Kinetic impact powered propulsion in postcards from cutty

Some rambling thoughts on the kinetic impact powered propulsion used in pfc...

postcards from cutty takes place in the year 2169, a mere two centuries after mankind first stepped on the Moon. I postulate a future where mankind has a small but permanent presence in space, where manned interstellar missions have taken place, but there has actually been a pull back from stations on Mars and Titan. Space colonization is mostly limited to habitats in Earth orbit and Venus orbit, with one station still on the Moon and another station still in Mars orbit. There are also a handful of "space hermits" who, like Cutty, have set off on their own on the outskirts of humanity. Cutty is the only one to venture beyond the main solar system.

But that's not what I'm going to ramble about here. I'm going to ramble about the space propulsion technology used. Cutty has set off to an Oort cloud object--a journey which has taken nine years at around 300km/s (this results in a round trip lightspeed delay of a week to her communications).

300km/s is an amazingly high velocity, well beyond what we can do today. But it's within what Cutty's spacecraft, the RMS Essex, is capable of--with outside assistance. It is fundamentally a large superconducting magnetic loop, with some spent fuel tanks strapped to it. Main propulsion is kinetic impact powered rocket. This involves puffing some inert sacrificial propellant (typically nitrogen), which very fast impactors collide with. The collision produces an explosion of charged particles--some of which are deflected by the ship's large magnetic field for thrust.

There are two basic directions this thrust can work in. The more intuitive direction is "downstream". The impactors come from the direction of the Sun, and the puffs of gas are directed a bit toward the Sun. This means the explosions occur on the Sun side of the ship, producing thrust away from the Sun.

The less obvious direction is "upstream". The impactors come from the direction of the Sun, but the puffs of gas are directed a bit away from the Sun. This means the impactors physically pass through the (unoccupied) center of the loop shaped ship, just before colliding with the propellant puff. The plasma explosion occurs on the other side of the ship, producing thrust inward toward the Sun. This direction is inherently less efficient, because the momentum of the impactors biases the explosion asymmetrically in the direction away from the Sun.

Because of this inefficiency, a stream of drones that were intended to accelerate the RMS Essex back toward the inner Solar System (from Titan) could instead be used to accelerate the RMS Essex outward at a much faster speed. These impactor drones were launched by Cutty's mother before they even arrived at Saturn to support their speedy "getaway" from Titan back to Earth orbit. But Cutty would use them to instead to boost outward. Drone speed was around 350km/s, the planned inward speed was going to be around 150km/s but Cutty ultimately used it for an outward speed of around 300km/s.

How do these drones reach 350km/s? This is done by kinetic impact powered propulsion also, taking advantage of the gravity well of the Sun. At 8.5 solar radii, escape velocity is over 200km/s. This means that two drones at perihelion, going in opposite directions, will have a relative velocity of 400km/s. That packs a heck of a lot of specific kinetic energy. What's actually used is one heavy drone in one direction, colliding with a stream of tiny glitter-like particles in the opposite direction. The heavy drone uses a magnetic loop for thrust, but it is much smaller and has a "donut" shape due to thick reflective layered insulation (to deal with intense sunlight). The heavy drone puffs sacrificial gas to collide with the glitter particles for thrust, accelerating from around 200km/s up to around 400km/s. As the drone pulls away from the Sun, the Sun's gravity does slow down the drone from 400km/s to 350km/s. The heavy drone then deploys its own payload of glitter-like particles in a stream. It is these glitter-like particles which will then be used by the client spacecraft to power its thrust.

This drive system provides a potent combination of high specific impulse and high thrust, with cheap propellant. However, the preparation lead time is very long. Depending on the starting point, it can take many years for the drones to reach the Sun. The typical route is via slingshot around Jupiter, lead time of around four years. This isn't really a problem for most commercial users, because there are prepositioned drones and glitter-like drone streams ready for customers to purchase on demand. But things can be more complicated for criminal jobs.

The Titan job required a fast getaway in order to avoid having the loot taken by competitors. These competitors may have had electric propulsion systems good for 50+ km/s of delta-v in pursuit; a cruise speed well in excess of that would ensure escape even if the pursuer had prepared a kinetic impact powered boost in the same vague direction. Normally, such massive overkill wouldn't be used for any space mission. Normal cruise speeds well under 50km/s are typical, because it's much cheaper.

Well, Cutty's mother died on Titan during the job, and Cutty took over the ship and decided to leave the main solar system entirely. The acceleration run drone stream was already on its way, arriving shortly after Cutty completed the job (acquiring the abandoned Titan Station's radioisotope generators). That solved half of her propulsion problem.

The other half is deceleration at her destination system. For this, Cutty had to release a pair of large drones before accelerating away from Saturn. These would take about two years to reach the Sun at around 20km/s, before zooming outward to catch up with Cutty by the time she reached the destination system. The resulting drone stream wouldn't be sufficient to brake the full mass of the RMS Essex. Cutty's plan is to dump most of the mass of the RMS Essex shortly before the braking run--mostly supplies of water, dry ice, and ammonia which could be replenished at the destination system...
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