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One bloody big step for a rail gun

The US Navy has tested its rail gun at 10 MegaJoules. Railguns will one day become the main armaments on US Navy vessels:

The technology uses high power electromagnetic energy instead of explosive chemical propellants (energetics) to propel a projectile farther and faster than any preceding gun. At full capability, the rail gun will be able to fire a projectile more than 200 nautical miles at a muzzle velocity of mach seven and impacting its target at mach five. In contrast, the current Navy gun, MK 45 five-inch gun, has a range of nearly 20 miles. The high velocity projectile will destroy its targets due to its kinetic energy rather than with conventional explosives.

A very big advantage of kinetic energy weapons is the reduction in size of a warships Achilles heel: the explosives magazine. With a railgun you would not need propellant charges.

The safety aspect of the rail gun is one of its greatest potential advantages, according to Dr. Elizabeth D’Andrea, ONR’s Electromagnetic Railgun Program Manager. Safety on board ship is increased because no explosives are required to fire the projectile and no explosive rounds are stored in the ship’s magazine.

I am not sure I believe you would get rid of all explosives as you might still want to lob an HE shell over the horizon and downwards on a target. If you are firing on a target 200 miles away, you cannot use direct fire unless you intend to blast a tunnel through a whole lot of water. That means the impact velocity on another ship using indirect fire would only be the normal terminal velocity of the falling shell. Nonetheless, the chance of a repeat of the HMS Hood disaster is much decreased.

What I would like to know is: has anyone done the calculations about direct fire at high elevation? Aircraft are naturally one of the targets. One wonders if it could reach out and touch something at a rather higher altitude.

For those unfamiliar with naval battles of WWII, the HMS Hood was sunk by one lucky salvo from the Bismarck that came straight down into the aft magazine. The ship was on the way to the bottom almost before the smoke cleared. There were (I believe) only 4 survivors.

Correction: It was 3 survivors.

53 comments to One bloody big step for a rail gun

  • James

    I thought it was three survivors. According to my google-fu it was three and they were Ted Briggs, Bob Tilburn, and Bill Dundas.

    The ‘plunging fire’ theory was adopted by the Inquiry at the time as being the most likely cause, but there are other explanations. Such as the possiblity of a handling error by the Hood’s own gunners that could have resulted in a magazine explosion.

    PS.

    The “the” in “the HMS Hood” is superfluous and incorrect. HMS at the time stood for His Majesty’s Ship and currently for Her Majesty’s Ship so you can simply say “HMS Hood was sunk” or “the Hood was sunk”.

  • J.M. Heinrichs

    You might ask MOD for info since the USN project is probably a successor to lab models at Kirkudbright; the ONR test model was delivered by BAE last year.

    A lower tech version is demonstrated at PowerLabs.

    Cheers

  • a.sommer

    http://www.navy.mil/management/photodb/photos/080131-N-0000X-004.jpg

    Check out the shockwave. How fast is that thing going?

  • Eric Tavenner

    As I recall from my time in the U S Navy the MK 54 5″/54 gun had a maximum range of 6.8 miles. It was the 16′/50 that had arange of about 20 miles. The 5″ is the gun caliber, the number after the / is barrel length in calibers.
    I do not think that explosive shells will ever be completely removed from Navy ships, one of the more important jobs is to provide artillery support for the Marines when they go all wild ass up onto the beach.

  • R. Richard Schweitzer

    Force security:

    Essentially providing security for the “platform,” which now shifts from Carriers to vessels possibly more vulnerable at least for a time.

    The initial efforts will probably be “which walnut shell has the “pea?”

    So far, this cannot be done from submerged status, though subs will likely be a source of force protection as things develop.

    That’s cryptography for ya.

  • R. Richard Schweitzer

    sorry that should have been cryptology.

  • Uncle Kenny

    “If you are firing on a target 200 miles away, you cannot use direct fire unless you intend to blast a tunnel through a whole lot of water. That means the impact velocity on another ship using indirect fire would only be the normal terminal velocity of the falling shell.”

    Is this an assumption on your part or have you done the math to conclude that all the kinetic energy of the railgun projectile would be dissipated at those ranges? How about the 50+ MJ versions than are being planned?

  • CountingCats

    Can that technology be integrated with this technology? Or will they compliment one another?

  • chuck

    “That means the impact velocity on another ship using indirect fire would only be the normal terminal velocity of the falling shell.”

    Lessee. Absent losses from atmospheric friction, the projectile will impact with the same speed as it had at the muzzle. This follows from conservation of energy together with the fact that the two ships are at the same distance from the earh’s center. Toss in frictional losses, and the estimated mach 3 might be about right.

  • chuck

    Check out the shockwave. How fast is that thing going?

    It’s hard to judge the angle of the shock cone from the photo. I’m going to make a wag of between mach 2 and 3.

  • Linda Morgan

    Here’s some video.

  • CountingCats

    “That means the impact velocity on another ship using indirect fire would only be the normal terminal velocity of the falling shell.”

    The article specifies impact at mach 5 after travelling 200 miles. That is somewhat more than terminal velocity.

  • Tim in TX

    “”If you are firing on a target 200 miles away, you cannot use direct fire unless you intend to blast a tunnel through a whole lot of water. That means the impact velocity on another ship using indirect fire would only be the normal terminal velocity of the falling shell.”

    Not unless you’re firing the weapon perfectly or near-perfectly vertical. Even at a very high angle, you’re going to start with and retain a pretty large horizontal component to the motion; at any sort of realistic elevation of the gun, you’re going to have even more.

  • Dishman

    What I would like to know is: has anyone done the calculations about direct fire at high elevation? Aircraft are naturally one of the targets. One wonders if it could reach out and touch something at a rather higher altitude.

    A muzzle velocity of Mach 7 gives a peak altitude (vertical fire) of about 200 miles, so it would have limited effectiveness as ASAT.

  • Julian Taylor

    Would there not still be a need for the Achilles heel? Certainly the technology does exist for an automated High Energy Electromagnetic Induction device slaved to an anti-aircraft target acquisition radar, thus no need for that massive SAM missile magazine. But what about ASW elements? Presuming that the railgun can not be used for anti-submarine duties then I suspect there would still be a need for a magazine for storage of torpedoes, both for the on-board chopper a for direct launch from the vessel.

  • Frederick Davies

    At such distances and speeds, the main problem is going to be targeting: hitting a plane 200 miles away is still very difficult, even for missiles, which have terminal guidance. It is unlikely they will be able to put guidance systems inside the rail-gun “shells” (“bullets” really), so the aiming of the rail-gun will have to be very accurate to be able to hit at long distances. The problem of targeting over-the-horizon is probably best left to missiles for now. On the other hand, this could make for a kick-ass medium-range anti-air (plane and missile) weapon, and I bet they are already thinking to use it as point defense against ballistic warheads (didn’t the Russians develop some anti-carrier ballistic missiles at the end of the Cold War?).

  • Nick M

    Tim in TX is absolutely right.

    People killed in places like Gaza by celebratory gunfire into the air are killed by bullets going not quite vertically up. If they are fired *exactly* vertically then they aren’t lethal (though probably hurt). Maximum range will be achieved by something like a 45deg angle so… It’ll still hit the target at a fair lick and cause a lot of damage.

    It isn’t related to Metal Storm.

  • Dale Amon

    James: Thanks for the info. I did indicate some uncertainty in my memory of the number and waffled between 3 and 4 before typing 3. Since it was not the main point of my story and my bed was calling loudly (it being after 3am) I left it at that.

    I am aware of the other ideas, one of which was used in a documentary but either way the cause was the magazines going up and the result was the Hood going down like a rock.

    I am far more interested in how this weapon system may have space defense applications if the final product can indeed chuck some mass 200 nautical miles. A railgun is a very nice anti missile defense and I am also wondering about LEO…

  • Methinks this is the beginning of the end for the anti-ship missile as this tech will also eventually make point defence just too damn effective. Could we see a return to a (sort of) heavy armoured battleship approach again for the generation-after-next of ships?

  • Dale Amon

    At the 45 degree angle (max range) the sin and cos components are equal so at least half the energy has gone into PE; in an orbital body that part would be fully recovered but in atmosphere all that you get back is the terminal velocity. That will be about 220+ mph for an aerodynamic body I believe. The other half of your energy is in the horizontal compoenent. Since there is no thrust during flight, the shell is dissipating energy for the entire flight. Just look at that shock wave. That’s energy getting sucked out of the system. I have serious doubts it is going any faster than a normal shell at that distance.

    One thing I do guarantee you: that shell is going to be friggning *HOT* when it hits. At night you would be seeing glowing arcs through the atmosphere from them radiating away their KE into heat.

  • Dale Amon

    The reason I did not try to calcuate this on my own is that I am not sure of the form of the dissipative term. I can say that it is proportional to area and fluid density; then you have the possibility of ablative mass loss; the effects of the shock wave and turbulence. Since this is a vector force counter to the direction of flight at all times it is constantly subtracting from both x and y components. In addition, the density of the fluid is dependent on y as well. This fluid resistance (air resistance) is not a linear term; for the moment it escapes me and I’m too bloody lazy this morning to reach up on the shelf for one of the Physics books, but it is at least proportional to v^2.

    Now you may have a point about low deflection shots towards targets just over the horizon. I’ll buy that, with the caveat that it will be traveling a lot slower after passing through that much sea level air. I have quite serious doubts about long range shots.

    And I reiterate, a firefight with these at night will look like something out of AC Clarke’s “Earth Light”.

  • Dale Amon

    I’ll toss out another one which I don’t have time to work on. Could there be ‘blind spots’? In your Physics courses you work these problems out as if you had an infinite plain and as you vary the deflection from 90 to 0 your impact point walks to every possible point out to max range and then back again.

    But if you are on a round body instead of a perfect infinite plain and your shell velocities are high enough, could there be points only reachable by high deflection shots? As a thought experiment, imagine you are on Ceres and that it had an atmosphere. Being ‘just’ over the horizon would be almost as good as being behind a mountain. Only high deflection shots could reach you: low deflection shots would all go over your head. Now the velocity of these shells may not be high enough relative to Earth’s gravity to show the same effect ,but I think it will be an effect that complicates the range tables more so than with old ‘slow’ shells.

    Of course if you can vary the firing velocity, you can also vary the target point so you may have the ability to dial the range continuously rather than by stacking a set of standard bags in the breech.

    There are a lot of interesting and as yet undisclosed angles to this technology.

  • Eric

    This thing really isn’t designed to be a point defense weapon. It’s designed to be the main battery for the equipped vessel. The projectiles will be guided, supposedly costing something in the neighborhood of $20k. They’ll be DU or tungsten, essentially a bit bigger version of the AT sabot rounds you see in modern armor.

    According to an article on strategypage that I can’t find anymore, they’re projecting an energy release similar to that of an 8-inch shell on impact (for a ballistic shot). That would be for the full-size version.

    Last time I looked into this they had some huge problems. For one thing, they couldn’t get it to last very long, as the huge magnetic fields would cause it to tear itself apart. Another problem was the enormous currents involved cause a bit of plasma from the projectile to get deposited on the rails, which means some kind of replaceable liner was thought to be necessary.

    And then there’s the power. Busing all those electrons around is something of an engineering challenge of its own.

    I wonder if this is close to being a weapon, or is it more of a proof on concept? And didn’t they cancel DDX, the ship for which it was being developed?

  • Plamus

    You are right, Dale, it’s proportional to v^2 for high-velocity (supersonic) projectiles. Precise calculations are a fugly mess. Stabilization becomes very important, as diameter (cross-section) is a factor in the drag, as is yaw angle. I was somewhat surprised to see relatively short, thick, and blunt-nosed projectiles in the videos – I would have expected a spear-type projectile, with higher mass and low cross-section, but maybe that’s where the tumbling becomes a bottleneck. Those guys seem to know that they are doing – exciting stuff.

    Here’s a competent-looking discussion of drag factors.

  • Freddy

    If you had a bloody big one of these things, would it be a good way to lift things to orbit ?
    (Not people, obviously, but dumb matter like reaction mass and raw materials for building space stations.)

  • Dale Amon

    Rail guns were originally thought of as a launch mechanism. They and mass drivers have been under discussion for 30 years for this purpose. If a 5″ gun succeeds, most of the problems will have been solved except for one: if you have more than orbital velocity at sea-level, you will very possibly need ablative shielding. People I talked to 20 years ago thought this might require up to 10% of the mass. Really negative sorts thought it would just act like a meteor in reverse, with the shards from the breakup going upwards :-)

    In any case this is not a mechanism for getting people into space; it would be for bulk supplies.

    The mass driver might be a people carrier if built at high enough altitude and with a long enough track to keep the acceleration within a tolerable range. You might use it for all the energy or you might use it as a ‘first stage’.

  • Alsadius

    CountingCats: Terminal velocity is dependent on the projectile, since it’s the point at which drag imparts a force equal and opposite to gravity. I can believe a hunk of solid steel having a terminal velocity of Mach 5, since it’s got a massive weight:cross-section ratio. If a human can attain Mach 0.25 or so in freefall, a projectile doing 20x that seems plausible.

    Perry: I can’t imagine heavily-armoured battleships will come back – if this technology gets up to battleship scale, then it’s going to have so much more effective power than a battleship shell ever did(as evidenced by having 10x the effective range) that no conceivable armour will be able to stop it. What I expect you’ll see is more of an eggshell-with-a-hammer approach – stick a single battleship-scale gun or turret on a much smaller ship(probably cruisers), and just admit you’re fucked if somebody actually hits you. Go for the lower cross-section, and put more of them on the field. There are still reasons for armour, but if we’re talking a 16″ shell with an impact velocity of Mach 5 or higher, you might as well armour against a nuclear bomb.

    Freddy: I was in a debate a couple years back that resulted in me working out the numbers for that. If you can manage a 120 km long launch tube(sloped at about 10-15 degrees above the horizontal) powered by a 1 GW nuclear reactor, you can accelerate, I believe, 1 ton of mass at 20g 243 times per low-earth orbit. It worked out to about a million and a half tons of lift per year. This is obviously the very large scale version of your idea, but it was fesable, and not at a high enough acceleration that it wrecked too many things. You might even be able to put humans into it – they’d black out, but 20 seconds at 20g is survivable, I think.

  • This thing really isn’t designed to be a point defense weapon.

    I understand that, but it is not hard to see how the technology would lend itself to point defence (essentially straight line trajectories at point defence ranges) and given the speed at which target solutions can be made these days, the Russian ‘supersonic approach’ starts to look like the wrong one.

  • Dale Amon

    A human being in free fall will hit the ground at 120 mph if flat on; or about 230 mph if going head down.

    I am not sure what the terminal velocity of a falling warhead is that is starting from 0mph, but I doubt it is anywhere near supersonic, especially not if it was shaped for surviving hypersonic ‘flight’ on the way up.

    Now if you have a spike of metal which is accelerated from space downwards, you will indeed have a hypersonic impact.

  • Roger Clague

    Freddie comments on using a kinetic gun to put things into orbit.

    I wonder if it could be designed to remove things such as spy or armed satellites from orbit.

  • When was the last time naval guns were fired in anger ?
    Lebanon 1982 ?

  • a.sommer

    I keep hearing rumors that the AEGIS radar can get returns off stuff in low orbit… so it’s entirely possible that railguns might eventually be used for that kind of thing.

  • Pa Annoyed

    “But if you are on a round body instead of a perfect infinite plain and your shell velocities are high enough, could there be points only reachable by high deflection shots?”

    Orbital velocity at sea level is about 7.9 km/s. Below this, the projectile always hits the ground. Above it (if not for air resistance) it could circle the Earth if fired horizontally, but will always hit if fired at high enough elevation. Only once you get to escape velocity at about 11 km/s could there be places you couldn’t hit because you were going too fast, and those places would be everywhere.

    “If you had a bloody big one of these things, would it be a good way to lift things to orbit ?”

    Current railguns are not fast enough for putting things in Earth orbit (although they could easily get to lunar orbit from the surface of the moon, which is what I think they were proposed for), but ram accelerators could be. Railguns can get the height, with their 3 km/s speeds, but not the horizontal speed for orbit which needs about 6-8 km/s. Ram accelerators on the other hand can make the 6 km/s needed for orbit, but are too long for easy use on ships. But whatever you use, you still need secondary rockets to adjust the trajectory for orbit. Under the influence of gravity alone, projectiles trace out ellipses, and an arc of an ellipse starting at ground level inevitably ends up at ground level if continued far enough.

  • chuck

    Even some WWII bombs broke the sound barrier:

    Most large Allied World War II aircraft bombs had very thin skins to maximise the weight of explosive which a bomber could carry—this was an improvement on the early part of the war when the actual HE content of British bomb designs was low. To be able to penetrate the earth (or hardened targets) without breaking apart, the casing of the Tallboy had to be strong. Each was cast in one piece of high tensile steel that would enable it to survive the impact before detonation. At the same time to achieve the penetration required, Wallis designed the Tallboy to be very aerodynamic so that when dropped from a great height it would reach a velocity higher than traditional bomb designs. In the final design the tail of the bomb was about half the overall length of the finished weapon—the bomb casing was some 10 feet (3 m) of the overall 21 foot (6 m) length. Initially the bomb had a tendency to tumble, so the tail was modified—the fins were given a slight twist so that the bomb spun as it fell. The gyroscopic effect thus generated stopped the pitching and yawing, improved the aerodynamics and improved accuracy. The improved design worked so well that it was found in development that it passed through the sound barrier as it fell. When dropped from 20,000 ft (6,100 m) it made an 80 ft (24 m) deep crater 100 ft (30 m) across and could go through 16 ft (5 m) of concrete.[1]

    As pointed out above, one of the key numbers is the density/cross-sectional area. Newton pretty much worked that out for projectile penetration and reaching terminal velocity in a fall isn’t all that different.

  • Dale Amon

    Okay, I’ll buy the tallboy. It’s about the right sort of shape for a shell as well. Keep in mind though that it still does not matter how fast you
    start out; your Vx is still not going to be any faster than termnal velocity of the same object if dropped. On that score you probably gain no more Vx final from 50 miles then you do dropping it from a Lancaster bomber at 20K feet.

    I’ve still got questions on how fast Vy falls off with t.

  • chuck

    your Vx is still not going to be any faster than termnal velocity of the same object if dropped.

    Oh, come. The question is if the projectile reaches terminal velocity before impact. I don’t see any reason to claim that the rail gun developers are so ignorant of drag that they would claim a speed of mach 5 on impact by mistake. Also, in a long shot, much of the trajectory is likely in the upper atmosphere, and atmospheric density falls off exponentially.

    Of course, with no atmosphere, shooting level at about 7.8 km/s (roughly mach 25 – leo orbital speed – ~38 MJ/Kg) one could reach any target in the world in about 45 minutes.

  • Errol

    When was the last time naval guns were fired in anger ?

    Possibly HMAS Anzac and HMS Marlborough at Al-Faw in April 2003. I chatted to some of the Marlborough crew involved in a subsequent port visit to NZ.

  • Sean O'Callaghan

    To attribute Hood’s loss to a ‘lucky’ salvo from Bismarck is a little unfair. The Germans were aiming for her – and they hit her! Given what happened to Queen Mary, Indefatigable, and Invincible at Jutland it shouldn’t have been such a big suprise.

  • Dishman

    Record velocity for a single human is in excess of Mach 1. That was jumping off a balloon at 21 miles up.

  • Eric Tavenner

    With the right shape hypersonic drag can be negligable. The SR71 used almost no fuel at high speed cruise, but burned huge amounts getting to speed.

  • CountingCats

    This distance business is a bit overstated, and surely is mentioned only to indicate theoretical capabilities.

    Fundamentally, this is the same thing as chucking rocks, just higher, faster and further. Once the rock has left your hands it cannot be guided. After travelling through 200 miles of random atmospheric fluctuations all you can hope to hit is something both stationary and large.

    This is more a means of obliterating things somewhat closer.

  • Freddy

    Dale, agreed about bulk supplies rather than people, but I always wondered about how a cheap route to orbit for bulk would impact on the costs for people.

    For example, how much of the design of the shuttle is constrained by the necessity of atmospheric braking to get back to the surface ?
    If you could refuel from a tanker in orbit, then the deorbit burn would become a really big delta v that sheds *all* your orbital velocity, instead of just enough to drop down to the top of the atmosphere.
    Then, the craft could just come down vertically under gravity. In this case, the craft wouldn’t need to be much stronger than a U2 or some such very high altitude airplane.
    In this case, I guess that the most physically stressful time for the spacecraft would be during launch instead of during reentry. This is still pretty intense, so obviously we are not talking about just modding a 747 for a new life as an orbiter.
    But there must be some savings to be made, which should reduce the cost of shuttle payload to orbit.

    Is there some specific meaning to the term “mass driver” beyond the obvious one ? (As in, a particular technology ?)

  • Freddy

    Alsadius, thank you. I suspect that 120km is too long to be feasible, given that you don’t want to bash it around by having the accelerator be anything less than straight. I guess if you halve the length of the accelerator, you have to quadruple the acceleration ?
    Regarding the size of the power plant : were you assuming doing this based on direct power from the plant ? Or were you using the power supply to charge up some enormous capacitors, which then power the accelerator ?

  • Freddy

    Dale, I take your point about ablative shielding, but surely this is just like a shuttle reentry in reverse ? Wouldn’t the ceramic tiles on the shuttle do the job perfectly well ?

  • Alsadius

    Freddy: The length was picked to be what was needed to attain a nice clean LEO, assuming the exit was at sea level, you neglect air resistance, and you accelerated at 20g. I was assuming it’d involve a gargantuan amount of excavation(5-6x the length of the Chunnel, and through worse material in all likelihood) to get it all underground, more likely than being required to build a structure that could handle those forces above ground. And no, halving the length only doubles the acceleration – v^2 = 2ad. And I was assuming direct feed – accelerating 1 ton at 20g required something around a gigawatt of power, and I was assuming 100% utilization of the accelerator(i.e., one ton every 20-odd seconds). After all, you’re hardly going to invest this much cash unless you’re needing a lot of stuff put into orbit.

    Also, I should disclaim – all these numbers are derived from memory. I’d stand behind my original calculations, but I can’t find them, and I’d really rather not re-do them. Thus, take my numbers as a first approximation, not an exact science.

  • Greg

    While there may be a reduction in explosives (no shell filling or propellant) there will still be just as much chemical fuel (the energy has to come from somewhere) aboard, it will just be in the form of *fuel*. Fuel for the ships engines, that is, which will have to generate the electricity to make all this work.

    Ships will need vastly increased bunkerage, or have vastly decreased (steaming) endurance. I predict a *major* push toward nuclear propulsion, along with railgun armament. It’s really the only way.

  • Of course nuclear, Greg. The obvious solution. I took that as a given :-)

  • Dale Amon

    chuck: I am quite sure the developers know the numbers, but I would feel fairly safe in making a guess that none of them wrote the press release. Someone talked to them and then some non-scientist interpreted it, made sure it was ‘clean’ and wrote it up. Things can happen in that loop, so do not always trust the numbers you get on a technological article. Even friends of mine who are pretty good can get things wrong because the numbers are just numbers to them.

    Dishman: Kittinger exceeded the speed of sound during his fall from 100,000 feet, but as he entered the lower atmosphere he slowed down to a normal skydiver speed. Had his parachute failed he would have impacted at about 120mph.

    Off topic but perhaps interesting: This is why there have actually been surivivors who fell out of airplanes. Very rare, but I know of at least two cases in WWII and a case of a stewardess; there was also at least one case of a survivor of a crashing bomber who jumped out on a snowy hillside before the crash..

    Eric: Not *that* negligible. It also runs rather hot from the air friction even at altitude. If you kill the engines I would bet it will slow down rather quickly.

    CountingCats: You would be surprised how accurate good artillery can be, even at ranges of 20 miles. Now at 200 I will make no claims as no one has done it other than with those old supergun tests of about 30-40 years ago.

    Freddy: The problem with the shuttle is the high wing loading. Because of its mass it has a lot of energy to dissipate on the way down and only has a limited amount of surface area to do it from. Another approach to re-entry is to have a much less dense craft with low aerodynamic loading. This spreads the same energy dissipation over a larger area and thus has a lower max temp. There are other ways to accomplish this as well, such as a skip approach where the energy is dissipated in bursts with radiative cooling breaks in between. As to powered landing from orbit… it is a nonstarter. The rocket equation simply *kills* you.

    The Mass Driver is a device invented by Dr O’Neill I believe and built by his grad students, one of whom was Eric Drexler. It is an elecromagnetic launch track where each ring is an electromagnet that is ‘fired’ just as the bucket reaches it. Even in the lab in the 70′s they got 90G’s out of it over a number of feet. It’s at the heart of the Space Studies Institute baseline plan to allow the economical use of lunar materials to build solar power sats in GEO.

    As to shuttle tiles: No. The shuttle sheds energy at high altitude in atmosphere which barely exists. It’s done re-entry before it gets as low as 100,000 feet. A surface fired projectile starts in sea-level air (or for tranport system on the highest mountain or plain you can find). This is a little bit like punching through a brick wall. It is also one of the reasons they put the first 2 miles of the test track in a Helium filled ‘tent’, to lower the density and reduce the energy loss. Note that Holloman AFB is at a goodly altitude to begin with. Perhaps a couple thousand feet MSL. (I was there in October btw).

    Greg: Although others have talked of this as a 5″ gun replacement, the first time I heard of rail guns for Naval use was as air defense on CVX, the next generation nuclear powered aircraft carriers. They have so much power to spare it isn’t even silly.

  • Given the accuracy they seem to be talking about it looks like it the projectile would have to have some kind of terminal guidance. At that point speed isn’t going to be a big problem if they aren’t going much faster than a run of the mill bomb (as Dale has told us) assuming the fins haven’t been burned off from when it was going quick. What do people think about the electronics though. There are going to be some fairly big currents induced in them (I assume) when this thing fires. Could they make the projectiles guided, or would the electronics get fried when it was fired?

  • a.sommer

    Given the accuracy they seem to be talking about it looks like it the projectile would have to have some kind of terminal guidance.

    My impression is that part of the appeal of these things was the ability to throw relatively inexpensive chunks of metal at target, instead of very pricey cutting-edge electronics…. but the US military isn’t noted for doing *anything* on the cheap.

    IMO, in a point-defense application, taking the shotgun approach would probably be cheaper and more effective than putting em-hardened terminal guidance electronics in each slug.

  • Blacksmith

    Couple comments:
    @Plamus: The ideal nose for the projectile is going to be round to reduce the heating effects there at the tip. I’ve heard that the bullet in these test cases was aluminum which would melt readily in the heating expected from a Mach 7 muzzle velocity if given a sharp pointed tip. It’s the same reason that ICBM warheads have a certain minimum radius for the tip.

    @Freddy: The railgun launcher might not be a bad idea if it can be looped around on itself at a high-enough radius of curvature (miles, most likely), and use a short enough projectile to prevent getting stopped up in the works. If you were to try moving humans that way, one might see a space-bullet brought up to speed in a reduced-pressure loop, then accelerated from a higher “initial” speed in a shorter offshoot track. This would build on other research of Dr. O’Neill’s (from before he began working on space habitats), in optimising and rerouting beams from supercolliders.

    On the other hand, if you’re sending up a capsule that has humans in it, how strong are your E and especially B fields going to be, from the human-interaction and safety standpoint?

    @Greg: DefenseTech or Strategy Page (and I can’t recall which) mentioned last week that there’s a proposal to build all future major surface combatants (DD and up) for the USN with nuclear power, as a combination of more power, better fuel economy, and it’s widely assumed, to make it easier to retrofit whatever final version of the railgun is adopted for use.

  • Freddy

    Dale :
    “As to powered landing from orbit… it is a nonstarter. The rocket equation simply *kills* you.”
    Dale, sorry, but I think we are at cross purposes.
    What I meant was that I know that a great big external fuel tank plus two solid rocket boosters gives a launching shuttle enough energy to reach both orbital altitude and orbital velocity.
    Once in orbit, some lesser amount of energy will be needed to shed orbital velocity only, i.e., to decelerate the shuttle to zero angular velocity relative to the ground.
    So if an orbiting shuttle could connect itself up to a (full) fuel tank that had been parked up there by some other means, it could decelerate itself to zero relative to the ground. It could then make its descent with no more stress than a parachutist.
    I can see the rocket equation killing you if you are trying to carry your own “deorbit fuel” up to orbit in the same craft; I don’t see how it applies when you have some other means to get the fuel up there.

    I think my basic point is that the current shuttle has to be very generalised: it has to handle the very different problems of launch, orbit and return in a single stand-alone mechanism. Once you have multiple mechanisms, you have room for specialisation and substantial improvements.

    This blog software is trying to make me say generalized instead of generalised. Ain’t gonna happen.

  • Freddy

    Blacksmith : “…how strong are your E and especially B fields going to be…”
    Good question – I haven’t a clue what the limits would be before they start interfering with my tender protoplasm.
    Probably more to the point, given that we are talking about exit velocities of the order of miles per second, the switch from the accelerator loop to the offshoot exit track would be an interesting bit of engineering.