Thursday, 3 March 2016

Soyuz TMA Re-Entry

Today's trivia dives into the detail of how Astronauts return from space.

On March 1st 2016 Scott Kelly (NASA) and Mikhail Korniyenko (RSC) returned to Earth in the Soyuz-TMA spacecraft. How does an astronaut return from orbit in this spacecraft?

Soyuz TMA spacecraft

The Soyuz TMA-M (transport, modified, anthropometric) is the latest in a very slow evolution of Russian human-rated spacecraft.

Soyuz TMA-17 before docking

Primary Functions

  • Carry up to 3 crew into a variety of orbits
  • Orbital transit
  • Docking
  • De-orbit and re-entry
  • Soft landing

Its design philosophy is based around automation. Most of the main tasks required for re-entry are automatic. It can also be fully controlled from ground control stations. Crew can override as required, for example if they encounter an emergency. A lot of its design is focused around many levels of redundancy. Automated systems have usually two backup systems and include some kind of reasonable contingency if all were to fail.

Artistic image of the 17 July 1975 Apollo Soyuz Test Program (ASTP) the first docking of a Russian and American spacecraft showing comparable scale of the Apollo and Soyuz spacecraft

The first Soyuz was developed at the same time as the American Apollo program as part of a secretive Soviet lunar mission which was eventually halted due to a number of reasons. This same craft has undergone three main design evolutions to spacecraft used today.

Its use as a crew transportation and return spacecraft for space stations was first proposed by the Russians as the Automatic Crew Return Vehicle for both the joint NASA/RSC project on the Mir space station and the International Stance Station. If the station was damaged by space debris or a medical evacuation was required, the Soyuz would be used for these tasks. With its 110+ manned launches it has an extensive track record.

Undock

Soyuz TMA-03M spacecraft (left) eases toward its docking with the Russian-built Mini-Research Module 1

Once the crew are onboard and suited up in their pressure suits, they will go through various checklists to prepare for the flight. This takes in excess of three hours, which may include time verifying there are no drops in pressure within Soyuz.

View from the on board computer system pilots view with information overlay

Once released from the station, springs inside the docking ring push the craft away from the station. The Soyuz is only able to use its reaction control thrusters once it is far enough (20m) away from the station to prevent covering the station in liquid propellant.

De-Orbit burn

The de-orbit burn is also automatic. Once the spacecraft position is accurately known, the computer calculates the angle and duration of burn to get the required landing trajectory to a pre-determined landing site.

This burn is critical as it determines the angle of descent into the atmosphere:

  • Too shallow and it will bounce off the atmosphere without showing down, resulting in overheating.
  • Too steep and the crew will experience too much deceleration force (in excess of 10g) which may be fatal for the crew.

Soyuz is made up of three modules, at the front is the Orbital Module, in the middle is the Descent Module and at the back is the Service Module which includes the solar panels

Once the de-orbit burn is complete, the Orbital and Service modules are jettisoned. Explosive bolts fire which push the jettisoned modules away from the descent module. They will burn up on the atmosphere on reentry. If for some reason the modules do not separate correctly, the Soyuz is designed so aerodynamic forces will break the modules apart in any case.

30 minutes later the spacecraft will cross the Kármán line at 100km and come into contact with enough atmosphere to start re-entry.

Re-Entry

Seen from the International Space Station, the Soyuz TMA-05M descent module begins to re-enter the Earth's atmosphere, leaving a plasma trail as the Expedition 33 crew streaks toward a pre-dawn landing on the steppe of Kazakhstan.

During re-entry the capsule will decelerate from orbital velocity (17,000mph) down to speeds where the parachutes can be deployed. This deceleration is the result of the capsule crashing into the atmosphere. The heat shield of Soyuz is rated to withstand the incredible temperatures of re-entry, however the shape is also crucial as well:

"If the reentry vehicle is made blunt, air cannot "get out of the way" quickly enough, and acts as an air cushion to push the shock wave and heated shock layer forward away from the vehicle. Since most of the hot gases are no longer in direct contact with the vehicle, the heat energy would stay in the shocked gas and simply move around the vehicle to later dissipate into the atmosphere." - Wikipedia

The re-entry is also completely automatic. The computer will use reaction control thrusters to create a steering effect, enough to keep it aligned with its target landing site. Should the computer fail, or an emergency arise such as depressurisation, the backup re-entry process will start a ballistic re-entry. This re-entry is faster because it is steeper but induces up to 8g of force on the crew.

Re-Entry as seen from the inside window of Soyuz Buzz Feed

Landing

Once the descent has slowed enough, the parachutes can be deployed. First two drogue chutes, followed by a main chute. This slows the capsule down to the required landing speed.

Around the same time the heat shield, and external window covers are jettisoned.

Just before landing 6 soft landing solid rockets fire to improve the landing which an astronaut still describe as "feeling like you are being hit by a truck".

During the early Soviet missions the landing site was less predictable. The cosmonaut would expect to be greeted by team parachuted in to assist. Now days helicopters and ground support vehicles arrive at the predetermined landing site and are in constant communication with the capsule as it lands.

Soyuz TMA-01M spacecraft shortly after the capsule landed

References:

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