From an email posted to Baen's Bar Honorverse dated June 1, 2005:

Hardening shipkiller missile drives

    A suggestion has been made that it would make sense to put drives into shipkiller missiles of sufficient power to make them immune to wedge kill by a counter-missile. Is this practical? Could it be done as the final stage of a MDM?

     

    No, it isn't. And it won't be -- ever. I believe we've had this discussion before. You cannot -- not "it would be hard," not "only possible with great difficulty," but cannot -- build a missile drive sufficiently powerful to take out another missile (or counter-missile) drive without being taken out itself. End of story. The physical size constraints make it impossible. That's why counter-missiles have used their impeller wedges as their primary missile-killing weapon for so long.