If you work out the rocket equation, it turns out that you can achieve the delta-v of this stage 0 with about a 30% increase in fuel (assuming a 3 km/s exhaust

If you work out the rocket equation, it turns out that you can achieve the delta-v of this stage 0 with about a 30% increase in fuel (assuming a 3 km/s exhaust velocity). Since the volume of a tank goes up with the cube of the linear dimensions, you need to increase the height and diameter of the boosters by only 10%. This seems to me to be a much easier task than engineering the whole mechanism of keeping a rocket hooked on a moving airplane, dealing with vibrations, make it sturdy enough so it does not collapse when it's not in a vertical position, and lots of other challenges associated with the unusual launch mode.