Propellants for rockets and guns

Fraunhofer ICT's core competences in the field of rocket propellants lie in formulation and production technology and the development of the property spectrum of solid propellants. Emphasis is placed on the development of low-signature, environmentally friendly composite propellants with a higher thermodynamic performance, to replace smoke-intensive AI/AP/HTPB propellants, and on low-signature, environmentally friendly composite propellants with lower detonation values as alternatives to double-base propellants. Developments so far include:

• High-performance, low-noise composite propellants for underwater propulsion
• Solid propellants containing ammonium nitrate, for booster and sustainer systems with high power density, low signature and low sensitivity
• Rapidly combusting propellants with high performance and low signature for rapid-acceleration missiles with significantly reduced jet plume signature
• Composite propellants with high performance and combustion speed for compact, high-performance engines, to increase the impact energy and penetration capacity of rockets and gun-launched projectiles

Different types of propellants and their properties

• ADN composite propellants with low smoke gas specification (AGARD: AA) and spectral radiation emission
• Standard AP composite propellants with HTPB, polyether and polyester urethane and GAP binders
• AN propellants with higher thermodynamic energy, lower sensitivity and low signature according to AA classification
• AP/CL20 propellants with higher energy density, higher combustion rates and lower jet plume signature according to AB classification
• AP/GAP propellants with combustion speeds above 100 mm/s at 130 bar

The rheological properties of gels fall between those of liquids and and those of solids. Gel propellants combine the advantages of solids, for example in terms of storage, with the dosability of liquids on introduction into the combustion chamber.

Gel propellants enable the construction of rocket engines with variable thrust strength, thrust control and thrust interruption combined with low signature, high specific performance and low sensitivity and vulnerability. Gel propellants are solid under normal environmental conditions. Under pressure and shear stress they become fluid, and their transfer from the tank into the combustion chamber enables controlled throughput and thrust.

Advantages and applications

Gel propellants offer particular advantages in applications where variable thrust strength and flexible operation at an extended range are needed, for example for missiles that fly over a battlefield slowly to identify the target, steer towards it and then destroy it with an amplified thrust phase on approach. The separation of the fuel and the oxidant, and the gel-like consistency of both substances, ensures good insensitivity and relatively safe handling. The performance, i.e. the specific impulse of gelled diergol fuel oxidant systems, is often higher than the performance of equivalent solid propellants. Both organic gel-forming agents and inorganic gelators can be used for gelation.

Modern gun propellants must fulfill a variety of different specifications. Besides the long-term availability of the raw materials and their compliance with the REACH regulation, it must be possible to deploy them safely and reliably over their whole military lifetime in different climatic conditions. Additionally, modern application scenarios are characterized by a strongly enhanced threat. The low sensitivity of the gun propellants is essential in the event of external attacks. The goal of our research activities is the development of new, improved propellant systems for a variety of applications. Our focus is on high-performance propellants with low sensitivity, temperature-independent combustion behavior, low erosivity, good long-term storage stability and preferably no material embrittlement at low temperatures, but also applicability in a wide caliber range.

Classic gun propellants based on nitrocellulose

Several mixers and kneaders of different sizes are available to homogenize the raw materials. Processing to create propellant strands is carried out using an extrusion press and extrusion manufacturing. Extrusion presses with different dimensions are available. Using the shear roll milling process, manufacturing can be carried out without a solvent.

In the development of novel gun propellants, special attention is paid to the selection of components and their processing, in order to fulfill increasingly stringent requirements. Using new energetic components such as plasticizers and crystalline energetic materials, the performance of propellant powders can be increased in a variety of ways. In the field of conventional gun propellants, formulations based on nitrocellulose and the energetic plasticizer DNDA-57 (DNDA: dinitro-diaza-pentane, -hexane and -heptane) have been developed for the mid-caliber range, which show temperature-independent combustion behavior and a low sensitivity. These propellants show a high performance and lower erosion during firing. The long-term storage stability of these propellant powders is significantly better than that of conventional formulations.A distinctive characteristic of these new powders is the almost temperature-independent combustion behavior. Unlike conventional powders, they show little or no increase in the combustion rate with rising environmental temperature.

Plastic-bonded propellants

All the propellant powders currently in use are based on nitrocellulose, which was discovered in 1846 and is synthesized from cellulose in a multi-stage process. However, naturally occurring raw materials such as cellulose have inconsistent properties depending on the climatic conditions where they are cultivated. Synthetic binders, by contrast, have the advantage of a consistent property spectrum. A promising approach to developing new, powerful gun propellants with low sensitivity is the application of thermoplastic elastomers (TPEs) as binders. These materials are composed of hard thermoplastic and soft elastomeric segments, and the combination enables specific properties. When external stress is applied the materials respond with a deformation, returning to their original form when the stress is removed.

Fraunhofer ICT covers the whole development spectrum for a new generation of plastic-bonded gun propellants. This includes the synthesis of suitable thermoplastic elastomers with energetic groups (ETPE), their combination with various plasticizer systems, formulation with energetic fillers, and powder manufacture and characterization. The energy density of the new plastic-based propellant formulations can be adapted by changing the quantities of the energetic components they contain, allowing these materials to be applied in a wide range of calibres. The new propellants also have a fIame temperature 600 K lower than that of conventional powders in the same performance category. A significantly decreased erosion of the barrel can therefore be expected. The ignition temperatures of about 200 °C are significantly higher than those of conventional propellants. ETPE propellant powders also show a good long-term stability. At Fraunhofer ICT the investigations to date have demonstrated the potential of a new generation of propellant powders based on energetic thermoplastic elastomers. Future activities in this field will focus on maturing this technology for real-life application.

Foamed propellants

Polymer foams filled with explosives have a very high burning rate due to their porous structure. By varying the polymer matrix, fillers and additives, foamed propellant charges with a wide range of material properties and performance data can be produced. Using the reaction injection molding process there is no restriction in geometry. These explosive-containing foamed propellants find application in the development of caseless ammunition or combustible cartridge cases, but also as container materials or as a protective layer with special properties. At Fraunhofer ICT reaction injection molding units have been constructed on a pilot scale and equipped for the processing of explosives. High-quality foamed charges filled with explosives can be manufactured with a high reproducibility.

New methods for the surface treatment of gun propellants

Particularly in the low- and mid-caliber range, gun propellants undergo surface treatment to modify their combustion behavior. Inert and energetic plasticizers and polymers are generally used. Conventional processes are time-consuming, or otherwise the surface coatings produced are relatively thick. Fraunhofer ICT is investigating new methods for the targeted surface treatment of propellant powders.

Fluidized bed coating enables coatings with a defined composition and thickness to be applied to the propellant grain. Subsequent layers can also be applied in a single process. The advantage of polymer coatings is that even on aging they don't migrate or permeate the propellant grain, affecting the combustion behavior.

Pressure impregnation of gun propellants

Pressure impregnation enables inert and energetic plasticizers with a defined penetration depth to permeate the propellant grain, in order to achieve particular interior ballistic effects. An autoclave is available for these investigations, which allows homogeneous pressurization of the material. The penetration depth can be varied, depending on the propellant type, the selected plasticizer and the conditions and duration of treatment.

Microstructure investigation in a given temperature range

Light microscopy enables investigation of the micromorphology of propellant powders or other raw materials. For this purpose a fully-automated rotational microtome is available to fabricate the necessary microtome sections, as well as a polarization microscope with incident and transmitted light sources for both bright- and dark-field analyses. The integrated digital camera provides high-resolution images and videos. A cooling and heating chamber enables investigations at temperatures between –196 and 420 °C, so the entire temperature range relevant for later application can be examined. Additional functions such as increased depth of sharpness mean that even uneven samples can be investigated.

Our offer

We develop tailored propellants and foamed propellants for your applications, and determine suitable manufacturing parameters and conditions. Customers benefit from our long-standing experience in the formulation of new energetic materials, from our detailed knowledge concerning the availability of new materials and their processing, and from a comprehensive characterization and quality assurance process adapted to the specific requirements of different propellants. This includes calculation of the performance data, oxygen balance and combustion temperature based on thermodynamic calculations, an evaluation of processability using rheological methods, adjustment of the pot life and curing cycles, the characterization of propellants through measurement of the web-size, pore and foam structure, characterization of the mechanical stability, evaluation of the embrittlement behavior, measurement of the chemical and thermal stability, investigation of the combustion behavior in a given temperature range, measurement of the sensitivity and investigation of the entire material property profile after targeted aging of the propellants.

In order to measure the service time of energetic materials, changes in the material properties are investigated and stabilization techniques are developed, for example intervention in the chemical reaction to prevent degradation processes in the material.

In order to investigate the behavior of energetic materials on aging, or the interaction of various energetic and non-energetic components, the changes in various material properties are observed over time. Analysis focuses for example on gas development, material decomposition or heat development resulting from chemical reactions. Based on these data, theoretical models are developed to increase understanding of the reactions on a molecular level. Stabilizers can then be used to adjust the processes on a chemical level, decelerating the decomposition.

Through an analysis of the chemical reactions and their kinetics, predictions can be made concerning known systems: for example the service life of a system under defined thermal stress (temperate climate, desert climate etc.). The measured values are modeled kinetically and then activation energies for the individual processes are determined. This enables calculation of the service life at constant temperatures and also temperature profiles (e.g. the so-called phoenix profile).

The service life incorporates all the desired properties of the system. Besides chemical lifetime (influenced for example by decomposition products that alter sensitivity to friction or impact), mechanical properties (e.g. fracture or stress-strain behavior), or physical properties (combustion speed) can also be determined by this method and incorporated into the service life prediction.

The REACH Regulation (Registration, Evaluation and Authorization of Chemicals) entered into force on 1 June 2007, as a pan-European chemicals legislation. In accordance with this regulation, substances raising particular concern are first placed on a candidate list (SVHC list, substances of very high concern) and may then, if additional selection criteria are met, be included in the list of substances subject to authorization (Annex XIV). After their sunset date, substances in Annex XIV require a temporary and cost-intensive authorization before they can be produced, placed on the market or used in the EU. For example phthalates, chrome(VI)- and lead compounds, which are applied in numerous commercial products, currently require authorization. These substances will therefore need to be replaced with suitable alternatives.

Fraunhofer ICT provides support in the search for substitutes. The suitability of the substitutes for a particular application can be assessed according to their chemical-physical properties, the available toxicological data, and through basic investigations and application-oriented tests. At Fraunhofer ICT, various mixing, kneading and processing units such as extrusion presses, extruders, presses and a wide variety of analytical applications for material characterization are available.