It's not about energy, it's about momentum and energy/weight ratio. Due to the energy density of chemical fuels, the vast majority of a space rocket's fuel is a

It's not about energy, it's about momentum and energy/weight ratio. Due to the energy density of chemical fuels, the vast majority of a space rocket's fuel is actually used to carry the _rest_ of the rocket's fuel to the altitude at which it will be burned. I'm not good with rocket physics, but I'm pretty sure that gaining a few kilometers of altitude and 1000 km/h of kinetic energy will have a large impact on the total amount of fuel/propellant required to reach orbit. Is someone in here good enough with rocket physics to comment on this? Of course, it would be even better if they could go to business-jet altitudes or above (15000 meters instead of the 9000 quoted in the article), and I am guessing from the number and size of the engines that this is what they are actually aiming at. But this is just idle speculation on my part.