Space shuttle Endeavour, mounted atop a modified Boeing 747 airliner, is scheduled begin the first leg its cross-country piggyback ride from California to Florida Wednesday.

Takeoff of the unique combo from Edwards Air Force Base's Runway 04 is slated for about 7 a.m. local time (10 a.m. EST), weather permitting.

File photo. Credit: NASA

The ferry flight's stops are not being revealed in advance due to security regulations, a NASA spokesperson said.

But a sprawling weather system across the southern United States will pose extra challenges to a traveling team of forecasters and pilots charged with looking after the spaceplane.

A cold front is producing widespread storms, high winds and cold temperatures along the ferry flight's projected route.

An Air Force C-17 cargo plane will fly about 100 miles ahead of the 747 and Endeavour to scout weather conditions along the flight path.

The flight must avoid precipitation and fly at low altitudes to satisfy temperature and air pressure constraints.

"We have to stay below the 15 degrees Fahrenheit (altitude) and we have to have it no lower than 8 psi of air pressure," said Frank Marlow, a veteran NASA ferry flight pilot. "So that means that we fly the aircraft across the country around 15,000 or 16,000 feet."

Marlow said those altitudes can be lower because of cold temperatures in winter.

Flying into rain could damage the sensitive tiles that blanket the orbiter to protect it from the heat of re-entry.

"We did go into 30 seconds of rain many, many years ago, and I believe that cost several million dollars worth of damage to the tiles," Marlow said.

Shuttle weather officer Kathy Winters is forecasting good conditions along the first two legs of the ferry flight path in the southwestern United States. An overnight stop is planned at about the halfway point of the nearly 2,500-mile trip, according to NASA.

The ferry flight is expected to continue Thursday, and Endeavour could be back at the Kennedy Space Center late Thursday or Friday.

"Some cloudiness and light turbulence is expected over the routes of the flight in the southeastern states, with a slight chance of thunderstorms," NASA said in a written statement.

Forecasters will continue to provide real-time adjustments to the weather outlook throughout the trip.

"We bring along a bunch of weather briefers," Marlow said. "They work for hours before we get to the briefs, so we know everything about the winds, the temperatures and the pressures up at altitude.

"We know everything about the takeoff field, the landing field and some intermediate fields so that if we had to make some sort of a divert we would know the weather at all the fields. It's very comprehensive and they do an excellent job on giving us the weather," Marlow said.

Marlow will command the first leg of Wednesday's ferry flight. He will swap places with pilots Bill Brockett and Jeff Moultrie during subsequent parts of the journey.

The 747 carrier aircraft typically carries a crew of four: two pilots, a 747 flight engineer and a shuttle systems expert.

Endeavour's ferry flight this week will include a crew of five. An extra engineer will be taking part in training exercises.

Endeavour will add about 220,000 pounds to the 747's normal weight. Most shuttles weigh between 195,000 pounds and 230,000 pounds at landing.

NASA's two 747 carrier aircraft were stripped of their interior furnishings and two stabilizers were added to the tail for improved handling.

But the extra weight decreases the margin of error for pilots flying with an attached orbiter.

"It has a little bit of an angle-of-attack, so that it's developing a little bit of lift," Marlow said. "But any time you develop lift, you're developing drag. When you fly the 747 with the orbiter on, if you lose an engine on takeoff, it's pretty critical to do things correctly because we have much less performance than a normal 747 would."

The weight also drastically cuts the 747's fuel mileage. Instead of nonstop transcontinental service, NASA's two 747's usually stop three or four times on a cross-country journey.

"It takes slightly less than one gallon of fuel per plane length in the cruise configuration at 15,000 feet," Marlow said. "It uses about 40,000 pounds per hour."

Crews will use vapor sniffers along the exterior of the 747 at each stop to detect traces of toxic hypergolic propellant from the shuttle's propellant tanks. Officials keep bystanders at a safe distance during the stops due to the threat from toxic fumes.

"We will land and they will come out with sniffers and they will check to see if there are any hypergols in the area and they'll clear it. If it's found to be dirty, we can throw these things over our heads so that we're breathing air and we can get out of the airplane safely," Marlow said.

NASA purchased its first 747 carrier aircraft from American Airlines in the 1970's. That plane, called NASA 905, was used during drop tests of the prototype shuttle Enterprise to test the ship on approach and landing.

NASA purchased another 747 from Japan Airlines after the Challenger accident and modified it to carry the shuttle. Named NASA 911, that aircraft will be used for Endeavour's ferry flight this week.

Both aircraft are 747-100 series planes.

"When they built the 747, Boeing built a winner," Marlow said. "The 747 is a beautiful airplane that handles very well without the orbiter on, and it also handles very well with the orbiter on."

Marlow said the shuttle does produce a noticeable vibration through the thick struts connecting the orbiter with the 747, but it doesn't cause any handling problems.

Endeavour landed Nov. 30 at Edwards Air Force Base about 60 miles north of Los Angeles. The shuttle was diverted there due to poor weather at the Kennedy Space Center.

NASA dispatched a team of shuttle technicians from KSC to help safe Endeavour and prepare the ship for its flight home. The work included retrieving hardware from Endeavour's crew cabin, purging the shuttle's fuel cells and removing residual maneuvering propellant.

Workers also bolted a sleek, white aerodynamic tailcone over Endeavour's main engines to help reduce drag during the flight.

High winds in the California desert Monday prevented engineers from lifting Endeavour inside the Mate-Demate Device at the Dryden Flight Research Center co-located at Edwards.

The gantry-like structure was specially built to lift shuttles for attachment to the 747 carrier aircraft.

Endeavour was finally mated to the aircraft Tuesday, wrapping up more than a week of prep work.

The shuttle attaches to the 747 in three places, the same locations that it is mated to the orange external tank during launch.

Electrical power from the 747 provides heat to the orbiter's systems during the flight. Cold temperatures could damage the shuttle's avionics, Marlow said.

Marlow, a 39-year NASA veteran, is one of six pilots certified to transport the shuttle using the 747.

The pilots trained on a flight simulator at United Airlines.

"They had the capability of putting in the drag and other characteristics for the 747 with the shuttle on-board. So the training that we got from United was excellent," Marlow said.

Pilots go to the simulator every six months and fly the real 747's every six weeks.

There is only one example of a similar piggyback configuration pairing a jumbo jet with a spacecraft. The Soviet Union briefly used an Antonov An-225 cargo plane to carry the Buran space shuttle across Russia in the 1980's.

"It's definitely a unique thing," Marlow said. "I think all of us are honored to be selected for this job."

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