Hacker News Reader

Comment by pneill | Hacker News Reader

Does anyone know why there is a flash of light and a puff of smoke when the gun fires? Since it's electromagnetic, you'd think you wouldn't see anything.

View on HN

A combination of three things: 1) The flash is air in contact with the super-hot rail ionizing instantaneously from the heat, and becoming plasma. 2) Friction (From the projectile against both the rail itself, and air) producing vast amounts of heat, which then a) causes water vapor in the air to flash to steam b) produces smoke from burning metal

I am not sure how much of the smoke is from 2)b), because I'm assuming the surface materials are designed to minimize friction and therefore surface burning.

[1] https://www.quora.com/When-a-railgun-is-fired-where-does-the...

Ah! Some good answers in the rest of this thread noting that this gun vaporizes its sabot - That's probably the biggest source of smoke

Nitpick on 2 a): you can't turn water vapor into steam, water vapor is steam.

Is it really that humidity in the air is classified as steam?

No steam references entropy or distribution of the molecules in the air. Concentrated water molecules visible as myst produced from hot water is steam. The water molecules evenly dispersed in the air is humidity.

>Concentrated water molecules visible as myst produced from hot water is steam.

Steam/water vapor/humidity is gas phase H2O and is transparent. Water droplets in air is often referred to as 'steam' or 'wet steam' but is liquid phase H2O suspended in air. If it mattered in context I assume it would be clearly stated as gas rather than liquid mist.

I doubt anyone would call the component of air that is 'humidity' by the word 'steam' but it seems it would be correct [1]. I am used to the term 'water vapor' for that gas phase H2O though.

[1] https://www.wordnik.com/words/steam

No, it's water vapour. Steam is a gas.

Water vapour is steam, i.e. it is the gaseous phase of water mixed with other gases (N2, O2, etc). Just like liquid water is still water if you mix it with alcohol. There is no possibility for a phase transition.

Railguns are not coilguns. Railguns maintain physical contact between the projectile and the rails for the entire length of the barrel. That's a lot of friction. Friction leads to heat, light, and a puff of smoke from material that has been ablated during the launch, if I were to guess.

This leads to one of the big problems with railguns, which is that you have to replace the rails frequently due to wear and tear.

They don't need to maintain physical contact. You can have an air gap, as long as the circuit shorts through the projectile.

The rails still wear down.

If you have a gap, then you end up with electrical arcing which generates even more heat

The old designs used to use copper behind the projectile. The current would make it vaporize and be a conducting gas/plasma thing to push the projectile along with the lorentz force.

I was stretching back in my memory about this stuff so had to read up again, the term they use is a "plasma armature" or "hybrid armature". I haven't been able to find out what this one uses in a quick search around.

A railgun, while its mechanism is electromagnetic, works on a base level by causing a massive high-current electrical short.

As a side effect (as in most electrical shorts), both the rails and some of the projectile are vaporized when the gun is fired due to both friction and electric arcing; and that's the smoke and fire you see.

This wear (which is signifcant per shot) is also the main obstacle to actually using railguns in any significant capacity in warfare (the other being the need for a massive current source, but nuclear generators and massive capacitor banks help).

Coilguns, on the other hand, don't actually touch the projectile they fire, and as such have a significantly cleaner output.

Why is no one developing coilguns? I've been wondering.

Coil guns require a lot more precision and accuracy in both manufacture and timing than rail guns, because you have to sequence the pulsing of the coils (compared to one massive burst of sustained power on a rail gun).

In a nutshell, you need to deal with most of the rail gun hard problems and then also deal with the added coil gun problems.

The air infront of the projectile.

When you compress a gas it creates heat. The projectile is moving so fast it super heats the air infront of it as it compresses it.

Also normally things that go the speed of this projectile (mach 6 I believe) are way up in the sky where the air is less dense. So theres a whole lot of air down here for it to shove out of its way.

The explosion is from the rapid expansion of the superheated gas after leaving the barrel.

although there is no gunpowder, the acceleration in a railgun is quite violent and the projectile needs to contact both rails while it is being accelerated. the smoke is probably just from the friction of the projectile against internal components. the flash could be from the same or an electrical arc (railguns use very high voltages).

another possibility for the "smoke": powerful railguns often use a pressurized gas to start the process of firing. if the projectile were stationary at the time that current was applied, it might be spot welded in place.

It's a wrapper (sabot, carrier, etc.) that ignites due to the speed.

Yea I was confused about that too. I expected it to just be a piece of Lead or Tungsten that launched out at high velocity. I don't understand the smoke/explosion either.

Reply on news.ycombinator.com