16
$\begingroup$

Lets say the advanced civilization never discovered rockets and for whatever reason never will.

Is it possible to create some kind of mechanical device to launch a 90kg person to the moon from earth? I would like it to be a massive catapult but It does not need to be a catapult (it can be some kind of electromagnetic launcher if it has to); however, it needs to be some kind of launcher where the person is given an initial velocity but the velocity cannot be sustained after it is launched (the object cannot burn fuel/energy to propel itself like a rocket after it is launched).

Assume the person launched is invincible, we don't need to worry about oxygen, safely landing, we also don't need to worry about the extreme forces on the person during the launch.

Edit: The reason why I say invincible "person" is because it makes the problem more fun. I am not interested in the survivability of the projectile. However, I am interested in the following:

1) The size and specifications of the device required to launch a human sized (90 kg) projectile to the moon.

2) If you build the launcher on the ground you will need to overcome air resistance. In that case I would like to hear about the initial velocity required to get to the moon while getting through our atmosphere. If the launcher is elevated, you will have to overcome the problem of elevating the launcher.

3) The method you come up with has to be theoretically possible but it does not necessarily have to be something that engineers can realistically build.

$\endgroup$
  • 29
    $\begingroup$ Julius Verne did the calculations for you in "From the Earth to the Moon" ;) $\endgroup$ – Renan Feb 19 at 21:52
  • 1
    $\begingroup$ This is a Google Search. Here is one good result: astronautix.com/j/julesvernemoongun.html $\endgroup$ – user535733 Feb 19 at 21:58
  • 1
    $\begingroup$ @Renan The only thing that doesn't really make much sense is the way g-forces are handled. It states the person is protected by hydrolic dampeners, but most structures (barred meta-materials) would fail with such a violent launch I reckon, let alone the poor sod having to ride the thing. $\endgroup$ – Hyfnae Feb 19 at 22:26
  • 3
    $\begingroup$ See Isaac Arthur's "Upward Bound" playlist on YouTube; he discusses mass drivers, space towers, space elevators, Lofstrom Loops, balloon/blimp launches, and more. Although without Newton's Third Law / rocketry, most of these are not much help. $\endgroup$ – TessellatingHeckler Feb 20 at 6:25
  • 1
    $\begingroup$ So... the person will just stay on the moon forever? $\endgroup$ – NotThatGuy Feb 20 at 15:33

11 Answers 11

28
$\begingroup$

To get to the moon you need to reach a speed of at least 7 mi/sec.

What about giant catapults?

A catapult will never reach anywhere near that kind of speed. It does not matter how long you make the arm, no man-made materials can survive anywhere near the sort of torsion that this would require.

What about railguns?

The world's strongest military grade railgun is the size of a bus and to maxes out at about 14.7MJ of kinetic energy. If you want to go with the absurd notion of just flinging an indestructible human body to the moon, you could put a person in a 500 lb metallic casket and fire them with a railgun about 1400 times as powerful as this. Figuring out the best way to scale this gets tricky since exact sizes are hard to come by, but my best estimate is that you are looking at a barrel that is about 10ft in diameter and 2.6miles long. This could be built on the side of a mountain and fired at just the right moment when the cannon comes into alignment with the moon; however, this is not a proper moon mission.

A more likely payload would be something on the scale of the Apollo Lunar Module. This would require something ~28,843 times as powerful as the strongest railgun in the world. My best guess here is that you are looking at a barrel that is about 25ft in diameter and nearly 9 miles long. Because the round is so heavy you can not use a curved firing tube to fire at a mostly flat plain and then turn it last minute to a more upward trajectory; so, even building up the side of a mountain may not work because you probably will not be able to get the firing arc you need. Really you could make the barrel even thicker and shorter and maybe fit it on a mountain, but the thicker you make the barrel the more it must contend with material strengths and heat dissipation, and even at this scale, I find this railgun's feasibility to be rather dubious. Also... you are still accelerating at way over human survivable speeds.

So, I'd call railguns feasible as per the OP, but probably not for real world applications.

What about light air guns?

These are typically faster and more efficient than railguns making them one of the fastest known cannons to modern technology. Unfortunately they max out at about 4.4mi/sec (the max expansion rate of compressed hydrogen); so, still no good.

What about Enhanced Hypervelocity Launchers?

I just made that word up, right? ... nope. SEE HERE These guys can reach speed of about 10mi/sec. Fast enough to get to the moon... maybe. While they fire fast enough to get to the moon, they have never fired anything bigger than one gram; so, they would probably also have critical material failures when scaled up to fire a man sized slug, much less a proper lunar capsole.

What about Pascal-B?

Alright, the possibility of launching a man or lander sized object at escape velocities using a nuclear explosion is doable if you REALLY want to stick with the idea that your astronaut is indestructible. The Pascal-B nuclear test fired a 2000 lb chuck of steel at 41 mi/sec. This is more than adequate to send a man to the moon, but we are talking about having him literally sit on a nuke! In terms of material integrities, you're probably better off sticking with a giant trebuchet.

In short, using modern technology as defined by the OP railguns might do it if they don't structurally fail due to square-cube law type issues, but doing anything worth doing in real life is pretty much a no go.

$\endgroup$
  • 2
    $\begingroup$ Pedantic: wouldn't the railgun actually be subject to a linear/square law? $\endgroup$ – Matthew Feb 19 at 21:56
  • 1
    $\begingroup$ "There is also the issue of air resistance in the barrel" - not if you take the air out (and ensure that at least the open end of the barrel is in space). The "person" will need appropriate protection against a vacuum for the journey to the moon anyway. $\endgroup$ – Steve Feb 20 at 7:53
  • 16
    $\begingroup$ One additional issue other than cannon integrity is that going at 30x the speed of sound through 1 atmosphere tends to damage the cargo. As in, "turn it into a fine mist" type of damage. Even if it hadn't been pancaked by the launch system just prior. $\endgroup$ – John Dvorak Feb 20 at 8:36
  • 8
    $\begingroup$ For potential top speed, you can't beat the nuclear potato cannon. Estimated takeoff speed: 41 mi/s. Probable fate: burned up on the way out of the atmosphere. But who knows, maybe it's on some ridiculous elliptical orbit around the Sun somewhere? $\endgroup$ – Sebastian Lenartowicz Feb 20 at 12:02
  • 1
    $\begingroup$ What are mi/sec? $\endgroup$ – Michael Feb 20 at 12:36
10
$\begingroup$

I'm going to ignore that your subject talks about a "person", because the body of your question basically says to do that.

It sounds like you want a space gun. The article has some information on plausibility, but the short version is that, if you want to reach the moon, or even just a stable orbit, you have issues with both the acceleration needed (but we're not launching a person, or at least, we're launching someone who is magically immune to all the bad stuff that happens to the human body under extreme acceleration) and with the air that's in your way. The latter is the big problem with imparting all your velocity at once, as opposed to taking engines up with you.

...and even if you get to the moon, the sudden stop at the end is going to be killer. Literally.

Also check out the "See Also" links in that article.

$\endgroup$
  • 2
    $\begingroup$ So, basically, if the person is invincible, then yeah it's totally possible. $\endgroup$ – boxcartenant Feb 19 at 21:06
  • 2
    $\begingroup$ @boxcartenant Not necessarily. No one has ever successfully made a space gun. As of today they are still in the realm of science fiction. $\endgroup$ – Nosajimiki - Reinstate Monica Feb 19 at 21:21
  • 1
    $\begingroup$ BTW, the way to overcome this problem is to make a really, really tall launch tube and somehow evacuate all the air from it. (If you can cap it, and jettison the cap just before the payload/ship/whatever gets to the end, that might work.) While obviously you'll need to make your payload/ship/whatever really well streamlined, at some point, the air simply can't get out of the way fast enough. (Mandatory reading: what-if.xkcd.com/1) $\endgroup$ – Matthew Feb 19 at 21:54
  • 2
    $\begingroup$ @Matthew you don't need to cap it if it pokes above the atmosphere. All you need to build is a 100km tall tube that can withstand high vacuum on the inside and supersonic winds on the outside, plus whatever dynamic forces get exerted on it during launch if it can even survive until that point. $\endgroup$ – John Dvorak Feb 20 at 8:45
  • 1
    $\begingroup$ @JohnDvorak, actually, "how tall does it need to be to not need a cap" is an interesting question. Presumably at some height, the pressure is low enough that continuous pumping can remove the gas that seeps in through the top. You might even be able to recover helium this way, which would make the thing useful for another reason. Of course building it is going to be quite the challenge... $\endgroup$ – Matthew Feb 20 at 15:44
5
$\begingroup$

Linear accelerator launchers (based on a combination of maglev and linear induction motors, usually) go back to the 1950s in fiction (Heinlein's Starman Jones from 1953, and The Moon is a Harsh Mistress from 1966). There are significant limitations in using these from within an atmosphere, but there are ways around them (route the rail up a very high mountain with an evacuated tunnel for the lower elevation part, for instance -- there are multiple mountains fairly near the Equator that exceed 7 km, above 90% of the atmosphere). The advantage of such a setup is that all the launch "fuel" comes from ground-based power sources (the electrical grid, supplied by fission, fusion, or solar/wind/tidal power), so the launch vehicle need only carry fuel for in-space maneuvering, and the acceleration can be within human launch limits (unlike a true "space gun" which is exceedingly difficult to implement with survivable acceleration).

$\endgroup$
4
$\begingroup$

The atmosphere of the Earth tends to destroy fast-moving objects. This is related to Newton's depth approximation, where we find that a projectile tends to penetrate through approximately its own mass of material before it stops. Newton fired cannonballs into sand, and found that they penetrated approximately as far as the column of sand that weighed the same as the metal ball. Faster impacts merely made wider craters. https://en.wikipedia.org/wiki/Impact_depth

The same holds true for atmospheres. Venus, for instance, has a minimum size of crater due to its thick atmosphere. An approaching meteor is slowed and then blows up long before it reaches the Venusian surface. https://www.universetoday.com/22521/craters-on-venus/

If you're flinging something straight up with acceleration only at ground level, you have to consider the total weight of the atmosphere above the object. Every square centimeter at sea level is pressed down by 1kg of atmosphere above it. If you managed to fling something really fast straight up, it would need to be extremely dense in order to complete the trip. A very long tungsten or DU rod might make it.

There are ways to reduce atmospheric resistance : a sealed vacuum launch tube, or a mountaintop launch site, or using rockets to accelerate over time instead of all at once.

$\endgroup$
1
$\begingroup$

It's technically possible, but there are a few difficulties with just shooting yourself straight there. So, as a baseline, you need about 15,000 m/s (meters per second) of ΔV for a traditional rocket to reach the Lunar surface. That number comes from the below slide from a 2012 presentation by NASA.

Slide from NASA describing orbital delta-V costs

I believe this includes deceleration burns and orbital maneuvers, so we can probably subtract a few thousand m/s. Lets just ballpark it and say 12,500 m/s ΔV. That means you need to launch a pod from the surface of the planet, directly at the moon and, even more important, precisely at the moon. You can't mess your aim up, as you can not make in flight course corrections! It's possible to make these calculations, but you need to know many different variables. Things like local weather, altitude, time of flight, velocity of the Earth, velocity of the Moon, and so on. You need to precisely aim your "space gun" so that when the package reaches the moon, the moon will have reached that point in space in the time between launch and arrival. Don't forget that there will be travel time for the package, and the moon will have moved out of it's previous location during that travel time.

There is also the technology aspect of it. Reaching escape velocity in the span of less than 1-2 seconds is something that is still currently not possible. From a quick google search, the fastest rail gun in the US Military shoots shells at 7,500 mph, which is equivalent to 3352.8 m/s. 3,352 m/s is only about a quarter of the velocity that we need to reach for our theoretical space gun. With the best tech that we have in 2020, we have rockets flying to mars versus guns that can barely reach orbit. Will the technology eventually get there? Sure, it's possible, but there would have to be some serious reason to dissuade scientists from researching rocketry, if the only goal is to get to the moon.

In short, it's theoretically possible, but it seems implausible that some civilization would go through the effort of developing some sort of space gun system when rocketry is just much easier.

As a note, you mentioned to assume the package was invincible. If this wasn't the case, then any human subject to these g-forces would be instantly killed, probably turned into a pancake at the floor of their ship.

$\endgroup$
1
$\begingroup$

Yes, a mechanical device called a Slingatron was proposed for space launches. I wrote about it some years back; the developer (Derek Tidman) felt that there was a huge bias in the establishment against this approach and he never made much progress with itSLingatron space launmcher.

However, a starup called SpinLaunch are now going with the same idea, so we may yet get to see if it works. If Tidman is correct, space launches may be possible using this type of device and fairly basic technology.

$\endgroup$
  • 1
    $\begingroup$ Interesting, didn't know this type of launcher had been seriously proposed. It goes back in science fiction to at least the 1937 Neil R. Jones story, "The Twin Worlds" - one of the Professor Jameson / Zorome series. Various artwork over the years has depicted the story and at least one illustration looks similar to the SpinLaunch art work (though the oldest art looks more like a giant ferris wheel). In the story it hurled capsules containing aliens and zoromes into space - biological humans were long extinct in this series so no mention of whether they could have survived a launch. $\endgroup$ – steevithak Feb 20 at 17:22
  • 2
    $\begingroup$ The Slingatron is expected to product 60,000 G on its payload: space.com/23015-slingatron-reusable-launch-system.html. Who wants to sign up for the first launch? $\endgroup$ – Lawnmower Man Feb 20 at 20:22
  • $\begingroup$ In order to lower the centripetal acceleration to 3g, you need to give the sling a 1,600 km radius. Good luck with that. $\endgroup$ – Lawnmower Man Feb 20 at 21:13
  • 2
    $\begingroup$ lol now I want to donate to the Slingatron on the hopes that they'll enter Punkin Chunkin $\endgroup$ – WernerCD Feb 20 at 22:31
  • $\begingroup$ This does not sound like a well thought out project. A good quality aircraft cable breaks under its own weight at about 60,000 feet in a 1G environment chain-cable.com/cables.htm. At 60,000G, anything longer than a foot will rip itself apart. Using omnicalculator.com/physics/centrifugal-force, we can see that this device would need a 700ft radius to reach the desired speeds at "only" 60,000G meaning you need cables 700 times as strong as steel to make it work. $\endgroup$ – Nosajimiki - Reinstate Monica Feb 21 at 21:45
1
$\begingroup$

The key problem is the amount of acceleration that your passenger can tolerate. You need to leave the atmosphere at as speed of at least 12.5km/s to be able to pass through the moon's L1 and enter its sphere of influence.

Lets say, your passenger can tolerate a 20g acceleration (approx. 200m/s^2). With this top acceleration, you need to sustain that acceleration for 12.5km/s / 200m/s^2 = 62.5s. The problem is, during that time, the capsule with the passenger will have traveled 1/2 * 200m/s^2 * 62.5^2s^2 = 390.625km. That's a damn long railgun/canon/whatever. I guess it's only feasible to build in the form of a maglev launcher that operates inside an evacuated tunnel.

Even if you increase the acceleration to about the highest a human has ever survived (deceleration at a car racing accident), which is 214g, you get an acceleration time of 12.5km/s / 2.14km/s^2 = 5.8s and a barrel length of 1/2 * 2.14km/s^2 * 5.8^2s^2 = 36.5km. Note, that while this extreme deceleration in the car accident was survivable with all the racing car safety measures in place, it didn't last very long. Only about 0.1s. What is survivable for such an extremely short time is not likely to be survivable for about 60x that time at all.


Of course, if you decide that you can make do with, say, a 100km evacuated barrel maglev launcher, you still have the problem that your passenger suddenly hits the air at the exit of the tunnel. If you just put the exit on a mountain, 8km above sea level, you still have about 35% of the sea level air density. And about 35% of the atmosphere left to pass through. That's too much. It will decelerate your projectile hard, the air molecules hammering into the projectile will quite instantly heat it to insane temperatures, and you won't reach the moon today. (You didn't use your convict for this test, did you?!?)

A much better altitude to have the exit would be at about 20km above sea level. This would leave you with about 4% to 5% remaining air pressure and atmosphere to pass through. Still a lot, but I guess that's about where the atmospheric deceleration and heating would reach manageable levels. Weather balloons routinely achieve such hights, so I guess it is theoretically doable to extend your maglev launcher to such a hight by supporting the barrel with helium filled balloons. Those balloons would be gigantic, though. And, making sure that the barrel remains straight would be a superb engineering challenge. I guess, building a Saturn V sounds like playing with lego by comparison...

$\endgroup$
0
$\begingroup$

To work out the heating from air resistance, think of the column of air above you and imagine being hit by it at orbital speed.

Pressure at sea level is about 100kPa, so there's about 10,000kg of air above every square meter. Orbit speed is 11km/s, so there's 600GJ of kinetic energy to absorb.

The heat capacity of steel is about 600J/kg k, so a million kg of steel would be heated to 1000 degrees by that much energy.

If you launch from the top of Everest, it's only a third as much. And you could just squeeze a person into a capsule with an area of a tenth of a square meter, so you'd only need a few hundred tons of heat shield.

Of course, real heat shields don't work like that, the surface gets much hotter, turns to plasma and ablates away. If it can reach 10,000 degrees, you'd only need a few tens of tons.

$\endgroup$
0
$\begingroup$

It can be some kind of electromagnetic launcher if it has to.

You are looking for a Space Fountain elevated Rail Gun.

Space Fountains technically are a 'rocket', just of another format. Rather than relying on combustion of fuel in a rocket engine to propel hot gas out the back, you instead rely on throwing around material that can interact with an electromagnet.

Throw material 'up' from a launcher solidly fixed on the ground, then have a section of your tower 'catch' it, and throw it back at the earth to propel the tower section up into the air.

In short, they are rockets without using rockets, and tied to plumbing on the ground.

But they can be powered by reusable materials, and connecting electrical lines can be easier and more efficient than trying to plumb highly flammable fuel and oxidizer lines on a 'questionable' structure...


To look at it another way, this is effectively a fancy series of rail guns used to lift and fire another rail gun in an attempt to avoid excessive energy loss on the final projectile rather than attempting to punch it through the air.

$\endgroup$
-2
$\begingroup$

From the other answers, any sort of gun/catapult seems to need such an enormous initial speed (muzzle velocity?) that it would be impractical. I have no idea of the physics, but if you were able to use (say) the sides of Mount Everest as the base of your "gun", maybe you would have a slight height advantage and a lower density of air to get through. I'd imagine the moment that you fire the device, the forces on the passenger would still be far too great to survive, and that's before you've dealt with any of the other issues such a device would have (ice being a likely one on Everest!).

Along similar lines, but maybe with less of the issues may be a Space Fountain...?

$\endgroup$
  • 3
    $\begingroup$ "I have no idea of the physics," kind of invalidates your whole answer, doesn't it? $\endgroup$ – Zeiss Ikon Feb 20 at 13:01
  • $\begingroup$ @ZeissIkon - I'm sure if someone spent the time to work it out, it would be as unworkable as the other gun/catapult ideas, but depending on how this information was to be used, it might look like a more feasible solution in a suitably fictional world. The real answer here was the Space Fountain. $\endgroup$ – Ralph Bolton Feb 20 at 14:25
  • $\begingroup$ But that and its relatives (Launch Loop, for instance) aren't really a catapult; they don't even work like one. A space fountain is mainly an alternative way to build a space elevator (one that doesn't, in theory, require an unreasonably strong material like carbon nanotubes). $\endgroup$ – Zeiss Ikon Feb 20 at 14:51
  • $\begingroup$ You need a 600km long accelerator to reach 11km/s at 10g: x=v squared / (2a) - from high school physics. $\endgroup$ – Robin Bennett Feb 20 at 14:56
-2
$\begingroup$

I think you should definitely consider a trebuchet over a catapult. They are far more superior siege engine, and able to launch a 90kg projectile up to 300m distance.

The moon being at 384 400 000 meters from the earth, you just need 1281333.34 time the power of a traditional medieval trebuchet. I would recommend:

  • build 1281334 trebuchet and connect them together to accumulate their power.
  • build a single trebuchet, simply 1281333.34 time bigger than a traditional one.

Good luck I you pursue the catapult way, I have no doubt it is a dead end, like everything catapult really.

$\endgroup$
  • 1
    $\begingroup$ Welcome to the site. I think the trebuchet vs catapult insight is helpful - but I'm not sure the specifics "build 1.2 million trebuchet" vs "build a trebuchet 1.2 million times bigger" necessarily equate to a realistic solution. Can you elaborate? $\endgroup$ – Zxyrra Feb 20 at 16:20
  • $\begingroup$ I'm convinced that with some clever system of mechanical advantage and pulleys, one can concentrate the power of his trebuchet army to send a single projectile 1.2 millions times further than a single trebuchet. (you can send 1.2 millions time further a single projectile, but not 1.2 millions projectiles at 300m) At the same time, virtualtrebuchet.com/#simulator allows us to see that building a trebuchet 1.2million time bigger may be a poor solution, but using a catapult doesn't seems a much more realistic solution. I mean, not even a single counterweight? $\endgroup$ – tmos Feb 20 at 16:41
  • $\begingroup$ Trebuchets can work together to fire a greater mass, but the max velocity is limited by the outside speed of the arm swing which is not compounding when using trebuchets together $\endgroup$ – Nosajimiki - Reinstate Monica Feb 20 at 18:31
  • $\begingroup$ Perhaps mimicking the functionality of the ground-portion of the launch-loop/Space-Fountain by firing trebuchets (or at least their projectiles) at the payload would be effective? $\endgroup$ – Ruadhan Feb 21 at 8:51

Not the answer you're looking for?Browse other questions tagged or ask your own question.