IRBM

The Intermediate-Range Ballistic Missile, or IRBM, is, as the name implies, a missile with mid range strike capabilities. It gives slightly more damage than the ICBM, due to its ability to carry more weight (and thus, more nuclear payload) since it doesn't travel too far. After researching the Trident, it becomes even more powerful, making the warhead divide into three different explosives soon before landing. It has 50% range, so build it near your first defense line.

Trivia

 * The IRBM model has some text written on its fuze.
 * The Basic model has the text "HI THERE!"
 * The Trident model has the text "DEAR JOHN", likely a reference to the novel.