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Two intergalactic superpowers, the Western Republic and the Eastern Empire, are in a Cold War. The Eastern Empire is comprised of mostly aquatic species, while the Western Republic is mostly comprised of terrestrials, including humans. The Eastern Empire has assembled 33 million FTL nuclear missiles pointed at the Republics territory. The Western Federation needs to have a weapon that can be equally destructive. Most Imperial Cities are submerged under 500 ft of water or more, so my question is, would nuclear weapons work or should they use another kind of weapon?

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    $\begingroup$ This sounds a little bit like a scale problem- galaxies are big, and cities are small. Intergalactic civilizations don’t fight over cities- they fight over supermassive black holes, quasars, and galactic quadrants. The Milky Way has around 300 billion stars, so the 33 million missiles aren’t much of a threat unless they’re each able to take out many solar systems in a fell swoop. Given that they’re still using nuclear warheads, you may want to rethink the scope of your civilizations. $\endgroup$ – Dubukay Apr 4 '18 at 22:35
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    $\begingroup$ Are the cities also filled with water? Does the republic intend to use these planets afterwards? Are some weapons considered taboo? What happens when the FTL engine fails? How powerful are the missiles? Are sonic weapons a thing? What about EMP? Are they weapons, tools of control or tolls of destruction? $\endgroup$ – Nuloen The Seeker Apr 4 '18 at 23:51
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    $\begingroup$ For the record, if you are capable of moving matter in real space at FTL speeds, putting a nuclear warhead on the end of that is pointless. The matter interactions between an atmosphere and a solid chunk of ANYTHING moving even close to the speed of light will be an extinction level event. It should set the atmosphere on fire and turn a big piece of the planet's crust into plasma and other exotic forms of matter. $\endgroup$ – Sean Boddy Apr 5 '18 at 2:22
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    $\begingroup$ I'm having trouble seeing why a mostly aquatic empire would, in any way, need to clash with a mostly terrestrial one. There's no reason, especially on a galactic scale, that these two would not happily co-exist, even to mutual benefit. Species/cultures at this tech level would not need to compete for resources. $\endgroup$ – StephenG Apr 5 '18 at 5:47
  • $\begingroup$ that's why I think we should not dump nuclear waste into the ocean, anyway I would suggest threatening them with plastic bags ;D (clinical trials with sea turtles and many aquatic species were successful!) $\endgroup$ – user6760 Apr 5 '18 at 7:25
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I believe what you have here is good, but as someone in the comments had said, galactic empires do not fight over cities. They fight over planetary control and galactic strongpoints. I would advise doing something, and yes this may seem like a rip off of Star Wars, but some sort of weapon that destroys aquatic planets by draining the water.

I have done some research on a nuclear weapon detonating underwater, and I am not sure if you are placing fleets on said planets, but here is what I found on nuclear weapons detonating underwater:

https://www.atlasobscura.com/articles/70-years-ago-the-us-military-set-off-a-nuke-underwater-and-it-went-very-badly

Now granted, this is a past incident. I am sure that in the future a fleet would not be severely harmed by nuclear detonations off from the water, but many ships were harmed from nuclear attacks, and missiles launched from the air could be hard to reach their targets.

So I would say construct some sort of super weapon that drains up water on planets, say something like a heat bomb or a way that it pollutes the water, like biological warfare to where the wildlife is destroyed. Or just simply make it to where underwater nuclear weapons are safer to be detonated.

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Water transfers force more efficiently than air, so pretty much any sort of blast from a device configured to perform as a depth charge will do.

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What you need is the equivalent of nuclear depth charges, which have already been developed. The SUBROC is actually an anti-submarine device, which is lauched from a sub, goes ballistic, then reenters the water at its target location, descending like a depth charge. You only need the air-to-submarine section of the weapon to work in your world, which should make it simple to modify to increase the power to an underwater city-buster.

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Underwater explosions have been studied by humanity for decades now. There is a short term for underwater explosion: undex.

Since water is (for practical purposes) incompressible, the energy of a blast suffers less dispersion over distance. In other words, the blast radius is much larger than what it would be, should the detonation happen in air.

About underwater nukes, you should read Wikipedia's main article on it, and then the one about Operation Hardtack's Wahoo Blast:

The nuclear device was positioned 500 feet deep in the Pacific Ocean (...) Within a second of detonation, a spray dome was created that reached a height of 840 feet [above the surface] after seven seconds (...) When the spray dome and base surge had dissipated, a foam patch could be seen spreading from the surface zero water to reach over 6,000 feet (...) The nuclear device had a blast that was calculated to be nine kilotons. All fallout stayed within the predicted fallout area with a maximum of 0.030 R/hr. The target ship at 5,900 yards was directly hit by the shockwave vibrating the entire ship and shaking it violently. The Moran, merchant marine ship moored at 2,346 feet away, was immobilized due to shock damage to its main and auxiliary equipment while also attaining minor hull damage.

That was a 9 kilotons blast, which is a relatively small yield. You can be sure that anything above the blast was obliterated. To the sides and further below, targets will take a hit from the blast wave.

But what if a larger payload was used? XKCD's What If number 15 was about detonating the Tsar Bomba (yield: 53 megatons, or 5,888.88 times the Wahoo test payload) underwater.

The explosion at the bottom of the Mariana Trench will create a quickly-expanding spherical cavity of hot steam. To figure out how big it gets, we can try a formula from the 1971 paper Evaluation of Various Theoretical Models For Underwater Explosion:

$$ Radius = (\frac{3}{4\pi})^{\frac{1}{3}} \times (\frac{40\% \times 53 \space megatons \space of \space TNT}{Mariana \space Trench \space pressure + 1\space atm})^{\frac{1}{3}} \approx 580 \space meters $$

If we use the same equation for a smaller depth of 500 feet (approximately 152 meters):

$$ Radius = (\frac{3}{4\pi})^{\frac{1}{3}} \times (\frac{40\% \times 53 \space megatons \space of \space TNT}{16 \space atm})^{\frac{1}{3}} \approx 2367 \space meters $$

I don't have any data to make further extrapolations, so from now on it's just pure speculation. I think the water column, fauna, flora and people within the blast would be vaporized, making for a big mushroom cloud reaching almost as high as the original Tsar Bomba test one (which peaked at 56 kilometers above sea level, nearly seven times the altitude of Everest's peak). The sheer pressure of the shockwaves will keep the surrounding water from caving in at first, but once that passes, water will rush in in a very catastrophic manner. Expect a whirlpool of biblical proportions. Both the initial blast and the later rush-in would cause tsunamis that would travel for dozens to hundreds of kilometers. If the sea in that area is around 150 feet deep, then the seafloor will become very hard, very plain glass, and the blast will be detected by sismographs multiple times as the shockwaves travel through the crust and mantle of the planet.

Oh and once that settles you will see a black rain. The water itself should not be much radioactive, but the salt in it should give you more problems than hypertension.

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I am not sure about a galactic war using FTL missiles to hit targets probably thousands of light years away.

There may be border defenses that can detect and stop approaching FTL missiles like the USA's "Star Wars" program might have been able to to stop ICBMs if completed.

And once past the border defenses each important target planet might have a planetary defense system designed to detect incoming FTL missile and try to stop them.

Thus FTL missiles might face lots of defenses on the way, often thousands of light years, to their targets, and might have a very hard time reaching those targets unassisted.

instead I imagine a galactic war using fleets of space battleships, both manned and unmanned, to attack planetary defense systems. If planetary defense systems have the capability to detect and stop FTL missiles, the planetary defense systems must be disabled to give the FTL missiles clear shots. So if a fleet of space battleships is detected approaching, the defending side may send a fleet of space battleships to fight the attacking side's fleet of space battleships, and stop them from disabling planetary defense systems to give FTL missiles a clear shot.

Thus it is possible that the decisive battles in any hot war between the two realms will be battles between fleets of millions of space battleships.

What kind of a warhead will a FTL missile have? A tiny pebble at FTL speed would have enough kinetic energy to smash a planet. No atomic bombs would be needed. Except that it is impossible to take a slower than light (STL) object and accelerate that object to FTL speeds. It would take infinite energy to accelerate even a subatomic particle to even the speed of light.

Therefore all possible FTL drives would somehow "cheat". They would get matter from star system A to star system B in less time than light takes to travel the distance without actually accelerating that matter to speeds faster than light. Therefore no possible FTL drive would give any matter impossible amounts of kinetic energy.

So it's back to using atomic bombs as the warheads?

Not necessarily.

The extinction of the dinosaurs and many other life forms was probably caused by the Chicxulub asteroid impact. And even if other theories are correct and the Chicxulub impact wasn't the main cause of the extinctions, it would have wiped out all life in an area hundreds and even thousands of miles wide.

The asteroid that struck Chicxulub was probably about 15 kilometers (9.3 miles) in diameter and struck at a speed of "only" a few tens of kilometers per second. The blast produced as much energy as ten billion Hiroshima A bombs.

According to my rough calculations, a rock only 15 meters or 50 feet in diameter travelling at the speed of the Chicxulub asteroid, just a few tens of kilometers per second, would release the energy of a Hiroshima bomb on any land or underwater city it struck.

The Tunguska event in 1904 produced a blast now estimated at 3 to 5 megatons, a few hundred times the blast of the Hiroshima bomb, and would devastate a large metropolis. It was probably produced by an asteroid or comet about 60 to 190 meters (196 to 620 feet) in diameter traveling at a few tens of miles per second.

So to produce a FTL missile, just attach a slower than light (STL) drive and a FTL drive to an asteroid. Use the STL drive to accelerate the asteroid to a speed of a few tens of kilometers per second. Then turn on the FTL drive, zoom to the target solar system, get very close to the target planet and in the correct relative position, and then turn off the FTL drive. The asteroid will still have the relative motion of tens of kilometers per second, and will slam into the target planet with devastating effects.

So you can use tiny asteroids to hit individual cities with nuclear bomb sized explosions, or asteroids a few kilometers in diameter to strike entire planets with devastating effects.

In the Lensman series a pair of planets with opposite velocities would be selected and giant FTL space drives would be installed on the planets and used to move them to opposite sides of the target planet, and then they would turn off the FTL drives. The intrinsic STL velocities of the two planets would cause them to smash into the target planet from opposite sides. But unless the target planets in your stories have fantastically advanced defense systems that would be billions of times overkill, trillions of times more mass than needed to devastate a planet.

Of course increasing the speed of the asteroid will increase the explosion it makes when it strikes.

Since the speed of light in a vacuum is 299,792.458 kilometers per second, that is 29,979.245 times a speed of 10 kilometers per second. Clearly an asteroid could be accelerated to tens or hundreds of times the speed that asteroids usually hit planets at, thus making its explosion a lot bigger.

It may be noted that neighboring stars often have high velocities relative to each other, so there might not be any need to artificially accelerate an asteroid to STL speeds to strike a planet in a neighboring star system, merely to use FTL speeds to get it near the target planet. Since stars mostly rotate around the centers of their galaxies in the same direction, stars on the opposite sides of their galaxies will be going in opposite directions with velocity differences on the order of hundreds of kilometers per second. And your Western Republic and Eastern Empire might be on different sides of your galaxy.

So if you are going to have an galactic cold war between two space realms using FTL missiles to threaten each other - like in the Cold War on Earth - you really don't have to worry much about the nature of your warheads. Normal atomic bombs or simple space rocks will do.

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Since this is a cold war, the Western Empire needs a doomsday weapon. They attack the atmospheres of the ocean planets.

An ocean cannot exist without an atmosphere, as it would quickly evaporate into space. There are a few methods that come to mind, depending on the empire's technical ability.

  1. A Solar Wind beam. Mars' atmosphere was stripped away by solar wind, assisted by the composition of its atmosphere and loss of magnetic field, but a similar method could work to hit multiple planets with manufactured or redirected solar wind.
  2. Bombarding the planet with magnesium pellets (if it has an oxygen atmosphere) triggering massive fires and burning of the atmosphere (more here)
  3. Gravity Gun - Reducing a planet's gravity would effectively dissipate its atmosphere, allowing the oceans to evaporate into space.

These would be pretty apocalyptic events on a planet-wide scale, maybe even for whole systems depending on the width of the beams/attacks.

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