ONE warm day in July last year, Derek Willis was in his garage in Newcastle
upon Tyne pondering a nasty problem. As a passionate rocket enthusiast, he had
designed and built dozens of small rockets. But he was fed up with handling the
dangerous propellants that blasted them into the sky. If he spilt a few drops by
mistake he could lose a hand or, worse, blow himself and the garage to
smithereens. There had to be an better way.
Willis was feeling peckish and decided to take a short break from his latest
project to think things over. So he put down his tools and picked up a chocolate
bar. His snack contained the thousands of tiny air-filled bubbles that gave the
bar its name鈥擜ero. As he munched, a thought struck him. What if the air in
the bubbles was pure oxygen, and the chocolate was a high-energy fuel? 鈥淗ey, I
thought, I could do it that way,鈥� recalls Willis.
He made a quick calculation and borrowed a cylinder of oxygen from a friendly
welder. He poured a few litres of engine oil into a strong metal cylinder and
threw in a cupful of epoxy-resin hardener. And using a home-made valve system,
he began to slowly pump in the oxygen.
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When he summoned enough courage to open the cylinder, what he saw amazed him.
Under the high pressure, the oxygen had diffused into the lubricant, turning it
from a viscous liquid into a foam. And the resin hardener had then catalysed a
polymerisation reaction in the oil, turning the sticky foam into a solid mass
that resembled dried clay.
Stuff of dreams
He removed a chunk, tested it and realised he had created his dream fuel. It
is relatively stable, safe and can be stored almost indefinitely, yet when burnt
it yields almost as much power as the solid propellants used on NASA鈥檚 space
shuttle. Best of all, the basic ingredients鈥攐xygen and engine
oil鈥攁re cheap. According to Willis鈥檚 calculations, his fuel, which he
calls Aerated Solid Propellant (ASPROP), costs about one tenth of the price of
equivalent propellants. If NASA used his 鈥渃hocolate-bar鈥� technology, Willis
suggests impishly, it could save itself $50 million on each shuttle
launch.
Since they must unleash huge amounts of energy, rocket propellants tend to be
made from combinations of fuel and oxidiser that are highly reactive and
inflammable. While large space agencies can afford the complex handling and
storage facilities required for fuels such as liquid oxygen and hydrogen,
amateur enthusiasts must resort to cheaper and sometimes more risky
alternatives.
One of Willis鈥檚 recent creations was a rocket engine powered by a potent mix
of kerosene and concentrated nitric acid. To ignite the mixture in the
combustion chamber, Willis had two choices. Either he could preheat the nitric
acid to boiling point, or he could add a third nasty鈥攁niline鈥攖o make
the propellant so unstable that it would immediately explode. Neither prospect
was particularly attractive. 鈥淚 wouldn鈥檛 have blown myself up,鈥� he says, 鈥渂ut
the development programme would certainly have been dangerous.鈥�
Propellants made from solids such as ammonium perchlorate also have
disadvantages. Once alight they tend to be difficult to control, and they are
extremely dangerous to handle. So hybrid systems, combining solids and liquids
or gases, are becoming popular among rocket enthusiasts. Hybrid propellants tend
to be safer, but they have one big problem: they generate less power than solid
or liquid propellants alone. The difficulty is getting the oxidiser and fuel to
mix thoroughly.
Willis鈥檚 approach seems to have overcome this. Rocket scientists measure how
much 鈥渙omph鈥� you get from a propellant by measuring its specific
impulse鈥攖he ratio of thrust to fuel consumption. By locking a high
concentration of oxygen gas inside an energy-rich oil, ASPROP produces a
specific impulse of 220 newton-seconds per kilogram, close to the value of 240
Ns/kg produced by the space shuttle鈥檚 solid-fuel boosters.
To help develop his propellant, Willis has set up an organisation called the
Space Quest Foundation. Part funded by Nestl茅鈥攖he makers of Aero
chocolate鈥攈e has patented the propellant and moved from his garage to
a small workshop.
He has also refined his manufacturing process. Willis quickly realised that
the key to success was to pump as much oxygen into the oil as possible. So he
has increased the gas pressure from the 2.1 megapascals (21 atmospheres) in his
original experiment to a crushing 21 megapascals. And Willis believes he can go
further. 鈥淲e鈥檙e experimenting to try to go up to 31 megapascals,鈥� he says.
As in a good champagne, the secret of his propellant is exactly the right
size of bubble. Higher pressures should give smaller bubbles, a faster rate of
burn and a higher specific impulse, but the incredibly high oxygen pressure
inside them creates its own problem. Even though the epoxy resin in the engine
oil sets it into a solid, the moment the pressure is released the outside of the
propellant begins to crackle and pop as the pressurised bubbles burst. 鈥淭he
whole thing starts to crack up,鈥� says Willis.
He has come up with a simple but elegant solution. Just before the propellant
sets hard, he spins the sticky mass at about 200 revolutions per minute. By
controlling the spin rate, Willis creates an uneven distribution of oxygen
bubbles, with more on the inside of the propellant and fewer towards the outside
(see Diagram). This is because the denser oil is thrown outwards under
centrifugal force, while the lighter oxygen bubbles accumulate at the centre of
the cylinder. Since the outer layer of the propellant has a higher overall
density and fewer bubbles, it is tougher and doesn鈥檛 crack when the oxygen
pressure is released. This brings another advantage. 鈥淭he [outside of the]
propellant is as strong as mild steel,鈥� says Willis, 鈥渟o you only need a thin
casing on the rocket.鈥�
Compared with corrosive acids and sensitive solid fuels, ASPROP is pretty
stable. The best way to ignite the fuel, says Willis, is to wrap a coil of wire
in paper, set it in a little crumbled ASPROP at the base of the main propellant
and pass a large current through it. The wire heats up to about 2000 掳C, at
which point the ASPROP bursts into flames.
Having created an improved propellant and designed a rocket that runs on it,
Willis is beginning to think big. He has begun negotiations with the Norwegian
Space Centre to use their launch facilities at And ya inside the Arctic Circle.
In about two years鈥� time, if all goes to plan, his four-stage rocket should lift
a 2-kilogram payload to an altitude of 200 kilometres. This could qualify him
for the $250 000 prize offered by the Foundation for the International
Nongovernmental Development of Space (FINDS) in Washington DC for the first
amateur launch to this altitude. Then, Willis says, he will exploit the cheaper
end of the launcher market鈥攑utting small satellites or scientific
experiments into low-Earth orbit.
Before his dream can come true, though, he must prove to the world that his
propellant works by launching a smaller prototype鈥攖he first ASPROP-powered
rocket. The propellant could prove spectacularly successful, but we may not find
out for a while, because the dead hand of bureaucracy now lies heavily on the
Space Quest Foundation. Britain鈥檚 Health and Safety Executive, which is
responsible for industrial safety, has halted his work until Willis is granted
an explosives licence.
The project鈥檚 source of funding has also caused problems. Nestl茅
provided Willis with sponsorship for his 鈥淣estl茅 Aerospace鈥� rocket. But
back in the 1980s, the company caused a storm when it exported powdered milk to
Third World countries. Protesters claimed that because it was used instead of
breast milk, it endangered the health of millions of babies. Determined to
prevent Nestl茅 from gaining publicity from sponsoring the project, they
are threatening to disrupt the launch.
Unwilling to load an already risky exercise with further problems, Willis
postponed the original launch in May. But now forces more powerful than
demonstrators or bureaucracy have taken a hand. 鈥淭he weather has been too
atrocious to launch,鈥� says Willis glumly. He could be in for a long wait. At
least he鈥檒l have plenty of time to munch on some snacks鈥攁nd maybe find
some more inspiration.
