Fact-based fiction: Electric, no-fuel propulsion for autonomous space drone
In an upcoming writing project, I will be focusing on an autonomous drone that picks through space wreckage. It has access to intermittent star light, but has no source of chemical propellant. The story is in the very distant future and the drone effectively has infinite time to perform its task. What sort of propulsion system might such a device employ that would offer reliable, predictable control including propulsion, braking, and rotation? Would it be as (finger quotes) simple as focusing the emission of photons? Are there other alternatives? Systems that would be glacially slow are in keeping with the theme, although faster or more agile systems are worth noting as well.
Specifically, I am looking for a list of proposed propulsion systems from actual experiments, theoretical physics, or from established science fiction. I am not looking for opinions on different systems. Links or references would be helpful in order to more fully explore the available options.
Edit: In a video, Scott Manley indicated that rotational devices have a maximum speed and can become saturated, and another force must be applied to relieve them. This isn't a problem in this context because the device has massive objects to push against.
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2 answers
I'd suggest the simplest and cheapest of them all: gravity. Recall that gravity pull is zero only at infinite distances. For convenience of the story, the drone will always be under the gravity pull of some celestial object.
The drone can design very elaborate gravity assisted maneuvers, planning fly-by next to the same space wreckages over the course of many millennia, if needed to. The only energy required is to run the drone brain to perform all the calculations.
The drone can design its trajectory by:
- modifying its shape: extending parts of its body (via arms, rails, ropes, threads), ejecting them via springs mechanisms (and rejoin aeons later), redistributing the mass within its body to increase, or decrease the angular momentum
- attaching and detaching from other objects: attaching to a piece of the wreckage to increase the mass, mechanically release a piece of the wreckage to impart itself an acceleration in the opposite direction, forcefully impact on space debris to cause a deceleration, or even to self impart a spin, or correct the current one
- timing a hitchhike on large astronomical bangs and ride on the supernova shock-waves like a boss
- heat up (lasers!) some celestial bodies to force them to eject mass, and thus make them move.
All these activities are pre-planned aeons in advance, as the drone calculates its infinite path through the universe. The secret to success is the infinite amount of time available, and enough celestial bodies to keep moving around. Forever.
All in all, given infinite time, a truly intelligent drone may never need to brake.
[ for the nostalgic, think of an intelligent version of the Voyagers ]
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Any light source is a propellant-less drive (often called a "photon drive"), it's just extremely energy inefficient.
With perfect conversion efficiency, the impulse of the device is (energy supplied)/(speed of light) and the thrust is therefore (power supplied)/(speed of light). That is, a 1 kilowatt photon drive offers about 3 micro-Newtons thrust. Really good LED light source get near 40% conversion of electrical energy to light energy.
Any inefficiency in conversion of electric power to light results is a corresponding loss of thrust.
No one has actually deployed a designed photon drive in which a on-board light source was used as a rocket, but:
- Test-bed scale solar sails have been deployed and work fine aside from the very low thrust to mass ratio.
- The Pioneer anomaly is considered solved in terms of a asymmetric thermal photon flux arising from heating in the probe's RTG.
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