Part 7 (1/2)

Obviously a new kind of rocket design must be found if we are ever to reach the stars Either we must radically increase the thrust of our rockets, or we need to increase the tie chemical rocket, for example, may have the thrust of several million pounds, but it burns for only a few ns, such as the ion engine (described in the following paragraphs), may have a feeble thrust but can operate for years in outer space When it comes to rocketry, the tortoise wins over the hare

ION AND PLASMA ENGINES

Unlike cheines do not produce the sudden, draases that propel conventional rockets In fact, their thrust is often measured in ounces Placed on a tabletop on Earth, they are too feeble to move But what they lack in thrust they more than make up for in duration, because they can operate for years in the vacuuine looks like the inside of a TV tube A hot filament is heated by an electric current, which creates a beam of ionized atoms, such as xenon, that is shot out the end of the rocket Instead of riding on a blast of hot, explosive gas, ion engines ride on a thin but steady flow of ions

NASA's NSTAR ion thruster was tested in outer space aboard the successful Deep space 1 probe, launched in 1998 The ion engine fired for a total of 678 days, setting a new record for ion engines The European space Agency has also tested an ion engine on its Smart 1 probe The japanese Hayabusa space probe, which flew past an asteroid, was powered by four xenon ion engines Although ungla-haul ent) between the planets In fact, ion engines may one day become the workhorse for interplanetary transport

A ine, for exanetoplasma rocket), which uses a powerful jet of plasineer Franklin Chang-Diaz, it uses radio waves and rees centigrade The superhot plasnificant thrust Prototypes of the engine have already been built on Earth, although none has ever been sent into space Soine can be used to power athe travel tins use solar power to energize the plasns use nuclear fission (which raises safety concerns, since it involves putting large amounts of nuclear materials into space on shi+ps that are susceptible to accident)

Neither the ion nor the plash power to take us to the stars For that, we need an entirely new set of propulsion designs One serious drawback to designing a starshi+p is the staggering amount of fuel necessary tospan of time before the shi+p reaches its distant destination

SOLAR SAILS

One proposal that may solve these probleht exerts a very se sail through space The idea for a solar sail is an old one, dating back to the great astronomer Johannes Kepler in his 1611 treatise Soh the physics behind a solar sail is si a solar sail that can be sent into space In 2004 a japanese rocket successfully deployed two small prototype solar sails into space In 2005 the Planetary Society, Cosmos Studios, and the Russian Academy of Sciences launched the Cosmos 1 space sail from a sub carried on failed, and the sail did not reach orbit (A previous attempt at a suborbital sail also failed back in 2001) But in February 2006 a 15-meter solar sail was sent successfully into orbit by the japanese M-V rocket, although the sail opened incoy has been painfully slow, proponents of the solar sail have another idea that e battery of lasers on the ht at a solar sail, enabling it to coast to the nearest star The physics of such an interplanetary solar sail are truly daunting The sail itself would have to be hundreds of miles across and constructed entirely in outer space One would have to build thousands of powerful laser bea continuously for years to decades (In one estimate, it would be necessary to fire lasers that have one thousand times the current total power output of the planet Earth) On paper a ht be able to travel as fast as half the speed of light It would take such a solar sail only eight years or so to reach the nearby stars The advantage of such a propulsion systey No nes of physics would have to be discovered to create such a solar sail But theprobleized by thousands of powerful laser beay that may be a century in the future (One proble back One would have to create a second battery of laser beams on a distant moon to propel the vessel back to Earth Or perhaps the shi+p could swing rapidly around a star, using it like a slingshot to get enough speed for the return voyage Then lasers on the moon would be used to decelerate the sail so it could land on the Earth) RAMJET FUSION

My own favorite candidate for getting us to the stars is the raen in the universe, so a raen as it traveled in outer space, essentially giving it an inexhaustible source of rocket fuel Once the hydrogen was collected it would then be heated to en would fuse, releasing the energy of a therine was proposed by physicist Robert W Bussard in 1960 and later popularized by Carl Sagan Bussard calculated that a raht theoretically be able toof force, that is, co on the surface of the Earth If the ra acceleration for one year, it would reach 77 percent of the velocity of light, sufficient to make interstellar travel a serious possibility

The requireine are easy to coas throughout the universe We also can calculate roughly howaccelerations That calculation, in turn, deteren gas With a few reasonable assumptions, one can show that you would need a scoop that is about 160 kilo a scoop of this size would be prohibitive on Earth, building it in outer space poses fewer problehtlessness

In principle the raine could propel itself indefinitely, ultialaxy Since tiht be possible to reach astrono the crew into suspended ani to clocks inside the starshi+p, the spacecraft would reach the Pleiades star cluster, which is 400 light-years away In twenty-three years it would reach the Androht-years froht be able to reach the limit of the visible universe within the lifetiht have passed on the Earth)

One key uncertainty is the fusion reaction The ITER fusion reactor, scheduled to be built in the south of France, coen (deuteriuy In outer space, however, the le proton surrounded by an electron The raine would therefore have to exploit the proton-proton fusion reaction Although the deuterium/tritium fusion process has been studied for decades by physicists, the proton-proton fusion process is less well understood, is more difficult to achieve, and yields far less power Sothe more difficult proton-proton reaction will be a technical challenge in the coineers, in addition, have questioned whether the ra effects as it approaches the speed of light) Until the physics and economics of proton-proton fusion are worked out, it is difficult to make accurate estin is on the short list of possible candidates for any mission contemplated to the stars

NUCLEAR ELECTRIC ROCKET

In 1956 the US Atoan to look at nuclear rockets seriously under Project Rover In theory, a nuclear fission reactor would be used to heat up gases like hydrogen to extreases would be ejected out one end of the rocket, creating thrust

Because of the danger of an explosion in the Earth's at toxic nuclear fuel, early versions of nuclear rocket engines were placed horizontally on railroad tracks, where the performance of the rocket could be carefully ine to be tested under Project Rover was the Kiwi 1 in 1959 (aptly nahtless bird) In the 1960s NASA joined with the AEC to create the Nuclear Engine for Rocket Vehicle Applications (NERVA), which was the first nuclear rocket to be tested vertically, rather than horizontally In 1968 this nuclear rocket was test-fired in a doard position

The results of this research have been mixed The rockets were very complicated and often ine often cracked the fuel bundles, causing the shi+p to break apart Corrosion due to burning hydrogen at high temperatures was also a persistent probleram was finally closed in 1972

(These atoer of a runaway nuclear reaction, as in a sh commercial nuclear power plants today run on diluted nuclear fuel and cannot explode like a Hiroshi+ma bomb, these atomic rockets, in order to create hly enriched uraniu a tiny nuclear detonation When the nuclear rocket program was about to be retired, scientists decided to perform one last test They decided to blow up a rocket, like a small atomic bomb They removed the control rods [which keep the nuclear reaction in check] The reactor went super-critical and blew up in a fiery ball of flaram was even captured on film The Russians were not pleased They considered this stunt to be a violation of the Liround detonations of nuclear bombs) Over the years the military has periodically revisited the nuclear rocket One secret project was called the Timberwind nuclear rocket; it was part of the military's Star Wars project in the 1980s (It was abandoned after details of its existence were released by the Federation of American Scientists) The main concern about the nuclear fission rocket is safety Even fifty years into the space age, cheo catastrophic failure about 1 percent of the tier and Colu fourteen astronauts, further confirmed this failure rate) Nonetheless, in the past few years NASA has resumed research on the nuclear rocket for the first tiram of the 1960s In 2003 NASA christened a new project, Proave fire to huh that funding was significantly cut to 100 million in 2006 The project's future is unclear

NUCLEAR PULSED ROCKETS

Another distant possibility is to use a series of mini-nuclear bombs to propel a starshi+p In Project Orion, mini-atomic bombs were to be ejected out the back of the rocket in sequence, so that the spacecraft would ”ride” on the shock waves created by these n could take a spacecraft close to the speed of light Originally conceived in 1947 by Stanislaw Ulaen bombs, the idea was further developed by Ted Taylor (one of the chief designers of nuclear warheads for the US military) and physicist Freeman Dyson of the Institute for Advanced Study at Princeton

In the late 1950s and 1960s elaborate calculations were made for this interstellar rocket It was estimated that such a starshi+p couldvelocity of 10 percent the speed of light But even at that speed it would take about forty-four years to reach the nearest star Scientists have speculated that a space ark powered by such a rocket would have to cruise for centuries, with awould be born and spend all their lives on the space ark, in order that their descendants could reach the nearby stars

In 1959 General Ato the size of an Orion spacecraft The largest version, called the super Orion, would weigh 8 ized by over 1,000 hydrogen bombs

But one major problem with the project was the possibility of conta launch Dyson estimated that the nuclear fallout from each launch could cause fatal cancers in ten people In addition, the electroreat that it could causeelectrical syste of the Limited Test Ban Treaty in 1963 sounded the death knell of the project Eventually the ner Ted Taylor, gave up (He once confided to me that he finally became disillusioned with the project when he realized that the physics behind mini-nuclear bombs could also be used by terrorists to create portable nuclear boh the project was canceled because it was deeerous, its namesake lives on in the Orion spacecraft, which NASA has chosen to replace the space Shuttle in 2010) The concept of a nuclear-fired rocket was briefly resurrected by the British Interplanetary Society from 1973 to 1978, with Project Daedalus, a preliminary study to see if an unmanned starshi+p could be built that could reach the Barnard's Star, 59 light-years from Earth (Barnard's Star was chosen because it was conjectured that it ht have a planet Since then astronoaret Turnbull have compiled a list of 17,129 nearby stars that could have planets supporting life The ht-years away) The rocket shi+p planned for Project Daedalus was so huge that it would have had to be constructed in outer space It would weigh 54,000 tons, nearly all of its weight in rocket fuel, and could attain 71 percent of the speed of light with a payload of 450 tons Unlike Project Orion, which used tiny fission boen bonited by electron bea it, as well as concerns over its nuclear propulsion system, Project Daedalus was also shelved indefinitely

SPECIFIC IMPULSE AND ENGINE EFFICIENCY

Engineers sometimes speak of ”specific impulse,” which enables us to rank the efficiency of various engine designs ”Specific ie in momentum per unit ine, the less fuel is necessary to boost a rocket into space Mo over a period of tie thrust, operate for only a few ines, because they can operate for years, can have high specific impulse with very low thrust

Specific iht have a specific impulse of 400500 seconds The specific ihest specific impulse ever achieved for a che a propellant en, lithiuine had a specific impulse of 1,640 seconds And the nuclear rocket attained specific impulses of 850 seconds

The maximum possible specific iht It would have a specific i the specific iines

TYPE OF ROCKET ENGINE

SPECIFIC IMPULSE

Solid fuel rocket 250

Liquid fuel rocket 450

Ion engine 3,000

VASIMR plasine 1,000 to 30,000 Nuclear fission rocket 800 to 1,000 Nuclear fusion rocket 2,500 to 200,000 Nuclear pulsed rocket 10,000 to 1 million Antimatter rocket

1 million to 10 ines, because they contain no rocket propellant at all, have infinite specific ih they have problems of their own) spacE ELEVATORS

One severe objection to ns is that they are so mammoth and heavy that they could never be built on the Earth That is why so thehtlessness would make it possible for astronauts to lift impossibly heavy objects with ease But critics today point out the prohibitive costs of assembly in outer space The International space Station, for example, will require upwards of one hundred launches of shuttle missions for complete assembly and costs have escalated to 100 billion It is thean interstellar space sail or ramjet scoop in outer space would cost many times that amount

But as science fiction writer Robert Heinlein was fond of saying, if you can make it to 160 kilometers above the Earth, you are halfway to anywhere in the solar system That is because the first 160 kilo to escape the Earth's gravity, cost by far the most After that a rocket shi+p can almost coast to Pluto and beyond

One way to reduce costs drastically in the future would be to develop a space elevator The idea of cli a rope to heaven is an old one, for example, as in the fairy tale ”Jack and the Beanstalk,” but it ht become a reality if the rope could be sent far into space Then the centrifugal force of the Earth's rotation would be enough to nullify the force of gravity, so the rope would never fall The rope would ically rise vertically into the air and disappear into the clouds (Think of a ball spinning on a string The ball seeal force pushes it away fro rope would be suspended in air because of the spinning of the Earth) Nothing would be needed to hold up the rope except the spin of the Earth A person could theoretically cliive the undergraduates taking physics courses at City University of New York the proble the tension on such a rope It is easy to show that the tension on the rope would be enough to snap even a steel cable, which is why building a space elevator has long been considered to be impossible

The first scientist to seriously study the space elevator was Russian visionary scientist Konstantin Tsiolkovsky In 1895, inspired by the Eiffel Tower, he envisioned a tower that would ascend into space, connecting the Earth to a ”celestial castle” in space It would be built bottoineers would slowly extend the space elevator to the heavens

In 1957 Russian scientist Yuri Artsutanov proposed a new solution, that the space elevator be built in reverse order, top-down, starting froeostationary orbit 36,000 miles in space, where it would appear to be stationary, and from which one would drop a cable down to Earth Then the cable would be anchored to the ground But the tether for a space elevator would have to be able to withstand roughly 60100 gigapascals (gpa) of tension Steel breaks at about 2 gpa,the idea beyond reach

The idea of a space elevator reached a much wider audience with the publication of Arthur C Clarke's 1979 novel, The Fountains of Paradise, and Robert Heinlein's 1982 novel, Friday But without any further progress, the idea languished

The equation changed significantly when carbon nanotubes were developed by chemists Interest was suddenly sparked by the work of Suh evidence for carbon nanotubes actually dates back to the 1950s, a fact that was ignored at the tier than steel cables, but also th necessary to maintain a space elevator Scientists believe a carbon nanotube fiber could withstand 120 gpa of pressure, which is co point This discovery has rekindled attempts to create a space elevator

In 1999 a NASA study gave serious consideration to the space elevator, envisioning a ribbon, about 1 , capable of transporting about 15 tons of payload into Earth'sobt Such a space elevator could change the econoht The cost could be reduced by a factor of ten thousand, an astonishi+ng, revolutionary change

Currently it costs 10,000 or more to send a pound of hly the cost, ounce for ounce, of gold) Each space Shuttle mission, for example, costs up to 700 million A space elevator could reduce the cost to as little as 1 per pound Such a radical reduction in the cost of the space program could revolutionize the e view space travel With a simple push of an elevator button, one could in principle take an elevator ride into outer space for the price of a plane ticket