Ars Mundi: The Gravity Highway

When a working principle of manipulating gravity was discovered, the general populace went mad. "It's just like Star Wars!!" the popsci websites and blogs shouted, rabid with the idea of being able to take to the stars in spaceships that could accelerate so fast as to get to lightspeed in mere days. As usual, the scientists had to destroy their ill-conceived dreams – gravitics still have to obey the conservation of momentum. Just like you can't push your car forwards while sitting inside it, you can't speed a ship up with a gravity generator on board. The reaction force has to go somewhere. Dreams of routine space travel were smashed. All the technology really offered was artificial gravity on-board spaceships, and zero-gravity chambers on Earth. Anything else was impossible, it seemed.

It took about five years for someone to figure out a solution to the problem: the gravity highway.

Gravitics works like creating an artificial mass. You create a gravity field that pulls all objects inside it in one direction. But it produces a force of equal magnitude and opposite direction on the generator itself. This is useful for artificial gravity – as long as the ship and the people in it are accelerating towards each other, you have gravity – but it was useless for the dreams of grav drives and contragravity. Anything carrying a grav generator ends up not moving an inch, as any acceleration it creates on itself is cancelled out by the reaction force. Gravity manipulation may be real, but so too is the conservation of momentum.

But what if you apply the field from an external source?

The idea was to use gravitic mass drivers, mounted on asteroids. This idea had been conceived before, with electromagnetic accelerators, but gravitic fields mean that the contents of the ship are in free-fall during acceleration, so the acceleration can be arbitrarily high without injuring or killing people on board. A ship manoeuvres itself into the gun's barrel, then the gun turns on and accelerates the ship at a few thousand Gs for a few seconds, the recoil force transferred to the asteroid the gun was mounted on. If you pick a big enough asteroid, you can throw lots of spaceships into space before it changes speed too much. But the extra bonus is that you can use the same system to decelerate incoming spacecraft. In this case, the asteroid moves towards the spacecraft it's slowing, so with some careful planning you can keep the asteroid in the same place by accelerating and decelerating spacecraft in the right directions!

With a string of these mass driver asteroid stations,  you can make a transport network, called a gravity highway. A ship's momentum is transferred along the chain of asteroids, respecting conservation laws while allowing extremely fast travel throughout the solar system. With creative use of gravitational slingshots around planetary bodies, even faster speeds can be obtained, allowing reasonably fast travel to even the outer planets. Soon, as the technology was perfected, you didn't even need to stop at each station: the grav generators would just deflect you in the direction you needed to go, almost halving travel times.

Travel along the network is super-fast, but establishing new nodes takes a long time. A spacecraft carrying the needed grav generators, powerplants, and structures must get there the old fashioned way, using conventional propulsion like nuclear rockets or fusion pulse drives. This can take months, even from a relatively close highway node.

Using the Gravity Highway
The gravity highway would likely be a defining element of a setting. It's best suited to an otherwise low-tech science fiction campaign, with humankind limited to a single system. It fits especially well at TL9, with gravitics as the emergent superscience coming early before the standard TL10^. The fact that the same type of gravitics can be used for artificial gravity opens up the solar system a lot – the main difficulty in living in space is gravity, and with artificial control over that, any asteroid, moon, or celestial body in the solar system is much more hospitable to life. This means that players can adventure anywhere from Earth, Mars, the asteroid belt, or the moons of Saturn without having to worry about the normal problems with spaceflight. The same setting should have other gravitic technology, but this probably only manifests as artificial gravity, zero gravity chambers, and grav guns. Most other applications in Ultra-Tech use gravitics for propulsion or contragravity, which isn't possible with this variation of the technology.

This all said, there's nothing wrong with using gravity highways at higher TLs or interstellar settings. They allow spacecraft with low deltaV or no FTL drives to travel rapidly around a star system. With enough time, power and money, you could even use them for interstellar travel! A series of high-yield grav accelerators in a row might be able to bring spacecraft to significant fractions of the speed of light (though they'd have to be really heavy, since they'd experience a huge reaction force from that much acceleration). A corresponding array in the destination star system slows the travellers down again, but if the destination doesn't have a gravity highway yet, magsails can be used like a parachute brake on the star's heliosphere, though this will take more time, and force the starship to rely on conventional drives afterwards.

While the gravity highway may define a setting, it isn't actually a big feature of a campaign setting. It's like a warp drive in a space opera setting – it's something the PC's use to get to where the adventure is. That said, like warp drives there are plenty of ways you can use it to make adventure. The most obvious is the hijack of one of the asteroid stations by malcontents, who use it as a giant grav gun to attack or threaten a planet or space station. With the kind of speeds the highway is capable of, a large enough rock fired at a planet becomes a weapon of mass destruction! With some tinkering of the computers to stop the proper deceleration and redirection, they can be potent – if disproportionately destructive – assassination tools. Secret nodes might be hidden, only accessible by broadcasting the correct passcode or sequence to the node that can redirect incoming ships there. There could even be entire "shadow networks" that are inaccessible from the main one, or accessible only in a few tightly controlled places.

The gravity highway also allows the GM to control how and where the PCs move about the solar system, if they want. Nodes will regularly need "reboosting", having lost too much momentum from kicking off spaceships. Nodes might malfunction, or safety concerns could lead to them being deactivated, only being repaired once a spacecraft with conventional drives can reach them several months later. Orbital mechanics could even be the problem – if the network is sparse in one region, it might be that the influence of a planet's gravity, or its distance from nodes in the network, render it difficult to get to.