THE SHUTTLE Discovery, carrying a five-man crew, four deliberately injured rats, 32 chicken eggs and a $100 million communications satellite, is finally ready for a delayed blastoff this week on the year's first shuttle voyage.

Says crewman Robert Springer: "We've got kind of a bread-and-butter mission."NASA managers at the Kennedy Space Center Friday cleared the orbiter for blastoff on the 28th shuttle mission at 6:10 a.m. MST March 11, more than two weeks behind schedule because of work to replace three suspect engine turbo-pumps.

The goal of the flight, Discovery's eighth and the third since the 1986 Challenger disaster, is the launch of a Tracking and Data Relay Satellite - TDRS - identical to one launched Sept. 29 during the first post-Challenger mission.

At the controls on Discovery's flight deck will be commander Michael Coats, 43, who served as co-pilot aboard Discovery for its maiden flight in 1984, and John Blaha, 46, a shuttle rookie who will serve as Coats' second in command.

Seated behind them will be James Buchli, 43, veteran of two previous shuttle missions, with rookie Springer, 46, seated to his right. Seated alone below on the shuttle's lower deck will be physician-astronaut James Bagian, 37, also a space rookie.

If all goes well, Discovery will glide to a dawn landing at Edwards Air Force Base, Calif., after a five-day, one-hour and seven-minute mission. At $375 million, the 28th shuttle flight will have cost $51,603 a minute.

With two successful post-Challenger flights behind them, Discovery's crew is eager to get off the ground.

"It's a relief now to be flying again and showing that we really are a spacefaring nation and that we have the capability to operate in space safely on a regular basis," Coats said in an interview. "We're anxious to prove that.

"It's been frustrating to leave the arena of space to the Russians for the past three years. We've got a lot of catching up to do. We've got a lot of ambitious plans and we believe the space program is critical to the country."

Launch had been scheduled for Feb. 23, but NASA managers delayed the flight in order to replace all three of the ship's high-pressure liquid oxygen turbopumps after a tiny crack was found in a pump used by one of the shuttle Atlantis' three main engines in December.

Atlantis was never in any danger, but managers were concerned because a major crack could cause a catastrophic engine explosion. While Discovery's pumps had no known problems, they were built the same way as the one aboard Atlantis that suffered a cracked bearing assembly.

NASA was unable to rule out the possibility that similar "stress corrosion" could occur in Discovery's pumps and, taking no chances, managers ordered all three replaced with pumps built using a technique that precludes the development of such cracks. The work was completed last week.

Once in orbit, the first priority for Discovery's crew will be the launch of the 4,637-pound TDRS satellite, built by TRW Inc. of Redondo Beach, Calif., and identical to one destroyed in the Challenger disaster.

Mounted atop an "intertial upper stage" - IUS - booster built by Boeing Aerospace Co. of Seattle, the TDRS spacecraft will be deployed from Discovery's 60-foot payload bay six hours and 13 minutes after blastoff - around 12:15 p.m. MST.

"The challenge really is the first 61/2 hours," Coats said. "Doing the things we have to do in that first six hours is a rush. If we can get through that without falling behind the timeline and get the satellite deployed safely, then we'll feel like we're over the hump."

The deployment will be filmed by Blaha using a wide-format IMAX camera, flown aboard two previous shuttle missions to take footage that later was used for two movies about spaceflight that are shown on giant, seven-story screens.

The camera will be used during Discovery's flight for a motion picture titled "The Fragile Earth."

"I don't think it will solve anything or dramatically change any directions," Blaha said. "But I think seeing a movie like that will give people a better perception, a better understanding of what we're doing to our planet. I think it'll help."

One hour after TDRS is gently pushed from its cradle in Discovery's payload bay, the first stage of the $45 million solid-fuel IUS booster will fire to begin the satellite's trip to a circular orbit 22,300 miles above the equator.

At such altitudes, a spacecraft's orbital velocity is synchronized with the rotation of the Earth and it thus appears to hang stationary in the sky, an ideal condition for communications satellites because it eliminates the need for complex - and costly - ground antennas capable of tracking moving objects.

Once on station, the TDRS satellite's two 16-foot antennas will open like umbrellas and a pair of rectangular solar panels will unfold, giving the spacecraft dimensions of 57.2 feet by 42.6 feet.

The satellite ultimately will replace an aging TDRS already in orbit, working in concert with a TDRS launched from the shuttle Discovery Sept. 29. With two such fully operational satellites in space, shuttle crews will be able to stay in radio contact with mission control over 85 percent of each orbit.

***** For the remainder of Discovery's mission, the astronauts will conduct a wide variety of engineering studies and experiments, including two dreamed up by high school students.

One, designed by Andrew Fras of Binghamton, N.Y., now a medical student at Brown University, will use four white rats to study how bone healing is affected by weightlessness.

About four days before blastoff, a dental drill will be used to burr tiny holes through the non-weight-bearing fibula bone in one of each rat's hind legs. The surgery will be conducted under anesthesia but the rats will be killed after their return to Earth so the bones can be studied.

Bone healing begins in earnest about four to five days after an injury and by drilling the holes well before blastoff, Fras should be able to gather valuable data about the processes governing bone growth in weightlessness, data that could one day help scientists learn how to treat injuries that otherwise could disrupt a long-duration spaceflight.

A second student experiment, sponsored by Kentucky Fried Chicken, involves 32 fertilized chicken eggs that will be carried into space to find out how the absence of gravity might affect developing chicken embryos. An earlier version of the experiment was destroyed in the Challenger disaster.

"We can grow a better crystal in space, so is Earth the best place to have children?" said John Vellinger, a Purdue University mechanical engineering student who proposed the experiment while in high school.

"Can we even have children up in space? We can look at the development of the embryo of the chicken egg. Certainly it's not going to answer these questions, but it can give us some valuable data as to whether this is an area that we really need to look at and examine for future space colonies."

The experiment called for 16 of the eggs to be fertilized nine days before launch, the remainder seven days later. Upon return from space, half of each group will be opened for examination to determine how zero gravity affected the embryo development. The remaining 16 eggs will be allowed to hatch.

Other experiments aboard Discovery include a long "heat pipe" in the payload bay to test a potential space station radiator design, an experiment to study plant growth in weightlessness and a protein crystal growth study aimed at helping researchers determine the precise structure of various proteins.

Such ultra-pure crystals, difficult or impossible to obtain under the influence of Earth's gravity, can help scientists determine the precise structure of critical proteins, data that can be used create new drugs.

"Of all the ones that I'm aware of we've ever done on the shuttle, that is the one that has the most potential immediate gain to society," Bagian said.