Bob Wall & Mark Bradley, Roxel UK
Roxel UK is the chief UK manufacturer of solid rocket propellant. The primary products are rocket motors made from either single (NC), double (NC & NG) or triple base (NC, NG & nitramines) powders which give low signature, minimum smoke propellants with good insensitive munitions (IM) properties.
Since 1990 Roxel UK has manufactured double base solid rocket propellant using NC/NG ‘paste’ as the principal energetic ingredient. This NC/NG paste is manufactured on site by Roxel UK and dried before use. Historically dry paste is used because 1) Everyone connected with UK Cast Double Base (CDB) propellant manufacture had used it, 2) Roxel UK and the wider industry was not aware of any viable alternatives, and 3) Casting powder containing solely NC was insufficiently energetic to meet customer needs. However from a safety standpoint, it was always suspected that if an incident was to occur it would be serious, especially given the labour intensive nature of drying NC/NG paste and the subsequent exposure time of the operatives.
In 2010 Roxel UK instigated a programme to formally assess the burning response of dry NC/NG paste to a variety of stimuli, especially to heat and electrostatic discharge. The results of this programme increased the impetus to find a replacement material and/or an alternative manufacturing process.
In 2012 Roxel UK became aware of the existence of a material known as nitrogelatin (NGel). Nitrogelatin is similar to the blasting explosive gelignite, having NG adsorbed on NC with ethanol also included in the matrix. Nitrogelatin is made up from 15% NC, 80% NG and 5% ethanol. Therefore nitrogelatin can be considered an NC/NG paste with a lower NC content when compared with those already used in propellant manufacture at Roxel UK, except having a different physical form. Nitrogelatin is both qualified for use in gun propellant manufacture and available in the same NC grades currently qualified for rocket propellant, lowering risk for eventual in-service use.
Nitrogelatin offers Roxel UK a number of advantages over NC/NG paste including improved handling, reduced number of processes and operator exposure, reduced energy use, consistency, burning response and operator health & safety. However, as NC/NG paste is considered to be a HT3 material and nitrogelatin is a HT1 material, this presents an unusual problem – proposing to make a process safer by introducing a theoretically more hazardous material.
Subsequent material trials performed at Roxel UK and at external testing facilities have shown that whilst ‘officially’ more hazardous, nitrogelatin has so far been demonstrated to be safer to use and handle in double base solid rocket propellant manufacture. It has been shown that the actual physical form of the material makes a significant difference to its burning response. Subsequently, small-scale propellant manufacture has been successfully demonstrated and production scale-up trials, including final propellant properties, are now underway. If these trials are successful, it is anticipated that nitrogelatin will replace NC/NG paste in future products.