To Orbit by Space Gun

Ever since the limitations of rockets were widely recognized, most prominently the fact that fuel requirements increase wildly as payload mass scales up, there has been perennial interest in “non-rocket space launch” technology. Most of this interest has been in the specific context of launching payloads from Earth surface to Earth orbit, which is the most challenging part of space travel. As outlined in my post on near-future interplanetary flight, even for a flight as long as Earth to Mars the energy to get to Earth orbit is still half of the total used for the entire trip. If a spaceship could dispense with the fuel required to reach orbit, spaceflight would become much cheaper and easier.

Obviously the energy required would still have to be provided from somewhere, thus all proposed non-rocket space launch technologies involve imparting energy on the vehicle from an external source. Solar sails at this point are probably the most well-known non-rocket propulsion technology. They capture energy produced by the sun, thus relieving the spaceship of the obligation of carrying its own fuel, greatly increasing efficiency. This is but one example of many, but one that we will explore in this post is perhaps the oldest proposed technology for non-rocket space launch.

Jules Verne in his famous and much-beloved novel From the Earth to the Moon (1865) chose to launch his astronauts from a giant cannon. While this often strikes modern readers as a peculiar way to reach orbit, keep in mind that in 1865 the entire field of space launch had yet to be investigated by modern science. That would only come half a century later when Konstantin Tsiolkovsky, the pioneer of the field, was the first to scientifically prove rockets could achieve spaceflight. In 1865 cannons were unrivaled in their ability to shoot projectiles at high speeds, and are still rather competitive even now. As late as the First World War the “ Paris Gun ” had unmatched ballistic power, launching 200-pound projectiles up to a distance of 80 miles. The shells attained a peak altitude of 26 miles and peak speed of 3681 mph. The altitude record in particular was only surpassed by the V-2 rocket in 1942.

Jules Verne, to his credit, did calculate in 1865 that a cannon could launch a projectile into space effectively enough for it to be a credible method of space launch. Unfortunately, it has since been demonstrated that the acceleration involved in such a shot is thousands of gees, far too high for a human to survive the launch. For perspective, humans can normally survive 5g of acceleration, up to 10g while wearing a “g-suit” that counteracts some of the effects. The g-forces involved depend on the barrel length; a longer barrel would lessen the acceleration, but as one NASA page points out even a mile-long barrel would still shoot out a projectile at 4000g. A barrel long enough to reduce the force to 10g could probably be built, but at that point it’s vastly cheaper and easier to just use a rocket.

A Space Gun for Orbiting Cargo

That’s not the end of the story, however. There are plenty of payloads to be launched into space that don’t involve humans. Would a Jules Verne-style space gun work for launching cargo? It would appear so. In fact, this has been demonstrated in real life. Project HARP in the 1960s, a military project to study ballistics of re-entry vehicles, used 16-inch guns to launch 400-pound test projectiles up to an apogee of 110 miles, thus reaching outer space and performing a suborbital spaceflight. The maximum speed was 3.6 kilometers per second, some way short of orbital velocity. However, the principle was demonstrated to work, even with a maximum acceleration, due to the barrel being only a few hundred feet long, of 15000g. Indeed, as far as I am aware the projectiles launched by Project HARP are to this day the only man-made objects to have reached space without using a rocket.

Fragile objects obviously couldn’t take the thousands of gees required for launch, but such forces wouldn’t be a problem for rugged objects or raw materials. This may prove to be significant in the future, since space guns have the potential to be much cheaper than currently existing rockets. A 2010 article in Popular Science reports that John Hunter, a notable recent booster of the technology, estimates the cost to orbit would be $250 per pound, compared to $5000 per pound for rockets. Hunter’s version of the gun has the interesting feature of being located mostly underwater, to enable easy swiveling to different angles for launch into different orbits.

One challenge of space gun launch is that without course correction the orbit has a perigee identical to the launch point, i.e. the Earth’s surface, thus any projectile will crash-land back on Earth after one orbit. The projectiles launched could be grabbed in orbit by another space vehicle (e.g. rendezvous with a space station in the first orbit) or a small rocket burn could be employed. The amount of delta-v required to circularize such an orbit would be less than one kilometer per second, very modest compared to conventional vehicles. This multi-stage model (with the gun serving effectively as the first stage) has been seriously proposed multiple times, including in Project SHARP (a government project in the 1980s and 90s that built on HARP but was cancelled before achieving much in the way of results) and Quicklaunch (proposed by John Hunter, one of the scientists who worked on SHARP).

Alternatively, it may be possible for a solar sail to provide the needed course corrections, in the same way that “statites” would use them. Laser beam propulsion may also be employed, as could all the other usual methods proposed for spacecraft propulsion.

Air resistance is another challenge; since a space gun projectile achieves most of its velocity as soon as it leaves the barrel, it has to cut through the thick lower atmosphere at near orbital velocity. This causes great air friction and thus heating that must be accounted for by sheathing the payload in a shell made out of strong and heat-resistant materials. This isn’t a real problem for a rocket, since by the time it achieves such speeds it’s already clear of most of the atmosphere. Sufficiently strong materials have been demonstrated (including in Project HARP), but this is yet another drawback to the technology, to the extent that Project SHARP’s scientists concluded that an initial launch of around half of orbital velocity with the rest being supplied by a second rocket stage was optimal.

As for the actual energy source involved in launching the projectile, Project HARP used ordinary chemical explosives, but more recent proposals have involved what is called a “light gas gun”, using the force of expanding hydrogen to push the projectile. This is considerably more efficient than ordinary explosives.

Going Nuclear

More efficient still may be the use of nuclear explosions rather than any chemical energy source. Believe it or not, this has (sort of) been demonstrated in real life. In 1957 the underground nuclear test code named Pascal B involved a test shaft with a steel plate (similar to a manhole cover) on top of it. The scientists involved calculated that the force of the explosion would send the steel plate upward at 66 kilometers per second, six times Earth’s escape velocity. They found the calculated speed interesting enough that they set up a high speed camera to monitor the plate, and the plate did indeed show up on just one frame, confirming that it had at least approached the speed they had calculated (at least that’s what is widely reported on the Internet). Unfortunately for any who might hope there’s a manhole cover out there on an interstellar trajectory, air friction almost certainly vaporized it into a streak of plasma long before it could reach space, though it’s possible some of the plasma made it.

Nevertheless, the principle was demonstrated. The manhole cover in question weighed around 2 tons, and the nuclear explosion had a yield of only 300 tons or so, rather small by nuclear standards. Bombs that are smaller still could be used in underground chambers to propel a projectile up a shaft (functioning as the barrel in this case) to a more reasonable velocity where they wouldn’t burn up. This is the same general principle that the far more famous nuclear pulse propulsion (associated with Project Orion) uses, and indeed may be to nuclear pulse what a chemical space gun is to a chemical rocket. If the gains in efficiency with nuclear space guns over a conventional model are anywhere near as large as nuclear pulse’s are over chemical rockets, this could be a very powerful method of launching space cargo.

Futurists and science fiction worldbuilders should take note of this, because nuclear space guns rarely if ever appear in scientific speculation about the future or in science fiction settings, yet may have the potential to be far superior to almost any known space launch technology. Only laser beam propulsion for small cargo (the lightcraft concept) or nuclear pulse propulsion for humans or large cargo would be superior, assuming there isn’t any showstopper issues that are unknown. As far as I am aware there has been no serious scientific research into nuclear space guns; indeed the research into chemical space guns is somewhat sparse, let alone any nuclear versions.

Even better, the negligible fallout issues associated with a ground nuclear pulse launch could be contained to be more negligible still with a nuclear space gun, as underground nuclear explosions (depending on a variety of factors) don’t release nearly as much fallout as ground explosions do. There may be quite a bit of potential with this sort of technology.

Toward a Man-Rated Space Gun

Of course, even if this potential is realized in a science-fiction setting or speculative real-world future filled with (nuclear or otherwise) Jules Verne-style space guns one might think the use of these launch systems would be forever restricted to hardier cargo, requiring people and more fragile cargo to reach orbit using other methods. Or would it? A speculative method currently being researched to insulate against g-forces that goes far beyond an off-the-shelf g-suit is to immerse human beings in liquid instead of in air. Water is much closer than air to the density of the human body, thus relative to air g-forces will act much more evenly in water, thus sparing the man from being smashed against the wall (as much).

The limiting factor using immersion is that there is still air in the man’s lungs, thus there still will be acceleration-induced compression of the air gaps within the human body. However, water immersion does do a lot of good. Researchers studying this problem have found that ordinary water immersion could enable a man to withstand up to 24g of sustained acceleration, much higher than currently used methods. While this couldn’t be used for space gun launch (as that would entail at least hundreds of gees), this technique could be used for high-speed or relativistic spaceflight, especially in interstellar contexts, where the time needed to accelerate to cruising speed at 1g is a significant fraction of the travel time.

There are still more exotic techniques that might be used to bring the survivable range of acceleration into space gun territory. To eliminate the squeezing of the air pathways in the lungs, it would be necessary to fill them with a fluid of comparable density to the human body. Believe it or not humans can actually breathe liquids; perfluorocarbon mixtures can transport enough oxygen to the human body to support life. A big problem with perfluorocarbons is that their high viscosity prevents carbon dioxide from leaving the lungs as much as it should, inducing respiratory acidosis after a time, thus they can only be breathed safely for a few hours at a time. Even worse for acceleration protection, they are twice as dense as the human body and so wouldn’t provide the needed protection against g-forces. Still, some scientists hold out hope that a fluid of the appropriate density will someday be discovered.

If that is the case, then what is the limit? The tissues of the human body differ in density, and the acceleration that would cause them to squeeze even under total liquid immersion is the ultimate limit that can be achieved. There is no hard data on this, but researchers believe it is in the range of hundreds of gees or perhaps even higher. This could enable spaceships accelerating to high speeds (or decelerating from them) to complete their maneuvers hundreds or thousands of times faster, assuming a breathing liquid of the appropriate density could be developed.

If such a fluid is developed someday, then there will be no obstacle to using space guns to launch human beings (or for that matter delicate cargo), as complete liquid immersion will enable them to survive the hundreds to thousands of gees required to launch them into orbit. Although launching men out of a cannon into outer space is completely unfeasible for now and likely into the near future, in the more distant future Jules Verne may yet get the last word.

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