# Viability of ballistically propelled “traveling salesman” space stations

I'm interested in the viability of the following scheme for (relatively) fast space commerce within the Solar System:

• have a fleet of space stations essentially built around a long linear accelerator.
• Stations don't have heavy-duty engines, but do have attitude engines able to (slightly) modify orbit.
• have ground stations on airless moons with similar (or longer) linear accelerators.
• send goods to stations by means of ground-based accelerators; station based accelerator is used to brake cargo.
• send goods to ground bases using station linear accelerator to kill speed of cargo (essentially eject it backward).
• both events, if done properly, will increase space station speed.
• use gravitational slingshot to modify space station trajectory towards next destination.
• use attitude controls to carefully trim transfer orbit to use next slingshot "right".

This way "reaction mass" is the cargo and space station will increase speed up to a maximum given by maximum speed attainable by ground station accelerators:

• in a first time uploading would give more speed to station because cargo "bullets" would be faster than S.S. and thus it has to slow them down gaining speed.
• at later time S.S. would be actually faster than cargo bullets, so it will slow down accelerating them to match its speed.
• at a certain speed the two effects will be equivalent and S.S. won't gain more speed.
• max speed depends on $\Delta v$ available in the various linear accelerating "cannons".
• gravity well deflections must be carefully computed to give the S.S. the right orbital change to send it on the right path to meet next destination.
• Small bodies (w.g.: asteroids) that don't have enough mass to significantly deflect orbit should rely on "passing by" S.S. directed to other sizable bodies (up/down-load process might be from a (relatively) long distance.

How much is such a system viable with not-too-far future technology (at least permanent space bases should be available).

• When I looked up "linear accelerators" it says that they are used to make X-rays or electrons for radiation therapy. It would be tricky to use one to slow down an incoming load of colorful pants. – Willk Sep 13 '17 at 23:18
• @Will, The concept he's talking about has been used in a variety of forms in science fiction. To keep it simple, think "rail gun," which is a "linear accelerator" vs. a particle collider which is a "loop accelerator." No, they're not scientific terms. – JBH Sep 14 '17 at 0:11
• @Will: what I was looking at is something like this; that is using, most likely EDS Maglev; I will update the question to clarify (it will take some time). – ZioByte Sep 14 '17 at 8:45
• This is the concept of the interplanetary transport network originally proposed back in the 1970s or 1980s (I forget which it's been a while since I saw the papers on the concept). The Aldrin Cycler is a modified version of the same concept. – a4android Sep 14 '17 at 9:26
• Re reading the OP again, it suddenly occurred to me that the addition of a spaceship between the mass driver and mass receiver complicates matters and adds nothing. Trade can be done by shipping dumb pods straight from one mass driver to a mass catcher at the target, and the process can be reversed, the mass catcher become the mass driver for return cargoes. – Thucydides Sep 15 '17 at 16:50

You are actually proposing a modified version of the "Aldrin Cycler", which is essentially a space station moving in a continuous orbit between the Earth and Mars. The primary advantage is the cycler can be massive enough to carry heavy radiation shielding, life support systems (like hydroponic farms) and other heavy equipment which would require a lot of energy to accelerate or decelerate.

Getting to and from the cycler is relatively easy, a small, light spaceship accelerated hard from Earth our Mars to catch up to the cycler, while other small, light spacecraft decelerate to enter Earth or Mars orbit. Since the ships don't have to carry lots of radiation shielding, life support or even supplies for more than a few days, they can be much smaller and lighter than a sip designed to go all the way between Earth and Mars.

Once you read the description, you realize the difference between the Cycler and the OP's proposal. Since the OP wants to catch and deliver cargo pods by accelerating or decelerating them agains the ship, the Cycler's orbit will be changed, while the original Aldrin proposal uses self propelled rockets to make the trip, hence little or no transfer of momentum to the cycler.

Orbital path of a cycler

So as a first pass, a simple cycler using shuttles to take on or deliver cargo at Earth and Mars would work.

The OP's proposal could work, however, given some very stringent conditions. By catching items shot from the Earth or Mars, the spaceship is essentially operating a "Hybrid mass driver", so the ship can gain or lose energy by catching and delivering mass from fixed points on the Earth and Mars. The cycler needs to follow a very carefully plotted orbit in order to maintain its path between the planets, so the amount of energy (mass X velocity^2) either being received by the cycler or delivered in the form of packages needs to be strictly limited to amounts which do not adversely change the orbital parameters. As well, the cycler must balance the receipt and expenditure of energy (taking losses into account) in order to prevent long term disturbances in its orbital path. In other words, if a Martian cancels their order from Amazon.earth, they will still get their package regardless. Failure to balance the inflow and outflow of mass and energy will rapidly throw the cycler off of its orbit.

• I think what I'm proposing is substantial modification of Aldrin Cycler, as it is essentially a fixed elliptical orbit between two planets, with a "Traveler Station" more massive than cargo; What I'm proposing is a "space billiard" kind of orbit with much higher speed (potentially much higher than escape speed from solar system) of a S.S. whose weight is comparable with cargo. Orbit computation would be quite tricky, but IMHO doable. Same for the docking system. I would like some cross-check on those aspects. – ZioByte Sep 14 '17 at 15:06
• The biggest issue is it requires terawatt levels of energy to move through the solar system that fast. The torch ship (which is what you have described) will need equally vast inputs of energy to decelerate into orbit or change into new orbits to new planets. Launching packages by railgun at these energy levels will be incredibly dangerous to the receiving parties, and probably not change the ship's orbit in any significant manner. Ships receiving a package shot at that speed would be like catching a modern artillery shell, but harder.... – Thucydides Sep 15 '17 at 2:44
• I think I failed to convey the idea through. Speed would be built slowly over time each time a cargo is exchanged. Ship would never get into planetary orbit, just use gravity well to be deflected toward next destination. I'm also not sure about the "torch" thing. I want to use a maglev rail to accelerate cargo both on (airless) planet/moon and on Station. Relative speed between cargo and Station would always be relatively small (the maximum that can be handled safely. Cargo is the reaction mass (beside trimming attitude engines. I'll try to add a drawing in the weekend. – ZioByte Sep 15 '17 at 6:08
• When you are talking about "much higher than escape velocity" in interplanetary flight, the escape velocity you are referring to is Solar Escape Velocity, which is over 72Km/sec. If you define your parameters more exactly then we have some basis to do calculations. And if you are sending cargos at relatively low speeds, you will not be getting very much energy input or output; in other words you have a ship with a very low ISP engine that cannot make large orbital manoeuvres to get quickly to the next destination. – Thucydides Sep 15 '17 at 16:48

Zio, I love the idea, but I expect it will need some modification to make the science work (the story, though... I love the idea for a story!)

Ignoring the details and oversimplifying for a moment, if you want a space station of mass M to move at velocity V then you need to ship cargo of mass M at velocity 2V to make up for the fact that upon catching the cargo you just doubled the mass. As the mass of cargo decreases, the shipment velocity must increase to accomodate the difference. Basically, if you ship cargo of mass 0.05M (5% of the station mass) then you must send it with a velocity of 21V (remember, I'm really simplifying the math. Our contributing physicists are probably choking on their after-dinner mints right now. But this is the basics of non-elastic collisions, as I recall from my decades-old college memories.). My point is, if you want to move a space station from Earth to Mars in a reasonalble time (like, a month), the force required to ship cargo to produce that acceleration (ignoring how you'd catch it) would leave a flaming streak through the atmosphere and the object caught would be a stream of carbon dust.

Making the matter worse, there is always a loss of energy when transforming one kind of energy into another. Let's assume you're using rail-gun style technology to accelerate a canister, the contents of which are your cargo. Electricity is used to generate magnetism (lost energy) which generates kinetic force (lost energy) which must overcome atmospheric conditions (lost energy) until it gets into space. Your space station cannot simply "catch" the bullet as the force involved would tear the station apart ... so we're creating an inverse magnetic field to convert the kinetic energy back into electricity (loss + loss). It's that new electricity that's being used to provide propulsion for the station (unless we use unobtanium to catch the bullet... but the contents are powder after the catch...).

Granted, the space station can use orbital and even atmospheric braking to actually enter orbit around a planet, reducing the need to use the accelerators for braking maneuvers, but I don't think that makes the solution viable enough.

So, using cargo shipments to contribute to the energy/fuel efficiency of the space stations is a great idea — but by themselves, the shipments either (a) cannot efficiently produce the energy necessary to move the station or (b) are doing so with so much energy loss that it would be economically more efficient to put an engine on the station.

In the end, while I adore the idea of travelling salesmen in ambulatory space stations, I'm afraid I must conclude that the propulsion solution is not viable. However, I'd keep the idea for moving cargo... you just need to add engines to the space stations. And you could use linear accelerators planetside and loop accelerators (like partical colliders) on the stations to tranship the cargo.

Dang! I love the idea!