Why sub-orbital flights aren’t going to happen anytime soon

Virgin Galactic sub-orbital space plane

The hype over supersonic and sub-orbital flying has been going on for at least 30-40 years, the promise being that people will be able to go anywhere in the world in a matter of hours. The usual example being London-Sydney in a few hours.

But as Charlie Stross explains in this excellent article, hypersonic and sub-orbital travel will be prohibitively expensive however it is done.

Let’s start with a simple normative assumption; that sub-orbital spaceplanes are going to obey the laws of physics. One consequence of this is that the amount of energy it takes to get from A to B via hypersonic airliner is going to exceed the energy input it takes to cover the same distance using a subsonic jet, by quite a margin. Yes, we can save some fuel by travelling above the atmosphere and cutting air resistance, but it’s not a free lunch: you expend energy getting up to altitude and speed, and the fuel burn for going faster rises nonlinearly with speed. Concorde, flying trans-Atlantic at Mach 2.0, burned about the same amount of fuel as a Boeing 747 of similar vintage flying trans-Atlantic at Mach 0.85 … while carrying less than a quarter as many passengers.

Rockets aren’t a magic technology. Neither are hybrid hypersonic air-breathing gadgets like Reaction Engines’ Sabre engine. It’s going to be a wee bit expensive. But let’s suppose we can get the price down far enough that a seat in a Mach 5 to Mach 10 hypersonic or sub-orbital passenger aircraft is cost-competitive with a high-end first class seat on a subsonic jet. Surely the super-rich will all switch to hypersonic services in a shot, just as they used Concorde to commute between New York and London back before Airbus killed it off by cancelling support after the 30-year operational milestone?

Well, no.

Quite. How are you going to transport these (mega-rich) people from the hypersonic terminal somewhere far away from civilization to the city they want to travel to? And how are you going to intercept a hypersonic jet which has been commandeered by terrorists?

None of it makes economic or logistical sense.

Concorde, from Wikipedia

The biggest problem with Concorde was the sonic boom it generated, which meant that it could only travel across oceans supersonically, When it was over land, Concorde was sub-sonic and still managed to rattle windows below it because of the enormous noise it made.

Concorde was able to break the sound barrier during its flights and was therefore able to cause a sonic boom. Sonic booms were a problem in North Cornwall and North Devon as these areas were underneath the flight path of Concorde. Windows would rattle and in some cases the “torching” (pointing underneath roof slates) would be dislodged with the vibration. It was proposed that Concorde, before it was decommissioned, could open up new flight paths to Australia. However the plans were scrapped due to the fact that few other countries would allow Concorde to fly at supersonic speeds due to sonic booms being disruptive and potentially damaging.

But Charlie Stross’ article is an excellent counter-blast to the hype that we would ever want to travel sub-orbitally or hypersonically. It really is hard science fiction and we’ve been sold it since the space age.


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