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I should say first that I don't believe this is a feasible launch method, otherwise NASA and other space agencies would be using it by now.

It's based on this BBC news story Saddam Hussein's Supergun but, luckily this monstrosity was never completed or even fully tested.

enter image description here

These giant cylinders are one of the few remaining pieces of a contender for one of the most audacious pieces of engineering ever designed: a “supergun” called Big Babylon, which could have fired satellites into orbit from a 156m-long barrel (512ft) embedded inside a hill.

Rather than thinking of the engineering aspects of the gun, what are the physics based reasons why we cannot arrange a series of linear explosions, with a valve type device to prevent blowback down the barrel at each stage and thereby maximising the upward boost to the payload to escape velocity.

Again, I would stress that I believe there are physical (rather than engineering) reasons this idea is not used today. I just don't know what they are. Is it as simple as the barrel would need to be unfeasibly long, even using the most powerful explosives we have available today?

The Project Harp Launch Gun was tested in the 1960s but never achieved more than half the escape velocity required.

Merci beaucoup, Jules Verne (1828-1905). From The Earth To The Moon

  • To my knowledge it is highly inefficient – Jaywalker May 25 '16 at 13:06
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    Not that any current launch mechanisms might be considered efficient, mind you... – Jon Custer May 25 '16 at 13:17
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    It's slightly unrelated, but I once heard of the single most ridiculous plan to counteract global warming imaginable, and it involved a gun like that. The plan was to send pieces of glass up between the earth and the sun, acting as "sunshades" to the earth. Problem is, that whoever had this "brilliant" idea, soon realized that the amount of glass required would be much more than all the sand in the earth, and that it is unfeasible to launch that many rockets, so he had another brilliant idea, to use these guns to launch ultra-thin pieces of glass that broke even if you lightly shook them :/ – Andreas C May 25 '16 at 14:11
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    @AndreasC Thanks for your comment. It's totally unrelated, but actually the single most ridiculous plan to counteract global warming is just to deny the problem exists. –  May 25 '16 at 15:43
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    It's a very popular plan though... – Andreas C May 25 '16 at 16:27
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    Several problems: (1) the acceleration required to get the object into orbit would destroy nearly all electronic components that are space-tested [need to be radiation hardened among other things]; (2) the muzzle velocity would be high enough heating of any exposed surface that it would require exotic materials just to withstand the heat; (3) nearly no control of projectile after exiting the barrel, thus subject to deflections by atmospheric variations; (4) etc. etc. – honeste_vivere May 25 '16 at 17:16
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    @JonCuster: Actually, rockets are fairly efficient: https://en.wikipedia.org/wiki/Propulsive_efficiency. If the final velocity is three times the exhaust velocity, the theoretical propulsive efficiency of a rocket is approx. 60%. Thermodynamic efficiencies of the engines are 40-70% these days, with a theoretical limit of roughly 75%. 1st stage empty mass on the Falcon 9 is estimated to be 23-26t (only twice the mass of the LEO payload of 13t) and the empty second stage is estimated at 4.7t. Since it also enters LEO, that material could theoretically be reused in orbit. – CuriousOne May 26 '16 at 06:55
  • there was a (IIRC) Nazi gun that used explosives placed along its barrel to further push the payload onwards. (I forget the details, it was one of those TV programmes you half watch), apparently it worked even though many people thought it wouldn't. They demonstrated a mini version in a lab which was remarkably efficient. Whether it'd get a payload into orbit is another matter (though I think firing it to a height where a rocket then takes over might work, if you don't blow the rocket up in the barrel!) – gbjbaanb May 26 '16 at 08:01
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    @gbjbaanb - The V3 cannon (wikipedia)? – Chris H May 26 '16 at 08:04
  • Related: Why don't we use catapults to get to space? on [space.se] – user May 26 '16 at 08:39
  • Related discussion of 'using' a nuclear explosion to launch a solid object into space: http://www.skepticblog.org/2011/12/15/united-states-beat-sputnik/ – pjc50 May 26 '16 at 08:58
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    Given what some of the answers say: achieving orbit requires a second impulse (and who was building it), I'd have to assume that the Babylon gun was to be used for de-orbiting satellites. – Mazura May 27 '16 at 16:22
  • @ChrisH, I just watched that episode: Bombing Hitler's Supergun. How much longer would it have to be, to reach a GEO satellite with its 140kg payload, given that its length was 130m and its muzzle velocity was 1.5kps? – Mazura May 27 '16 at 16:31
  • @Mazura, I don't think it ever worked anyway. But it wasn't optimised for launch unlike the latter US designs. I haven't seen it on TV, just Wikipedia. – Chris H May 27 '16 at 18:12
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    @honeste_vivere Self-guiding artillery has been around for years. Accelerating the electronics would be difficult, but perhaps not out of the realm of possibility. – JS. May 27 '16 at 23:38
  • @JS. - True, but no artillery travels at $\gtrsim$13 km/s. There are micro electronic components that can withstand upwards of ~50000 g's (e.g., in bunker buster bombs), but those would not work in the high radiation environment of space. Most space-tested electronics look like they are over 10 years old because truly micro electronics suffer too much from single event upsets etc. for scientific spacecraft, let alone a military or commercial spacecraft. – honeste_vivere May 28 '16 at 16:28

7 Answers7

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Other answers don't mention the fact that no single impulse (e.g, like being fired from a gun) can launch a projectile into orbit. A purely ballistic projectile fired from a gun must either crash back into the planet, or it must escape from the planet altogether.

In order to achieve orbit, at least two impulses must be applied to the projectile. The first one (from the gun) launches it into an elliptical trajetory that returns to the surface, and then the second impulse must be applied by a rocket motor to "circularize" the orbit at the moment when the projectile reaches the apogee of the initial ellipse.

Solomon Slow
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    I know that this answer is not very physics-y---it contains no math or citation to support my claim---but is it wrong? (i.e., why the downvote?) – Solomon Slow May 25 '16 at 18:24
  • Not downvoted by me anyway James, I think someone misread it, payload needs a rocket included. –  May 25 '16 at 19:21
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    This is a good point. It might be that if you take atmospheric friction into account you could arrange fir some kind of good final trajectory. However the real purpose of these giant guns is of course suborbital hops: cheap ICBMs basically. –  May 25 '16 at 19:48
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    If the Earth had no atmosphere you would be right. Yet if fired into the atmosphere and then slowed by shock heating that might be the second impulse needed. – Lawrence B. Crowell May 26 '16 at 00:34
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    This is very right. Even the friction argument in the above comments can't save gun launches. Friction always acts opposite the direction of motion, and retrograde thrust will never raise perigee. –  May 26 '16 at 02:57
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    Sailboats do interesting things with friction, maybe our hypothetical satellite could too. – Stig Hemmer May 26 '16 at 08:45
  • @StigHemmer sailboats use wind to gain kinetic energy with respect to the water. But the only wind you have as an orbiting satellite is a very faint 11 km/s headwind. Sailboats can move against wind, but only because they have water to push against and even then they move mostly sideways. – John Dvorak May 26 '16 at 09:30
  • Thanks James, I read From the Earth To The Moon as a kid and loved it, thus the question. –  May 26 '16 at 11:02
  • @ChrisWhite Also airplanes can not fly. (Of course lift still can't get the perigee above the athmosphere, and so still does not solve the problem) – Taemyr May 26 '16 at 11:38
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    @ChrisWhite is correct, in order to circularize an orbit, you need to apply a prograde impulse when at the apogee. Friction will never work that way since it's always a retrograde force. If you're skeptical, give Kerbal Space Program a try and it'll help you understand how this stuff works. – Thane Brimhall May 26 '16 at 15:24
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    If you were on a planet with no air, you could theoretically fire a gun directly straight in front of yourself, and if the bullet shot off at the right speed, it would perfectly orbit around the planet at gun height above the ground until it hit you in the back.

    The relevance being that on a planet with air to slow down the forward motion, you just have to calculate the proper angle of launch such that the projectile retains precisely the right amount of momentum to maintain orbit when it reaches apogee.

     – industry7 May 26 '16 at 19:46
  • While friction is inherently a negative force lift isn't. Thus you can use the atmosphere to lift your periapsis. However, barring third-body encounters your orbit will always include the last point at which it was altered. You could use a lifting body to put your periapsis in the upper atmosphere but no higher. You'll still need to circularize. – Loren Pechtel May 27 '16 at 05:46
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    @industry7 Your hypothetical airless planet scenario works because the projectile is already in orbit at the instant it leaves the gun. The perigee of a purely ballistic projectile can be no higher than the gun that launched it. A gun on the surface cannot produce an orbit that does not intersect the surface. – Kevin Krumwiede May 27 '16 at 06:45
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    Worth reading the article: “Rather than throwing away the first stage of a rocket, using a large gun for the first stage would enable this hardware to be reused and easily serviced.”. This seems to accept that a gun would require a second burn. – Gusdor May 27 '16 at 09:42
  • This argument was covered in the article linked in the question, so I'm not sure why it's considered something that prevents us today from using the design - they knew about this limitation and didn't consider it a limitation. – Adam Davis May 27 '16 at 18:30
  • Turned this non-answer over to the community wiki. I only meant to point out a complication that other answers were ignoring. And that's all it is, a complication---not an insurmountable, theoretical obstacle. Gerald Bull thought a gun-launch would be a viable way to put satellites into orbit. (Either that, or he was scamming Saddam Hussein, which does not sound like a smart thing to do. But then, come to think of it, helping Saddam turned out not to be too smart either...) Anyway, I'm sure he knew more about ballistics than I will ever know so,... Have fun with it. – Solomon Slow May 27 '16 at 19:34
  • The question clearly asks whether or not a series of explosions can get the projectile up to escape velocity. This answer just says that you need a 2nd burn to circularize orbits. That's true, but has absolutely nothing to do with the question. How does it have so many upvotes? – Shane May 27 '16 at 20:58
  • KSP taught me that you are indeed correct, no matter how hard I try. – BruceWayne May 29 '16 at 04:53
  • Ok, I was trying to provide a bit of explanation along with my answer, but since you guys didn't like my explanation maybe you'll listen to Nasa scientists...

    https://www.researchgate.net/publication/234224858_Temporary_orbital_capture_of_ejecta_from_comets_and_asteroids_Application_to_the_Deep_Impact_experiment

    "This experiment [Deep Impart], intended to ..., mimics a natural cratering event on the surface of the comet. Ejecta from such a cratering event may ... or be trapped in temporary orbits around the nucleus"

    Temporary, b/c solar wind eventually blows it away...

     – industry7 Jun 08 '16 at 18:38
33

Anything launched into orbit by such a gun needs to travel at orbital velocity (in fact above orbital velocity) in the lower atmosphere. That's generally undesirable, to put it mildly: there will be really serious heating.

  • And really serious drag. You need to fire a very big projectile to get it through the atmosphere no matter what the launch velocity. – Loren Pechtel May 27 '16 at 05:47
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Aside from the interior ballistic aspects of these various projects, it was quickly realized that any satellites launched by gun would have to withstand high g-loadings during firing of the gun and the size and mass of the satellite would be greatly constrained by the dimensions of the bore of the gun and the maximum impulse which could be provided by the propellant without damaging the gun.

Special designs for satellites were prepared so that sensitive electronics would not be damaged by being fired from a gun, and recognizing that the gun could not provide sufficient velocity to reach orbit, satellites with booster rockets were designed to fire after being flung aloft by the gun.

The project ended for various reasons, some budgetary, some political. The escalating war in Vietnam caused funds for a lot of research projects to be cut, and this project was originally a joint effort between the U.S. and Canada. When relations between the two countries hit a rough patch over differing policies regarding Vietnam, the project became ripe for being eliminated.

user16622
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I think the heart of the question is whether one could arrange a continuous combustion of propellant along the length of the barrel. In that way the acceleration occurs along the length of the barrel in a more gentle way. Since the expanding gases from the propellant in a shell casing expand and the pressure of the expanding gases declines along the way it means the primary force or acceleration loading is not at the start of the projectile motion.

You still have a huge acceleration. Suppose the barrel is $100$m in length and assume the projectile has orbital velocity ($\simeq 10^4m/s^2$) at the end of the barrel. Then using the elementary equation $2ad = v_f^2 - v_i^2$ the acceleration is then $$ a = \frac{v^2}{2d} = \frac{10^8m^2/s^2}{200m} = 5.0\times 10^5m/s^2. $$ This is the average acceleration, which if you design the firing of propellant correctly it might be the actual acceleration that is nearly constant. This is considerable.

There is an additional problem. The projectile as it leaves the gun will be slowed by the large shock wave it produces in the atmosphere. So you would need to fire the projectile at a higher acceleration to account for this loss.

Lawrence B. Crowell
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One more nasty factor: What is the expansion speed of your propellant. Take the Jules Verne approach and your spacecraft falls far short no matter how much powder you put in the gun because the expansion velocity is too low. Your craft will never exceed the expansion velocity of the propellant.

Note, however, that you don't have to use explosives (or combustible gas mixtures--the best chemical space-gun designs I've seen used gas, there was no boom) to build a space gun. Consider the railguns the navy is working on--supergun type speeds, nothing in the launcher goes boom. You still need gun-hard electronics, though.

For a fixed launcher you can use a linear motor rather than a railgun.

One more headache to keep in mind: A simplistic aiming of a space gun cuts through a lot more atmosphere than the 14.7 psi you will go through if you went straight up. This suggests another approach: Make your launch angle as steep as possible, over 80 degrees would be ideal if you could build the needed vertical velocity. What's that I hear from the peanut gallery about needing orbital speed?? If your vertical velocity is high enough you can get away with very little horizontal velocity. Go nearly straight up at 18,000 mph and you'll fall back. Go nearly straight up at say 24,500 mph and it's another matter--the objective is to go out as far as possible consistent with not getting your orbit wigged by the sun. You only need enough horizontal velocity to travel 4000 miles during this hop (and you can take days on the hop, that's not much speed at all), come back down just skimming the atmosphere and then do an aerocapture maneuver. You'll still need a circularization burn at the end but you've encountered far less drag than if you had gone for orbit directly.

Now, if you're on a body without an atmosphere the linear motor really shines. You still need to circularize but you aren't fighting mega-drag, nor do you have to worry about things like the shockwave of your craft destroying the launch system. An ejection angle of zero is fine, thus there's no length limit on the booster. You no longer need gun-hard electronics, such a system is even useable for manned transport. (Wrap your boost track clear around the moon and your manned speed is only limited by the centripetal force as you go around the track. If I haven't messed up the math that's enough to provide anything from solar escape to solar impact.)

Loren Pechtel
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    Thanks for a great answer, and the time you put into it. I mean this in a good way, but have you been reading the same SF books as me? –  May 27 '16 at 08:53
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Orbital insertion is hard enough with ships where the initial stages can be aimed in directions favorable to the final orbit.

This design calls for a fixed launch direction, which would be a terrible waste of on-board fuel for all but those satellites whose orbits coincide with the gun's trajectory.

Even that can be overcome with fuel. The real issue is size and scale. The proposed design is meant to launch lightweight satellites of relatively small volume - maximum 1 meter in diameter including any casing or shell needed for the launch.

Today's satellites are often much larger than cars and buses on the road - easily over 2 meters in diameter, and weighing much more than this small gun could handle.

You could, in theory, make the gun larger and longer, but in addition to the exponentially greater costs of construction and fuel, you find that the energy required to launch goes up exponentially.

At first glance this shouldn't be a problem, since the same issue exists for our current launches, however this design has a particular drawback - the vehicle must be going much faster at the beginning of the shot than at the end, as it will expend significant energy travelling through the dense atmosphere of the lower layers of the atmosphere.

Our current rocket technology can start off relatively slowly, burning less fuel, and ramp up as they get faster and as they start passing through thinner atmosphere.

This means that the exponential increase in energy required affects both, but the gun requires an even greater exponential increase because it's starting out so fast.

Adam Davis
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Let's say you have got such gun. Next logical step will be to install on the satellite a smaller gun that would shot-back several small shells and so accelerate the satellite, indeed? If this small on-board gun would use really many small shells (size of molecula) then your are getting just a traditional rocket. Apparently it is not much difference for energy required if you launch a rocket from traditional platform or gun it, but engineering difficulties for the latter technology are massive.

dmafa
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    I dont see that being the next logical step.. – James T May 25 '16 at 15:56
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    There's a fairly significant difference: the gun & projectile approach does not need to lift any propellant / reaction mass. –  May 25 '16 at 17:19
  • I don't think that respecting energy the difference is as significant. 1st stage of a rocket (plus possible booster) does exactly same as the gun, namely provide initial impulse to rest of the rocket and then disengages once run out of fuel. So, at point of 1st stage disengagement, rest of the rocket has some speed and has no propellant, no tank, engine etc. Exactly as if it was gunned. I agree that with the gun you can save weight of fuel for the 1st stage, but instead you have to use certain powder. Idea with gun is nice, but for me enegry efficiency of gun vs rocket doesn't look massive. – dmafa May 25 '16 at 18:05
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    @dmafa I think this needs actual calculation. A Saturn V weighed about 3,000 tonnes wet, and could get about 140 tonnes into LEO (and about a third of that to TLI). It's not clear to me that a gun could not do better in some theoretical sense (in particular, operating outside an atmosphere: it's clearly not practical with an atmosphere I think). –  May 25 '16 at 19:58
  • I added a down vote because this might be theoretically true, but has nothing to answer the question. And yes, there's a huge impact of having to take energy and reaction mass along (that's why there's an exponential in the equation) – Daniel Jour May 25 '16 at 22:44
  • @tfb Well, let's try. Never calculated any gun before. First search for energy capability of gun powder. Taken data of 7.62x39mm Kalashnikov round, wiki gives bullet weight 7.9 g, velocity 730 m/s. Calculate kinetic energy mv^2/2 = 0.0079730*730/2=2100 J. Same energy is written in wiki, so I am happy with the number. Divide energy by filling weight of 1.6 g, obtain ratio 2100/0.0016=1.3e6 J/kg. To my understanding it is what we can get out of gun powder. – dmafa May 25 '16 at 22:48
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    @dmafa, look up the Tsiolkovsky rocket equation. Then think about what it means. Really think about it. That natural log isn't in there to look pretty. Carrying your fuel with you is expensive. – Mark May 25 '16 at 22:53
  • Second question: what energy do we need to lift 140 t to LEO? Multiply it by 7.9 km/s squared and divide by 2, obtain 140000*(7900)^2 / 2 = 4.35e12 J. How much powder is needed for this energy? Divide it by the ratio of 1.3e6, and obtain 3.3e6 kg, that is 3.3e3 t = 3,300 tonnes. This number looks pretty close to 3,000 tonnes of Saturn V weight. So, I don't see any profit for a gun as energy effective alternative to a rocket. Agree, the calculation is far not accurate, just did it in simplest manner as far as I can. If you can do it better, then ok. – dmafa May 25 '16 at 22:56
  • @dmafa I strongly suspect that round uses cordite or equivalent as its propellant, rather than gunpowder. I haven't read the book for years but I think even Jules Verne used guncotton (nitrocellulose) - he loved the stuff. – Chris H May 26 '16 at 08:16
  • @dmafa and who said that the gun must be powered by gunpowder? – Crowley May 26 '16 at 08:38
  • @Chris H and all: Guys, can anybody roughly estimate how much of cordite or any other (the best existing) propellant is needed to fill the gun to make same momentum as a rocket does using certain amount of jet fuel to deliver same payload to LEO? To my rule of thumb calc (above) it is about same mass of propellant. I respect Tsiolkovsky equation, well, have to pay this log price. All I say is that a gun will not provide enough significant advantage (x times better) vs a rocket in respect of needed propellant mass to make it interesting to deals with troubles of gun launch technology. – dmafa May 26 '16 at 09:18
  • I am happy to change my opinion if any of you can justify needed amount of any existing gun propellant (to your choice) to lift up 140 tonnes as Saturn V did (or any other rocket ever flew to LEO, again to your choice) and compare that to amount of used rocket fuel. So far I see no benefit of a gun. See calculation above why. – dmafa May 26 '16 at 09:36
  • @dmafa I have a strong suspicion you're right at least as regards starting on the surface of earth. The most plausible SF version I saw was moon-based. I think the US proposal used a gas-air propellant. Rather than for heavy lift, I think this system was for many smaller satellites. – Chris H May 26 '16 at 10:04
  • @Chris H, I just noted that solid stuff is much less effective propellant than liquid. Cannot explain why, I am not quite good in chemistry. That's why solid propellant for gun launch is inherent disadvantage of the idea itself, it is evident even before one start to think about technology difficulties. Downvoters simply don't admit this point without providing any argument, ok. However if, as you say, they were going to use gas-air in the project, that could improve the situation. I still rather believe in liquid, but this point is uncler until someone do a gas system and test it. – dmafa May 26 '16 at 17:04
  • P.S.: discovered that gas as jet propellant in reality was tested at Tupolev Tu-155 plane. I don't hope that test data are published anywhere as comparison to liquid fuel plane, unfortunately. Would be interesting to know outcome they got of the tests. Still aircraft is different application than space launch. – dmafa May 26 '16 at 17:35
  • @dmafa Fuel is cheap. Consider the Falcon 9: Full cost, ~$60 million. Fuel: ~$200k. Not even 1% of the rocket. A gun launch leaves the launcher at home ready to fire again. – Loren Pechtel May 27 '16 at 05:51
  • @Loren Pechtel, Agree. To my knowledge most of costs go down to maintain launch platform and associated infrastructure. Not sure that costs will be lower for a gun, since it is anywhere hard equipment to make (require very tight tolerance to grind barrell, beyond nowday technology), but even bigger question is how many times the gun can shot before repair. Mechanical and temprerature stresses are huge, maybe the barrell will do only one launch, then need grind again. If so, then maintain costs will be higher than for rocket. Anywhere this is unknown matter until someone do it in reality. – dmafa May 27 '16 at 10:00