The catch is that intercepting offensive missiles in boost phase is challenging. The stage lasts only 3-5 min. In the 1980s and early 1990s, the Strategic Defense Initiative proposed the Brilliant Pebbles program, which envisioned a constellation of about 2,000 satellite-like interceptors, with solid rocket motors on board to propel them toward the incoming missiles (
AW&ST Feb. 26, 1990, p. 62). But the vastness of low Earth orbit and the limitations of those thrusters meant that only a handful of Brilliant Pebbles interceptors would be in range of any single ICBM launch. An enemy that launched multiple ICBMs nearly simultaneously could defeat the system.
But propelling space-based interceptors with large solid rocket motors—especially an XB-34—could change the orbital math, expanding the reach of each missile over a broader coverage area.