The Emperor protects!

By the way, the Thunderhawk machinery described in Gav Thorpe story "Know your foe"
As the Thunderhawk plummeted deeper into the thick cloud of Slato’s skies the passage became smoother, and half a minute later the pilot activated the standby lights. The padded restraints arched up into the wall above Ramesis’s head with a hiss of hydraulics and he stretched his arms, the servos within his powered armour whirring quietly as they matched the movement. He felt pressure on his back as the Thunderhawk’s machinery implanted his backpack into the socket along his armour’s spine, then dropped the ablative shoulder pads down on either side of his head. Now fully armoured, Ramesis stood up and walked steadily along the decking of the Thunderhawk, passing his gaze over the twenty-six assembled Space Marines. Each was conducting their own pre-battle rituals: checking weapons, comms or armour one last time, wishing each other the Emperor’s benevolence or just praying quietly.

and in his "Renegades"
Gessart stepped backwards into one of the flight alcoves along the flanks of the hull. Mechanics hissed as servo-arms came down from the ceiling and detached the Space Marine's backpack and plugged it into the Thunderhawk's system to recharge. Even the compensating muscle-like fibre bundles of his power armour felt lighter without the backpack's reactor weighing him down. A quick check of the suit's systems in his visor display confirmed that his armour had internal power for several hours; more than enough for them to reach the strike cruiser in orbit.
...The Space Marines had discarded their helmets, and their backpacks were locked into stowage positions to recharge from the gunship's engines while their armour functioned on its own internal power source. Except for Hephaestus, who was in the cockpit piloting, they were all sitting with their heads bowed in prayer, each silently mouthing their own chosen catechisms to the Emperor and their primarch, Lion El'Jonson. Aware of the subdued mood, the boy quelled his excitement and seated himself at the rear end of the gunship, away from the intimidating presence of the Space Marines.
Soon light glared through the ports as the Thunderhawk docked, accompanied by the clang of clamps sealing to the hull and bringing the craft safely inside the Blade of Caliban. Roused from their reverie, the Space Marines stood, and each backed onto his armour's power pack. With a hiss of hydraulics and the clank of locking mechanisms, automatic arms implanted the backpacks into their armour once more. They reached beneath the bench and picked up their helmets, uniformly carrying them under their left arm. As the assault ramp crashed down onto the decking, they filed out slowly into the docking bay. Techpriests and half-machine servitors moved to and fro, checking the Thunderhawk, giving praise to the Machine-God for its safe return and sprinkling it with holy oils from heavy censers.