Use of Payloads

Well the payload of a rocket is generally goods and devices being carried to a destination....Here in this illustration the rocket is carrying 2 satellites, a DemoSat and Nanosat-2, the satellites will be carried into proper orbit placement and deployed...The booster engines, 3 in all, will be jettisoned as they are used up in reaching the destination, then the 2nd stage engine will finish proper positioning, then it too will jettison...

Here is an animation showing the entire process.....

https://www.youtube.com/watch?v=5v7SeAYumoI

Balance mass devices are generally used when a capsule is approaching a planet for entry into the atmosphere....

"Balance Mass Devices traditionally see the jettisoning of weights from the spacecraft upon entry into a planetary atmosphere. For example, when the Curiosity Rover approached Mars, two tungsten weights totalling 150kgs were expelled from the spacecraft. The jettisoning of such weights help to shift the center of balance of the spacecraft, placing it at an angle on its approach to the planet's surface. This angled entry helps the spacecraft to generate lift, which subsequently aids in a more controlled landing. "

http://www.outerplaces.com/universe/technology/item/6011-nasa-announces-$20000-prize-for-new-mars-balance-mass-challenge#sthash.h36VhDB2.dpuf

Mars rover steps to landing....

"Starting a minute later, small thrusters on the back shell will halt the spin the spacecraft maintained during cruise and approach phases. Nine minutes before entry, the back shell thrusters orient the spacecraft so the heat shield faces forward, a maneuver called "turn to entry."

After the turn to entry, the back shell jettisons two solid-tungsten weights, called the "cruise balance mass devices." Ejecting these devices, which weigh about 165 pounds (75 kilograms) each, shifts the center of mass of the spacecraft.Offsetting the center of mass when the spacecraft is in Mars' atmosphere allows MSL to fly through the atmosphere at an angle, generating lift. That lift is used for "guided entry" to steer out of unpredictable variations in the atmosphere and improve landing precision.As the spacecraft interacts with the upper atmosphere, thrusters on the back shell can adjust the angle and direction of tilt, letting the spacecraft fly a series of "S" curves. Those curves reduce the horizontal distances the spacecraft covers as it descends. Using fewer of them prevents undershooting the target area; using more of them prevents overshooting the target area. The guided entry maneuvers can also correct for drift to the left or right due to winds.More than 90 percent of the deceleration before landing results from friction with the atmosphere of Mars before the parachute opens. Peak heating occurs about 80 seconds after atmospheric entry, when the temperature at the external surface of the heat shield will be about 3,800 degrees Fahrenheit (about 2,100 degrees Celsius). Peak deceleration occurs about 10 seconds later.After the spacecraft finishes its entry maneuvers, a few seconds before the parachute is deployed, the back shell jettisons more tungsten weights to shift the center of mass back to the axis of symmetry. This set of six weights, the "entry balance mass devices," each has a mass of about 55 pounds (25 kilograms).The parachute deploys about 255 seconds after entry, at about 7 miles (11 kilometers) high and a velocity of about 900 miles per hour (402 meters per second). After about 24 more seconds, the heat shield separates and drops away when the spacecraft is at an altitude of about 5 miles (8 kilometers) and traveling about 280 miles per hour (125 meters per second).The Mars Descent Imager begins recording video of the ground beneath the spacecraft. The rover, with its descent-stage "rocket backpack," is still attached to the back shell on the parachute. The terminal descent sensor, a radar system mounted on the descent stage, begins collecting data about velocity and altitude. The back shell, with parachute attached, separates from the descent stage and rover about 80 seconds after heat shield separation. At this point, the spacecraft is about 4,800 feet (1,450 meters) above the surface dropping at about 180 miles per hour (80 meters per second). All eight throttleable retrorockets on the descent stage -- called Mars landing engines -- begin firing for the powered descent phase.After the engines have decelerated the spacecraft to about 1.7 miles per hour (0.75 meters per second), the descent stage maintains that velocity until touchdown. Four of the eight engines shut off just before nylon cords begin to spool out to lower the rover from the descent stage in the "sky crane" maneuver.The rover separates its hard attachment to the descent stage, though still attached by the sky crane bridle, at an altitude of about 66 feet (20 meters), with about 12 seconds to go before touchdown."

http://www.nasa.gov/centers/langley/exploration/langley-msl-role.html