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US Navy ready to deploy laser for first time

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Re: US Navy ready to deploy laser for first time

#61

Post by Filligree »

UnoriginalGuy wrote: post #47
Filligree wrote: post #42

Earlier quoted context omitted.

It will still be travelling sideways when it hits the target, and unless your target is a long, long way off it'll be travelling sideways with significant speed. Railgun projectiles, like cannon shells, travel on a parabolic curve - not a half-circle.

Cannon shells cannot be fired over the horizon either. Firing over the horizon will convert the projectile's forward momentum into height, it will reach whatever its apex is, run out of momentum and then fall due to gravity. The angle is just too steep. The only way to make this an effective weapon is to fill it with explosives (since then you've just created a mortar). But firing explosives from a railgun has its ow…

I'm going to assume that the designers have considered this, and that even though it's over-the-horizon the projectiles will still have significant momentum when they hit.

Re: US Navy ready to deploy laser for first time

#62

Post by vilhelm_s »

UnoriginalGuy wrote: post #47
Filligree wrote: post #42

Earlier quoted context omitted.

It will still be travelling sideways when it hits the target, and unless your target is a long, long way off it'll be travelling sideways with significant speed. Railgun projectiles, like cannon shells, travel on a parabolic curve - not a half-circle.

Cannon shells cannot be fired over the horizon either. Firing over the horizon will convert the projectile's forward momentum into height, it will reach whatever its apex is, run out of momentum and then fall due to gravity. The angle is just too steep. The only way to make this an effective weapon is to fill it with explosives (since then you've just created a mortar). But firing explosives from a railgun has its ow…

You seem to be saying that in order to fire a projectile farther, one needs to make launch angle closer to vertical. But that is not the case, unless the distances get very large you always achieve the longest distance by firing at a 45 degree angle, so half the velocity is in the horizontal direction. Making it more vertical than that will make the range shorter.

For extremely large distances (probably not relevant for naval railguns), the curvature of the earth becomes significant. But the correction goes in the other direction, it is optimal to fire at an angle more horizontal than 45 degrees. In the limiting case, the optimal angle for an ICBM is 22 degrees [1], and they will re-enter the atmosphere more horizontal than veritical [2].

[1] http://www.fas.org/rlg/garwin-aps.htm [2] http://www.fas.org/nuke/guide/usa/icbm/Slide92.JPG

Re: US Navy ready to deploy laser for first time

#63

Post by dwiel »

Kynlyn wrote: post #57
kiba wrote: post #18

Earlier quoted context omitted.

I am excited about future weapons, but I lament the existence of the military industrial complex and the military innovations that goes with them.

Do you also lament the creation of things like the fore-runner of the Internet that was created by the military industrial complex? Or the countless other advancements we use every day that were driven by military R&D? Or are you just completely and totally unaware of the significant contributions that military R&D has made to civilian lives?

The option isn't military or no military, it is military or spend the money on something else. There are plenty of other ways the same money could have been spent which resulted in similar civilian improvements.

Re: US Navy ready to deploy laser for first time

#65

Post by Consultant32452 »

TeMPOraL wrote: post #45
bluedino wrote: post #26

Earlier quoted context omitted.

According to a quick internet search, $25,000 a round. That sounds insanely expensive but cruise missiles are half a million dollars each, not sure what an LRAP costs or what 'traditional' artillery shells used on ships cost.

I guess half of that is amortized R&D costs, and then some more cost padding so that various people in the military supply chain can have their cut.

Also, I'd assume no one is mass producing these rounds yet, so they're all small scale production which greatly increases the unit price. Once the military puts in the first order for a few million rounds the price will go down a lot.

Re: US Navy ready to deploy laser for first time

#66

Post by Consultant32452 »

UnoriginalGuy wrote: post #47
Filligree wrote: post #42

Earlier quoted context omitted.

It will still be travelling sideways when it hits the target, and unless your target is a long, long way off it'll be travelling sideways with significant speed. Railgun projectiles, like cannon shells, travel on a parabolic curve - not a half-circle.

Cannon shells cannot be fired over the horizon either. Firing over the horizon will convert the projectile's forward momentum into height, it will reach whatever its apex is, run out of momentum and then fall due to gravity. The angle is just too steep. The only way to make this an effective weapon is to fill it with explosives (since then you've just created a mortar). But firing explosives from a railgun has its ow…

I'll try to explain it with simplified physics. Let's assume for a moment that the projectile is fired 50 ft off the surface at a speed of mach 8, or about 9k fps. If we assume no resistance vertical or horizontal at a 50ft drop the projectile is travelling downward at 55 fps by the time it hits the ground. As you can see, the horizontal velocity is still orders of magnitude greater than the vertical velocity. As you start to deal with curvature of the earth your drop distance will increase, but not enough to overcome the 9k fps in horizontal velocity any time soon. The horizon is 3 miles off, or 16k ft. Less than 2 seconds of travel time for our projectile. Of course, there's always going to be resistance, so at some distance the horizontal velocity line will cross paths with the vertical terminal velocity, but I doubt that happens at any practical firing distances.

Re: US Navy ready to deploy laser for first time

#67

Post by waiquoo »

wavefunction wrote: post #29
Havoc wrote: post #23

Earlier quoted context omitted.

>A laser weapon is not an over-the-horizon weapon I'd imagine the long term plan is to roll both lasers and rail guns. The combo seems ideal. Cheat laser shots to swat missiles and drones out of the surroundings whilst bombarding everything further away with the railgun. I bet with a bit of clever tech you can pull off some insane over-the-horizon railgun shots. >Phalanx CIWS which is also computer controlled, it is…

I assume rather than rail-guns, we will simply park laser batteries in NEO calibrated or perhaps just powerful enough to punch through the intervening atmosphere and water vapor to strike their targets. Of course I am just a sci-fi enthusiast in this regard, so take this comment for what it's worth... (I am not at all excited about the weaponization of space, but unfortunately feel it is inevitable given "politicians…

https://en.wikipedia.org/wiki/Project_Thor#Project_Thor

Re: US Navy ready to deploy laser for first time

#68

Post by UnoriginalGuy »

vilhelm_s wrote: post #62
UnoriginalGuy wrote: post #47

Earlier quoted context omitted.

Cannon shells cannot be fired over the horizon either. Firing over the horizon will convert the projectile's forward momentum into height, it will reach whatever its apex is, run out of momentum and then fall due to gravity. The angle is just too steep. The only way to make this an effective weapon is to fill it with explosives (since then you've just created a mortar). But firing explosives from a railgun has its ow…

You seem to be saying that in order to fire a projectile farther, one needs to make launch angle closer to vertical. But that is not the case, unless the distances get very large you always achieve the longest distance by firing at a 45 degree angle, so half the velocity is in the horizontal direction. Making it more vertical than that will make the range shorter. For extremely large distances (probably not relevant…

> You seem to be saying that in order to fire a projectile farther, one needs to make launch angle closer to vertical.

I didn't say anything even close to that. Not even ballpark.

I said that if you fire at a high angle you won't have much kinetic energy when the projectile finds its target. This topic is about firing non-self-propelled projectiles (cannon balls, railgun pellets, etc) from a ship or some other platform near sea level, several people are claiming that you can fire at a high enough angle to escape the horizon while still maintaining enough forward momentum to do significant damage, I am saying that at those angles the forward momentum is largely consumed as height.

The projectile will travel away and up, and then after it reaches its apex where it has consumed a lot of its forward momentum to elevate itself it will fall back to earth only at the speed gravity carries it at (minus wind resistance).

So while, yes, you can likely hit a ship or other target beyond the horizon, you cannot do so with more kinetic force than the projectile's weight being carried down by gravity (which is typically not very much relative to the kind of speeds we're talking about when initially fired).

Re: US Navy ready to deploy laser for first time

#69

Post by vilhelm_s »

UnoriginalGuy wrote: post #68
vilhelm_s wrote: post #62

Earlier quoted context omitted.

You seem to be saying that in order to fire a projectile farther, one needs to make launch angle closer to vertical. But that is not the case, unless the distances get very large you always achieve the longest distance by firing at a 45 degree angle, so half the velocity is in the horizontal direction. Making it more vertical than that will make the range shorter. For extremely large distances (probably not relevant…

> You seem to be saying that in order to fire a projectile farther, one needs to make launch angle closer to vertical. I didn't say anything even close to that. Not even ballpark. I said that if you fire at a high angle you won't have much kinetic energy when the projectile finds its target. This topic is about firing non-self-propelled projectiles (cannon balls, railgun pellets, etc) from a ship or some other platfo…

I think you have an inaccurate understanding of how projectile motion works.

First, there is no particular angle that you need to fire to "escape the horizon". The horizon is not a well-defined distance (it depends on how high how above sea-level you are), and in any case you never need to fire at a steeper angle than 45 degrees.

Second, momentum is not "used up" to elevate a projectile. If you ignore air resistance, a cannon shell will hit the ground with exactly the same speed as it was launched (no matter what angle it was launched at). For example, if you fire it straight up, then the kinetic energy of the projectile when it lands (again, disregarding air resistance) is exactly the height it reached times the force of gravity. And that is exactly the speed it had when it was initially fired (which is how it could reach that height).

Re: US Navy ready to deploy laser for first time

#70

Post by UnoriginalGuy »

Consultant32452 wrote: post #66
UnoriginalGuy wrote: post #47

Earlier quoted context omitted.

Cannon shells cannot be fired over the horizon either. Firing over the horizon will convert the projectile's forward momentum into height, it will reach whatever its apex is, run out of momentum and then fall due to gravity. The angle is just too steep. The only way to make this an effective weapon is to fill it with explosives (since then you've just created a mortar). But firing explosives from a railgun has its ow…

I'll try to explain it with simplified physics. Let's assume for a moment that the projectile is fired 50 ft off the surface at a speed of mach 8, or about 9k fps. If we assume no resistance vertical or horizontal at a 50ft drop the projectile is travelling downward at 55 fps by the time it hits the ground. As you can see, the horizontal velocity is still orders of magnitude greater than the vertical velocity. As you…

You seem to have forgotten the actual problem scope.

Nobody is claiming that railguns (or cannons) don't work. The issue is that to fire a projectile over the horizon the projectile's angle has to be increased.

This poses one of two issues:

- You fire the projectile SO fast you overshoot the target

- You fire the projectile to hit a target just over the horizon but to do so it no longer contains enough energy to do measurable damage to the target.

Try a diagram and see if that helps:

http://i.imgur.com/jWihwxf.png

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