Mark Anson - Below Mercury

She watched the primary flight display intently, her hand resting on the sidestick, ready to break contact at the first sign of trouble, while Wilson monitored the rising levels of liquid oxygen in the main tank.

Minutes passed. Creaks rang through the spaceplane’s structure as the tank adjusted to the sudden chill of tonnes of cryogenic liquid flowing into it. Boiling liquid oxygen streamed away in a long plume behind the spaceplane through the open vent lines. The spaceplane rocked back and forwards slightly, the noise of the engines rising as the autopilot compensated for the increase in mass.

‘Nine zero tonnes,’ Wilson said, ‘still venting.’

‘Keep it coming,’ Clare murmured.

A shower of white ice broke off from the refuelling boom, and was snatched away instantly by the spaceplane’s slipstream. The boom was so cold now that water vapour from the air was freezing onto its outer surface.

‘One hundred tonnes. Nearly there,’ Wilson said. ‘Stand by to break contact.’

A few seconds later, a fountain of pure liquid oxygen gushed out from the venting tubes, leaving a spectacular vapour trail of boiling oxygen behind the spaceplane.

‘Tank full,’ Wilson confirmed, ‘break contact.’

Clare disengaged the autopilot and reduced thrust, and the spaceplane detached from the refuelling boom in a shower of ice and vapour. Clare let the spaceplane fall behind the tanker, and then banked away to the right.

‘Mercury Two Zero Seven, contact broken, moving away. Thanks for the fill, guys.’ Wilson bent back to the navigation display, looking for the rendezvous with the waiting propane tanker.

‘Tanker Eight One. Have a good day.’

Thirty minutes later, over the empty expanses of the western Pacific Ocean, the Olympus-240 spaceplane followed close behind a second tanker.

The transfer of the last few tonnes of super-cold liquid propane fuel was complete. Clare continued to hold contact with the tanker, keeping the fuel tanks topped up while they waited for their orbital climb clearance from Guam Centre, far behind them in the ocean.

High above them, the invisible plane of the Sun and planets crept higher in the sky, until it was almost directly overhead. They hung there for what seemed like an age, floating in the clear blue skies behind the tanker, waiting for the moment when their launch window would open, the narrow sliver of time where the spaceplane’s projected climb path would neatly intersect the orbital plane of the space tug. Finally the clearance came through, the voice faint with distance:

‘Mercury Two Zero Seven, Guam Centre. Clear to launch, left turn onto zero six seven, clear through flight level one one four and orbital climb to three two zero kilometres. Orbit inclination plus two three decimal five, insertion point Sierra One at zero one zero two Zulu.’

‘Okay, let’s get moving,’ Clare said. ‘Breaking contact.’ The spaceplane shuddered slightly as she detached from the tanker and banked away onto their new course, lining the spaceplane up with their orbital path. She nudged the thrust levers forward slightly to give them more airspeed.

‘Clear launch, heading zero six seven, clear climb through one one four to three two zero kilometres, inclination plus two three decimal five, Sierra One insertion at zero one zero two Zulu, Mercury Two Zero Seven.’ Wilson released the transmit and watched the tanker slide away into the distance, propane vapour trailing from its refuelling boom.

‘Visor up,’ Clare commanded, and the heat-resistant visor moved slowly up to cover the windows, smoothing the airflow over the nose and shielding the main cockpit windows from the heat of the hypersonic climb. The interior of the cockpit darkened as the visor closed over them; its small windows gave only the minimum necessary view forward. The roar of air over the spaceplane lessened as the airflow smoothed out.

‘Cockpit lighting.’ Clare’s voice seemed louder in the muffled quietness of the cockpit as she turned to the passengers. ‘Okay, gentlemen, it’s time to prepare for the climb. I want you to check your seat straps, close and lock your faceplate, and cross-check with your neighbour.’

Matt’s pulse quickened as he pulled his seat straps tight over his shoulders and closed his faceplate; the climb was seconds away now. The sound of his breathing was loud inside his helmet as he gave a thumbs-up sign to Rick and received one in return.

Wilson’s voice came over their headsets.

‘Tanker reports clear of launch area. Orbital climb checklist complete.’ His eyes scanned the instruments in an unbroken sequence from behind his faceplate, checking fuel temperatures, engine readiness, and the navigation display for any stray aircraft in the launch area.

They were the only human beings in a five-kilometre radius of the launch. The spaceplane held nearly 160 tonnes of cryogenic propellants and was now a flying bomb; if those propellants were to mix and suddenly ignite, the resulting explosion would obliterate the spaceplane, its crew, and any aircraft that happened to be nearby.

Clare took one last look round the instruments, put her hand on the sidestick, and took a deep breath.

‘Everyone ready? Okay. Set mode to orbital climb. Tanks up to full pressure.’

On the engine displays in the centre of the cockpit, pressure readings swung into the green part of their arcs. The spaceplane’s structure creaked again as the fuel tanks were pressurised with helium, ready to force the propellants into the turbopumps.

‘Full pressure. Ready for ignition,’ Wilson confirmed.

‘Roger. Starting climb.’ Clare eased the thrust levers all the way forward to their maximum thrust setting. The engines’ roar increased, and the spaceplane started to accelerate, climbing into the sky.

Clare watched the thrust and airspeed build up, making small adjustments to the spaceplane’s pitch.

‘Afterburners.’

Wilson pressed the four switches below the thrust levers, and the afterburners lit up in a thunder of blue flame, pushing the crew into their seats. Clare raised the nose further, until they were climbing at thirty degrees, arrowing up into the sky.

The spaceplane shook as its speed built up to Mach one. They were passing through the supersonic transition region, where the aerodynamic forces on the craft’s structure changed rapidly, and shock fronts formed, streaming out behind the spaceplane in invisible compression waves. The engine intakes moved, becoming narrower, slowing the air so that it could flow safely into the whirling blur of the engines.

Suddenly, they were through; the buffeting faded as the supersonic flow stabilised around the craft, and the roar around the cockpit grew strangely quiet. Far behind them, and unheard in the cockpit, a dull, rolling boom echoed across the empty skies.

‘Mach one. Engine management to auto.’ The spaceplane’s flight smoothed out now. Behind them, the roar of the engines could be heard through the structure of the craft, but there was no sound coming from outside; just the faint hiss of the boundary layer, where the supersonic flow dragged over the spaceplane’s skin. Clare watched the airspeed rise until they were at Mach two. Her right hand hovered over the autopilot engage button.

‘You’re in the launch box. Vee three.’ Wilson confirmed, watching the figures in his primary flight display change from magenta to white.

‘Commit.’ Clare pressed the autopilot button, and gently released her grip on the sidestick.

Under the control of the autopilot, the spaceplane accelerated towards three times the speed of sound. The climb sequence and engine management from here was entirely automatic, the flight computer adjusting constantly to get the optimum flight trajectory and engine configuration. As speed increased, the air entering the engine intakes was compressed by the sheer speed of the spaceplane through the air, and the hot, compressed air was diverted directly to the afterburners, turning the engines into giant ramjets.