The proximate cause of my objection was someone asserting that Virgin Galactic 's business model is ultimately targeting sub-orbital flights between continents, rather than brief bouts of free-fall tourism for the rich. At first glance, this isn't an obviously stupid assertion: enough folks have signed up for the sub-orbital tourist package that there's clearly demand, various companies have been buying patches of isolated terrain as sites for spaceports ( even in Scotland ), and there's a British start-up proposing to build an air-breathing hypersonic carrier craft for satellite launches and passenger travel. It's a perennial dream technology that keeps coming back from the dead, because the idea of flying from Heathrow to Sydney in three hours instead of 22 is obviously appealing to those of us who occasionally fly LHR-SYD.

One of the failure modes of extrapolative SF is to assume that just because something is technologically feasible, it will happen: I'm picking on sub-orbital passenger travel as an example of this panglossian optimism because I got sucked into a thread on twitter the other day and I think it's worth explaining my objection to it in a format that permits me to write more than 140 characters at a time.

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.

Firstly, this is the post-9/11 age. Obviously security is a consideration for all civil aviation, right? Well, no: business jets are largely exempt, thanks to lobbying by their operators, backed up by their billionaire owners. But those of us who travel by civil airliners open to the general ticket-buying public are all suspects. If something goes wrong with a scheduled service, fighters are scrambled to intercept it, lest some fruitcake tries to fly it into a skyscraper.

It's going to be a lot harder to intercept a hypersonic service, to say the least. If nothing else, the reaction time will shrink by an order of magnitude. Today, it takes perhaps 2-5 minutes to get an RAF QRA Typhoon-II into the air. It can then go supersonic and overhaul a subsonic target at relatively high speed. From Coningsby or Leuchars a Typhoon-II can reach just about any spot over the UK in 15-20 minutes, in which time a subsonic airliner can travel perhaps 100-200 miles. The picture is very different for a hypersonic passenger craft. In 20 minutes such an aircraft would travel somewhere between 1000 and 3000 miles. None of today's military aircraft are up to the job of intercepting it, and indeed, active radar can't even track it effectively—for that, you'd need something on the order of a cold war ballistic missile warning radar system, designed to provide advance notice of an ICBM strike.

A hypothetical hijacker interfering with the flight profile of a hypersonic transport wouldn't need to deviate from their flight plan 20 minutes before it crashes into a target; it could be a last-minute gambit. So the security surrounding such flights is going to be intense, they're only going to be allowed to fly on well-established schedules (no short-notice bizjet-equivalents need apply!), and they're going to fly in and out of spaceports some distance from the destination city. For example, there's a proposal to use the former RAF Lossiemouth in Scotland as a spaceport for the UK. It's a good site for polar orbit satellite launches (north of Moscow but with far more clement weather), but it's nearly 600 miles from London. Similarly, the New Mexico spaceport isn't exactly next door to Los Angeles.

There are some places where it may be quite difficult to build a spaceport suitable for civil sub-orbital transport. There's lots of cheap land in New Mexico or Australia, (and, at a pinch, in northern Scotland), but it's unlikely to be easy to find land for a spaceport at an affordable price within a few hundred miles of Beijing or Seoul or Tokyo. But that's beside the point. It may be technically possible to build and operate hypersonic intercontinental passenger services. But they'll be scheduled, regular services, and subject to stringent passenger security checks at all times (think in terms of flying El Al, all the time). Moreover the spaceport will be at least an hour away from your nearest hub by non-hypersonic transport—possibly several hours away by high speed rail (never mind automobile).

And now for the killer: the inconvenience factor.

First class air travel by civil aviation is a dying niche today. If you are wealthy enough to afford the £15,000-30,000 ticket cost of a first-class-plus intercontinental seat (or, rather, bedroom with en-suite toilet and shower if we're talking about the very top end), you can also afford to pay for a seat on a business jet instead. A number of companies operate profitably on the basis that they lease seats on bizjets by the hour: you may end up sharing a jet with someone else who's paying to fly the same route, but the operating principle is that when you call for it a jet will turn up and take you where you want to go, whenever you want. There's no security theatre, no fuss, and it takes off when you want it to, not when the daily schedule says it has to. It will probably have internet connectivity via satellite—by the time hypersonic competition turns up, this is not a losing bet—and for extra money, the sky is the limit on comfort.

I don't get to fly first class, but I've watched this happen over the past two decades. Business class is holding its own, and premium economy is growing on intercontinental flights (a cut-down version of Business with more leg-room than regular economy), but the number of first class seats you'll find on an Air France or British Airways 747 is dwindling. The VIPs are leaving the carriers, driven away by the security annoyances and drawn by the convenience of much smaller jets that come when they call.

For rich people, time is the only thing money can't buy. A HST flying between fixed hubs along pre-timed flight paths under conditions of high security is not convenient. A bizjet that flies at their beck and call is actually speedier across most intercontinental routes, unless the hypersonic route is serviced by multiple daily flights—which isn't going to happen unless the operating costs are comparable to a subsonic craft.

Concorde made money because a predictable daily London-New York route could carry a lot of traffic, and if you flew LHR-JFK-LHR you could grab four extra daylight hours in New York, making a workday commute for high-powered meetings just about feasible. There's no obvious equivalent productivity gain for hypersonic transports around the Pacific rim or between the North and South hemisphere capitals. (If anything, hypersonic travel may make jet lag worse.) Let's not forget that attempts to operate Concorde on other routes didn't show a profit. London to Sydney was viable in a little under 18 hours, with refuelling stops—but there wasn't enough passenger traffic to justify the heroic logistics. Flying LHR-SYD, Concorde burned 500 tons of fuel in each direction, and the plane required 12 hours of ground maintenance per hour of flight time: British Airways managed one flight per week before they dropped the service. If I could have afforded a first class LHR-SYD ticket at the time, common sense would have told me to spend an extra four hours on a 747 with a seat that turned into a lie-flat bed, instead of 18 hours crammed into a Concorde (in seats jammed together like economy on a 747).

Merely supersonic bizjets for the rich might well be viable if they can be operated between regular airports in a manner compatible with current security requirements. (They'll almost certainly need passenger screening, though, as the margin for intercepting them will be much narrower than with current subsonic airliners.) Virgin Galactic's sub-orbital pleasure hops are unlikely to be problematic, as long as the end-points lie nowhere near major population centres. The military will love the technology (although the military think nothing of building and flying bombers that cost on the order of $1M per hour to operate). But point-to-point sub-orbital passenger services are, I think, going to remain a pipe dream for the foreseeable future. Costs too much for the inconvenience.

And this is my classic worked example of roads not taken. Just because something is technologically possible, it does not follow that it will inevitably happen. Someone has to want it enough to pay for it—and it will be competing with other, possibly more attractive options.