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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. Concept

Fan Boy
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    If the rail gun is in Orbit already how does it negate the effects of atmosphere? – Organic Marble Nov 08 '18 at 04:17
  • 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. – Fan Boy Nov 08 '18 at 04:22
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    The rail gun is not meant for launching things into earth orbit but rather launching then into orbit around other planets. – Fan Boy Nov 08 '18 at 04:32
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    @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. – uhoh Nov 08 '18 at 04:41
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    Ah, so the atmosphere does not impede the launching of projectiles from the gun. Thanks! – Organic Marble Nov 08 '18 at 04:46
  • Yes, sorry i got a little ahead of myself there. I just Found this site did not know there was a place where i could get my questions answered by a community of people who actually knows things that I only hypothesize about. I will do better in the future. – Fan Boy Nov 08 '18 at 05:02

4 Answers4

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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.

uhoh
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Ingolifs
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    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 – Antzi Nov 08 '18 at 06:14
  • I've just asked Ideal shape for a long, skinny reaction mass for LEO to cis-lunar and beyond? (a “space rail gun”) – uhoh Nov 08 '18 at 06:32
  • 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. – Fan Boy Nov 08 '18 at 06:34
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    @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. – uhoh Nov 08 '18 at 06:48
  • 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. – Fan Boy Nov 08 '18 at 06:54
  • @FanBoy The answer doesn't say it's impossible; it merely explain one of the main drawbacks (ie you don't save on propellant at all). – Antzi Nov 08 '18 at 07:18
  • @Antzi if the station is in a high enough orbit, lets say around hubble range, you could hypothetically launch several probes before you would have to burn back to the original orbit height. – Fan Boy Nov 08 '18 at 07:24
  • Indeed, this is the frustrating thing with orbital mechanics and any general idea of a 'refuelling base'. The propellant exhausted in having to make the extra stop (creating the elliptical intercept orbit, then equalising velocity to circularise the orbit, refuelling, then creating the elliptical intercept orbit to the destination, then matching velocities again) is often a significant fraction of what you'd get back by refuelling. By placing the mass driver in a higher (presumably still circular) orbit, you're increasing the amount of fuel you need to get there. – Ingolifs Nov 08 '18 at 07:26
  • I wonder how many launches from a mass driver situated on the moon you could get before it breaks free from the regolith and/or begins to significantly affect the moons orbit. – Magic Octopus Urn Sep 29 '19 at 02:40
  • You can avoid Newton's third law by simply making your mass driver fire in both directions. Fire a dummy load of the same mass at the same speed but opposite velocity vector. You can make the 'dummy' side smaller because a dummy load can tolerate high G's. However, this also doubles the amount of mass you have to bring to the thing anyway. Another serious problem will be heat - a mass driver would generate a lot of heat very quickly, and it's really hard to dump excess heat in space. – Dan Hanson Sep 29 '19 at 23:19
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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.
Steve Linton
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  • 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. – Fan Boy Nov 08 '18 at 11:53
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    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. – Steve Linton Nov 08 '18 at 12:23
  • 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? – Fan Boy Nov 08 '18 at 13:17
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    @FanBoy this is more or less exactly the proposal discussed in https://space.stackexchange.com/questions/25751/is-solar-power-electromagnetic-propusion-possible-the-orbital-mass-accelerator/ – Steve Linton Nov 08 '18 at 14:25
  • You don't need a reboost, you need to shoot a cargo off in the other direction soon after to balance it out. – GdD Nov 08 '18 at 15:59
  • @GdD that's what the third paragraph is about. If you shoot off a cargo backwards (with respect to your own orbit) it doesn't get anywhere useful. That's only helpful if you can manage to accelerate cargo as it flies through your launcher at 14 km/s relative velocity. – Steve Linton Nov 08 '18 at 16:13
  • @SteveLinton it doesn't change anything in the reasoning, but isn't it 1sec ($v = 1/2at^2 , v = 1/28000t^2, 4000 = 1/28000t^2, 4000 = 4000t^2, t^2 = 1, t = 1$) and so also 8MW (kinetic energy / time = $1/2mv^2 / t = 1/21v^2 / t = 0.5(4000^2) / 1 = 8000000 $ = 8MW)? – BlueCoder Nov 08 '18 at 16:47
  • @BlueCoder no $v = at$ $s = 1/2 a t^2$ – Steve Linton Nov 08 '18 at 16:49
  • @SteveLinton Right! Then the answer just need one edit, as it now reads $v = 1/2 at^2$ instead of $s = 1/2at^2$ :) – BlueCoder Nov 08 '18 at 16:58
  • You could perhaps shoot a sacrificial mass out the other side on a trajectory to burn in the atmosphere to re-boost the gun back to its original orbit. – Dave Nov 08 '18 at 18:39
  • @Dave yes, that's true. if you have a 4 km/s railgun it's $I_{sp}$ is better than all but the most efficient chemical rockets, so you may as well use it as your reboost. That means doubling the launched mass of all payloads, of course, unless you are getting mass shipped down from the Moon or somewhere. – Steve Linton Nov 08 '18 at 20:32
  • @Steve Linton the link you provided was spot on, answered everything in such a way that I understood it. Thanks. – Fan Boy Nov 09 '18 at 02:16
  • @SteveLinton Interesting! So to understand how much the railgun is beneficial (beyond the technical difficulties to build it) we could conceptualize it as an "added stage" to the rocket coming from the earth, weighting 0 Kg (you don't have to carry it from Earth) and having a very high ISP? (that we haven't calculated yet) – BlueCoder Nov 09 '18 at 08:25
  • (With the caveat that the real stages of the rocket from Earth cannot provide more deltaV than the one needed to go to the railgun LEO orbit - i.e. the rocket has to "stop" at the station before starting the railgun "added stage") – BlueCoder Nov 09 '18 at 08:29
  • Railgun ISP: let's assume that you have to fire the same mass of the satellite to perfectly keep your orbit (I would guess this is not exactly true but let's take an ideal simplified situation). According to the rocket equation $\Delta V = v_{exh}ln(m_0/m_1)$ with the two masses being the initial mass (satellite+sacrificial mass) and the final mass (satellite only), so $4000 = v_{exh}ln(2/1), v_{exh} = 4000 / ln(2), v_{exh} = 5797, ISP = v_{exh}/g = 5797 / 9.81 = 590$. – BlueCoder Nov 09 '18 at 08:44
  • A chemical rocket can have a 450 ISP (e.g. Delta IV, Atlas V), so I guess the high complexity and cost of the railgun might not justify this "small" increase, compared to other methods to increase dV once in space.. (e.g. ion trusthers with ISP in the thousands)... although this method accelerates you a lot faster...even too much faster for some applications :) – BlueCoder Nov 09 '18 at 08:48
  • But you can double the ISP by doubling the velocity of the railgun, which needs 4 times the acceleration and also 4 times the power.. this could be interesting until the acceleration or the power is too much to handle.. – BlueCoder Nov 09 '18 at 08:53
  • @BlueCoder the rocket equation doesn't directly apply here, because all the thrust is happening in one go. The "exhaust velocity" is simply the velocity at which the reaction mass leaves the railgun, so the $I_{sp}$ is just over 400, not quite as good as LH2/LO2. On the other hand, if you can build an 8 $km/s$ railgun, you are up in NERVA territory. Note that this is the $I_{sp}$ of the railgun considered as a means of propelling itself, the concept doesn't make much sense for the payloads. – Steve Linton Nov 09 '18 at 10:42
  • @BlueCoder we should probably move this conversation to chat or try and formulate it as questions and answers. – Steve Linton Nov 09 '18 at 10:43
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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.

Antzi
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  • 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. – Fan Boy Nov 08 '18 at 07:38
  • To answer your points in order; – Fan Boy Nov 08 '18 at 07:40
  • Which is easily solved by a bigger rocket. The price difference between falcon 9 and falcon 9 heavy is quite small after all... – Antzi Nov 08 '18 at 07:43
  • 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. – Fan Boy Nov 08 '18 at 08:02
  • What do you mean it would make up for that ? Yes, you won't need to fly a refuel mission on every launch, but you will still need them. 3: You know have an additional launch vehicle to plan for.
    • – Antzi Nov 08 '18 at 08:22
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    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 – Antzi Nov 08 '18 at 08:24
  • One rocket brings three probes to the station for launch at different times/planets. By combining the probes into one launch you just cut down on work hours and fuel, logistics etc;. After building the station you can factor the need for re-insertion after so may missions or so much mass and fuel it to a specific target number. – Fan Boy Nov 08 '18 at 08:28
  • Not seeing where the additional launch vehicle comes into play. Launch from earth to station, station to target. – Fan Boy Nov 08 '18 at 08:30
  • @FanBoy I think he meant that the probe has to be designed for two launch "vehicles", the rocket from earth to station and the railgun from station to target. Two requirements to accomodate, being carried by a rocket and being fired by a railgun vs designing a normal probe that is directly launched to target by a single bigger rocket. – BlueCoder Nov 08 '18 at 09:59
  • I think might be over simplifying the requirements on my end and and for that I apologize. Like I said I'm totally new to this type of discussion board and the things I have worked out in my mind I'm not well versed enough to translate them so that other people can see it. I promise I will get better. Everyone's feedback is greatly appreciated. it has given me a lot to think about and started some new ideas for me to explore and pose to the group. – Fan Boy Nov 08 '18 at 10:13