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Florida

FOR the space shuttle, the journey home begins high above the island of
Madagascar. The pilot turns the shuttle on its side so that a wing and the nose
can begin carving their way into the atmosphere. 鈥淵ou have to knife your way in.
Otherwise you鈥檇 skip right off the atmosphere like a rock skimming over water,鈥�
says Bryan O鈥機onnor, who piloted the shuttle in 1985.

It is a manoeuvre the pilot cannot afford to get wrong. The shuttle is a
notoriously poor glider so any navigational errors are difficult to correct. If
the shuttle veers more than 1300 kilometres off course鈥攁 distance it
travels in less than three minutes in orbit鈥攊t could never make up the
lost ground. And if it failed to reach its destination at Cape Canaveral in
Florida, or any back-up landing sites, it would have no option but to
crash-land. The crew would be unlikely to survive.

The shuttle鈥檚 shortcomings as a glider don鈥檛 end there. When it slams into
the atmosphere at 27 000 kilometres per hour, it heats up. It gets so hot that
the gas around the shuttle ionises, forming a plasma. Radio waves cannot pass
through this plasma, blacking out communications for several agonising minutes.
If anything goes wrong during this period, ground control is helpless to
intervene. Other spaceplanes such as the Russian space shuttle, the Buran, and
the Japanese vehicle, Hope-X, have similar shortcomings.

But spaceplanes need not be like this. Researchers at NASA say that in the
next century it will be possible to build vehicles using ceramic materials that
can glide so efficiently that they could land almost anywhere on the planet
after entering the atmosphere. These planes will be sleek-bodied and
sharp-nosed鈥攖he way spaceplanes ought to look. They will also be safer and
easier to fly鈥攁nd they will not suffer from communications blackouts
during re-entry.

Earlier this year, NASA began tests of the ceramics that will make this
possible by mounting them on the nose of a ballistic missile. Now it hopes to
carry out further tests by fitting the material to a top secret spaceplane left
over from a military space programme.

But why are the shuttle and its relations such clumsy fliers? The reason is
that they share the same fat, blunt-bodied shape. This shape is important
because it prevents the vehicles from overheating during re-entry. The large
surface area of the nose and leading edges of the wings creates a large shock
wave, which prevents the hottest gases touching the surface. So the hottest
parts of the shuttle鈥攖he nose and leading edges鈥攔each only 1700 掳C,
which can be easily handled by the shuttle鈥檚 thermal protection tiles.

Unfortunately, that large shock wave also creates huge amounts of drag. For
example, the surface area of the shuttle鈥檚 nose is 64 000 times bigger than that
of a sharp-bodied vehicle like Concorde. So the drag it creates is 64 000 times
greater, says Paul Kolodziej, an engineer on the project at NASA鈥檚 Ames Research
Center in Mountain View, California.

The ratio between lift and drag determines how fast an unpowered spaceplane
will drop. For the shuttle it is 1.5鈥攆or every metre the shuttle falls it
travels 1.5 metres forward. By comparison, a recreational glider travels 30
metres for every metre of altitude lost. 鈥淢ost hypersonic vehicles have really
lousy lift to drag ratios. Apollo had one of 0.3. What we really need for a
global range is 3.6 or more,鈥� says Kolodziej.

Better ratios are possible by designing a spaceplane with a sharp nose and
wings, like Concorde. During re-entry, the leading edges would create a tiny
shock wave and little drag. But the nose and leading edges would heat up to 2800
掳C, far higher than any conventional material can withstand.

Enter ultra-high-temperature ceramics (UHTCs), brittle compounds of either
zinc or hafnium, and boron, silicon and carbon. They were first developed 30
years ago by the US Air Force and tested in the 1970s. The research was
eventually abandoned because the material tended to fracture unexpectedly when
heated rapidly鈥攏ot a property that endears the material to aerospace
engineers. Picture a blowtorch against glass, says Kolodziej. For a material
that would have to survive being chilled to -129 掳C and then rapidly heated
to thousands of degrees, this was obviously a severe disadvantage.

Today, scientists know far more about the properties of brittle solids like
UHTCs and the way they fracture. NASA believes that this increased
understanding, together with improvements in the way UHTCs are made and better
design could make them aerospace engineers鈥� dream material. So Kolodziej and a
team led by Joan Salute, also at Ames, have started a programme called SHARP, or
slender hypervelocity aerothermodynamic research probes, to find out more.

To make UHTCs, engineers start with compounds of hafnium or zirconium and
boron which are commercially available in powdered form. The trick is to make
the powders with a specific grain size and then to mix them with silicon carbide
in precise proportions. Heating the mixture to over 1600 掳C at high pressure
produces the UHTC, a greyish-black material that is brittle like a glass, with
the thermal conductivity of stainless steel and a melting point of more than
2800 掳C.

Salute and her team have turned to a small company based in Rhode Island
called White Materials Engineering to refine the production process. The
details, including the exact firing temperature and distribution techniques, are
closely guarded secrets. 鈥淭here are tricks of the trade that you pick up along
the way. For instance, what do you mix the material in so that you don鈥檛
contaminate it?鈥� says Jeff Bull, an engineer on the SHARP project.

He and his colleagues are helped by improvements in the powders, which are
cleaner than they used to be, with better distributed particle sizes. The
resulting ceramic materials are more uniform than they have been in the past,
says Bull, so they are less likely to fail because of manufacturing defects.

Nonetheless, they will fail at some point in the future. The problem is that
they give no warning. 鈥淐eramics are brittle and so shatter or crack,鈥� says Bull.
This kind of failure is always sudden and always catastrophic. A catastrophic
failure during re-entry would almost certainly endanger the vehicle. In the
past, this unpredictable nature of brittle fracture frightened engineers
away.

Today, brittle fractures are better understood: it turns out that brittle
fracture can never be predicted. 鈥淚t is essentially a statistical process,鈥� says
Bull. 鈥淭he trick is to reduce the chances of a fracture with careful design.鈥�
For example, the likelihood of a fracture in small volumes of material is much
less than in large volumes. So making ceramic components in small parts
significantly reduces their chance of failure.

Temperature gradients and the stresses they create can also be more easily
handled in small volumes. In fact, the size of the piece can be matched to the
thermal stress it is likely to suffer. 鈥淚f you do it right, thermal shock just
isn鈥檛 an issue,鈥� says Bull.

Engineers also believe they have a better grasp of the chemical changes that
take place during re-entry. These can even work in their favour. For example,
the surface of the hafnium ceramic oxidises into a flexible layer 2 millimetres
thick that can bend without cracking. This is useful because the temperature
gradient is greatest near the surface. 鈥淭hese things were not taken into
consideration in the design 30 years ago,鈥� says Bull.

The ceramics held up better than expected during NASA鈥檚 first test earlier
this year鈥攁 suborbital hop in which the vehicle re-entered at a speed of
up to Mach 12. During the test, engineers monitored the temperature and pressure
around the hafnium UHTC fitted on the nose of a ballistic missile. They had
expected the hafnium diboride nose-tip to fail at an altitude of about 30
kilometres because of the steep angle of entry. In the event, it survived until
it reached 20 kilometres, said Salute. The vehicle eventually splash-landed and
was not recovered. 鈥淲e hit the ocean at Mach 5 so there was very little left,鈥�
says Bull.

Next June, Salute and her team will try a more ambitious plan. This time they
want to recover the missile head from the sea so they will have to use
parachutes for a soft landing in the water. 鈥淲e鈥檒l home in on a beacon and pull
it out of the water,鈥� says Bull.

Coming up with a design to survive even a gentle impact will be the real
trick. When the re-entry vehicle hits the cool Pacific waters it will still be
smouldering at several hundred degrees C. The ceramic might well shatter from
the shock.

Instead of a nose cone, the June experiment will test two wings protruding
from a missile鈥檚 nose. 鈥淭hese things could still be quite toasty when they reach
the surface. You don鈥檛 want them quenching in the water,鈥� says Bull. So the
SHARP engineers are working on a system to automatically retract the wings into
a watertight case before they hit the ocean. 鈥淭hat鈥檚 pretty tricky and it鈥檚
still on the drawing board,鈥� he adds.

The data from these tests will be invaluable. One of the biggest barriers to
the use of UHTCs is lack of data on how they perform. One of the goals of the
SHARP programme is to thoroughly fill that gap so engineers can feel confident
when using it in their designs.

Some unexpected benefits are beginning to show up. At 4.5 grams per cubic
centimetre, hafnium UHTC is more than twice as dense as the shuttle鈥檚 thermal
protection material.

This is not the disadvantage it might seem. Most of a spaceplane鈥檚 weight is
at the back where its engines are housed. But because the centre of gravity must
be near the middle, vehicles such as the shuttle have to carry ballast to make
up the difference. Since UHTCs would only be used in the nose and on the leading
edges of the wings, they could take the place of ballast. Standard thermal
protection tiles would be used everywhere else.

Another advantage is that because the vehicle generates only a small shock
wave, it is not enveloped in a plasma. This means that communications will be
possible throughout the descent鈥� essential if ever a fleet of spaceplanes
is to operate. 鈥淚f you want to land at Memphis, Chicago or Dallas, you鈥檒l need
communications during re-entry so that you know that the vehicle is doing what
it鈥檚 supposed to do,鈥� says Kolodziej.

NASA does not expect the new materials to revolutionise American space travel
in the near future. Instead, they are being considered for a series of
experimental spacecraft that would take to the skies in the 21st century. These
vehicles would be the successors to the X-33 reusable spaceplane, a blunt design
that dates back to the 1950s.

The SHARP team is planning to test UHTCs on a spaceplane in 1999. This should
provide a more realistic test because it will be a lifting re-entry rather than
a ballistic one that simply plummets to the ground. 鈥淭hat flight will be the
grandest of them all,鈥� says Bull. If it proves successful, O鈥機onnor鈥檚 successors
at the controls of future spaceplanes will never again have to worry about
navigational errors high over Madagascar.