Supersonic mechanical propulsion.

General Questions
#1

Just a random question that popped into my ghead while doing a boring job.

Is it possible to launch a missile at supersonic speeds using just mechanical (rather than chemical, nuclear, magnetic etc) energy? I know the question is somewhat vague because there’s no fine line between the types of energy, but basically I want to know if I can make an arrow travel at mach 1.1 using just springs, levers, pulleys etc?

My initial thought was that it’s impossible. A force can only travel along a spring or lever at the speed of sound in that material, so any prejectile can’t be going much faster than the speed of sound in air, and after factoring in friction, inertia etc. it’s going to be slower.

But then I thought that if I rigged up a sled, and put a catpault on the sled, and put a bow on the catapault arm etc. then I could overcome that problem by adding the speed of each component with some clever timing.

So any engineers or physicist got any idea? Has anyone ever manged to get a missile to go FTS uisng just mechanical energy? Could it be done?

#2

Design a whip that releases it’s tip just as it “cracks.”

The actual design is no trivial matter, though.

Tris

#3

The crack of a whip is caused by supersonic travel, so it should certainly be possible. (Though I’m not going to try to dream up a method using this just now).

#4

What about a rail gun?

I’m asking. Don’t know enough about them or physics. Dopers to the rescue!

#5

Even if this were a limitation, the speed of sound in, say, steel is much greater than it is in air.

#6
      • When you say “mechanical energy”, you mean some thing that is powered by yourself “cocking” it, I presume. And when you say “travel”, I am guessing you mean something that is launched at that high of a speed. Since you specifically mentioned an arrow…
  • There are a couple different airguns that are manually-cocked and can push pellets at speeds above the speed of sound. These are spring-piston airguns that do not use external sources of air or any other power; you have to cock the barrel once for each shot. Probably the two most famous models that will do this are the Theoben Eliminator and the Weirauch HW-90. In the US, Beeman sells a version of the Eliminator named the Crow Magnum, and also a version of the HW-90 named the RX-2. Both these guns use a gas-ram for a spring; there are now other mechanical-spring guns that shoot just-slightly transonic but they tend to have long-term durability problems (-so I suppose there’s something to consider).
  • The ability to shoot at such high velocities doesn’t turn out to be of much use for airguns. The pellets use thin skirts that tend to fail at higher (breech) pressures, and the pellets buffet as they decellerate only a few feet from the muzzle and this decreases their accuracy drastically, but there are a few that can do it. I own one, an older Beeman RX, but I tend to shoot it at much-lower veolcities (as the pressure of gas-rams is adjustable, so is the gun’s power level and cocking effort). Here’s a thought though: with the ram pressure all the way up, the airgun takes about 50 lbs of effort to cock, to launch an ~8 grain pellet at about 1200 FPS, which works out to around 19+ ft-lbs. A regular 22LR cartridge has about 120 ft-lbs of power, launching a 40-grain bullet about 1150 FPS…
    ~
#7

I think a rail gun counts as “magnetic”! Certainly gets you above mach 1.

It could be achieved quite easily using a gas gun - pump air into a high pressure container, by hand if necessary, and use it to shoot your projectile down a sufficiently long barrel. But I think this solution is not in the true spirit of the OP.

Another solution is to build up velocity in a rotating system, e.g. a flywheel with your projectile at one edge and a counterweight for balance at the other edge. When you have it spinning sufficiently fast, let the projectile go. The speed of sound in the flywheel material is not a limiting factor (not that it really is anyway, as Mangetout points out) but the strength of the material is. Don’t want the thing flying apart on you! A low-loss mechanical transmission that would allow you to build up high rpms from a mechanical source of energy would take some designing though. Perhaps a big windlass with a tapering spindle geared to the flywheel, and the windlass is attached to a weight hanging off a high cliff? As the cable unwinds from the windlass, it moves down from the thin end to the thick end so the velocity ratio increases as the speed increases.

Spring-driven systems are limited by the fact that the spring tends to accelerate its own parts as well as the projectile (e.g. the limbs of a crossbow) and the energy density stored in springs generally won’t let them self-accelerate to really high speeds. Blake’s idea of stacking crossbows on crossbows would work in theory - this is a way of building up KE incrementally in the projectile. There may be some cunning variations on the same theme.

A dropped weight can have a terminal velocity above mach 1 if it’s dense and streamlined. A lead “bomb” at the edge of a HIGH cliff could exceed the speed of sound by the time it hit the ground. There’s a bit of a shortage of nine-mile high cliffs.

#8

I’d have thought some kind of trebuchet (essentially a whip with a tip that detaches, as Triskadecamus and nanoda Mentioned) would be better than a flywheel, because of the issue of keeping the whole thing together while you get up to speed; a trebuchet/whip arrangement could actualy be designed to be sacrificial - the projectile is launched supersonically, destroying the launcher in the process.

#9

Quite possibly! I don’t know enough about the supersonic whipcrack to judge - the volume of material that actually exceeds the sound barrier, the direction it’s going in etc. I’d be worried that you’ll end up trying to detach something rather small and low density in a random direction.

Rather than have an actual flywheel, you could have symmetrical slings tied to an axle with drawn steel wire. I’ll get the trusty envelope out at some point and run some numbers to see if it’ll hold together, at least in theory.

Alternatively, you could have a horizontal Stonehenge slab, and a vertical stonehenge slab standing on one end of the horizontal one. Put a well-greased lemon pip where the two meet and then topple the vertical slab to squirt the pip into orbit!

[Mainwaring]I think we’re getting into the realms of fantasy here, Jones…[/Mainwaring]

DougC’s examples show that it is possible to have a hand-held, hand-powered weapon firing supersonic ammo, but it uses pneumatics to transfer energy from the spring to the pellet. I’m still hoping that there may be a cunning way to achieve this purely mechanically, but I’m stumped at the moment.

#10

Air pressure is mechanical. Although I know what you mean.

#11

But would the sled get its energy? There is no such thing as mechanical energy… machinery only transfers existing energy. Without some kind of energy source, springs, pulleys, and levers tend to just lie around creating fire code violations. And your OP looks like it intends to rule out all energy sources. Maybe you meant human effort… it’s a more interesting question but it is a form of chemical energy. Tell us exactly what energy sources are allowed and maybe you will get better answers.

#12
      • I got to idly discussing this with another airgun geek online–wether it would be possible to make a mechanical supersonic BB gun.
  • The idea I had was similar to a spring-piston BB gun: these use a piston in an air cylinder, cocked back against spring tension. When you pull the trigger the piston rushes forward, and the air in front of the piston gets pushed into the back-end of the airgun’s barrel, pushing the pellet out as you’d expect.

…The BB gun idea I was pondering was instead of an air piston, use a LARGE (1? 2?-inch?) ball bearing that is pushed forward by spring tension, and a chain of progressively smaller ball-bearings, until the last one in the line is a regular-BB sitting in the back end of the airgun “barrel” (-since there is no air-propulsion, the barrel would not be a cylinder so much as perhaps just 3 pieces of metal rail or stiff wire, to guide the BB in a particular direction while not suffering from a vacuum behind it, as a regular cylindrical barrel would).

  • Problem is though I did not know the math to do the transfer-business; would only the last ball get launched? And I dunno how much energy you could transfer across two bearings of different size. I am guessing from the desktop-hanging-ball toys it would… maybe. All the balls on the desktop toys are the same size you see, I dunno if this would matter or not. I do not have the facilities to construct such a thing however. And I suspect that all the balls in the line would have to be ball-bearings (hardened steel) or else the weakest one is going to get pounded…
    ~
#13

This is my first post. As a longtime lurker, I specifically registered so that I could post to this thread because it happens to go along with what I was thinking of writing for a short story. When the OP asked about firing a missle with only mechanical energy, I interpretted him to mean that this missle would be sent UP.

I’ve read other posts on here about escape velocity and the energy required to get something into space and forever leave this planet, but what I really want to know is whether this is possible to actually engineer something that could send a projectile (something as small/light as a BB pellet perhaps?) into space using ONLY human power. 100 people jumping on a see-saw ain’t gonna cut it, but I have to believe that this could be accomplished without breaking the bank or putting people in danger. What are the options? :confused:

#14

Sure. Use pedal-driven generators to generate electricity, and make hydrogen and oxygen by electrolysis. Use mechanical pumps to compress the gases, and put it in a rocket.

I don’t see any other way. Even if you launch a projectile at escape speed, it would immediately lose kinetic energy due to friction with air. This is worse for smaller projectiles (larger surface-to-volume ratio, which means greater deceleration).

#15

Very large compressed gas gun could do it (I was reading about one of these ‘super guns’ a few years ago that used compressed gas to fire projectiles at something like a mile per second). Usually they use energy to compress the gas, then the gas goes through a series of smaller diameter tubes to further increase the pressure. I suppose a mechanical means could be found though to compress the gas (maybe a large weight). Store the energy in a chamber, fire a discarding sabot type projectile and bobs your uncle…supersonic projectile with mechanical means.

-XT

#16

Okay, maybe I was thinking of something more visual and fun for this human-powered launch… something that doesn’t require storing any potential energy (like a gas chamber) but is still possible.

Okay, bear with me for using some fanciful imagination:

How about a town getting together and using 10,000 blow guns all linked together with tubing to shoot something out the top of a home-built chamber?

Is there a way to “sequence” several thousand slingshots so that all their energy is harnessed to work together at the same time?

Can you sequence those see-saws that acrobats use to launch people in the air… even if it’s just to launch a small object into space?

If you had 1,000 people on stationary bicycles winding up a “super-coil” that would fling something into space, would it work? (okay this breaks the “no-using-potential-energy” rule above, but I like the image!)

This is going to be a love story, but the science (which is only a small part of it) should be real… and funny. :slight_smile:

#17

Well one “simple” way to send something up very fast is with hydrostatic pressure. First, build a long vertical tube in a deep oceanic trench. Then slowly pump the water out. This can be done with human power. Place your projectile in the bottom of the empty tube. It should form a water-tight seal, but the still be able to easily slide. Finally, open a valve below the projectile, and the water pressure will force it up extremely rapidly. For better results, evacuate the air from tube as well (put a disposable cap on top).

Obviously, there are a lot of engineering details to be figured out, but the basic principle should work.

(This is not an original idea, but I forget who came up with it first.)

#18

Thanks, matt. I couldn’t remember how rail guns were supposed to work.

Fun thread!

#19

Brilliant! You’re wasted here. You should be out there solving the world’s problems!

You don’t get to only launch the end ball - I think all the balls emerge from the end of the thing, but you do get a velocity increment from each collision. The collisions have to be elastic, which means that your balls must not permanently deform or dent. I think cylinders would be better than balls, to increase the contact area during the collisions. And you may have to make them out of diamond. But I love the idea!