The most remarkable, and little-noticed, aspect of the first 2004 flight
of SpaceShipOne, was the fuel. It was not liquid hydrogen, as in the
space shuttles, but rubber. To be exact, it was polybutadiene, a
component of the synthetic rubber from which squash balls are made. It
can neither explode nor pollute and is wholly safe to handle.
A palm-sized slab of polybutadiene––as light as
three ounces––has enough energy if burned in a combustion chamber to
light a 100-watt light bulb for twelve hours.
Rubber-based fuel burned by liquid oxygen produces a
hybrid engine which cannot cause a catastrophe like the explosion that
blew up the space shuttle Challenger when halfway to orbit. Neither does
it pollute the air as it rises. A typical space shuttle launch releases
more than 230 tons of hydrogen chloride into the atmosphere. The hybrid
motor produces none.
Its only truly regrettable drawback is that it cannot
be obtained from the old motor tires that litter the countryside. For
these, when burned, give off the unpleasant pollutant sulphur dioxide.
A hybrid rocket works quite differently from the
solid or a liquid-fuelled variety that NASA uses. In the latter, the
fuel and the oxidiser that makes it burn are mixed together and form a
solid propellant ``grain'' inside the combustion chamber. This has led
to frequent disasters when electrostatic discharges have caused
explosions.
A liquid rocket stores its oxidiser and fuel––both
as liquids––in separate tanks. If they come into contact before a
controlled ignition they can explode.
But a hybrid motor, combining solid polybutadiene
with liquid oxidiser, has no such drawbacks. It can be carried by lorry
through city streets without risk, and non-essential workers do not need
to be evacuated when it is being readied for launch.
A box of twelve squash balls typically costs about
£15. A tankful of liquid hydrogen by contrast, together with the price
of storing it and safely transporting it, must cost tens of millions. It
would be difficult to imagine a rocket fuel so cheap and so versatile as
rubber-based polybutadiene.
Whether future astronauts are likely to soar into
orbit on columns of fire from burning rubber or from a lift made of
buckyballs is still a matter of some debate.
But in either case, farewell to the platitude that
space travel is so expensive that only governments can afford it. Before
long, history will not be of one world but of many.
