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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

#71

Post by Symmetry »

UnoriginalGuy wrote: post #54
Symmetry wrote: post #50

Earlier quoted context omitted.

I'd assume that even with a laser they'd do most of the targeting work with a radar, in which case it's pretty easy to distinguish the target from the background. Just look at the Doppler shifts of the returns and don't worry about anything that isn't moving with respect to the radar. Missiles might indeed hide below the horizon on their approach, but that makes them just as invisible to visual ship-based detection a…

Anti-ship missiles are designed specifically to make for small radar cross-sections and to fly below radar range.

Yes, but "below radar range" in this context means below the horizon, which even from the top of a ship is only 15 km away or so. At mach 4 a Sunburn can cover that distance pretty quickly. I'm sure the low radar visibility make a big difference when hiding from AWACs on the approach, but since radar returns scale with the fourth power of distance I don't see how a missile could possibly have such a low cross section as to be invisible at as close a range as 15 km.

Re: US Navy ready to deploy laser for first time

#72

Post by jloughry »

TeMPOraL wrote: post #46
UnoriginalGuy wrote: post #17

This seems misleading: > Both costs pennies on the dollar compared with missiles and smart bombs A laser weapon is not an over-the-horizon weapon, so its spear of effect is quite a bit smaller than a "missile and smart bomb." It might be able to displace a Mark 45 gun or the Phalanx CIWS point defence system, but neither of which are missiles or smart bombs, and both fire relatively inexpensive (but heavy) ammunition…

Could it be possible to reflect the laser with a mirror? If so, maybe the navy could use UAVs with mirrors to guide the laser for over-the-horizon shots.

When a high power (megawatts) laser hits a surface, even a mirrored surface, the effect is more like a hammer than a reflection. This was studied extensively during the SDI program. Chrome plating missiles is, in the long term, ineffective; the beam transfers energy kinetically like a bullet, not through heating effects.

That said, it might work for now against kilowatt lasers.

Edit: an interesting and non-theoretical example of catastrophic optical damage is the "fiber fuse" effect: a spot of light burns its way along the length of a strand at about 1m/sec (destroying it as it goes). See this video (the narration is in Japanese):

http://www.youtube.com/watch?v=BVmIgaafERk

Re: US Navy ready to deploy laser for first time

#73

Post by UnoriginalGuy »

vilhelm_s wrote: post #69
UnoriginalGuy wrote: post #68

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. 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,…

> First, there is no particular angle that you need to fire to "escape the horizon".

You cannot make up fictional quotes then chastise me for my lack of understanding based on those quotes. I'm not even going to respond to that.

> Second, momentum is not "used up" to elevate a projectile.

That is where you're mistaken. The higher the angle that the projectile is launched at the more energy that gets transferred into elevation.

So when firing straight forward you transfer almost no energy from forward momentum into height, which is why when you stand straight, stick out your arm, and fire at a target down range you have to adjust for gravity.

As you point the "gun" further and further upwards the "bullet" will travel higher and higher but start to hit the ground at no more than its terminal velocity. This is because energy is going towards height from forward speed until forward speed is near zero.

> 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).

Then where does the energy come from in order to gain their altitude? If we take what you just said literally, then a mortar is an impossible kind of weapon, since you have no way of making the projectile travel upwards, only forwards.

The problem with that is that you just contradicted your own post. Earlier you said that 45 degrees was the optimal angle for distance, correct? So you acknowledge that at different degree elevations projectiles travel different distances, would this also be correct? Therefore if we combine these two points together: that different degrees cause projectiles to travel different distances, and that that travel requires energy, you must acknowledge that that energy has to come from the projectile's forward momentum.

If you acknowledge that forward momentum is converted into height at higher degree elevations (as you did earlier, see 45 degree point) you must also acknowledge that energy is being eaten for this additional height.

> 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.

That is incorrect. If you fire a projectile straight up it doesn't come down "times the force of gravity." In fact things cannot exceed their terminal velocity which is 1x gravities unless they're being artificially accelerated (e.g. firing a rocket straight down can go beyond 1x gravities, but a falling brick cannot exceed 1x gravities).

Re: US Navy ready to deploy laser for first time

#74

Post by vilhelm_s »

UnoriginalGuy wrote: post #73
vilhelm_s wrote: post #69

Earlier quoted context omitted.

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,…

> First, there is no particular angle that you need to fire to "escape the horizon". You cannot make up fictional quotes then chastise me for my lack of understanding based on those quotes. I'm not even going to respond to that. > Second, momentum is not "used up" to elevate a projectile. That is where you're mistaken. The higher the angle that the projectile is launched at the more energy that gets transferred into…

The image you posted in the sister comment made me see what's going on. You are arguing according to the Aristotelian theory of projectile motion, while the other commenters assume the Galilean theory. See e.g. this link: http://www.met.reading.ac.uk/pplato2/h-flap/phys2_2.html#sec... .

(In the presence of strong air resistance, the Aristotelian theory is not a bad approximation. If there is air resistance, a falling object will indeed gradually slow down to a terminal velocity, like you write in your last paragraph.)

Re: US Navy ready to deploy laser for first time

#75

Post by drjesusphd »

prestadige wrote: post #15
deletes wrote: post #3

With a laser operating on about 30 kilowatts of electricity - and possibly three times that in the future - the cost amounts to a few dollars per shot, Thompson said. The important part. Power is plentiful on a ship. No need to pay for and carry ammunition. The cost for a single rocket or cannon volley is usually in the thousands of dollars.

Indeed, and although the US Navy has plenty of power already, I'm interested to know how the polywell prototype fusor that they've been sponsoring is coming along. http://en.wikipedia.org/wiki/Polywell

I reckon it's going quite well for Polywell...

It's classified. No need to take it seriously as real science without real peer review.

Re: US Navy ready to deploy laser for first time

#76

Post by spingsprong »

lutusp wrote: post #60
spingsprong wrote: post #56

Earlier quoted context omitted.

Weapons that are designed to blind have been illegal for a long time. And these types of lasers are so powerful that if one hit you in the face, you'd probably lose more than just your eyesight.

Yes, but as the range increases, the chance of death declines, but the chance of blinding continues out to a much greater range. If there's really an international law against weapons that blind, it's about the be seriously violated.

There's laws against weapons designed to blind, but weapons that occasionally blind as an accidental side effect, are legal.

I mean, bullets and shrapnel can and has blinded people, yet bullets and bombs remain legal.

Re: US Navy ready to deploy laser for first time

#77

Post by TeMPOraL »

UnoriginalGuy wrote: post #49
TeMPOraL wrote: post #46

Earlier quoted context omitted.

Could it be possible to reflect the laser with a mirror? If so, maybe the navy could use UAVs with mirrors to guide the laser for over-the-horizon shots.

That is such a crazy concept it just might work. The logistics of it are mind boggling, but other than mirror-heat I cannot see too many huge issues with it right off the bat (aside from logistics).

> The logistics of it are mind boggling

Could you elaborate? I don't think it would be just as simple as putting a big mirror on a Predator drone, but then I can't see it being that much more complicated.

Re: US Navy ready to deploy laser for first time

#78

Post by Symmetry »

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…

Rather, in order to hit further away targets you need to initially fire at more than 45 degree. A higher trajectory gets you into thinner sections of the atmosphere more quickly so you lose horizontal velocity to air resistance less quickly. Air resistance doesn't seem too big a problem to us in our day to day lives, but increases as the square of velocity so at the speeds that artillery shells travel it will tend to start out as a much stronger force than gravity.

What ICBMs (and space_bound rockets) do is to travel vertically initially to get out of the thickest part of the atmosphere before starting to put on rotational velocity. You'll notice the article you linked specifies a reentry angle of 22 degrees, not a launch angle of that. A missile has an advantage over artillery in that it can apply thrust over time, and wait to start accumulating horizontal velocity until it has gained some height.

EDIT: A better way to explain things. When a bullet leaves the barrel of an artillery piece the forces on it are dominated by air resistance. As it travels it's kinetic energy decays exponentially with distance due to air resistance, and eventually it's traveling slowly enough that gravity becomes an equal source of acceleration as it settles into terminal velocity.

Re: US Navy ready to deploy laser for first time

#79

Post by Havoc »

ChrisOstler wrote: post #38
Havoc wrote: post #35

Earlier quoted context omitted.

>I think "most of that" is a little overly optimistic. Target tracking is a complex problem I disagree. The current complexity of target tracking comes mostly from that fact that you need to work out firstly where the thing is, secondly where its heading (in 3 dimensions & in real time) and third how fast. Having a +- zero flight time means you only need to do the first step. You can always rinse & repeat if its a la…

That assumes that aiming the laser is instantaneous, too. I have no idea if that is the case or not.

>That assumes that aiming the laser is instantaneous, too.

I was simplifying quite a bit & making assumptions yes. Another slightly sketchy assumption is that the laser fires instantly...chances are there is some kind of power up delay.

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