A long narrow body and little delta wings make Pegasus look more like a cruise
missile than a satellite launcher. And like a cruise missile, it begins its
missions slung beneath the belly of a large jet aircraft. On 8 March this year,
some 100 kilometres off the Californian coast, a modified Lockheed TriStar
airliner flying 12 kilometres above the ocean released a Pegasus rocket carrying
a small US Air Force research satellite. For a few seconds the Pegasus glided,
unpowered, on its stubby wings. Only when it was a safe distance away did the
first-stage rocket motor ignite, pushing it through the sound barrier. Two more
stages accelerated it towards the magic speed of Mach 25 needed to reach space.
Within 11 minutes of the release, Pegasus had placed its cargo into orbit some
800 kilometres above the poles.
Since March, there have been further successful Pegasus launches, and more
are scheduled. The rocket is built by the aerospace company Orbital Sciences,
based in Washington DC, which claims it can launch small satellites weighing up
to 450 kilograms at about half the cost of other rockets. Because Pegasus is
launched from the air, there is no need for expensive ground-based launch
facilities. And because the rocket begins its journey high above the ground it
does not have to carry the fuel that conventional rockets need to burn to punch
their way through the densest part of the atmosphere. Best of all, the modified
airliner that carries Pegasus鈥攅ffectively the rocket鈥檚 first
stage鈥攊s cheap to operate and completely reusable, unlike conventional
launchers.
But launches could be cheaper still if, as well as flying payloads into
orbit, Pegasus could fly back through the atmosphere, land and be ready to fly
again within hours. Orbital Sciences, NASA and a number of other organisations
and companies that helped develop Pegasus believe that this kind of
aircraft-like performance could make space flight as routine as air travel is
today.
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Last year, NASA, Orbital Sciences and the aerospace giant Rockwell
International joined forces to develop a small experimental air-launched, winged
rocket that could do the job. It would have to be capable of withstanding
temperatures in excess of 1300 掳C during re-entry and, since it would be
uncrewed, be able to fly autonomously at speeds ranging from a few hundred
kilometres per hour to Mach 25. Above all, it would have to be quick, simple and
cheap to maintain. The aim was for the vehicle, designated the X-34, to fly
sometime in 1998. But the alliance proved an unhappy one, and after months of
squabbling over the design Rockwell dropped out. By the end of the year, the
X-34 was facing cancellation.
Back on course
But this August, NASA and Orbital Sciences announced a less ambitious
$50-million scheme for a craft that will test the technologies needed for
a reusable launcher, but will not itself leave the atmosphere. The plan is for
the vehicle to be powered by an experimental reusable rocket motor that will
take it up to 80 kilometres above the Earth and to speeds of up to Mach 8.
Though this is only a third of the speed that an orbiting craft would experience
during re-entry, the vehicle will be designed to heat up to re-entry
temperatures as it passes through the air. The vehicle will fly autonomously and
glide back to base when its fuel runs out, ready for another launch within 24
hours of landing. The first flight is planned for 1998 at the US Army鈥檚 White
Sands Missile Range in New Mexico.
Some of the most critical work for the X-34 is being carried out at NASA鈥檚
Ames Research Center, south of San Francisco, where engineers are putting the
finishing touches to the insulating tiles that will protect the craft from the
heat of re-entry. The tiles were developed to protect the Mars Pathfinder during
its plunge into the Martian atmosphere in July 1997, but engineers say they will
be equally suitable for Earth re-entry. Dan Rasky, the mechanical engineer who
heads the thermal protection systems group at Ames, expects the tiles to survive
as many as 25 flights at temperatures up to 1300 掳C. 鈥淏ut you don鈥檛 know for
sure until you put them on a vehicle and fly it,鈥� he says.
The tiles are made of a material called SIRCA, or 鈥渟ilicone impregnated
reusable ceramic ablator鈥�. The base material is a matted block of glass fibres
that has been fired in an oven at about 1200 掳C for one hour. Each tile is then
soaked in a silicone resin, a substance containing silicon, oxygen, carbon and
hydrogen, which leaves a rubbery coating on the fibres. The tiles are then
fitted to the vehicle, even though the silicone resin cannot do its job of
protecting the craft until it has been fired at a higher temperature. 鈥淵ou don鈥檛
fire it in an oven,鈥� says Rasky. 鈥淵ou fire it on the vehicle when it re-enters.鈥�
The silicone reacts with oxygen in the atmosphere forming water vapour and
carbon dioxide. This layer of gases acts as a thermal barrier, reducing the
amount of heat reaching the tiles. This process also leaves a strong, glassy,
black coating over the tiles, which radiates heat. Some material is inevitably
lost in this ablative process, but engineers expect each tile to last for
several flights without having to be replaced. 鈥淭he X-34 will show how many
times it will work and whether there are any surprises,鈥� says Howard Goldstein,
a chemical engineer who helped to develop SIRCA in 1992.
One as yet unknown factor is the rate at which the tiles will be eroded by
tiny particles in the atmosphere. The tiles can survive a certain amount of
erosion because the continuous production of glass fills in any pitting in the
material. But at Mach 8, even water droplets abrade the surface like a
sandblaster and only flight testing will tell how effective the self-repair
mechanism will be. 鈥淭here are all sorts of real-world expectations that you have
a hard time duplicating,鈥� says Rasky.
Easy-fit tiles
When the tiles do need replacing, the job should be much easier than it is
with the protective tiles on the space shuttle, which are moulded and fired,
sprayed with a ceramic coating and then heated a second time. 鈥淭he difficulty
there is that when you do that second firing the material warps slightly,鈥� says
Rasky, and this makes the shuttle tiles difficult to install. This is one reason
why the shuttle costs around $400 million every flight. SIRCA tiles do
not need to be fired a second time before they are fitted, so there is no
warping. This makes them simple to install on the X-34鈥檚 nose and the leading
edges of its wings, where the heating will be greatest.
Engineers are still tweaking the SIRCA formula. The latest sample is
currently being tested in a wind tunnel at Ames that simulates the intense
conditions of re-entry. So far, the sample has survived 15 simulated flights at
Mach 8 and 1300 掳C. Ten more are planned.
To navigate, the X-34 will use the Global Positioning System, a constellation
of satellites broadcasting signals that give an accurate fix on its position.
But ensuring that the uncrewed craft will be able to glide smoothly through the
atmosphere and then land safely poses a series of challenges for the design
team. Designs that are stable at hypersonic speeds become unstable at lower
velocities. So the X-34 must make continual adjustments using flaps on its
wings. 鈥淭he autonomous landing will be a challenge,鈥� says Bob Lindberg, who
manages the X-34 project. Understanding how best to fly in these conditions is
crucial and the techniques can only be perfected in real flight tests.
Speed check
Knowing what corrections to make will only be possible if the vehicle鈥檚 speed
and attitude can be measured precisely, and gathering these data poses a
problem. Unlike conventional aircraft, the X-34 cannot rely on external booms to
take the air-pressure readings that conventional aircraft use to calculate
airspeed. 鈥淗aving a nose boom out in front can screw up the air flow,鈥� says
Jerry Budd, an engineer at NASA鈥檚 Dryden Flight Research Center in the Mojave
desert, who is directing the efforts to develop an alternative. And that is not
even the biggest problem. In a craft capable of hypersonic flight, there is also
heating to contend with. 鈥淗ow do you come up with something that won鈥檛 melt?鈥�
Budd asks.
So in place of conventional sensors, the X-34 will have an array of 24
carefully positioned air inlets, flush with its body, with pressure sensors
fitted inside. Air entering the holes, each less than 1 millimetre across, will
be directed along an arrangement of tubes designed to cool it down before it
reaches the sensors. 鈥淚f one hole gets plugged by a bug you still have 23,鈥� says
Budd. From the pressure pattern over the array, an onboard computer will
calculate the speed and attitude of the rocket, and adjust the control surfaces
accordingly. This will be especially important at speeds below Mach 1, when the
vehicle is at its least stable. 鈥淚t鈥檚 like balancing a broom on the tip of your
finger,鈥� explains Lindberg.
When it comes to interpreting the pressure patterns at hypersonic speeds the
designers will be venturing into the unknown. Wind-tunnel tests and computer
simulations of the hypersonic flow may give an indication of what to expect, but
only flight data will tell them for sure. 鈥淚t should work fine but until you
actually do it, it鈥檚 still just an educated guess,鈥� says Budd.
For propulsion, the X-34 will use a completely new engine. Known as the Fast
Track engine, it is being developed at NASA鈥檚 Marshall Space Flight Center in
Huntsville, Alabama. Fast Track, which is fuelled by kerosene and liquid oxygen,
looks and works like a conventional rocket motor. But unlike most conventional
engines, it is designed to be reusable as well as both simple to build and easy
to maintain.
Take for example the bell-shaped nozzles that direct the thrust. The nozzles
on most engines are lined with a network of tubes carrying low-temperature fuel
to prevent overheating. Since these are expensive to manufacture, the Fast Track
engine does without. Instead, its nozzle and even its combustion chamber are
made from layers of a cheap, cloth-like silica material which chars and boils
away like the SIRCA tiles on the vehicle鈥檚 body. Initially, the entire engine
will be taken out and renovated after each flight. Eventually, technicians hope
to get away with exchanging only the nozzle and combustion chamber, like a pit
crew putting fresh tyres on a racing car.
For all its ingenuity, the X-34 has already attracted heavy criticism. The
present contract between NASA and Orbital Sciences is for only two flights,
which will be at the relatively modest speed of Mach 4. NASA has a
$15-million option to extend the contract to include a further 25
flights at Mach 8, but to do this it will have to find a testing ground that is
more than 800 kilometres long, far longer than the White Sands range.
Given the funding that is going into competing reusable launch systems (see
鈥淭he race for reusable rockets鈥�), there is no guarantee that a full-scale X-34
capable of going into orbit will ever be built. 鈥淚s the X-34 in the critical
path? They don鈥檛 have a convincing argument at this time,鈥� says Charles Miller,
vice-president of the Space Frontier Foundation, a New York-based lobbying group
that promotes the exploration and colonisation of space. According to Miller,
NASA sees it mainly as a public relations backup in case the X-33 runs into
trouble, and as a source of much-needed work for NASA offices left out of the
X-33 project.
Some critics find the X-34 faintly laughable. 鈥淚t鈥檚 basically a cruise
missile with landing gear,鈥� is one dismissive summary. But while it will not fly
as fast or as high as the X-33, it is scheduled to fly first. For that reason,
if for no other, all eyes will be on it.
* * *
The race for reusable rockets
WITH the space budget being squeezed, value for money is the big driving
force behind NASA鈥檚 quest for a reusable launcher. At present it has three
projects under way, one of which is the X-34. Even if none of them comes to
fruition, they should contribute to a pool of technologies for the designers of
a cheap replacement for the space shuttle to draw on.
Experience with the shuttle has shown that reusable does not automatically
mean cheap. In the early 1970s, when NASA was developing the shuttle, it
promised that its new craft would slash the cost of placing payloads into orbit
from around $2000 per kilogram to less than $50, and that each
launch would cost no more than $2 million. Twenty years on, the shuttle
costs around $400 million to launch and each kilogram of payload costs
more than $25 000 to place in space. Much of this money goes on
overhauling the engines after each launch and replacing parts that cannot be
used again. So now NASA is going back to the drawing board.
For the past two years, NASA has been funding a project to fly a demonstrator
version of a reusable rocket known as the Delta Clipper-Experimental or DC-X
(see 鈥淪pace Clipper comes of age鈥�, New 杏吧原创, 12 August 1995, p
27). The vehicle, which takes off and lands vertically, like the rockets in a
Flash Gordon film, was designed and built in 18 months at a cost of $60
million, about the price of two space shuttle toilets. Last June, it
demonstrated the ability to fly again within 26 hours of landing, far better
than the shuttle and almost as good as commercial airliners.
However, the DC-X project still has a long way to go before success is in
sight. The vehicle has not flown higher than a few thousand metres, and is not
designed to to reach the speed of sound, let alone the hypersonic speed needed
to escape into orbit. Earlier this year, during a routine landing, one of the
DC-X鈥檚 legs failed to deploy. The vehicle toppled over and was seriously damaged
in the resulting fire. With no replacement vehicle planned so far, the future of
the project is uncertain.
NASA鈥檚 other project is the X-33, a $1 billion programme to build a
small-scale version of a reusable rocket capable of carrying humans into space.
The vehicle, known as the VentureStar, will test a revolutionary new engine (see
鈥淔light of the aerospike鈥�, New 杏吧原创, 6 July, p 36) as well as an
untried thermal protection system at speeds of up to Mach 15. The first flight
is due in 1999, when it will take off vertically and land horizontally, like the
shuttle.