I don't know the details of either SSME's or the Delta IV core engine designs. It may be that a different injection/combustor design, plus differences in the operating pressures have something to do with it. I believe I recall the SSME's operate at about 2,975 psia, which when they were designed was quite high. Don't know about the Delta IV engines. A lower P-sub-c might make quite a difference.<br /><br />Also, I don't know the expansion ratios of the nozzles on both birds. It may be that the Shuttle engines are optimized for a different altitude as they burn through the entire launch sequence, whereas the Delta IV has upper stages that are optimized for higher altitudes.<br /><br />Also, as someone else pointed out, the opaque exhaust of the solid rocket boosters drowns out the relatively clear exhaust of the SSME's, as is also the case when the Delta IV has SRM's on the sides.<br /><br />Hypergolics have a different colored exhaust, depending on the propellants. In the vacuum of space there is also no reaction with atmosphere.<br /><br />Ad Luna! Ad Aries! Ad Astra!<br />Trailrider