Secrets of the Mystery Gun that Shelled Paris | Hacker News Reader

Secrets of the Mystery Gun that Shelled Paris

blog.modernmechanix.com

Be sure to read -DOUG-'s comment at the end of the article!

I was snooping around that web site after reading the fascinating article, and found this article in the 'Scary' section. Food for thought no?

http://blog.modernmechanix.com/2008/06/05/the-computer-data-...

I love reading about the historical future of technology. Post it and I'll vote it up!

Sputnik's perigee was only about 155 miles. Could this gun have shot a shell into orbit if they'd made it sufficiently light?

I programmed a simulator that asked this very question a few years back just for kicks. I was wondering what types of things might help us reach low earth orbit on the cheap.

The answer is no -- any kind of ballistic shot needs some kind of tangential thrust applied at some point to achieve orbit.

You can shoot something really far with a cannon and still it hits the ground. It's possible to shoot and hit things on the other side of the earth like this, but you're going to have a huge apogee!

Any kind of ballistic shot? That doesn't seem right. If you can make the initial impetus arbitrarily powerful and the projectile arbitrarily light, surely at some point you cross the threshold of being able to get it into orbit.

obviously you are right, that's why we can see Earth as a blue planet from the space - the photons bounce of the ground and leave the atmosphere without tangential thrust. But theoretically if we were the only body in space they would eventually fall back. It's needed only when you want to escape the gravity field by exceeding the escape velocity.

Thrust for massless photons?

Photons escape earth's gravitational pull because the gravity is not large enough to bend space to collapse the photon back onto the earth.

One orbit. Consider the projectile just as it leaves the barrel, if it is in orbit then this point is on its orbit. It will return. The earth will have rotated the gun away, but the projectile is still going to come back into the atmosphere where it will lose too much energy to continue.

Any useful orbit needs to be above the atmosphere and would have to be shot from there.

You could use a secondary nudge of some sort to round out the orbit once it is in space and keep the projectile up, but making such a device that will survive the initial acceleration is unlikely.

What if we took advantage of the drag that the atmosphere exerted on the object as it ascended to change it's orbit or to move it into a useful orbit?

Edit: Never mind, that would only work if we wanted to lower the orbit.

Lowering the orbit isn't the problem, if it's already fast enough. The problem is that the end of the velocity change is still going to be in the atmosphere.

I think this can get you most of the way there, using some form of atmospheric steering. Best case you could get your nearest approach tangent to the highest spot of atmosphere. You still need a bit more to avoid the atmosphere entirely, but your are mostly there.

"making such a device that will survive the initial acceleration is unlikely"

Bear in mind that using an electromagnetic launcher you could accelerate (relatively) gradually over a length of track, the so-called "Mass Driver" concept.

The friction with the air at its highest speed (and where the air is densest) would still be a problem.

Which is why you run it up the Himalayas, right? :)

Nope. At least where I see from the simulator. Either it's gone completely -- escape velocity -- or it falls right back down. I think the problem is that orbiting bodies are continually "falling over the horizon", ie, there is a high degree of speed parallel to the ground underneath them for orbiting to work. (Or a low speed with a small pull of gravity, etc. But always some significant forward motion)

You can do some interesting things with packages that have thrusters on them. If I remember correctly you could have a 300-400km railgun shoot boxcar-sized payloads into orbit with very little additional thrust. It's a helluva cheap way to send mass into space -- once you spend a couple hundred billion for the railgun setup (which is still a good deal, btw)

The arguments about why it wouldn't work to shoot a projectile into orbit sound convincing, so I suspect they're right. But what actually happens as you launch a projectile at a 45% angle with increasing velocities?

I couldn't find the simulator, but as I recall, from any point on the surface, whatever initial velocity you give a projectile will impact the surface at some time later, if it doesn't achieve escape velocity.

A good way to think of it is this way: if you were able to point the cannon tangential to the ground, ie horizontal, and you were to fire the projectile at exactly orbital speed for that elevation, then you would achieve orbit at ground level. If, however, you elevate the cannon at all from horizontal, you create an oblong orbit that intersects the surface at some point (even if the orbit extends thousands of miles up into space) That's because all orbits are elliptical to some degree. No matter what the angle or power, you're just creating an orbit with some degree of stretch that eventually comes back around to where you launched. A 45-degree launch from one place would just be the same as launching at 90-degrees from another place on the globe.

BTW, this makes for a nice hobby program to write over the weekend. I ended up adding angle, initial velocity, and projectile thrust. I also added scaling where you could either go very small, like a model rocket launch, or zoom out to include the moon. I was very surprised at the sizes of the earth and moon and the distances between them.

Probably not, no. The biggest guns (see Wikipedia on Project HARP) can get a payload to 100 mi, but we can't yet combine that with the horizontal speed necessary to orbit. The problem (I gather) is the atmosphere. Sputnik was traveling along its orbit at about 18k kph, 10 times the velocity quoted for this shell, or about Mach 25 at sea level. Your muzzle velocity will have to be much higher to get it outside the atmosphere going that fast, and the shell will have to withstand ~30 seconds of shuttle-reentry-like heating and buffeting.

Possible, definitely, but not with this gun.

Is there some practical limit to the muzzle velocity of a gun?

Not really, no. Especially if you use barrel-less electromagnetic designs. The Navy is testing a rail gun that fires 3.2 kg at 9k kph.

Again, it's the acceleration and heating on the projectile that I would worry about. Then again, I think Project HARP found that electronics survived the firing better than they expected in their fairly crude sand and epoxy suspensions.

One strategy would be to conventionally launch a big satellite factory robot with the most fragile parts (fuel, solar cels, glass instruments, ...) and then do cheap gun launches with the more durable parts at your leisure.

The issue with a gun to get things into orbit is that you need to provide enough force right at the beginning to get into space. That would destroy most anything. So you could shoot something into orbit, but it will be a largely mangled hunk of metal by the end of it.

If I were in 1910, trying to get something into space, I'd use a hot air balloon to go a few miles, then shoot a gun. Ideally, it would be done at the equator, where the tangential speeds are higher and getting into orbit is easier.

We can deal with mangled hunk of metal problem in version 2. For now I'm just asking whether one could get anything into orbit by shooting it from this gun.

Step 1: figure muzzle velocity required to get into orbit.

Step 2: make a basic model for air resistance, which feeds back into velocity

Step 3: does that feedback converge or diverge? If divergent, impossible.

Step 4: make that material

I don't know enough aerodynamics to model high speed air resistance well.

The way we handle it coming back to earth is to add a layer that will be shed in heat to protect the object. That adds mass.

No, Daniel's right. Or the true answer is yes, any time you throw something up into the air, it's in orbit. It's just that the orbit intersects the Earth's surface. Orbits are (to a good approximation) closed ellipses, so anything shot from the Earth's surface will come back to surface level no matter how fast you shoot it.

Another way to think about the problem: An orbit is determined by both the energy and angular momentum of the object. (A circular orbit has the highest angular momentum of any orbit with the same semimajor axis.) From a point injection of these, like from a gun, you can get enough energy but never enough angular momentum to get the perigee higher than the Earth's surface. For that you need some tangential thrust (at a location that's not the apogee).

The most energy-efficient way to get into orbit consists of two tangential impulses. The first one raises the apogee to the desired height, the second one is injected at apogee to circularize the orbit. (Though it's strictly only applicable for changes in orbit, not from launches from the ground: http://en.wikipedia.org/wiki/Hohmann_transfer_orbit)

If I was setting out to do it, I'd think about a three part system. First would be the gun on the surface, the second would be a launch vehicle, and the third would be a satellite in orbit to act as a target. The gun fires the launch vehicle at the target. The satellite (I picture it as a donut, with the projectile aimed at the center) uses an electromagnet to "fix" the launch vehicle's orbit. Aside from whatever physics I am ignorant of, I think the big question mark here is power as I would expect the draw of the magnet to be beyond what you could plausibly provide with solar panels. It's also questionable how well this would scale. It doesn't seem like something you would ever think of using to launch crewed vehicles or things like the Hubble, but if the marginal cost of your launches is low, it seems like individual cargo capacity is less important. You could launch raw materials, small parts and assemblies, and maybe even things like human consumables.

The influence of magnetic forces decays at r^2 in distance, making any sort of magnetic tractor beam extremely unfeasible.

I'm having trouble explaining it in words, but I'm thinking of it as more of a funnel you aim at than a discrete beam.

See my note above - the problem is the need to circularize the orbit so the projectile doesn't just hit the other side of the Earth.

EDIT: I just had a thought, if you have a facility in orbit, you could use a gun to shoot perfectly vertically to the orbital height, then capture the projectile before it fell back. Nope, that still wouldn't work - The capturing vehicle would need to add velocity to the projectile to raise its velocity from the eastward component of the radial velocity of the launch site to the orbital velocity.

You've described a Sky Hook. It's a great idea: http://en.wikipedia.org/wiki/Skyhook

You're right that you don't want to accelerate it too much when caught.

One of the coolest applications of sky hooks is _landing_ when there is no atmosphere.

You extend or contract the tether to decrease or increase the angular momentum, and balance it with the decreasing potential energy of landing. You can land softly without rockets.

Hard to deal with recoil issues when firing from something that is not anchored. You'd have to have a pretty massive balloon.

The fundamental problem is that even with enough velocity, and it would need to be substantially higher than even the Paris gun's, is the need to circularize the orbit, otherwise it will just dive back down to ground level on the other side of the Earth.

There's some more information here too: http://en.wikipedia.org/wiki/Paris_Gun

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