Propulsion is one of the major areas for energetic materials research and development. While this includes gun propulsion, more work is being done on chemical rocket propulsion. This covers all aspects from ingredients though to performance and the management of life. There are many strands of research related to rocket propulsion and work is taking place internationally. This special issue is an attempt to illustrate the range of work and of the groups engaged in it. It can only be a snapshot but may encourage readers to investigate further. While solid propellants form a major part of the technology other options such as liquid and hybrid are important, and this importance has been shown in the development of non-traditional commercial systems. The range of requirements are likely to become more diverse and different aspect will become important. The choice of propulsion system is not just limited by the available technology but must be a response to the mission requirements. Naturally the most important requirement is to meet the performance need within the cost limits and the available technology. The other important factors include the desired lifetime after production, and both vulnerability and safety requirements. To these must be added the need to minimise the environmental impact. This is of increasing importance. Environmental impact is important in many areas. It includes not just service use but both in production and disposal at the end of life. This naturally affects the choices made for propulsion systems. It is hard to claim minimal impact when the system involves the burning of large quantities of fuel with the consequent emission of combustion products, many of which will affect the environment. However, the choice of ingredients and the design of the system can reduce that impact significantly. Overall, the requirements can be listed as follows: • Maintaining or improving performance – all aspects o Thrust including thrust profile; o Combustion; Ignition • Reduced vulnerability to meet Insensitive Munitions requirements • Improved safety in manufacture, storage, and handling • Definition and management of safe life • Improved stability in multiple environments • Reduced cost through improved manufacturing • Reduced environmental impact including the impact of REACH etc. • Disposal These can easily be seen to be difficult to reconcile especially within the necessary cost limitations. This range of requirements can act as constraints and these constraints should drive research into the various aspects of the technology. The papers in the special issue illustrate some the current activities. These range from the design and choice of ingredients to their efficient and effective mixing. There is also active work on ignition and on the ballistic properties and combustion. Papers also include work to understand the behaviour of the system components. This allows prediction of performance in the broadest terms as well as management of risk. The first of the two reviews is particularly relevant as the authors address new directions in processing where Resonant Acoustic Mixing is seen to offer better more environmentally and cost effective routes to high quality solid propellants. As is clear from both the review and other papers the method shows high promise. There are still issues to be addressed, particularly that of scale. Can this method be scaled up to meet production needs? That too is being addressed and to date there appear to be no real showstoppers. However, this needs thorough testing to ensure safety and consistency of quality. One of the factors that needs careful examination is the range of application for the RAM technique to determine how generally it can be used. Most techniques have limitations, sometime intrinsic and sometimes derived from the characteristics of the ingredients used. Often what is used is what is readily available, and changes might be possible if there is a good reason. Research is needed to establish the limitations and whether these can be adjusted through changes in the properties of the ingredients used and the methods of their introduction. Therefore, research work on both new materials and new forms of existing materials is important to define future systems. There is also a need to combine the various characteristics to achieve the best solution for the requirements at that time. Such solutions may change with time and therefore a critical awareness of developments is needed. The other review on burning rate suppression and management covers an equally important area. Management of combustion is critical for achieving the mission and for ensuring the safety of both system and launch vehicles and crew. It is important to understand how materials behave and how that behaviour can be managed. This area covers several aspects and some of those are covered in papers in this issue. It is not just a matter of understanding the combustion of propellants of all types, but also how to optimise that combustion so that the performance is both optimal and predictable. Small changes can make a significant difference. The paper on extinction of combustion addresses some of the concerns and problems. While steady state combustion is important ignition is equally critical as is the effect of ablation on insulation. We are including papers discussing some of these aspects, as this area is a very active one. Managing life and safety is also covered with papers on the effect of shock and fracture on solid propellants. These aspects form part of the overall assessment of safety. Such examinations support Insensitive Munitions requirements but also address the general safety of motor systems. The safety need goes beyond the IM requirements and includes safety in handling and storage. Any rocket motor contains the largest mass and volume of energetic materials in the systems, which while normally UN Hazard Class 1.3 poses a significant risk especially when combined with both a warhead and the various ignition systems. Managing that in all circumstances poses a problem, a problem that has driven research for decades. The safety of rocket systems has improved, and the useful service life is better understood, so that the risk is more manageable in most circumstances. There are always outliers however, and risk assessment requires that there is understanding that can be used to predict and therefore manage those risks. Those risks change with changes in the ingredients; the formulations; the motor design and expected service life. It remains a moving target and one that is continuing to drive research. While research into the various aspects of rocket propulsion in response to requirements and the identification of gaps is and remains important, it is also important to see how these individual areas are linked and how the results in one can affect others. For example, new processing methods or different ingredient forms can change the options available for formulation and manufacture. Similarly, new modelling approaches can affect the prediction of both performance and vulnerability, but all are linked and that needs to be recognised so that the system can benefit from the results obtained in each component area. This is an important area of research, so much so that some results cannot be reported openly for various reasons. This issue does however illustrate some of the work and hopefully will encourage discussion. Prof. Adam Cumming, University of Edinburgh