The biggest modification to the ML-1 was on the platform's base, where engineers increased the size of a exhaust duct to a rectangle stretching and strengthened the surrounding structure. The SLS will weigh more than twice as much as the planned Ares I rocket. The Ares I rocket would have featured a single solid-fueled first stage, while the SLS will include two large solid rocket boosters and a powerful core with four RS-25 engines. The base of the ML-1 is high, long, and wide. The ML-1 also features a Launch Umbilical Tower (LUT) with several arms that will permit servicing of the SLS on the launch pad, and will swing away from it at launch.