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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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    $\begingroup$ "but has no source of chemical propellant." - why wreckage can't be a source of reactive mass? Why hydrogen present in space can't be a source of reactive mass? Or else you handwave or stick with EM-Drive. I mean you could clarify that moment. $\endgroup$
    – MolbOrg
    Jan 13, 2018 at 0:10
  • $\begingroup$ @MolbOrg I can't answer that question without telling more of the story. $\endgroup$
    – Jack Stout
    Jan 13, 2018 at 0:16
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    $\begingroup$ @MolbOrg, if you think about it, why he couldn't use sources of propellant is irrelevant. He asked a question that meets our site's expectations for a good question. Now it's our job to answer it on his terms. $\endgroup$
    – JBH
    Jan 13, 2018 at 0:24
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    $\begingroup$ @JBH it is not our job. And the thing I'm asking is to determine which terms those actually are. Does he mean just chemical propulsion or he mean reactionless types only? It significantly affect the answer, and to make the question better, it makes sense to clarify that aspect. $\endgroup$
    – MolbOrg
    Jan 13, 2018 at 0:44
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    $\begingroup$ Needs clarification. Do you want a reactionless drive (e.g. handwavium per current physics), or just not a chemical propellant? If the latter, ion drives using collected material are an option (but where do you get the energy?), as are solar sails. $\endgroup$
    – jamesqf
    Jan 13, 2018 at 4:17

8 Answers 8

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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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  • $\begingroup$ Some of the 'lab grade' LED devices are even passing the 80% mark. Micronewtons of thrust is on par with some small ion thrusters, maybe we'll see a photon thruster one day soon... $\endgroup$
    – Samwise
    Jan 13, 2018 at 21:22
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    $\begingroup$ For a photon drive, the emitted wavelength is irrelevant. As such, you can infrared radiation just as well as visible light. However, your can easily turn electric energy into heat radiation, at conversion rates approaching 100%, especially when in space (no heat losses due to heat conduction or convection). So, all you need to do is, power a strong heat radiator and place it in the focus of a highly reflective mirror. (The mirror needs to be designed for the predominant wavelengths of the radiation source.) This should easily give you a photon drive with an efficiency of more than 50%. $\endgroup$
    – cmaster - reinstate monica
    Mar 22, 2019 at 19:54
  • $\begingroup$ @cmaster Then the challenges are (a) collimation (which is not particularly hard, I think) and (b) keeping the mass down while keeping anything from melting when you pump a lot of power through it (which may be harder, though I haven't figured it). $\endgroup$
    – dmckee --- ex-moderator kitten
    Mar 22, 2019 at 22:11
  • $\begingroup$ @dmckee (a) collimation: That's what the mirror is for. Same system as your car's headlights. (b) keeping mass down: You only need a thin surface as a radiator. And you just need to make the radiator big enough so that it stays a tad short of becoming too hot. The real weight problem is the mirror which needs to have quite a large surface. $\endgroup$
    – cmaster - reinstate monica
    Mar 22, 2019 at 22:50
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At this time there are no working propellantless thrusters that I know of1. The closest would be an ion thruster. However, there are some other technologies.

There may be others. You could perform a Google search for "reactionless thrusters" or "propellantless thrusters" to see.

1 But the Resonant Cavity Thrusters are close...

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    $\begingroup$ "Propelentless" and "reactionless" are not synonymous. Photon drives qualify for the former, but the latter necessarily violated the conservation of momentum which photon drives do not. The interesting thing about the EMDrive is that it's purported thrust/power ration is several orders of magnitude about that of a photon drive. It's also come down each time we've seen a more rigorous test which is (sadly) not a good sign. The alleged Gyroscopic Inertial Drive is nonsense of the purest kind. Anything involving the quantum vacuum is technobable at best, and further nonesense at worst. $\endgroup$
    – dmckee --- ex-moderator kitten
    Jan 13, 2018 at 0:37
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    $\begingroup$ These are in no way "promising technologies". If you want to refer to ideas from science fiction, say so, but don't refer to thoroughly debunked nonsense in a way that will mislead a casual reader. $\endgroup$
    – Chris Hayes
    Jan 13, 2018 at 11:31
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    $\begingroup$ Resonant Cavity Thrusters are a hoax, at best. They are not a working drive. $\endgroup$
    – kingledion
    Jan 13, 2018 at 13:47
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    $\begingroup$ Slightly fictional there are Bussard Ramjets, which pick up their fuel, interstellar hydrogen, in space instead of carrying it on board. They just need some kind of booster to get started and solar battery/electric power to manage the magnetic ramscoop until power can be gained from the ramjet itself. Oh also some kind of solar sail combined with solar power for the electronics to control the angle and deployment of the sail.(Ack, already an answer) $\endgroup$
    – Todd Wilcox
    Jan 13, 2018 at 14:00
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This probe could use a combination of a solar sail to get to a debris field and the use of electromagnets once it is near debris. By using multiple electromagnets with very well-calculated "bursts" of attraction and repulsion from various pieces of metallic debris, the probe could slowly make it's way around the debris field. Imagine an architecture with several "arms" which have electromagnets on them. One might be pushing at the same time another is attracting. By powering the magnets in very controlled bursts, the probe would be able to alter it's trajectory.

Side effects would include throwing pieces of debris off their own orbital paths, but if the debris field is not over a populated planet, that is not such a big problem. There are actually a number of metallic asteroids in the asteroid fields in our solar system, which makes this not totally implausible, especially if the drone was designed to operate in asteroid and debris fields.

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    $\begingroup$ how do you push with an electromagnet? $\endgroup$
    – ths
    Jan 13, 2018 at 12:57
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    $\begingroup$ @ths Since the magnetic field of an electromagnet can be turned on and off and be changed in size, you can turn it on and expand it to push against any nearby ferromagnetic material. As the foreign objects are pushed away, thrust will be generated in the opposite direction. $\endgroup$
    – Todd Wilcox
    Jan 13, 2018 at 14:03
  • $\begingroup$ "and expand it to push against any nearby ferromagnetic material." I don't believe that is correct in general. You can arrange the field to push against any nearby object with a permanent field, but unmagnetized ferromagnetics will polarize in response to the field of the electromagnet and will be attracted. $\endgroup$
    – dmckee --- ex-moderator kitten
    Jan 13, 2018 at 16:55
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    $\begingroup$ Not going to work very well when there's an average distance between objects in our asteroid belt of 2 million miles. $\endgroup$
    – Mazura
    Jan 13, 2018 at 17:57
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    $\begingroup$ @Mazura Your source gives 2 million miles between asteroids larger than 1 km. Surely this answer can also work with asteroids that are smaller than 1 km. $\endgroup$
    – David K
    Jan 14, 2018 at 15:56
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What is the drone's range of operations?

Where the drone is, and what kinds of celestial bodies are nearby, will drastically impact the propulsion options available.

Things that will work everywhere are essentially limited to various kinds of reaction wheels and photon drives. Reaction wheels are great for changing orientation, and pointing the photon drives, but won't actually push you anywhere. While there are a few far-out options being investigated, e.g., by NASA's Breakthrough Propulsion Concepts program, there are currently no other propellantless propulsion systems available within the realm of known physics.

However, if it has intermittent access to starlight for power, might it also have access to stellar wind? If so, the drone may be able to use a magnetic sail for both propulsion and breaking. Unless you have a large array of magsails, thrust will always be away from the source star in that case--tacking against the wind, like you can do with a solar sail, doesn't work so well with magsails--but that can still be useful, especially if the drone is intended to engage in interstellar travel, and needs a cheap way to decelerate into a target system, and a boost for leaving a source system.

If the drone is ever in the vicinity of a planet with a magnetic field, or sufficiently close to a star to make use of the star's magnetic field, then the probe could use an electrodynamic tether both for propulsion and breaking (in which case, it would generate excess power, which could be directed to a photon drive or for other purposes). This would be most useful for spiraling in towards or out from the host planet/star. If the drone is capable of giving itself a net electric charge (for which a simple electron gun would suffice), then it could manipulate its charge to generate turning forces in magnetic fields as well. This would even be useful for interstellar navigation (perhaps more so than for maneuvering around a planet), if the drone has access to a reasonably good map of the interstellar magnetic environment. Turning in the galactic magnetic field by charging the ship is an approach that has been considered for some interstellar exploration mission designs as a means of allowing the ship to approach the target star "from behind" and thus make use of a laser sail, powered from the home system, for both acceleration and deceleration.

When the drone is in proximity to other large bits of matter (like asteroids, or comets, or the actual wreckage that it's actively looking for), there are of course plenty of "simple" mechanical options. It can crawl around, it can use shock-absorbing legs to break and "land", and it can push off with legs. A grappling device (or suite of grappling devices for different surfaces) attached to a Very Long Tether (possibly one which could double as the previously mentioned electrodynamic tether) may be useful for grabbing onto Stuff from an intermediate distance (say, up to a few hundred or thousand kilometers). Such a tether could also be used in reverse to allow the drone to steal an object's rotational energy and give it a stronger fling towards a new destination than it could achieve with legs alone.

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  • $\begingroup$ Maybe a base station with a huge magnetic field (either a permanent magnetic, an electromagnet using superconductors or a combination of the two) would allow probes with tethers to be able to navigate in or out (radially). $\endgroup$
    – David Elm
    Jan 13, 2018 at 22:17
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JBH has them pretty well covered. I am fond of the Quantum Vacuum Thruster.

Another one that involves shooting stuff: Reverse fusion ion drive http://www.halfbakery.com/idea/Reverse_20fusion_20ion_20drive#1276103681

Accumulated energy is used to form mass (perhaps small and easily acceleratable electrons?) which is then accelerated away to propel. It is not a very efficient use of energy but you can accumulate a lot of energy with infinite time. Glacially slow might apply.

One more is the notorious Alcubierre drive.(https://en.wikipedia.org/wiki/Alcubierre_drive) This is usually invoked as a legitimate way to move faster than light by warping space in front of the moving object. Objections to this are the vast amounts of energy it would take and very energetic and destructive particles it might produce.

But there is no reason this same mechanism could not be used to move slower than light. It would be safer. This concept is bandied about here: Using an Alcubierre warp drive strictly for sub light travel

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  • $\begingroup$ Ooooh! I've not heard of the reverse fusion ion drive before. Thanks! $\endgroup$
    – JBH
    Jan 13, 2018 at 0:59
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There is only one propellant that imparts momentum and has no rest mass

And that propellant is photons. Photons have a momentum of

$$p = \frac{h}{\lambda}$$ Where $h$ is the Planck constant and $\lambda$ is the wavelength.

There are plenty of problems with photonic engines, which I discuss here. But if you want no-fuel propulsion in a fact-based manner, this is the only way to do it.

Your propulsion unit is basically a laser. You point the laser in one direction, fire it with very high power, and you are imparted momentum in the other direction.

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    $\begingroup$ While lasers are the light-source of choice for science fiction whiz-bang, if you were building one today you would probably use a non-lazing LED as the source. Sure you give up a smidgeon of efficiency due to weaker collimation, but the device is simply more efficient. $\endgroup$
    – dmckee --- ex-moderator kitten
    Jan 13, 2018 at 1:20
  • $\begingroup$ @dmckee You want all the photons pointed directly away from the desired direction of thrust. An LED will send photons over some arc, thus lowering the net thrust in the desired direction. The collimation is the whole deal, you want the photons going in only one direction. Hence, a laser. $\endgroup$
    – kingledion
    Jan 13, 2018 at 1:24
  • $\begingroup$ It's trivially easy to buy LEDs with less than 5 degree FWHM beam width. In fact, in some bands it's hard to buy wide beam devices. And because conversion efficiency factors directly into the thrust it would be the limiting issue in a design today. But you'll never use to to lop a enemy ship in half at any decent range. $\endgroup$
    – dmckee --- ex-moderator kitten
    Jan 13, 2018 at 1:31
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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:

  1. 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
  2. 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
  3. timing a hitchhike on large astronomical bangs and ride on the supernova shock-waves like a boss
  4. 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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Your drone has access to both starlight and shipwrecks, so a fairly simple solution would be to have it harvest from each shipwreck it visits and create slugs. It can then use those slugs as "propellant" by firing them out of railguns or coilguns arranged around its body. Firing a rail- or coilgun only requires some sort of ferrous slug and a bunch of electricity, which is well within the drone's means. There's a related question about the feasibility of railgun propulsion over here on space.se.

This makes even more sense if the drone has effectively infinite time to complete its task; after all, something that's expected to operate on the timescale of "forever" is going to have multiply redundant self-repair functions which can easily be turned to the task of harvesting materials from wrecks and creating propulsion slugs.

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