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UK Propellants Research and Future Needs

Dr Adam S. Cumming Dstl

Fort Halstead Sevenoaks

Kent TN14 7BP UNITED KINGDOM

Solid propellants began to undergo a revolution in the early eighties when new materials began to be investigated. These offered the chance of meeting vulnerability, signature and performance goals, and the UK invested significant amounts of research money in studying the options. As the propulsion requirements continue to change it is time to review what the UK, within a world-wide context, has achieved, and to examine future priorities. This paper will outline the progress made by the UK overall, and consider where gaps remain if the technology is to meet present and future requirements.

INTRODUCTION

Within a military context, any research must meet a need or requirement. This can include responding to a perceived capability gap, but can also be a response to more complex requirements including non-military imperatives. As there is also a relatively long lead time between initial investigation and application, this is also an issue that must be addressed, and investigated as a consideration of the art of the possible at any given time.

The United Kingdom has, in common with other nations, spent a considerable amount of time in investigating such longer-term options in the broader field of energetic materials, and their specific application in rocket propellants amongst others. The changing world in which we find ourselves requires that we look at what has been achieved and what the current drivers are for application, as well as where research and applications may develop in the future.

While the prime driver remains performance, this can be expressed in different forms. It is common for propellant designers to use specific impulse etc as a measure, but a more effective measure is the actual performance in a given system. However, this is now only part of the story and a more complete list is given in Table 1 below. This describes the various ‘performance’ drivers that require to be addressed at present to meet present and possible future systems needs. It reflects the preoccupations of the NATO and other nations, and some are necessarily less urgent than others. Nevertheless they present a challenge to the formulator as well as the system engineer, modeller and synthetic chemist.

It is important to accept that these requirements will evolve to meet both new requirements as well as reflecting the changing possibilities offered by developing technology.

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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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Table 1: Performance Issues for Present and Future Propellants

Thrust The ability to provide the system delivery as specified.

Vulnerability The need to comply with Insensitive Munitions requirements.

Thrust control The ability to responds flexibility to mission demands.

Mass/thrust ratio The increasing need to get more out of less – i.e. fixed volume motors.

Signature Broadly includes both control and detectability issues including smoke, IR etc.

Initiation Effective and controllable ignition with minimum weight penalty.

Combustion Stability Control of combustion to ensure ease of performance and effective use.

Predictability The ability to predict both performance, including ballistics and vulnerability.

Characteristics Mechanical properties appropriate to use.

Lifetime Predictable and accurate lifetime, to meet actual service needs.

Disposability The ability to cost effectively dispose of systems at the end of life.

Environmental Impact Reduction of pollution in manufacturing, service, and disposal to meet legislation etc.

There are of course several methods of achieving these goals, and it is very difficult to achieve all. The priorities remain thrust, thrust control and vulnerability, for service use, though lifetime issues are and will remain critical since logistics rely on the ability to provide what is wanted, where it is wanted when it is needed. Some of these issues are encapsulated within the ‘Whole Life Cost’ concept. Unfortunately obtaining a satisfactory whole life cost is difficult, and options must be explored even with just qualitative data.

There are also several scientific issues to be addressed if some sensible choices are to be possible. It has been proposed that many of these issues can be addressed using novel materials – energetic binders and plasticisers; novel oxidisers and fillers, and these appear to offer designers options to meet future needs. However, it is essential to understand the strengths and limitations of such new materials if their use is to be properly assessed. Other options such as air breathing and hybrid systems exist and sensible comparisons need to be made if proper systems decision as are to be made.

The UK programme has included synthesis, mainly of energetic binders, characterisation of ingredients, formulation studies and some system assessments aimed at providing designers with potential solutions to needs. While some of this has been undertaken within what were government establishments, much of the work has been performed in partnership with industry and in collaboration with other nations. This has been described in detail elsewhere [1,2].

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INGREDIENTS AND COMPOSITIONS

The UK has developed commercial routes to energetic binders produced by dinitrogen pentoxide nitration. These offer potential for next generation explosives and propellant compositions, and as part of the option assessment the UK (both government and industry) has been conducting research into the application of the energetic binder PolyNIMMO to rocket propellants. This was later extended to include PolyGLYN. Collaboration with other nations provided a comparison with GAP and other materials.

The stability and compatibility of both PolyNIMMO and PolyGLYN have been reported elsewhere [3,4]. PolyNIMMO proved to be acceptable for use, and while initially PolyGLYN degraded rapidly, a structural modification was produced, enabling the ICI produced material to be used in formulation studies [4]. Once the decomposition route was identified, it was possible to devise a reaction to modify the end groups. This had the additional benefit of slightly increasing the functionality. The improvements in stability of the cured material are illustrated in Figure 1.

Figure 1: Improvement in PolyGLYN Stability.

The potential advantages of using PolyGLYN are illustrated in Figures 2 and 3 below. There are clear performance and loading benefits to be gained if these predictions are matched by reality.

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Figure 2: Specific Impulse of CL20 Propellants (calculated).

Figure 3: Impulse Density of CL20 propellants (calculated).

While these figures are encouraging, they need to be examined in the light of actual system performance and requirements, where the other issues listed in Table 1 must be considered. Since such compositions and materials are still at a relatively early stage of development, there are many questions that need to be answered.

One advantage that such materials have is a potential for the significant reduction in signature. Not only does this affect the environmental impact, it affects both control, and detection. There are therefore implications for both defence and offence in such systems if the predicted benefits are obtained. Figure 4 displays two predictions of plume signature, based on the know characteristics of both traditional propellants and using a standard motor for the sake of comparison. This is therefore merely an indication of the potential impact on signature, and will require to be demonstrated experimentally. Such trials are planned for the next year, and will hopefully confirm these benefits.

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Figure 4: Plume Signature of Traditional CDB vs. Plume Signature of Novel Propellant.

NEW MATERIALS

In addition to new organic fillers and binders, the technology for producing sub-micron or nanometric metal particle has been developed. Results published [3,4] recently suggest that these may or may not have significant effects on the combustion and hence performance of composite propellants. Many groups including those in the UK are examining this. One of the factors affecting the assessment is the variability in the materials available, and the UK is focusing on material produced within the UK.

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This material is produced by a plasma process and has been sufficiently well characterised to enable applications in propellants to be examined.

The size of the agglomerates ranged from less than 5 µm to more than 200 µm. The particle size is observed to be approximately 100 nm ± 50 nm, however the particles still appeared to be clumped together, indicating finer units.

The particle size is observed to be approximately 100 nm ± 50 nm.

The powders specific surface area = 20 – 30 m2g-1, as determined by gas adsorption (BET):

The Variation of Specific Surface Area of Idea Nanometric Aluminium Powder with Particle Size

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The composition of the material is ~65 wt% metal and 35 wt% oxide. XPS estimates an oxide layer <5nm in thickness. Metal and oxide Al peaks are noted.

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Weight Percentage of Oxide Versus Particle Diameter for Aluminium Nanopowder

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It is hoped that it will be possible to demonstrate whether these materials have a real role to play in future composite propellants, but any decision is as yet premature. These considerations will have to address both current and future requirements, including boost phase motors for the other propulsion systems that are being investigated.

MECHANICAL PROPERTIES AND OTHER CHARACTERISTICS

While it is clear that these compositions offer performance benefits, there are other issues to be addressed if they are to be considered for service applications. The need to use plasticisers in considerable quantity means that there are unresolved questions over future use. Work is in hand to attempt to address this, and much of that is being undertaken within the WEAG forum [1].

As an example, Tables 2 and 3 describe two compositions based on UK binders and Ammonium Nitrate. The work was moderately successful, but the figures indicate some of the issues yet to be addressed.

Table 2: Propellant Formulations

AN21 AN28

SCAN 50 50

RDX 20 20

MEN42 20 20

PolyNIMMO (trifunc) 7.4 –

PolyGLYN – 7.4

N100 2.6 2.6

DBDTL +15ppm +15ppm

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Samples of the AN28 composition were subjected to impact, friction, vacuum stability, burn rate and tensile tests. The results obtained showed no significant differences between the two binders although the density of the PolyGLYN based material is higher which is important in respect of the weight of material it is possible to get into any given system. The results obtained are detailed in Table 3.

Table 3: Comparative Properties of AN21 and AN28

AN21 AN28

BAM Impact J 14 24

BAM Friction N >360 >352

Vacuum Stability (40 hours at 80ºC) ml/g 0.06 0.16

Vacuum Stability (40 hours at 100ºC) ml/g 0.92 0.87

Ignition temperature ºC 197 181.8

Density ml/g 1.59 1.67

Burn rate (Parr Bomb) at 80 bar, mm/s 4.2 4.1

n value 0.71 0.64

Stress at Max Load, MPa, Ambient 0.29 0.18

Strain at Max Load, %, Ambient 6.33 6.2

Stress Modulus, MPa, Ambient 6.45 3.4

Stress at Max Load, MPa, 60ºC 0.22 0.14

Strain at Max Load, %, 60ºC 6.55 6.8

Stress modulus, MPa, 60ºC 4.85 2.5

It is only if such materials can meet performance requirements, that investment in improving other characteristics is justified. This requires both prediction and experimental proof. This is being obtained, and the work on refining the intrinsic properties is underway.

INSENSITIVE MUNITIONS

Within the UK the signature of STANAG 4439 on Insensitive Munitions has lead to the implementation of an IM Policy. This is stated as follows:

The vulnerability of the munitions in the MOD inventory will be reduced over time to meet the requirements of STANAG 4439 (Policy for Development and Assessment of Insensitive Munitions).

All new munitions requirements are to stipulate compliance with the criteria for IM set out in STANAG 4439.

All legacy munitions are to be kept under review to identify opportunities to achieve IM compliance at Mid Life Update, refurbishment or re-provisioning.

In both instances formal dispensations at Two Star Level is required for any non-compliance, either in the requirement definition or in the procurement solution.

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At present IM compliance is a major issue for all future systems, and while it remains a systems issue, being approached using packaging, mitigation and motor construction, the use of appropriate propellant solutions plays a significant part in the systems integration studies. Any new propellant must offer an IM benefit to be considered seriously, or should be capable of contributing to an IM solution. Such characteristics must not limit or damage the ability to undertake the mission.

It is clear that any propellant system must be properly characterised through diagnostic tests, if possible, to permit modelling and prediction of both performance and response to threats. Only in this way can an assessment of the system be provided, to enable its acceptance for service use.

LIFETIME AND DISPOSAL

Finally, any system must meet not just the planned lifetime but should also have it life understood sufficiently to allow measurement and prediction to be undertaken dynamically. That requires a clear understanding of the probable failure modes of any system; an understanding of how it will respond to its environment, and how changes to that environment will affect the life and probability of failure. Dynamic measurement of temperature, humidity and shock/vibration are required, and these measurements need to be related to sufficient understanding of the system to enable an adequate prediction to be made.

Failure may or may not be due to the propellant, but an understanding of the characteristics of the propellant is required if proper, assessment of ageing and lifetime is to be made. The UK has undertaken significant work on the assessment of failure [5], and developed tools for assessing mechanical degradation, within rocket motors.

Knowledge of the nature, characteristics and hazards associated with any propellant as essential if effective disposal is to be undertaken. Without this knowledge design of disposal methods is difficult, and the associated risks are higher. The UK is developing a ‘Design for Disposal’ philosophy which attempts to ensure that an acceptable disposal route is devised for any munition entering service. This was addressed in OB Proceeding P115, and forms part of the UK input to STANAG 4518. If it is employed successfully and properly it affects the choice of materials and the way in which they are used, and is therefore part of a proper systems approach to munitions.

CONCLUSIONS

In recent years the UK has attempted to examine the direction of its propellant research; to assess the benefits and application opportunities and to define the issues affecting how the research can be carried out.

The programme is now directed towards desired outcomes, which match systems needs, and the issues associated with these are being identified. New requirements and developments are modifying this approach which must be an iterative exercise. However, if the developing approach is successful, then it should be sufficiently flexible to respond to changes. It will take into account novel materials and approaches, and provide a means of assessing these effectively, and will include the acceptance that the needs of interoperability require that there must be a significant amount of collaborative research if true benefits are to be realised.

The new materials produced by the UK programme have the potential to meet future systems needs, but that requires to be proven in a way that convinces the systems engineers, and this remains a challenge for not just the UK but for the entire community.

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ACKNOWLEDGEMENTS

This paper would not be possible without the support of many people. In particular, QinetiQ, Tetronics, Nobel Enterprises, Royal Ordnance Rocket Motors Division, and the members of WEAG CEPA 14 all deserve thanks.

REFERENCES

[1] A European Co-Operative Programme for New Rocket Propellants; Y. LONGEVIALLE, SNPE Propulsion, France NATO AVT Workshop, Aalborg, Sept 2002.

[2] Chemical Problems of Next Generations Propellants; D. WAGSTAFF, Royal Ordnance, UK; NATO AVT Workshop, Aalborg, Sept 2002.

[3] Nano-Particles in Aluminium Combustion; I.G. ASSOVSKIY, Semenov Institute of Chemical Physics, Moscow; Proc 29th IPS, Westminster CO, 2002, p 219.

[4] Heterogeneous Combustion with Nanomaterials; A.N. PIVKINA et al, Semenov Institute of Chemical Physics, Moscow; Proc 29th IPS, Westminster, CO, 2002, p97.

[5] Evaluation of Mechanical Degradation in Rocket Motors using Embedded Stress Sensors; J. THEOBALD, D. TOD, QinetiQ Ltd, UK, Proc 33rd ICT Conference, Karlsruhe, 2002, V31.

British Crown Copyright 2002

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SYMPOSIA DISCUSSION – PAPER NO: 5

Discusser’s Name: J. Docherty

Question: How much of the information from research and modeling will be made available to industry to prevent repetition of research on topics such as disposal etc.?

Author’s Name: Adam Cumming

Author’s Response: The information will be available to UK Integration Program Teams (IPTs) and contractors as far as possible. Much of the work was carried out in collaboration with other parties including UK industry.

Discusser’s Name: Luigi DeLuca

Question: A very brilliant talk! Are you investigating nano ingredients other than Aluminum?

Author’s Name: Adam Cumming

Author’s Response: Yes we do. We plan to publish a report in due course.

Discusser’s Name: Ron Derr

Question: Table 1 of your paper lists issues for present and future propellants. I was surprised that cost was not included as a primary issue in the UK program.

Author’s Name: Adam Cumming

Author’s Response: You are correct. Cost is a major issue, so much so that I is taken as a given. It ought to be included! Cost needs to be considered as a part of whole life costing which includes purchase and is offset by other benefits accruing from life precision and other issues, at least in theory.

Discusser’s Name: Klaus Menke

Question: What do you think are the most likely propellant systems to be applied in future? In what field of applications are the greatest benefits to be expected?

Author’s Name: Adam Cumming

Response: From the UK viewpoint, we will look for tactical systems that have a family of ingredients, which may be employed with energetic binders and fillers, and new grades of these. However, a “killer” application is required where these are needed: 1) New grades of materials of nano-materials in existing systems and 2) reduced cost and new materials.

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