Non-rocket space launch (NRS) is a launch into space where some or all of the needed speed and altitude are provided by something other than expendable rockets. A number of alternatives to expendable rockets have been proposed. In some systems such as skyhook, rocket sled launch, and air launch, a rocket is used to reach orbit, but it is only "part" of the system.
Present-day launch costs are very high – $10,000 to $25,000 per kilogram from Earth to low Earth orbit. As a result, launch costs are a large percentage of the cost of all space endeavors. If launch costs can be made cheaper the total cost of space missions will be reduced. Fortunately, due to the exponential nature of the rocket equation, providing even a small amount of the velocity to LEO by other means has the potential of greatly reducing the cost of getting to orbit.
Getting launch costs down into the hundreds of dollars per kilogram range would make many of the proposed large scale space projects such as space colonization, space-based solar power and terraforming Mars possible.
- 1 Comparison
- 2 Static structures
- 3 Tensile structures
- 4 Dynamic structures
- 5 Projectile launchers
- 6 Reaction drives (jets and unconventional rockets)
- 7 Buoyant lifting
- 8 Hybrid launch systems
- 9 See also
- 10 References
- 11 External links
|Method(a)||Publication year||Estimated build cost
|Estimated cost to LEO
Metric tons per year
|Technology readiness level|
|Conventional rocket||1903||700 – 130,000||4,000 – 20,000||≈ 200||9|
|Non-rotating Skyhook||1990||< 1||2|
|Hypersonic Skyhook||1993||< 1(c)||1,500(d)||30(e)||2|
|Orbital ring||1980||15||< 0.05||2|
|Launch loop (small)||1985||10||5,000||300||40,000||2+|
|Launch loop (large)||1985||30||5,000||3||6,000,000||2+|
|Ram accelerator||2004||< 500||6|
|Slingatron||100||2 to 4|
|Laser propulsion||2||100||550||3000||Up to 4|
|Microwave propulsion||1||< 100||600|
(a) References in this column apply to entire row unless specifically replaced.
(b) All monetary values in un-inflated dollars based on reference publication date except as noted.
(c) CY2008 estimate from description in 1993 reference system.
(d) Requires first stage to ~ 5 km/s.
(e) Subject to very rapid increase via bootstrapping.
(f) Requires Boeing proposed DF-9 vehicle first stage to ~ 4 km/s.
(g) Based on Gen-1 reference design, 2010 version.
(h) Jules Verne's novel From the Earth to the Moon. Newton's cannonball in the 1728 book A Treatise of the System of the World was considered a thought experiment.
In this usage, the term "static" is intended to convey the understanding that the structural portion of the system has no internal moving parts.
A space tower is a tower that would reach outer space. To avoid an immediate need for a vehicle launched at orbital velocity to raise its perigee, a tower would have to extend above the edge of space (above the 100 km Kármán line), but a far lower tower height could reduce atmospheric drag losses during ascent. Satellites can orbit temporarily in elliptical orbits dipping as low as 135 km or less, yet orbital decay causing reentry would be rapid unless altitude was later raised to hundreds of kilometers. If the tower went all the way to geosynchronous orbit at approximately 36,000 km, or 22,369 miles, objects released at such height could then drift away with minimal power and would be in a circular orbit. Building a tower to that extreme height is not possible with current materials on Earth. The concept of a structure reaching to geosynchronous orbit was first conceived by Konstantin Tsiolkovsky, who proposed a compression structure, or "Tsiolkovsky tower".
A parallel-sided structure made of conventional brick and stone cannot reach past approximately 2000 meters, since the bricks at the bottom would be crushed under the cumulative weight of the bricks above them. Other materials could allow the tower to reach a greater height, especially if the structure tapers (i.e. the upper parts are narrower than the bottom), but with current construction techniques, cost increases exponentially with construction height. Buckling may be a failure mode before exceeding a material's nominal compressive yield strength (though designs such as with a truss may help compensate), but, aside from that and aside from design against weather, the theoretical scale height of a structure is the allowable load of its material divided by the product of density and local gravitational acceleration, where needed material cross-section increases by a factor of e (2.718...) over each scale height.
For common plain carbon steel under a typical allowable stress limit, its scale height is ≈ 1.635 kilometer. A 4.9 kilometer high tower (3 × its scale height) of such would accordingly mass at least 20 times the weight supported at its top (as e3 ≈ 20). In contrast, an example of a more expensive high-performance aerospace material, Amoco T300/ERL1906 carbon composite, has a scale height of 54 kilometers at a safety factor of 2, though construction challenges including wind loading would apply. Earth's atmosphere has approximately 50% of its mass under 6 kilometers elevation, 90% below 16 kilometers, and 99% below 30 kilometers of altitude.
Natural mountains reach up to 9 km altitude. As of 2013, the tallest man-made structure is the Burj Khalifa which is 829.8 m tall. A tower or other high-altitude facility could form one component of a launch system, such as being the base station of a space elevator, or a support pillar for the distal part of a mass driver or the "gun barrel" of a space gun.
Alternatives other than compressive structures, such as tethers hanging down from high-altitude balloons or superconductor-based magnetic levitation, could take advantage of how: the characteristic length of Kevlar and some other macro-scale material performance in tension (instead of compression) is up to hundreds of kilometers; compressive buckling becomes no longer applicable; and setup might be simpler. Inflatable, kinetic, and electronic structures could also be options.
Tensile structures for non-rocket spacelaunch are proposals to use long, very strong cables (known as tethers) to lift a payload into space. Tethers can also be used for changing orbit once in space.
Orbital tethers can be tidally locked (skyhook) or rotating (rotovators). They can be designed (in theory) to pick up the payload when the payload is stationary or when the payload is hypersonic (has a high but not orbital velocity).
Endo-atmospheric tethers can be used to transfer kinetics (energy and momentum) between large conventional aircraft (subsonic or low supersonic) or other motive force and smaller aerodynamic vehicles, propelling them to hypersonic velocities without exotic propulsion systems.
The skyhook is another proposed low cost launch system. Skyhooks come in two types; rotating, and non-rotating. The non-rotating skyhook is a much shorter version of the planetary surface to geostationary orbit space elevator that does not reach down to the surface of the parent body, is much lighter in mass, can be affordably built with existing materials and technology, and in its mature form, is cost competitive with what is thought to be realistically achievable using a space elevator. It works by starting from a relatively low altitude orbit and hanging a cable down to just above the Earth's atmosphere. Since the lower end of the cable is moving at less than orbital velocity for its altitude, a launch vehicle flying to the bottom of the skyhook can carry a larger payload then it could carry on its own. When the cable is long enough, single stage to skyhook flight with a reusable launch vehicle becomes possible.
David Smitherman, of NASA Marshall Space Flight Center, included the non-rotating skyhook concept in his paper "Space Elevators, An Advanced Earth-Space Infrastructure for the New Millennium", NASA/CP-2000-210429, calling it a "Low Earth Orbiting Space Elevator." 
Rotating skyhooks are skyhooks that rotate around their center of gravity such that the lower end of the cable has an even lower tip speed than the lower end of a non-rotating skyhook that is in the same orbit and has the same length. The advantage of this is an even greater velocity reduction for the launch vehicle flying to the bottom end of the rotating skyhook which makes for an even larger payload and a lower launch cost. The two disadvantages of this are: the greatly reduced time available for the arriving launch vehicle to hook up at the lower end of the rotating skyhook (approximately 3 to 5 seconds), and the lack of choice regarding the destination orbit.
A space elevator is a proposed type of space transportation system. Its main component is a ribbon-like cable (also called a tether) anchored to the surface and extending into space above the level of geosynchronous orbit. As the planet rotates, the centrifugal force at the upper end of the tether counteracts gravity, and keeps the cable taut. Vehicles can then climb the tether and reach orbit without the use of rocket propulsion.
Such a cable could be made out of any material able to support itself under tension by tapering the cable's diameter sufficiently quickly as it approached the earth's surface. On Earth, with its relatively strong gravity, current technology is not capable of manufacturing tether materials that are sufficiently strong and light. With conventional materials, the taper ratio would need to be very large, escalating the total launch mass to a very large degree, and making conventional materials fiscally infeasible. However, recent concepts for a space elevator are notable for their plans to use carbon nanotube or boron nitride nanotube based materials as the tensile element in the tether design. The measured strengths of those nanotube molecules are high compared to their linear densities. They hold promise as materials to make an Earth-based space elevator possible.
The space elevator concept is also applicable to other planets and celestial bodies. For locations in the solar system with weaker gravity than Earth's (such as the Moon or Mars), the strength-to-density requirements aren't as great for tether materials. Currently available materials (such as Kevlar) are strong and light enough that they could be used as the tether material for elevators there.
An endo-atmospheric tether uses the long cable within the atmosphere to provide some or all of the velocity needed to reach orbit. The tether is used to transfer kinetics (energy and momentum) from a massive, slow end (typically a large subsonic or low supersonic aircraft) to a hypersonic end through aerodynamics or centripetal action. The Kinetics Interchange TEther (KITE) Launcher is one proposed endo-atmospheric tether.
A space fountain is a proposed form of space elevator that does not require the structure to be in geosynchronous orbit, and does not rely on tensile strength for support. In contrast to the original space elevator design (a tethered satellite), a space fountain is a tremendously tall tower extending up from the ground. Since such a tall tower could not support its own weight using traditional materials, massive pellets are projected upward from the bottom of the tower and redirected back down once they reach the top, so that the force of redirection holds the top of the tower aloft.
An orbital ring is a concept for a space elevator that consists of a ring in low Earth orbit that rotates at slightly above orbital speed, and has fixed tethers hanging down to the ground.
In the 1982 Paul Birch JBIS design of an orbital ring system, a rotating cable is placed in a low Earth orbit, rotating at slightly faster than orbital speed. Not in orbit, but riding on this ring, supported electromagnetically on superconducting magnets, are ring stations that stay in one place above some designated point on Earth. Hanging down from these ring stations are short space elevators made from cables with high tensile strength to mass ratio. Paul Birch found that since the ring station can be used to accelerate the orbital ring eastwards as well as hold the tether, it is possible to deliberately cause the orbital ring to precess around Earth instead of staying fixed in inertial space while the Earth rotates beneath it. By making the precession rate large enough, the orbital ring can be made to precess once per day at the rate of rotation of the Earth. The ring is now "geostationary" without having to be either at the normal geostationary altitude or even in the equatorial plane.
A launch loop or Lofstrom loop is a design for a belt-based maglev orbital launch system that would be around 2000 km long and maintained at an altitude of up to 80 km (50 mi). Vehicles weighing 5 metric tons would be electromagnetically accelerated on top of the cable which forms an acceleration track, from which they would be projected into Earth orbit or even beyond. The structure would constantly need around 200 MW of power to keep it in place.
The system is designed to be suitable for launching humans for space tourism, space exploration and space colonization with a maximum of 3 g acceleration. Some other Launch Loops are developed in 
Pneumatic freestanding tower
One proposed design is a freestanding tower composed of high strength material (e.g. kevlar) tubular columns inflated with a low density gas mix, and with dynamic stabilization systems including gyroscopes and "pressure balancing". Suggested benefits in contrast to other space elevator designs include avoiding working with the great lengths of structure involved in some other designs, construction from the ground instead of orbit, and functional access to the entire range of altitudes within the design's practical reach. The design presented is "at 5 km altitude and extending to 20 km above sea level", and the authors suggest that "the approach may be further scaled to provide direct access to altitudes above 200 km".
A major difficulty of such a tower is buckling since it is a long slender construction.
With any of these projectile launchers, the launcher gives a high velocity at, or near, ground level. In order to achieve orbit, the projectile must be given enough extra velocity to punch through the atmosphere, unless it includes an additional propulsion system (such as a rocket). Also, the projectile needs either an internal or external means to perform orbital insertion. The designs below fall into three categories, electrically driven, chemically driven, and mechanically driven.
A mass driver is basically a very long and mainly horizontally aligned launch track for spacelaunch, targeted upwards at the end.
It would use a linear motor to accelerate payloads up to high speeds. Sequential firing of a row of electromagnets accelerates the payload along a path. After leaving the path, the payload continues to move due to inertia.
StarTram Generation 2 is a proposal for an evacuated tube at 22 km for launching vehicles into space, held up by a large current in superconducting cables that repels another set of cables on the ground with an opposing current flow. Other versions of the concept would fire vehicles from a tube exiting on a mountain peak.
However, even with a "gun barrel" through both the Earth's crust and troposphere, the g-forces required to generate escape velocity would still be more than what a human tolerates. Therefore, the space gun would be restricted to freight and ruggedized satellites. Also, the projectile needs either an internal or external means to stabilize on orbit.
John Hunter of Quicklaunch is working on commercialising the 'Hydrogen Gun', a new form of mass driver which proposes to deliver unmanned payloads to orbit for around 5% of regular launch costs (i.e. $500/lb or US$1,000/kg) and perform 5 launches per day.
A ram accelerator also uses chemical energy like the space gun but it is entirely different in that it relies on a jet-engine-like propulsion cycle utilizing ramjet and/or scramjet combustion processes to accelerate the projectile to extremely high speeds.
It is a long tube filled with a mixture of combustible gases with a frangible diaphragm at either end to contain the gases. The projectile, which is shaped like a ram jet core, is fired by another means (e.g., a light gas gun) supersonically through the first diaphragm into the end of the tube. It then burns the gases as fuel, accelerating down the tube under jet propulsion. Other physics come into play at higher velocities.
Blast wave accelerator
A blast wave accelerator is similar to a space gun but it differs in that rings of explosive along the length of the barrel are detonated in sequence to keep the accelerations high. Also, rather than just relying on the pressure behind the projectile, the blast wave accelerator specifically times the explosions to squeeze on a tail cone on the projectile, as one might shoot a pumpkin seed by squeezing the tapered end.
In a slingatron, projectiles are accelerated along a rigid tube or track that typically has circular or spiral turns, or combinations of these geometries in two or three dimensions. A projectile is accelerated in the curved tube by propelling the entire tube in a small-amplitude circular motion of constant or increasing frequency without changing the orientation of the tube, i.e. the entire tube gyrates but does not spin. An every-day example of this motion is stirring a beverage by holding the container and moving it small horizontal circles, causing the contents to spin, without spinning the container itself.
This gyration continually displaces the tube with a component along the direction of the centripetal force acting on the projectile, so that work is continually done on the projectile as it advances through the machine. The centripetal force experienced by the projectile is the accelerating force, and is proportional to the projectile mass.
In pneumatic launch systems, a projectile is accelerated in a long tube by air pressure, produced by ground-based turbines or other means.
Reaction drives (jets and unconventional rockets)
In air launch a carrier aircraft carries the space vehicle to high altitude and speed, before release.
This technique was used on the X-15, SpaceshipOne and other launches.
The main disadvantages are that the mothership tends to be quite large, and separation within the airflow at supersonic speeds has never been demonstrated, thus the boost given is relatively modest.
A spaceplane is an aircraft designed to pass the edge of space. It combines some features of an aircraft with some of a spacecraft. Typically, it takes the form of a spacecraft equipped with aerodynamic surfaces, one or more rocket engines, and sometimes additional airbreathing propulsion as well.
Some air-breathing engine-based designs (cf X-30) such as aircraft based on scramjets or pulse detonation engines could potentially achieve orbital velocity or go some useful way to doing so; however, these designs still must perform a final rocket burn at their apogee to circularize their trajectory to avoid returning to the atmosphere.
Other, reusable turbojet-like designs like Skylon which uses precooled jet engines up to Mach 5.5 before employing rockets to enter orbit appears to have a mass budget that permits a larger payload than pure rockets while achieving it in a single stage.
Laser propulsion is a form of beam-powered propulsion where the energy source is a remote laser system which can be ground-based, airborne, orbital, or a combination of these. While climbing out of the atmosphere, the surrounding air can provide the reaction mass. This form of propulsion differs from a conventional chemical rocket where both energy and reaction mass come from the solid or liquid propellants carried on board the vehicle.
The concept of laser propelled vehicles was introduced by Arthur Kantrowitz in 1972.
Nuclear pulse propulsion
Nuclear pulse propulsion is a theoretical method of spacecraft propulsion that uses nuclear explosions for thrust.
This technique was proposed in the 1950s and 60s, most notably by General Atomics in the form of Project Orion. The idea of Orion was to react small directional nuclear explosives against a large steel pusher plate attached to the spacecraft with shock absorbers. Efficient directional explosives maximized the momentum transfer, leading to specific impulses in the range of 6,000 seconds, or about thirteen times that of the Space Shuttle Main Engine. With refinements a theoretical maximum of 100,000 seconds (1 MN·s/kg) might be possible. Thrusts were in the millions of tons, allowing spacecraft larger than 8 × 106 tons to be built with 1958 materials.
The system appeared to be entirely workable when the project was shut down in 1965, the main reason being given that the Partial Test Ban Treaty made it illegal (however, before the treaty, the US and Soviet Union had already detonated at least nine nuclear bombs, including thermonuclear bombs, in space, i.e., at altitudes over 100 km: see high altitude nuclear explosions). There were also ethical issues with launching such a vehicle within the Earth's magnetosphere. Calculations showed that the fallout from each takeoff would kill between 1 and 10 people,
However, using its (proposed) ability to launch very large payloads as a basis for the rapid deployment of a Space Elevator could mean that only a limited number of Earth based launches would be required to provide safe and cheap access to space. Once the Space Elevator was constructed, the launcher could be re-deployed for fast interplanetary (or in extreme versions, even inter-stellar) transit.
Balloons can raise the initial altitude of rockets.
However, balloons have relatively low payload (although see the Sky Cat project for an example of a heavy-lift balloon intended for use in the lower atmosphere), and this decreases even more with increasing altitude.
The lifting gas is usually helium, which is expensive in large quantities. This makes balloons an expensive launch assist technique. Hydrogen could be used as it has the advantage of being cheaper and lighter than helium, but the disadvantage of also being highly flammable.
Buoyant space port
By using big balloons (e.g. as a sky anchor) it may be possible to construct a space port in the stratosphere. Rockets could launch from it or a mass driver could accelerate payloads into the orbit. This has the advantage that most (about 90%) of the atmosphere is below the space port.
A SpaceShaft is a proposed atmospherically buoyant structure that would serve as a system to lift cargo to near-space altitudes. It is conceived to have multiple platforms distributed at several elevations that would provide habitation facilities for long term human operations throughout the mid-atmosphere and near-space altitudes.
A SpaceShaft would be designed to have the capability to lift cargo to space or near-space altitudes. For space launch, it would serve as a non-rocket first stage for rockets launched from the top, with the rocket part of the launch system being significantly smaller than if launched from the surface.
Hybrid launch systems
Separate technologies may be combined. The NASA in 2010 suggested that a future scramjet aircraft might be accelerated to 300 m/s (a solution to the problem of ramjet engines not being startable at zero airflow velocity) by electromagnetic or other sled launch assist, in turn air-launching a second-stage rocket delivering a satellite to orbit.
All forms of projectile launchers are at least partially hybrid systems if launching to low Earth orbit, due to the requirement for orbit circularization, entailing a large delta-v to raise perigee (e.g. a substantial rocket burn, taking more than half the launched mass in propellant alone, plus the mass of the motor), or in some concepts much more from a rocket thruster to ease ground accelerator development.
Some technologies can have exponential scaling if used in isolation, making the effect of combinations be of counter-intuitive magnitude. For instance, 270 m/s is under 4% of the velocity of low earth orbit, but a NASA study estimated that Maglifter sled launch at that velocity could increase the payload of a conventional ELV rocket by 80% when also having the track go up a 3000‑meter mountain.
Forms of ground launch limited to a given maximum acceleration (such as due to human g-force tolerances if intended to carry passengers) have the corresponding minimum launcher length scale not linearly but with velocity squared. Tethers can have even more non-linear, exponential scaling. The tether-to-payload mass ratio of a space tether would be around 1:1 at a tip velocity 60% of its characteristic velocity but becomes more than 1000:1 at a tip velocity 240% of its characteristic velocity. For instance, for anticipated practicality and a moderate mass ratio with current materials, the HASTOL concept would have the first half (4 km/s) of velocity to orbit be provided by other means than the tether itself.
Combining multiple technologies would in itself be an increase to complexity and development challenges, but reducing the performance requirements of a given subsystem may allow reduction in its individual complexity or cost. For instance, the number of parts in a liquid-fueled rocket engine may be two orders of magnitude less if pressure-fed rather than pump-fed if its delta-v requirements are limited enough to make the weight penalty of such be a practical option, or a high-velocity ground launcher may be able to use a relatively moderate performance and inexpensive solid fuel or hybrid small motor on its projectile. Assist by non-rocket methods may compensate against the weight penalty of making an orbital rocket reusable. Though suborbital, the first private manned spaceship, SpaceShipOne had reduced rocket performance requirements due to being a combined system with its air launch.
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