5.4.4: The Present Day

The role of solid motors changed in the 1990s. Apogee motors are becoming less common as comsats take advantage of the operational flexibility of liquid propellant systems, but it seems clear that the relative simplicity of the solid motors will ensure a niche for them for the forseeable future. The Shuttle and expendable launch vehicles with profiles matched to it created a new market for perigee kick motors. Notable among these in the US are UTC's Orbus series: the big Orbus 21 makes up the first stage of Boeing's IUS and the single stage of OSC's Transfer Orbit Stage, and the Orbus 7 used as the perigee stage for some commercial satellites. Thiokol's Star 48 and big sibling Star 63 have also been used in this role. The Boeing IUS is the largest solid upper stage system in use, with an Orbus 21 motor topped by an Orbus 6. It has seen service on both Shuttle and Titan.