Today’s question for Dr. Science is, “What are starports for? Lots of starships call at my hab, and we don’t have one.”

Starports and starships have surprisingly little to do with one another.

If there were only starships and drifts, and perhaps the odd rock, we’d have no need of starports. The starships could simply pull up alongside their destinations and shift their cargo about with longshorebots and lighter OTVs and a few stout lads working out of docks and locks. Running a few insulated lines would take care of fueling, and in this scenario, no doubt the passengers – spacers all – would be happy enough to take a walk over. And outside local space, no-one cares where you heave to.

No, starports exist because the galaxy is full of planets, and because large numbers of people are perverse enough to want to live on them. (See my earlier column, Yes, They Store Their Air On The Outside (And Why We Can’t).)

They do have lots of facilities for starships associated with them – cageworks, chandlers, refueling depots, orbital warehouses, freight transshipment nodes, and suchlike – because it’s often convenient to keep them together in a central location, and because it helps pay the bills. But what starports are actually for is solving the interface problem.

One of the less believable realities of space travel is that – on most highly populated worlds, other than a few moons – the depth of the gravity well and the thickness of the atmosphere is such that it takes every bit as much delta-v to climb from the surface into orbit and as it does to make transit between a system’s worlds. The depth of the well and the passage through the atmosphere impose even more constraints on the structural strength and hull forms of starships, in ways that handicap them for operation in the space environment; most starships that are in operation today could neither support their own weight at the bottom of a planetary well, nor withstand the rigors of atmosphere entry. The need to transport freight and passengers between these two disparate environments is the essence of the aforementioned interface problem.

And so starports straddle this line, possessing both a dirtside half (the Down, or downport) and an orbital half (the Orbital, or highport), each composed of a variety of specialized facilities in close formation. The Orbital houses many starship service facilities, but the majority of its business is transferring freight and passengers to and from its counterpart. Except for relatively new colonies and those worlds with the wealth and traffic volume to support a space elevator (or more than one; Seranth has six elevators supporting its ring-city), this falls into a familiar pattern.

Freight is simple enough. Some worlds opt for pure mass-driver launch facilities, and some prefer laser-launchers, but wherever it can, the Imperial Starport Authority prefers to opt for the maximal efficiency of a hybrid system. Should you visit the freight terminal of any major downport, you’ll find it rather unimpressive in itself, despite the sheer size of the building, because it is merely the front end of an enormous mass driver – miles in length! – or array of mass drivers, ending at the peak of a mountain high enough to get the muzzle of the drivers above the thickest part of the planetary atmosphere – and if no mountain is conveniently located for the starport architects, an artificial one will be constructed for the purpose. Around the muzzle of the mass driver, a complex of gigawatt-range phased-array pulse lasers provides additional power and control.

Every few seconds, a freight container is taken from the outgoing queue, and locked into place within a reusable aeroshell, which provides both the streamlining necessary to penetrate the atmosphere, and ablative remass for the latter part of its flight. This aeroshell is then loaded into the mass driver and accelerated up to orbital velocities, with the mountaintop array selectively lasing the ablative remass (pulsed plasma propulsion) to provide guidance and additional delta-v as needed. (The degree to which it is needed varies by cargo: heavy hardbulk can withstand high accelerations, and as such most of the acceleration can be provided by the efficient mass driver, whereas more delicate cargoes require gentler acceleration for longer, and thus proportionately more of the total delta-v is provided by the lasers.) Upon its arrival in orbit, the aeroshell is caught by the muzzle of another, rather smaller, mass driver, this time operating in reverse, and converting the aeroshell’s residual kinetic energy back into electrical energy. Once it has been braked into the receiving station, the aeroshell is stripped off and sent for reconditioning and refueling, while the container is dispatched to the incoming queue, and thence to the appropriate orbital warehouse.

Ground-bound freight follows the reverse process, being accelerated by the small orbital mass driver onto a re-entry trajectory targeted upon the muzzle of its groundside partner; on its way down through the atmosphere (it is designed to be stable stern-down for reentry), the laser array and ablative remass are again called upon to provide guidance and, if necessary, additional deceleration. Plunging into the barrel of the mass driver, the reverse process is again used to brake it to a stop at the freight terminal, where the aeroshell is again stripped off and reconditioned, and the container routed onward to its final destination.

These systems are often operated in pairs, enabling the efficiency of using the captured gravitational potential energy of freight moving downwell – captured by the mass driver to the greatest extent that engineering and thermodynamics permits – to partially power the ascent of upwell freight. As you can imagine, a pair of these systems sending and receiving containers every few seconds, every hour of the day, every day of the week, can move an awful lot of freight!

Passengers, though, are more fragile than most freight. (And rather less comfortable stepping into the breech of Heaven’s Own Sluggun, whatever the numbers might say.) They prefer to travel on shuttles, vehicles specifically designed to cope with the interface problem – with all that atmosphere in the way, you can’t just hop in a commutersphere or ride a candle!

But atmospheres aren’t all bad news. Given the depth of a planet’s well, you might expect that the shuttles would have to be huge lumbering ships to carry all the remass they needed to climb up to orbit; but since they spend so much time in atmosphere, they can use the atmosphere itself as remass, and only carry the little they need for the very end of their journey. Most shuttles have trimodal nuclear engines. They start out as simple tilt-turbine ducted fans when they leave the ground, until they can achieve the speed and altitude necessary to start using their reactors to heat the air directly, becoming nuclear-thermal scramjets, and this mode carries them up through hypersonic speeds to the very edge of space. At this point, before the air becomes too thin for them to function, they switch over to using their internal supply of remass, becoming true nuclear-thermal rockets until they dock with the highport and deliver their passengers. Refueling there, they land again using the same engine modes in the reverse order, and the cycle repeats.

It’s ironic, then, that the features most commonly associated with starports in the public mind – the enormous graphite-and-cerametal pads with their massive hidden cradles, the blast-deflecting berms, the “hot” shafts with their billowing wash-down sprays, and so forth – are those dating back to an earlier age of space, when planets truly were the center of civilization and mighty ships rose heavenwards on pillars of atomic fire, now sadly reduced to a minority of any starport’s business, handling a few special loads, private yachts, and those small tramp traders which service early colonies and outposts that cannot yet afford full starports of their own. But even they share this one commonality: a need to get to and from the planetary surface.

In the end, they’re all about the planets.

Dr. Science

– from Children’s Science Corner magazine