These rockets are mainly used to increase the thrust of the rocket. The solid fuel used by these rockets is hydroxyl-terminated polybutadiene (HTPB).
The material used to make the rockets is graphite- epoxy shell that is about 4 to 5 times lighter than metals. These shells provide the outer structure for the solid rocket motor fuel. These rocket motors are known as graphite- epoxy motors, or "GEMs."
For the Rover A (spirit) mission, the GEMs are 1016 mm (40 in) in diameter and are fueled with approximately 12,000 kg (26,400 lbs) of hydroxyl-terminated polybutadiene. Each GEM will provide an average thrust of 498,000 N (112,000 lbs).
For the Rover B (opportunity) mission, the GEMs are 1168 mm (46 in) in diameter. The GEMs for the Delta II 7925H are also longer than the GEMs used on the Delta II 7925. The Delta II 7925H GEMs provide approximately 25 percent more thrust than the standard Delta II GEMs.