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Impulse Grid

Also known as a Dynamic Momentum Grid

Impulse Grid
Image from Steve Bowers
The Chuley (47 Arietis) system, showing a few selected elements of the local Impulse Grid telpher beams (blue) and boostbeams (green), superimposed on the planetary orbits (red). Also shown are some of the deflection stations, and a few of the many shepherd arrays in this system.
An Impulse Grid, also known as a Dynamic Momentum Grid, is a network of mass beams, used for transport, energy transmission, and waste heat management inside a star system. The use of smart particles, or beamlets, allows the grid to dynamically reconfigure its structure to adapt to its users' requirements while efficiently redistributing momentum between them.

Along with datanets, impulse grids are one of the most common examples of in-system infrastructure in the Terragen sphere.

Beamlets

An individual beamlet consists of four elements: a length of superconducting wire, a thin film of diamond that acts as a dielectric light sail, computronium, and micromechanical systems to change configuration. Because beamlets can merge and divide, sharing their elements between one another, there is no particular size. However, the smallest useable beamlets (which can no longer divide) have approximately 20 micrometers of superconducting wire and 100 square micrometers of light sail.

The superconductor allows the beamlet to magnetically couple to external structures such as ships or reflector stations, transferring its momentum. However, it also has many other capabilities. It can store energy, serve as a magnetic sail or plasma magnet drive, accelerate the beamlet against planetary or stellar magnetic fields, and transmit or absorb infrared and microwave radiation. There are, however, trade-offs between these different functions.

The light sail, of course, allows the beamlet to gain momentum from visible light. Usually this will be light from the star, but lasers can also steer the beamlet from a distance. By using its light sail and magnetic sail in conjunction, the beamlet can tack in different directions.

Beamlets in free space can harvest momentum from the local star's light and solar wind using their light sails and magnetic sails. To accomplish this, the beam is shaped like a narrow ribbon to maximise the surface area presented to the star. Momentum harvesting allows beams to traverse more complex and useful paths that would be possible with orbital mechanics alone. It also allows the beam to compensate for deflection by heavy traffic, and provides additional energy to the beam (indeed, impulse grids often generate surplus energy which can be used to power other infrastructure).

While each beamlet has a relatively low intelligence, it can network with those surrounding it using infrared signals, giving the beam as a whole considerable collective intelligence.

Shepherds

Shepherds are macroscopic vehicles consisting of superconducting cables, energy storage, fabricators, a mattercache, an optical phased array, and sometimes a conversion reactor, with a total mass anywhere between a few kilograms and hundreds of tonnes. They traverse the impulse grid, trading energy, momentum, and beamlets with the beam itself to help balance and reconfigure the network. They absorb damaged beamlets and synthesise new ones, and help to split or merge beams if required. With their OPAs, they can absorb and transmit lasers over long distances to share energy with each other and with other vehicles traversing the impulse grid. In some cases, streams of shepherds can constitute a mass beam in themselves, carrying significant momentum.

Deflection Stations

Deflection stations are large masses used to change the path of a beam onto a more useful orbit. The station doesn't orbit the star (or moves at less than orbital speed), and its weight is balanced against the force of the beam. Deflection stations are often asteroids fitted with magnetic mirrors, but they may also be habitats in their own right, other useful infrastructure, or parts of a megastructure.

Transportation

Any ship equipped with superconducting loops can couple to an impulse grid and use it to accelerate. The ship does not need to remain coupled to the beam for its entire journey, of course — once accelerated up to a suitable cruise velocity, it can coast through free space.

If a ship is only using the kinetic energy of the beam to accelerate, it is limited to the velocity of the beam. However, if the ship expends additional energy, it can continue to accelerate by actively pushing against the beam. The energy may be stored onboard the ship, transmitted by laser from the shepherds, or even provided from the superconducting batteries of the beamlets themselves. This energy will not be lost — when the ship decelerates at the end of the journey, its kinetic energy will be absorbed by the impulse grid and used elsewhere.

A bigger limitation is deflection: accelerating ships will exert a force on the beamlets, deflecting them from their original orbit. Impulse grids use multiple techniques to manage deflection.

Impulse grids used for transportation tend to use two configurations:

In the telpher configuration, two parallel beams travel in opposite directions at close to orbital velocities (10-100 km/s). The beams have a high mass density, and accelerate a continuous stream of traffic. Ships exerting a large force collect a portion of the forward-moving beam and then eject it into the backward moving beam. If two ships on the same telpher beam need to pass each other, one will need to temporarily detach from the beam. The detached ship can still accelerate, however, if it absorbs a stock of beamlets ahead of time and then ejects them while detached.

In the boostbeam configuration, a beam is aimed directly at a specific ship. The ship deflects the entire beam as it accelerates. Boostbeams have much lower mass densities and higher velocities (on the order of 1000 km/s) than telpher beams, and are most useful for flexible routing of individual ships where traffic is lower. The boostbeam does not have to be in the same direction as the ship's acceleration if it is deflected obliquely. In a reciprocal boostbeam configuration, two ships can accelerate away from each other or decelerate towards each other by passing beamlets back and forth in a closed loop. (See also Interstellar Boostbeams).

The two configurations are often used in conjunction: A ship can accelerate along the telpher beam for part of its journey, then receive boostbeam acceleration. Shepherds can produce boostbeams by redirecting a small portion of a telpher beam, which allows boostbeams to emitted from the most convenient locations. Once a boostbeam has been deflected by a ship, it has a limited capacity steer itself, and may be used to help accelerate another ship elsewhere, or absorbed back into a telpher beam.

Telpher beams can also serve as interstellar catapults. Routes in the outer system are often longer than 5 AU, so a small ship accelerating along them at 1000g can easily reach a significant fraction of lightspeed.

Energy transmission and storage

Impulse grids are also used to transmit and store energy. This is a natural part of its use as a transportation system — accelerating ships absorb energy from the grid, while decelerating ships impart energy to it.

While local conversion reactors are a common alternative, impulse grids allow energy that would otherwise be wasted — such as from interstellar ships decelerating via boostbeam or fusion nucleosythesis — to be transmitted to locations that can use it.

Impulse grids can store large amounts of energy in a system if it isn't needed, by moving deflection stations to higher orbits.

The maximum energy density of a beam's superconducting storage is approximately 1.5 MJ/kg (10% that of a specialised superconducting battery, because of competing design factors). The kinetic energy density of a beam is between 50-5000 MJ/kg for the telpher configuration (operating close to orbital velocities), and approximately 10^6 MJ/kg for the boostbeam configuration. However, boostbeams generally have a much lower mass per unit length than telpher beams.

Heat management

Because the beamlets collectively have a high surface area, they function as very effective radiators for waste heat. A mass beam can be directed through a heat exchanger, absorbing heat which it will radiate out into space as it continues its journey for use elsewhere. Most ships riding a mass beam will use it as a cooling system

Beamlets can't be heated beyond temperatures of 500K, or they lose their superconductivity and cease to function effectively. To remove waste heat of higher temperatures, the simplest solution is to use passive radiator plates made of graphene, steel or tungsten, connected to superconductor loops through a thermal insulator. The radiators can then be loaded with waste heat and sent out to ride the beam until they cool down. If large amounts of heat are being dispersed, the beams will be visible from a distance as orange or red glowing lines.

However, such temperature gradients are an effective source of useful work. In a technique pioneered by the Negentropy Alliance, small vehicles hold thermal batteries which are loaded with waste heat at a high temperature. The vehicles radiate this heat into space at a much lower temperature, and use the thermal gradient to power useful tasks as they travel, such as computation, industrial manufacture, or charging the superconducting batteries of passing beamlets.
 
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Appears in Topics
InfrastructureInterplanetary TransportPropulsion Technology
Transportation
 
Development Notes
Text by Liam Jones
Initially published on 11 September 2025.

 
 
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