NEXT week the first of a new generation of small Japanese rockets is due to lift off from the space centre on the tiny island of Tanegashima, 115 kilometres south of the Japanese mainland. The three-stage vehicle, called J-I, will eventually carry satellites weighing up to a tonne into orbit. But for its maiden flight, J-I will carry a cargo that will influence the shape of the next generation of Japanese spacecraft and have a major impact on the nation鈥檚 long-term ambitions in space.
HYFLEX is an experimental hypersonic vehicle that is critical to Japan鈥檚 plan to launch its own space shuttle, called HOPE, early next century. It is designed to study the hostile conditions that HOPE will encounter as it re-enters the Earth鈥檚 atmosphere. Once J-l reaches an altitude of 110 kilometres, HYFLFX will separate from the launch vehicle and begin gathering data as it flies over the Pacific Ocean at speeds of up to Mach 15.
HOPE is the brainchild of the National Space Development Agency of Japan (NASDA) and the National Aerospace Laboratory (NAL) in Tokyo. It will carry out experiments in space, ferry cargo to and from the international space station and service satellites in orbit. But unlike the American space shuttle, HOPE will have no crew, so it must be able to carry out all its tasks remotely, with the minimum of help from the ground. While the designers of HYFLEX will be watching closely as it tears through the upper atmosphere, other teams of engineers will be testing systems that will allow HOPE to navigate, fly and land automatically.
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Lift and drag
By the time HOPE is launched, the development team expects to have spent 150 billion yen (拢1 billion), but it has not yet worked out how much each launch will cost. 鈥淥ur target is to make it the same or cheaper than the American space shuttle,鈥� says Kazuo Joko, a senior engineer at NASDA鈥檚 Office of Space Transportation Systems. This is no mean sum. The American shuttle can cost up to $400 million to launch, but Joko says that economic necessity will drive down the cost of operating reusable spacecraft to a tenth or even a hundredth of this amount. 鈥淭his is just the first step for Japan,鈥� says Joko. 鈥淥ur aim in building HOPE is to test and discover ways to build cheaper spacecraft in the 21st century.鈥�
Understanding hypersonic flight is a crucial part of this process. When it re-enters the atmosphere, HOPE will be travelling at around Mach 26 and friction will heat parts of the vehicle to more than 1500 掳C. At this temperature, gas molecules in the shock wave surrounding the craft will break up, forming a plasma of ions and electrons. As the molecules break up, the aerodynamic properties of the gas will change, making the forces of lift and drag that HOPE experiences significantly different from those faced at lower temperatures by ordinary aircraft. These differences are known as the 鈥渞eal gas effect鈥�.
Another difficulty is that plasmas are impervious to radio waves, so HOPE must be capable of coping on its own during re-entry. There is also a risk that the temperature of the plasma will increase to much higher levels than necessary. Parts of the vehicle will be covered by a heat shield made of ceramic tiles containing rare-earth metals. At high temperatures, these act as catalysts that can increase rates of chemical reactions in the plasma. If not controlled, this action could increase the temperature of the surrounding plasma by 60 per cent.
While HYFLEX will help engineers to understand some of these problems, it will not solve them all. One reason is that, of necessity, the vehicle is very different from HOPE.
For a start, HYFLEX has been designed to fit inside the fairing of J-I and so is shaped like a stubby cylinder with two short fins and a snub nose. By contrast, HOPE will look like the American shuttle, albeit only a third of the size. In addition, HYFLEX will not fly anywhere near the speed of HOPE. The design team has limited the speed of the experimental vehicle in order to prevent a complete radio blackout, so that it can relay data to the ground throughout its flight.
HYFLEX will be in the air for only 10 minutes. An onboard computer guidance system will monitor the craft鈥檚 attitude and make adjustments by moving two flaps at the back of the craft and small thrusters on each side. Although the unconventional shape of the vehicle creates lift at hypersonic velocities, at subsonic speeds it becomes unstable. So once its mission is over, the vehicle will descend by parachute into the Pacific Ocean where it will be picked up by a support ship.
Keeping cool
The Japanese team hopes that data gathered during this brief flight will help to fill in critical details of the shuttle鈥檚 design. At speeds of around Mach 15, HYFLEX will be subjected to temperatures of 1400 掳C, which the researchers will use to test the performance of a number of thermal protection materials, such as ceramic tiles and carbon-based composites. Onboard sensors will measure the temperature of these materials and how the heat spreads through the structure. Joko and his team hope to finalise details of the shuttle鈥檚 heat shielding within 18 months.
Other sensors aboard HYFLEX will measure air pressure at different points around the craft. The engineers are anxious to know if such measurements could be used to calculate the vehicle鈥檚 angle of inclination, possibly providing a backup to the craft鈥檚 inertial guidance system.
While the exact distribution of heat-shielding materials on HOPE鈥檚 outer skin has yet to be decided, the craft鈥檚 basic shape has already been settled. In the past, any aircraft design would have undergone extensive tests in a wind tunnel before being finalised. But at NAL, the designers of HOPE have pioneered a different approach.
Armed with some of the most powerful computers in the world, they have developed a way to simulate the high-speed air flow around different designs for HOPE using a 鈥渘umerical wind tunnel鈥�. When the results from this NWT are compared with those from real hypersonic wind tunnels, they agree almost exactly, says Masao Shirouzu who works on the computerised system. 鈥淲e are extremely confident in our results,鈥� he says.
With conventional wind tunnels, the designers would probably only manage to carry out sixty runs a month. But with the NWT, they are carrying out a hundred a month, which gives them more scope for revising their designs. The computerised system can also simulate how different designs heat up during re-entry, how catalytic effects add to this heating and even how the real gas effect influences the aerodynamics of the vehicle.
Shirouzu has tested dozens of HOPE designs that appear to be identical, but on closer examination have minuscule differences, for example, in the shape of the wings or the angle they make with the body. It turns out that these changes can have a huge impact on the way parts of the vehicle heat up. The NWT has allowed the design of the craft to be fine-tuned in a way that was impossible twenty years ago when the American space shuttle was being designed.
But HOPE鈥檚 design still has some curious quirks. 鈥淭he shape that is good for re-entry is not good for aerodynamics,鈥� says Yukimitsu Yamamoto, who heads the hypervelocity aerodynamics laboratory at NAL. To minimise friction during re-entry, for example, the wing area must be smaller than is ideal for aerodynamic flight. In addition, HOPE is designed to enter the atmosphere in a 鈥渂elly-flop鈥� position with its nose high in the air. To make it stable in this attitude demands careful positioning of the aircraft鈥檚 centre of mass, relative to its centre of aerodynamic lift. But this arrangement has the unfortunate effect of making the craft unstable at slower speeds. 鈥淭he vehicle is not stable below Mach 2,鈥� says Yamamoto.
To ensure that HOPE can fly at subsonic speeds, it will be fitted with a computerised system that measures the position and attitude of the craft and adjusts its flight surfaces automatically. The system will use 鈥渇ly-by-wire鈥� electronics similar to those employed in many modern aircraft. But while aircraft can also manoeuvre by adjusting engine thrust, HOPE does not have this luxury 鈥� the researchers decided not to give it an engine. Designing an engine for the spacecraft would have been enormously expensive, says Joko. 鈥淚n addition,鈥� he says, 鈥渁 vehicle without an engine is much easier to design.鈥� In practice, because it does not have an engine, HOPE鈥檚 navigation system will have to sample the craft鈥檚 position and make any compensatory adjustments at a rate of 40 times a second.
The team has now built a scale model of the shuttle the size and weight of a small car, and fitted it with a prototype computer guidance, control and navigation system. In March, the model will be hoisted to an altitude of 1500 metres by a helicopter, accelerated to 160 kilometres an hour and dropped. The team hopes to make around thirty such flights at the Woomera Flight Test Centre in southern Australia, one of the few places in the world with a runway long and wide enough.
If all goes to plan, HOPE will be launched early in the next century. The research team plans to make use of Japan鈥檚 heavy-lifting rocket, H-II. However the present version of that rocket is designed to place a 2-tonne satellite into geostationary orbit. A much more powerful version will be needed to carry the 20 tonnes of HOPE into a low Earth orbit. NASDA engineers are now working to upgrade H-II.
At present, the HOPE project is about two years behind schedule, but Joko is sanguine about the delay. 鈥淭he launch timetable is not as important as developing the new technologies we require,鈥� he says. 鈥淪ometimes,鈥� he adds with a grin, 鈥渄elays allow us to find cheaper ways to solve problems.鈥�
