Originally Posted by
sceh
Given that SpaceX can launch anything cheaply and land on land anywhere or at sea, what exactly is the point of this beast? Blue Origin has yet to reach orbit but when it does it will also be cheap. Using this beast to launch a rocket from 35000 feet seems silly now
The delta-v speed acquired in low atmosphere is the most expensive in terms of fuel factor, mostly because of drag.
For most rockets, the dynamic pressure starts to decrease around 13 km of altitude.
To put things in perspective, the burn-out altitude of Ariane V solid rocket booster is 66.7 km with a speed of 2020 m/s. For a smaller rocket like Atlas V 551, it is 37.7 km. So, for an even smaller rocket, starting from stratospheric limit can mean avoiding the complexity of attaching a solid propellant rocket.
Also, when climbing to a LEO orbit, the decrease in geopotential energy is almost linear. This means that starting from 10 km is a 2.5% saving on geopotential energy.