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Imagine in the far future that there are space battles. In this question user: m t said that in the future it would be too expensive to do stuff in space so people would focus on "offense, intelligence, and first-strike capabilities, leaving very little for defense." But say it we wanted to make a defensive system. So I had this idea:

Imagine a sliding door and somebody shooting NERF bullets at it. You could move the door back and forth blocking the bullets. So what if we did that in space, except the sides of the door would have magnets or rockets to propel it side to side, like this:

Rocket style:

                ___________________________________________             
               [                                           ]           
     (Rocket)>=[                                           ]=<(Rocket)
               [___________________________________________]                

Magnet style*:

                   ___________________________________________             
                  [                                           ]
(-)    (Magnet+)>=[                                           ]=<(Magnet-)   (+) 
                  [___________________________________________]                

*I would think that you could turn the +,- sides on and off so you could repel either side to make it move.

Would this work? I know that it would be hard to replace them if the enemy launched a ton of missiles, but is there a way to replace them fast and quick as well as being affordable?

So here:

  1. Can we do it?
  2. Is it good enough to stop missiles?
  3. Is it affordable?
  4. And would the propulsion system side to side work? (If not, are there better ways)
  5. What materials should it be?
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  • $\begingroup$ is this armor for a world, ship, person, or something else (what scale are you looking at)? $\endgroup$ – depperm Sep 4 '15 at 20:23
  • $\begingroup$ @depperm For ships in space. Pew Pew! $\endgroup$ – Kit Sep 4 '15 at 20:28
  • $\begingroup$ I was thinking "amor" was some dashing space pirate lover. $\endgroup$ – Serban Tanasa Sep 4 '15 at 20:31
  • $\begingroup$ Having a movable shield for a ship would seem to be a huge/expensive/illogical way of protection. Having a thicker hull or anti-missile weapons would seem more viable and effective. $\endgroup$ – depperm Sep 4 '15 at 20:35
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    $\begingroup$ Have you run the physics? If you know the distance between detection of a projectile and collision, the mass of the armor, and a few other things like that, you can calculate useful things like how fast the armor must move, and how much energy it takes to move it. Those numbers might create some insight. However, it is worth pointing out that the US's ballistic missile defense system is basically exactly the system you describe, except instead of using heavy armor, we just put something small and agile in the way. $\endgroup$ – Cort Ammon - Reinstate Monica Sep 4 '15 at 20:40
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The first problem with things in space is that everything moves fast - REALLY fast. For example, when you're near a planet, it's not that gravity is just too weak to pull you in - it's that you're going really fast and are counteracting gravity. To put a number to it, the international space station is traveling at 17,000 MPH.

With today's bi-propellant technology, if your shield were in the ISS's orbit and 90% of it was fuel and a missile was coming in behind you, the best you could do is go roughly 6,000 MPH in the other direction. However, you wouldn't actually go 6,000 MPH in the other direction: you would instead start falling back to Earth irritatingly quickly.

Changing orbits with today's technology is very difficult.

Furthermore, missiles flying in space are limited in what they can do because they have to carry all their fuel with them and accelerate it as well. This is why ballistic missiles are just ballistic: it's hard to affect changes in your trajectory when you would also have to carry fuel... so they just don't carry fuel and fly. We then lob little things at them to get in their way (EKV: Exoatmospheric Kill Vehicle, which is literally a thing that gets in the way of incoming missiles and runs into them - sound familiar?). These are incredibly expensive and rely heavily on the other target being predictable (in this case, ballistic) so they can minimize fuel usage and cost.

But you asked about the future...

Why did I say everything above? Because, in the future, if you have interplanetary travel, you must have solved the engine inefficiency problem and made it trivial to produce engines that can just do whatever you want. However, your space ships still have structural issues, so they can only maneuver so quickly (unless, I suppose, the entire hull is lined with small engines so it can perfectly distribute forces). Presumably they may have also created some sort of an inertial damper to let them maneuver more quickly.

So let's say the enemy missiles and you both have the technology to make everything maneuver exactly how you want: you still lose because you're still playing defense and defense is always reactionary. If you get your shield right in the way of the missile (because your technology lets you do this effectively), then their missile would just change its course (because their technology also lets them do that). So you figure out how their algorithm works and make your shield predict theirs, so they change theirs. *(Note: this is exactly how defense works nowadays too)*.

Regardless of your level of technology between now and then, you CAN put a shield (whether it be something already in space or something like EKV) between you and them, but you always have a disadvantage that they will have the same maneuverability abilities and they control the game (again, defense is always reactionary). Plus your [big metallic] shield tends to get destroyed when it gets hit. Plus yours has to be a lot more expensive because it has to be more capable than theirs, so they can always exhaust your resources by just firing lots of them at you.

A shield like that really isn't any more effective than shooting their missile with your missile - except that your missile doesn't have to happen to be in the right spot [as much] as a shield does.

This, by the way, is the genesis of energy weapons.

Some more related examples

Using some numbers from wikipedia...

The US's "Minuteman" ballistic missiles cost $7 Million and [technically] only take a few people to operate them from a single silo.

The US's EKV program is hard to estimate the cost of, but [1] says they cost about $90+ Million each. It costs more than 10 times as much to defend against a single ICBM as it does to launch one. But this isn't the whole story, because to be able to hit a flying thing, you first have to detect the flying thing (with radars, which have limited range and precision) and transmit that information (which can be jammed) and launch against it, so you have the cost of all of those pieces too.

This cost differential has always been and probably always will be so.

[1] - http://mostlymissiledefense.com/2012/07/24/ballistic-missile-defense-how-much-does-a-gbi-interceptor-cost-july-24-2012/

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In other answers, I mentioned shilding based on superconductor flux pinning. That would work for intentional things being thrown at you as well.

That is, defense is similar to shielding on fast spacecraft, which is a technology that will be developed so it is available for repurposing. I can imagine a protagonist group in a story naturally trying to adapt shielding to use for defense against attack.

Think of the phenomenon as an improvement/refinement to the "magnet" idea. The plate is tasked to an intercept and moved by using the magnetic flux lines as a long arm; the plate is pinned to the far end.

The plate absorbs energy by breaking. And the pieces are still pinned and now individually controllable, down to a minimum size.

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