Electrically powered spacecraft propulsion

From Wikipedia, the free encyclopedia

For vehicles other than spacecraft that are propelled by electric means, see Electric vehicle.

 

6 kW Hall thruster in operation at the NASA Jet Propulsion Laboratory.

 

An electrically powered spacecraft propulsion system uses electrical energy to change the velocity of a spacecraft. Most of these kinds of spacecraft propulsion systems work by electrically expelling propellant (reaction mass) at high speed, but electrodynamic tethers work by interacting with a planet's magnetic field.^[1]
Electric thrusters typically use much less propellant than chemical rockets because they have a higher exhaust speed (operate at a higher specific impulse) than chemical rockets.^[2] Due to limited electric power the thrust is much weaker compared to chemical rockets, but electric propulsion can provide a small thrust for a long time.^[3] Electric propulsion can achieve high speeds over long periods and thus can work better than chemical rockets for some deep space missions.^[2]
Electric propulsion is now a mature and widely used technology on spacecraft. Russian satellites have used electric propulsion for decades.^[4] As of 2013, over 200 spacecraft operated throughout the solar system use electric propulsion for stationkeeping, orbit raising, or primary propulsion.^[5] In the future, the most advanced electric thrusters may be able to impart a Delta-v of 100 km/s, which is enough to take a spacecraft to the outer planets of the Solar System (with nuclear power), but is insufficient for interstellar travel.^[2][6] Also, an electro-rocket with an external power source (transmissible through laser on the solar panels) has a theoretical possibility for interstellar flight.^[7][8] However, Electric propulsion is not a method suitable for launches from the Earth's surface, as on average the thrust for such systems is too weak.

Contents

• 1 History
• 2 Types
  • 2.1 Ion and plasma drives
    • 2.1.1 Electrostatic
    • 2.1.2 Electrothermal
    • 2.1.3 Electromagnetic
  • 2.2 Non-ion drives
    • 2.2.1 Photonic
    • 2.2.2 Electrodynamic tether
    • 2.2.3 Unconventional
  • 2.3 Steady vs. unsteady
• 3 Dynamic properties
• 4 See also
• 5 References
• 6 External links

History

The idea of electric propulsion for spacecraft dates back to 1911, introduced in a publication by Konstantin Tsiolkovsky.^[9] Earlier, Robert Goddard had noted such a possibility in his personal notebook.^[10]
Electrically powered propulsion with a nuclear reactor was considered Dr. Tony Martin for interstellar Project Daedalus in 1973, but the novel approach was rejected because of very low thrust, much weight needed to convert nuclear energy into electrical equipment and as a result a small acceleration, which would take a century to achieve the desired speed.^[11]
The first in-space demonstration of electric propulsion was an ion engine carried on board the SERT-1 (Space Electric Rocket Test) spacecraft,^[12][13] launched on 20 July 1964 and it operated for 31 minutes.^[12] A follow-up mission launched on 3 February 1970, SERT-2, carried two ion thrusters, one operated for more than five months and the other for almost three months.^[12][14][15]
By the early 2010s, many satellite manufacturers were offering electric propulsion options on their satellites—mostly for on-orbit attitude control—while some commercial communication satellite operators were beginning to use them for geosynchronous orbit insertion in place of traditional chemical rocket engines.^[16]
One such satellite system is the Boeing 702SP which can be launched as a pair on a lighter-weight dual-commsat stack—two satellites conjoined on a single launch—and was specifically designed to take advantage of the lower-cost SpaceX Falcon 9 launch vehicle.^[17][18] The first two commsats of this design were launched aboard a SpaceX rocket from Cape Canaveral, Florida, at 10:50 PM on 1 March 2015.

Types

Ion and plasma drives

Main article: Ion thruster

This type of rocket-like reaction engine uses electric energy to obtain thrust from propellant carried with the vehicle. Unlike rocket engines, these kinds of engines do not necessarily have rocket nozzles, and thus many types are not considered true rockets.
Electric propulsion thrusters for spacecraft may be grouped in three families based on the type of force used to accelerate the ions of the plasma:

Electrostatic

If the acceleration is caused mainly by the Coulomb force (i.e. application of a static electric field in the direction of the acceleration) the device is considered electrostatic.

• NASA Solar Technology Application Readiness (NSTAR)
• Electrostatic ion thruster (Ion Bombardment)
• Colloid ion thruster
• Contact ion thruster^{[citation needed]}
• Field Emission Electric Propulsion
• Microwave (Radiofrequency) ion thruster^{[citation needed]}
• Plasma separator ion thruster^{[citation needed]}
• Radioisotopic ion thruster^{[citation needed]}
• Nano-particle field extraction thruster
• Hall effect thruster
  • SPT – Stationary Plasma Thruster
  • TAL – Thruster with Anode Layer

Electrothermal

The electrothermal category groups the devices where electromagnetic fields are used to generate a plasma to increase the temperature of the bulk propellant. The thermal energy imparted to the propellant gas is then converted into kinetic energy by a nozzle of either solid material or magnetic fields. Low molecular weight gases (e.g. hydrogen, helium, ammonia) are preferred propellants for this kind of system.
An electrothermal engine uses a nozzle to convert the heat of a gas into the linear motion of its molecules so it is a true rocket even though the energy producing the heat comes from an external source.
Performance of electrothermal systems in terms of specific impulse (Isp) is somewhat modest (500 to ~1000 seconds), but exceeds that of cold gas thrusters, monopropellant rockets, and even most bipropellant rockets. In the USSR, electrothermal engines were used since 1971; the Soviet "Meteor-3", "Meteor-Priroda", "Resurs-O" satellite series and the Russian "Elektro" satellite are equipped with them.^[19] Electrothermal systems by Aerojet (MR-510) are currently used on Lockheed Martin A2100 satellites using hydrazine as a propellant.

• Arcjet
• Microwave arcjet
• Resistojet

Electromagnetic

Main article: Plasma propulsion engine

If ions are accelerated either by the Lorentz force or by the effect of an electromagnetic fields where the electric field is not in the direction of the acceleration, the device is considered electromagnetic.

• Electrodeless plasma thruster
• MPD thruster
• Pulsed inductive thruster
• Pulsed plasma thruster
• Helicon Double Layer Thruster
• Variable specific impulse magnetoplasma rocket (VASIMR)

Non-ion drives

Photonic

Photonic drive does not expel matter for reaction thrust, only photons. See Laser propulsion, Photonic Laser Thruster, Photon rocket.

Electrodynamic tether

Main article: electrodynamic tether

Electrodynamic tethers are long conducting wires, such as one deployed from a tether satellite, which can operate on electromagnetic principles as generators, by converting their kinetic energy to electric energy, or as motors, converting electric energy to kinetic energy.^[20] Electric potential is generated across a conductive tether by its motion through the Earth's magnetic field. The choice of the metal conductor to be used in an electrodynamic tether is determined by a variety of factors. Primary factors usually include high electrical conductivity, and low density. Secondary factors, depending on the application, include cost, strength, and melting point.

Unconventional

The principle of action of these theoretical devices is not well explained by the currently-understood laws of physics.^[21]

• Quantum Vacuum Plasma Thruster
• EmDrive

Steady vs. unsteady

Electric propulsion systems can also be characterized as either steady (continuous firing for a prescribed duration) or unsteady (pulsed firings accumulating to a desired impulse). However, these classifications are not unique to electric propulsion systems and can be applied to all types of propulsion engines.

Dynamic properties

Further information: Reaction engine § Energy use

Electrically powered rocket engines provide lower thrust compared to chemical rockets by several orders of magnitude because of the limited electrical power possible to provide in a spacecraft.^[3] A chemical rocket imparts energy to the combustion products directly, whereas an electrical system requires several steps. However, the high velocity and lower reaction mass expended for the same thrust allows electric rockets to run for a long time. This differs from the typical chemical-powered spacecraft, where the engines run only in short intervals of time, while the spacecraft mostly follows an inertial trajectory. When near a planet, low-thrust propulsion may not offset the gravitational attraction of the planet. An electric rocket engine cannot provide enough thrust to lift the vehicle from a planet's surface, but a low thrust applied for a long interval can allow a spacecraft to maneuver near a planet.

See also

• Magnetic sail, a proposed system powered by solar wind from the Sun or any star
• List of spacecraft with electric propulsion, a list of past and proposed spacecraft which used electric propulsion

S-F Spacecraft: Cole/Helios nuclear pulse vehicles

http://up-ship.com/blog

Feb 112010

As a followup to THIS and THIS, here’s an example of what might be in the actual book… the section on the Cole/Helios internal nuclear pulse propulsion system (if you want to see more on Cole/Helios, check out issue Volume 1, Number 3 of Aerospace Projects Review. Also much better-rez images, and more of ‘em).
<>  The final book would likely have several different illustrations (as well as larger ones), largely for copyright reasons. A book of this type would necessarily have to be graphics-heavy, and getting licensing for a whole bunch of art would probably be cost prohibitive (for example, a major arospace company who’s name begins with “B” and ends with “ing” typically wants about $400 per illustration… *far* beyond my, or pretty much anybodies,  means). In Word format, the following fills up 11 pages. Multiply that by the outline previously published, and you’ll get a hint as to the scope and size of the book.
………………..
The Helios Concept
The notion of using atomic weapons as part of a powerful spacecraft propulsion system arose shortly after the dust had settled from the first atomic blast in New Mexico during the summer of 1945 With an energy denisty vastly greater than any conceivable chemical propellant, the idea of using the blast from an atomic bomb to push a spacecraft was simply too attractive to pass up.
While studies of the Orion concept were underway, work was underway elsewhere on a related but quite different propulsion system. From 1959 to 1961, Dandridge Cole, a visionary engineer at the Martin Company in Denver, Colorado, produced theoretical studies of vehicles propelled by contained nuclear explosions. In this concept, the atomic device would be detonated within a large spherical chamber; a nozzle would direct the blast in an orderly stream directly aft. The clear advantage of this system was that a vast proportion of the bomb’s energy could be harnessed, whereas the Orion concepts used only ten percent or so of the bomb’s energy.
Coles studies focused on four very theoretical concepts; no real engineering design work seems to have been done.
Cole’s “Model I” vehicle was dominated by a 130 foot diameter steel sphere. With a wall thickness of 0.5 inches, the sphere alone weighed 1,000,000 pounds. The payload, shock absorbers and propellant were contained within a structure mounted forward of the steel sphere. The Model I was intended for use as a space vehicle not a launch vehicle, although it was considered possible for the Model I to boost itself into Earth orbit. A standard mission was to leave Earth orbit and soft-land on the moon, drop its payload, return to Earth and aerobrake into orbit. Payload was to be 350,000 pounds. Alternate missions would have taken the Model I from Earth orbit to Mars or Venus; mission velocities of 26,000 ft/sec were possible.
The atomic bomb “energy capsules” had a yield of 0.01 kilotons, or the equivalent of 10 tons of explosive power. A total of 2,400 of these energy capsules would be carried. In order to provide useful thrust, 858 pounds of water would be pumped into the chamber just before each detonation. This would be done through transpiration cooling channels in the wall of the sphere. By using the power of the energy capsules to heat water, the Model I gained two important features: firstly, the water helped cool the sphere, and secondly, the water served as a reaction mass for the bomb’s energy to work with. Had a propellant not been used, the bomb would have served as an effective means of heating the chamber, but would have done little towards accelerating the vehicle. A total of 2,060,000 pounds of water was carried onboard.

Cole’s Model 1 (Martin, 1961)
The wall thickness of the Model I steel sphere was dictated by the potential use of the vehicle within Earths atmosphere. At sea level, 85,000 pounds of air would be contained within the vessel; the walls would be subjected to a sudden overpressure of 310 psi. For purely space missions, where the sphere would not see operation within an atmosphere, the wall thickness, and thus weight, could be reduced substantially.
Cole preferred using a Nova class chemical rocket booster to loft the Model I to orbit, but considered self-launch. By increasing the pulse rate, thrust would be increased to where the thrust to weight ratio was adequate for boost. Cole also mentioned that since the total energy release during boost would be less than that released by a single multi-megaton bomb, then atmospheric contamination by radioactive particles would be correspondingly less. What Cole didn’t mention, however, was that in order to get a nuclear device to detonate with only 0.01 kilotons yield, it must be made intentionally inefficient, and hence rather filthy.
Engine performance for the Model I was not spectacular, and was in fact considerably lower than for the baseline Orion vehicle. Thrust was 800,000 pounds, with a specific impulse of 931 seconds. Thrust-to-weight for a Model I operating at one pulse per second was 0.25, which was adequate for lunar landing missions. Orion performance was expected to be far superior. NERVA-style nuclear thermal engines would be capable of similar performance, with far less fallout.
Cole’s Model II was a straightforward evolution of the Model I. The thrust chamber weight was reduced to 200,000 pounds by using stronger materials, and specific impulse was increased to 1,150 seconds by replacing the water propellant with hydrogen. The number of energy capsules would be increased to 5,800; these were expected to cost $10,000 each. The mass of hydrogen expelled with each pulse was 558 pounds. Payload would be increased to 2,920,000 pounds. Vehicle gross weight was 6,720,000 pounds.
The Model IIa was also sketched out. This was a simple tenfold mass scaleup of the Model II. This included scaling up the thrust chamber to 282 feet diameter, and increasing energy capsule yield to 0.1 kilotons. The main advantage of this scaleup was derived from the fact that the energy capsules would not cost substantially more to have an increased yield.  This would also result in a slightly increased specific impulse of 1,350 seconds.
The fourth, and most entertaining, of Coles designs was the Nuclear Pulse Jet. This was, in essence, a pulsejet similar to that used on the V-1 “Buzz Bomb” of World War II , scaled up and using atomic bombs instead of chemical fuel. This vehicle would be used specifically for Earth surface to orbit operations.

Sketch of Nuclear Pulse Jet (Martin)
No design details were provided for this vehicle, apart from the basic concept. By raising pulse rate to two per second and using atmospheric air for propellant, average thrust of the Nuclear Pulse Jet was to be 9,660,000 pounds. Coles comments on atmospheric contamination are a clear anachronism, pointing out that the late 1950s-early 1960s was a different era, when people were more willing to take risks for potentially great rewards. “It may be decided that the relatively minor health hazard from this contamination is far less a matter for concern than, say, the exhaust from 50 million automobiles, and should be accepted philosophically as one of the penalties we pay for civilization and progress.” Cole also proposed the use of “clean” bombs, something still not available (at least so far as is known). Very low-yield bombs have been built, but they use much the same mass of very expensive fissile material as much higher-yield bombs. Very low yields are achieved by making the fission reaction inefficient and incomplete… unfortunate side effects include a great deal of very radioactive fallout and considerable variability in yield.
Little interest was shown by the USAF, NASA or Martin higher management, and Cole’s vehicle concept faded away. It appears that no true engineering studies or detailed vehicle design efforts were undertaken, although a considerable amount of art was created of the concept.
While Cole and Martin did no further work of note on the internal detonation concept, the idea received further study at Lawrence Livermore Laboratory in California. Starting late in 1963, this program ran under the name Project Helios. While this program also did not apparently produce engineering studies of vehicle configurations, it did produce more detailed studies of pulse unit and thrust chamber requirements. The thrust chamber design was given highest priority. Also, artwork of conceptual Helios vehicles was produced. While no engineering data has been produced to back these paintings up, they are of a much higher technical order than those produced at Martin and appear fairly realistic given the propulsion system.
The initial Helios study concept called for a vehicle of 1,500,000 pounds in Earth orbit, intended for a manned mission to Mars. Payload delivered to mars orbit (the Helios vehicle itself would not go to the Martian surface) was on the order of 50 tons; total mission velocity to be achieved was 60,000 ft/sec. The vehicle would be assembled in Earth orbit from components launched by conventional boosters.
For the first vehicle iteration (Jan 1964) the optimum number of pulses would be about 4000; each would have a yield of only 4 tons (0.004 kilotons). Each charge would mass about 100 pounds. Five hundred pounds of hydrogen would be introduced into the 20 to 50 foot diameter thrust chamber; a small amount of carbon would be added to increase radiation absorption. The chamber was initially expected to mass about 80 tons. Thrust would be two or three million pounds (nine or thirteen million Newtons) for 0.1 seconds; pulses would occur every ten seconds… averaging out to two to three hundred thousand pounds of thrust.. By January of 1965, the chamber was 30 feet in diameter; the pulse unit yield was 5.1 tons (0.0051 kt), and the hydrogen propellant was reduced to 150 pounds per pulse.
Considerable effort was put into calculation of thrust chamber wall conditions and structural design. Wall temperatures would peak at about 14,000 K slightly less than two milliseconds after detonation, with a pressure maximum of 5.25 kilobars at the same instant. These were instantaneous values, and rapidly dropped to manageable levels. Calculations suggested that 4.5% or more of the energy of each pulse would be retained in the chamber walls as heat after propellant discharge. This was about the same as the allowable maximum heat retention, which implied that further analysis would have to be done. Several potential chamber wall materials were tested. One that showed considerable promise was a laminate of 0.062 inch thick titanium alloy (Ti – 5 Al – 2.5 Sn) sheets, bonded with a silver alloy. It was found that a six-ply laminate had 80% of the toughness of a single same-thickness sheet of the same alloy, while it had 6 times the fracture toughness of bar stock of the same alloy.
The pulse units assumed for the vehicle were quite simple in overall design. A rather simple sphere only one meter in diameter, most of the unit was a low density high explosive shell. In the core was a 2 kg uranium sphere; it was jacketed by a higher density shell of high explosive. This combination would produce an explosive yield equivalent to 5.1 tons of TNT.

Pulse unit (Not To Scale)
While the amount of uranium expended with each pulse was rather small, during the course of a baseline mission several tons would be used. Also, while the pulse units were able to use a small quantity of uranium to produce the desired effect, it was an inefficient system… ten times that amount of uranium would have produced more than one thousand times the energy release. So, in effect, while the Helios concept would have generated a high specific impulse and made efficient use of the energy available, the way in which it used valuable fissionable material was very inefficient. Only a small fraction of the uranium would properly fission; the rest would be vaporized, leaving a trail of radioactive waste behind the Helios vehicle.
Also standing in the way of the Helios concept were the throat conditions experienced by the large thrust chamber. Throat temperatures of 4000 Kelvin were expected, well beyond the ability of known materials to handle. Also, the radiative heating on the walls of the chamber would also have been very high, stressing the wall material nearly to the maximum allowable tolerance. These heating problems, in conjunction with the high engine mass of internal detonation systems, compared poorly to external detonation systems. Also, despite Dandridge Cole’s models, the internal detonation systems do not scale up well.
As a result of these difficulties, and the Nuclear Test Ban Treaty, work on Helios was halted 1965. No similar concepts have since come to light.

Uses:

A “Cole/Helios” style nuclear pulse engine could be used for both ground launched designs as well as pure in-space systems. With the ability to use atmospheric air as reaction mass, the Cole-type engine could be used for atmospheric cruise as well as launch.
The engine should be a fairly simple affair… a spherical detonation chamber married to a nozzle. The size of the nozzle and the throat would depend on the role of the engine… is it strictly an in-space propulsion system, or is it intended to operate within an atmosphere? If the latter, the nozzle will be shorter, with a lower expansion ratio than an equivalent in-space system. Despite the unconventional nature of the energy source, the basic performance of the Helios engine is fundamentally like that of a conventional liquid rocket engine, and can be calculated using the same processes described in section XXX. This type of engine scales down rather poorly, limited by the ability to create a reliable, consistent and fully self contained pulse unit… a sub-kiloton nuclear device of consistent yield is difficult to manufacture. Unless a “nuclear hand grenade” is developed (which is of course conceivable in a science fiction world), anything much smaller than what was described for Helios is unlikely.
For an air breathing Cole engine, inlets would obviously be required. The flow path to the engine should be as straight and unobstructed as possible. Flow straighteners may be used to make sure that the airflow into the engine is as consistent as possible. The exact mechanisms for injecting air into the engine can vary substantially… a vast multitude of small injectors spread around the periphery of the spherical chamber is likely the best approach, but a smaller number of larger, simpler injectors may also be used if a secondary engine coolant system is employed. A combination of a few discrete air injectors coupled with a vast number of liquid injectors would probably work well.
In operation, a Helios-style engine would pulse once or less per second. Unlike Orion, Helios not only needs to fire a nuclear pulse unit, but also a large quantity of reaction mass; engineering difficulties with this would limit pulse rate. Compared to Orion, the need for complex shock absorbers would be reduced. While Orion involved a mechanical impulse against the pusher plate that would last only microseconds, Cole/Helios engines would spread the impulse out over around one tenth of a second. In this case, it would probably be easier and more cost efficient to put the payload on shock absorbers.

Appearance:

With each pulse there would be a momentary blinding flash from dead center of the detonation chamber. The reaction mass would flare to incandescence and flood out of the engine as a conventional exhaust. With water as the reaction mass, the exhaust would be largely indistinguishable from a conventional hydrogen/oxygen rocket engine, with the exception that the exhaust plume would last for only a small fraction of a second (no more than about 0.1 seconds). The throat was expected to get extremely hot, and would thus glow white hot for several seconds at least. Cooling would best be done by evaporating a fluid through the throat, which would produce a super heated but largely non-propulsive vapor between pulses.
Since the propulsion system operates by pulses separated by noticable periods of time, from large fractiong of a second to several seconds, structural flexing could quite likely be noticable.
External bracing is quite likely and appropriate, depending upon the vehicle. For a vehicle intended for atmospheric flight, especially an atmospheric “cruiser,” any such structural reinforcement would most likely be encase in an aerodynamic shroud.

Examples (Real World):

Martin Company artwork showing Nuclear Pulse Jet in operation. The vehicle seems to be designed as a seaplane. For scale reference, note the flight of Martin P6M SeaMaster bombers that are about to get vaporized, and the shadow of the spaceplane on the water. The ship below appears to be the S.S. United States, which had a length of 990 feet. This image is not to be taken as a faithful representation of contemporary Martin design efforts, but as pure marketing.

Coles “Aldebaran,” as painted by space artist Roy Scarfo. In 1959, Dandridge Cole envisioned craft such as this being the backbone of the space launch industry in the 1980-1990 timeframe. The Aldebaran was to be able to carry 60,000,000 pounds of payload into low Earth orbit, or soft-land 45,000,000 pounds on the Moon. Scale is shown by comparing the Aldebaran to the liner SS. United States; the helicopter shown loading cargo into the Aldebaran also helps show the substantial size envisioned. Clearly, if the vehicle could carry 60 million pounds of payload, it would need a bigger payload loading door than the one shown. Curiously, a secondary cockpit or observation deck is shown on the vertical fin.

Coles “Macrolife” concept spacecraft, painted in 1960. By using contemporary rocket vehicle growth curves, Cole expected vessels of this size to be built by the end of the twentieth century. Carrying 10,000 or more colonists, the “Macro-Life” vehicles would be fully self-contained and self-sufficient, needing only to stop off at the occasional asteroid or comet for raw materials. Note that the nozzle appears to be very short, and that the throat appears to be quite large, a good fraction of the diameter of the chamber.

This Lawrence Livermore “Helios” concept seems to show, at first glance, a ground launch vehicle. However, the exposed truss-structure around the engine bell argues against that; what was probably the goal here was to put the crew compartment and payload as far from the detonation chamber as possible. This design also puts the pulse units between the crew and detonations, providing further radiation protection. Also visible are support rods on the pulse units, which were, presumably, to assure that the unit was properly positioned within the chamber for detonation. This sort of setup would assure that the pulse rate would be low.
This was clearly a design for a Mars vehicle. At the extreme front of the Helios is a landing craft that looks much like a model rocket; tucked in behind that is a winged Earth return vehicle of a type studied for several other Mars vehicles. The tanks strapped to the side of the vehicle are for liquid hydrogen propellant storage. (courtesy Livermore Labs)

This shows what is clearly a design meant for on-orbit assembly. The three toroids appear to be inflatable structures, most likely habitation volume for the crew. This vehicle may have been meant to spin to provide artificial gravity.
Volume for pulse units and propellant seems to be very limited. Again, though, it appears to be a Mars vehicle, owing to the winged lander (which appears to have legs for a tail-landing). What the jointed arms are for is something of a mystery… engine gimbal seems unlikely, and they would make poor shock absorbers.  (courtesy Livermore Labs)

A very unusual configuration. What was hoped to be gained by this layout is unclear; being clearly an on-orbit assembled design, length is fairly unimportant, and this puts the crew quite close to the detonations and radioactive exhaust. Note the “afterburner.” Exactly how this was meant to work is unclear.  (courtesy Livermore Labs)

Helios reconstructions based on the preceding artists impressions. (copyright 2007, Scott Lowther)

Examples (fictional):

N/A
NOTE: this section would describe known examples of the concept as used in prior books, Movies, TV series, etc. Commentary and criticism on the designs and usage would be provided.

Background References:

Martin, A. and Bond, A. “Nuclear Pulse Propulsion: A Historical Review of an Advanced Propulsion Concept,” Journal of the British Interplanetary Society, Vol. 32, pp. 283-310, 1979
Martin/Cole Studies:
Cole, D. “The Feasibility of propelling Vehicles by Contained Nuclear Explosions,” Advances In the Astronautical Sciences, Vol. 6, pp. 726-742, 1961.
Yaffee, M. “Martin Proposes Nuclear Rocket Plan,” Aviation Week, January 25, 1960, pp. 34-35.
Cole, D. Beyond Tomorrow – The Next Fifty years In Space, Amherst Press, Amherst Wisconsin, 1965.
Helios Studies:
Helios Quarterly Report No. 1, January 27, 1964, , University of California, Lawrence Radiation Laboratory, Livermore, California
Helios Quarterly Report No. 2, June 4, 1964, , University of California, Lawrence Radiation Laboratory, Livermore, California
Helios Progress Report No. 3, January 6, 1965, University of California, Lawrence Radiation Laboratory, Livermore, California
Helios artwork courtesy Laboratory Archives, University of California, Lawrence Livermore National Laboratory

Nuclear lightbulb

From Wikipedia, the free encyclopedia

Nuclear gas core closed cycle rocket engine diagram, nuclear "light bulb"

 

A nuclear lightbulb is a hypothetical type of spacecraft engine using a Fission reactor to achieve Nuclear propulsion. Specifically it would be a type of Gas core reactor rocket that separates the nuclear fuel from the coolant/propellant with a quartz wall. It would be operated at such high temperature (approx. 25,000°C) that the vast majority of the electromagnetic emissions would be in the hard ultraviolet range. Fused silica is almost completely transparent to this light, so it would be used to contain the uranium hexafluoride and allow the light to heat reaction mass in a rocket or to generate electricity using a heat engine or photovoltaics. ^[1]
This type of reactor shows great promise in both of these roles. As a rocket engine it, like all nuclear rocket designs, can greatly exceed the power density of a chemical rocket. However, it also does not involve the release of any radioactive material from the rocket, unlike other nuclear designs which would cause nuclear fallout if used in a planetary atmosphere (e.g. Project Orion). As a method to generate electricity, nuclear lightbulbs are extremely efficient because higher-temperature heat contains more Gibbs free energy than the low-temperature heat produced in current fossil-fuel plants and water-cooled nuclear reactors. The theoretical specific impulse (I_sp) range from 1500 to 3000 seconds.

See also

• Gaseous fission reactor
• Nuclear material
• Atomic physics

Interstellar mission spaceship designs require tens of megawatts per kilogram of power density

November 03, 2013

Project Icarus: Specific Power for Interstellar Missions Using Inertial Confinement Fusion Propulsion

Current chemical energy systems yield low specific energy which lead to energy conversion systems with a low performance design, less than 10 km/s.

Higher specific power drastically shortens mission times.

Interstellar travel

From Wikipedia, the free encyclopedia

A Bussard Ramjet, one of many possible methods that could serve as propulsion for a starship.

 

Interstellar space travel is manned or unmanned travel between stars. Interstellar travel is conceptually much more difficult than interplanetary travel: the distance between the planets in the Solar System is typically measured in standard astronomical units (AU) — while the distance between the stars is typically hundreds of thousands of AU, and usually expressed in light years. This means that some combination of huge travel time (lasting from years to millennia) and great speed (some percentage of the speed of light) would be required. These speeds are far beyond what current methods of spacecraft propulsion can provide.
A variety of concepts have been discussed in the literature, since the first astronautical pioneers (such as Konstantin Tsiolkovsky, Robert Esnault-Pelterie and Robert Hutchings Goddard). Given sufficient travel time and engineering work, both unmanned and manned interstellar travel require no break-through physics to be achieved, but considerable technological and economic challenges need to be met. NASA, ESA and other space agencies have been engaging in research into these topics for decades, and have accumulated a number of theoretical approaches.
(Intergalactic travel, or travel between different galaxies, would be even more difficult.) The concept of interstellar travel via starships is a staple of science fiction.

Contents

• 1 Challenges
  • 1.1 Interstellar distances
  • 1.2 Required energy
  • 1.3 Interstellar medium
  • 1.4 Travel time
  • 1.5 Communications
  • 1.6 Manned missions
  • 1.7 Time dilation
  • 1.8 Constant acceleration
• 2 Prime targets for interstellar travel
• 3 Proposed methods
  • 3.1 Slow missions
    • 3.1.1 Generation ships
    • 3.1.2 Suspended animation
    • 3.1.3 Extended human lifespan
    • 3.1.4 Frozen embryos
    • 3.1.5 Mind uploading
  • 3.2 Island hopping through interstellar space
  • 3.3 Fast missions
  • 3.4 Unmanned
    • 3.4.1 Nanoprobes
  • 3.5 By transmission
• 4 Propulsion
  • 4.1 Rocket concepts
    • 4.1.1 Nuclear fission powered
      • 4.1.1.1 Fission-electric
      • 4.1.1.2 Fission-fragment
      • 4.1.1.3 Nuclear pulse
    • 4.1.2 Nuclear fusion rockets
    • 4.1.3 Antimatter rockets
    • 4.1.4 Rockets with external energy sources
  • 4.2 Non-rocket concepts
    • 4.2.1 Interstellar ramjets
    • 4.2.2 Beamed propulsion
    • 4.2.3 Pre-accelerated fuel
  • 4.3 Speculative methods
    • 4.3.1 Hawking radiation rockets
    • 4.3.2 Magnetic monopole rockets
    • 4.3.3 Faster-than-light travel
• 5 Designs and studies
  • 5.1 Project Hyperion
  • 5.2 Enzmann starship
  • 5.3 NASA research
  • 5.4 Hundred-Year Starship study
• 6 See also
• 7 Notes
• 8 Further reading
• 9 External links

Challenges

Interstellar distances

The main challenge facing interstellar travel is the immense distances between the stars.
Astronomical distances are often measured in the time it would take a beam of light to travel between two points (see light-year). Light in a vacuum travels approximately 300,000 kilometers per second or 186,000 miles per second.
The distance from Earth to the Moon is 1.3 light-seconds. With current spacecraft propulsion technologies, a craft can cover the distance from the Earth to the Moon in around eight hours (New Horizons). That means light travels approximately thirty thousand times faster than current spacecraft propulsion technologies. The distance from Earth to other planets in the Solar System ranges from three light-minutes to about four light-hours. Depending on the planet and its alignment to Earth, for a typical unmanned spacecraft these trips will take from a few months to a little over a decade.^{[citation needed]}
The nearest known star to the Sun is currently Proxima Centauri, which is 4.23 light-years away. (However, there may be undiscovered brown dwarf systems that are closer.^[1]) The fastest outward-bound spacecraft yet sent, Voyager 1, has covered 1/600th of a light-year in 30 years and is currently moving at 1/18,000th the speed of light. At this rate, a journey to Proxima Centauri would take 80,000 years.^[2] On such timescales, motion of the stars would become relevant; for example, in about 31,000 years the star Ross 248 would start to become closest to us and by 36,000 years would be as close as 3.02 light years.^{[citation needed]}
A better understanding of the vastness of the interstellar distance to one of the closest stars to the sun, Alpha Centauri A (a Sun-like star), can be obtained by scaling down the Earth-Sun distance (~150,000,000 km) to one meter (~3.3 ft). On this scale the distance to Alpha Centauri A would still be 271 kilometers or about 169 miles.
This means some combination of great speed and long travel time are required. The time required by propulsion methods based on currently known physical principles would require years to millennia.

Required energy

A significant factor contributing to the difficulty is the energy which must be supplied to obtain a reasonable travel time. A lower bound for the required energy is the kinetic energy K = ½ mv² where m is the final mass. If deceleration on arrival is desired and cannot be achieved by any means other than the engines of the ship, then the required energy at least doubles, because the energy needed to halt the ship equals the energy needed to accelerate it to travel speed.
The velocity for a manned round trip of a few decades to even the nearest star is several thousand times greater than those of present space vehicles. This means that due to the v² term in the kinetic energy formula, millions of times as much energy is required. Accelerating one ton to one-tenth of the speed of light requires at least 450 PJ or 4.5 ×10¹⁷ J or 125 billion kWh, without factoring in efficiency of the propulsion mechanism. This energy has to be generated on-board from stored fuel, harvested from the interstellar medium, or projected over immense distances.
The energy requirements make interstellar travel very difficult. It has been reported that at the 2008 Joint Propulsion Conference, multiple experts opined that it was improbable that humans would ever explore beyond the Solar System.^[3] Brice N. Cassenti, an associate professor with the Department of Engineering and Science at Rensselaer Polytechnic Institute, stated at least the total energy output of the entire world [in a given year] would be required to send a probe to the nearest star.^[3]
The habitual illumination energy requirement for each person is estimated to be 12 kilowatts (12,139.7 Watt).^[4][5] Other long-term energy requirements are still being investigated.^[6]

Interstellar medium

A major issue with traveling at extremely high speeds is that interstellar dust and gas may cause considerable damage to the craft, due to the high relative speeds and large kinetic energies involved. Various shielding methods to mitigate this problem have been proposed.^[7] Larger objects (such as macroscopic dust grains) are far less common, but would be much more destructive. The risks of impacting such objects, and methods of mitigating these risks, have been discussed in the literature, but many unknowns remain.^[8]
Virtually all the material that would pose a problem is in our solar system along the disk that contains the planets, asteroid belt, Oort cloud, comets, free asteroids, macro and micro-meteroids, etc. so any device or projectile must be sent in a direction opposite of all of this material. The larger the object humans send, the greater the chances of it hitting something. One option is to project something very small where the chance of it striking something is virtually non-existent in the vacuum of interplanetary and interstellar space.^[9][10]

Travel time

It has been argued that an interstellar mission which cannot be completed within 50 years should not be started at all. Instead, assuming that a civilization is still on an increasing curve of propulsion system velocity, not yet having reached the limit, the resources should be invested in designing a better propulsion system. This is because a slow spacecraft would probably be passed by another mission sent later with more advanced propulsion (Incessant Obsolescence Postulate).^[11] On the other hand, Andrew Kennedy has shown that if one calculates the journey time to a given destination as the rate of travel speed derived from growth (even exponential growth) increases, there is a clear minimum in the total time to that destination from now (see wait calculation).^[12] Voyages undertaken before the minimum will be overtaken by those who leave at the minimum, while those who leave after the minimum will never overtake those who left at the minimum.
One argument against the stance of delaying a start until reaching fast propulsion system velocity is that the various other non-technical problems that are specific to long-distance travel at considerably higher speed (such as interstellar particle impact, possible dramatic shortening of average human life span during extended space residence, etc.) may remain obstacles that take much longer time to resolve than the propulsion issue alone, assuming that they can even be solved eventually at all. A case can therefore be made for starting a mission without delay, based on the concept of an achievable and dedicated but relatively slow interstellar mission using the current technological state-of-the-art and at relatively low cost, rather than banking on being able to solve all problems associated with a faster mission without having a reliable time frame for achievability of such.
The travel time could be reduced to a millennium using solar sails, or to a century or less using nuclear pulse propulsion.
An interstellar ship would face manifold hazards found in interplanetary travel, including vacuum, radiation, weightlessness, and micrometeoroids. Even the minimum multi-year travel times to the nearest stars are beyond current manned space mission design experience. The fundamental limits of spacetime present another challenge. The distances between stars isn't a problem in and of itself.
However, more speculative approaches to interstellar travel offer the possibility of circumventing these difficulties. Special relativity offers the possibility of shortening the travel time: if a starship with sufficiently advanced engines could reach velocities approaching the speed of light, relativistic time dilation would make the voyage much shorter for the traveler. However, it would still take many years of elapsed time as viewed by the people remaining on Earth, and upon returning to Earth, the travelers would find that far more time had elapsed on Earth than had for them. (For more on this effect, see twin paradox.)
General relativity offers the theoretical possibility that faster-than-light travel may be possible without violating fundamental laws of physics, for example, through wormholes, although it is still debated whether this is possible, in part, because of causality concerns. Proposed mechanisms for faster-than-light travel within the theory of general relativity require the existence of exotic matter.

Communications

The round-trip delay time is the minimum time between an observation by the probe and the moment the probe can receive instructions from Earth reacting to the observation. Given that information can travel no faster than the speed of light, this is for the Voyager 1 about 17 hours, and near Proxima Centauri it would be 8 years. Faster reaction would have to be programmed to be carried out automatically. Of course, in the case of a manned flight the crew can respond immediately to their observations. However, the round-trip delay time makes them not only extremely distant from, but, in terms of communication, also extremely isolated from Earth (analogous to how past long distance explorers were similarly isolated before the invention of the electrical telegraph).
Interstellar communication is still problematic — even if a probe could reach the nearest star, its ability to communicate back to Earth would be difficult given the extreme distance. See Interstellar communication.

Manned missions

The mass of any craft capable of carrying humans would inevitably be substantially larger than that necessary for an unmanned interstellar probe. For instance, the first space probe, Sputnik 1, had a payload of 83.6 kg, while spacecraft to carry a living passenger (Laika the dog), Sputnik 2, had a payload six times that at 508.3 kg. This underestimates the difference in the case of interstellar missions, given the vastly greater travel times involved and the resulting necessity of a closed-cycle life support system. As technology continues to advance, combined with the aggregate risks and support requirements of manned interstellar travel, the first interstellar missions are unlikely to carry earthly life forms.
A manned craft will require more time to reach its top speed as humans have limited tolerance to acceleration.

Time dilation

Main article: Time dilation

Assuming one can not travel faster than light, one might conclude that a human can never make a round-trip further from the Earth than 40 light years if the traveler is active between the ages of 20 and 60. So a traveler would never be able to reach more than the very few star systems which exist within the limit of 10–20 light years from the Earth.
But that would be a mistaken conclusion because it fails to take into account time dilation. Informally explained, clocks aboard ship run slower than Earth clocks, so if the ship engines are powerful enough the ship can reach mostly anywhere in the galaxy and go back to Earth within 40 years ship-time. The problem is that there is a difference between the time elapsed in the astronaut's ship and the time elapsed on Earth.
An example will make this clearer. Suppose a spaceship travels to a star 32 light years away. First it accelerates at a constant 1.03g (i.e., 10.1 m/s²) for 1.32 years (ship time). Then it stops the engines and coasts for the next 17.3 years (ship time) at a constant speed. Then it decelerates again for 1.32 ship-years so as to come at a stop at the destination. The astronaut takes a look around and comes back to Earth the same way.
After the full round-trip, the clocks on board the ship show that 40 years have passed, but according to Earth calendar the ship comes back 76 years after launch.
So, the overall average speed is 0.84 lightyears per earth year, or 1.6 lightyears per ship year. This is possible because at a speed of 0.87 c, time on board the ship seems to run slower. Every two Earth years, ship clocks advance 1 year.
From the viewpoint of the astronaut, onboard clocks seem to be running normally. The star ahead seems to be approaching at a speed of 0.87 lightyears per ship year. As all the universe looks contracted along the direction of travel to half the size it had when the ship was at rest, the distance between that star and the Sun seems to be 16 light years as measured by the astronaut, so it's no wonder that the trip at 0.87 ly per shipyear takes 20 ship years.
At higher speeds, the time onboard will run even slower, so the astronaut could travel to the center of the Milky Way (30 kly from Earth) and back in 40 years ship-time. But the speed according to Earth clocks will always be less than 1 lightyear per Earth year, so, when back home, the astronaut will find that 60 thousand years will have passed on Earth.

Constant acceleration

Regardless of how it is achieved, if a propulsion system can produce 1 g of acceleration continuously from departure to destination, then this will be the fastest method of travel. If the propulsion system drives the ship faster and faster for the first half of the journey, then turns around and brakes the craft so that it arrives at the destination at a standstill, this is a constant acceleration journey. This would also have the advantage of producing constant gravity.
From the planetary observer perspective the ship will appear to steadily accelerate but more slowly as it approaches the speed of light. The ship will be close to the speed of light after about a year of accelerating and remain at that speed until it brakes for the end of the journey.
From the ship perspective there will be no top limit on speed – the ship keeps going faster and faster the whole first half. This happens because the ship's time sense slows down – relative to the planetary observer – the more it approaches the speed of light.
The result is an impressively fast journey if you are in the ship. Here is a table of journey times, in years, for various constant accelerations.

Destination	1g	2g	5g	10g	Planetary time frame (all in years)
Alpha Centauri	4	2.8	1.8	1.3	5
Sirius	7	5	3	2.2	13
Galactic Core	20[13]	11	5	2	30,000

Note again, that times observed from the planetary frame of reference (which applies to both departure and destination points) are very different from those observed in the space craft, and that from the ship frame of reference there is no limit on the top speed.^[14]

Prime targets for interstellar travel

There are 59 known stellar systems within 20 light years from the Sun, containing 81 visible stars. The following could be considered prime targets for interstellar missions:^[15]

Proposed methods

Slow missions

Potential slow manned interstellar travel missions, based on current and near-future propulsion technologies are associated with trip times, starting from about one hundred years to thousands of years. The duration of a slow interstellar journey presents a major obstacle and existing concepts deal with this problem in different ways.^[20] They can be distinguished by the "state" in which humans are transported on-board of the spacecraft.

Generation ships

Main article: Generation ship

A generation ship (or world ship) is a type of interstellar ark in which the crew that arrives at the destination is descended from those who started the journey. Generation ships are not currently feasible because of the difficulty of constructing a ship of the enormous required scale and the great biological and sociological problems that life aboard such a ship raises.^{[21][22][23][24]}

Suspended animation

Scientists and writers have postulated various techniques for suspended animation. These include human hibernation and cryonic preservation. While neither is currently practical, they offer the possibility of sleeper ships in which the passengers lie inert for the long years of the voyage.^[25]

Extended human lifespan

A variant on this possibility is based on the development of substantial human life extension, such as the "Strategies for Engineered Negligible Senescence" proposed by Dr. Aubrey de Grey. If a ship crew had lifespans of some thousands of years, or had artificial bodies, they could traverse interstellar distances without the need to replace the crew in generations. The psychological effects of such an extended period of travel would potentially still pose a problem.

Frozen embryos

Main article: Embryo space colonization

A robotic space mission carrying some number of frozen early stage human embryos is another theoretical possibility. This method of space colonization requires, among other things, the development of a method to replicate conditions in a uterus, the prior detection of a habitable terrestrial planet, and advances in the field of fully autonomous mobile robots and educational robots which would replace human parents.^[26]

Mind uploading

A more speculative method of transporting humans to the stars is by using mind uploading or also called brain emulation.^[27][28] Frank J. Tipler speculates about the colonization of the universe by starships transporting uploaded humans.^[29] Hein presents a range of concepts how such missions could be conducted, using more or less speculative technologies, for example self-replicating machines, wormholes, and teleportation. ^[27] One of the major challenges besides mind uploading itself are the means for downloading the uploads into physical entities, which can be biological or artficial or both.

Island hopping through interstellar space

Interstellar space is not completely empty; it contains trillions of icy bodies ranging from small asteroids (Oort cloud) to possible rogue planets. There may be ways to take advantage of these resources for a good part of an interstellar trip, slowly hopping from body to body or setting up waystations along the way.^[30]

Fast missions

If a spaceship could average 10 percent of light speed (and decelerate at the destination, for manned missions), this would be enough to reach Proxima Centauri in forty years. Several propulsion concepts are proposed that might be eventually developed to accomplish this (see section below on propulsion methods), but none of them are ready for near-term (few decades) development at acceptable cost.^{[citation needed]}

Unmanned

Nanoprobes

Near-lightspeed nanospacecraft might be possible within the near future built on existing microchip technology with a newly developed nanoscale thruster. Researchers at the University of Michigan are developing thrusters that use nanoparticles as propellant. Their technology is called “nanoparticle field extraction thruster”, or nanoFET. These devices act like small particle accelerators shooting conductive nanoparticles out into space.^[31]
Given the light weight of these probes, it would take much less energy to accelerate them. With on board solar cells they could continually accelerate using solar power. One can envision a day when a fleet of millions or even billions of these particles swarm to distant stars at nearly the speed of light, while relaying signals back to earth through a vast interstellar communication network.

By transmission

Main article: Teleportation

If physical entities could be transmitted as information and reconstructed at a destination, travel at nearly the speed of light would be possible, which for the "travelers" would be instantaneous. However, sending an atom-by-atom description of (say) a human body would be a daunting task. Extracting and sending only a computer brain simulation is a significant part of that problem. "Journey" time would be the light-travel time plus the time needed to encode, send and reconstruct the whole transmission.^[32]

Propulsion

Rocket concepts

All rocket concepts are limited by the rocket equation, which sets the characteristic velocity available as a function of exhaust velocity and mass ratio, the ratio of initial (M₀, including fuel) to final (M₁, fuel depleted) mass.
Very high specific power, the ratio of jet-power to total vehicle mass, is required to reach interstellar targets within sub-century time-frames.^[33] Some heat transfer is inevitable and a tremendous heating load must be adequately handled.
Thus, for interstellar rocket concepts of all technologies, a key engineering problem (seldom explicitly discussed) is limiting the heat transfer from the exhaust stream back into the vehicle.^[34]

Nuclear fission powered

Fission-electric

Nuclear-electric or plasma engines, operating for long periods at low thrust and powered by fission reactors, have the potential to reach speeds much greater than chemically powered vehicles or nuclear-thermal rockets. Such vehicles probably have the potential to power Solar System exploration with reasonable trip times within the current century. Because of their low-thrust propulsion, they would be limited to off-planet, deep-space operation. Electrically powered spacecraft propulsion powered by a portable power-source, say a nuclear reactor, producing only small accelerations, would take centuries to reach for example 15% of the velocity of light, thus unsuitable for interstellar flight during a single human lifetime.^[35][36][37]

Fission-fragment

Fission-fragment rockets use nuclear fission to create high-speed jets of fission fragments, which are ejected at speeds of up to 12,000 km/s. With fission, the energy output is approximately 0.1% of the total mass-energy of the reactor fuel and limits the effective exhaust velocity to about 5% of the velocity of light. For maximum velocity, the reaction mass should optimally consist of fission products, the "ash" of the primary energy source, in order that no extra reaction mass need be book-kept in the mass ratio. This is known as a fission-fragment rocket. thermal-propulsion engines such as NERVA produce sufficient thrust, but can only achieve relatively low-velocity exhaust jets, so to accelerate to the desired speed would require an enormous amount of fuel.^[35][36][37]

Nuclear pulse

Main article: Nuclear pulse propulsion

Based on work in the late 1950s to the early 1960s, it has been technically possible to build spaceships with nuclear pulse propulsion engines, i.e. driven by a series of nuclear explosions. This propulsion system contains the prospect of very high specific impulse (space travel's equivalent of fuel economy) and high specific power.^[38]
Project Orion team member, Freeman Dyson, proposed in 1968 an interstellar spacecraft using nuclear pulse propulsion which used pure deuterium fusion detonations with a very high fuel-burnup fraction. He computed an exhaust velocity of 15,000 km/s and a 100,000 tonne space-vehicle able to achieve a 20,000 km/s delta-v allowing a flight-time to Alpha Centauri of 130 years.^[39] Later studies indicate that the top cruise velocity that can theoretically be achieved by a Teller-Ulam thermonuclear unit powered Orion starship, assuming no fuel is saved for slowing back down, is about 8% to 10% of the speed of light (0.08-0.1c).^[40] An atomic (fission) Orion can achieve perhaps 3%-5% of the speed of light. A nuclear pulse drive starship powered by Fusion-antimatter catalyzed nuclear pulse propulsion units would be similarly in the 10% range and pure Matter-antimatter annihilation rockets would be theoretically capable of obtaining a velocity between 50% to 80% of the speed of light. In each case saving fuel for slowing down halves the max. speed. The concept of using a magnetic sail to decelerate the spacecraft as it approaches its destination has been discussed as an alternative to using propellant, this would allow the ship to travel near the maximum theoretical velocity.^[41] Alternative designs utilizing similar principles include Project Longshot, Project Daedalus, and Mini-Mag Orion. The principle of external nuclear pulse propulsion to maximize survivable power has remained common among serious concepts for interstellar flight without external power beaming and for very high-performance interplanetary flight.
In the 1970s the Nuclear Pulse Propulsion concept further was refined by Project Daedalus by use of externally triggered inertial confinement fusion, in this case producing fusion explosions via compressing fusion fuel pellets with high-powered electron beams. Since then lasers, ion beams, neutral particle beams and hyper-kinetic projectiles have been suggested to produce nuclear pulses for propulsion purposes.^[42]
A current impediment to the development of any nuclear explosive powered spacecraft is the 1963 Partial Test Ban Treaty which includes a prohibition on the detonation of any nuclear devices (even non-weapon based) in outer space. This treaty would therefore need to be re-negotiated, although a project on the scale of an interstellar mission using currently foreseeable technology would probably require international co-operation on at least the scale of the International Space Station.

Nuclear fusion rockets

Fusion rocket starships, powered by nuclear fusion reactions, should conceivably be able to reach speeds of the order of 10% of that of light, based on energy considerations alone. In theory, a large number of stages could push a vehicle arbitrarily close to the speed of light.^[43] These would "burn" such light element fuels as deuterium, tritium, ³He, ¹¹B, and ⁷Li. Because fusion yields about 0.3–0.9% of the mass of the nuclear fuel as released energy, it is energetically more favorable than fission, which releases <0.1% of the fuel's mass-energy. The maximum exhaust velocities potentially energetically available are correspondingly higher than for fission, typically 4–10% of c. However, the most easily achievable fusion reactions release a large fraction of their energy as high-energy neutrons, which are a significant source of energy loss. Thus, while these concepts seem to offer the best (nearest-term) prospects for travel to the nearest stars within a (long) human lifetime, they still involve massive technological and engineering difficulties, which may turn out to be intractable for decades or centuries.
Early studies include Project Daedalus, performed by the British Interplanetary Society in 1973–1978, and Project Longshot, a student project sponsored by NASA and the US Naval Academy, completed in 1988. Another fairly detailed vehicle system, "Discovery II",^[44] designed and optimized for crewed Solar System exploration, based on the D³He reaction but using hydrogen as reaction mass, has been described by a team from NASA's Glenn Research Center. It achieves characteristic velocities of >300 km/s with an acceleration of ~1.7•10⁻³ g, with a ship initial mass of ~1700 metric tons, and payload fraction above 10%. While these are still far short of the requirements for interstellar travel on human timescales, the study seems to represent a reasonable benchmark towards what may be approachable within several decades, which is not impossibly beyond the current state-of-the-art. Based on the concept's 2.2% burnup fraction it could achieve a pure fusion product exhaust velocity of ~3,000 km/s.

Antimatter rockets

An antimatter rocket would have a far higher energy density and specific impulse than any other proposed class of rocket. If energy resources and efficient production methods are found to make antimatter in the quantities required and store it safely, it would be theoretically possible to reach speeds approaching that of light. Then relativistic time dilation would become more noticeable, thus making time pass at a slower rate for the travelers as perceived by an outside observer, reducing the trip time experienced by human travelers.
Supposing the production and storage of antimatter should become practical, two further problems would present and need to be solved. First, in the annihilation of antimatter, much of the energy is lost in very penetrating high-energy gamma radiation, and especially also in neutrinos, so that substantially less than mc² would actually be available if the antimatter were simply allowed to annihilate into radiations thermally. Even so, the energy available for propulsion would probably be substantially higher than the ~1% of mc² yield of nuclear fusion, the next-best rival candidate.
Second, once again heat transfer from exhaust to vehicle seems likely to deposit enormous wasted energy into the ship, considering the large fraction of the energy that goes into penetrating gamma rays. Even assuming biological shielding were provided to protect the passengers, some of the energy would inevitably heat the vehicle, and may thereby prove limiting. This requires consideration for serious proposals if useful accelerations are to be achieved, as the energies involved (e.g., for 0.1g ship acceleration, approaching 0.3 trillion watts per ton of ship mass) are very large.

Rockets with external energy sources

Rockets deriving their power from external sources, such as a laser, could bypass the ordinary rocket equation, potentially reducing the mass of the ship greatly and allowing much higher travel speeds. Geoffrey A. Landis has proposed for an interstellar probe, with energy supplied by an external laser from a base station powering an Ion thruster.^[45]

Non-rocket concepts

A problem with all traditional rocket propulsion methods is that the spacecraft would need to carry its fuel with it, thus making it very massive, in accordance with the rocket equation. Some concepts attempt to escape from this problem (^[46]):

Interstellar ramjets

In 1960, Robert W. Bussard proposed the Bussard ramjet, a fusion rocket in which a huge scoop would collect the diffuse hydrogen in interstellar space, "burn" it on the fly using a proton–proton fusion reaction, and expel it out of the back. Though later calculations with more accurate estimates suggest that the thrust generated would be less than the drag caused by any conceivable scoop design, the idea is attractive because, as the fuel would be collected en route (commensurate with the concept of energy harvesting), the craft could theoretically accelerate to near the speed of light.

Beamed propulsion

This diagram illustrates Robert L. Forward's scheme for slowing down an interstellar light-sail at the destination ^[47] system.

 

A light sail or magnetic sail powered by a massive laser or particle accelerator in the home star system could potentially reach even greater speeds than rocket- or pulse propulsion methods, because it would not need to carry its own reaction mass and therefore would only need to accelerate the craft's payload. Robert L. Forward proposed a means for decelerating an interstellar light sail in the destination star system without requiring a laser array to be present in that system. In this scheme, a smaller secondary sail is deployed to the rear of the spacecraft, while the large primary sail is detached from the craft to keep moving forward on its own. Light is reflected from the large primary sail to the secondary sail, which is used to decelerate the secondary sail and the spacecraft payload.^[48]
A magnetic sail could also decelerate at its destination without depending on carried fuel or a driving beam in the destination system, by interacting with the plasma found in the solar wind of the destination star and the interstellar medium.^[49] Unlike Forward's light sail scheme, this would not require the action of the particle beam used for launching the craft. Alternatively, a magnetic sail could be pushed by a particle beam^[50] or a plasma beam^[51] to reach high velocity, as proposed by Landis and Winglee.
Beamed propulsion seems to be the best interstellar travel technique presently available, since it uses known physics and known technology that is being developed for other purposes,^[15] and would be considerably cheaper than nuclear pulse propulsion.^{[citation needed]}
The following table lists some example concepts using beamed laser propulsion as proposed by the physicist Robert L. Forward:^[48][52]

Geoffrey A. Landis proposed for interstellar travel an interstellar probe with supplying the energy from an external source (laser of base station) and Ion thruster.^[45][53][54]

Pre-accelerated fuel

Achieving start-stop interstellar trip times of less than a human lifetime require mass-ratios of between 1,000 and 1,000,000, even for the nearer stars. This could be achieved by multi-staged vehicles on a vast scale.^[43] Alternatively large linear accelerators could propel fuel to fission propelled space-vehicles, avoiding the limitations of the Rocket equation.^[55]

Speculative methods

Hawking radiation rockets

In a black hole starship, a parabolic reflector would reflect Hawking radiation from an artificial black hole. In 2009, Louis Crane and Shawn Westmoreland of Kansas State University published a paper investigating the feasibility of this idea. Their conclusion was that it was on the edge of possibility, but that quantum gravity effects that are presently unknown may make it easier or make it impossible.^[56][57]

Magnetic monopole rockets

If some of the Grand unification models are correct, e.g. 't Hooft–Polyakov, it would be possible to construct a photonic engine that uses no antimatter thanks to the magnetic monopole which hypothetically can catalyze decay of a proton to a positron and π⁰-meson:^[58][59]

π⁰ decays rapidly to two photons, and the positron annihilates with an electron to give two more photons. As a result, a hydrogen atom turns into four photons and only the problem of a mirror remains unresolved.
A magnetic monopole engine could also work on a once-through scheme such as the Bussard ramjet (see below).
At the same time, most of the modern Grand unification theories such as M-theory predict no magnetic monopoles, which casts doubt on this attractive idea.

Faster-than-light travel

Artist's depiction of a hypothetical Wormhole Induction Propelled Spacecraft, based loosely on the 1994 "warp drive" paper of Miguel Alcubierre. Credit: NASA CD-98-76634 by Les Bossinas.

 

Main article: Faster-than-light

 

Scientists and authors have postulated a number of ways by which it might be possible to surpass the speed of light. Even the most serious-minded of these are speculative.
According to Einstein's equation of general relativity, spacetime is curved:

General relativity may permit the travel of an object faster than light in curved spacetime.^[60] One could imagine exploiting the curvature to take a "shortcut" from one point to another. This is one form of the warp drive concept.
In physics, the Alcubierre drive is based on an argument that the curvature could take the form of a wave in which a spaceship might be carried in a "bubble". Space would be collapsing at one end of the bubble and expanding at the other end. The motion of the wave would carry a spaceship from one space point to another in less time than light would take through unwarped space. Nevertheless, the spaceship would not be moving faster than light within the bubble. This concept would require the spaceship to incorporate a region of exotic matter, or "negative mass".
Wormholes are conjectural distortions in spacetime that theorists postulate could connect two arbitrary points in the universe, across an Einstein–Rosen Bridge. It is not known whether wormholes are possible in practice. Although there are solutions to the Einstein equation of general relativity which allow for wormholes, all of the currently known solutions involve some assumption, for example the existence of negative mass, which may be unphysical.^[61] However, Cramer et al. argue that such wormholes might have been created in the early universe, stabilized by cosmic string.^[62] The general theory of wormholes is discussed by Visser in the book Lorentzian Wormholes.^[63]

Designs and studies

Project Hyperion

Project Hyperion, one of the projects of Icarus Interstellar.^[64]

Enzmann starship

Main article: Enzmann starship

The Enzmann starship, as detailed by G. Harry Stine in the October 1973 issue of Analog, was a design for a future starship, based on the ideas of Dr. Robert Duncan-Enzmann.^[65] The spacecraft itself as proposed used a 12,000,000 ton ball of frozen deuterium to power 12–24 thermonuclear pulse propulsion units.^[65] Twice as long as the Empire State Building and assembled in-orbit, the spacecraft was part of a larger project preceded by interstellar probes and telescopic observation of target star systems.^[65][66]

NASA research

NASA has been researching interstellar travel since its formation, translating important foreign language papers and conducting early studies on applying fusion propulsion, in the 1960s, and laser propulsion, in the 1970s, to interstellar travel.
The NASA Breakthrough Propulsion Physics Program (terminated in FY 2003 after 6-year, $1.2 million study, as "No breakthroughs appear imminent.")^[67] identified some breakthroughs which are needed for interstellar travel to be possible.^[68]
Geoffrey A. Landis of NASA's Glenn Research Center states that a laser-powered interstellar sail ship could possibly be launched within 50 years, using new methods of space travel. "I think that ultimately we're going to do it, it's just a question of when and who," Landis said in an interview. Rockets are too slow to send humans on interstellar missions. Instead, he envisions interstellar craft with extensive sails, propelled by laser light to about one-tenth the speed of light. It would take such a ship about 43 years to reach Alpha Centauri, if it passed through the system. Slowing down to stop at Alpha Centauri could increase the trip to 100 years,^[69] while a journey without slowing down raises the issue of making sufficiently accurate and useful observations and measurements during a fly-by.

Hundred-Year Starship study

The 100 Year Starship (100YSS) is the name of the overall effort that will, over the next century, work toward achieving interstellar travel. The effort will also go by the moniker 100YSS. The 100 Year Starship study is the name of a one year project to assess the attributes of and lay the groundwork for an organization that can carry forward the 100 Year Starship vision.
Dr. Harold ("Sonny") White^[70] from NASA's Johnson Space Center is a member of Icarus Interstellar,^[71] the nonprofit foundation whose mission is to realize interstellar flight before the year 2100. At the 2012 meeting of 100YSS, he reported using a laser to try to warp spacetime by 1 part in 10 million with the aim of helping to make interstellar travel possible.^[72]