Last week a British engineering company called Reaction Engines unveiled the Lapcat A2, a prototype jet that could carry 300 people at speeds up to Mach 5. At 3,400 miles per hour, that's fast enough to make 'day trips to Australasia' a realistic possibility. More importantly, Reaction Engines also believe the plane would be more environmentally friendly than conventional aircraft, through a series of innovations more usually associated with space travel.

One such innovation is the Scimitar, an engine that is a combination of turbojet and ramjet, and runs on liquid hydrogen. Rather than CO2, the engine would emit water vapour and nitrous oxide. Obviously nitrous oxide is still a greenhouse gas, so it's not exactly a 'green' jet, but overall emissions would be lower. The plane would also be lighter than a 747, and by climbing to a higher altitude and flying at the edge of space, far more efficient.

Managing Director or Reaction Engines Alan Bond explains how it would operate in practice:

... the A2 is designed to leave Brussels international airport, fly quietly and subsonically out into the north Atlantic at mach 0.9 before reaching mach 5 across the North Pole and heading over the Pacific to Australia. -- Guardian

Unsurprisingly, the design is controversial. One particular traveler-oriented complaint is that it has no windows. The heat generated by the high speeds make normal airplane windows impossible, and the small, thick variety used on the space shuttle are too heavy. Others point to the failure of hypersonic travel, not least in the one example we have - Concorde was notoriously inefficient at low speeds, and because it was so loud it could only fly at top speed over water. Fine for the London to New York route that it plied, not so good for India.

But of course, the underlying question, from a green perspective, is this: do we really need 'day trips to Australasia'? If it was as easy for us Brits to get to Australia as it is to get to Spain, any increase in efficiency would surely be negated by the rise in demand (a phenomenon described by something called the 'Khazzoom Brookes Postulate', or 'Jevons Paradox', which Craig explains fully here). Until the rise of cheap flights, nobody had thought they needed to make weekend breaks to New York. Come to think of it, until the invention of the railways, nobody had thought of commuting. The telegram invented news, just as much as the communication needs created the telegram. The average man on the medieval street didn't realise he needed to know the time until the clock was invented. You see where I'm going with this - technology creates needs just as much as it fulfils existing ones.

It would be great if the southern hemisphere was more accessible than it is, but there would be such a jump in the number of people prepared to travel that a liquid hydrogen jet engine would not be enough to offset the damage, and certainly not enough to reduce our current emissions. In that sense, although the Lapcat A2 is undoubtedly a formidable piece of technology, it's one the world still can't afford.