
The Pentagon’s electromagnetic ‘rail gun’ makes its public debut - Shivetya
http://www.washingtonpost.com/news/checkpoint/wp/2015/02/06/the-pentagons-electromagnetic-rail-gun-makes-its-public-debut/
======
politician
Mach 7 is ~1.48 miles per second. The escape velocity from the Moon is ~1.47
miles per second [1,2]. (Let's launch stuff from asteroids and from the moon
back at Earth (except, let's be sure not to bombard ourselves.))

[1] [http://www.idialstars.com/evmc.htm](http://www.idialstars.com/evmc.htm)

[2]
[http://en.wikipedia.org/wiki/Escape_velocity#List_of_escape_...](http://en.wikipedia.org/wiki/Escape_velocity#List_of_escape_velocities)

~~~
dshankar
Not to mention, this is capable accelerating a projectile to Mach 7 within the
friction of Earth's atmosphere. Without one, perhaps it would be capable of
faster velocities.

~~~
politician
Haha, that's right, I forgot that Mach is a measurement of speed through a
fluid and the local speed of sound. Obviously, Mach 7 on the Moon is greater
than Mach 7 on Earth. I guess that means we can fire larger payloads with the
same platform.

~~~
jerf
"Obviously, Mach 7 on the Moon is greater than Mach 7 on Earth."

Well... Mach 7 through the Moon's atmosphere is probably best described as
"undefined" rather than greater than anything....

~~~
politician
Lunar atmospheric pressure is non-zero [1] (but to your point, it's really
really small).

[1]
[http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html](http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html)

~~~
zachrose
Would very low pressure mean that Mach 1 on the Moon is be much faster than on
Earth?

~~~
tsm
Yes, exactly

~~~
lutorm
Actually, the speed of sound through a gas only depends on temperature, not
pressure. (But the Moon's "atmosphere" can hardly be described as a gas
anyway...)

~~~
ecma
At least google stuff like this before posting it (and maybe include a
reference) because this is unequivocally wrong. Speed of sound in a gas is
calculated with the Newton-Laplace equation [1] which refers to K, the bulk
modulus which is derived from the pressure and the adiabatic constant for
ideal gases. It also involves rho, the density of the medium which is why
gases and non-gaseous fluids have very different results.

[1]
[http://en.m.wikipedia.org/wiki/Speed_of_sound#Equations](http://en.m.wikipedia.org/wiki/Speed_of_sound#Equations)

------
abstractbill
This can make a projectile move at mach 7, which is a little more than 20% of
the Earth's escape velocity.

Are there physical limits that would prevent us from building a bigger one and
using it to launch small things into space? Or "just" engineering problems?

~~~
empy
Elon Musk @elonmusk · Jan 26 Final one: anything launched by a railgun (if you
could ever reach ~ Mach 27) would explode upon exiting the barrel in our dense
atmosphere
[https://twitter.com/elonmusk/status/559629011983147008](https://twitter.com/elonmusk/status/559629011983147008)

~~~
infogulch
What if you have an enclosed track in a partial vacuum that runs up along a
mountain and exits where the air is thinner? Everest is 8.8km tall, and air at
that height has 1/3 the pressure as at sea level [1]. If you can build a 1km
tower on top to extend the track, you can get down to 1/4 the pressure.

[1]: [http://www.engineeringtoolbox.com/air-altitude-pressure-
d_46...](http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html)

~~~
Phlarp
Sounds like a modified hyperloop

------
ourmandave
I watched the video with the slow motion shot of the projectile coming out
with a large ball of fire behind it. It then goes on while the casing falls
away.

But I don't understand, why is there a fireball if it's all electromagnetic?

~~~
ethanbond
Probably from dumping 32 megajoules of energy into the rails/barrel. I
wouldn't be surprised if part of it is melting/igniting the actual hardware
itself and part of it is due to the friction of the projectile against the
air.

Remember that this is still a prototype and one of the big concerns since day
one has been lifetime of the system. Essentially, it's probably still melting
its own hardware (at least to some degree) every shot.

~~~
tjradcliffe
32 MJ is actually not all that much... it's almost exactly the energy in a
single litre of gasoline (44 MJ/kg, 32 MJ/l).

The important thing about a railgun is not the total power put into it but
that the power is dumped into the projectile continuously over the course of
acceleration, which makes much more efficient use of energy than conventional
weapons, which produce large initial accelerations followed by rapidly
decreasing gains as the projectile moves along the barrel and the hot gases
behind it expand (the limit of conventional technology is the so-called "light
gas gun" that uses hydrogen as a propellant to maximize the speed of sound in
the accelerating medium and produce a relatively flat pressure curve along the
barrel.)

~~~
metaphorm
well, that gives you an idea just how much chemical potential energy there is
in gasoline. its pretty incredible stuff.

~~~
deeviant
It's a pretty incredible amount of energy. Just think if you put a gallon of
gas in a car than hit the petal, then lets take away (or at least mostly
minimize so that the car has same aerodynamics as a artillery shell) air
resistance for fun. The car accelerates until it burns through a gallon of
gas, so for maybe, what 10-30 minutes depending on the vertical it's
constantly accelerating, I can even imagine what speed it could get up to if
you discount mechanical limitations of the engine/tires/etc, but let's say at
least 1000 mph, conservatively. Then car runs into a brick wall. That is the
amount of energy in a gallon of gas.

~~~
stolio
For fun, KE = (1/2)mv^2 = 32 MJ -->[mathing intensifies]--> a 1,000 kilogram
car moving at about 565 mph carries 32 MJ of energy, which is what 1 liter of
gasoline carries.

A gallon is about 4 liters, so at 100% efficiency it could get a 1,000kg car
to ~1,130 mph or a 1,500kg car to ~925 mph.

For reference, the same calculations tell us a quarter-pounder with cheese
could get a 165 lbs (75kg) human up to 540 mph.

~~~
eridal
now that's fast food!

------
lettergram
Jesus, 110 nautical miles...

110 nautical miles = 126.586 miles

That's equivalent to shooting a projectile from:

\- _Baltimore to Philadelphia (102 miles = 88.6356 nautical)_

\- _San Francisco to Sacramento (87.3 miles = 75.86 nautical)_

\- _Joliet to Champaign (113 miles = 98.1943 nautical)_

\- _(almost) Pittsburgh to Cleveland (132.7 miles = 115.31315 nautical)_

\- _Tampa to Orlando (84.1 miles = 73.0809 nautical)_

and so on...

Simply amazing how far these things can fire.

~~~
anigbrowl
I'm sure they'll show up on ground-based artillery soon enough, and there will
be an arms race in the things, because for many theaters they could give one
side an overwhelming advantage. Besides the range, the other benefit is cost:
railgun launches cost 1-2% of equivalent missile launches, with fewer risks,
eg if the enemy bombs your ammo dump, it's much less of a big deal because
it's not full of high explosive. These risks matter a lot; in the history of
modern naval warfare, the biggest risk to a warship was having its magazine
hit, which would usually tear the ship in two.

~~~
lettergram
Ground-based artillery isn't really piratical, it would require a large power
draw from a grid, or its own power source.

In the case of drawing power from a grid, that wouldn't be practical (or at
least, a good idea) in a war time scenario. In the case of having its own
power source it would be much to large to be mobile.

It works well for ships because many are already nuclear powered and being on
the water they have much more mobility.

~~~
deeviant
I don't really see how being at sea makes the energy requirement logistics
easier...

~~~
ForHackernews
Many naval vessels already have a nuclear reactor onboard.

~~~
deeviant
Any many land base grids have nuclear reactors of much high capacity, plus
hydro power, plus solar, plus fossil fuels...

Anyways.

~~~
ForHackernews
Which is great if you happen to be conducting your war in a developed, first-
world nation with a solid power grid, and you can count on your enemies to
play nice and not disable or degrade that power grid.

~~~
deeviant
There is a great amount of precedent here. Total war was waged in Europe,
attempts by all sides were made to disable or degrade the enemies grid, to
varying levels of success, but it was never completely suppressed. The same
with rail, which I bring up because if you had one of these on a train with
electrified track, it would be a pretty significant threat. I mean you could
devastate London from Marquise(or from a train rolling down the coast of
France.)

~~~
ForHackernews
I guess rail guns will be an important munition if WWI breaks out again.

~~~
deeviant
Right, because WWI was the last time artillery was used...

------
stolio
I remember hearing about a plan to build a giant rail-gun (or coil, or
whatever) into the side of a mountain and try to send stuff into space.

The issue was A) the barrel would be evacuated of air and the piece of glass
covering the end would send glass everywhere and B) the resulting sonic booms
would destroy the local ecosystem.

Looking for sonic boom energy levels (up to 100 megawatts/sq. meter!) I
stumbled across this gem:

> In 1964, NASA and the FAA began the Oklahoma City sonic boom tests, which
> caused eight sonic booms per day over a period of six months. Valuable data
> was gathered from the experiment, but 15,000 complaints were generated and
> ultimately entangled the government in a class action lawsuit, which it lost
> on appeal in 1969.
> ([http://en.wikibooks.org/wiki/Acoustics/Sonic_Boom#Perception...](http://en.wikibooks.org/wiki/Acoustics/Sonic_Boom#Perception_and_noise))

------
Keyframe
I'm more amazed by that camera tracking shot than by the amazing gun itself.

------
ddstanley
Yes, that's great, because we didn't have enough ways to kill people.

~~~
ffn
Not that I'm saying killing people or sowing destruction is good, but tons of
"good" or productive inventions we use today have their origins or
inspirations as weapons of destruction. Who is to say, years later, the work
on EM field concentration developed here for the rail gun can't be used to
(for example) "tune" people's brains out of seizures.

------
lifeisstillgood
Isn't the political implications here on Earth the more relevant (though less
fun)?

Novel large weapons (I'm thinking lasers, hyper velocity cruise missiles and
this and it's ilk) all seem set to unbalance military strategy on a scale we
have not seen since the USNavy told a couple of pilots "I bet you can't sink
those spare dreadnoughts"

A naval vessel parked thirty miles of the coast of most countries that is able
to destroy incoming targets and fire large lumps of metal at seven times the
speed of sound at major cities seems like the return of gunboat diplomacy.

Add to which now that lasers seem to be at the 25KW range from a truck, I do
wonder what 25 KW will do to a human head. Especially one tracked by facial
recognition.

Oh by the way, Washington DC apparently has big zeppelin based radar arrays
hanging above it these days - hoping to spot things travelling at Mach 7
before it's too late. So these thoughts have been occurring to folks for quite
a while

~~~
Crito
I'm sure the Navy is really enjoying their new toy and will have lots of fun
playing with it, but I honestly don't think it will change politics much.

For most countries in the world, there is little practical difference between
a navy ship 30 miles off their coast with a railgun, and a navy ship 30 miles
off their coast with cruise missiles. _Even if_ they could shoot down the
cruise missile _(which most countries couldn 't do with much reliability)_,
the fact that the cruise missile was flying through their airspace in the
first place implies active military conflict. That's a situation that
_already_ escalated beyond diplomacy.

Think of it this way: Which countries would shrug off the US threatening them
with stealth bombers and cruise missiles, but _wouldn 't_ shrug off the US
threatening them with a railgun? Russia or China? Maybe France or the UK? Just
about any country that plausibly might be in a position react that way is a
nuclear power anyway.

------
zerooneinfinity
That guy talking about the flux capacitor was great. Seriously though, could
this technology being translated to guns as well?

~~~
desdiv
There are fully functional coilgun prototypes out there that's (arguably)
lethal:

[https://www.youtube.com/watch?v=TWeJsaCiGQ0](https://www.youtube.com/watch?v=TWeJsaCiGQ0)

The fine print is that they're coilguns, not railguns; their operating
principles are entirely different.

~~~
ajuc
This seems to have about the same energy of bullets as some slingshots, and is
considerably more expansive and complicated.

[https://www.youtube.com/watch?v=jMuI634RAYI](https://www.youtube.com/watch?v=jMuI634RAYI)

It also shares some problems - namely the lack of riffling, and the bad
accuracy/bullet stability problems over long distances (but it also probably
can't shoot with enough energy for it to matter).

I think till we solve batery problem - portable railguns and coilguns will be
about as realistic weapons as slingshots.

~~~
darkmighty
I think the military are already using coilguns but for mortars. With small
munitions a very small fraction of the nergy deliverable by the
batteries/capacitors is imprinted on the actual projectile -- a lot gets lost
on the coils -- and there's not enough time to use a significant amount of
energy. With mortars you can use bigger projectiles which have better magnetic
properties and a lot more mass (more time to gain kinetic energy).

~~~
desdiv
_I think the military are already using coilguns but for mortars._

It's still in early R&D stage unfortunately (or fortunately, depending on your
perspective):

[http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA481646](http://www.dtic.mil/cgi-
bin/GetTRDoc?AD=ADA481646)

[http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA610358](http://www.dtic.mil/cgi-
bin/GetTRDoc?AD=ADA610358)

------
protomyth
The US Navy has a Science and Technology Expo? I wonder what else they showed
and how open it is?

"The lack of gunpowder and explosive warheads eliminates some significant
safety hazards for Navy crews, officials say." \- that's probably an
understatement.

~~~
chiph
Not only that, but because you don't need room for the powder (and all the
safety armored bulkheads & systems around it), you can store a lot more of
these projectiles on board. So far as warheads are concerned, E=1/2 m v^2
means kinetic energy is probably just as effective, given enough v.

I think I read elsewhere the rate of fire is 5 or 6 a minute. Combine that
with the 160km range, and you've got quite a good shore bombardment system.
I'm wondering how well it'd do ship vs. ship, over the horizon. Presumably
they'd have a drone near the target so they could adjust fire.

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higherpurpose
Everyone please applaud this cool new way to _kill people_.

~~~
aetherson
A railgun is a direct-fire weapon with no explosives -- it won't be used to
carpet-bomb civilian targets or wage drone-like terror campaigns. It's pretty
much "a way for a warship to blow up another warship."

That's not to necessarily say we should celebrate any weapons technology, but
as far as weapons technology goes, this is one that fits pretty solidly in the
role of "militaries fighting other militaries." Sure, I guess that the US Navy
could use it to blow up an unarmed civilian vessel, but honestly the US Navy
could do that anyway.

~~~
ProAm
When was the last time we shot at another warship with a warship? (honest
question)

~~~
yourapostasy
Possibly
[http://en.wikipedia.org/wiki/Battle_of_Bubiyan](http://en.wikipedia.org/wiki/Battle_of_Bubiyan)

If ISIS grows to the point they field brown-water and/or littoral asymmetric
warfare vessels (very large numbers of small craft carrying highly lethal
missiles), then the expanded ammo capacity and stand-off capabilities of the
railgun would likely be welcomed by the sailors on the US vessels.

