At something like 286 tonnes at liftoff, I believe that Antares with Orb-3 may be the second heaviest-ever rocket to fail and fall back on or near its launch pad in United States history, but it may be the heaviest-ever liquid stage to fail this way in the U.S..
Number one has to be, hands down, Titan 34D-7 which weighed about 680 tonnes-plus when it blew apart at T+16 seconds above SLC 4E at VAFB on April 18, 1986. But its liquid stages only weighed about 178 tonnes.
Delta 241, a 7925 with GPS 2R-1 that failed at T+7 seconds on January 16, 1997 above Cape Canaveral LC 17A, weighed 232 t…