Solventless Extruded Double Base (EDB) propellant charges. A review of the properties, technology, and applications

EDB propellant may be manufactured by solvent or solventless processes. Generally, the former is used for small diameter gun propellants (<15 mm diameter, although the web size is a more important factor) and the latter for larger diameter gun propellants and charges used in gas generators/rocket motors. The solventless process has safety and quality advantages and provides a reduced environmental impact, which when combined with continuous processing, offers a cost-effective alternative to the solvent manufacture of gun propellants, even at relatively low diameters (e.g. 5 mm). Solvent processing is impractical at larger diameters (generally greater than ~25 mm), due to the difficulties of solvent removal, so propellant charges for ejector seats, rocket motors and gas generators are normally manufactured exclusively by the solventless process.
Until the 1990’s EDB charges as large 180 mm diameter were produced and used in service, but in recent years there have been few applications above 130 mm and the most significant production has been for 70 mm (2.75”) rockets/missiles. However, there are several other niche applications for such charges, e.g. ejector seats and boost/blip motors for missiles.
EDB charges are used mainly due to their efflux having minimum smoke, attractive oxygen balance and low corrosivity. The propellant class also has desirable burn rate characteristics, since they exhibit plateau burning and have a low temperature sensitivity to burning rate (πk). Whilst their energy density is significantly less than composite type propellants their ballistic performance can, in certain applications, be largely compensated by the plateau and πk advantages. External ballistic modelling results are presented that illustrate the performance advantages that EDB propellants can provide, compared to more energetic propellants that are also not Minimum Smoke.
For rocket motors, the plateau behaviour is generally achieved by the inclusion of lead salts as ballistic modifiers, although these have had increasingly restricted availability and lead-free alternatives are not yet generally in use (despite many years of R&D effort). Since this is a critical point, the review includes an assessment of the progress on lead free ballistic modifiers.
The manufacturing processes, both traditional batch and continuous, are described including the process validation performed to establish continuous manufacturing processes for medium and large calibre solventless gun propellants. For rocket propellant charges, the inhibition methods (that are generally also required) are reviewed including a consideration of the feasibility of case bonding such charges for 70 mm rocket and other applications. Novel methods of bonding sections of EDB together, to achieve a diameter that would normally only be possible with a very large press, are also described. Other advanced manufacturing techniques, such as stamping complex shapes, are also considered. The strengths and weaknesses are considered together with potential developments in processing & materials (e.g. synthetic/nano cellulose).
The various current and recent applications for such charges are reviewed: aircraft (ejector seats, anti-stall devices, JATO), Gas Generators (turbine starters, automotive air bags), solventless gun propellants for medium and large calibre including the unusual application of such a gun propellant in the 16” diameter HARP and SLRC guns, Air to Ground Rockets (70mm/2.75” and Zuni 5”) and missiles (e.g. Sustain motors for Seawolf, HARM, guided 2.75”; plus the large number of missiles that use EDB eject charges).
The Insensitive Munitions (IM) aspects for gun and rocket/missile applications are considered. At the propellant level, EDB’s have quite good insensitiveness thus typically resulting in Hazard Division 1.3 systems. The paper includes a discussion on the factors related to IM for such propellants. Consideration is also given to the maturity and potential further implementation of other Min Smoke propellants, etc, to predict the future opportunities for solventless EDB propellants in the next five years. This includes an assessment of some historical issues, when propellant and motor technology advances have been attempted, since the low risks with EDB technology provides an attractive solution even when introducing new products.