solid rocket propellants for improved im response – recent

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Solid Rocket Propellants for Improved IM Response – Recent Activities in the NIMIC Nations

Michael Fisher and Dr. Michael Sharp NATO Insensitive Munitions Information Center (NIMIC)

NATO Headquarters, B-1110 Brussels BELGIUM

Tel: (32-2) 707.54.16 Fax: (32-2) 707.53.63

Email: [email protected]

SUMMARY

As part of a systems approach to meeting Insensitive Munitions requirements, the NIMIC member nations have expended a great deal of effort on the development and qualification of less sensitive rocket propellants. The desired characteristics of such “IM propellants” are discussed, in terms of general methods of tailoring the propellants to counter thermal and mechanical (shock/impact) threats. Examples that illustrate the application of these methods in real, useful propellants are presented, along with the corresponding results of component and/or full-scale IM tests, where possible. Tradeoffs between reduced vulnerability and other propulsion system design drivers (e.g., performance, cost, signature, environmental concerns) are also discussed.

New energetic materials and propellant ingredients that show promising IM characteristics are presented, along with a look at future research and development efforts and technology trends in this area. The conclusion is that energetic materials and propellant formulations are available that can provide the rocket motor designer with the required performance while also reducing sensitivity.

INTRODUCTION

Today, it is generally accepted by the international IM research and development community that hazards reduction is a systems problem, and, therefore, the approach most likely to significantly reduce a munition’s sensitivity and vulnerability to IM stimuli is a systems approach. For a solid rocket propulsion system, such an approach is likely to consist of the following elements, as illustrated in Figure 1.

Venting Device*

Ignition System Less Sensitive Propellant Motor Case

Bore Mitigant*

* Examples of Mitigation Techniques Intumescent Coating*

Figure 1: Components of a Systems Approach to IM for Rocket Motors.

RTO-M

Paper presented at the RTO AVT Specialists’ Meeting on “Advances in Rocket Performance Life and Disposal”,held in Aalborg, Denmark, 23-26 September 2002, and published in RTO-MP-091.

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In addition to the motor design attributes highlighted in Figure 1, a systems approach to IM can also be influenced by munition storage and packaging concepts (e.g., packaging/storage configurations, barriers and deflectors, pumice, munition container vents). So far, we have only mentioned the characteristics of the munition itself, but as Boggs[2] points out, the approach must also take into account the influence of the hazard stimuli (fast and slow heating, bullet and fragment impact) and the environment on the munition response.

+

+ Munition

Stimuli

Environment Response

Motor Case Propellant Mitigation Devices Container/Storage Effect of Warhead

Figure 2: IM Systems Approach.

A methodology has been developed for using a series of hazard assessment protocols[3] to consider the hazards in a logical, ordered procedure that allows the designer to identify the likely response paths, and determine what information is needed and what tests can be used to provide it. Using this methodology, an understanding of the reaction mechanisms may be achieved, and confidence in the assessment of likely responses can be increased.

Obviously, choosing the best propellant from an IM perspective is not a simple process. Besides considering the influence of other system components and factoring in a scientific evaluation of the hazard/munition interaction, the designer must also weigh the IM requirements against other system constraints when tailoring propellants for specific applications. As Stokes[1] suggests, performing the appropriate trade-offs between hazard reduction and the other considerations listed in Figure 3 involves a “subtle understanding of the interaction of the normal (i.e. non-IM) propellant selection criteria with their influence on desensitizing the propellant to the hazard threats.” The question to answer, then, is can the designer reduce propellant sensitivity to IM hazards without compromising other system requirements?

COST & MATURITY

MOTOR PERFORMANCE

COMBUSTION STABILITY

ENVIRONMENT, END OF LIFE IM HAZARDS

SIGNATURE

PROCESSING, AGING

USEABLE IM PROPELLANT

Figure 3: Rocket Motor Design Tradeoffs.

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IM PROPELLANT CHARACTERISTICS [1,2,4]

Despite the fact that the rocket propellant is only one aspect of a munition system, there are specific characteristics that have been identified as contributing factors to the achievement of IM status, or IM-ness as it is often called. Certainly, there are various, equally valid, ways of categorizing these characteristics. For example, they could be grouped according to the specific IM stimuli against which they are effective, or for which type of propellant their use is appropriate. For the purposes of this report, the IM propellant traits are categorized (roughly) according to the general method by which they affect a formulation’s IM sensitivity.

Using this means of categorizing propellant characteristics might lead us to three general approaches to reducing sensitivity:

Changing the friability or “toughness” of the propellant.

Propellants with good elongation properties (particularly at low temperature limits), those that absorb energy and deform with minimum damage, tend to perform well against shock and impact threats.

Using ingredients with low glass transition temperatures (Tg).

Managing the Partitioning of Energy

o Reducing the solids loading. In general, decreasing the total solids level (for an equivalent energy level) improves the mechanical properties and decreases detonability of the propellant. This can be accomplished in several ways:

•

•

•

Increasing the density of crystalline oxidizers (development of new oxidizers).

Use of an energetic binder system, which allows a corresponding decrease in solids loading without decreasing total energy.

Use of high-density additives to maintain or increase propellant density-impulse while decreasing the total level of solids.

o Controlling particle size and distribution – using particle distributions that are optimum for binder wetting and particle-to-particle bond strength. For example, fine grinding of nitramines reduces shock sensitivity.

o Using less sensitive solid ingredients (i.e., reducing the level of nitramines, reducing or changing ballistic modifiers, reducing the amount of ammonium perchlorate). This may involve the development of new ingredients, or the use of new combinations of existing propellant ingredients, such as energetic plasticizers.

o Using low ignition temperature binder materials; binders that melt at temperatures below the melting temperatures of the solid additives.

Developing extinguishable propellants – propellants that smolder or extinguish at atmospheric pressure.

Detailed descriptions of the mechanisms by which these methods improve IM-ness are beyond the scope of this paper; however, more information can be found in the listed references. These approaches to tailoring propellants for IM are not new, as they have been proposed, discussed and debated in the IM and propulsion communities for quite a few years. So, it should be interesting to take a look at how various organizations have made use of these methods in the development of IM propellants over the past decade. The next section of this paper will present some examples, highlight the approaches used to reduce

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sensitivity to IM stimuli, and provide IM test results that illustrate the degree of success (or failure) achieved by each example. Information will also be presented, where available, concerning the effects of reducing IM-ness on the other relevant design drivers.

IM PROPELLANT EXAMPLES

1) HTPE [6,7] (Alliant Techsystems, NAMMO Raufoss AS, NAWCWPNS China Lake)

A family of propellants, based on a binder system containing hydroxyl-terminated polyether (HTPE) polymer, has been developed and demonstrated over a range of motor sizes and configurations during the past 10 years. These propellants were developed as less sensitive replacements for “garden variety” HTPB/AP propellants currently used in many tactical missile rocket motors. Reduced smoke and aluminized versions of the propellant were originally developed by Alliant Techsystems under contract to the Naval Air Warfare Center Weapons Division, China Lake. Manufacturing capability for these

propellants was later transitioned to NAMMO Raufoss AS in Norway, for use in the development program for the Evolved Sea Sparrow Missile (ESSM) rocket motor.

Reduced Solids Loading o Energetic Plasticizer

Reduced Level of AP o Energy Partitioned by

Using 2nd Oxidizer (AN) Good Elongation Properties

The basic approach to IM-ness employed in this example is the reduction in solids loading accomplished by using an energetic plasticizer that is compatible with the HTPE polymer. This permits the HTPE propellant to maintain a theoretical specific impulse at or above that of a comparable HTPB propellant while significantly reducing the level of solids, as shown in Figure 4. x Sensitivity is further reduced by the replacement of a portion of the AP with ammonium nitrate (AN).

77%

HTPE

89%

HTPB

Theo

retic

al Is

p (s

)

Solids Loading (%)

Figure 4: HTPE’s Advantage: Reduced Solids Loading at the Same Level of Energy.

The HTPE propellants produced by Alliant and Raufoss have performed very well in design tradeoffs relative to their HTPB counterparts. Performance, mechanical properties, signature and service life requirements for tactical applications have been met. The properties achieved for HTPE propellants have matched or exceeded those of the comparable HTPB’s in critical design tradeoff areas, as noted in Table 1.

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Table 1: Comparison of Properties – HTPB vs. HTPE

Characteristic HTPB HTPE

Isp * ρ (lb-s/in3) reduced smoke ≥15 ≥15

Burn Rate @ 1000 psi (in/s) 0.3 – 1.5 0.3 – 1.2

Pressure Exponent ~ 0.5 ~ 0.5

Failure Stress @ 77ºF (psi) 120 170

Failure Strain @ 77ºF (%) 40 50

Modulus @ 77ºF (psi) 600 500

Pot Life (hours) 10 20

Shock Sensitivity (NOL Card Gap) 0 cards 0 cards

As previously stated, these HTPE propellant formulations have been tested in a variety of configurations, at the sub-scale or generic level, as well as in full-scale prototype motors, from 5-inch diameter to the 10-inch diameter ESSM size. Several of these configurations have undergone the full range of IM tests, and have generally performed quite well, especially when combined with graphite composite motor cases. Table 2 presents the results of a series of IM tests conducted on 10-inch diameter analog test motors, and offers a comparison between the IM-ness of similar configurations using HTPB and HTPE propellants.

Table 2: Comparison of IM Test Results: HTPB vs. HTPE

Configuration (10-inch Analog Motors w/Graphite Composite Cases) IM TEST

HTPB HTPE

Slow Cookoff Explosion Burn

Fast Cookoff Burn Burn

Bullet Impact Deflagration Burn

Fragment Impact Explosion Extinguish

The IM gains illustrated by the test results for full-scale ESSM motors have not been as dramatic as those presented in Table 2, although the responses measured were certainly less violent than the expected response from a similar, HTPB-filled motor. The main reason for ESSM’s failure to demonstrate more dramatic gains is the use of a monolithic steel motor case instead of a composite one. This illustrates the importance of integrating the choice of a less sensitive propellant into an entire systems approach to IM-ness, as the propellant alone will not provide an IM solution.

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2) HTPE with High-Density Additive [5] (Alliant Techsystems) This version of the Alliant HTPE propellant was developed with high performance in mind, to extend the range of potential applications for this propellant family. Bismuth Trioxide (Bi2O3) was added in amounts ranging from 10-21% to increase the density-impulse of the propellant, which is an important measure of performance for volume-limited systems. Earlier efforts using this approach, conducted at China Lake and sponsored by the US Navy Insensitive Munitions Advanced Development (IMAD) Program, demonstrated that the addition of Bi2O3 could improve IM response. The expected performance gains were achieved without compromising other propellant parameters, including shock sensitivity, processability, and ballistic properties.

Figure 5: Bullet Impact Test.

Initial IM tests indicated that, with some tailoring, the high density HTPE’s will provide similar IM response characteristics to those exhibited by the baseline HTPE’s. Figure 5 shows the result of a bullet impact test (three 50 cal. AP rounds, velocity = 2533 ft/s) conducted on a 5-inch diameter, graphite composite cased motor filled with HTPE propellant containing 21% Bi2O3. The motor ignited and burned in a controlled manner resulting in a Type V response (as judged by Alliant personnel). The result of a fragment impact test was also judged to be a passing response. A third motor containing the same propellant failed a slow cookoff (SCO) test with an explosive response. In a

subsequent SCO test with a slightly modified Bi2O3-HTPE formulation, a mild burning response was achieved, similar to the response of the baseline HTPE propellant.

Reduced Solids Loading o Energetic Plasticizer o High Density Additive

Reduced Level of AP Good Elongation Properties

Uses Less-sensitive Ballistic Modifier

Low Tg Binder

3) Butacene [8] (SNPE) Butacene is a prepolymer designed for use in high burning rate composite propellants. Instead of using more conventional burn rate catalysts (e.g., iron oxide, liquid ferrocenes), Butacene consists of a low molecular weight HTPB onto which a ferrocene catalyst has been chemically grafted. This avoids the problems associated with conventional catalysts that are not bonded to the polymer,

Uses Less-sensitive Ballistic Modifier

Low Tg Binder

namely, the tendency to migrate to the propellant surfaces and bondlines, and increase sensitivity. While these propellants are not formulated specifically with IM-ness in mind, they do seem to provide a good compromise between high performance and reduced sensitivity.

For composite propellants possessing good mechanical properties (i.e., tough propellants with good elongation over the tactical temperature range), the thermal threats are typically the most difficult to solve. According to data presented by SNPE, butacene-based propellants may provide improvements in the response to fast and slow cookoff, as compared with the typical responses of high-energy HTPB propellants. However, full compliance with IM requirements (i.e., burning reaction only) in thermal environments will likely require additional approaches, as discussed earlier in this paper. Likewise, meeting bullet/fragment impact requirements may also require further improvements, since butacene-based propellants are not expected to provide reduced responses to these threats.

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Information on the safety characteristics of butacene-based propellants is presented in Table 3.

Table 3: Safety and Hazards Characteristics of Butacene-based Propellants

Propellant: Total Solids Loading, % 85 86 88

Burn Rate @ ambient pressure, mm/s 4.2 3.5 2.2

Self-ignition temperature, ºC 213 215 247

Friction Sensitivity, N 90 60 110

Cookoff (SNPE test 41) Critical temperature, ºC Reaction type

160 Combustion

170 Combustion

172 Combustion

SCO (SNPE test 85) Temperature of reaction, ºC Reaction type

181 Combustion

178 Combustion

185 Combustion

SCO (5 kg model, 3 mm steel confinement) Type II

FCO (5 kg model, 3 mm steel confinement) Type IV or V

4) Insensitive XLDB, “Nitramites” [8,9] (SNPE, CELERG) SNPE (and later CELERG) have developed a family of energetic low-signature cross-linked doublebase propellants known as nitramites. These formulations are based on an energetic binder (hydroxyl-terminated polyester or polyether plasticized with NG), filled with a nitramine such as RDX or HMX. The minimum smoke varieties contain no AP. Since nitramites use such highly energetic ingredients, they have been very sensitive to shock threats, specifically exhibiting XDT or “bore effect” responses in center-perforated rocket motor configurations. To combat this sensitivity to shock, SNPE formulated nitramite variants containing lower amounts of nitramine filler, lower sensitivity energetic plasticizers (TMETN and BTTN instead of NG), and, for reduced smoke versions, AN in place of some of the nitramine. Gains in insensitivity were accompanied by losses in specific impulse on the order of 1 to 2%. Although these formulations exhibit good responses to thermal stimuli, and are designed to prevent bore effect responses to bullet impact, they are still vulnerable to sympathetic detonation due to their significant nitramine content.

Reduced Nitramine Level o AN Used to Replace a

Portion of Nitramines Less-Sensitive Plasticizers

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In a demonstration program for a hypervelocity missile concept, CELERG chose a propellant, designated C1767, from the SNPE family of insensitive nitramites. Using this propellant, two 140 mm diameter, composite cased mock-ups were subjected to IM testing. A fast cookoff test, conducted in accordance with STANAG 4240, resulted in a burning reaction with no projection of fragments (see Figure 6). A bullet impact test, conducted in accordance with STANAG 4241, resulted in no reaction. Both results were considered Type V. The result of the bullet impact test is presented in Figure 7.

Figure 6: Nitramite Fast Cookoff – Burning Response.

Figure 7: Nitramite Bullet Impact – No Reaction.

5) HTCE/Polyether [10,11,12] (Pratt & Whitney Space Propulsion – CSD, NAWCWPNS China Lake)

CSD and NAWC, China Lake, have formulated and tested several aluminized and reduced smoke propellants based on the polymer known as HTCE (a block copolymer of polytetrahydrofuran and polycaprolactone), a lower cost alternative to HTPE. The approaches used to reduce vulnerability to IM threats are the same as those previously mentioned for HTPE propellants, with the added effect of a reduced level of AP solubility in the binder, which is thought to decrease propellant sensitivity.

The basic formulation of CSD’s UTP-32,070 aluminized propellant is presented in Table 4. This propellant and a comparable HTPB-based formulation were loaded into 8-inch


Reduced Level of AP o Energy Partitioned by

Using 2nd Oxidizer (AN) Good Elongation Properties

Reduced AP Solubility in Binder

diameter graphite composite cases and subjected to IM testing as part of an investigation of propellant alternatives for a US Navy missile propulsion development program. The IM test results for both propellants, described in Table 5, illustrate the IM advantages gained by the use of the HTCE propellant, particularly in the slow cookoff response. The HTCE propellant was subsequently chosen for further development and testing in a tactical motor configuration.

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Table 4: UTP-32,070 Formulation

Ingredient Weight % Polymer, polyether/polyester Cross-linker Plasticizer, mixed Bonding agent Cure catalyst Isocyanate

16.0

Oxidizer, AP Fuel, Aluminum

84.0

Table 5: UTP-32,070 IM Test Results

*The pass/fail results were officially determined by the Ordnance Hazards Evaluation Board, NAWCWPNS, China Lake.

Burn only no overpressure, no lethal debris outside test area: 377°F cook-off

Burn only, no over-pressure, no debris outside test area

Burn only, no over-pressure, no lethal debris outside test area

Burn only, moderate initial reaction with no overpressure

Polyether/polyester UTP-32,070

2.8 psi overpressure, lethal debris outside test area: 400°F cookoff, partial detonation

No overpressure, no lethal debris outside test area, burning

No overpressure, no lethal debris outside test area, moderate reaction

No overpressure, no lethal debris outside test area, moderate reactionHTPB UTP-33,000

Slow Cookoff*Fast Cookoff*Fragment Impact*Bullet Impact*

IM TestsPropellant

*The pass/fail results were officially determined by the Ordnance Hazards Evaluation Board, NAWCWPNS, China Lake.

Burn only no overpressure, no lethal debris outside test area: 377°F cook-off

Burn only, no over-pressure, no debris outside test area

Burn only, no over-pressure, no lethal debris outside test area

Burn only, moderate initial reaction with no overpressure

Polyether/polyester UTP-32,070

2.8 psi overpressure, lethal debris outside test area: 400°F cookoff, partial detonation

No overpressure, no lethal debris outside test area, burning

No overpressure, no lethal debris outside test area, moderate reaction

No overpressure, no lethal debris outside test area, moderate reactionHTPB UTP-33,000

Slow Cookoff*Fast Cookoff*Fragment Impact*Bullet Impact*

IM TestsPropellant

PASS PASS PASS

PASS PASS PASS PASS

PASS PASS PASS

PASS PASS PASS PASS

FAILFAIL

The NAWCWPNS version of the HTCE propellant, a reduced smoke formulation designated IMAD-116, uses 10% ammonium nitrate (AN) as a secondary oxidizer. China Lake reported some problems with low temperature strain capability, after subjecting the propellant to arctic cycling. To solve this problem, some of the HTCE polymer was replaced with a polyether. The resulting propellant exhibited excellent processing and mechanical properties, and met both safety and performance requirements. NOL card gap testing showed IMAD-116 to be a zero card propellant. IM testing has not yet been conducted.

Table 6: IMAD-116 Formulation

Ingredient Weight % HTCE/BuNENA or TMETN (1:2) 18 Ammonium Nitrate 10 Ammonium Perchlorate (tri-modal) 71 Curative and catalyst 1

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6) ARC-9131 [13] (Atlantic Research Corporation) The Atlantic Research Corporation (ARC) has reported the development of a slightly aluminized (5% Al), 65% nitramine filled, high-energy, low-smoke propellant based on a PEG binder plasticized with mixed nitrate esters. The partitioning of the energy between the solid and liquid ingredients seems to have produced a relatively insensitive formulation, based on tests conducted by ARC. EIDS substance tests produced no reaction to bullet impact, extinguishment in fragment impact and fast cookoff, and a rupturing of the test pipe and subsequent extinguishment in slow cookoff. IM tests were also conducted using two system demonstrators: one using a thick steel case, and one using a thin steel case. Table 7 summarizes the IM test results for both configurations. This formulation is currently being used by ARC as an insensitive explosive fill, highlighting the trend in energetic materials toward common characteristics between propellants and high explosives.


Nitramine Particle Size Distribution

Extinguishable

Less-Sensitive Plasticizers o Replaced NG with

TMETN / TEGDN

Table 7: IM Test Results for ARC Propellant

IM Test Thick Steel Case Thin Steel Case Slow Cookoff TBD Type V Fast Cookoff Type V Type V Bullet Impact Type V Type V Fragment Impact TBD Type V Sympathetic Detonation Pass (No Detonation) Pass (No Detonation)

7) EDB [8,9] (SNPE & CELERG) SNPE and CELERG have demonstrated two reduced sensitivity extruded double-base (EDB) propellants for an air-to-ground application. One of the formulations uses lead salts to modify ballistic behavior (SD 1175) and the other, designated SD 1178, uses a lead-free modifier. The approach to reducing IM vulnerability was to replace the NG commonly used in EDB propellants (such as NOSIH AA2) with low-sensitivity nitrate ester plasticizers. The nitrate esters used were trimethylolethane trinitrate (TMETN) and triethyleneglycoldinitrate (TEGDN). The development goals were to improve IM-ness while reproducing the performance of conventional US propellants (e.g., NOSIH AA2). Table 8 offers a comparison between AA2 and the two SNPE propellants, SD 1175 and 1178.

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Less-Sensitive Plasticizers o Replaced NG with

TMETN / TEGDN


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Table 8: Extruded Double-base Formulations

Weight % Ingredient

NOSIH AA2 SD 1175 SD 1178 NC + NG 89.6 - - NC + TMETN - 92.7 - NC + TMETN/TEGDN - - 94.4 Stabilizer 2.0 1.8 1.5 Plasticizer 4.3 1.8 - LC 12-15 4.0 - - Salt no. 1 - 1.8 - Lead free modifier - 1.8 3.0 Additive (for combustion stability) - 1.8 1.0

Wax 0.1 0.1 0.1

CELERG has conducted a series of IM tests on SD 1175 and SD 1178 loaded into aluminum, composite and steel strip laminate cases. Test items loaded with NOSIH AA2 and a CELERG NG-based reference propellant (SD Reference) were also tested, for comparison. The results of the IM test series, presented in Table 9, demonstrate the relative insensitivity of these propellants. No detonations were observed, and no propagation occurred in the sympathetic detonation trials. Only slow cookoff produced a violent response.

Table 9: IM Test Results for SD 1175 & SD 1178, with SD Reference for Comparison

Configuration IM Test Propellant Case FCO SCO BI SD

AA2 Aluminum Burning Explosion Burning No PropagationSD Reference Aluminum Burning Burning No Reaction Propagation SD Reference Strip Laminate Burning No Reaction Propagation SD 1175 Aluminum Burning Burning No Reaction No PropagationSD 1175 Composite Burning Explosion No Reaction No PropagationSD 1175 Strip Laminate Burning No PropagationSD 1178 Aluminum Burning Burning (violent) Burning No Propagation

WHAT’S NEXT

In the previous section, a few examples of reduced-sensitivity propellants were presented to indicate the level of progress that has been made by the IM community in their efforts to develop IM propellants. Although great progress has been made, solutions are still sought in a number of areas. The following section of this paper is an overview of the ingredients and formulations currently under investigation as possible solutions.

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Oxidizers There are three main compounds that are currently being investigated as future oxidizers by the propellant community. These are ADN, HNF and CL-20. All of these oxidizers could be used for low signature propellants. Unlike AP, none contain chlorine and so are also desirable from an environmental point of view.

1) ADN [14-17] ADN is showing promise as a replacement oxidizer for AP. It has a high excess of oxygen, but has a lower density than the other two oxidizers mentioned below. It has a high burn rate and can be used for high specific impulse propellant applications. It is also manufactured from inexpensive starting materials, so it is likely to be attractive from a financial point of view. ADN is produced in prilled form

Heat of formation = –148 kJ/mol Density =1.818 g cm-3 Melting pt ≈92°C Auto Ignition T. = 160°C %CO + O2 = 25.8

NH4NNO2

NO2

to improve thermal stability (0.3% hexamine added as a stabilizer). From a safety and IM point of view, it is less sensitive to impact than RDX, HMX and CL-20, and is not sensitive to friction or electrostatic discharge.

There are some problems that have been reported with this material. For example, it is light sensitive. ADN is also very hygroscopic; however, this is resolved by encapsulation in a polymeric binder. ADN has some compatibility problems. It is reported to be compatible with HTPB but reacts with isocyanates, which are used extensively to cross-link (cure) polymeric binder systems, including HTPB. Reaction can be limited using less reactive isocyanates, and reaction ceases on completion of curing. The melting point is low and the density is less than that of the other two materials presented here. It is reported to be relatively shock sensitive (although a value is not reported).

2) HNF [18-21] HNF is a powerful solid oxidizer and hence is also a potential replacement oxidizer for AP. It has a high excess of oxygen and a higher density than ADN. The ballistic properties are good: replacing AP with HNF gives a 3-4% increase in performance. However, there are a number of problems that have been reported that need to be solved. The thermal stability above 60°C needs to be improved. It is also sensitive, and does not appear to be de-sensitized by the binder. The material tends to

Heat of formation = –72 kJ/mol Density =1.870-1.910 g cm-3 Melting pt ≈115°C DSC onset 121-129°C %CO + O2 = 21.8 O2N C

NO2

NO2

N2O5

form needle-like crystals that are undesirable for processing into propellant. It is also highly reactive, and is incompatible with isocyanates, resulting in problems in curing polymer composites. Despite this, HNF/Al/HTPB mixtures have been made.

3) ε-Cl-20 [17,22,23] CL-20 is very interesting from a specific impulse point of view. It has a lower oxygen balance than the other two oxidizers, but has a higher density and heat of formation. The sensitivity properties of CL-20 are not good (it has similar impact sensitivity to PETN). However, it is desensitized when incorporated into formulations. Formulations are reported to have sufficient safety properties to be handled in large amounts.

Heat of formation = 372 kJ/mol Density = 2.04 g cm-3 Melting pt > 195°C %CO + O2 = 10.9

NNN N

N N

NO2NO2

NO2

NO2 NO2

NO2

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Oxidizers – Summary A comparison of the relative performance of the three oxidizers along with AP is given in Figure 8.[15] All the oxidizers compare well to AP. Of the three, ADN is likely to give the best IM properties. Compositions containing large amounts of CL-20 are likely to have a 1.1 hazard division, so they are unlikely to meet IM SD criteria. Unfortunately, HNF has the most problems but work is continuing to try to improve this product.

Binder PolyNIMMO / GAP Azide

Figure 8: Oxidizer Performance.

A Note on Nano-Aluminum Nano-aluminum (nano-sized particles) has received much attention recently because when it is used to replace conventional aluminum (micron sized particles), it gives significantly enhanced burn rates. Reports of increases in burn rates of 100% and above have been made.[24-27] Higher burn rates could be expected to result in more violent response from rocket motors when initiated by accidental or combat stimuli. However, this may not be the case. A recent study of polymer nano-composite materials concluded that mechanical properties improve when substituting conventional micrometric aluminum with nano-sized material in an Al/GAP formulation.[28] This could help reduce the violence of response, since good mechanical properties have been shown to reduce the likelihood of DDT. There were no observed changes to the thermal properties (glass transition temperature and decomposition temperature).

Energetic Polymeric Binders and Formulations Energetic binder systems containing both energetic polymers and plasticizers are being developed to meet future performance requirements. As mentioned earlier, they are also interesting as they can allow reduced solids loading. Table 10 presents some of the binders that are being developed for use in energetic materials, high explosives, gun propellants and rocket propellants.

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Table 10: Density and Heat of Formation of Selected Ingredients [23,29-31]

Ingredient ρ [g cm-3] ∆Hf [kJ/mol] Tg [°C] PolyAMMO 1.17 46 -40 PolyBAMO 1.30 440 GAP 1.29 138 -45 PolyNIMMO 1.31 -335 -30 to -35 (Di, Tri) PolyGlyn 1.47 -285 -35

These polymers can be used to produce cross-linked polymeric matrices (polyurethanes) using isocyanates. However, they can also be used to produce block co-polymers. These are not cross-linked and can be melted (i.e., thermoplastic elastomers, TPEs). They are attractive for use in energetic materials because they allow one to reprocess the propellant during production and give the possibility of easy recovery of the energetics at the end of life.

Some examples of developmental formulations are given in the table 11 below.[17,22,32]

Table 11: Developmental Propellant Formulations using various Binder Systems

Basic Formulation (binder, plasticizer(s), oxidizer/fuel)

Specific Impulse*[s]

Density Impulse [s g cm-3]

Burn rate [mm/s]

HTPB/AP/Al 264.5 464 9.0 (@7MPa) GAP/AP/Al 492 9.9 (@7MPa) GAP/AN/Al 261.5 463 GAP/AN/CL-20/Al 263.7 475 GAP/Cl-20/Al 273 521 GAP/ADN/Al 274.2 491 GAP/HNF/Al 272.6 492 PolyNIMMO/AP/Al 486 9.5 (@7MPa) PGA/TMETN/BTTN/RDX (63%) 238.5 405 GAP/TMETN/BTTN/RDX (60%) 242 411 14.6 (@7MPa) GAP/TMETN/BTTN/CL-20 (60%) 252 452 20 (@7MPa) HTPB Reduced smoke 247 421

*Theoretical

SNPE has also reported Nitramite formulations. These are reduced vulnerability formulations that gain this status through reducing nitramine filler content by using AN as the primary oxidizer. The energetic plasticizers TMETN and BTTN are used along with the energetic binder GAP to boost performance.[8] A combustion reaction was observed for one formulation in response to thermal threats, fast and slow cook-off. They also have a large critical diameter, which helps make them resistant to sympathetic detonation. These compositions have met MURAT*** requirements.

SNPE also reported GAP/TMETN/BTTN/RDX and GAP/TMETN/BTTN/CL-20 formulations (included in Table 11).[32] These are reported to give burning type reactions to slow cook-off at 118°C and 110°C respectively. They also gave satisfactory IM responses to bullet impact. The GAP/RDX combination gave improved burn rates and GAP/CL-20 gave a 7% higher volumetric impulse than HTPB propellant.

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ATK Thiokol has reported the use of block copolymers, TPE’s, consisting of polyAMMO soft block and polyBAMO hard block. A propellant containing 80% solids (65%AP and 15%Al) with 20% binder was reported to give favorable ballistic performance.[33] Since this publication, further work is reported on TPEs based on copolymers of polyBAMO with GAP and polyGlyn.[29] These two materials are more energetic and have higher burn rates. The GAP containing combination is reported to give the highest burn rates.

There are a number of energetic plasticizers that are available for use, some of which are included in the formulations above. The appropriate choice from among these plasticizers will be determined by a number of factors such as: sufficient plasticization effect, low migration rate, compatibility, processing, and hazard properties. The table below gives some properties of current plasticizers.[34-38]

Table 12: Plasticizer Properties

Density [g cm-3]

O2 Balance %

Melting Point [°C]

Drop WeightImpact [cm]

BTTN 1.52 -16.6 -27 4.7 TMETN 1.46 -34.5 -3 8.8 MEN42 1.4 BuNENA 1.2 -104.0 -27 to -28 BDNPA/F 1.39 -15 95.7, 98.4 DGTN 1.52 -18.5 K10 19.0, 19.6 TEGDN 1.39 -58 DEGDN 1.38 -40.8 2

BTTN is quite sensitive to drop weight impact and has been used in conjunction with TMETN to give more acceptable properties.[39] DGTN was reported recently[39] as a possible replacement for BTTN. It has improved safety-handling properties. In addition, it is also reported to give better propellant safety and mechanical property characteristics than BTTN.

CONCLUSION

Development of new rocket propellants is primarily driven by the need for greater performance. However, there are several other design drivers that must be addressed when developing new formulations, one of which is IM-ness. In this paper, NIMIC has discussed some of the current formulations and new materials that are being considered by the propulsion community. It has been shown that fielded and developmental formulations exist, utilizing currently available ingredients and design approaches, that can indeed allow the designer to reduce propellant sensitivity to IM hazards without compromising other system requirements.

However, it is crucial to recall that the propellant is only one part of the route to IM. As previously discussed, the propellant must be integrated into a system approach – only then will the designer be able to develop munitions possessing true IM-ness.

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REFERENCES

[1] Operational Hazards and Solution Trade-Offs; B. Stokes, NIMIC, Brussels, Belgium; Presented in Session 1 (Insensitive/Low Hazard Rocket Propellants) of the AIAA Solid Rocket Technical Committee Lecture Series, Reno, Nevada, USA, January 1994.

[2] Propellants to Meet Insensitive Munitions Requirements; T. Boggs, Naval Air Warfare Center Weapons Division, China Lake, CA, USA; AIAA Solid Rocket Technical Committee Lecture Series, Reno, Nevada, USA, January 1994.

[3] NIMIC: Custodian of the Hazard Assessment Protocol; M. Fisher, NIMIC, Brussels, Belgium; NIMIC Document # L-69, November 2000.

[4] Research on New Energetic Materials; R. Miller, Department of the Navy (USA), ONR; International Defense & Technologie, September 1994.

[5] High Density HTPE Propellants; T. Comfort and K. Hartman, Alliant Techsystems, Allegany Ballistics Laboratory, Rocket Center, WV, USA; NDIA Insensitive Munitions & Energetic Materials Technology Symposium, San Antonio, TX, USA, November 2000.

[6] IM Evaluation of the Evolved Sea Sparrow Missile Propulsion Section; K. Fossumstuen & G. Raudsandmoen, NAMMO Raufoss AS, Raufoss, Norway; K. Hartman & T. Comfort, Alliant Techsystems, Allegany Ballistics Laboratory, Rocket Center, WV, USA; 1999.

[7] Insensitive Munitions Technology for Small Rocket Motors; K. Hartman, Alliant Techsystems, Allegany Ballistics Laboratory, Rocket Center, WV, USA; RTO Symposium on Small Rocket Motors and Gas Generators for Land, Sea and Air Launched Weapons, Corfu, Greece, April 1999.

[8] Energetic Insensitive Propellants for Solid and Ducted Rockets; G. Doriath, SNPE-CRB, Vert-le-Petit, France; AIAA Journal of Propulsion and Power, Vol. II, No. 4, July-August 1995.

[9] Demonstrators for Insensitive Tactical Rocket Motors; R. Coléno et al, CELERG, Saint-Medard en Jalles, France; NDIA Insensitive Munitions & Energetic Materials Technology Symposium, Bordeaux, France, 2001.

[10] Insensitive Reduced Smoke Propellant with Low-Cost Binder; M. Chan, T. Bui, A. Turner, Naval Air Warfare Center Weapons Division, China Lake, CA, USA; NATO Research and Technology Agency (RTA) Applied Vehicle Technology (AVT) Panel Meeting on Advances in Rocket Performance Life and Disposal, Aalborg, Denmark, September 2002.

[11] Propellant Development for Insensitive Munitions: IM Testing; H. Weyland and M. Jones, Pratt & Whitney Space Propulsion, Chemical Systems Division, San Jose, CA, USA; NDIA Insensitive Munitions & Energetic Materials Technology Symposium, Bordeaux, France, 2001.

[12] Low Cost Binder for IM Applications; D. Tzeng and M. Jones, Pratt & Whitney Space Propulsion, Chemical Systems Division, San Jose, CA, USA; NDIA Insensitive Munitions & Energetic Materials Technology Symposium, Cleveland, OH, USA, 1998.

[13] Private communication between M. Fisher (NIMIC) and K. Graham (ARC), August 2002.

[14] A Cooperative Effort to Develop Manufacturing Processes for Spherical ADN; Sjoberg, P.; Wardle, R.; Highsmith, T.; NDIA/IMEMTS 2001 Insensitive Munitions & Energetic Materials Technology Symposium Bordeaux, France, October 8-11, 2001.

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[15] NEXPLO BOFORS Dinitramide Datasheet.

[16] Energetics Research at FOI, Sweden; Janzon, B.; Lamnevik, C.; Lamnevik, S.; Östmark, H.; NDIA/IMEMTS 2001 Insensitive Munitions & Energetic Materials Technology Symposium Bordeaux, France, October 8-11, 2001.

[17] New Solid Propellants Formulations and Production Processes. Nouvelles compositions de propergols solides et procédés de production.; Doriath, G.; d’Andrea, B.; Presented at the 5th Symposium International, 22-24 May, 1996, Carré des Sciences, Paris – La Propulsion dans les Transports Spatiaux; Propulsion in Space Transportation.

[18] Aerospace Propulsion Products, APP; HNF Datasheet.

[19] The Effect of Different Crystallisation Techniques on Morphology and Stability of HNF; Veltmans, W.H.M.; van der Heijden, A.E.D.M.; Bellerby, J.M.; Rodgers, M.I.; ICT Energetic Materials-Analysis, Diagnostics and Testing, 31st International Annual Conference of ICT, June 27-June 30, 2000, Karlsruhe, Federal Republic of Germany.

[20] Improvement of Hydrazinium Nitroformate Product Characteristics; Rodgers, M.J.; van der Heijden, A.E.D.M.; Geertman, R.M.; Veltmans, W.H.M.; ICT Energetic Materials – Modelling of Phenomena, Experimental Characterization, Environmental Engineering, 30th International Annual Conference of ICT June 29-July 2, 1999, Karlsruhe, Federal Republic of Germany.

[21] Ballistic Properties of HNF/Al/HTPB Based Propellants; van der Heijden, A.E.D.M.; Keizers, H.L.J.; van Vliet, L.D.; van Zelst, M.; Groenewegen, W.; Lillo, F.; Marcelli, G.; ICT Energetic Materials – Ignition, Combustion and Detonation, 32nd International Annual Conference of ICT, July 3-July 6, 2001, Karlsruhe, Federal Republic of Germany.

[22] New Energetic Molecules and Their Applications in Energetic Materials; Nouvelles molécules énergétiques et leurs applications dans les matériaux énergétiques.; M. Golfier, H. Graindorge, Y. Longevialle, H. Mace; ICT Energetic Materials-Production, Processing and Characterization. 29th International Annual Conference of ICT, June 30-July 2, 1998, Karlsruhe, Federal Republic of Germany.

[23] Development Prospects of Solid Propellant Motor Systems for Future Missiles; Tréhu, O.; Besser, H.-L.; Bayern-Chemie, Aschau am Inn, Germany; Protac, La Ferté – St. Aubin, France.

[24] Burn Rate Studies of Composite Propellants Containing Ultra-fine Metals; Lessard, P.; Beaupré, F.; Brousseau, P.; ICT Energetic Materials – Ignition, Combustion and Detonation, 32nd International Annual Conference of ICT, July 3-July 6, 2001, Karlsruhe, Federal Republic of Germany.

[25] ‘Activated’ Aluminum as a Stored Energy Source for Propellants; Ivanov, G.V.; Tepper, F.; The fourth International Symposium on Special Topics in Chemical Propulsion, May 27-31, 1996 – Stockholm, Sweden, – Challenges in Combustion and Propellants – 100 years after Nobel.

[26] Comparative Studying the Combustion Behaviour of Composite Propellants Containing Ultra Fine Aluminium; Simonenko, V.N.; Zarko, V.E.; ICT Energetic Materials – Modelling of phenomena, Experimental characterization, Environmental engineering. 30th International Annual Conference of ICT, June 29-July 2, 1999, Karlsruhe, Federal Republic of Germany.

[27] Propellant Burning Rate Enhancement and Thermal Behaviour of Ultra-fine Aluminum Powders (Alex); Accentuation de la vitesse de combustion de propergols et du comportement thermique de

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poudres d’aluminium ultra-fines (ALEX); M.M. Mench, C.L. Yeh, K.K. Kuo; ICT Energetic Materials-Production, Processing and Characterization, 29th International Annual Conference of ICT, June 30-July 2, 1998, Karlsruhe, Federal Republic of Germany.

[28] Polymer Nanocomposites: ETPE/Alex®; Diaz, E.; Brousseau, P.; Ampleman, G.; Prud’homme, R.E.; 29th International Pyrotechnics Seminar in Westminster, Co, USA held from 14-19 July 2002.

[29] The Synthesis and Combustion of High Energy Thermoplastic Elastomer Binders; Braithwaite, P.; Edwards, W.; Sanderson, A.J.; Wardle, R.B.; ICT Energetic Materials – Ignition, Combustion and Detonation, 32nd International Annual Conference of ICT, July 3-July 6, 2001, Karlsruhe, Federal Republic of Germany.

[30] Novel Energetic Polymers Prepared Using Dinitrogen Pentoxide Chemistry.; Arber, A.; Bagg, G.; Colclough, E.; Desal, H.; Millar, R.; Paul, N.; Salter, D.; Stewart, M.; Technology of Polymer Compounds and Energetic Materials – 21st International Conference of ICT 1990 – Fraunhofer- Institut fur Chemische Technologie (ICT) – July 3-July 6, 1990 – Congress Center, Stadthalle, Weinbrenner-Saal, Karlsruhe, Federal Republic of Germany.

[31] Novel Energetic Monomers and Polymers Prepared Using Dinitrogen Pentoxide Chemistry; Colclough, E.; Golding, P.; Millar, R.; Norman, P.; Stewart, M.; ADPA Symposium on Compatibility and Processing, Cavalier Resort Hotels, Virginia Beach, Virginia, USA, October 23-25, 1989, Page 235.

[32] The Use of New Molecules in High Performance Energetic Materials; Longevialle, Y.; Golfer, M.; Graindorge, H.; Jacob, G.; NDIA, Insensitive Munitions & Energetic Materials Technology Symposium Proceedings, November 29-December 2, 1999, Tampa, FL, Event 055.

[33] Poly (BAMO/AMMO) TPE as a Solid Propellant Binder; R. Scott Hamilton, Robert B. Wardle, Louis F. Cannizzo, Wayne W. Edwards; 1996 JANNAF Propulsion Meeting, Volume 1, Albuquerque, New Mexico, 9-12 December 1996.

[34] NENAs – New Energetic Plasticizers; Simmons, R.L.; From : Proceedings of the 1994 NIMIC Workshop held at the NATO HQ, Brussels, Belgium, June 22-June 24 1994.

[35] The NIMIC Coordinated Characterisation Programme: A Program and Data for the Characterisation of New Ingredients for Energetic Material.; Sanderson, A.J.; NIMIC Document L-06.

[36] Explosives; Meyer, R.; 4th revised and extended edition, Published by VCH.

[37] BDNPA/BDNPF, Aerojet Material Safety Data Sheet 7/86.

[38] Plasticised PolyGLYN Binders for Composite Energetic Materials.; Cliff, M.D.; Cunliffe, A.V.; ICT Energetic Materials – Modelling of Phenomena, Experimental Characterization, Environmental Engineering. 30th International Annual Conference of ICT, June 29-July 2, 1999, Karlsruhe, Federal Republic of Germany.

[39] Development of Very Energetic Binder System Ingredients (PGN and DGTN); Sanderson, A.J.; Cannizzo, L.F.; Hajik, R.M.; Highsmith, T.K.; Martins, L.J.; ICT Energetic Materials – Ignition, Combustion and Detonation, 32nd International Annual Conference of ICT, July 3-July 6, 2001, Karlsruhe, Federal Republic of Germany.

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NOMENCLATURE

ADN Ammonium dinitramide AN Ammonium nitrate AP Ammonium perchlorate ARC Atlantic Research Corporation BAMO/AMMO Bis-azidomethyloxetane/Azidomethyl-methyloxetane copolymer BI Bullet impact Bi2O3 Bismuth trioxide BTTN Butanitriol trinitrate BuNENA Butyl nitroxyethyl nitramine CL-20 (HNIW) Hexanitroisowurtzitane CSD Chemical Systems Division of Pratt & Whitney Space Propulsion DDT Deflagration to Detonation Transition DGTN Diglycerol tetranitrate EDB Extruded doublebase EIDS Extremely Insensitive Detonating Substances ESSM Evolved Sea Sparrow Missile FCO Fast cookoff FI Fragment impact GAP Glycidyl azide polymer HMX Cyclotetramethylenetetranitramine HNF Hydrazinium nitroformate HTCE Copolymer of polytetrahydrofuran and polycaprolactone HTPB Hydroxy-terminated polybutadiene HTPE Hydroxy-terminated polyether IM Insensitive Munitions IMAD Insensitive Munitions Advanced Development Program ISP Specific impulse NAWCWPNS Naval Air Warfare Center Weapons Division NC Nitrocellulose NG Nitroglycerine NIMIC NATO Insensitive Munitions Information Center PEG Polyethylene glycol PGA Polyglycol adipate PolyAMMO Poly azidomethyl-methyloxetane PolyBAMO Poly bis-azidomethyloxetane Polyglyn Polyglycidyl nitrate PolyNIMMO Polynitrato methyl methyloxetane RDX Cylcotrimethylenetrinitramine SCO Slow cookoff SD Sympathetic detonation (reaction of adjacent munitions) TEGDN Triethylene glycol dinitrate Tg Glass transition temperature TMETN Trimethylolethane trinitrate TPE Thermoplastic elastomer XDT “X” to Detonation Transition (delayed transition) XLDB Crosslinked doublebase

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