Weather and risk management

Forecasting the toxicity hazards which may arise from activities at the site is the responsibility of Range Safety, using data provided by the CSG weather station...

A Computational Methods expert in the CNES/CSG Range Safety and Environment department is responsible for forecasting the toxicity hazards which may arise from activities at the site. He carries out this mission using specialised software which analyses, in particular, the data provided by the CSG weather station.

Forecasting toxicity hazards

The site is laid out in such a way that there is no real toxic risk under nominal conditions, a fact that is confirmed by experience, as well as by the data gathered by the Sarrim software program and the weather database. Any high-risk concentrations of toxic products are contained within the CSG zone and are below external regulatory limits. All operations considered hazardous at the Space base are codified by Range Safety, which determines the need for meteorological forecasts. This code also covers the Pariacabo port area, where Ground Range Safety only authorises an operation to proceed if the wind is moving towards a well-defined sector, sparing the town of Kourou from any toxic risk.

A difficult but essential simulation

The most difficult thing to model is the atmospheric dispersion of the cloud released by the solid boosters during launch, during testing at the BEAP (booster engine test stand), or even during periodic burning-off at the Regulus facility. As commercial software is not specialised enough, over the last six years CNES and Aria Technology have been developing a software program which is specific to the phenomenon of propellant combustion: Sarrim (Stratified Atmosphere Rocket Release Impact Modelling). Not only must this software determine the concentration of the toxic cloud on the ground, but it must also simulate the movement and stabilisation height of the combustion cloud. The gases resulting from combustion rise, as they are extremely hot. The higher the cloud, the lower the concentrations of toxic products on the ground. Atmospheric data from 0 to 5 km in altitude are provided by the soundings carried out by the weather station. Every 100 m, the sounding balloons take readings of wind speed and direction, hygrometry, pressure and temperature. The software also has to integrate other parameters, such as launcher characteristics and the combustion generated, as well as the water from the 'deluge system' which is pumped into the flame trenches. This kind of simulation is performed for each launch, beginning at T-8 hours, in order to determine any trend which may be confirmed or invalidated by other simulations carried out closer to liftoff. The final sounding is analysed 30 minutes before launch. If it appears that any pollution risk might reach a zone such as the Agami observation site, people there are evacuated just after the solid boosters are jettisoned, in order to avoid any problems. Mainly due to chlorine, these are usually experienced as strong odours, or even stinging eyes, although the concentrations are not dangerous as the olfactory threshold is well below nuisance threshold.

One of Ground Range Safety's other missions relates to testing at the BEAP. Unlike an actual launch, the gases here are all released at ground level over a period of two to three minutes. There is a chance that any problem could affect the main road. The decision about whether to allow a solid propellant motor to be ignited at the BEAP is only taken after checking that the concentrations of toxic gases outside the CSG perimeter are within regulatory limits.

Reliability of toxic dispersion forecasts

The reliability of Sarrim's results is confirmed by all the environmental measurement campaigns undertaken after each launch or test at the BEAP. These involve alumina and hydrochloric acid sensors taking readings in the field, which are then compared with results forecasted by Sarrim, as well as the sounding carried out a few minutes after the launch. The values calculated by Sarrim and those measured in the field are always similar; Sarrim’s are, if anything, more pessimistic.

Developing the partnership between Range Safety and the weather station

Currently, simulating the toxic cloud dispersion is performed using soundings obtained before liftoff (T-2 hours). This means that the estimate of any imminent toxic hazard is produced using measured atmospheric data, under the hypothesis that atmospheric conditions are maintained. This two hour delay is difficult to reduce, given the time it takes to release the balloon then process the data. The feasibility of a diagnostic approach, in which the imminent toxic risks would be estimated in duplicate, using the sounding data and the forecasts from the weather station, is currently being studied. This feasibility study is the subject of the CARPAT campaign (Caractérisation ARPège pour les Applications de Toxicité or Arpege characterisation for toxicity applications), which is presently underway.

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