Rocket science is not just about optimising fuel, combustion and thrust generation in a rocket engine: rocket plumes require significant study and are just as scientifically captivating as they are visually. Rocket plumes are unique in that they are the only direct anthropogenic emissions at high altitudes and in particular in the ozone layer. ESA has initiated studies but the issue of the quantification of atmospheric impact is complex, requiring a complete knowledge of the aerothermodynamics of rocket plumes including combustion, expansion in the nozzle and development in the atmosphere. Flight experiments are difficult and costly while for an accurate computational representation the small scale plume must then be integrated into relatively large scale climate models without loss in fidelity.
What happens in a plume depends on a number of factors: firstly a rocket nozzle is designed to optimise its thrust. To do this the nozzle must expand the high pressure combusted fuel to a low pressure exhaust at its exit plane – the maximum thrust is reached when the nozzle expands the combustion gases to the same pressure as the atmosphere outside the nozzle. Since the pressure in the atmosphere decreases dramatically for most of the launch the pressure at the nozzle exit is either higher or lower than the atmospheric pressure. This leads to an interesting phenomenon in the nozzle exit region plume. Since the plume is travelling at supersonic speeds relative to the external atmosphere the difference in pressure produces shockwaves and expansion waves which in turn form areas of afterburning combustion in the plume.
The general shape of the plume also is affected by the atmosphere. At low altitudes the plume is compact but as the altitude increases the plume gets wider and wider. Interestingly, in space when small thrusters are used to re-orientate or maneouver a spacecraft the plume is so wide that significant work is required to ensure that the plume does not contaminate optical surfaces etc. Not only is altitude (and hence atmospheric pressure) influencing the shape but winds and eddies in the atmosphere change the rate at which the plume expands or mixes with the atmosphere. Most of the chlorine in a solid rock booster plume is in the form of CL2 but sunlight converts this to Cl which is a very efficient ozone depleter. The impact of night and day launches was therefore a question posed during recent studies.
Studies conducted by partners including CERFACS, DLR, FMI, Imperial College, IUP & ONERA have provided ESA with significant insight into the impact of ESA launcher plumes on the atmosphere. These studies have introduced small scale plumes into global climate models using two methods: utilising subgrid models and transient development of the plume to climate model grid scales. Both methods have required particular development for this application. Thus far the results have shown that the impact of gaseous species such HCL and CO2 are consistent with previous studies however thanks to these studies we now have evidence of a chemical reaction at the surface of exhaust alumina particles that converts inactive chlorine species, especially HCl, into chlorine radicals.
So whats next? A flight test through an ESA plume to verify our computations…..