Another weird science post: Laser Recoil!

[clsage]

Re: Another weird science post: Laser Recoil!

 

Which raises the question "What do you serve droids in a bar?" Y'know, considering they don't eat or drink.

 

After seeing the "prequels", my interpretation is that it is a holdover/prejudice from the days of the Clone War and the "human (even if they were cloned) versus droid conflict" there in.

 

IMNSHO. YMMV.

 

-Carl-

[Susano]

Re: Another weird science post: Laser Recoil!

 

Which raises the question "What do you serve droids in a bar?" Y'know, considering they don't eat or drink.

 

(I fixed your typo for you Susano)

 

Electronic stimulants?

 

And I found a second typo....

[Yansuf]

Re: Another weird science post: Laser Recoil!

 

Have we had a threadjack here?

[Lawnmower Boy]

Re: Another weird science post: Laser Recoil!

 

Everyone be calm. As soon as the authorities have met the legitimate demands of the Vulcan Popular Liberation Front (Provisional Sons of Vulcan Brigade), this thread will be returned to its scheduled subject.

[Susano]

Re: Another weird science post: Laser Recoil!

 

You won't take me alive, copper!

[Xavier Onassiss]

Re: Another weird science post: Laser Recoil!

 

Have we had a threadjack here?

 

Like you have to ask at this point? This thread has been well-and-truly jacked.

[Xavier Onassiss]

Re: Another weird science post: Laser Recoil!

 

You won't take me alive' date=' copper![/quote']

 

I gave them your home address.

[Cancer]

Re: Another weird science post: Laser Recoil!

 

You won't take me alive' date=' copper![/quote']

 

Of course, if they send the titanium agents after you rather than those bendable copper ones, you have no hope.

[Susano]

Re: Another weird science post: Laser Recoil!

 

I gave them your home address.

 

:facepalm:

[RexMundi]

Re: Another weird science post: Laser Recoil!

 

Now, Laser Recoil or Particle Beam Recoil which one would be more noticeable?

 

~Rex

[Chimera 12]

Re: Another weird science post: Laser Recoil!

 

Now, Laser Recoil or Particle Beam Recoil which one would be more noticeable?

 

~Rex

 

Intuition suggests particle beam recoil. Most non-photon particles you'd want to fire would have a non-zero rest mass and would need to be accelerated to a useful speed first, so a particle blaster should have more 'thrust' and thus recoil than an equivalent laser. That said, I'll freely admit to not having crunched any actual numbers on this.

 

(And either way I'm discounting any mechanical side effects of whatever loading action we'd be using. "Just plug in a battery and it goes" works better for some hypothetical energy weapons than for others.)

[RexMundi]

Re: Another weird science post: Laser Recoil!

 

Yeah but then shouldn't any Laser large enough to prove to be more damaging then a map pointer, even though the recoil is significantly less because of the way a photon works, still, because of the sheer amount of energy released, provide significant recoil? I mean, looking at what we have now, then scaling the "Sci Fi Effect", it seems that there should be some issues. Some math nerd needs to crunch numbers my puny jock brain is not comprehending the big picture.

 

~Rex

[Xavier Onassiss]

Re: Another weird science post: Laser Recoil!

 

With particle beams, special relativity rears its ugly head. You're going to want particles accelerated close to light speed, so their mass is going to increase a lot. At 99% of light speed, they're seven times more massive than at rest. At 99.9%; multiply their mass x22. Recoil will increase correspondingly.

[Cancer]

Re: Another weird science post: Laser Recoil!

 

Yeah but then shouldn't any Laser large enough to prove to be more damaging then a map pointer' date=' even though the recoil is significantly less because of the way a photon works, still, because of the sheer amount of energy released, provide significant recoil? I mean, looking at what we have now, then scaling the "Sci Fi Effect", it seems that there should be some issues. Some math nerd needs to crunch numbers my puny jock brain is not comprehending the big picture.[/quote']

 

(recoil force in newtons) = (laser power in watts) / (3e8)

(3e8 is the speed of light in m/s, of course)

pounds of thrust = 0.2248 * (newtons of force)

 

So a 10 MW laser continuously fired gives you 0.0075 pounds of continuous thrust, that is, recoil.

 

To generate the 7.65 million pounds of thrust (34 million newtons) that the first stage of the Saturn V generated, you need 10^16 watts of laser power, which is a factor of several hundred more than the sum of all of Earth's electrical generation.

[RexMundi]

Re: Another weird science post: Laser Recoil!

 

That's a Lot. But is there really more of a Recoil out of let's say, a Lightning Bolt, then a Laser Beam?

 

~Rex

[Cancer]

Re: Another weird science post: Laser Recoil!

 

Well, lightning makes for explosive heating (and subsequent expansion) of the air, which will give a quite a kick. Your laser might do the same, but that is in no way included in the photon-recoil statement above.

[RexMundi]

Re: Another weird science post: Laser Recoil!

 

So if a Laser is fired in the Air, we get the explosive heating etc etc as well. Hmmmm...

 

~Rex

[Cancer]

Re: Another weird science post: Laser Recoil!

 

Came across a little tangential note about photon rockets in a textbook. Actually, I think this applies to any brute-force rocket trip, but it's easiest with photon rockets. Warning, math stuff.

 

You have a photon rocket for propulsion. Assume that your rocket has to carry the power source. You may also assume 100% efficiency in converting fuel to laser rocket energy for this discussion.

 

Let gamma stand for the relativistic gamma, that is, [1 - (v/c)^2]^-1/2. For those not used to special relativity, gamma approaches infinity as velocity approaches c, and it is 1 at zero velocity. Gamma = 10 corresponds to 99.5% of the speed of light.

 

Next, let f be the mass fraction of your rocket which is payload; everything you want to keep is in this fraction (passengers, rocket structure, weapons, etc.). Everything else is fuel (in this case, the mass you are going to convert to energy of your photon rocket exhaust). This fraction f absolutely has to be less than 1 by definition (unless you aren't going anywhere). It could be zero, but then you aren't delivering anything you want. (Incidentally, if your fuel-to-momentum conversion is not a photon rocket, then you need the factor for that conversion to be included here. That factor is 1 -- and hence omitted here -- for 100% conversion of reaction mass to photons.)

 

It can be shown (strictly from momentum-conservation and mass-energy conservation) that if you want to achieve a particular value of gamma, then your payload fraction can be computed from this gamma (in this 100% conversion efficiency situation) by solving the equation

 

f^2 - 2 * gamma * f + 1 = 0.

 

For the example of the earlier-mentioned gamma = 10, that means f = 0.05. Even with photon rockets you still have a large fuel mass to carry.

 

If all your rocket is going to do is:

1. Accelerate from rest to cruising speed
   
2. Decelerate from cruising speed to rest (presumably at the remote location)
   
3. Accelerate back up to cruising speed (for the return trip)
   
4. Decelerate back to rest (at the end of the return trip)
   

then the fuel for each of these acceleration episodes counts as "payload" for all of the previous episodes, so for each acceleration episode you reduce your payload fraction at the beginning by another factor of the needed fuel mass. This means that a rocket with enough fuel for out-stop-come back, in effect, your true payload at launch is only f^4.

 

So again, if gamma = 10 (99.5% of c), for a photon rocket carrying sufficient fuel to do a full out-and-back trip without restocking, then your rocket fuel mass is approximately all of your launch mass; the returning payload is at best 6.25 millionths of the initial launch mass.

[dmjalund]

Re: Another weird science post: Laser Recoil!

 

This is why Larry Niven had Ramjets which scooped fuel from the interstellar medium

[Susano]

Re: Another weird science post: Laser Recoil!

 

This is why Larry Niven had Ramjets which scooped fuel from the interstellar medium

 

Which, IIRC, have been discredited as a viable propulsion system.

[DusterBoy]

Re: Another weird science post: Laser Recoil!

 

Does anyone know how to calculate momentum? Do you simply multiply speed by mass. or what?

[Xavier Onassiss]

Re: Another weird science post: Laser Recoil!

 

Which' date=' IIRC, have been discredited as a viable propulsion system.[/quote']

 

 

They've been discredited as a relativistic propulsion system. The process of scooping fuel creates drag, which bollixes the idea of cruising anywhere near light-speed. They might still work at lower fractions of C.

[Old Man]

Re: Another weird science post: Laser Recoil!

 

Does anyone know how to calculate momentum? Do you simply multiply speed by mass. or what?

 

p = mv, i.e. momentum equals mass times velocity.

[Cancer]

Re: Another weird science post: Laser Recoil!

 

In the classical limit. For masses moving relativistically, it's rest mass times speed times gamma.

 

For radiation -- which has rest mass = zero --

(energy) = (momentum)*(speed of light).

[Susano]

Re: Another weird science post: Laser Recoil!

 

They've been discredited as a relativistic propulsion system. The process of scooping fuel creates drag' date=' which bollixes the idea of cruising anywhere near light-speed. They [i']might[/i] still work at lower fractions of C.

 

Thanks for the clarification.