Satguns - railgun on a satellite with evasive capabilities

Question

So, railgun cold war coming up: http://www.popsci.com/an-electromagnetic-arms-race-has-begun-china-is-making-railguns-too

And I imagine that the powers that be will be less shy about using these cheaper, non-mass-destruction weapons, as opposed to expensive ICBMs-that-go-big-boom.

So in my story, preparing for that probable first strike, that would be aimed at taking out the adversary's railguns, the US would like to protect its railguns by placing them on small satellites in orbit. The purpose of this is to enable complete evasion (dodging) of incoming railgun shots (incoming from Earth's surface). Also, I really really want a super cool Satgun that can shock the bad guys by "stepping aside" from their supposedly unstoppable attack.

However, the requirements for an effective railgun on a satellite seem to come in direct conflict with the evasive capabilities. I'm not going to ask for hard science here, because I realize this may be quite beyond our current tech level. So sci-fi it is! The story can be set at any point between 2020 and 2100.

My question is: How would the US build a Satellite that can both use its own railgun and evade railgun shots coming from the Earth's surface?

Clarifications:

Despite what the link/s above may suggest, I'm not set on China being the adversary in the story, and I'm only thinking about the US because they seem to have the most advanced tech in this field, and therefore the greatest chance of pulling a Satgun off.

The railguns (both on Satguns and on Earth) are your everyday easygoing 32MJ guns such as the ones that the US Navy currently tells us about: http://www.popularmechanics.com/military/research/news/a27455/us-navy-railgun-more-powerful/

The slugs are not guided, they just fly up/down real quick.

My Satguns need to be:

1. For the purpose of evasion:

   • As small as can be
   • Highly maneuverable (depends on whether we can detect an incoming slug very early on)
   • Capable of receiving warning on an incoming slug on time for a successful evasive maneuver (actual sensors don't have to be on board, but the data about a specific slug headed towards a specific satellite needs to reach the satellite on time)

2. For containing and using their railgun:

   • Big enough to contain the railgun/s and energy supply (The smallest thing that I can think of that'll give enough juice is a nuclear reactor)
   • Big enough for ammo storage - at least a hundred slugs (since you can't easily replenish depleted ammo in space)

   • Handle recoil (I've read some cool ideas about using recoil energy rather than absorbing it)

   • • • • • • • • • • • • • • • BOOM ! ! !

Answer 1

There are problems with orbital weaponry and especially an orbiting railgun.

1: Orbits are predictable. They follow Newtonian mechanics. If you want to hit something in orbit, you can calculate its position at any given time.

2: Orbits are palindromes. Once you calculate an orbit, you can put something (e.g. a satellite killer) into that same orbit going the opposite direction. The orbit depends on the mass and velocity of the satellite. If your satellite killer is heavier than its prey it can move faster than its prey and so share the same orbit going the opposite direction.

3: The shotgun approach works in space. Missile defense systems all have multiple impactors that fly out, like a cloud of pellets from a shotgun. Even an evasive target might not evade them all.

4: A "scorched earth" (scorched space?) anti satellite strategy would deploy loads of gravel into space, to scour given orbits of inhabitants. http://www.halfbakery.com/idea/Not-So-Brilliant_20Pebbles#1171766306

5: The tremendous recoil of such an energetically launched projectile will tend to push the orbiting satellite gun out of its trajectory. The satellite railgun must either be so massive that the recoil force does not move it much, or have engines able to compensate.

6. Railgun rails are harshly used by the projectiles and by current arc s- current railguns suffer a lot and the rails must be cleaned of debris between each firing.

The solutions: many small dispensable cheap one-shot railguns, launched in an enormous variety of orbits stable and unstable.

Each railgun would be ideally launched from an earthbound railgun, or perhaps from a rockeloonannon balloon cannon rocket hybrid. These one-shot guns would have graphite rails (for decreased weight) and be launched with a charged capacitor capable of powering one shot. It is expected that after this shot the railgun will be blasted backwards, probably out of its orbit.

For attitude control and to aim the small railguns would have onboard gyroscopes spun up before launch. These would not allow maneuvering but would allow rotation to aim the gun.

Orbits would be many and varied. Conventional orbits for satellites would be used, as well as highly elliptical orbits.

from https://earthobservatory.nasa.gov/Features/OrbitsCatalog/page2.php

These elliptical orbits have the advantage that the incoming small railguns are moving very fast during their approach - they will be hard to see coming.

One could consider small railguns in lunar orbits as well.

A group of railguns in a given orbital path could have a sentinel: the furthest forward or back watches along the path. If a massive satellite killer is coming along the same orbit, the sentinel will discharge its projectile at the satellite killer.

The idea is that there will be too many of these cheap one-shot guns to get them all, even with "pebbles" type weapons that take entire orbital paths out of commission.

Answer 2

For what you write it seems You need a "use once" satellites (railguns can use up 100 slugs in matter of seconds).

That is good because you cannot "absorb" recoil; momentum must be preserved in an "isolated system", so if you fire a slug in one direction satellite will move in the opposite direction. no way around that.

To avoid recoil you should have two railguns firing identical projectiles in opposite directions, probably impractical.

What you can do is to properly balance railgun so that firing will not cause satellite to spin wildly and accept fact that after usage it will be pushed helplessly out of orbit.

note: this is reason why satellite armament is usually recoilless energy weapons (e.g.: lasers).

To "dodge" incoming fire you have opposite problem: you need a high reaction mass to change suddenly your impulse; fortunately you already have a heavy subsystem "loosely coupled": the power reactor.

You can have power reactor connected with (relatively) long cables and "dodge" by "pushing away" (explosive bolts or compressed gas spring) reactor with as much force as possible without harming internal machinery. This will leave center of mass fixed, but "two halves" of satellite would "jump aside" in opposite direction. If needed you can haul pieces together again using winches on connecting cables and repeat at next attack. There's a chance that some cable might be struck and cut, but that would be a very good aim and can be countered "simply" by having redundant cabling.

Answer 3

I'm not sure if your question makes sense when you look at the details.

• ICBMs deliver fission or fusion warheads. There has been some thought about conventional warheads for Prompt Global Strike, but that is generally seen as an inefficient use of an expensive, one-shot delivery system.
• Railguns would fire kinetic energy rounds or chemical explosives. KE rounds rely on the mass and speed of the projectile, here the extra speed of a railgun really helps. Chemical rounds rely on the filler, here the extra speed of the railgun mostly helps to extend range.
• A kinetic orbital bombardment system also relies on kinetic energy, but much of that comes from the orbital energy of the sat. First a lot of energy is expended to get the sat into orbit, than a little bit to brake it onto a reentry path. That energy comes from the rocket motor during launch, then it is "stored" while the sat is in orbit.

So it might make a little bit of sense to use a railgun on a sat to quickly "de-orbit" a KE projectile, but the bang isn't from the railgun.

Regarding your questions, shooting an orbital target with a surface gun would run into all the problems of a "supergun" project.

• If all the energy is imparted on launch, the projectile cannot maneuver and even a very small course adjustment of the target will cause a miss.
• If the projectile can maneuver, the railgun is basically the first stage of a rocket system. The upper stage(s) must be able to survive the high acceleration of the launch.

So it is relatively easy to make the sat evasive. A small rocket engine, a little tank, there you are.

Answer 4

There is an inherent problem with your idea: railguns with any sort of range and striking power are freaking huge! To give you an idea, here is an illustration of a real proposal for an orbiting railgun called "Have Sting" which was one of the various ideas floated during the research phase of the Strategic Defense Initiative in the 1980's:

Have Sting with Space Shuttle orbiter to scale. Illustration by Scott Loather

The enormous length is to provide the proper armature for the railgun itself. The cone shaped item is a small nuclear reactor to provide "hotel" power while on station and power the large tracking arrays. Actual combat power was cleverly provided by having liquid hydrogen and oxygen aboard. Several rocket engines can be seen, which allow the platform to slew around and orient to the target, but the burning H2/LOX mixture powered a large turbine as well, which provided megawatts of electricity for short bursts when firing.

Obviously Have Sting would be highly visible in orbit and easily tracked from the ground, but the railgun projectiles would be devastating at thousands of kilometres range, so theoretically the Have Sting could provide for its own self defence and fire on rockets climbing into orbit with ASAT payloads.

Now your conception is for "small" railguns in orbit. While even screws, nuts and bolts and even paint flecks can be traced from the ground, a satellite with is the size of a "Cubesat" (10×10×11.35 cm cubic units, has a mass of no more than 1.33 kilograms per unit) will be much harder to track than "Have Sting" and can be built in large numbers.

1U Cubesat unit

Now the armature will be very short and there isn't much room for capacitors, tracking systems or much else. Because of the small size and lack of consumables, micro railguns like this would have very short service lives in orbit, and may be held on the ground and launched in bundles of 10-100 at a time to be dispersed in orbit when needed.

3U Cubsat. A weaponized one may have to be at least this large

Such a railgun satellite will only be good as either an "escort" satellite for a much larger and more expensive one, firing its single shot at any unidentified satellites or space vehicles approaching, or as an "assassin", moving in with a swarm of fellow cubesats and attempting to overwhelm an enemy spaceship with a multitude of shots to kill any defending satellites and strip away any armour protection it might have. Since orbital velocity is 7 Km/s, the amount of kinetic energy that even a ball bearing sized round would have would be orders of magnitude greater than a rifle bullet. Attempting to fire and hit a target by firing against the direction of orbit means the pellet is striking with whatever velocity the railgun imparts + the 7 Km/s the enemy platform is orbiting at.

So tiny railguns that can fly in swarms could provide a different means of fighting a space war, but the ranges, timeframes and other factors will be extremely different than what we think of today.