
The most powerful mobile electromagnetic railgun built by a non-government - nkurz
http://imgur.com/a/GrAiE
======
mangeletti
This is absolutely not 27,000 joules. 27,000 joules would produce a temporary
wound channel nearly large enough to destroy the ballistic gel block, and then
it would travel through the entire block, sending the block a foot or two into
the air. Based on the penetration of the projectile into the gel, but taking
the obtuse nature of the projectile into account, I'm left to conclude this
was more along the lines of 1000 joules... which makes me think the "50mph"
that the author claims the CO2 provides might be more along the lines of
200mph, and that the magnetic acceleration aspect of the gun is actually doing
nothing at all.

In other words, we might be looking at a slightly high-powered potato gun
being described by a misinformed author. Don't believe me? Take a look at the
delay between the shot and the entry hole in the car door gif. That's
commensurate with a projectile moving at freeway speed, not something with
27,000 joules, which for an aluminum projectile at that size, would be
something like +4,000 feet per second.

EDIT: Updated mph to take @avian's math into account - the ballistic gel
penetration supports that as well.

~~~
hatsunearu
It's probably 27kJ measured by the stored charge in the capacitor... which
probably aren't delivered to the fullest extent.

You can see the wires jump--that's inductive magic in action. To get any sort
of quick magnetic pulse into the railgun, you need high voltage[1] and low
transmission line inductance; neither of which are satisfied here.

A guy I know works on high power pulse mode lasers and they achieve ridiculous
rising edges with thin twisted pair wire. Lots of resistive loss but hey at
least the current rising edge is nice and fast.

[1]: a few kilovolts.

~~~
mangeletti
Perhaps it is, but the inefficiency of the machine shouldn't really come into
play when describing how powerful it is. Otherwise, I could call my car a 900
horsepower Honda Accord, based on the gas it burns, since the engine probably
isn't even close to 30% energy efficient.

~~~
iaw
They actually typically report horsepower and torque measurements for cars at
the engine and not at the wheels. There's an additional efficiency loss in the
drive train not accounted for in marketing numbers.

------
mathgenius
"The power must be monitored and disconnected before the maximum rating of the
capacitors is exceeded!"

As a teenager I used to love plugging in little electrolytic capacitors into
the wall socket. Like, rated for 16v capacitors hit with (in australia) 240v
AC. Man those things explode big time :-)

I did actually try this with big capacitors (still low voltage) hoping for a
proportionally bigger explosion, but those guys just fizzled.

Alright, just about now I'm thankful the internetz didn't exist back then i
could have done some real damage to myself. This rail-gun looks stupidly
dangerous.

~~~
DanBC
In theory big capacitors have vents so they should fizzle rather than explode.

~~~
HeyLaughingBoy
Even small ones (47uF or so) have safety slits to release overpressure. Maybe
that's a recent change, though. I don't think I have any caps older than about
20 years, so I can't verify.

------
grecy
Many things are being asked that were answered by the creator in the Reddit
thread yesterday.

[https://www.reddit.com/r/DIY/comments/3s4o4j/diy_250_lb_gian...](https://www.reddit.com/r/DIY/comments/3s4o4j/diy_250_lb_giant_mobile_railgun_27000_joules/)

~~~
soylentcola
Yeah, only posting the image thread misses the main discussion. Thanks for the
link.

------
Already__Taken
> Two years were required to acquire 56 of this capacitor (This project began
> in February, 2013). Two more capacitors were purchased as backup, in case of
> failure. The total cost was $2,600. The MSRP on this type (new) is
> approximately $850 per capacitor ($50,000 in total).

My question is who does that and cheaps out on the copper to connect them?

~~~
madaxe_again
That's a lot of expense on capacitors. When we made a rail gun at school, we
built our own capacitor bank with big glass cider bottles. Main problem we had
was the projectile (ball bearing) welding itself to the rails. Solved by
rubbing them with graphite before each firing. No idea what the efficiency
was, but we lodged several bearings deep in a brick wall.

They made us dismantle it as it was unsafe - so we re-use the capacitors and
made a tesla coil.

~~~
pjc50
_own capacitor bank with big glass cider bottles_

What was the construction of these - plate inside/outside made of foil?

~~~
ethbro
Link for reference:

[https://en.wikipedia.org/wiki/Leyden_jar](https://en.wikipedia.org/wiki/Leyden_jar)

------
ChuckMcM
That looks like a really fun gizmo. I'm surprised it took him that long to get
the capacitors though. Another fun way of storing energy is a flywheel on an
axle driving a generator. You can mechanically get the flywheel up to speed
while the coil of the generator is open circuit, and then power your apparatus
with it by shorting the output coil. If I did the math right and you used an
old 20" car rim and weighted it up to 25kg (55 lbs) you would need to spin it
at 2,500 RPM to store up 27kJ of energy. Lawn mower motor could do that, I've
seen it on Battlebots :-).

But perhaps more interesting slug. The NRL uses a conductive "sabot" in the
gun which then launches the projectile (which is not plasma at that point) our
the end. Seems like some improvement to be made there.

Also there is the issue of erosion of the gun rails. How many shots before you
need new rails?

The best thing though is the really awesome small talk at parties, "I was
tuning up the rail gun to manage the shock wave that develops on the leading
edge of the slug when it goes supersonic in the barrel you know? And then I
noticed hey, the plasma is creating a partial vacuum from the heat, and the
shock wave I'm seeing comes when it hits the air at the end of the rails, and
I think hey someone has to have solved this problem ..."

------
aidenn0
I suppose the author counts the University of Texas as being government? They
were able to propel a 185g projectile at 1900m/s which is nearly 700kJ of
projectile energy, wheras this appears to be at best 1kJ of projectile energy.

[edit] Just found that in the 80s they achieved an 8.8MJ launch; their later
designs were lower energy but could fire repeatedly.

~~~
thrownaway2424
I believe the UT people are also behind the US Navy's railgun project. They
also developed a railgun howitzer in the 90's. Perhaps that qualifies them as
"government".

~~~
aidenn0
I didn't realize they were behind the Navy one too; it would be interesting to
hear why they switched from flywheels to capacitors for power delivery.

------
TeMPOraL
HV capacitors seem to be cheap here (Shenzhen). I guess I know what's my next
side project.

~~~
baldfat
Better get real good life insurance. I think most people fail to see all the
dangers in this project. This thing terrifies me and wouldn't be within 40
feet of those capacitors. Electricity doesn't always behave the way we
intended it, and this would probably be fatal if it discharges incorrectly or
accidentally.

~~~
grecy
> _This thing terrifies me and wouldn 't be within 40 feet of those
> capacitors._

Good for you. Sailing around the world hundreds of years ago was dangerous.
Going to the moon 45 years ago was dangerous, tons of things are still VERY
dangerous.

Just because it's beyond your personal level of safety-acceptance, it doesn't
mean there are not hundreds of people lining up to do it. You mentioning it's
dangerous adds absolutely nothing to the discussion or possible learning
opportunities.

All you're doing is detracting from someone who's actually _doing_ something.
Rather than being so negative, why don't you go out and do _something_ and
document it as well as this guy and share it for everyone like he has?

~~~
jscheel
All you're doing is getting pissy at someone who has pointed out legitimate
safety concerns. You think sailing around the world didn't involve planning
and safety considerations, or that going to the moon did have a safety talk or
two? Yeah, there are always people lining up to do things. And sure, plenty of
risks were taken. But there's absolutely nothing wrong with thinking about
safety. They didn't say they wouldn't do it, they said they would maintain a
safer distance.

~~~
grecy
> _They said they would maintain a safer distance_

And I like tofu.

Neither of those pieces of information add anything useful to the discussion.

~~~
jscheel
Why doesn't it? There are plenty of instances where bad safety practices with
high energy have led to injuries. In fact, the parent comment had more
information relevant to the discussion than yours did. By your measure, your
comment brought absolutely nothing to the discussion, and you would have been
better to simply ignore the comment you didn't like. Policing what people say
in that manner is subjective and tends to lean more towards your personal
dislike for the comment than for any stringent criteria of quality. All that
said, I like tofu too. Maybe we can find a tofu discussion on HN to carry on
in.

~~~
grecy
It bugs me that so many posts on HN immediately get shot down with "It's not
safe", which is not constructive at all.

(see: self driving cars, drone delivery, etc.)

Just saying "it's not safe" is not useful, and I suppose I wish people would
do it less. Yes, safety is a consideration for sure, so a comment that talks
at length about how to make it safer is useful. Simply saying it's not safe is
not constructive.

We should have a HN tofu meetup

~~~
jscheel
Fair enough!

------
adaml_623
"During the second test, the projectile snagged before fully entering the
rails and was partially reduced to a molten spray of vaporized aluminum. This
issue was resolved for future launches with looser fit tolerance."

For me a machine that can create a molten spray of vapourised aluminium (fixed
spelling) is actually almost as interesting as a rail gun. :-)

Dear FBI, purely for artistic purposes.

~~~
thechao
The discovery of the elements is an interesting story. Even better, is all the
craziness surrounding the _naming_ of the elements! There are two (classical)
naming methods for elements we care about for this story: those elements whose
names end in -ium (strontium, gallium, et al), and those elements whose names
end in -um (platinum, tantalum).

Humphrey Davy (the discover of Al) originally use aluminium (1808) but, later,
switched to aluminum (1812); the latter spelling became his preferred
spelling. The British objected to the -um variant, and insisted on -ium. Davy
(himself) seemed to consistently choose the -um variant after 1812.

When the mass production of Al became feasible (another great story!) the
inventor (... or, most effective capitalizer, I guess), used the -um variant;
this happened in North America. Since that was the effective product name, it
became the most commonly used variant in North America.

Now, which is correct? The answer is: both! While I only have a few examples
to look at, the dictionaries I have admit both spellings are used, and note
which spelling to use in which circumstance, i.e., aluminium in Britain, and
aluminum in North America.

In fact, for some programs if your English locale is US then 'aluminum' is
recognized as the proper spelling, and 'aluminium' is marked as 'misspelled'.
However, switching your locale to UK, the opposite is true!

~~~
dpark
The original name given was not "aluminium" (nor "aluminum"), but "alumium".
Wikipedia confirms this and provides citations.

[https://en.wikipedia.org/wiki/Aluminium#Different_endings](https://en.wikipedia.org/wiki/Aluminium#Different_endings)

The story I've heard is that Davy started with "alumium", but added the "n"
because the pronunciation was deemed too awkward, yielding "aluminum". Others
pushed for "aluminium" because it sounded more consistent with other elements'
names.

~~~
thechao
Unfortunately I can't edit my post; I would add that I am repeating a story I
have heard---it is broadly the same as Wikipedia, as you point out. I am
certainly not an authority!

------
Already__Taken
Also if you're not concerned about the 330fps legal limit paintball markers
are manufactured to you can probably adjust the regulators (not just the
velocity screw you mentioned) to get a much higher initial kick. Or possibly
use your own valve, that tank must output around 400-800psi before the marker
regulates it down to somewhere like 200-300.

And avoid feeding liquid CO2 into the marker. If there's a siphon tube in the
tank it's fine that way up otherwise have the valve pointing up-ish.

------
ris
Somebody buy that guy a high speed camera.

------
sawwit
The coming decades will surely going to be interesting as we are getting
better at arbitrarily manipulating matter in a cheap way. It seems to me that
the power of the individual could just become too big. There is a good reason
why we decentralize important decisions: Decisions of individuals are often
just too prone to errors and malice.

Once individuals can synthesize viruses, armed drones, atomic bombs and AIs in
their cellars, there will be a myriad ways of things going terribly wrong (in
addition to the myriads of things that can already go wrong).

~~~
Natanael_L
"The cost of destroying the world falls by the day"

Paraphrased, can't remember the source

~~~
300bps
May be true but not applicable to this story about a rail gun that is more
dangerous to the person setting it up and firing it than any potential target
of it.

The thing couldn't even shoot through two sides of a car door.

~~~
TeMPOraL
True, but note that this project can be done by any random teenager (closer to
12 than 18 years old) with no access to specialized materials or equipment.
Aluminium and copper is around everywhere, and you can pry HV capacitors from
off-the-shelf equipment (as a kid I used to get them from spent single-roll
analog cameras). Instructions you can find on-line. In contrast, making a
working gun at home (and more importantly, ammo!) is much more difficult
(though still doable), requiring some specific chemicals and a lathe if you
want to finish it in under a decade. Though the latter is changing as well, as
the lathes are more precise, cheaper and easier to get than they used to be.

And this is still very low in the category of actual damage - close to the top
I'd put a resourceful teenager halting a desalination plant or crashing the
power grid in some country using only his laptop. Actually, I'm still a bit
surprised it hasn't happened yet given the laughable state of infosec in the
industries.

The point of the quote is - our technological advancements tend to raise the
amount of damage a single person can do by using products of the system
against the system itself. And this is a big problem, because while you can
hope for _some_ level of control over countries or even corporations, with 7
billion people on this planet, some accident is bound to occur eventually, and
individuals hell-bent on destroying the world pop up by pure chance.

~~~
dpark
> _note that this project can be done by any random teenager_

This thing is optimistically as powerful as a handgun. Any random teenager
with over $50K and two years of time to sink into this kind of project can get
a gun.

A teenager with $50K can obtain a car and just run people over, too. That's
both more likely and more effective.

> _The point of the quote is - our technological advancements tend to raise
> the amount of damage a single person can do_

Our technological advancements also tend to raise the amount of "the sky is
falling" paranoia. When drones first started getting popular, I saw several
articles about how terrorists were going to build smart bombs with GPS-guided
drones. This is a legitimately plausible scenario. The question is whether
someone with the resources to build a drone with sufficient range and carrying
capacity to do real damage could do the same without drones, and the answer is
most certainly yes.

New tech doesn't necessarily mean more danger, even if it means new types of
danger.

~~~
TeMPOraL
$50k would be the retail cost. The guy got his for < $4k.

> _Our technological advancements also tend to raise the amount of "the sky is
> falling" paranoia._

I've noticed that too. I don't want to sound paranoid about technology in
general, and I'm not worried about terrorists doing their terrorist things.
I'm worried what an average random individual could do, whether by accident or
on purpose. Most of the popular dangers don't multiply the power of an
individual enough. But biotech does.

~~~
dpark
> _The guy got his for < $4k._

Thanks. I missed that. How did he manage to get these for 1/10th the cost?
Still, I'm pretty sure a black-market handgun or a beater car can each be
obtained for <$4k.

> _Most of the popular dangers don 't multiply the power of an individual
> enough. But biotech does._

I could see that. I'm not sure how much biotech is really going to be
accessible, though. I think much of it will end up locked down (legally), and
I suspect that the regulations will be fairly effective. There are already
biological agents that people can harvest, but it's relatively uncommon.

~~~
TeMPOraL
> _Thanks. I missed that. How did he manage to get these for 1 /10th the cost?
> Still, I'm pretty sure a black-market handgun or a beater car can each be
> obtained for <$4k._

I don't know, but one of the way I used to source (much smaller) HV capacitors
for free was disassembling flash modules from single-use cameras that were
thrown away after film was extracted from it. Generally, there are ways. In my
city there is an electronics market where you can source a lot of parts (some
extracted from old devices) cheap. I haven't asked for how much they sell
those big HV capacitors in Shenzhen yet, but I'm willing to bet it won't be
anywhere near $800/pc.

> _I 'm not sure how much biotech is really going to be accessible, though. I
> think much of it will end up locked down (legally), and I suspect that the
> regulations will be fairly effective._

I don't know either; we'll find out in a decade. But I don't see it
effectively locked down in the longer (10+ years) run, unless something slows
down current wave of progress that is happening around biotech equipment. Like
with 3D printers, a lot of things you needed shit ton of money for can now be
done with DIY equipment in a garage. This gave rise to community biolabs
(biohackerspaces).

> _There are already biological agents that people can harvest, but it 's
> relatively uncommon._

True. I ascribe this to people generally not being evil or having murderous
instincts. What worries me more are accidents and incentives-induced-
stupidity. As an example of the latter, the most common bioattack people are
doing right now is coming to work sick. It's actually mind-blowing that so
many companies (outside IT) force people directly or indirectly to come in
sick, even though it results in other employees getting infected and overall
productivity in the company taking a hit.

------
emeraldd
It would be very interesting to see what could be done with a much smaller
projectile. I'd imagine there's a sweet spot for maximum energy transfer and
that 22grams is somewhere above that.

~~~
kragen
Presumably, you would like the reverse EMF from the projectile moving through
the magnetic field to provide the majority of the load voltage in the circuit
and be large enough that the current continues at over half its max until the
projectile makes it out of the barrel.

If the projectile is too large, then you will get all of the energy out of the
capacitors, but the slower motion will result in a lower back EMF, and
consequently most of the energy will be dissipated resistively, probably
mostly in the arc between the rails and the projectile.

If the projectile is too small, then almost all of the power in the circuit
will be reactive, but the projectile will leave the barrel while the
capacitors are still mostly charged.

Calculating this is somewhat complicated by the fact that the voltage across
the projectile will be highest as it leaves the barrel, because that's when
it's cutting the maximum amount of magnetic flux per millisecond; but that's
exactly the point where the voltage across the capacitors is lowest, since
that's when they stop discharging.

Maybe you could remedy this problem by using a flywheel-driven homopolar motor
instead of capacitors. Failing that, probably the best you can do is
discharging the capacitors to something like half voltage, transferring three
fourths of their energy into the arc and the projectile.

Your two arcs, assuming a length on the order of millimeters, are going to
suck up minimally 50 or so of your volts, which is one reason to use higher
voltage — so that the relatively fixed voltage drop across the arcs doesn't
hurt efficiency so badly.

[https://en.wikipedia.org/wiki/Tesla_%28unit%29](https://en.wikipedia.org/wiki/Tesla_%28unit%29)
suggests that the maximum field you can reasonably expect to achieve across
the rails is on the order of 500 milliteslas. If you were going to keep using
400 volts, discharged down to 200 at the time of exit, then the optimum would
be to have a back EMF at 500 milliteslas of about 150 volts at whatever the
muzzle velocity is. 150V / (500 mT * 20mm) = 15 km/s, about Mach 45 and about
twice the speed of, say, the International Space Station.

Okay, so if we have, say, 15kJ of projectile energy at 15 km/s, how massive is
the projectile? 15 kJ / ((15 km/s)²/2) = 130 mg. One difficulty with this is
that small, light projectiles slow down more from air resistance, which is
significant when you're talking about flying fast enough to suffer ablative
erosion from turning the air in front of your projectile into plasma.

So, you're right. 22 grams means that the capacitors finish discharging far
too early, so almost all the energy would be dissipated in the resistance of
the arc. Except actually I think most of it ends up in the flapping cables,
which weigh a lot more than 22 grams. You could probably get a noticeable
improvement in the penetration just by tying the cables to boards so they
can't flap around and generate so much back-EMF of their own. (They'll still
generate a significant amount due to self-inductance, which you can reduce but
not eliminate by tying the opposite-direction cables very closely together.)

But wait! What if the magnetic field is weaker? Achieving 500 milliteslas is
pretty hard, after all. MRI machines have a field on the order of a tesla,
which creates hazards of its own, though that's mostly due to the field being
distributed over a large volume. If it's only 50 mT, then you need the
projectile to be going _ten times as fast_ to build up that same 150 volts of
back-EMF: 150 km/s. And that, in turn, means that your sweet-spot weight goes
from 130 mg to about 40 mg.

You could improve the situation for massive projectiles, at the expense of arc
losses, by using a _lower_ voltage and higher capacitance. And of course a
stronger magnetic field would help too, but I don't think that's very
practical.

I don't know, maybe I've missed something in my calculations here. You'd think
that barrel length would enter into it somewhere, for example.

------
jokoon
I wonder if a small railgun could be made portable for infantry, and how much
power per shoot it would be. Because the home made ones don't seem that much
powerful in term of muzzle energy.

~~~
hderms
Well they're being used by the Navy now to shoot down missiles, as far as I
know. I think the tradeoffs are that they are super heavy and require a lot of
energy but they can accelerate projectiles extremely fast. Very useful if
you're trying to shoot down something that's also moving obscenely fast but
not so useful if you just want to poke a hole in a living target running at 10
miles an hour.

------
Kristine1975
Reminds me of Neal Stephenson's novel "The Big U", where (among other plots)
students build a railgun.

------
rplnt
I parsed the title as:

> Most powerful mobile railgun was built by a non-government (i.e. this is the
> most powerful mobile railgun)

when in reality it probably is:

> Most powerful mobile railgun that was built by a non-government (i.e. there
> are more powerful mobile railguns)

Is the title ambiguous or is it just me?

~~~
Rifu
The way it's written now (The most powerful mobile electromagnetic railgun
built by a non-government) seems pretty non-ambiguous in implying that more
powerful mobile railguns have been built by governments.

~~~
logfromblammo
If you want to get technical (and who doesn't?) the giant railguns mounted on
navy warships are mobile. They're definitely more powerful than the device in
the article.

I suspect that government defense contractors have repeatedly built man-
portable prototypes that just don't measure up favorably against the .50-cal
rifle. The government then mothballs the project again, until better
electronics technology comes along. It also seems obvious that anything that
did measure up would be classified as a state secret, so we wouldn't
necessarily even know about it.

So the implication I got is that government-built devices are purposefully
excluded, because we have no way of really knowing how powerful the most
powerful government-built mobile railgun is.

------
leni536
>The current flows in a C-shape through the device. This C-shape produces
outward force in all directions, like water flowing through a bent rubber
hose.

One really shouldn't expect any measurable force from this effect. I don't
claim that this effect doesn't exist, but electron mass is really small and
electrons usually travel really slowly in metals (mean velocity, not to be
confused with Fermi-velocity, which is usually quite large), even at
relatively large currents. I can make a simple calculation for the author's
construction if anyone's interested.

One can build guns based on induction and Lorentz-force but I think using the
electrons' inertia is completely unfeasible.

------
junto
I imagine that someone is going to have a visit by the FBI.

That or a job offer.

~~~
adrianN
Is it illegal in the US to build your own railgun? I know that in Germany
there is a limit on the Joules you can put in a projectile without a licence,
I think something like 8 Joules. This gun would violate that, I guess.

~~~
Rexxar
Applied strictly, this probably means that some Olympic sports require a
license (hammer throw, discus throw, shot put and javelin)

~~~
amalcon
8 Joules is a _really_ small amount of energy. I wouldn't be surprised if an
average healthy child could put that much energy into a ball by kicking it. A
quick back-of-envelope calculation says a 0.45kg ball (FIFA standard ball)
traveling at 6m/s (~13MPH) has more than 8 Joules of energy.

~~~
revelation
Then again, a ball is an awfully heavy and big projectile.

------
clentaminator
Metal Gear?!

------
ck2
Kind of amazing you can build this and not get a visit by any government
entity.

Of course right now it is far less powerful that any shotgun/rifle you can buy
without any background check at a gun show.

We are more than a bit insane about weapons in this country.

~~~
mindcrime
_We are more than a bit insane about weapons in this country._

How so? We purport to be a "free country" and part of that entails the right
to own weapons. Just because some people spazz out with irrational fear every
time the hear the word "gun" doesn't mean that the rest of us have a problem.
Consider that, by far, the vast majority of firearms owned in the US will
never be used in the commission of a crime. I don't have the numbers in front
of me, but somebody worked out the math and based on existing statistical
trends, it's a minuscule percentage of firearms that will ever be used to
commit a crime.

