Using lasers to stop bullets

There's a real fence that uses lasers to cut off the wings of insects to keep them out.

Is it possible to extend this technology to burn bullets into ash before they hit anything? Limit on minimum distance the bullet has to burn to stop?

• This uses a similar principle to stop rain, but maybe it can be adapted to fit your needs. what-if.xkcd.com/119 Commented Mar 8, 2015 at 8:24
• Remember that vaporizing doesn't get rid of either mass or inertia - you now have a chunk of molten metal flying through the air at high speed...it won't penetrate so well and it will slow down faster...but it still exists. You would need to zap it some distance from the target. Commented Mar 8, 2015 at 17:34
• @TimB That is melting, not vaporization. That said, inhaling super-heating, gaseous lead is not conducive to long term health, either. Commented Feb 1, 2019 at 5:09
• @Xavon_Wrentaile Sure. Depending on how much melts, how much vaporizes, how much re-congeals, etc. Commented Feb 1, 2019 at 13:39

There are two ways to stop the bullet using a laser, vaporisation (which will cause the bullet to lose its momentum due to air resistance) and ablation (which blasts material off the surface to slow it down).

For the purpose of this answer, we assume a bullet has a speed of 1000m/s and weighs 4 grams (5.56mmx45 NATO), and the bullet has to be stopped within a distance of 100m (0.1s)

Vaporisation

Using the enthalpies of vaporisation, melting and specific heat of lead from this page, it will take ~3400J to heat the bullet by 2000 degrees, and a further ~3.5kJ to melt and vaporise the bullet. A further amount of travel will probably be necessary for the lead vapor to dissipate its momentum.

This requires a power output of $\text{6.9kJ/0.1s = 69kW}$ of power delivered to the bullet. Actual laser power required will depend on the efficiency of the laser and how well the heat transfer occurs. If the lead vapor takes a long time to dissipate, it might be necessary to provide more power to allow it the time to dissipate.

Ablation

This paper gives specific velocity changes for lasers ablating material. No values for copper (FMJ) or lead (hollow point) are given, so aluminium was used as the closest material for laser ablation.

This paper cites the previous paper, and suggests that the ablation rates were in the order of magnitude of 0.01 to 0.1 $\text{J m}^{-1} \text{ s g}^{-1}$ after unit conversion.

Therefore, assuming an ablation rate of 0.1 $\text{J m}^{-1} \text{ s g}^{-1}$, it will take $\text{40kJ/0.1s = 400kW}$ of total laser power to stop the bullet.

The actual mode of stopping the bullet appears to depend on the relative power of the laser. A high intensity pulse (MW range) will tend to favour ablation, while a relatively lower intensity pulse/higher material absorbance will tend to favour vaporisation.

• Will the flare from a 69Kw laser itself cause damage? Bullets are usually metallic, and metals are shiny Commented Mar 9, 2015 at 3:08
• Note that with extremely accurate targeting, you would not need to ablate or vaporize the entire bullet, but just enough on one side to divert it enought to miss the target. Commented Mar 9, 2015 at 5:29
• If you use ablation you don't need to stop the bullet. Ablate one side of the bullet to produce thrust at an angle and the bullet will miss. This is exabated by the fact that you will cause the bullet to tumble. Commented Mar 9, 2015 at 11:37
• @Taemyr While you are correct, the question did ask for "stop the bullet". Commented Mar 9, 2015 at 19:00
• @Taemyr "miss" is sometimes worse than a hit. In Desert Storm (Iraq 1), a scud missile was intercepted and destroyed that, on later analysis would have fallen harmlessly in the ocean. The debris ended up landing in a barracks of US forces in Riyadh, accounting for nearly half the casualties in that war. Commented Feb 1, 2019 at 16:14

With a powerful enough laser you can vaporize the bullet almost instantly.

There's not much to say besides that, someone else might try to calculate how much power you need to produce such effect on a beam with the diameter needed to cover the whole bullet. But instead of a fence you could try something like a small scale anti-projectile laser. You need a way to detect and track bullets and a laser on a gimbal with fast enough reaction time to track the bullet and sufficient power over range to vaporize it quickly. By using a fence, you need to cover the whole height with laser beams, stacking laser sources one over the other. This might not provide enough time for the laser to completely vaporize it, or this limitation might preclude the use of less powerful lasers.