REDESIGN OF THE space shuttle`s booster rockets will probably get the craft in orbit again by 1988, but there is no guarantee against another space disaster, experts caution.
Though a faulty joint in the right-hand rocket booster was the initial cause of the Challenger disaster in January, other systems in the craft have always been highly experimental and potentially dangerous–most notably the huge, detachable tank filled with liquid hydrogen and liquid oxygen, which have a high potential for explosion when mixed.
The safety fixes announced 11 days ago by the National Aeronautics and Space Administration are aimed just at making the booster rockets safer. The space agency is looking into measures to reduce other risks in the shuttle, but engineers familiar with the craft doubt that substantial improvements can be made.
”This is a tremendously complex system, and there will be more failures,” predicted Merrill Beckstead, a consultant on rockets for the U.S. Navy. ”When the next one fails, we just have to hope it`s not a catastrophic failure, so that the astronauts can abort the mission and get back alive.”
Other experts stress that the shuttle must henceforth be treated like the hazardous test vehicle it has always been, and that only people with valid scientific roles should be sent up in it.
The booster design changes should prevent a repeat of the type of malfunction that doomed the Challenger. During that tragically brief mission, hot gases leaked from a failed joint and touched off an explosion of the liquid fuel tank.
That explosion, 74 seconds after liftoff, hurled the shuttle`s crew of seven, including teacher Christa McAuliffe, on a nine-mile plunge toward the Atlantic Ocean.
Ironically the rocket boosters, which burn solid fuel, were among the best understood components of the shuttle and those thought least likely to fail, according to experts. Thus, fixing the problems that led to the Challenger disaster is not a great engineering challenge, they maintain.
Harry Hilton, a solid fuel expert at the University of Illinois, said NASA`s fix doesn`t represent a technological innovation. ”They`ve had the capability to do this for a long time,” he said.
The real question is whether the safety features can be implemented as modifications of the boosters already built and available to NASA. This is the space agency`s expectation, but testing may indicate the redesign will require building new boosters from scratch, Hilton said.
But even when the solid rockets are fixed, they won`t be fail-safe.
American experience with solid rockets, both in the manned and unmanned space programs, has shown a failure rate of about once in 70 launches. Engineers say that designing solid rockets that fail only once in 100 times may be about as reliable as current technology can make them.
Solid rockets are like giant inverted roman candles that burn steadily until they are spent. They provide no means of throttling for control. Together with the liquid fuel system, they provide the thrust to get the shuttle off the ground.
After the boosters are jettisoned 127 seconds after liftoff, the liquid system propels the shuttle into orbit. The empty liquid fuel tank is then detached from the orbiter.
Liquid jet fuel would be safer for use in the shuttle than liquid hydrogen
and oxygen, but it does not provide sufficient thrust. Paul Chiu, an engineer at the U. of I. at Chicago, has proposed a new engine design he believes would provide enough thrust using jet fuel to power the shuttle.
Chiu`s design is under consideration by NASA, he said, but it would be several years before it will be developed sufficiently for use on the shuttle. Even if it does prove workable, Chiu said, his jet fueled shuttle would still require added boost from solid fuel rockets during liftoff.
A long-term reassessment of overall shuttle safety is unlikely to come up with anything substantive, the engineers say. This is because most schemes to increase safety were given a thorough consideration when the shuttle was first designed in the 1970s. They were rejected because it was concluded that they introduced more hazards than safety.
In the period after liftoff, when the two booster rockets are firing, the crew members are captives of their cabin, with no safe way of ejecting from the craft. Engineers concluded that it is close to impossible to eject four to eight people from an out-of-control shuttle without them crashing into one another or incinerating in rocket exhaust.
Facing the immediate problem of getting the shuttle flying again, NASA and its shuttle contractors have focused on joints where segments of the rocket are joined, the source of the leak that caused the Challenger explosion.
To improve joint safety, NASA proposes installation of an extra flange it calls a ”capture feature” that will hold the segments tightly together, discouraging movement even during the extreme pressures of liftoff.
The redesign, which will be tested on the ground in scaled-down versions of the rockets this fall, also calls for improvements in sealants and in insulation to ensure that heat from the burning solid fuel at the core of the rocket does not leak out and reach its metal skin.
Below-freezing temperatures played a crucial role in the disaster by making two ”O rings,” which seal pressures inside the rocket, too brittle to perform properly. Engineers at NASA and at Morton Thiokol Inc., the Chicago-based contractor that builds solid rocket boosters for the space agency, plan to use new cold-resistant materials to replace the rubber in the O rings.
The redesign adds a third O ring that is intended to use pressure from the rocket gases to seal the joint tighter instead of allowing a gap to form, as happened on the fatal Challenger flight. The new design eliminates the need for putty to fill an insulation gap at the joint.
Another safety addition to the field joint is a weather seal and heater strip to warm the joint in cold weather and to prevent water from seeping in before launch.
The immediate redesign also seeks to improve the safety of the solid rockets` nozzles, nozzle joints, ignition system and insulation. A general reassessment of the shuttle`s solid rockets was undertaken not only because of the Challenger explosion, but because of the April explosion of an unmanned Titan rocket carrying a secret Air Force payload.
Because the Titan rocket`s design is similar to the shuttle`s solid rockets, its explosion–apparently caused by insulation peeling away from the rocket`s inner wall–suggests that flaws may not be limited to the rocket`s joints.
NASA also plans to test alternate improvements so they will be available should the planned redesigns fare poorly in the tests.
Despite the uncertainties, rocketry engineers both within the space agency and outside it say they expect the immediate redesign–or quick fix–of the booster`s flaws to produce a rocket that won`t explode.
”I think this design will preclude this disaster happening again,” is how John Thomas, manager of NASA`s booster design team, expressed it.
NASA has also invited the rocket industry, including Thiokol`s competitors, to submit designs for completely new boosters, which lift the shuttle from the earth during its first 127 seconds of flight before dropping clear of the shuttle and falling into the ocean below.
But David Winterhalter, acting director for shuttle propulsion at NASA`s Washington headquarters, said the request for new designs is aimed at improving booster efficiency, not safety.
The reassessments of America`s space program that followed the Challenger explosion have gone far beyond fixing the immediate cause of that disaster. President Reagan has already adopted a recommendation of advisers that the shuttle stop carrying private payloads.
That controversial decision is intended to reduce launch pressure on the shuttle fleet. That pressure to launch frequently, eventually achieving a
”pay-as-it-goes” status, was seen as taking precedence over concern for human safety.
A presidential commission, headed by former Secretary of State William Rogers, called for a review of the space agency itself after it investigated the explosion. The commission questioned NASA`s ability to uphold safety standards in the face of financial and flight scheduling pressures for the shuttle.
Outside watchdog committees–created by the National Research Council, the research arm of the National Academy of Science–have been another response to the new distrust of the space agency.
One committee of scientists and engineers is monitoring each step of the booster redesign. Another will assess possible launch rates for space shuttles and study distribution of payloads between shuttle flights and unmanned flights.
Another recommendation of the Rogers commission called for NASA to improve the safety of its space shuttles through such measures as crew-escape systems and launch-abort measures.
Taking note of earlier, rejected proposals to enhance human safety, the Rogers commission urged a new look at them. But they aren`t likely to be more appealing this time around, said Robert Thompson, who was shuttle program manager for NASA when the spaceship was first designed.
”We didn`t discard escape features lightly,” said Thompson, now with the McDonnell Douglas astronautics corporation in California.
”There was a lot of discussion of each feature as to whether we were helping ourselves or hurting ourselves with it,” he said. ”We didn`t drop a safety feature just in order to save money or add pounds of payload.”
Ironically, there was a concern that individual safety systems themselves might make the total spaceship less safe. ”Safety devices can cause mischief,” said Thompson.
Even though the operating systems functioned properly, on one of the shuttle`s 24 successful flights a high-temperature sensor failed, shutting down a main engine early and putting the ship into a lower orbit than planned. That failure of a device intended to improve safety not only hampered the mission`s goals, but had it occurred earlier, it could have threatened crew safety.
As a general philosophy, Thompson said, it is wiser to engineer a spaceship that functions properly in the first place, rather than load it down with safety features that take over when something goes wrong.
”You`re kidding yourself by trying to orchestrate failures,” Thompson said. ”They don`t tailor themselves to abort modes. If you`re riding a bus and it goes over the side of a mountain, it isn`t equipped with ejection seats to save you. Instead, they try to keep buses from going off the mountain in the first place.”
”NASA`s job now is to try to be realistic with the public,” said an engineer who serves on one of the oversight committees. ”We have auto accidents, jetliner accidents, and we`re going to have spaceflight accidents. ”The astronauts know the risks. If we`re going to fly men in space, things are going to go wrong.”
In addition to technical problems, any system as complex as the shuttle is always vulnerable to human error. The shuttle Columbia came within 31 seconds of disaster on Jan. 6, when it was almost launched with insufficient fuel to reach orbit, a mistake NASA attributed to ”operator fatigue.”
”Rocket launching is inherently risky,” stressed Beckstead, the consultant for the Navy. ”You`re taking the most energetic materials known to man, short of atomic power, and consuming millions of pounds of this material within a few minutes.
”It is a tremendous engineering feat to do that and fail only one time in 100, but I would hate to be riding on the rocket the one time it fails.”
