Semtex has an R.E factor of 1.66

1. Iran Air Flight 655 (IR655) was a civilian jet airliner shot down by U.S. missiles on 3 July 1988. U.S. Navy's guided missile cruiser USS Vincennes, killed all 290 passengers and crew aboard, including 66 children, ranking it ninth among the deadliest disasters in aviation history. In 1996 the U.S settled with for an average of $213,103.45 per passenger to pay in compensation to the families of the Iranian victims. The United States has never admitted responsibility, nor apologized to Iran.
2. Semtex, the explosive that destroyed Pan Am Flight 103 is a general-purpose plastic explosive containing mostly RDX (Research Department Explosive) and PETN (Pentaerythritol tetranitrate). It has an R.E factor of 1.66. Outside of nuclear a nuclear bomb, Semtex has the third highest R.E factor but is much easier to manufacture and used a lot more readily than the first two.
3. Relative Effectiveness factor (R.E factor) compares the explosive prowess of a compound to TNT.