Non-rocket space launch (NRS) is a launch into space where some or all needed speed and altitude is provided by something other than conventional chemical rockets. A number of alternatives to rockets have been proposed. In some systems, such as rocket sled launch and air launch, a rocket is involved to reach orbit, but it ignites after helpful initial altitude or velocity is obtained in another manner.
Transport to orbit is a large factor in the expense of space endeavors. If launch can be made much cheaper the total cost of space missions will be reduced. Present-day launch costs are very high – $10,000 to $25,000 per kilogram from Earth to low Earth orbit. Since theoretical minimum intrinsic energy costs are orders of magnitude less than those of rockets, the financial costs may conceivably be similarly slashed.
Many proposed space projects are hamstrung by the high cost of rocket launch. These include space colonization, space-based solar power and terraforming Mars. Bringing the cost of launch down would remove a major obstacle to such projects.
In addition to reduced costs, other expected benefits may be increased safety and reliability of launches to the point where it would allow for disposal of radioactive waste in space.[not verified in body]
|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|
|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.
Static structures 
In this usage, the term "static" is intended to convey the understanding that the structural portion of the system has no internal moving parts.
Space tower 
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 far lesser 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 2000 meters as bricks at the bottom would be crushed under the weight. Other materials could allow the tower to reach a greater height, if a tapered structure, but 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. The current tallest man-made structure is 0.8 kilometer. 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, may 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 may be simpler. Inflatable, kinetic, and electronic structures may also be options.
Tensile structures 
Tensile structures for non-rocket spacelaunch are proposals to use long, very strong cables (known as tethers) to drag a payload into, or fling it toward, space. Tethers can also be used for changing orbit once in space.
Orbital tethers can be tidally locked (skyhooks) 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.
Space elevator 
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.
Theoretically 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 to build a space elevator. The taper ratio would need to be so large that the total mass of conventional materials needed to construct the cable would be far too great. 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 strength of these molecules is high compared to their densities and they hold promise as materials to make an Earth-based space elevator possible.
The 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.
Hypersonic skyhook 
A hypersonic skyhook is a relatively short tether that reaches from just above the edge of space to its design length.
Without the two end ballasts, a space elevator would still be in geosynchronous orbit, and thus stationary relative to the ground. If that tether were to be shorter and still reach the surface, the center of gravity would need to drop also. This would cause the lower tip to have a velocity in the orbital direction. The shorter the tether is, the faster becomes the lower tip velocity. With higher tip velocity, lower material properties are needed to make a practical design but the less benefit is obtained from this method. Eventually, any such design becomes a balance between the expense of providing the velocity to the payload at pick-up and the expense of launching the mass of the tether and power plant as dictated by available materials. Also, the lower tip is raised out of the atmosphere to avoid heating problems.
A reference design was published using materials similar to Spectra 2000 and relying on one Titan IV launch to orbit a fully functional hypersonic skyhook. To keep the tether weight within the launch capacity, a payload pick-up velocity of 5 km/s was assumed.
A SpaceShaft is a proposed atmospherically buoyant structure that would serve as a system to lift cargo to near-space altitudes. It would support 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 is comparable to a maritime oil spar platform. Although a SpaceShaft is also described as a structure, it is not a space tower because it does not stand on foundations in contact with the surface of the planet as to support compressive forces caused by weight. A platform at the top of a SpaceShaft either spaceplanes or spacecrafts with built-in propulsion systems could be launched.
A SpaceShaft would have the capability to lift cargo to space or near-space altitudes, but it would not itself be able to launch the cargo into orbit or any higher altitude. It is an elevator to space, but not not a space elevator as defined by the most common modern international use of the phrase.
The SpaceShaft was originally proposed at the 2nd Eurospaceward Conference on December 2008 in Luxembourg as part of a possible transportation method for the CNT tether spools that a space elevator system will need for its deployment from space.
A rotovator is a rotating tether such that the retrograde velocity of the tip fully cancels the orbital velocity. To a stationary payload, it appears as though the tether tip decelerates as it drops straight down from the sky, and then accelerates back upwards. The payload must grapple the tip of the tether during that short duration when the tip has come to a stop. Hans Moravec's description of this was "a satellite that rotates like a wheel".
Hypersonic rotovator (HASTOL) 
Similar to skyhook, a rotovator can be a much shorter tether than its stationary equivalent and this allows it to pick up its payload at hypersonic speeds. The Hypersonic Airplane, Space Tether, Orbital Launch (HASTOL) is one design for this.
Endo-atmospheric tethers 
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.
Dynamic structures 
Space fountain 
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.
Orbital ring 
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.
Launch loop 
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.
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.
Projectile launchers 
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.
Mass driver 
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.
Rail gun 
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.
Space gun 
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.
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.
Ram accelerator 
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.
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.
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) 
Air launch 
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 
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.
Buoyant lifting 
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 it is possible to construct a space port in the stratosphere such as a sky anchor. Rockets can start from it or a mass driver can accelerate payloads into the orbit. This has the advantage that most (about 90%) of the atmosphere is below the space port. One prototype has been made by JP Aerospace. Project Tandem is a space ladder to a balloon runway at 86,000 feet, with a plasma thrusting plane flying into space.
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, at a minimum entailing several percent of total delta-v to raise perigee (e.g. a tiny rocket burn), 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.
See also 
- "SpaceCast 2020" Report to the Chief of Staff of the Air Force, 22 Jun 94.
- A Fresh Look at Space Solar Power: New Architectures, Concepts, and Technologies. John C. Mankins. International Astronautical Federation IAF-97-R.2.03. 12 pages.
- Robert M. Zubrin (Pioneer Astronautics), Christopher P. McKay. NASA Ames Research Center (1993?). "Technological Requirements for Terraforming Mars".
- Tsiolkovsky, Исследование мировых пространств реактивными приборами - The Exploration of Cosmic Space by Means of Reaction Devices (Russian)
- Hirschfeld, Bob (2002-01-31). "Space Elevator Gets Lift". TechTV. G4 Media, Inc. Archived from the original on 2005-06-08. Retrieved 2007-09-13. "The concept was first described in 1895 by Russian author K. E. Tsiolkovsky in his "Speculations about Earth and Sky and on Vesta."" Citation copied from Space elevator article. As of Oct 2012, the archived page was still good, but the original no longer functioned.
- "The Hypersonic Skyhook", Analog Science Fiction / Science Fact, vol. 113, no. 11, September 1993, pp. 60-70.
- "A Non-Synchronous Orbital Skyhook", Hans P. Moravec, Journal of the Astronautical Sciences, vol. 25, Oct-Dec 1977
- Paper, AIAA 00-3615 "Design and Simulation of Tether Facilities for HASTOL Architecture", R. Hoyt, 17-19 Jul 2000.
- Paper, NIAC 3rd Ann. Mtg, NIAC subcontract no. 07600-040, “Hypersonic Airplane Space Tether Orbital Launch – HASTOL”, John E. Grant, 6 Jun 2001.
- "Orbital Ring Systems and Jacob's Ladders - I-III". Note: in 1980s money.
- Launch Loop slides for the ISDC2002 conference
- Johansen, US patent #6913224, Method and system for accelerating an object, 5 Jul 2005
- "The Startram Project"
- "Quicklaunch Inc."
- "Slingatron, A Mechanical Hypervelocity Mass Accelerator"
- Jordin Kare (2009). "HX Laser Launch".
- "NASA researcher Kevin Parkin discusses microwave space propulsion". nextbigfuture.com. 15 february 2011.
- www.vectorsite.net > [4.0] Space Guns v1.1.4 / Chapter 4 of 7 / 1 jun 2008 / Greg Goebel / Public domain
- Kenneth Gatland. The Illustrated Encyclopedia of Space Technology.
- Hirschfeld, Bob (2002-01-31). "Space Elevator Gets Lift". TechTV. G4 Media, Inc. Archived from the original on 2005-06-08. Retrieved 2007-09-13. "The concept was first described in 1895 by Russian author K. E. Tsiolkovsky in his "Speculations about Earth and Sky and on Vesta"."
- In his book "Structures or why things don't fall down" (pub[clarification needed] Pelican 1978 - 1984[clarification needed]), professor J. E. Gordon considers the height of the Tower of Babel. He wrote: brick and stone weigh about 120 lb per cubic foot (2000 kg per cubic metre) and the crushing strength of these materials is generally rather better than 6000 lbf per square inch or 40 megapascals. Elementary arithmetic shows that a tower with parallel walls could have been built to a height of 7000 feet or 2 kilometres before the bricks at the bottom were crushed. However by making the walls taper towards the top they … could well have been built to a height where the men of Shinnar would run short of oxygen and had difficulty in breathing before the brick walls crushed beneath their own dead weight."[clarification needed]
- Structural Methods. Retrieved May 26, 2011.
- "Atmosphere Table". Retrieved April 28, 2011.
- What is a Space Elevator?
- Bradley C. Edwards. IAC-04-IAA.3.8.2.01. THE SPACE ELEVATOR DEVELOPMENT PROGRAM.
- Non-Synchronous Orbital Skyhooks for the Moon and Mars with Conventional Materials, Hans Moravec, 1978
- "Space Shaft: Or, the story that would have been a bit finer, if only one had known...". Knight Science Journalism, Massachusetts Institute of Technology. Retrieved April 21, 2011.
- "The SpaceShaft". www.spaceshaft.org. Retrieved April 21, 2011.
- "3rd International Conference on Space Elevator, CNT Tether Design, and Lunar Industrialization Challenges". Luxembourg: EuroSpaceWard. December 5–6, 2009. Retrieved April 21, 2011.
- A. Yunitskii, “General Planetary Transport System”, “TM” [clarification needed] (Technology for Young), no. 6, 1982 (in Russian). www.ipu.ru/stran/bod/ing/sovet2.htm; pictures: www.ipu.ru/stran/bod/ing/soviet_ris.htm.
- "Orbital Ring Systems and Jacob's Ladders - I-III"
- Keith Lofstrom's launch energy paper from the International Space Development Conference in 2002 http://www.launchloop.com/isdc2002energy.pdf</a>
- Quine, B. M.; Seth, R. K.; Zhu, Z. H. (2009-04-19). "A free-standing space elevator structure: a practical alternative to the space tether". Acta Astronautica 65 (3–4): 365–375. doi:10.1016/j.actaastro.2009.02.018. hdl:10315/2587. (P. 7.)
- Quicklaunch website
- Book, "Slingatron - A Mechanical Hypervelocity Mass Accelerator"; Derek A. Tidman; ISBN 978-1-4276-2658-5; Aardvark Global Publishing LLC (November 1, 2007)
- Käsmann, Ferdinand C. W. (1999). Weltrekord-Flugzeuge [World Speed Record Aircraft] — Die schnellsten Jets der Welt (in German). Kolpingring, Germany: Aviatic Verlag. p. 105. ISBN 3-925505-26-1.
- Gerard K. O'Neill (1981). 2081 : a hopeful view of the human future.
- NASA. "Emerging Technologies May Fuel Revolutionary Launcher". Retrieved 2011-05-24.
- "The Maglifter: An Advanced Concept Using Electromagnetic Propulsion in Reducing the Cost of Space Launch". NASA. Retrieved 24 May 2011.
- "Constant Acceleration". Retrieved 24 May 2011.
- U.S. Air Force Research Report no. AU ARI 93-8: LEO On The Cheap. Retrieved April 29, 2011.
- "SpaceShipOne". Encyclopedia Astronautix. Retrieved May 25, 2011.
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