Non-rocket space launch (NRS) is a launch into space where some or all needed speed and altitude is provided by non-rocket means, rather than simply using conventional chemical rockets from the ground. 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.
Transportation to orbit is one factor in the expense of space endeavors; if it can be made more efficient the total cost of space flight can be reduced. Present-day launch costs are very high – $10,000 to $25,000 per kilogram from Earth to low Earth orbit.[1] Since theoretical minimum intrinsic energy costs are orders of magnitude less, major cost reduction is conceivable. To settle outer space, e.g. space exploration and space colonization, much cheaper launch methods are required. Another benefit may be increased safety and reliability of launches, which, in addition to lower cost, would avail for space disposal of radioactive waste. Once having overcome the Earth gravity barrier, vehicles may instead use other, non-rocket-based methods of propulsion, e.g. ion thrusters, which have a higher propellant efficiency (specific impulse) and potential maximum velocity than conventional rockets, but are not suitable for spacelaunch.[2]
Contents |
| Method(a) | Publication year |
Estimated build-cost US$B(b) |
Payload Size kg |
Estimated cost to LEO US$/kg(b) |
Capacity metric tons per year |
Technology readiness level |
|---|---|---|---|---|---|---|
| Conventional rocket[1] | 700-130,000 | 4,000-20,000 | ≈ 200 | 9 | ||
| Space elevator | 2004 | |||||
| Hypersonic Skyhook[3] | 1993 | <1(c) | 1,500(d) | 30(e) | 2 | |
| Rotovator[4] | 1977 | 2 | ||||
| HASTOL[5][6] | 2000 | 15,000(f) | 2 | |||
| Orbital ring[7] | 1980 | 15 | <0.05 | 2 | ||
| Launch loop[8] (small) | 1985 | 10 | 5,000 | 300 | 40,000 | 2+ |
| Launch loop[8] (large) | 1985 | 30 | 5,000 | 3 | 6,000,000 | 2+ |
| KITE Launcher[9] | 2005 | 2 | ||||
| StarTram[10] | 2001 | 20(g) | 35,000 | 43 | 150,000 | 2-4 |
| Rocket sled launch, e.g. turbo fan skyramp | 4 | |||||
| Ram accelerator | 2004 | <500 | 6[11] | |||
| Space gun[12] | 1865(h) | 0.5 | 450 | 500 | 6 | |
| Slingatron[13] | 100 | 2 | ||||
| Spaceplane | 1992 | 10-15 | 12,000 | 3,000 | 7 | |
| Laser propulsion[14] | 2 | 100 | 550 | 3000 | up to 4 |
(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.[15]
In this usage, the term "static" is intended to convey the understanding that the structural portion of the system has no internal moving parts.
Compressive structures for non-rocket spacelaunch are proposals to use long, very strong structures like guyed antenna towers or artificial mountains up which payloads can be raised.
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.[16] If the tower went all the way to geosynchronous orbit at approximately 36,000 km, objects released at such height could then drift away with minimal power and would be in a circular orbit, though a tower to that extreme height is not doable with current materials on Earth. The concept of a structure reaching to geosynchronous orbit was first conceived by Konstantin Tsiolkovsky,[17] 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.[18] 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.[19]
For common plain carbon steel under a typical allowable stress limit, its scale height is ≈ 1.635 kilometers. A 4.9 kilometer high tower (3x 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.[19][20]
Natural mountains reach up to 9 km altitude. The current tallest man-made structure is 0.8 kilometers. 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 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.
A skyhook is a tidally locked tether, i.e., it rotates once each time it orbits around a planet or moon.
An example use of a skyhook is that a payload, launched from the ground, can be attached to the base of the skyhook, which is then carried to orbit. This means that a single stage to skyhook approach can be employed, and a high-specific impulse drive or propellantless electromagnetic tether can be used to make up the momentum debt of the payload- or payload flow can be balanced from the moon.
A space elevator a.k.a. beanstalk or "synchronous skyhook" is a stationary skyhook. It focuses on tensile structures (tethers) reaching from the ground to geosynchronous orbit.
The most common proposal is a tether, usually in the form of a cable or ribbon, spanning from the surface near the equator to a point beyond geosynchronous orbit. Neglecting perturbations, it would be possible to design such a tether to barely touch the ground while remaining in orbit. All proposals however have additional ballast placed at the two ends to provide stability. As the planet rotates, the inertia 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 structure could hypothetically permit delivery of cargo and people to orbit at a fraction of the cost of launching payloads by rocket.
Current technology is not capable of manufacturing materials that are sufficiently strong and light enough to build an Earth-based space elevator as the total mass of conventional materials needed to construct such a structure would be far too great. Recent proposals for a space elevator are notable in their plans to use carbon nanotube-based materials as the tensile element in the tether design, since the theoretical strength of carbon nanotubes appears great enough to make this practical, if strength on the macro scale can be obtained comparable to the figures for individual nanotubes.
Another difficulty includes shielding the passengers from the Van Allen belts which would require extremely heavy and comprehensive shielding to prevent significant health issues such as cancer and may prevent manned launch for quite some time irrespective of the other issues.
Current technology may be able to support elevators in other locations in the solar system however, and other designs for space elevators exist that use current materials.
A partial space elevator can also be less than the full length from orbit to the surface[21] Subsystems for an orbiting space elevator have been demonstrated with the X-43 [2] and TIPS [3] flights.
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[3] 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. Though the reference design was limited to an Initial Operating Condition of 1,500 kg payload size, at a maximum rate of about one payload each 17 days, the prime limitation to higher capacity was power plant size. One launch of additional power plant would almost double the available power and capacity.
The problem is getting the payload to the altitude (100 km) and velocity (5 km/s) required for pick-up.
A SpaceShaft is a proposed atmospherically levitating structure that would serve as an elevator system to near-space altitudes. It will 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 also a candidate of the technologies cataloged for non-rocket spacelaunch.[22][23]
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, (note; pure weight becomes significant at the elevation where buoyancy becomes null).[24] On the contrary, it is a very dynamic system since it would be constantly moving upwards. A SpaceShaft is not an orbital insertion system, as is assumed to intrinsically be the case with the Brad Edwards' proposed Centrifugally Extended Carbon-Nanotube Tether Space Elevator. However, from a platform at the top of a SpaceShaft either spaceplanes or spacecrafts with built-in propulsion systems could be launched.[23]
Because of the orbital insertion incapability of the SpaceShaft,[25] some people do not regard a SpaceShaft as a true space elevator, but this is a questionable criticism.
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 the popular ISEC space elevator system will need for its deployment from space. The system has never been adopted by the proponents of the ISEC model. However, the group has become a common face among the European groups that advocate the use of space elevators.
A bolus is a tether that rotates more than once each time it orbits around a planet or moon. Bolus typically rotate in the same sense as they orbit such that the lower tip has a retrograde motion relative to the center of gravity.
Bolus tethers give in almost all ways have the same benefits as skyhooks. However, due to the retrograde velocity, the lower tip can achieve a specified Mach number with a shorter tether. This, despite the rotational forces, produces lower stresses in the tether so that lower strength to weight ratio materials can be used for the same results.
A rotovator is a bolus 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 upward. 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."[4] With current materials a rotovator to reach Earth's surface is impractical however; but is possible on other interplanetary bodies such as Mars and the Moon.
Similar to skyhook, a spinning bolus space tether 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.
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.[9]
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.
The first design of an orbital ring offered by A. Yunitsky in 1982.[26]
In the 1982 Paul Birch JBIS design[27] 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 3g acceleration.[28]
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".[29] 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. 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.
A rail gun is a pair of conductive rails with a projectile between them.
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.[10]
A space gun is a method of launching an object into outer space using a large gun, or cannon.
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.
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.
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 gasses with a frangible diaphragm at either end to contain the gasses. 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 gasses as fuel, accelerating down the tube under jet propulsion. Other physics come into play at higher velocities.
In a slingatron,[13][30] 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 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.
Early spaceplanes were used to explore hypersonic flight (e.g. X-15).[31]
Some air-breathing engine-based designs (c.f. 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.
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.
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.[32] This has the advantage that most (about 90%) of the atmosphere is below the space port and that rockets start much closer to space. As a rough estimate, a rocket that reaches an altitude of 20 km when launched from the ground will reach 100 km if launched at an altitude of 20 km from a balloon.[33]
Separate technologies may be combined. NASA in 2010 suggested that a future scramjet aircraft might be accelerated to 300 m/s (a solution to how ramjet engines do not start at zero airflow velocity) by electromagnetic or other sled launch assist, in turn air-launching a second-stage rocket delivering a satellite to orbit.[34]
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.[10]
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.[35]
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.[36] 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.[5]
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.[37] 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.[38]
|
||||||||||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||||||