7 JULY 1995. It is dawn at the White Sands Missile Range, a vast expanse of desert in New Mexico where the US Air Force is testing an extraordinary vehicle. Amid white clouds of boiling hydrogen stands the world鈥檚 first reusable rocket 鈥� the Delta Clipper-Experimental or DC-X. The hydrogen is cooling the rocket鈥檚 motors to minimise the thermal shock when the liquid hydrogen and oxygen fuel floods into them prior to ignition. 鈥淭hey do the same for the shuttle engines,鈥� explains Dale Shell, a US Air Force major who is second-in-command of the test.
This will be the Air Force鈥檚 final test of the DC-X before NASA takes over. The DC-X was originally part of the Strategic Defense Initiative Organisation, better known as the Star Wars project. But in November 1993, the Pentagon decided it had no use for the craft and would rather spend its money elsewhere. Last year, NASA stepped in with $1 million to finish tests and an offer to continue the project. The space agency plans to replace the aluminium fuel tanks and parts of the rocket鈥檚 internal structure with new lightweight materials and fit the craft with a network of microsensors to see how the new materials perform during flights. With the information from these and other tests, scientists hope to design a reusable launch vehicle known as the X-33 that will be light enough to reach orbit with a single stage and yet cheap and reliable enough to make space flight as easy as air travel is today.
Today鈥檚 flight will simulate the manoeuvres that a full-sized Delta Clipper would have to go through after returning from orbit 鈥� to rotate from a horizontal re-entry position to a vertical landing position. The plan is to fly to 2600 metres, turn the DC-X on its side with its nose 10 degrees below the horizontal, rotate it through 180 degrees and finally return it to the vertical position for landing. The rotation will cause the fuel to slosh around in the tanks and there is a danger that the flow to the engines will be interrupted.
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Time critical
But the scientists are confident of success. 鈥淭his what the DC-X is designed to do,鈥� says David Schweikle, who heads the team from McDonnell Douglas which built the vehicle (see 鈥淲ill the Space Clipper stay a dream?鈥�, 快猫短视频 28 May 1994). The fuel tanks contain baffles that minimise the movement of fuel and direct it towards the engine intake. 鈥淚f we lost an engine, we could recover the craft but it would be time critical,鈥� he says.
So far, the flight preparations have gone to plan. Engineers are even talking about demonstrating the DC-X鈥檚 rapid turnaround time. 鈥淚f all goes well we could launch the DC-X a second time today,鈥� says Jess Sponable, the USAF officer who oversees the project.
But unknown to Sponable and his 30-strong flight crew, the vehicle鈥檚 radar altimeter is malfunctioning. The device measures the distance above the ground, providing information which the vehicle uses to judge when to apply full thrust to slow the rate of descent during landing.
As all eyes turn to the launch pad, a bright red flame flickers from within the clouds of cryogenic gases. The DC-X rises slowly from this maelstrom, producing no visible flame in its wake. At a height of 2700 metres, 30 seconds into the flight, it tilts first to point away from the watching crowd and then back towards it. The manoeuvre takes 15 seconds during which the craft swoops 200 metres towards the ground.
But now there is a problem with the descent. The craft is plummeting unexpectedly quickly at 70 metres per second compared with the planned rate of no more than 50 metres per second 鈥� a result of the faulty altimeter readings. 鈥淪low down, baby,鈥� shouts a cameraman in the crowd. A few metres above the ground, the DC-X flight control system spots the error. The engines roar at full thrust but it is too late. The craft slams into the ground raising enough dust to hide it from view.
As the clouds part, it is clear that the DC-X has survived in one piece. The craft is designed to withstand landings that generate seven times the force of gravity on impact 鈥� for example, if the engines fail and the craft lands by parachute only. Part of the landing gear is made of a honeycomb material that can collapse by up to 30 centimetre to absorb the impact. This landing used only 15 centimetres. 鈥淭his can be repaired within a couple of hours,鈥� says Paul Klevatt, a former engineer on the project.
The crew quickly assesses the flight. 鈥淭he altimeter problem had nothing to do with the rotation manoeuvre 鈥� that was damned near perfect,鈥� says Schweikle. But the DC-X will not fly again today. The heavy landing has also split part of the bodywork and repairs will take weeks.
Fitted with wooden splints to hold the damaged bodywork in place, the DC-X is laid on its side ready to be transported by truck to its birthplace at the McDonnell Douglas aerospace laboratories in Huntingdon Beach, California. Under NASA supervision, McDonnell Douglas will modify the craft at a cost of $50 million and rename it the DC-XA.
Most of the changes are designed to make the DC-XA lighter than its predecessor. At the moment, the DC-X鈥檚 strength is provided by a set of aluminium beams inside the structure. This is a clumsy, heavy design. It is far better to design the bodywork to act as the weight-bearing structure and do away with the beams, says Curtis McNeal, who is one of the aerospace engineers overseeing the DC-XA programme. NASA has developed an extraordinarily light composite material for this purpose that consists of a strong aluminium honeycomb sandwiched between two layers of lightweight graphite epoxy. Instead of redesigning the entire craft, engineers will test the composite material in part of the craft鈥檚 midriff known as the intertank area.
The aluminium fuel tanks will also be replaced. A new liquid hydrogen tank has already been constructed from graphite epoxy which is 60 per cent lighter than aluminium. Liquid oxygen, however, tends to react with most organic compounds. So NASA is testing graphite epoxy to see whether it can be used to store liquid oxygen. The results are promising but they are not yet conclusive, says McNeal.
In the meantime, the DC-XA will be fitted with a liquid oxygen tank fashioned from an alloy of aluminium and lithium that is 38 per cent lighter than aluminium alone. The tank is being made by the Russian space company RSC Energiya which built the Russian space shuttle, the Buran.
McNeal says that RSC Energiya is extremely well organised but that the Russian transport infrastructure is hopeless. 鈥淰ital tools were lost en route for two months,鈥� he says. The Russians say the tank will be ready by September when it will be hauled by truck from RSC Energiya鈥檚 factory near Moscow to Frankfurt and then flown to the US. 鈥淭hat鈥檚 a pretty tough schedule,鈥� he adds.
Once the tanks are in place, the DC-XA will be fitted with over 200 microsensors to monitor the temperature and stress throughout the craft. 鈥淥ur fibre-optic stress sensors add almost no weight at all,鈥� McNeal points out.
The sensors are part of a vehicle 鈥渉ealth鈥� management system that will be vital for operating reusable rockets in future. Every time the space shuttle flies it has to be completely overhauled and recertified to fly again, a process which can take months, involving thousands of engineers, says McNeal. In future, on-board sensors will indicate when parts need replacing. Apart from routine maintenance that can be carried out by a handful of engineers in a few hours, these rockets will be ready to fly again and again, much as aircraft do today.
In addition to the main engines, the DC-X has four smaller auxiliary thrusters that use gaseous hydrogen and oxygen as fuel. This has to be stored separately from the liquid fuel and requires its own heavy tanks that take up large amounts of room. NASA plans to get rid of these by using liquid fuel from the main tanks and converting it into gas before pumping it to the smaller thrusters. McNeal says the equipment that will do this has been cannibalised from the Buran.
The DC-XA is due to fly in the spring next year and McNeal is confident that it will match the DC-X鈥檚 perfomance. 鈥淲atching the DC-X fly was the crowning point in my career,鈥� says McNeal, who learnt his craft designing missles. 鈥淚t鈥檚 always nice to fly something and have it come back,鈥� he grins (see Graph).
