# Internal Mass Driver Engines in Cislunar Space

An internal mass driver engine uses electrical power to accelerate reaction mass in a mass driver and to accelerate a spacecraft that way. Some designs assume relatively large exhaust particles (dust or grains) at relatively low velocity (in the double or single digit kilometers per second).

Would the use of such an engine in relatively congested space (including LEO, GEO, and in the future the Lagrange Points and lunar orbit) be a significant danger to the infrastructure?

• Grr. Mass drivers are already in use in cislunar space, namely ion engines. Exhaust hit is painful if it happens, yet neutral xenon/krypton atoms used in modern drives are too dispersed in space to cause real damage more than what's being caused by solar wind already. And, if your engines get exhaust velocity of more than 100 km/s, there is no risk of your exhaust hitting Earth "back" if it didn't hit Earth or nearby objects right away. Commented Oct 31, 2023 at 6:41
• @Vesper, see the linked description. It assumes larger particles and slower exhaust. I'll edit.
– o.m.
Commented Oct 31, 2023 at 6:45
• @JBH, the question is "is this engine too dangerous to use?" I can edit to leave this the single question mark ...
– o.m.
Commented Oct 31, 2023 at 18:12
• That works for me. Vote retracted and cheers.
– JBH
Commented Oct 31, 2023 at 18:49

relatively low velocity (in the double or single digit kilometers per second).

The distinction is important.

In general, if your exhaust velocity $$v_e$$ is greater than the local escape velocity $$v_{esc}$$ minus your current orbital velocity $$v_o$$, your exhaust might escape the local gravitational sphere of influence. If $$v_e > v_{esc} + v_o$$ then your exhaust will escape the local gravitational SoI, unless it hits something first.

Orbiting at the distance of the moon, $$v_{esc} \approx 1.4 \pu{km/s}$$ and $$v_o \approx 1 \pu{km/s}$$ so if $$v_e > 2.4 \pu{km/s}$$ then it'll probably be shooting off into interplanetary space, at least. It would be surprising to have an exhaust velocity of less than that, because the specific impulse of your engine would be rather poor, and there are probably better means of propulsion both in terms of efficiency and not polluting the neighbourhood.

If $$v_e \approx 10 \pu{km/s}$$, then your exhaust will be on an escape trajectory if your orbital radius is at least ~23000 km (which is lower than geosynchronous). Using a grit-cannon at lower altitudes is probably undesirable, given that space starts getting crowded down there, but clearly a double-digit km/s exhaust velocity will not generate fresh space junk around Earth.

Would a direct hit with the exhaust endanger space infrastructure?

Well, yes, obviously. However, in general you'll be pointing your big engines either tangentially to your orbit (for prograde/retrograde burns) or perpendicular to your orbital plane (for plane change manoevers) and as such the exhaust plume will generally be aimed away from what you're orbiting, which means it'll be flying through less busy space, meaning you don't have to worry so much about shooting someone

There are some exceptions, such as being in a lowish, boring orbit in the equatorial plane, in which case some of your thrust could pass through geostationary orbits, so as soon as you're closer than GEO you probably want to be very careful about using your grit-cannon. Other exceptions include thrusting towards the Earth from a Lunar orbit.

would there be a practical risk that exhaust particles exceed escape velocity of the planet and come back on a return orbit around the sun, decades or centuries later?

If you're operating as far out as the Moon, or an Earth-Sun Lagrangian point, then it is almost guaranteed that your exhaust will escape into a heliocentric orbit. If it is at least 12 km/s it might escape into interstellar space, but in order to guarantee that escape it would need to be going a good 72 km/s and that looks a lot more like serious nuclear rocketry than a simple mass driver, to me. It seems likely, then, that most of the exhaust will hang around, to some degree.

Is it likely to be a problem though? Well, space, as the good book says, is big. Very big. Low Earth Orbit is comparatively snug... there's not very much of it, and stuff there has a low orbital period and so there's lots of chances for things to hit you. Things in an Earthlike heliocentric orbit, on the other hand, get at most 1 chance to hit you every 6 months (as opposed to 45 minutes for LEO), and in general periods will be much, much longer.

Given a year or more to expand, even a tightly collimated exhaust beam is going to be spreading right out into a very diffuse cloud so even if you did hit it, you're unlikely to hit more than a very small proportion of the exhaust. Depending on the size of the exhaust particles, you may find that the Yarkovsky or Poynting-Robertson effects will tend to sweep them in towards the Sun, making them someone else's problem over longer timescales.

You talk of dust and grit, and that sort of stuff is already the sort of thing that spacecraft need to be concerned about... we get regularly peppered with cometary debris in significant quantities... far more than you might reasonably expect to produce by flying a bunch of mass drivers about. A spacefaring civilization of the sort that will be producing enough mass driver exhaust to become a realistic problem will have had enough experience and expertise to build appropriate shielding to the point where it seems unlikely to be anything more than an irritation. Maybe they'll have moved to a better propulsion system, too.

• Yarkovsky effect requires a decently large body to influence properly, as small bodies get evenly heated pretty quickly by heat conductivity over small distances. It's said it works on bodies over 10 cm (up to 10 km) radius, and a mass driver of this caliber would be more useful as a weapon than as an engine. But Poynting-Robertson effect is right on the spot. Commented Nov 1, 2023 at 8:11
• So there could be responsible operators, who tend to use high-Isp mass drivers, mostly in the outer system, and use AI, human (or both AI and human) navigation safety officers to check before thrusting on departure from Earth. And then there are irresponsible operators, who use low-Isp mass drivers, betting that nothing will go wrong and that nothing can be proven if it does.
– o.m.
Commented Nov 5, 2023 at 8:09
• @o.m. "it depends". Problem is, stuff in space is very observable, and most things you would want to do take quite a long time. It might well be possible to reconstruct where the debris that just hit you came from, depending on who is watching what, and how traffic control is handled. Also, even being in possession of a high-thrust, low-velocity mass driver seems like something that would get you side-eyed in busy space. Commented Nov 5, 2023 at 18:11

I will have a quick go at a reply, and maybe others can improve on this.

This is not something you would do in LEO, though people have considered using dead satellites as reaction mass. If they do, I hope they atomise the debris, but I imagine they know their job and are well aware of Kessler Syndrome. Any extra debris is bad, but several tons of space dust fall to Earth each day and we live with that.

Escape velocity from the Moon is 2.4 Km/sec. If you are out beyond the moon, and your exhaust velocities are greater than this, anything you emit will probably keep going into solar orbit. That is large, and the chance of getting anything back from there is pretty small. If there are other craft nearby, then a polite spaceman always thinks of others when plotting their courses, and keep their jets pointed safely away from others. A less neighbourly sort might use their propulsion system as a weapon.

If you are out at lunar distances and your jet has a spread of a degree, then this would be 6500 Km wide at Earth, which is about the Earth's radius. I imagine you might avoid any burn in this direction in case a larger object got caught up in the jet. However, other than at lift-off, we rarely fly directly away from Earth.

Oxygen will be produced in vastly greater quantities than you have any other use for, being a byproduct of refining most metals and other elements, and liquid or solid oxygen makes a plausible propellant for mass drivers.

Oxygen is of course only stable in liquid or solid form under pressure or at extremely low temperatures, and will not survive exposure to sunlight for very long. In the cislunar environment, it will likely evaporate in seconds...at "double or single digit kilometers per second", perhaps within double or triple digit kilometers, depending on how it's dispersed. The resulting cloud of gas will rapidly expand to harmless densities.

• I like this idea, but it does seem odd to use solid or liquid oxygen in a mass driver, as opposed to using something like an arcjet or resistojet or some other kind of gas or plasma-producing electrical rocket or even a solar thermal rocket if you really felt like it. Mass drivers seem like they'd be better suited to less volatile stuff, like regolith. Commented Oct 31, 2023 at 16:29
• @StarfishPrime mass drivers might have a major advantage for using solid reaction mass, but squirting a bit of liquid from a tank into the bucket as it starts its trip down the driver has some advantages over manufacturing, storing, and transferring solid projectiles from some vast storage hold. And if it reaches even high single-digit km/s, the mass driver will have higher specific impulse, with no ionization losses. Commented Oct 31, 2023 at 20:02
• Modern-day tech ought to be able to deliver 500 seconds Isp from an oxygen-fuelled arcjet, though thrust would be low. But then, sustained thrust from a high-velocity mass driver seems likely to be awkward, and by the time that problem was solved I'd expect arc-jettery to improve, too. Honestly though, in the inner solar system a mass driver seems like an odd choice... given the large amount of sunlight and limited available mass, beam and plasma propulsion seem more sensible options. Commented Oct 31, 2023 at 21:09
• 500 s is only 4.9 km/s, and there's issues with just scaling up the exhaust velocity of thermal approaches like arcjets, like rapidly increasing thermal and cooling losses, unwanted ionization, etc. For increasing specific impulse further, you're going to want a completely different underlying technology, like a plasma rocket. Those can in principle use oxygen, but it's not ideal, and this is a setting that apparently can get similar performance from mass drivers. Commented Oct 31, 2023 at 23:52
• 500s is an example of something you could make right now, compared to a practical mass driver delivering regular 5km/s+ payloads, which is a rather more difficult ask. Oxygen was simply given as an example, because you used the same material. But as I said, beam and plasma propulsion seem like more sensible options. Commented Nov 1, 2023 at 14:37