STRANGE ideas were a speciality of the science fiction writer Robert A.
Heinlein. When he died in 1989, he left behind descriptions of brain-eating
slugs and fantastical space vehicles carved from the inside of meteors. So it is
only fitting that it was at a memorial service for Heinlein that aerospace
engineer Gary Hudson bumped into Tom Clancy, the author of thrillers such as
Clear and Present Danger and Op-Center. Hudson
eventually talked him into contributing $1 million towards a bizarre
spacecraft.
Now, almost a decade and $16 million later, Hudson is about to begin
building this craft: a reusable rocket with a crew of two that will be blasted
into space by a weird rotating engine, and will return to Earth tail-first
beneath four whirling rotor blades like a giant metal sycamore seed. “Gary
Hudson is doing something revolutionary,” says NASA administrator Daniel Goldin,
who met Hudson last year. “I love it. It’s in your face. We don’t have enough
people who are willing to put their life on the line with original ideas.”
Hudson’s revolutionary approach to rocket design comes from his desire to
capture a chunk of the market for satellite launchers. Over the next 10 years,
telecommunications companies are planning to put almost two thousand satellites
into orbit, and this means big money. The going rate for a satellite launch is
anything up to $11 000 for every kilogram delivered into orbit, and
anyone who can shave a few dollars off that price tag stands to make a
killing.
Advertisement
High-priced monsters
The main reason prices are so high is that today’s rockets are heavy,
inefficient monsters. As they climb through the atmosphere they shed engine
stages like used sweet wrappers. Even NASA’s “reusable” shuttle relies on
disposable propellant tanks and bolt-on solid rocket motors. They are released
when they’ve done their job, and either burn up in the atmosphere or fall into
the ocean.
A variety of schemes have been devised to make launches cheaper. One company
plans to refuel its delta-winged vehicle in midair to save weight at launch.
Another proposes saving fuel by towing its craft to an altitude of 10 000 metres
before firing its rocket engines. But Hudson believes he can slash the launch
price to below $2200 per kilogram by using a radically different kind of
launch vehicle, which he calls the Roton.
Instead of relying on the heavy and expensive turbopumps that are used in
conventional rockets, Hudson makes his whole engine spin. As it does so, simple
centrifugal force will throw kerosene fuel and liquid oxygen into the engines’
combustion chambers. To save more weight, Hudson is building the Roton’s conical
body from graphite composites, which are considerably lighter and stronger than
the aluminium that is normally used. And if Hudson’s calculations are correct,
his ship should weigh less than the equivalent rocket, yet be able to carry 25
per cent more fuel—a difference that should allow it to reach space in one
go.
Hudson’s spinning engine is undoubtedly ambitious. It consists of 96
beer-can-sized combustion chambers mounted around the edge of a rotating
disc some 7 metres in diameter. There are two types of combustion chamber:
boosters and sustainers. The slightly smaller boosters are designed to be most
efficient during takeoff, while the larger sustainers become more efficient at
higher altitudes.
Before launch, the engine disc will be spun up to more than 700 revolutions
per minute by a hydraulic motor attached to the launch pad. As the engine
rotates, kerosene fuel and liquid oxygen are thrown into the combustors from
storage tanks inside the Roton’s body. To keep the engine spinning after
lift-off, the combustors are offset by 1.5 degrees from the vertical, just like
a giant Catherine wheel.
The propellants are fed to the engine through rotating fluid seals attached
to the central hub. The issue of engine seals has become highly sensitive since
the shuttle Challenger blew up in 1986 just after liftoff. This was caused by
the escape of hot gases through an O-ring seal on one of its engines. But Hudson
is quick to point out that there is no comparison between the seals on the
Challenger’s engine and those on the Roton. He says the seals on the Roton will
experience a pressure of only 20 pounds per square inch (about 1.4
atmospheres)—about a tenth of the pressure felt by those used in existing
rocket engines—and will operate at room temperature too.
The design sounds complex but Hudson is confident that it will fly: “It’s
fairly straightforward to rotate this 4000-pound [1.8-tonne] engine,” he says.
“The main engine bearing is an off-the-shelf product usually used on massive
boring mills.” He also believes that building a spinning engine will be a lot
simpler than developing reliable turbopumps from scratch. “That would take three
to four times as much money,” he says. “And we expect this system to be about 10
per cent lighter.”
But before even thinking about flight tests, Hudson will have to prove that
his design is capable of withstanding high temperatures and extraordinary forces
without shattering into thousands of red-hot fragments. The periphery of the
disc experiences an astonishing 1800g as it spins, and the walls of the
combustor will easily reach 800 °C. The combustors will be built of the same
heat-resistant zirconium copper alloys used in conventional rocket engines, but
some warping from the intense heat and strong forces is inevitable, and has to
be allowed for in the engine’s design.
Nothing lasts forever
To provide some cooling, the combustors will be chilled with the same supply
of liquid oxygen that is used to generate thrust inside the chambers—by
simply routeing the cold liquid through the walls of the chambers before it is
burned. Even then, Hudson accepts that the engines will not last forever. “If we
have to change them every 20 flights we wouldn’t like it, but it wouldn’t be a
catastrophe,” he says. “But we’re going for 100 flights.”
Hudson will shortly be testing the Roton’s combustion chambers on a
centrifuge built by his company, the Rotary Rocket Company, outside the town of
Mojave in the desert north of Los Angeles. If the tests are successful, engine
construction will begin in earnest later this month.
To keep the weight down, the Roton’s liquid oxygen tank will be made from
carbon-epoxy composites instead of the usual aluminium alloy, and will form part
of the rocket’s outer wall. To build it, Hudson has enlisted Scaled Composites,
also in Mojave, a company headed by Burt Rutan who is a master at building
lightweight aircraft with composites.
Engineers at Scaled Composites expect the material to be strong enough. “The
big thing that composites allow you to do over metals is to orient the fibres to
take a high load in one direction,” says Bob Williams, who is managing the Roton
programme at Scaled Composites. Aluminium has the same strength in all
directions “so you have to pile on more”, he explains. The designers calculate
that pressure from the liquid oxygen inside the tank will add to its rigidity
and strength. “The liquid oxygen tank will be part of the primary structure,”
says Williams. “There won’t be a structural aeroshell around it.”
The decision to use a carbon composite storage tank is particularly brave
because of the danger of chemical reaction between its carbon structure and the
concentrated oxygen inside. If the two come into contact, says Williams, they
could rapidly oxidise: “That’s a polite way of saying an explosion,” he
says.
To keep them apart, engineers at Scaled Composites insist they have found a
way around the problem. They have developed a chemical lining just a few
hundredths of a millimetre thick that can be sprayed or painted onto the inside
of the tank. Hudson refuses to reveal the exact chemical composition of the
liner, simply describing it as a “polymer-like liquid”.
With the potential consequences of failure so devastating, the lining has
been extensively tested. “What we did was go out and build tanks and panels,
then we put liquid oxygen on one side, pressurised them and checked for leaks,”
says Hudson. “We even heated one side to simulate re-entry.” Samples of the
lined composite have been tested 120 times without failure, and now engineers
are ready to begin using them to build the tanks. “The industry said you
couldn’t do that but we have done it for about a half-million dollars,” Hudson
says proudly.
Building the Roton and getting it into orbit is only half the battle: for all
its advanced technology, the concept stands or falls by whether the rocket can
get back to Earth in one piece. And it’s here that Hudson’s design really goes
out on a limb. Attached to the Roton’s nose cone will be four 15-metre-long
rotor blades made from heat-resistant Inconel alloy. During its ascent through
the atmosphere, the blades are kept safely tucked against the side of the
vehicle. Only when the Roton has ejected its satellite payload and is ready for
re-entry will these blades be deployed.
They are in for a testing time. As the rocket strikes the top of the
atmosphere base-first at Mach 25, friction will convert the air molecules into a
shock wave of hot plasma. The engine and the rotors would be destroyed if they
came into contact with the plasma wave, but Hudson is confident that the design
team has done its homework to make sure that this won’t happen. Computational
fluid dynamics suggests that the heat and shock waves generated during re-entry
will be deflected away from the engine by a disc-shaped heat shield. “The
combustors are protected by the geometry of the engine,” he says. “The heat
shield establishes a shock wave, but the combustors are slightly recessed into
the shield.” And to keep its temperature down to below 700 °C, Hudson plans
to cool the shield and combustors with a constant trickle of water which will
flow across their surfaces in tiny channels etched into the metal.
But the rotor blades are a different matter. Fully deployed, they would sit
right in the firing line where the plasma would instantly slice through them. So
the engineers at the Rotary Rocket Company have designed them to trail behind
the vehicle at a 60-degree angle, making the whole thing resemble an inside-out
umbrella. In this position, the rotors will stay shielded within a pocket of
relatively cool air that is created by the Roton’s base. But they will still be
exposed to enough of the airflow to spin slowly, and this, Hudson says, will
“assist with vehicle stabilisation during re-entry”.
The atmosphere will bring the Roton down to subsonic speed by the time it
reaches an altitude of around 10 kilometres. As it does so, the angle of the
rotor blades will be changed so that they spin faster and faster in the airflow.
Since its rotors are unpowered, the Roton will start to behave like a helicopter
under “auto rotation”—the technique that helicopter pilots use to avoid
falling like a stone when they have suddenly lost power. Small thrusters
substituting for a tail rotor will offset the torque of the main rotor and allow
the craft to change its orientation and glide down towards its intended landing
site.
Keep on spinning
When helicopter pilots land under autorotation they disengage the clutch and,
at the last minute, adjust the angle of the rotor blades so that airflow not
only keeps them spinning but also maximises the lift. Hudson says that as the
Roton approaches the ground, its pilot will do the same, slowing the Roton to a
rate of descent of less than 1 metre per second. A soft landing is vital, as it
will allow the Roton to get away with using lightweight landing legs.
If using a rotor to slow the craft’s re-entry sounds ambitious, it is
positively tame compared with Hudson’s original concept. This envisaged using
rotors as an integral part of the launch system, too. Powered by small rocket
engines mounted on the tips of the blades, the rotor would have supplied 90 per
cent of the rocket’s thrust at takeoff. This would have carried the Roton to an
altitude of about 5000 metres, where the rotary engine would fire. But to lift
the 135-tonne craft, the rotors would have needed to be 19.5 metres across,
instead of the 15 metres in Hudson’s current design. “That doesn’t sound like a
very big difference,” says Hudson, “but the technical risk of building a much
larger-bladed Roton was greater than we wanted to bear for this first
.”
So last year, after much hand wringing, Hudson reluctantly scaled back his
plan and shelved the idea of rotor-assisted launch. A team of engineers is
continuing to brainstorm the approach, but the first Roton will lift off under
rotary rocket power alone. “We’ll go back to the bladed Roton later for
passenger-only vehicles,” says Hudson.
Radical though it seems, the idea of using rotors to help rockets re-enter
the atmosphere and land was first proposed as far back as the 1950s. Ten years
later, NASA studied the idea of deploying a folded or telescoped rotor from a
space capsule, and its designs were eventually tested to Mach 15 in a wind
tunnel at NASA’s Ames Research Center in California. “None of the concepts used
rotors for ascent,” says Hudson, “but the work has been very useful to us.”
Hudson’s colleagues elsewhere in the aerospace industry will be looking on
with interest. “He has a basic concept that fundamentally could work but it’s
quite challenging,” says Robert Zubrin, an engineer with the aerospace
consultancy Pioneer Astronautics of Indian Hills, Colorado. “If he succeeds,
he’s going to leave quite a few people behind.”
Even Goldin, one of Hudson’s most vocal supporters, is not convinced that his
Roton will succeed. But if it does, NASA would buy the Roton’s launch services,
he says. “It’s going to shake up this aerospace industry that’s been too
dependent on the federal government.”
The Rotary Rocket Company have so far raised $17 million and expect to
receive another $30 million later this year. If all goes to plan, the
first engine should be ready for flight by March next year, and a complete Roton
should debut a year later, says Geoffrey Hughes, marketing director at the
Rotary Rocket Company’s headquarters in Redwood Shores, near San Francisco. And
even if it takes a year or two more, it may not matter. As Hughes sees it, the
Roton will not just serve the satellite revolution—it will fuel it. Once
you bring down the launch costs, companies that hadn’t conceived of building
satellites will start, he says.
And he doesn’t see it stopping at scientific and telecoms satellite. “We will
operate our prototypes initially, but after a few years we intend to sell to any
comers,” says Hudson. “That would include space tourism firms.”
