The two Solid Rocket Booster's (SRB's) provide the main thrust to the Space Shuttle during the lift-off and ascent stages, providing the system with 71.4 percent of the total thrust at lift-off. The boosters propel the shuttle to the altitude of 28 miles (150,000 feet) at a speed of 3,094 MPH and at a thrust of 5.8 million pounds. Each SRB provides a thrust of 3.3 million pounds (14,678,000 N) at standard sea level. The boosters burn an ammonium per chlorate (oxidizer) and aluminum powder mixture, which are both formed into a paste when ignited. The SRB's support the entire weight of the orbiter and ET when they are attached to the mobile launch platform (MLP). The Solid Rocket Boosters are also reused after every shuttle mission, when they separate and return to the Atlantic Ocean via  parachutes. The boosters are jettisoned just over two minutes into the flight and are recovered about 120 miles (200 kilometers) downrange from the launch site. The SRB's are recovered by two ships, the UTC Liberty and UTC Freedom, from the Atlantic and then are returned to Port Canaveral to be reprocessed and reprocessed for a future shuttle flight.