What kind of military spacecraft can we create with current technology?

[Nothalogh]

The biggest fattest Orion you can build, loaded down with several hundred of the closest thing to a torchmissile that we can build (A NSWR powered rocket with a 100mt orion charge, optimized for cutting instead of propulsion)

I don't care if you've got shields powered by fairydust and world peace, if the first one doesn't do the job, one of the next hundred will.

[SomeGuy123]

2 hours ago, Nothalogh said:

The biggest fattest Orion you can build, loaded down with several hundred of the closest thing to a torchmissile that we can build (A NSWR powered rocket with a 100mt orion charge, optimized for cutting instead of propulsion)

I don't care if you've got shields powered by fairydust and world peace, if the first one doesn't do the job, one of the next hundred will.

The problem isn't destroying the enemy ship.  If it's made out of matter and barring some unknown to current science and physics principle, that part is easy. 

The problem is if the aliens are competent enough for interstellar travel, they can probably rebuild their ship - it'll take your Orion weeks to months to get there - to have a honking gigantic space laser with incredible range.  It would then proceed to snipe each torch missile from a million kilometers away.  

[Nothalogh]

1 minute ago, SomeGuy123 said:

The problem is if the aliens are competent enough for interstellar travel, they can probably rebuild their ship

Correct, and just like the old days of naval battles, you've gotta overwhelm their damage control

[kerbiloid]

10 hours ago, SomeGuy123 said:

Ok, let's assume the aliens don't have force fields or antigravity or anything else that known science doesn't say is possible.

So what do they have? Well, they have interstellar flight.  And they must have used anti-matter pion propulsion to do it.  (or maybe a black hole engine).

So there's actually one big advantage we'd have on Earth.  Warning time.  Their ship would take decades to decelerate, and leave a flare of gamma rays as it does so.  Both engine types emit much of their exhaust energy in the form of gamma rays.

Ok, what else do the aliens have?  They probably have molecular nanotechnology, and they probably can copy themselves very rapidly given the needed resources.  But their starship itself is probably very small and light at the end of an interstellar voyage, having burned off most of it's mass during the trip as propellant and expended stages.

If that's true - again, I'm assuming plausible limitations based on current scientific knowledge - then it's not hopeless.

Basically you have to assume that the alien ship is higher quality than anything we can make on earth.  It can probably reconfigure itself and repair damage very rapidly.  So you need at least 100 : 1 odds.  If the alien ship weighs 1000 tons of payload*, you need to send 100 kilotons of warships to intercept it.  And you have to reach it before it can dock with an oort cloud object and grow rapidly into an unstoppable fleet.

What would the warships be?  Orion nuclear pulse battleships, armed with lasers and nuclear howitzer rounds.  Obviously.  There's no stealth in space so you just have to get into range.  Realistically you'd probably want to try to ram the alien ship.  You've got to assume that long before you get into range, the alien ship would probably reconfigure itself into a gamma ray laser with incredible range or something.  Maybe 100 : 1 odds isn't enough...

* you calculate the payload mass of the alien ship with spectroscopy to analyze their engine flare, then you look at how fast it is decelerating.

It certainly sounds like an interesting scenario.  

In the sci-fi book which I'm currently reading (Robert Ibatullin, "Rose and worm") (I'm spoilering it because anyway yet haven't found if it's translated into English, otherwise just would give a link).
 

Spoiler

The alien fleet was accelerated by homeworld (star-powered?) laser and sails (so, this IR radiation gave them up to humans when they already were in half way to the Earth), with no signals.

Then they began braking with magnetoplasmatic chutes/retroengines powered by the interstellar ionized gas flow (their speed was 0.5 c, so they could)..This caused a synchrotron radiation which gave up to humans that they are braking and not trying to communicate. So, they were correctly treated as an invasion fleet with estimated time of arrival T+200years.

Humans built a near-Sun fleet of (sun-powered?) lasers and a fleet of missiles equipped with sails, going to intercept them in Kuiper Belt when they arrive.
But soon realized that sneaky aliens, before begin braking, released 100-kg kinetic shells made of carbon, moving to the Earth with 0.5 c. So, a century before the alien fleet arrival, the Earth was eliminated with kinetic bombardment, only occasional spacebases and .tribes survived.

When the alien fleet speed dropped down and the ion flow had gotten weak, they disabled the magnetochutes and continued braking with what you've described: either Annie or Hollie engines.
In such manner they happily arrived to the Kuiper Belt.

But human survivors engaged the laser transmitters and launched laser/sail shells in two waves. Shells were invisible to aliens due to the alienships torch radiation.
First wave was homing into the alien engines, but missed. Alien realized that they're under attack and disengaged engines getting invisible for the second wave.
But the first wave measured and transmitted to the second wave their exact coordinates, so the second range successfully hit the alienships with kiloton-range direct kinetical hits.

But sneaky aliens were sneaky, and later the humans realized that they not just disengaged engines, but decoupled the ships (or maybe emergency capsules) themselves from the engine modules.
So, the missiles hit the engines, but aliens were not destroyed.
They colonized near-Jupiter asteroids and began producing invasion bots mining the asteroid matter. But this was giveb up to humans by thin comet tails appearing due to gas wastes.

The book describes how it then was going in this setting.

 

Edited September 21, 2016 by kerbiloid

[SomeGuy123]

11 hours ago, kerbiloid said:

In the sci-fi book which I'm currently reading (Robert Ibatullin, "Rose and worm") (I'm spoilering it because anyway yet haven't found if it's translated into English, otherwise just would give a link).
 

I like it.  It's a very grim and depressing setting - but it also would make an awesome setting for a video game.  Maybe right after the primary fighting has stopped.  The reason is that video games need a setting where the amount of total art and assets needed is feasible for a studio of a practical size.  That's why the whole Earth, undamaged, with all the people and all the cities can't be in a feasible game - it would take too long and too much money just to model all that stuff statically.  Much less try to predict how geopolitical politics would go once the game starts.

What is feasible is if the earth has been pounded into a smoking crater oozing magma.  A few successful alien attacks have eliminated all the mega-group of survivors, leaving tiny outposts with a few hundred people in each scattered throughout the asteroid belt.  Something you could actually practically model, about the size of Diamond City in FO4.  

It would be a derivative of KSP - you'd start out somewhere and have access to various scrap parts and have to device a spacecraft to get around in the asteroid belt.  Instead of science to unlock stuff you'd go find wrecked equipment leftover from the fighting and have to scavenge for components...

Naturally in the plot of the game there's a surviving group of aliens, and they are willing to talk, and your character gets a choice to either wipe them out or work with them.

Edited September 21, 2016 by SomeGuy123

[kerbiloid]

9 minutes ago, SomeGuy123 said:

I like it.  It's a very grim and depressing setting

The book is quite optimistic, though. A high cyberpunk utopia (despite of two space civil wars between human survivors while they are waiting for aliens.).

Edited September 21, 2016 by kerbiloid