Who says you need a rocket to access space?

There have been plenty of alternative means of getting into space proposed over the years that don't employ traditional chemical rocket engines. The list presented here isn't exhaustive but representative, and they fall into four broad categories—cannons, external-energy vehicles, static structures and dynamic structures.

Cannons

The original concept for this came from Jules Verne, in his book From the Earth to the Moon, in which his lunanauts ride to their destination inside a luxuriously appointed gunpowder cannon shell that is blasted all the way to our sister orb. He was remarkably prescient in many aspects of the mission (e.g., the "launch" occurred from Florida, with a three-man "crew"), but the proposed barrel length of his gun would have been insufficient to reduce the acceleration as necessary to deliver his passengers in non-pancake form.

The problem with cannons is that unless they have improbably long barrels, they are very high-acceleration, and thus the payloads have to be extremely rugged (and, generally, small). In addition, they must have some means of doing a second impulse when they reach space (e.g., a rocket engine and propellants), or they will simply fly up into the heavens and then impact the earth as they come around the other side. Beyond that, unless the muzzle of the gun is above the atmosphere (e.g., at the top of a very tall mountain or artificial tower), they must be able to handle the heating from the drag as they traverse it at hypersonic velocities. Gunpowder is probably not a practical energy source for space missions, but other types of cannons might be. Electromagnetic rail guns can accelerate a payload by running a very high current between through parallel "rails" in opposite directions, generating a magnetic field between them that propels a magnet down the "barrel." It has been tested for decades as a weapon by the military, and proposed as a means of providing the velocity needed to get into orbit. It has very high electrical power requirements for any reasonably sized projectile (megawatts or even gigawatts), which are usually provided by dumping the energy in a spinning flywheel in a very short amount of time. Light-gas guns use a large-diameter piston driven by conventional explosives and a burst disk, to allow a working gas (usually hydrogen) to be forced into a narrow tube when the pressure reaches the needed threshold, driving the projectile down the length of it. The most powerful example, in terms of projectile velocity, is the Super High Altitude Research Project (SHARP) developed at Lawrence Livermore Laboratory in the 1990s. It demonstrated 3 kilometers per second for 5-kilogram bullets, and there were plans for a faster version, up to orbital velocities for smaller payloads, that were unfunded. There is a new company that proposes to use the technology for space launch, with an undersea launch tube and a muzzle at the surface.

External-Energy Vehicles

If a vehicle doesn't have to carry its own energy source with it, it can be much smaller and lighter. The late Arthur Kantrowitz proposed beaming a multimegawatt laser at a rocket, which the beam would heat to burn off parts of it off (ablation), or to vaporize the air behind it, providing a reaction to lift the vehicle all the way to orbit. The concept has since been refined by Leik Myrabo at Rensselaer Polytechnic, with small-scale demonstrations, and proposals for launch systems with very high propellant efficiency (specific impulse) of hundreds or even thousands of seconds, dramatically reducing the size and cost of the launcher. Interestingly, some of Myrabo's concepts resemble flying saucers. It could also be used, more conventionally, to remotely heat a more conventional propellant (such as hydrogen), but this, though still an improvement over conventional chemical propulsion, would reduce the advantages of not having to carry tankage.

Another external energy source could be nuclear-pulse propulsion. The Orion concept of the 1960s, by Freeman Dyson and others, featured small nuclear explosions behind the vehicle, generated by dumping small bombs overboard, which would push on a damped plate, shoving it out of the atmosphere (and perhaps on to another planet, such as Mars). The ride would be smoothed by having a high frequency of detonations, just as an internal combustion engine smooths out its explosions with hundreds of them per minute and a flywheel. The concept was tested at subscale using dynamite, but doing atmospheric testing of the full-scale nuclear system would have been problematic at best, and the entire concept was shelved after the atmospheric test-ban treaty went into force.

Static Structures

One type of static structure is a compressive tower into space. Sci-fi author and engineer Geoff Landis has proposed a tower whose top would be above the earth's atmosphere (100 km), the top of which would be used as a launch site for a conventional rocket, which would be optimized for vacuum and have no need to worry about atmospheric drag. Of much more interest, though, is the so-called space elevator. First popularized in Arthur C. Clarke's novel Fountains of Paradise, it is a tower over 22,000 miles high, all the way to and beyond geostationary orbit (GEO, where most communications satellites reside). It would be held in tension by the centripetal acceleration of the "anchor weight" above GEO, and mounted to the earth at the equator. One would ride either into low-orbit altitude, where you would have to board a rocket to put you into orbit (gravity is still 90 percent of earth's there), or all the way to GEO, where one would be weightless and already in orbit. Above that altitude, one could actually be flung into higher orbits, perhaps all the way to earth escape velocity at sufficient heights. We don't yet have materials with the tensile strength to make this possible, though diamondoid carbon fibers might theoretically come close. Also, the speed of the elevator is a function of the power, so affordable trips all the way to GEO might be slow, perhaps taking weeks. Better have a good supply of elevator music and snacks.

Dynamic Structures

There are two types of dynamic structures for space access. Devices such as the Space Fountains and the Launch Loop maintain structural integrity from electrodynamic effects or the momentum of the moving parts, and payloads and passengers ride them to orbit. More interesting are Rotovators. In this concept, a large structure orbits the earth, and rotates in the direction of the orbit like a wagon wheel with no rim, and the "hub" in a circular orbit. As it rolls, each "spoke" reaches down and dips into the atmosphere, and then rises up again with the rotation. A rocket launches up to meet it, performs a rendezvous with it as it stops, and then is carried up into space and high orbital velocities. Similarly, it drops other vehicles returning to earth, and they drop safely and gently down using their propulsion. It is an admittedly delicate ballet. In the extreme case, it would reach all the way to the ground, and you could just grab on at the bottom, but don't miss it—there's no "hold door open" with this elevator.

This content is created and maintained by a third party, and imported onto this page to help users provide their email addresses. You may be able to find more information about this and similar content at piano.io