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.))
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.
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.
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...)
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.
The maximum velocity occurs the moment the projectile leaves the gun, so the only time air friction has any chance of reducing the maximum velocity is while the projectile is still in the gun. My guess is that on Earth, air friction's effect on the maximum velocity is negligible.
(The projectile will slow down a lot faster in the Earth's atmosphere than it will on the moon, but that fact has no bearing on the maximum velocity.)
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?
A space railgun is theoretically possible, but you'll still need a rocket to circularize the orbit.
In practice, a rocket goes up (out of most of the atmosphere) before going fast. A projectile fired out of a railgun would be going at its fastest while still in the atmosphere, and would thus be very hot / inefficient at the gun's "muzzle"
Heinlein's "The Moon is a Harsh Mistress" discusses such a solution for sending mined material back from the moon, which seems like a more feasible application.
The whole system is indeed possible, though. See this discussion:
If you wanted to get really clever, you could reach at least some orbits without a rocket (other than small corrections) by taking advantage of three-body interactions with the Moon or Sun. This would require even more velocity than just getting into low-Earth orbit, I imagine.
The short answer is that it is a plausible mechanism for g-hardened payloads, but probably not for soft payloads (like human beings). The problem is that accelerating up to orbital velocity at the maximum survivable continuous rate (~4 or 5 g) still takes hundreds of miles, which is probably an infeasible length for a barrel. Additionally, if it's not somehow supported so that it rises above the atmosphere, the de-acceleration from drag at the exist of the barrel is also too extreme.
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
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.
But why can't we use the railgun to replace just the first stage of a rocket? Due to the tyranny of the rocket equation that would mean outsized savings in fuel and payload.
It would be exiting the barrel at far less than mach 27 so this wouldn't be an issue.
Even without a first stage, wouldn't that mean that the railgun would need to account for dealing the energy needed to accelerate a couple hundred thousand tons of rocket fuel to any reasonable speed? As per Newtons 3rd law?
This is correct. And the power needed for larger rockets is well beyond what we currently generate with traditional power plants on earth.
For instance, the space shuttle with SRBs and SSME's firing would require more than five Hoover Dams to match (11.7 gigawatts, vs 2.074 gigawatts). The Three Gorges Dam puts out 18.3 gigawatts, but the Saturn V first stage did somewhere around 190 gigawatts.
You could get away with less of course, since with rockets the first stage must lift the immense mass of itself, but with a railgun you will have significant efficiency losses with all of the energy conversions involved. I would expect a heavy-lift railgun to consume at least a significant percentage of a large nation's entire power supply. So we'd be looking at ludicrously sized capacitor banks, or some very large power plants that sit idle without anything to do while we aren't launching rockets.
No doubt that's true in the individual case, but surely you'd attempt to run a "train" into the atmosphere with this kind of thing? Meaning that each projectile is slipstreaming behind the one in front.
It still might not be possible, but it seems like friction would be much less of a problem. Although starting up would be a significant issue with many thousands of projectiles hitting earth at high velocity - you'd want to run it 24x7 indefinitely.
That doesn't sound like a low cost project, though; certainly well beyond our current capabilities. But you might achieve very nice energy efficiency with it.
This is my imagination vs your imagination, so neither of us really has anything resembling an argument, but I'm wondering if in such a scenario the following projectiles might not find themselves running into the plasma wake left by the preceding projectiles. As such, your scenario might actually be worse than firing things one at a time.
The plasma wake will be less dense than the atmosphere displaced by the leading projectile, but it'll be much hotter and comprised of [#] much heavier atoms.
[#] "comprised of" is a correct usage, for anyone sucked in by the poor guy obsessed with editing it out of Wikipedia (many words have opposing meanings and some great English poetry depends on this; the term causes no confusion amongst native speakers; etymology is irrelevant to modern usage...)
>[#] "comprised of" is a correct usage, for anyone sucked in by the poor guy obsessed with editing it out of Wikipedia (many words have opposing meanings and some great English poetry depends on this; the term causes no confusion amongst native speakers; etymology is irrelevant to modern usage...)
The tyranny of the minority, in this case, is comprised of a single man, whose idiosyncratic reign of grammatical terror is now reaching out from Wikipedialand into the Greater Internet.
In my mind, I'm picturing this being used to shoot pretty much blocks of solid materials, not anything delicate, so the only issue is how much they are slowed down; by the time we could build something like this, fabricating in space is not going to be an issue.
Of course, by the time we could build this (assuming its even possible), mining asteroids could already have surpassed it.
The more likely limitation is the human body. We can only take so much acceleration until we lose consciousness or worse. This would necessitate a much longer "runway" for people to use safely. No comment on unmanned use.
And note that "lose consciousness" is around 10g (depending on orientation, health, and many other factors) and "die" is something like 20-40g for more than a small period of time. A gun like this generates thousands or tens of thousands of g, which is more along the lines of "turned into a fine paste smeared on the back wall of the capsule."
Other people have mentioned why you wouldn't want to use a railgun to get a payload into orbit in one go. However, the exponential nature of the rocket equation means that every km/s you can get to start with has a large effect on what fraction of your rocket has to be fuel.
It covers the speed you need, but not the velocity. For most tasks, launching directly away from Earth with minimal post-launch fuel capacity is not very helpful.
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?
The projectile has so much kinetic energy that it turns everything it touches into plasma. That includes the air it travels through. At the point of launch there is also very substantial material ablation that feeds a plasma bloom.
In fact, one of the primary engineering challenges in the design of a rail gun is minimizing ablation of the gun itself across firings. Rail guns slowly eat themselves so you want a design where (1) the ablation is slow enough that you can still get many shots off before it is non-functional and (2) the ablation is localized in easily, cheaply, quickly replaceable parts.
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.
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.)
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.
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.
its way more energy than that! car engines aren't even close to 100% efficient. all the heat that blows out your exhaust pipe is energy that the engine couldn't capture. ditto for heat of friction on the mechanical components, and heat that radiates into the air from the engine block.
I think the gun is specifically designed so that won't happen, since the current has to go through the projectile (else you're just "short circuiting" the weapon)
Oh, fair enough. I'm not familiar with how railguns work. I paid a quick visit to Wikipedia and the current is supposed to go through the armature of the projectile, so breakdown of the air inside the shaft is detrimental.
> One big question this video begs is, what causes the giant fireball? Rail guns are supposed to be powered solely by electricity, and don't use explosives of any kind for propellant. Babb told PopSci the answer: The flames are from pieces of the projectile disintegrating; the 7-pound slug is jammed so firmly between the rails that when it's fired, pieces shear off and ignite in the air.
My first guess was that the explosions are coming from the projectile punching through those walls they put up, or the speed of a projectile superheating the air through which it's travelling to the point of igniting the air around it.
The latter guess seems way off base, but I'd love to learn exactly why!
I wonder what the effective range of this thing actually is because the curvature of the earth is going to come into it when talking about these distances.
The curvature is roughly 8 inches per mile, ~1 meter per 5 miles. Something 6 stories tall (~18 meters), like say a large capital ship would be below the horizon if it was 90 miles away. Granted rail guns don't shoot in a straight line but you aren't going to get much ballistic drop in the time it takes a Mach 7 projectile to get to its destination...
Mounting the gun higher on the ship would help, but then you are dealing with a very large equal-and-opposite force high above your center of gravity...
[EDIT] On second thought, with a velocity of 1.48 miles per second you are talking about 81 seconds of flying time at it maximum range, which is quite long enough for our friend gravity to drop the projectile and "bend" it around the horizon. Holy shit that's pretty incredible.
Naval gunnery has had to take account of that for a long time; pretty sure that 110 mile range is taking the ballistic trajectory into account. It's not like this thing fires the projectile at mach 7 in a straight line, than after 110 nm it suddenly stops.
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.
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.
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.
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.)
>I'm sure they'll show up on ground-based artillery soon enough...
No it won't, not unless you're going to postulate the existence of some magical power source that fits on a truck and delivers megajoules of power in a fraction of a second.
Firing something that distance is easier than hitting anything specific once the projectile gets there. Projectiles must have some kinds of internal guidance system (that works at Mach7) or else the Navy is just going to be slamming random meteorites down anywhere within very large swaths of countryside.
I think you underestimate the military's ability to solve the equations for ballistic trajectories. Don't forget, the very first digital computers existed for the sole purpose of calculating firing tables. They've been at this game a looooong time.
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...)
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.
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
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.
Gunboat diplomacy never went away. When East Timor was reclaimed from the Indonesian cough 'militias' cough, the US parked a fleet in international waters near Jakarta, for example.
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.
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).
UT Austin did a lot research for a tank-caliber railgun; in fact a lot of novel developments for pulsed energy from flywheel storage happened there.
Rifle size is currently a non-starter as a significant issue for rifles is the weight of the ammunition, and any electrical source of energy is likely going to be heavier than modern gunpowder.
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.
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.
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.
I don't think this is really about ship to ship combat, I think this is a replacement for the big guns of the Iowa class in their fire support role during an amphibious assault or supporting ground operations otherwise. It would be substantially cheaper to replace smart munitions dropped from airplanes, which is what primarily fulfills that mission now, with equally-accurate cheap projectiles rapidly fired from ships offshore, over the horizon, reserving the airplanes for special situations.
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.
The fact remains that the people on the other end of this weapon are still people, and all weapons of this kind are still a deadweight loss, economically speaking.
Think about it in these terms: you have a pile of money and a bunch of smart people. You can invest in a) creating a new technology that will make human lives better and trade with other people to increase global wealth or b) creating a new technology that will kill the people you would have traded with in the other scenario.
It is possible to both recognize that a certain amount of killing people is likely necessary in this crazy mixed up world of ours, and still be saddened by the waste and stupidity that that necessity represents. It is a huge error in economic thinking to forget that killing people is necessarily the result of waste and stupidity, either on their part or ours.
Personally, I won't work on anything whose primary purpose is killing people (and I've turned down jobs and contracts because of that) and I'm always a little saddened when I see good engineers investing their lives in such a profoundly unproductive activity.
With you on the larger point, but for the sake of argument, creating weapons technology isn't necessarily deadweight. Military applications have driven all sorts of technological developments that ultimately resulted in civilian economic growth (the Internet probably being the most salient here). Even this contraption probably required development of new compact power systems that might have all sorts of uses eventually.
And it may well be that the Internet would have happened anyway, but there seem to be some inventions that nobody would have had the drive to produce if not for wars... nuclear power might be a good example.
Nuclear power plausibly would have been created without military incentives--it just isn't that hard, and all the pieces were in place before Manhattan. Nuclear weapons are somewhat harder (uranium bombs are relatively easy, plutonium harder, fission much harder.) So if we're making imaginary arguments, I'm going to imagine a world where nuclear power was invented via civilian research, but limited military research meant no nuclear weapons :-)
The important thing is that it doesn't have to be military research that drives any of this stuff. If you want spin-off benefits--and who doesn't?--invest heavily in space exploration. It would have precisely the same odds of civilian spin-off benefits as military research, and would involve building stuff that a) isn't primarily intended to destroy itself and whatever productively useful thing it is aimed at and b) would get us closer to spreading humanities eggs across multiple baskets.
So "spin off benefits are good" is not a very strong argument for military research. It's a strong argument for research, but why not fund something productive rather than destructive?
One concern is that it makes it cheaper for the USN to blow up a vessel; this is way cheaper than the short-range cruise missiles it replaces, so it can be deployed more often.
The costs are not that significant a concern right now [1]. So saving money on the attacks will allow the government to spend more on useful functions, and not so much on more attacks.
Because price is almost certainly a non-factor. If the Navy intends on sinking a ship, they're going to do it. They're quite well funded. I can almost promise you that the price of a missile is not coming up in the discussions which take place in the war room.
Ah, I see the confusion; I'm not talking about the Navy sinking a ship today. I agree with that. However, as we get better at sinking ships, the Navy's total capability for sinking ships increases.
This weapon, due partly to decreased per-shot cost, is likely to result in either smaller ships or larger destructive capacity per-ship.
In the event that the USN is capable of sinking every single civilian ship on the planet right now, in which case that doesn't matter much, but if we haven't reached that point yet, then it does make a difference.
I promise you that aggregate logistics, including munitions costs, are important in war planning, operations, and strategy, even if they aren't important considerations in individual tactical decisions.
Well, it is cool, just on its merits as a machine. The fact that it's designed for killing people doesn't suddenly make it un-cool, nor does it mean we're somehow obligated to ignore it or pretend it isn't interesting.
There's a difference between admiring something because it kills, and admiring something for other reasons, that just happens to kill.
I understand your sentiment, but ultimately I fear that the only way humans will stop doing this is to genetically engineer it out of ourselves. Until then, I'd rather be stronger than weaker, since I cannot control what other humans want to do to me.
I don't like it, but I don't particularly feel like being conquered, either.
You might not. One of the problems with dropping tungsten rods from space is how large the rods must be to achieve the acceleration necessary to bust bunkers. With a space-based solar-powered platform, you could accelerate much smaller projectiles for similar effective damage. The logistics may be much easier -- for example, the rods might fit on a SpaceX capsule.
[1] http://www.idialstars.com/evmc.htm
[2] http://en.wikipedia.org/wiki/Escape_velocity#List_of_escape_...