Members of the EU-funded GRAIL project have spent the past three years investigating the creation of a green solid propellant for space launchers. Their unexpected findings could make alternate, liquid propellants the only viable route to green space launches.
The case for using solid propellants in space propulsion technology is easily pleaded: they are the most cost effective, competitive and reliable solution out there, period. However, there is one element in those propellants that increasingly receives bad press: ammonium perchlorate (AP).
Further details: The greening of solid rocket propellants?