Orion ISP at low altitude

In this post calculations based on page 55 of Project Orion show that the orbital test vehicle's low altitude pulse units average size is 108 pounds. The 664 lb vacuum pulse units have an ISP ranging from 3000-6000, but the chart has no ISP data for the smaller units. We can calculate the implied ISP which will vary with cycle rate, at .8 seconds per pulse the result is 15,800 seconds.