The US Navy forbids hypergolic fuels, liquids where the fuel and oxidizer ignite on contact with each other, for shipboard use, but since the US MDA is looking at ground baseing, they are considering liquid fuels for these interceptors.

Basically, because you can throttle liquid fueled rocket motors, it means that you can, for example, shut down at some point in the intercept, and restart the motor for improved kinematics for the end game, or to extend range, etc.

I still see missile defense as a big hole that people are dumping money into, particularly hit to kill.