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Originally Posted by mugaliens
What I'd be interested in knowing is whether or not the length of firing is pre-set (as in a solid fuel motor) or if it can be set in flight, either on board, via ground station, or both.
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The de-orbit burn is not solid.
NASA - Russian Soyuz TMA Spacecraft Details
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The propulsion compartment contains the system that is used to perform any maneuvers while in orbit, including rendezvous and docking with the Space Station and the deorbit burns necessary to return to Earth. The propellants are nitrogen tetroxide and unsymmetric-dimethylhydrazine. The main propulsion system and the smaller reaction control system, used for attitude changes while in space, share the same propellant tanks.
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I don't know who can control the burn length. Since it's the main engine and is used in various parts of the mission, it performs burns of various durations. I'd expect the deorbit burn length to be modifiable.
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Wikipedia narrative about problems during the TM5 mission (to Mir) makes it sound pretty flexible, in this battle of erroneous navigation computer and manual control:
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They jettisoned the orbital module and made ready for deorbit burn to return to Earth. However, unbeknownst to the cosmonauts or the Mission Control Center in Korolev (TsUP), the guidance computer was using the docking software of the Bulgarian Mir mission in June. The deorbit burn did not occur at the appointed time because the infrared horizon sensor could not confirm the proper attitude. Seven minutes after the scheduled time, the sensor determined that the correct attitude had been achieved. The main engine fired, but Lyakhov shut it down after 3 seconds. A second firing 3 hours later lasted only 6 seconds. Lyakhov immediately attempted to manually deorbit the craft, but the computer shut down the engine after 60 seconds.
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