Recoiling Spacecraft Weapons & Effects

It is my understanding that for recoil to occur, you have to have something against which to recoil? In space, there is no gravity to speak of, and no matter against which to recoil. Unless of course, you are counting on dark matter and even then, not all space will have equal amounts of dark matter...

If that makes sense...

FOTSGreg said:

So, what I'm really thinking of here is an unknown weapon system that not only affects the enemy, but also obeys known physical laws and also effects the spacecraft firing it. For example, if a 400 kt spacecraft fires weapons doing 400 kt of damage to an enemy, it also provides 400 kt of "thrust" to the firing ship whether or not it's weapons actually hit anything.

Click to expand...

For that you really need to define what you mean by "kt". Generally right, although it depends a lot on the weapon system. A purely kinetic weapon will impart the same amount of kinetic energy on the firing ship as it carries itself, while a laser only produces negligible thrust, and a missile obviously none.

Obviously, it's far less efficient than lasers, for example, but the idea is that the firing of the weapons adds a vector to the firing ship that cannot be accurately predicted and thus adds to the "chaos" factor of a 3-dimensional battle scenario.

Thoughts?

Click to expand...

Going to be pretty irrelevant. Unless the ship is firing a serious percentage of its mass, the acceleration from the recoil is going to be negligible. And projectiles of that size would be serious overkill against similar ships.
That's not to say the recoil wouldn't move the ship noticeably, just that it'd be extremely minor compared to the acceleration of an actual engine.

FOTSGreg said:

For example, if a 400 kt spacecraft fires weapons doing 400 kt of damage to an enemy, it also provides 400 kt of "thrust" to the firing ship whether or not it's weapons actually hit anything.

Click to expand...

An easy way to do the maths is to think of conservation of momentum.

eg: Your ship fires a projectile weighing 2 tonnes. The speed of the projectile is 100km/hr.

[Momentum = Mass * speed]

So the momentum of the projectile is 2*100 = 200 tonnes * km/ hr.

The speed that your ship will get pushed in the opposite direction is:

[Speed = Momentum / Mass]

So if your ship weighs 100 tonnes, you will get pushed backwards at a speed of 2 km/hr. (200 / 100)

That's assuming that your ship starts with a nominal 'resting' position - but that's the basics.

----
One of the really interesting limitations in space battles may well be getting rid of heat - one of the limitations is how much waste energy you can radiate away ... and firing weapons is likely to generated a lot of waste energy.

So if I fire a 100 Mega Joule laser at you (and the laser is 99% efficient) I still have to get rid of a Million Joules of waste energy - or I cook myself.

That means that I need radiation cooling fins - basically plates of metal whose job it is to radiate out energy. Assume that I have cooling fins like that that can dissipate the million joules of energy, so I'm fine.

All you have to do is fire your own laser - a measly 1 Mega joule - at my cooling fins and you'll prevent them from dissipating any more energy. In fact - your tiny laser will disable my massive weapons - yet do no damage !

So by keeping your lasers shining on my weapons heatsink, you'll be doing me no damage .. but you'll be ensuring that my main laser will self-destruct if I try and fire it !

Yes - there are counter measures, etc.

Good luck !

Mac

thothguard51 said:

It is my understanding that for recoil to occur, you have to have something against which to recoil? In space, there is no gravity to speak of, and no matter against which to recoil. Unless of course, you are counting on dark matter and even then, not all space will have equal amounts of dark matter...

If that makes sense...

Click to expand...

The concept of inertia is usually used to make sense of the situation involving masses in space.

http://en.wikipedia.org/wiki/Inertia

thothguard51 said:

It is my understanding that for recoil to occur, you have to have something against which to recoil?

Click to expand...

Nope. For the same reason that rockets work in space, without anything to push "against". You could, in fact, fire a standard rifle in space (the oxidizer being part of the cartridge charge), and it would recoil in the normal fashion.

Perhaps this will help, from the Clarion Blogg

http://clarionfoundation.wordpress.com/2010/09/16/spectech-how-rockets-work/

And from the article...
Exchanging Momentum

A fundamental, immutable law of physics is that you can’t get something for nothing; conservation of energy. I have talked about this before, and sure will mention it again because in governs everything around us. In order to change your current course (note that standing still counts as a current course, just one with a speed of zero) you have to push against something. On earth we have air and water and ground to propel us, but what about space? In space you have to bring your own thing to push against.

Momentum is a quantity representing a mass and how fast that mass is moving. The bigger you are and the faster you move the more momentum you have. This value, like energy, is conserved (at least in most cases). Starting out with some mass and some speed you can sacrifice some of you mass for more speed. This is, in a nutshell, a rocket.

Does this make sense?

I wouldn't so much say it helps, as that it completely confuses the issue as well as two physical concepts.
There is conservation of energy, which is the first law of thermodynamics, and there is the law of conservation of momentum. The two are only in so far related as the 1LoT is pretty much the most basic physical law of all and thus related to everything.
"Sacrificing mass for more speed" Makes no sense whatsoever, and having air or ground to propel you is also inaccurate.

Anyway, what the law of conservation of momentum states is that a closed system always conserves total momentum. What that means in practice is that whenever any mass is accelerating in a direction, a proportional acceleration of some other mass in the opposite direction needs to happen so the total stays the same. In the case of a rocket, the exhaust gets accelerated out of the back of the rocket, and in return, the rocket accelerates forward.

In the case of a weapon, the bullet gets accelerated out the front, and in return, you experience recoil.

For any significant distance in space, you'd want a missile. And unless time is a factor(target able to course correct faster than the missile, which would make it useless at any range) a planet-based missile platform would be sufficient. "Real" space combat has some very cool and unique characteristics, and while space opera dogfighting is inefficient and ineffective, kinetic missiles (just a big metal slug with a rocket engine) can be very scary once you start measuring their velocity in percentages of light speed.

As to adding chaos, there's two options: If it obeys all laws of physics, a computer could pwith reasonable precision (accounting for unknown density of the projectile) determine the firing ship's new course in a fraction of a second, and thus correct both it's course, and it's future firing trajectory before any human on board could react; if it doesn't obey physics, then it's magic and can do whatever you want.

The interesting stuff is what happens when the firing ship or the target are already traveling so fast that they can't easily correct their course to dodge or aim without subjecting any humans inside to lethal G forces or exceeding their engines capability. At that point you see your doom coming and are powerless to stop it possibly years in advance.

thothguard51 said:

Perhaps this will help, from the Clarion Blogg

http://clarionfoundation.wordpress.com/2010/09/16/spectech-how-rockets-work/

And from the article...
Exchanging Momentum

A fundamental, immutable law of physics is that you can’t get something for nothing; conservation of energy. I have talked about this before, and sure will mention it again because in governs everything around us. In order to change your current course (note that standing still counts as a current course, just one with a speed of zero) you have to push against something. On earth we have air and water and ground to propel us, but what about space? In space you have to bring your own thing to push against.

Momentum is a quantity representing a mass and how fast that mass is moving. The bigger you are and the faster you move the more momentum you have. This value, like energy, is conserved (at least in most cases). Starting out with some mass and some speed you can sacrifice some of you mass for more speed. This is, in a nutshell, a rocket.

Does this make sense?

Click to expand...

I had no idea the physics that inspired attendees at Clarion was so Aristotelean. No wonder people are so eager to go to Clarion; its like going to a living, working Medieval University! A dizzying glimpse of the past just as you head off to write some gritty urbane fiction.

I thought the first sentence of the quote applied more or less to this topic...

I stand corrected...

the equivalent yield of a 20-kiloton nuclear explosion without the attendant explosive effects

Click to expand...

By '20-kiloton explosion' - you are talking about an explosion that is equivalent to '20-kilotons' of TNT? The standard conversion is:

One kiloton of TNT is equivalent to 4184 GigaJoules of energy.

So 20-kilotons is 83.7 Gigajoules

I'm assuming that your weapon is applying 83.7 Gigajoules to the target - rather than simply applying a trigger to some kind of fuel source on the target.

To put this value in perspective - at retail prices in the US - that is $2 million worth of energy every time you pull the trigger !

Mac

FOTSGreg said:

My thinking was that this could be combined with tactical thrusting in order to provide a random thrust vector for the firing spacecraft.

Click to expand...

Any kind of thrust from a laser is going to be absolutely negligible. While you can use a photon drive to accelerate a vehicle, you'll want much different specifications than for a weaponized laser. For one thing, you only need a tiny fraction of the energy output of a photon drive to have a decent weapon. In the reverse, any laser weapon will generate negligible thrust.

I'm trying very hard to avoid certain "magical" devices for controlling the weapons "heat", but such an intense amount of energy should have some secondary effects

Click to expand...

Oh boy, in that case i hate to tell you that you can't use cooling lasers that way. It kind of violates the 2LoT.

There's two kinds of projectile-firing weapons - the basic gun, in which the expanding gases from a chemical explosion in a tube with an opening at one end forces the projectile out the open end.

Then there's the "recoilless rifle" or Bazooka, a tube open at both ends that fires a rocket. The gases (and reaction force) go out one end and the projectile goes out the other end.

Other types of weapons that are "popular" involve electromagnetic propulsion, and in these the projectile and the launcher push against each other, providing recoil. Since there's no gas escaping that would carry away the opposite momentum, there's no way to avoid recoil. Well, you could shoot another projectile in the opposite direction, or fire a rocket engine in the opposite direction.

FOTSGreg said:

Oops! Thanks, Lhun. Back to the drawing board.

Click to expand...

When trying to come up with cooling, you have to remember that the only thing that can get rid of waste heat is passive radiating. Any kind of heat pump (and thermodynamically, a cooling laser is just another heat pump, like the chiller of a fridge or an AC) will generate waste heat in excess of the heat it moves around.
Heat sinks or venting chemical lasers aren't essentially different either. A heat sink just provides a useful environment to radiate heat into, and venting the laser medium gets rid of the hot material instead of just the heat.

Anyway, if you go with magical cooling lasers (hey, still less fantasy than blaster and less stupid technobabble than phasers or deflector dishes, so it can work) the one thing i'd really have a problem with wouldn't be the thermodynamics, but the idea that you'd have to avoid friendly fire from cooling lasers, or even the whole idea of the cooling laser firing backwards. It's just another laser! Even if the design of the laser generator dictates that the main laser beam and cooling laser beam are at a 180° angle to each other, stick a mirror on it and get two offensive lasers for the price of one! Any sensible design wouldn't treat it as a weapons laser with a 40% efficiency and a cooling laser to be shot into space, but two weapons lasers with a combined determined by whatever amount of waste heat the cooling laser can't remove.