To elaborate, it's not the IUS that's forbidden, it's the Centaur. There had at one time been plans to put Centaurs in the Shuttle's payload bay, with the IUS being sort of a temporary solution until the neccesary alterations were made to the Shuttle. (Centaurs run on LH2/LOX, so they need special accomodations. IUS is solid-propellant, and thus much simpler.) But this was already fading by the time of STS-51L, partly because of the complexity, and then the loss of Challenger was pretty much the final nail in the coffin. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em> -- The Tenth Doctor, "Blink"</font></p> </div>