Up to three crew members can return to Earth from the International Space Station aboard a Soyuz TMA spacecraft. The vehicle lands on the flat steppe of Kazakhstan in central Asia. The return to Earth takes less than 3.5 hours.
Lightening the Load on the Way Down
[image-51]The Soyuz TMA spacecraft is composed of three elements attached end-to-end -- the Orbital Module, the Descent Module and the Instrumentation/Propulsion Module. The crew occupies the central element, the Descent Module. The other two modules are jettisoned prior to re-entry. They burn up in the atmosphere, so only the Descent Module returns to Earth.
The Orbital Module provides the crew with extra living space during the trip to the station. It contains systems vital to rendezvous and docking with the station's Pirs Docking Compartment or other port: a docking mechanism, a hatch and rendezvous antennas. Once the Soyuz departs, the Orbital Module is no longer needed, so it is jettisoned about three hours after undocking.
The Instrumentation/Propulsion Module is shed at the same time, about half an hour after its engines perform their final task -- a deorbit burn that drops the Soyuz from orbit. With it go the spacecraft's two solar arrays. This module contains the primary guidance, navigation and computer systems for the vehicle.
[image-78]A secondary guidance, navigation and control system in the Descent Module enables the crew to maneuver the vehicle after the Instrumentation/Propulsion Module has been jettisoned. The Soyuz commander can pilot the module using a rotational hand controller that manages the firing of eight hydrogen peroxide thrusters on the vehicle's exterior. This system is deactivated 15 minutes before landing, when the parachutes are deployed.
Cushioning the Landing
Having shed two-thirds of its mass, the Soyuz reaches Entry Interface -- a point 400,000 feet above the Earth, where friction due to the thickening atmosphere begins to heat its outer surfaces -- three hours after undocking. With only 23 minutes left before it lands on the grassy plains of central Asia, attention in the module turns to slowing its rate of descent.
Eight minutes later, the spacecraft is streaking through the sky at a rate of 755 feet per second. Before it touches down, its speed will slow to only 5 feet per second, and it will land at an even lower speed than that. Several onboard features ensure that the vehicle and crew land safely and in relative comfort.
[image-94]Four parachutes, deployed 15 minutes before landing, dramatically slow the vehicle's rate of descent. Two pilot parachutes are the first to be released, and a drogue chute attached to the second one follows immediately after. The drogue, measuring 24 square meters (258 square feet) in area, slows the rate of descent from 755 feet per second to 262 feet per second.
The main parachute is the last to emerge. It is the largest chute, with a surface area of 10,764 square feet. Its harnesses shift the vehicle's attitude to a 30-degree angle relative to the ground, dissipating heat, and then shift it again to a straight vertical descent prior to landing.
-- Landing engines
The main chute slows the Soyuz to a descent rate of only 24 feet per second, which is still too fast for a comfortable landing. One second before touchdown, two sets of three small engines on the bottom of the vehicle fire, slowing the vehicle to soften the landing.
Further cushioning the impact of landing are the crew seats with their custom-fitted liners. The liners are made preflight, individually molded to fit each person's body -- this ensures a tight, comfortable fit when the module lands on the Earth. When crew members were brought to the station aboard the space shuttle, their seat liners were delivered with them and transferred to the existing Soyuz spacecraft as part of crew handover activities.
TMA Improvements for Landing
[image-126]The Soyuz TMA spacecraft is a replacement for the Soyuz TM, which was used from May 1986 to November 2002 to take astronauts and cosmonauts to Mir and then to the International Space Station beginning in November 2000.
The TMA increases safety, especially in descent and landing. Two new engines reduce landing speed and forces felt by crew members by 15 to 30 percent, and a new entry control system and three-axis accelerometer increase landing accuracy. Instrumentation improvements include a color "glass cockpit," which is easier to use and gives the crew more information, with hand controllers that can be secured under an instrument panel. All the new components in the Soyuz TMA can spend up to one year in space.
Descent module structural modifications, seats and seat shock absorbers were tested in hangar drop tests. Landing system modifications, including associated software upgrades, were tested in a series of airdrop tests. Additionally, extensive tests of systems and components were conducted on the ground.