Media playback is unsupported on your device Media caption British astronaut Tim Peake speaks to David Shukman

British astronaut Tim Peake is going through a gruelling round of final training before liftoff to the International Space Station next month.

Here at Russia's Star City centre near Moscow, he and two fellow astronauts endured three hours in a Soyuz simulator this morning.

Dressed in spacesuits, the three men clambered into the tiny capsule for a rehearsal of the end of their mission.

This is when the crew uncouple from the ISS and descend back to Earth.

Just before the exercise began, I suggested to Tim Peake that he looked far more sombre than he had during a big media day in London last week when he seemed to enjoy fielding light-hearted questions.

"It is a serious business," he said, sitting on the steps leading to the spacecraft.

"There's a fun side to space and what we do and the educational programmes we run but when it comes to actually getting into a Soyuz rocket - and the operational tasks that we have to perform - you need to be focused and serious."

Asked about the greatest challenges during the simulation, he said: "The most difficult thing to deal with is multiple failures.

"If you have just one failure then you can work through it as a crew. But if you have several it's much harder to keep track of all the problems."

Tim Peake: Career in brief

Born on 7 April 1972 in Chichester, West Sussex

Studied at Chichester High School for Boys, leaving to attend the Royal Military Academy in Sandhurst

After graduating, he served in the Army Air Corps and flew Apache helicopters

Retired from the Army in 2009 and was employed as a senior helicopter test pilot for AgustaWestland

Selected as a European Space Agency astronaut in May 2009 and completed basic training the following year.

Has been training for his six-month mission aboard the International Space Station, scheduled for launch on 15 December 2015

Enjoys skiing, scuba diving, cross-country running, climbing, and mountaineering, and he is interested in quantum physics and aviation

Tim Peake: How I became a British astronaut

Tim Peake: 360 degree photos

Watching the process relayed by CCTV to the simulator control centre, all three of the astronauts looked busy and tense.

During the exercise, instructors created a series of pretend challenges including a leak in the spacesuit worn by US astronaut Tim Kopra, a slow leak of the capsule itself causing depressurisation - and at the same time a computer failure.

Emerging later, Tim Peake looked quite tired and slightly unsteady on his legs - a normal reaction after sitting hunched in such a confined space.

But he was pleased with the way the team had coped with the malfunctions that the instructors had faced them with - and was keen to emphasise how the training is a positive experience.

"I always feel more confident when I come out of the simulator - they throw these emergencies at you but we coped with them and we got back safely," he told me.

Unlike Tim Kopra and the Russian mission commander Yuri Malenchenko, this will be Tim Peake's first flight into space.

But Tim Kopra said: "Tim is an ideal crew member - he's so sharp and so friendly. He's a great crew member to work with."

The trio will continue their training here at Star City before being flown on 30 November to Baikonur, the Russian space launch centre in Kazakhstan, Central Asia.

There they will be kept in quarantine to minimise any risk of picking up a cold or flu.

Last week, Tim Peake told me his greatest worry, as the Russian winter starts to bring snow and ice, was slipping and twisting an ankle.

"I'm going to keep myself safe with bubblewrap," he said.

The launch is set for December 15.

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