tested in later research. When the burn rates at each pressure are averaged together a new plot is created, shown in Figure 9.



Figure 9. Average Baseline DCPD Propellant Burn Rate.

In Figure 9 the scatter seen in Figure 8 is removed producing a much clearer view of the burn rate of DCPD-based composite propellant. As can be seen in both figures the burn rate at 600 psig does not follow the trend of increasing pressure leading to increased burning rate. This may be due to a burning phenomenon at this pressure or experimental errors noted above. By excluding the results from the 600 psig tests a better curve fit with a trend line is possible (r-squared value 0.9463 vs. 0.7633). However, by eliminating the results from the 500 psig tests a trend line with a curve fit r-squared value of 0.7908 is created. Tabulating the average burn rates for the baseline DCPD composite propellant strands is done in Table 6. The standard deviations for the burn rates at each pressure show that there is a reasonable variance in burn rate at each pressure. This is most likely due to the differences from batch to batch of propellant. Equation 2 displays St. Robert’s Law for DCPD using the data gathered.

Table 6. Average Burn Rates for Baseline DCPD Composite Propellant Strands. Chamber Pressure

(psig) Average Burn Rate

(in/s) Standard Deviation

Number of Samples

500 0.7777 0.1162 5 600 0.6860 0.0494 7 800 0.9526 0.1630 6 1000 0.9845 0.2100 15 1400 1.0549 0.1621 9

4756.00356. Pr = (2)

As DCPD is a new binder for composite propellants it is necessary to compare its performance with other existing composite propellant binders. Hydroxyl-terminated polybutadiene (HTPB) was selected for comparison as it is widely used in a variety of composite propellant applications. Strands of HTPB composite propellant were produced (same aluminum



and AP ingredients, course-to-fine ratios, solids loading, mixing method) and combusted in the same manner as the DCPD baseline composite propellant strands to generate Figure 10.

Figure 10. DCPD – HTPB Baseline Burn Rate Comparison.

Each of the two binders tested, DCPD and HTPB, shown in Figure 10, use the same sizes and loadings of solid materials not directly related to the binder. In addition, they are manufactured in the same manner. Therefore the difference seen in the burn rates of the two composite propellants should emanate principally from the difference in binders. As seen in Figure 10 and listed in Table 7, DCPD-based composite propellant burns at double the rate of HTPB-based composite propellant. These results are very significant as past research of DCPD as a solid fuel for hybrid rocket motors had DCPD burning at nearly half the rate of HTPB.9 It would seem that the incorporation of solid oxidizer and aluminum fuel particles directly into the DCPD binder matrix has a substantial effect on the burn rate.

Table 7. Average Burn Rates for Baseline HTPB Composite Propellant Strands. Chamber

Pressure (psig) Average

Burn Rate (in/s) Standard Deviation

Difference From Baseline DCPD

Number of Samples

600 0.3505 0.0032 -95.71% 2 1000 0.4334 0.0079 -127.18% 4 1400 0.5746 0.0226 -83.60% 2

In addition to evaluation of the baseline DCPD composite propellant, Crawford Bomb tests were done to evaluate the burning rate of DCPD composite propellant loaded above the baseline mixture. It is expected that the increase in solid materials should lead to an increase in the burning rate of the DCPD-based composite propellant. As was discussed earlier the higher solids loading slightly reduced the total stress-strain capacity of the DCPD composite propellant.



Therefore certain applications may have to balance the increased burn rate with reduced mechanical properties.

As was expected the burn rate for the higher solids loading DCPD composite propellant is higher than the baseline mixture. The increase in burn rate is approximately 13.5% from the baseline (80% solids) to the higher solids loading (85% solids). For the baseline mixtures, the average burn rate at 1000 psig was 0.9845 in/s while the higher solids loading saw an average burn rate of 1.1377 in/s at the same pressure.

When evaluating the use of a plasticizer in a composite propellant the effect on the burn rate must be taken into consideration. Since the plasticizer is almost always a different material than the binder it may cause an alteration in the burn rate. Of the three plasticizers considered only evaluations of Plasticizer #1 in a 5-to-1 Monomer-to-Plasticizer ratio have been conducted thus far.

The difference in burn rate at 1000 psig between the average baseline mixture and the DCPD propellant with Plasticizer #1 was only 0.84% indicating that this plasticizer had minimal impact on the burn rate. However, this is not necessarily nor likely the case for all DCPD plasticizers.

VIII. Conclusion

Dicyclopentadiene (DCPD) has shown potential as a viable binder for composite propellant. While the monomer exists as a hard wax-like solid at room temperature, it is a low viscosity liquid when heated above 90⁰F (32.2⁰C) and forms an incredibly tough plastic with excellent tensile strength when polymerized. DCPD is able to successfully incorporate a large amount of solid material (85% solids loading demonstrated) while still retaining a large portion of its structural strength.

Dicyclopentadiene composite propellant has also proven to be a very strong binder mechanically. Baseline mixtures of DCPD composite propellant exhibited an average maximum tensile stress 1.5 times that of similarly loaded HTPB composite propellant while enduring 77% of the strain at peak stress. This ability to handle a significant portion of strain of an HTPB sample while far exceeding the stress load is an exceptional gain over the previous phase of research.10 In one batch of propellant, though, the strength of the DCPD binder was dramatically increased to an average maximum stress of over 1800 psi, nearly five times that of HTPB.

It was also shown that DCPD composite propellant is able to accept several different plasticizers that can alter the mechanical properties of the propellant. Three plasticizers were selected and added to DCPD composite propellants in various amounts. These plasticizers were successfully able to blend with the DCPD monomer and form a more flexible composite propellant.

In addition to its strength advantage, DCPD also exhibits a significant burning rate advantage over a comparable HTPB-based propellant. Dicyclopentadiene-based composite propellant exhibited a burn rate twice that of its HTPB counterpart, nearly one inch per second at 1000 psig without addition of any burning rate catalysts. Higher burning rates will allow for



simpler grain designs or provide reduced action times for short duration firings. The combination of high strength and burning rate make DCPD an attractive alternative for many composite solid propellant applications.

IX. Acknowledgements

The authors gratefully acknowledge the support for this work through the Missile Defense Agency under contract numbers HQ0006-06-C-7513 (Heather Simko, CoTR) and W9113M-09-C-0192 (Heather Simko and Jason Calvert, CoTR) and the State of Indiana 21st Century Research and Technology Fund.

X. References

1. Davenas, Alain, “Development of Modern Solid Propellants”, Journal of Propulsion and Power. Vol. 19, No. 6, November-December 2003.

2. Geisler, R., “My Memoirs of Solid Propellant Development at the Air Force Rocket Propulsion Laboratory.” AIAA/SAE/ASME/ASEE 29th Joint Propulsion Conference and Exhibit. June 1993. AIAA 93-1783

3. Kessler, M. R., Larin, G. E., Bernklau, N., “Cure Characterization and Viscosity Development of Ring-Opening Metathesis Polymerized Resins.” Journal of Thermal Analysis and Calorimetry, Vol. 85, (2006), 1, pp. 7-12

4. Davidson, T.A., Wagner, K.B., “The Polymerization of Dicyclopentadiene: An Investigation of Mechanism” Journal of Molecular Catalysis A: Chemical 133, (1998), pp. 67-74

5. Davidson, T.A., Wagner, K. B., Priddy, D. B., “Polymerization of Dicyclopentadiene: A Tale of Two Mechanisms.” Macromolecules, Vol. 29 No. 2. 1996. pp. 786-788

6. Humble, Ronald W., Grubbs, Robert H., Woodson, Charles S., (To A.P.T. Aerospace, LLC), “Rocket fuels based on metal hydrides and poly-DCPD.” U.S. Patent 6,383,319, May 7, 2002.

7. Riser, D., Hunter, J., Rast, R.. “A Microtensile Test Specimen for Testing Double-Base Sheetstock.” AIAA/SAE/ASME/ASEE 28th Joint Propulsion Conference and Exhibit. July 1992. AIAA 92-3723

8. F. N. Kelley, "Solid Propellant Mechanical Properties Testing Failure Criteria and Aging." Advances In Chemistry Series, pp. 188-&, 1969.

9. Humble, Ronald W. “Fuel Performance Enhancements for Hybrid Rockets.” AIAA/ASME/SAE/ASEE 36th Joint Propulsion Conference and Exhibit. July 2000. Huntsville, AL. AIAA-2000-3437

10. Austin, B.L., Heister, S.D., Meyer, S.E., Corpening, J.H., Trebs, A., Wernimont, E., Linke, K., "Novel Binder Materials for IM-Compliant Solid Rocket Motors", Missile Defense Agency Phase I SBIR Report, Contract No.HQ0006-06-C-7513, April 30, 2007.

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