Of Lightsails and Solar Arrays

by Paul Gilster on May 8, 2006

Rudolph Meyer’s work on solar arrays and ion propulsion elicited quite a few e-mails asking for further information. I don’t yet have the Acta Astronautica paper that spells out the details — nor do I know just how detailed Meyer gets — but I’ll try to provide some answers soon. In the interim, I was startled to realize that Geoffrey Landis, who commented on the Meyer design for New Scientist, had actually gone into this concept at some length as long ago as 1989.

In fact, Landis’ key paper “Optics and Materials Considerations for a Laser-propelled Lightsail” (available here) was presented at the 40th International Astronautical Federation Congress in that year. Landis speculated on a lightweight sail that focuses power on a small solar array, noting that a basic problem with laser-propelled lightsails is their low energy efficiency:

The energy efficiency may be greatly improved, at the cost of a reduction in specific impulse, by combining the laser sail with a photovoltaic powered electric (ion) engine. Ion engines in principle have no physical limits on the specific impulse, although extremely high specific inpulses require proportionately high energy consumption. Such a laser-powered rocket would have the ability to decelerate at the target star (with some loss of efficiency), and could also greatly decrease the amount of power required from the laser.

Landis then presents a schematic for a rocket like this, with solar array mounted so that the sail acts as a mirror to focus light on it. A little later in the paragraph, he presents a related idea (internal references deleted):

An alternate version would be to form thin-film solar cells directly on the sail. The specific impulse of such a system can be extremely high as long as the mass flow rate of reaction mass is low; but even with extremely low mass flow rates the energy efficiency of the sail can be greatly improved…

Landis examined the idea again at an American Physical Society meeting and presented further thoughts at the legendary “Interstellar Robotic Probes: Are We Ready?” conference hosted by Ed Belbruno in 1994. At the latter, he discussed a “…laser-powered rocket, where the laser is converted into electrical energy, which is used to power an electric propulsion system.” Here’s the conceptual figure Landis used then, with his original caption:

Hybrid sail/electric concept by Geoffrey Landis

Image: Fixed laser, at left, illuminates a light-weight solar array, shown here as a centrifugally-tensioned thin-film membrane supported by tension wires. Power from the array is fed to an ion engine.

Clearly, the combination of solar sail and electric propulsion has been around for a while. Indeed, one of the fascinations of interstellar propulsion studies is the sheer range of brainstorming they generate. Landis’ papers drew on Robert Forward’s ideas about laser-propelled lightsails, and it was Landis who did key early work on refining our ideas about the best sail materials for that job. How Meyer’s work advances our knowledge of lightweight arrays and their potential uses is something we’ll talk about again once more facts become available.

{ 14 comments }

pfdietz May 8, 2006 at 6:40

One of the things driving this idea is the knowledge that photovoltaic cells can be made very very thin. Crystalline silicon is actually a bad material for thin PV cells, since as an indirect bandgap semiconductor the absorption length for photons in the relevant energy range is around 100 microns. Alternate materials, like CuInSe2, have absorption lengths of less than 1 micron. Texturing leading to light trapping (by total internal reflection) can also be used to reduce the thickness of the cell by a factor of up to (index of refraction)^2 by giving photons many chances to be absorbed before escaping the cell (it also helps recapture photons produced by carrier recombination).

James Benford May 8, 2006 at 21:08

I spoke to Geoff Landis this weekend at the NSS/PS meeting at LAX. He says that the PV in the concept is VERY sporty, so high risk. PV s are not particularly low mass/area, is the problem.

qraal May 9, 2006 at 7:47

Wonder how the laser-powered ion drive compares to Jordin Kare’s mini-laser sail pellet drive?

pfdietz May 9, 2006 at 8:24

PV s are not particularly low mass/area, is the problem.

Current PV arrays, certainly. The fundamental physics doesn’t require them to be heavy, however. Just a small matter of engineering. :)

Administrator May 9, 2006 at 8:27

Kare’s mini-sail concept — SailBeam — uses a laser to push micro-sails that serve as the ‘pellets’ or particles sent to the spacecraft. And the outbound vehicle would then use an onboard laser to vaporize the stream of sails into a plasma that could push a magnetic sail; it could conceivably use a pusher plate to absorb the energy as well. The beauty of the mini-sail is that you do away with the huge deployment problems created by larger sail ideas. These hybrid sail concepts seem very fruitful to me, particularly as we begin to see what kind of materials would be best adapted to sail work.

qraal May 10, 2006 at 7:37

Landis’ paper claims the efficiency of the laser-powered ion-drive beats a laser-sails if the speed is under 0.5c and if the vehicle deccelerates at the target. Higher than 0.5c and the laser-sail is more efficient.

Perhaps a hybrid – ion-drive, laser-sail then mag-sail to deccelerate – will be the way to go for a relativistic mission?

Even a probe propelled at 0.2 c would be amazing. Naturally the energy limited mass ratio would be the optimal 4.92, thus a Vex of 0.126c, which Landis says is within range of an ion-drive. But how best to extract energy from the laser beam? Space elevator studies suggest using lasers tuned to the band-gap energies of the PVs, with some suggesting 0.8 efficiency or higher.

But how do we ensure beamed energy drives aren’t weaponised?

Adam

Administrator May 10, 2006 at 8:48

I particularly like the deceleration via magsail idea. Landis has also explored particle beam concepts using magsails that seem plausible, and the beauty of magsails is that they’re a deep space technology that seems emminently testable on actual missions in the not so distant future. All subject to budgetary constraints, of course, but I’d better not start on that…

pfdietz May 11, 2006 at 9:02

An idea I’ve mused around but haven’t written a paper on is a laser ion sail. In this idea, the laser beam is tuned to the atomic resonance of some ions trapped in a magnetic field. Singly ionized alkali earth elements (magnesium, calcium, etc.) may work well, since they have a single electron outside a closed shell core, much like neutral alkali elements, and excitations of this electron lead to very strong absorption lines.

The cross section for absorption at an atomic resonance can be enormous (even with thermal broadening), so this sail could in principle be very low in mass. Laser light incident on this (cool) plasma would be absorbed and reradiated, mostly to the side toward the source (since photons emitted inward would be reabsorbed and do a random walk through the plasma; if the plasma is thick they will tend to come out the side they came in on). Because the sail is not solid — indeed, it’s already a plasma — it can become very hot without being destroyed. If the laser is tuned appropriately, it may even be possible to cool the plasma by selectively scattering off ions whose thermal motion is toward the laser. I suspect a similar laser refrigeration scheme could be used with conventional light sails as well.

A laser-powered ion engine may be more ‘efficient’, but if it is much heavier than a photon-pressure laser sail it may end up requiring more laser energy for a given payload mass.

Administrator May 11, 2006 at 9:11

This sounds quite interesting to me. In fact, it sounds like something that could be submitted to NIAC for possible Phase I funding, once you’ve gotten farther into the physics and have written the paper you mention. I’m not qualified to speak on how effective such a sail might be, but I hope you’ll keep us posted as you continue to explore this notion. And maybe some readers can jump in with their thoughts on the laser-ion sail.

Timothy J Mayes July 11, 2006 at 2:34

I believe that the best solution is to combine the use of a microwave
(maser) sail with a nuclear electric ion propulsion system . Nuclear
electric powered linear particle accelerators containing up to 1000 grids
in sequance charged with alternating electric voltage can accelerate
a beam of protons up to an exchaust velocity of more then 90% of the
speed of light. The proton beams can be neutralized by electron emitters
and expelled to generate thrust to accelerate an interstellar vehicle forward. The onboard nuclear power plant using nuclear fission can generate perhaps 2000 watts of electric power per kilogram of its mass.
For a vehicle with a mass of 1,000,000 kg to 3,000,000 kg the nuclear
power plant could generate up to 2 gigwatts of electric power to accelerate the proton exchaust beam. The on board nuclear fission powered electric power source may have only enough electric energy to accelerate microgram mass quantities of protons to more then 90 % of light velocity and thus generate a few hundred to a few thousand newtons of thrust.
There could also be an ultra-lite parabolic microwave antenna of perhaps some square kilometers of area attached to the interstellar space craft .
Microwave (maser) trans mitters on earth, or the moon could beam multi-terawatts of electric power to the interstellar vehicle in the form of maser
(microwave frequancy laser beams ) . These beams would be reflected to the focus of the parobolic microwave (sail ) antenna . There would be coils of copper wire located at the focus of the parabolic microwave antenna. When the microwaves hit these copper coils they will induce a direct electric current in them containing muti-terawatts of electric power
every second. Rectifiers will be used to convert this direct current into an alternating electric current to power the linear proton particle accelerators.
Multiterawatts of electric power will enable the acceleration of multi-milligram, to multi-gram mass qauntities of protons to more then 90% of the velocity of light. This will allow the generation of several hundred thousand to several million newtons of thrust from the hydrogen ion(proton) exchaust beam. This will enable accelerations of up to .10 G to be achieved by the star ship. A .10 acceleration rate if mantained for 10
years will accelerate the starship to 77 % of light velocity in 10 years or
72 % of light velocity in 7 1/2 years. The mass of the star ship is comparable to on of the saturn 5 rockets used to send apollo space crafts to the moon. I discovered many years ago that the design of the ancient viking long boats contained a method of achieving interstellar space flight
by means of making an analogy with with a space faring celestial viking long boat. In this analogy the nuclear electric ion propulsion system is the equivelent of the viking long boats oars which were used to navigate the fords, and harbors of scandinavia , and which were used by vikinggs to launch the long boats. The microwave sail antenna in this starship design is by this analogy, the equivelent of the curved giant sqaure, or triangular striped sail used by the viking long boats more then 1000 years ago to catch the wind and propell the long boats at up to 20 knotts per hour.
In this analogy both the natural sun light , and the microwave light of the maser beams is the equivelent of the wind that hit the sailsa of viking longboats and propelled them across the oceans more then 1000 years ago. By Timothy J Mayes 7-11-2006

Kevin Reed November 13, 2006 at 15:30

Hello,

Interesting disussion. We have made 9.5% efficency, 6 micron thickness solar cells with record power density of 4300 W/kg. We hope to improve this to 10,000 W/kg by reducing the thickness to 2 micron and some small gain in efficency to 11%.

KLReed

Adam November 13, 2006 at 20:32

Hi Kevin

Wow! Those power densities are incredible. What would their in-space survivability be like though under a rain of protons and micro-meteorites?

And do you have a web-page covering your work?

Adam

Mark D. Wagner May 15, 2007 at 19:57

The world’s most powerful laser placed on the moon’s south pole in constant
sunlight to power it can accelerate a Laser-Sail Starship with a 600 mile
diameter sail spun around it to 50% the speed of light. Star Pilgrims
fleeing the Dying Earth will be frozen safely using a new hibernation
method which allows a unique state of water frozen without crystals. NASA
is developing a magnetic force field to protect astronauts traveling to
Mars from solar and cosmic radiation. Since there is only 25 years of oil left
until the collapse of our oil society and technological civilization all efforts
must be maid to plan to build these lifeboat Starships to carry a few people
to an Earth-like planet to save mankind from extinction.
See http://www.newearthexpedtion.com to verify most of these statements,
Mark D. Wagner

ljk May 16, 2007 at 7:26

Mark, I cannot seem to access that Web page you list.

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