I've perused some of the articles and questions on magnetic acceleration using rail guns and they all seem to say that it's possible just not practical.
I wanted to know what are the drawbacks to building an orbiting rail gun and using it to accelerate a ship or satellite to a reasonable distance to the target before using more conventional propulsion? That negates the drawbacks of atmosphere, does it not? I've added a rudimentary image to help illustrate what I was proposing. The station would be built in LEO and the payload would be protected by a magnetically shielded fairing which would be ejected after launch. After gaining the acceleration the payload could make slight course corrections or boosts as needed until it reaches it's target where it could then do an orbital insertion or brake for landing. 
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8$\begingroup$ If the rail gun is in Orbit already how does it negate the effects of atmosphere? $\endgroup$– Organic MarbleCommented Nov 8, 2018 at 4:17
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$\begingroup$ in a similar manner in which the building of the ISS was accomplished, a giant rail gun could be built in orbit and 'loaded' with a specific payload to be fired on the correct trajectory to intercept the target body. Once close enough traditional propulsion could be used to insert it into orbit or slow it down for landing. reducing fuel usage and load. $\endgroup$– Fan BoyCommented Nov 8, 2018 at 4:22
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1$\begingroup$ The rail gun is not meant for launching things into earth orbit but rather launching then into orbit around other planets. $\endgroup$– Fan BoyCommented Nov 8, 2018 at 4:32
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2$\begingroup$ @OrganicMarble I'm guessing the OP read about rail guns on Earth's surface, and is now prosing to "build an orbiting rail gun". In this case it is the staging in space rather than on Earth that negates the effect of the atmosphere. $\endgroup$– uhohCommented Nov 8, 2018 at 4:41
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1$\begingroup$ Ah, so the atmosphere does not impede the launching of projectiles from the gun. Thanks! $\endgroup$– Organic MarbleCommented Nov 8, 2018 at 4:46
4 Answers
Concepts for mass-drivers or railguns situated on the moon exist.
What you're asking about is a mass driver space station in low earth orbit that a payload or ship launched from earth can dock with, and then be propelled into deep space.
The delta-V needed to go from LEO to an escape trajectory is about 3 km/s, and to go from there to a Hohmann transfer to the outer reaches of the solar system requires a further 5 or so km/s. I'm sure a mass driver could be made to provide such delta-Vs, though it would have to be rather long in order to not smush the payload with its massive acceleration.
One big problem I see is Newton's third law. Due to recoil, the orbital mass driver will have the same momentum backwards as the craft will have going forwards. This means its orbit will be altered. If the mass driver is much heavier than the mass it accelerates, this won't result in a delta-V change too great, but it's conceivable even a small change could put it into an orbit that intersects with earth's atmosphere. Now you could counteract this by sending up a refueling rocket so the mass driver station can burn to negate the delta-V change, but I feel like if you're going to do that, you might as well use that fuel to accelerate the craft directly, without the mass driver. The other alternative is that you launch a different probe in the opposite direction with the same momentum as before, but the coincidence of having two probe missions with exactly the opposite delta-V requirements is... unlikely to say the least.
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1$\begingroup$ One of the only theoretical advantage would be if you intended to shoot lots of very lightweight small satellites without onboard propulsion and very high speed such as Breakthrough Starshot $\endgroup$– AntziCommented Nov 8, 2018 at 6:14
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$\begingroup$ I've just asked Ideal shape for a long, skinny reaction mass for LEO to cis-lunar and beyond? (a “space rail gun”) $\endgroup$– uhohCommented Nov 8, 2018 at 6:32
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$\begingroup$ If you fire the probe from a high enough orbit the resulting braking maneuver would just make the orbit lower. which could all be worked out pre-launch. and also the the station could pick up some momentum it self by launching during it's descending orbit. I was thinking that the Station would be larger than anything it launches therefore cancelling out the reciprocating force. $\endgroup$– Fan BoyCommented Nov 8, 2018 at 6:34
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2$\begingroup$ @FanBoy in space there is no damping, so even forces and torques that you might think of as small will have consequences. They may take hours or days to show up as an orbit change or loss of proper attitude, but they have to be mitigated or things will go bad. Every action has an equal and opposite reaction, etc. $\endgroup$– uhohCommented Nov 8, 2018 at 6:48
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$\begingroup$ Understood, a small course change here translates to miles down the trajectory. The speed of the craft and the length of time can be factored in along with possible gravitational influences and calculated maneuvering burns can be applied without (hopefully) being detrimental to the mission. $\endgroup$– Fan BoyCommented Nov 8, 2018 at 6:54
Let's look at some numbers. For the sake of argument let's target a delta-V from the railgun of 4 km/s enough to get to low lunar orbit or Mars transfer orbit.
If the railgun is 1km long, that would mean an acceleration of about 800g for half a second (using $v^2 = 2as$ and $s = 1/2 a t^2$), so, more than a little uncomfortable for humans, and imposing some restrictions on the design of unmanned payloads. Per kilogram of payload, this also means an average power of about 16MW during the launch, so we're going to need some chunky capacitor banks.
This delta-V would have to be prograde (forwards along the orbit of the launcher). Launching a payload that is initially stationary with respect to the launcher in any other direction would require a much higher delta-V to reach the same targets. A satellite in LEO will reenter within an hour if it is slowed in its orbit by as much as 90 m/s (that's for the ISS orbit, your mileage may vary). So to avoid this happening we either need to be able to reboost the launcher within a few minutes of a launch, or we need the launcher to mass at least 50 times the payload mass.
One possible solution is to launch payloads that are not initially stationary with respect to the launcher. For example a payload in the same orbit, but moving in the opposite direction could be launched to the moon or Mars transfer with a delta-V of 4 km/s retrograde to the launcher, thereby reboosting the launcher. However that requires the payload entering the launcher accurately at 14 km/s relative velocity and, because it is in the launcher much less long the acceleration and power needed are higher -- about 12800g for about 60 ms, and a power per kilogram of 2GW. These seem like fairly insuperable problems.
So realistically, this is only useful
- for very robust payloads -- not humans or delicate scientific instruments
- for very small payloads -- a few kg at most
- when you have a cheap way to reboost the launcher frequently.
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$\begingroup$ Not versed enough in astrophysics to decipher your comment in a reasonable amount of time, but I understand the point that you are making. what if there was a group of floating rings spaced and aligned at the necessary distance and the payload was accelerated along this line at the desired target? Picking up speed as it cleared each ring?This would eliminate the need for a dedicated station and the rings would be equipped with OM thrusters to get them into the proper alignment. $\endgroup$– Fan BoyCommented Nov 8, 2018 at 11:53
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2$\begingroup$ So making the launcher longer reduces the acceleration and peak power. Making launcher a bunch of independently flying sections might make it easier to build and launch, and help a bit with things like lunar tidal forces, but it makes it harder to operate and doesn't really make much difference to the basic physics. If you leave long gaps between the sections to reduce mass, then your acceleration just comes as a series of violent jolts as you pass through each section, giving you back the problems with non-robust payloads. $\endgroup$ Commented Nov 8, 2018 at 12:23
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$\begingroup$ Your comment about opposing directions but same orbit got me to thinking. An orbiting ring that boosts the vehicle as it passes through and is also boosted into a higher opposing orbit respectively, the orbits keep intersecting going into higher and higher orbits until the vehicle fires it's own propulsion taking it off to it's desired target. The ring could be the same mass as the vehicle or the charge can be adjusted to the right resonance for the desired effect. Plausible? $\endgroup$– Fan BoyCommented Nov 8, 2018 at 13:17
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1$\begingroup$ @FanBoy this is more or less exactly the proposal discussed in space.stackexchange.com/questions/25751/… $\endgroup$ Commented Nov 8, 2018 at 14:25
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$\begingroup$ You don't need a reboost, you need to shoot a cargo off in the other direction soon after to balance it out. $\endgroup$– GdDCommented Nov 8, 2018 at 15:59
For the purpose of orbiting other planets; this solution adds lots of complexity, without (almost) any benefit.
- You still need propellant, both on the craft and the railgun
- The probe still need a thruster to enter the other planet orbit
- The probe needs to be designed according to the railgun spec
- You need to build, maintain, that railgun
- You need to rendez vous with the railgun (it takes LOTS of paperwork to allow a craft nearby the ISS)
- You add a single point of failure
- ...
Since the added benefit is only to reduce the propellant needed on the probe, this sounds like a VERY high price to pay.
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$\begingroup$ An added benefit would be faster time to target seeing as the probe would only have to carry fuel for rendezvous with the station and subsequent braking and insertion having picked up acceleration without having to use stored fuel, thus creating a lighter craft for earth based launch. $\endgroup$– Fan BoyCommented Nov 8, 2018 at 7:38
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$\begingroup$ Which is easily solved by a bigger rocket. The price difference between falcon 9 and falcon 9 heavy is quite small after all... $\endgroup$– AntziCommented Nov 8, 2018 at 7:43
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$\begingroup$ 1.Given. The number of launches before higher orbit re-insertion would make up for that. 2:Already stated in the proposal. 3:Probes are always built with the launch vehicle in mind. 4: See picture. 5:There will always be paperwork. 6:Murphy's Law should be factored into any training/planning. You can.t plan for everything but that doesn't mean you shouldn't try. $\endgroup$– Fan BoyCommented Nov 8, 2018 at 8:02
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1$\begingroup$ Look, I'm not saying it's not possible, I'm just saying that it is incommensurably more complex than just adding more fuel on the speacraft or launching with a bigger rocket $\endgroup$– AntziCommented Nov 8, 2018 at 8:24
The reason this is not useful is because if you want to go somewhere in space and you have plenty of money to build stuff, it's better have an ion engine and gently accelerate to where you want to go whilst using very little fuel. (Or maybe something more exciting like nuclear propulsion). The railgun with its rapid acceleration and large initial cost (which would also need an ion thruster to reboost itself!) is not helping much, but adds large fixed costs.
