Apr 8, 1999

Setting Sail for the Stars

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Cracking the whip and unfurling gray sails are among new techniques under discussion at the 1999 Advanced Propulsion Research Workshop


April 8, 1999: A fixture in any movie involving a trek across the desert is a store with a sign warning, "Last chance for gas, next 200 miles." Right now, the sign at the edge of the solar system reads, "Last chance for gas, next 4.3 light years." If you're using an interstellar sail, you can ignore the sign and coast to the next solar system. Since the mid-1980s, scientists have been considering sailing to the stars with a "breeze" produce by lasers or microwave transmitters.

Right: A "conventional" solar sail, fully deployed and cruising into interstellar space. Innovative ideas for "gray" and electromagnetic sails may leave this concept in the interstellar dust. (NASA)

"A propellant-free system is very attractive because the main problem with interstellar travel is the weight of the propellant," said Geoffrey Landis of the Ohio Aerospace Institute at NASA's Glenn Research Center. He spoke Wednesday morning to the 10th annual Advanced Propulsion Research Workshop held by NASA, Marshall, the Jet Propulsion Laboratory, and the American Institute of Aeronautics and Astronautics being held Tuesday-Thursday at the University of Alabama in Huntsville.


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The original notion of space sails is to unfold a large aluminum coated Mylar blanket, face it to the sun, and let sunlight and the solar wind push the craft deeper into space. But that takes a long time to get anywhere. In 1984, Dr. Robert Forward, , vice president of Tethers Unlimited in Seattle and an advocate of sails and tethers for space propulsion, proposed giving Mother Nature a hand by using high-power laser or microwave transmitters that would beam for a few days or weeks to speed the probe on its way.


Gray sails could provide a better ride

Forward's Starwisp concept would have used a mesh of superconducting aluminum wires to receive its "push" from microwave photons, and then reflect to produce an equal magnitude thrust. This would propel the craft from Earth orbit past Neptune, at 1/20th the speed of light, in just a week. Since then, Forward and others have been rethinking the concept.

"My major message is, that's wrong, don't use it" said Forward as he pointed at the equation he used in his initial studies. Since 1984, he has determined that the sail material would absorb a significant amount of the energy, weakening the structure and possibly letting it collapse.


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Forward now proposes putting that absorption to work in a "gray sail" made of carbon. The sail would absorb the light, getting a push from it, and reradiate it as infrared energy. With the sail oriented properly to the source, this would generate a significant amount of thrust in the desired direction.

A mission to interstellar space could be accomplished with a combination sail. An aluminum coating - just 70 atoms thick - would serve as a traditional reflective solar sail to boost the spacecraft out of Earth orbit, then cancel its solar orbital velocity so it plunges on a near-miss trajectory towards the sun.


Right: The sunshade for the Next Generation Space Telescope is not as large as a sail for space propulsion, but will provide valuable technical lessons on how to build one. (NASA)

As it passes just 3 solar diameters from the sun's visible surface, the aluminum would evaporate, exposing the carbon structure underneath. The carbon would absorb sunlight and heat to 2,000 K (almost 3,600 deg. F). Radiating infrared light would accelerate the craft at 14 times Earth's gravity (the Space Shuttle reaches a maximum of 3 G during launch).

"The trajectory is nearly a straight line" away from the sun, Forward said. He is proposing a laboratory demonstration using a 1 kilowatt microwave beam to levitate a 2.5 cm (1 in.) square, 02.5 micron-thick carbon film in a vacuum chamber.



Wild & crazy - yet disciplined

Whether you view today as the "good old days" or "the dark ages," space transportation has to become cheaper, faster, and more frequent to really open the "highway to space."

Speaking to the keynote banquet for the 10th annual Advanced Propulsion Research Workshop, Art Stephenson the director of NASA's Marshall Space Flight Center outlined some of the goals for improvements in space transportation.

He said that one goal is to increase the safety of space travel to be comparable to flights on commercial airliners within 40 years. Other goals include increasing a vehicle's life span to 10,000 missions, and reducing the turnaround between missions to a few hours with a crew of two persons.

"What we're talking about here is a revolution in space transportation," he said, with respect to both Earth-to-orbit and orbit-to-deep space propulsion.

Referring to a 1940s-style graphic shown at the start of his talk, Stephenson said that "We're in the good old days, looking to the future." He later cautioned that "We're really in the dark ages" with regard to what's been done so far to reduce spacecraft mass and improve transportation.

Some of the advances will come from a planned series of technology demonstration flights, such as the X-34 small launcher demonstration, the X-37 hypersonic flight demonstrator, and the rocket-based combined cycle engine in 2005. The latter effort supports work on a launcher that would incorporate a magnetic-levitated sled to accelerate the vehicle (below), then engines that would work as a rocket, then a ramjet, then a rocket again, all to reduce weight and flight costs.



The path just starts there. Showing an artist's concept of the microwave-powered Lightcraft that NASA/Marshall is partially funding in advanced studies, Stephenson said, "This is way out thinking. But it's the kind of thinking we should be doing to get an elevator to low Earth orbit."

The "way out thinking" is getting favorable reviews. Stephenson said that NASA Administrator Dan Goldin "complimented Marshall for working on wild and crazy propulsion concepts. But, we must combine wild and crazy ideas with disciplined engineering."

Other Propulsion Stories this week
Apr 6: Ion Propulsion -- 50 Years in the Making - The concept of ion propulsion, currently being demonstrated on the Deep Space 1 mission, goes back to the very beginning of NASA and beyond.
April 6: Far Out Space Propulsion Conference Blasts Off - Atoms locked in snow, a teaspoon from the heart of the sun, and the stuff that drives a starship will be on the agenda of an advanced space propulsion conference that opens today in Huntsville.
April 7: Darwinian Design - Survival of the Fittest Spacecraft
April 7: Coach-class tickets for space? - Scientists discuss new ideas for high-performance, low-cost space transportation
April 8: Setting Sail for the Stars - Cracking the whip and unfurling gray sails are among new techniques under discussion at the 1999 Advanced Propulsion Research Workshop
April 12: Reaching for the stars - Scientists examine using antimatter and fusion to propel future spacecraft.
April 16: Riding the Highways of Light - Science mimics science fiction as a Rensselaer Professor builds and tests a working model flying disc. The disc, or "Lightcraft," is an early prototype for Earth-friendly spacecraft of the future.



A new use for radio

Landis also finds carbon sails attractive in a reworked approach to Forward's Starwisp concept. Landis proposes using millimeter-wave radio to push a carbon sail. Millimeter-wave transmitters are more efficient than lasers, so less power would be needed to run the system.

"If you're pushing terawatts into space," Landis said of the beaming system, "it's expensive."

A lens to focus the millimeter waves (using techniques similar to those that steer phased-array radar beams) would only have to be 185 km wide, as compared to a 50,000 km fresnel lens that would be required for a system.

The sail itself would be made of carbon fibers, or possibly with variants of the high-temperature superconductors that have been in development since the early 1990s. The transmitter technology already is becoming available through megawatt-power, 1,110 gigahertz (0.78 mm wavelength) gyrotrons developed for fusion power experiments.

Landis suggested a laboratory demonstration using a 2 cm (4/5th inch) diameter cone. Shaping it so it would stay on the beam "is a tricky design problem, but it's a design problem with a solution," he said.


A precursor space mission, carrying a 1 kg (2.2 lb) payload on a 10x10-meter sail would take 20 hours to accelerate. In three weeks, it would pass the orbit of Pluto and continue outward to the Oort cloud of comets surrounding the solar system. Reaching a star would take 400 years, so it's only good as a demonstration.

"It's still science fiction," Landis said, "but it's near-term science fiction."

Even closer at hand is a concept to sail without a deploying a sail, but throwing a switch and generating one around the spacecraft. In an approach called Mini-Magnetospheric Plasma Propulsion - or M2P2 - a probe would imitate nature to get the solar wind to push it into deep-space.

"The enabling technology is pretty much available today," said Dr. R.M. Winglee of the University of Washington Winglee works in the geophysics program which studies the magnetosphere, the region of space around the Earth where the solar wind is deflected by the Earth's magnetic field.

Sailing in a bubble

"What we're proposing to do is create a magnetic bubble to deflect the solar wind," Winglee explained.

Magnetic sails were proposed by Robert Zubrin, inventor of the Mars Direct concept. Such sails are limited, so Wingless suggests injecting plasma (ionized gas) that would drag the magnetic field lines out and generate a bubble 30 to 60 km (18-36 mi) in diameter.


The magnetic field of 0.1 Tesla could be generated by a conventional solenoid, and the helicon plasma source "is amazingly simple." With a bottle of just 3 kg (1.5 lb) of helium as the plasma fuel, the magnetic bubble could be operated for three months. The size of the bubble would expand and contract with variations in the solar wind, so the force on the 100 kg spacecraft would stay constant at 1 Newton (about a quarter pound). The 3 kilowatts of power to run the magnet and plasma generator would be powered by solar cells.

Left: An artist's concept shows how Earth's magnetic field deflects the solar wind and forms the immense magnetosphere around the planet. Scientists may imitate nature and generate a mini-magnetosphere around a space probe and let the solar wind accelerate it into deep space. The solar wind exerts no appreciable push on the Earth because of the Earth's great mass. (NASA)

Winglee calculates the specific impulse (a measure of efficiency), would be tens of thousands of seconds. That's 10 to 20 times better than the Space Shuttle Main Engine.

"We can go faster and lighter than anyone else," Winglee said.

How fast?

If launched in 2003, M2P2 would go past the heliopause, where the solar wind runs into the interstellar wind, by 2013. That's a distance of more than 150 times the distance from the sun to the Earth. Voyager 1, launched in 1977, will get there in 2019.

Winglee said that adding dust particles to the magnetic bubble would enhance the thrust, and accelerate the M2P2 even faster for a mission to another star.

After giving his briefing, Winglee received a glowing recommendation from sail advocate Forward: "I just love the audacity of that concept."

Crack the whip to Mars and back

Forward also is closely involved in developing a precise interplanetary game of "crack the whip" that could send payloads to the Moon or Mars.

"Our goal is to develop a public transit system in space," said Robert Hoyt, president of Tethers Unlimited.

Hoyt and Forward believe that an interlocking, well-timed series of rotating tethers could carry payloads from low Earth orbit to the surface of the Moon with almost no rocket power involved.


People used to the smoke and fire of rockets may try to decode tether as an acronym for an exotic rocket. It's not. A tether is a flexible line or rope connecting two objects. Scientists have known for some years that if two bodies were rotating at opposite ends of a tether, they will snap into different orbits if the tether is broken. That happened with the unfortunate loss of the Tethered Satellite System when it was flown on the Space Shuttle in 1996. If the bodies are moving fast enough, one could be sent on its way to the planets.

Right: At artist's concept traces the trajectory for a payload dispatched from the HEFT tether orbiting Earth to the Lunavator Facility that will place it on the Moon. (Tethers Unlimited)

Under a contract to the NASA Institute for Advanced Concepts, Tethers Unlimited is defining a Cislunar Tether Transport System. The first step is appropriately named HEFT - High-strength Electrodynamic Force Tether, 90 km long in orbit around the Earth. At the other end of the line is the Lunavator Facility, a 200 km tether - plus counterbalance and central mass - in orbit around the Moon.

At the start, HEFT has a 1,000 kg payload at one end of the tether. It can start rotating by momentum exchange with payloads coming back from the Moon, reeling the tether in, or by using the tether itself as part of an electric motor (explained in a few paragraphs).

When the payload is swung out from Earth, HEFT releases and the payload sails on to the moon. A little bit of rocket power is used on the way since tidal forces and other effects will usually require midcourse corrections.

The payload arrives at the Moon, just in time to meet the Lunavator Facility as its tip is swinging outward. At this point, the Lunavator orbits well clear of the Moon. To deliver the payload, the central mass winches its way to the end with the counterbalance. Now the center of mass is very close to one end of the tether. The other end, with the payload, swings down to the surface and deposits the package at zero velocity.

That may sound impossible, but think of the edge of an automobile tire. It meets the road at zero speed (but is traveling at twice your car's speed when it rotates to the top). With the package delivered, the Lunavator Facility redistributes its masses in preparation for the next arrival. Or, it can pick up a package, at zero speed, and sling it back to Earth.

Early work on the Cislunar Tether Transport System led Forward to extend the idea to Mars.

"When Rob Hoyt first started his calculations, he was throwing the payloads too hard," Forward said. "He had to slow them down or otherwise they would escape from the combined Earth-Moon gravity field. After doing some calculations, I found that ordinary Spectra [the tough, light-weight fishing-line material used in the tethers] could throw payloads to Mars."


So he started designing for the next phase, the Mars-Earth Rapid Interplanetary Transport Tether (MERRITT) system. Forward has made another advance in that the outbound payload need not reach orbit. It can make an atmospheric ascent to 150 km (90 mi) where the EarthWhip picks it up and throws it outward to Mars. Like the Lunavator, the EarthWhip touches the payload at nearly zero relative speed, the center of mass shifts to balance the arrangement, and the tether releases the payload at the right instant to send it to Mars.

Arrival at Mars is the reverse, with the MarsWhip stage dropping the payload into the Martian atmosphere to glide or parachute to its destination.

"It will get you in," Forward said, "You don't need a deorbit propellant." The Martian atmosphere rules out tethers going directly to the surface, at least for the foreseeable future.

The trip to Mars could be made in 116 to 162 days, depending on the speed of the whip tip. With aerobraking to slow the craft on arrival at Mars and just before contacting the MarsWhip, the craft can make the trip in as little as 94 days by increasing the speed of the EarthWhip.

"We have a new idea," Forward said. "It looks pretty solid, and it looks pretty promising."

A step down before stepping up


The first step on this trip will be taken in August 2000 when Marshall Space Flight Center flies the Propulsive Small Expendable Deployer System (ProSEDS), a 15 km tether that will act like a small electric motor to lower the orbit of an expended rocket stage faster than natural atmospheric decay.

Left: NASA/Marshall engineer Les Johnson inspects part of the deployment mechanism for the ProSEDS tether mission. (NASA)

"We believe that an electrodynamic tether has a lot of applications," said Les Johnson, the principal investigator at NASA/Marshall.

ProSEDS' tether will expose the last 5 km of wire to make an electrical connection to the plasma (electrified gas) surrounding the Earth. As the rocket stage (the second stage of a Delta II that will launch an Air Force satellite) orbits the Earth, the wire cuts through the Earth's magnetic field. With electronics on the stage completing the circuit, the tether thus generates an electrical current at the expense of its speed, thus lowering its altitude.

"After ProSEDS, there may be a commercialization of this concept," Johnson said, "with operators putting these onboard spacecraft to deorbit rocket stages without using fuel."

NASA is also studying an Electrodynamic Tether Upper Stage that - by proper control of the electrical current - could boost satellites to higher orbits, then return itself to a lower orbit to deliver more satellites. A highly profitable application could be on the International Space Station where a low-cost electrodynamic tether could save about $1 billion a year in the cost of supplying reboost propellants.


Web links

Far out propulsion conference blasts off -- Apr. 6, 1999. The 1999 Advanced Propulsion Research Workshop begins this week.

Ion propulsion over 50 years in the making -- Apr. 6, 1999. A review of ion propulsion's long history, culminating in the technology on board Deep Space 1.

Survival of the fittest spacecraft -- Apr. 7, 1999. Scientists use artificial intelligence to "breed" better spaceships.

A coach-class tickets for space? -- Apr. 7, 1999. Scientists discuss new ideas for high performance low-cost space transportation.

Setting Sail for the Stars -- April 8, 1999. Cracking the whip and unfurling gray sails are among new techniques under discussion at the 1999 Advanced Propulsion Research Workshop

Reaching for the stars -- April 12, 1999. Antimatter and fusion are the subjects of new fuels for rocket engines of the future.

MSFC Advanced Space Transportation Programs Office.

Leftover Instruments Will Pave Way for New Propulsion Test (March 22, 1998) Well understood and well used scientific insturments will help verify a new instrument as they all fly on JAWSAT.

Spacecraft may fly on "empty" (Jan. 22, 1999) Using a propulsive tether concept, spacecraft may be able to brake or boost their orbits without using onboard fuel. A NASA/Marshall project, named "ProSEDS," is slated to demonstrate braking, by accelerating an expended rocket toward re-entry.

Lecture series to cover Physics for the Third Millennium (Feb. 2, 1998) Lectures on science in the next century will be held at Marshall Space Flight Center during February 9-12, 1998. Relativistic physics, and next generation propulsion techniques are among the topics.


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Author: Dave Dooling
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