Tuesday, November 22, 2011

Mars Science Laboratory Launch Overview

The Mars Science Laboratory (MSL) rover Curiosity is scheduled to launch from Cape Canaveral Air Force Station this Saturday, November 26 at 10:02 EST. The following information is provided by NASA and discusses the Atlas launch vehicle, factors that went into setting the launch date, and the sequence of events involved in the launch.

Launch Vehicle
A two-stage Atlas V-541 launch vehicle will lift the Mars Science Laboratory spacecraft from Launch Complex 41 at Cape Canaveral Air Force Station, Fla. The vehicle is provided by United Launch Alliance, a joint venture of Boeing Co. and Lockheed Martin Corp. The thee numbers in the 541 designation signify a payload fairing, or nose cone, that is approximately 5 meters (16.4 feet) in diameter; four solid-rocket boosters fastened alongside the central common core booster; and a one-engine Centaur upper stage.

Centerpiece of the first stage is the common core booster, 106.5 feet (32.46 meters) in length and 12.5 feet (3.81 meters) in diameter. It has a throttleable, RD-180 engine from a joint venture of Pratt & Whitney Rocketdyne, West Palm Beach, Fla., and NPO Energomash, Moscow. Thermally stable kerosene fuel (type RP-1) and liquid oxygen will be loaded shortly before launch into cylindrical fuel tanks that make up about half of the total height of the vehicle. The common core booster can provide thrust of up to about 850,000 pounds (3.8 million newtons) at full throttle.

Four solid rocket boosters strapped onto the common core booster add to the thrust that will be produced by the first stage. Each of these boosters is 64 feet (19.5 meters) long and 61 inches (155 centimeters) in diameter, and delivers about 306,000 pounds (1.36 million newtons) of thrust.

Two interstage adaptors connect the first stage of the Atlas with its Centaur upper stage. The Centaur has a restartable RL-10 engine from Pratt & Whitney Rocketdyne. This engine uses liquid hydrogen and liquid oxygen and can provide up to about 22,300 pounds (99,200 newtons) of thrust. The Centaur can control its orientation precisely, which is important for managing the direction of thrust while its engine is firing. It carries its own flight control computer and can release its payload with the desired attitude and spin rate.

The spacecraft will ride into the sky inside a protective payload fairing atop the Centaur stage. With the payload fairing on top, the vehicle ready for launch will stand approximately 191 feet (58 meters) tall.

The first Atlas V was launched in August 2002. An Atlas V-401 sent NASA’s Mars Reconnaissance Orbiter on its way to Mars on Aug. 12, 2005, from the same launch complex where the Mars Science Laboratory will be launched.

Launch Timing Factors
As Earth and Mars race around the sun, with Earth on the inside track, Earth laps Mars about once every 26 months. Launch opportunities to Mars occur at the same frequency, when the planets are configured so that a spacecraft launched from Earth will move outward and intersect with Mars in its orbit several months later. This planetary clockwork, plus the launch vehicle’s power, the spacecraft’s mass, and the desired geometry and timing for the landing on Mars were all factors in determining the range of possible launch dates.

An additional factor in determining the launch period for the Mars Science Laboratory was the launch period from Aug. 5 to Aug. 26, 2011, for NASA’s Juno mission to Jupiter. The Juno launch on Aug. 5 used an Atlas V-551 vehicle from the same launch complex as the Mars Science Laboratory, Launch Complex 41 at Cape Canaveral Air Force Station. Consideration was given to the minimum time required to process the Mars Science Laboratory launch vehicle for its launch following Juno’s departure. This was an important factor in the choice of the type of trajectory to Mars and the timing of the launch period for the Mars Science Laboratory.

One priority for choice of a launch period within the range of possible dates has been to have the landing occur when NASA orbiters at Mars are passing over the landing site so they can receive radio transmissions from the Mars Science Laboratory spacecraft during its descent through the atmosphere and landing. If the landing is not successful, this strategy will provide much more information than would be possible with the alternative of relying on transmissions from the Mars Science Laboratory directly to Earth. Landing on Mars is always difficult, with success uncertain. After an unsuccessful attempted Mars landing in 1999 without definitive information on the cause of the mishap, NASA set a high priority on communication during subsequent Mars landings.

The selected launch period for the Mars Science Laboratory begins Nov. 25, 2011, and ends Dec. 18, 2011. The durations of the daily launch windows during this 24-day launch period vary from day to day. On the first day in the launch period, the launch window is 103 minutes long. The windows will shorten to 44 minutes by the end of the period. Within each window, regardless of length, unique launch opportunities will occur once every five minutes.

The first launch opportunity on Nov. 25 is for liftoff at 10:25 a.m. EST.

Launch Sequences
The launch time, called “T Zero,” is 1.1 seconds before liftoff. Ignition of the Atlas V-541 first-stage common core booster is at 2.7 seconds before T Zero, or 3.8 seconds before liftoff.

The four solid rocket boosters ignite 3.5 seconds after ignition of the common core booster (0.3 second before liftoff). They burn for about a minute and a half, and then their spent casings are jettisoned in pairs several seconds after burnout.

The common core booster engine of the first stage continues to burn until about 4 minutes and 20 seconds after liftoff. During that burn, at about 3 minutes and 25 seconds after liftoff, the payload fairing around the spacecraft is jettisoned. The booster engine finishes its work at an event called the booster engine cutoff. At that point, these engines have consumed about 623,000 pounds (284,000 kilograms) of propellant in less than five minutes and, with the help of the solid rocket boosters, taken the spacecraft to an altitude of Mars Science Laboratory Launch 27 Press Kit about 98 miles (158 kilometers) down range from Cape Canaveral about 307 miles (494 kilometers).

After coasting for a few seconds following booster engine cutoff, the launch vehicle’s second stage Centaur separates from the first stage, which drops into the Atlantic Ocean. About 10 seconds after that separation, the Centaur engine is started for the first of two burns. That burn lasts about 7 minutes and inserts the Centaurand- spacecraft stack into a parking orbit. The shape of the parking orbit is an ellipse varying in altitude from 103 miles (165 kilometers) to 165 miles (265 kilometers). However, the spacecraft will not complete even one orbit. After completion of the Centaur main engine’s first burn (main engine cutoff 1), the stack coasts in the parking orbit until it reaches the proper position for start of the second Centaur burn, depending on launch date and launch time. This coast lasts 14 to 30 minutes, depending on launch timing — about 20 minutes for a launch at the initial Nov. 25 opportunity at the opening of the launch window.

The second Centaur burn, continuing for nearly 8 minutes (for a launch at the opening of the Nov. 25 launch window), lofts the spacecraft out of Earth orbit and on its way toward Mars. The burn ends with main engine cutoff 2. Three minutes and 43 seconds after that cutoff, pyrotechnic actuators and push-off springs on the second stage of the Atlas release the spacecraft with a separation velocity (relative to the launch vehicle) of 0.6 miles per hour (0.27 meters per second) and a spin rate of about 2.5 rotations per minute. By that point, 43 minutes after liftoff for the first Nov. 25 opportunity at the opening of the launch window, the two stages of the Atlas have accelerated the spacecraft to about 22,866 miles per hour (10.22 kilometers per second) relative to Earth. Shortly after that, the separated Centaur performs its last task, an avoidance maneuver taking itself out of the spacecraft’s flight path to avoid hitting either the spacecraft or Mars.

Throughout the launch sequence, radio transmissions from the Atlas to NASA’s Tracking and Data Relay Satellite System enable ground controllers to monitor critical events and the status of the launch vehicle and the spacecraft. The Mars Science Laboratory spacecraft cannot begin its own direct transmissions until after it separates from the launch vehicle and an antenna on the spacecraft is exposed. One minute after spacecraft separation, the flight software transitions the Mars Science Laboratory to cruise phase and turns on the X-band transmitter. The spacecraft does not start transmitting data until five minutes after that, allowing time for the warming up the amplifier and configuration of the telecommunications system.

The first antenna station of NASA’s Deep Space Network to receive communication from the spacecraft is near Canberra, Australia. Use of an antenna operated by the private-sector United Space Network on the Indian Ocean island nation of Mauritius provides a bonus opportunity for earlier establishment of communications with the spacecraft.

Data received from the spacecraft in the initial acquisition will provide the first evaluation of the spacecraft’s health in cruise mode, including confirmation that the cruise-stage solar arrays are producing electricity. Once the spacecraft is in a stable state, cruise phase activities can begin.