The Soviets conceived of a wide array of responses during the SDI era. In 1997 Russia deployed the Topol-M ICBM which utilized a higher-thrust engine burn following take-off, and flew a relatively flat ballistic trajectory, both characteristics intended to complicate space-based sensor acquisition and interception. The Topol fires its engine for only 150 seconds, about half the time of the SS-18, and has no bus, the warhead is released seconds after the engine stops. This makes it far more difficult to attack.