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At the end of a runway on a small air base in Spain's southern countryside a NASA crew is busy building a barrier net designed to catch a space shuttle.

The device is new and is being installed at the recommendation of investigators who probed the Jan. 28, 1986, Challenger disaster.Similar catch nets are also being installed on an airstrip at Zaragoza in northern Spain and at the international airport in Banjul, Gambia, in the event something goes awry during the upcoming launch of the shuttle Discovery, scheduled Sept. 29.

The primary shuttle abort landing site, on a Moroccan military base at Ben Guerir, has a runway long enough to allow a fully loaded shuttle to roll to a stop without difficulty, so no net is being installed there. But the runways at the other sites could prove to be too short.

Whether the nets would be needed if one of the abort sites were used depends, in part, on how much weight the orbiter is carrying at the time. "We're looking at a weight that approaches 240,000 pounds," said Don Carlson, the National Aeronautics and Space Administration's ground operations manager at Moron. "The payload has a large impact on total weight."

With the payload deployed and most of the on-board fuel exhausted, the shuttle would weigh about 180,000 pounds, he said. "We're marginal depending on if we have a heavyweight payload. If we have a lot of fuel on board during boost out of Kennedy, it's going to be touch and go whether it gets to the end of the runway or not" in the event of an emergency return to an abort site, Carlson said.

Discovery's payload will include a tracking relay data satellite similar to the one aboard the Challenger when it exploded shortly after launch and killed all seven crew members. The satellite will complete a two-satellite shuttle tracking network that will allow ground crews to have unin terrupted communication ability with the shuttle, Carlson said.

NASA currently relies on ground tracking stations around the world that allow voice communication with astronauts in the shuttle about 20 percent of the time, Carlson said.

Regardless of the shuttle's payload, the astronauts and ground controllers can manipulate the shuttle only after the main engine is exhausted, which takes two minutes from the time the shuttle is launched.

If a malfunction is detected during or shortly after two minutes, NASA officials would try first to turn the shuttle around and glide it back to the Kennedy Space Center. If an abort is ordered and the shuttle has gone too far across the Atlantic to glide back to Florida, the next option would be to bring it down at one of the four TransOceanic Abort Landing Sites that have been operational during all 25 shuttle missions so far.

If an abort site has to be used this time, the barrier nets will increase the safety to the crew and people on the ground at the air base if the shuttle rolls past the end of the runway.

At the Moron Air Base, home of a small contingent of Spanish F-5 fighters and a revolving force of military aircraft from the United States, NASA will have about 17 people standing by along with another 45 from the Defense Department, Carlson said.

NASA crews also will be standing by at the two African abort sites. The Zargoza site will not be activated for the Sept. 29 Discovery mission, said Chris Hasselbring, Lockheed manager at the Moron site.

The barrier is made of a 300-foot-wide net containing vertical nylon strips 25 feet tall that are strung along cables. The cables are connected to coiled dampers called "water twisters" that quickly slow the shuttle down after it gets caught in the net.

The net's vertical strips allow the shuttle's nose to slip through before they catch the vehicle by the wings.

The net design is such that the shuttle's nose completely clears the net so the cockpit is clear if a rescue is needed, Hasselbring said.

The catch net is only a small part of the equipment now being assembled at the abort sites. An elaborate satellite communications system that links the African and Spanish sites to the Kennedy Space Center was installed along with a microwave scan beam landing system that would send guidance signals to the shuttle's computer, which would then make landing calculations.

"We came back here in May to do site activation," Hasselbring said. The barriers were installed, then removed temporarily leaving only the concrete foundation. "That's when we actually ran a lot of cable and buried it and checked a lot of systems to make sure they're going to work okay."

In June NASA tested scale models of the barrier net and shuttle, Carlson said, "so we've had a lot of people over here over a few months."

Clearing some of the electronic equipment through customs at the African sites has taken several months, Carlson said. Most of the equipment will be removed again after the mission is ended. "Virtually the only things to stay in are the concrete foundations."

Carlson said the barriers may not be needed on future missions because NASA plans to install drag chutes on the shuttles. The barriers may still be set up before launches just as an added precaution, Hasselbring said.

In addition to the four abort sites, NASA has established 15 to 20 emergency landing sites that could be used if something goes wrong later in the mission and the shuttle can't be flown directly to Kennedy. No special manpower or equipment is sent to any of those sites.