The purpose of this work has been to prepare chemically stable composite propellants with very high burning rates. Propellants with polydimethylsiloxane and polydimethyldiphenylsiloxane as binder, n-hexylcarborane as combustion catalyst and ultrafine ammonium perchlorate as oxidiser have been prepared in small scale and tested regarding to sensitivity and burning rate. The propellants are chemically very stable and resemble ordinary HTPB/AP propellants in sensitivity. Very high burning rates have been measured. With NHC contents of 0% -15% and a pressure of 20 MPa burning rates of 100 mm/s-280 mm/s have been obtained, and at a pressure of 150 MPa burning rates higher than 900 mm/s were obtained. The curing of the propellants have, however, been poor. To obtain acceptable mechanical properties, the curing problems must be solved. A method for measuring the burn rate on small samples of propellants with low strength was used.