So recently I was doing research on Magnetic Coil based weaponry in the Halo universe, and while reading about how the "Super Mac" worked, I noticed that the theoretical speeds at which the Tungsten Steel slugs weighing around 3,000 ton were being fired, which is about 12,000 km/s or 4% the speed of light, could be repurposed to fire spacecraft at faster speeds than many conventional forms of travel in space. Let me also clarify, I understand that within the Halo universe there are Translight space drives, but I mean this to be theorized in the present to near future, not millennia away. I would like to know the communities thoughts on this form of space travel. (Sorry if my question is confusing, I'm not a professional writer :D)


So to clarify exactly what a "Super Mac" is, it is a Magnetic Acceleration Cannon. The science behind how it works is a little more complicated than I can admit is understand, but the best way that I can describe it in my word is that it is a long metal cylinder wrapped in a coil made of magnetized metal that when given a current creates a magnetic field that when an object of the opposite magnetism is placed in the field, it suspends the object in an almost Zero Gravity environment which creates a frictionless environment opposite to how a gun works by firing a bullet through a cylinder that has friction.

  • $\begingroup$ Welcome to Worldbuilding! It might be helpful if you included a short explanation of how the magnetic coil based weaponry works as well as what a super Mac is, as well as links explaining the same in more detail. Thanks! $\endgroup$ – Mithrandir24601 Sep 6 '17 at 16:54
  • $\begingroup$ You're correct, sorry, I was writing this from the perspective of everyone reading this would know the Halo universe and lore as extensivly as me $\endgroup$ – BattleDoge Sep 6 '17 at 16:56
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What is being described in the OP and various answers is a variation of a mass driver. Since many of the salient points have been mentioned, here are a few which should be taken into account:

  1. The mass driver is difficult to build, particularly the high speed electrical switching needed to energize and reenergize coils in fractions of a second. (This is why the US Navy is experimenting with railguns, which are conceptually and physically simpler)
  2. In order to have a reasonable efficiency, superconducting coils may be needed. This makes the design more complex and provides more possible points of failure
  3. If the mass driver is not fixed on a planet or asteroid, it will recoil when the spacecraft is fired. This needs to be taken into account and some form of mitigation will be needed (rocket thrusters etc.)
  4. The corresponding "mass catcher" at the receiving end needs to be similarly scaled to the mass driver (essentially a mirror image). If humans are being launched, both the mass driver and mass catcher will need to be hundreds of kilometres long in order to make the launch at under 3 G for the passengers.

enter image description here

Artist's concept of a mass driver in orbit

There is no reason this should not work, assuming that the resources are available to build the infrastructure and there is enough traffic to amortize the cost of building the units. Even building the first leg will provide a large advantage to the users, you don't need any fuel to launch, so the ships's mass is reduced which means the amount of fuel needed to decelerate and then fly the return leg is correspondingly reduced. Turned the other way around, the mass driver can serve as the rocket motor for a spaceship, which means that almost anything can be the reaction mass.

enter image description here

Mass driver as the engine for a mining ship retrieving an asteroid

This idea is also scalable. Marshal Savage's book "The Millennial Project" had a portion near the end where he postulated some far future civilization would use mass drivers the length of the Solar System to fire manned pods to distant stars, where they could be received by corresponding mass catchers. The size of such a mass driver is staggering: 3000 AU in length, and taking a month of acceleration at 10"g" to bring the pod to .99 c. The mass catcher at the other end will be equally large, and need an equal amount of time to decelerate the pod (but can recover a large fraction of the kinetic energy while doing so).

So using mass drivers to launch or power spacecraft is quite possible, you need to be aware of the large sizes and energy requirements for launching humans out of them, however.

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What you're talking about is shooting people out of a gun. The problem with this is that guns get their projectiles moving at very high speeds very quickly. Far too quickly for people to handle.

To put this is perspective humans can survive brief exposure to 100 g of acceleration during a crash. A 1km long space cannon with a muzzle velocity of 6km/s would provide 1,800 g of acceleration to it's projectile. The spacegun you are describing has 2,000 times that muzzle velocity.

You will run into similar problems on the end of the journey. When the 3,000 ton slug collides with it's target both it and the target are going to be destroyed. Unless there is a system to slowly decelerate the projectile any occupants are going to be dead on arrival.

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  • $\begingroup$ Okay, so let's say theoretically we had the technology to displace the pressure felt from the 1,800 g's of acceleration, would it be possible then for a craft say the size of the space shuttle, which weighs 82.5 tons, to survive being shot from this "space gun"? Also, this craft that is being fired from this platform would come with de-acceleration thrusters to help slow the craft down before arrival at what targeted world or solar system, and not decelerating in the system, like decelerating millions if not trillions of miles from said target. $\endgroup$ – BattleDoge Sep 6 '17 at 16:48
  • $\begingroup$ @sphennings Your point is further supported by the fact that the cannon will have not only higher acceleration than a crash, but also that the acceleration along a long "gun" could also last longer. If it lasted longer, the effects are worse. If it did not last longer, then you are essentially smashing everything in the bullet as if hit with a human-sized fly-swatter. At some point (not sure what that point is), we won't just die, we'll be splatter. $\endgroup$ – Loduwijk Sep 6 '17 at 16:49
  • $\begingroup$ Another valid point is that this idea is not new at all. Didn't Jules Verne write about this long ago as a method to get to the moon? This is essentially the same thing, just a bigger, stronger cannon and longer distances, but the answer is still the same as it was then. $\endgroup$ – Loduwijk Sep 6 '17 at 17:41

@sphennings is right. One gun is too much. But I want to salvage your idea BattleDoge! It can work, but you just need to ease into it.

  1. Have your coil. A little coil. Shoot your ship out of it. Gently, so your passengers will be ok.

  2. A little ways away, in space there is another coil. You have aimed carefully. As the ship traverses the second coil (threads the needle), it is again gently accelerated.

  3. As the number 3 would suggest, there is a third coil which again gently accelerates your passengers.

You repeat with additional accelerator coils in sequence until you get your ship to the desired acceleration.

  1. It is time to start slowing down. Near your destination there are again coils. Make sure you go through the middle part or they won't work! Each one gently decelerates you in the mirror image of your initial process. Where did these coils halfway across the universe come from, you may ask? Robots with different methods for deceleration were sent ahead, and built them for you.
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    $\begingroup$ If you just accelerated people slowly over a longer distance. Why not do it at constant acceleration to reduce the effects of jerk upon the passengers. Do that and you've described a train. $\endgroup$ – sphennings Sep 6 '17 at 16:52
  • $\begingroup$ So instead of having the coils built with 500-600 meters of each other, it would be individual coils in small stations spread 500-600 miles apart per say? $\endgroup$ – BattleDoge Sep 6 '17 at 16:53
  • $\begingroup$ @sphennings is right again - one could have a long low acceleration coil and they leave it at the desired speed. And you should wait a few days before bestowing that green check! Spaced out coils in space and threading the needle is cooler and offer more storyline potential. This spaced coils system was stolen from Heinlein (Tunnel in the Sky?) and a surface train, where spaced coils are necessary to add back momentum lost to air resistance. $\endgroup$ – Willk Sep 6 '17 at 18:02
  • $\begingroup$ When designing your (de-)accelerator stations, keep Newton's 3rd law of motion in mind. Whenever they accelerate a ship, they will also be accelerated into the opposite direction with the same amount of force. So you better put some thrusters on them to keep their positions. $\endgroup$ – Philipp Sep 6 '17 at 20:02

It's quite reasonable. You need to make it longer and weaker. See Heinlein's "The Moon is a Harsh Mistress" for some of the details.

Catching at the other end is a bit trickier. You will need some kind of larger structures to funnel them in. I can't see hitting a space capsule sized target from interplanetary distances. Midcourse adjustment thrusters?

The mass of your launch/catch systems has to be very large compared to the mass of the capsule. An airless moon works well. A hefty asteroid. E.g. Luna to Phobos

As a transport system, you need ultra reliability. If the catcher isn't operating when a capsule arrives, you end up with a capsule hitting the base of the mass launcher at umpteen km/s. Serious fender bending will result.

At 2 G acceleration (20m/sec/sec) it takes 50 seconds for each kps. In system speeds are under 100 kps, would be 5000 seconds. d = at^2/2 10 * 5000^2 = 50 million meters. or 50 thousand kilometers. Ok. It will a bigger project than the Eiffel tower. But 10 kps would be 500 seconds, which is only 500 km long.

You don't get much by increasing acceleration. It takes 4 times the acceleration to half the length of the launcher. I'll take an hour and a half at 2 G over 20 minutes at 8 G's if I have a choice.

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