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Photonic Laser Thruster Promises Earth to Mars in a Week
serutan writes "Using lasers to drive spaceships has been a subject of interest for many years, but making a photonic engine powerful enough for practical use has been elusive. Dr. Young Bae, a California physicist, has built a demonstration photonic laser thruster that produces enough thrust to micro-maneuver a satellite. This would be useful in high-precision formation flying, such as using a fleet of satellites to form a space telescope with a large virtual aperture. Scaled up, a similar engine could speed a spacecraft to Mars in less than a week."
Are we talking about "accidentally cut Venus in half" scaled up? Typically the downside of photonic thrust has been the low power to weight ratio, so for a laser powerful enough to propel a ship to Mars (don't forget that it has to both accelerate and decelerate) that fast I have to wonder just how powerful the laser has to be.
I read the internet for the articles.
At least now we'll have a way to beat the Kzinti when we make first contact.
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As I recall, the computations for reaching Mars in a week were predicated on One-G acceleration. i.e. Earth normal gravity for a ship in transit. To slow down, you simply spin the ship at the halfway point and accelerate in the opposite direction.
If (and I stress *if*) this invention is not so much hyperbole, it could change the face of space travel forever. We could build interplanetary starships (in this context, ships that never land on a planet) that would be limited only by their power-generation capabilities and not by their reactive fuel. Which means that we could build a ship with a large nuclear powerplant on board, and it could cruise the solar system for as long as its Uranium/Plutonium fuel held out.
Of course, we still need to solve the problem of high cost of launch, but that little issue would be easier to solve if we actually had somewhere to go once we got in orbit. Scaling up the number of launches would almost certainly bring the price per launch down. In fact, the reason why the Space Shuttle never reached its promised price-per-kilo is because it was predicated on regular launches that never materialized. Starships could change all that. Especially if the cost of moving personnel and equipment was marginalized by carrying more of them per trip.
For example, I always figured that a special module could be fitted to the Shuttle's cargo bay to carry as many as 60 people to the ISS. Given that the Shuttle has to be man-rated for flight, carrying people makes a lot more sense than hauling around equipment that's better served by a Delta or Atlas rocket.
How exciting! And probably too good to be true.
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I did quite a bit of reading on spacecraft propulsion recently (specifically Nuclear pulse propulsion and basically what I got out of it is that if you have a massive energy source (say, antimatter) you're better off just blowing it up and riding the blast wave. You can get extremely high thrust AND specific impulse that way, which is not possible with almost any other engine technology (either high thrust and low specific impulse like chemical rockets, or low thrust and high specific impulse like ion engines). NPP (and its derivatives) is basically the best way we know of right now to get high enough performance for interplanetary, or even interstellar, missions.
NPP originally started with using nuclear explosions, but more recent research has focused on inertial confinement fusion and even antimatter-catalyzed fusion. The obvious extreme is using antimatter-matter detonations and riding the blast wave, which I'm fairly certain would be more efficient and yield better performance than taking that energy and pumping it into a laser.
Twice the average speed if you want constant acceleration.
Bingo! 160 km/s somewhere between Earth and Mars absolutely qualifies as solar system escape velocity! I'm a little rusty, but isn't it 400 km/s from the surface of the sun, and around 15 km/s out past Pluto? Voyager II was doing 16 km/s when it left the building...
Since you're using photon pressure, the reaction mass is zero. With sufficient energy, you could travel anywhere in the universe. But unfortunately, Thrust = Power / speed of light.
Even a 1 Newton thruster requires 300 MW at 100% efficiency.
You've gotta scale up the power plant to get more thrust, and it's already going to be pretty massive (I believe that puts it on the order of a medium sized commercial nuke plant.) so I just don't see you reaching Mars in a week. Proxima Centauri in a lifetime, perhaps, but no way on the mars thing.
Of course, since he's talking about a laser, it's possible he means to have the equipment on the ground (or moon, or earth orbit) and propel a much smaller craft. With sufficiently focused optics, you could propel a small probe the whole way to mars (in a week? My envelope just ran out of space...), though it would require some pretty heat-resistant mirrors. Fortunately, the energy requirements for that Newton drop by half when you factor reflection into the equation.
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Thrust is the derivative of momentum with respect to time, and momentum is conserved, so in an open-loop drive F=dp/dt=d/dt(E/c)=(1/c)dE/dt, so power (dE/dt) is force times C.
But here's where the novel part comes in. Every photon is bounced back and forth thousands of times between the spacecraft and a mirror. The mirror experiences the same force as the spacecraft but in the opposite direction. The spacecraft's momentum comes from "pushing against" the mirror, rather than "pushing against" the exhaust photons.
For every photon with momentum E/c, the spacecraft gets a momentum kick of E/c when it emits the photon, 2E/c when the photon bounces off it again after a round trip to the mirror, 2E/c again on the next round trip, and so on until the limits of the optics lose the photon out into space. If the drive could really deliver the thousands of photon reuses Dr. Bae talks about, then the power requirements drop to more like 1E10 watts.
OK, fitting that into ten metric tons means we still need antimatter.
The problem with all of this is scale, right? The energy required to send larger and larger objects would be impractical.
So, what's the smallest thing we can send, then? How small can we make a satellite that can send some information back?
It may not be useful for transporting people to the other end of the universe in a practical amount of time, but I'm sure sending a probe that can check up on Mars every week or so would be of some sort of slight interest to researchers...
Of course, there's the issue of the touchdown...
His institute seems to have a lot of promising ideas, but no real substance. It has three major projects, one of which relies on the photon thruster and some kevlar straps to toss around satellites, and some sort of undeveloped nano-microscrope.
http://www.baeinstitute.com/
Bullshit, I indeed smell.
But what about the heat? It's quite difficult to cool off lump of metal in a vacuum without discarding hot material to do so. Even if you could feasibly power a craft to Mars with this, how would you stop yourself from arriving as Astronaut McNuggets?
Aide-toi, le Ciel t'aidera - Jeanne D'Arc.
If .5G could get you to your turnaround point in 3.5 days, that would mean you'd be going about 1500 km/s when you get there. That's equivalent to 1e6 MJ/kg, or 3.6 MW/kg. Sayth the Wiki that a nuclear fission plant can provide that kind of energy density, and to spare. Not sure about the power density, though, nor about the shielding requirements for human habitation. But from my inexpert viewpoint, the energy requirements look like they'd scale.
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I think the problem is that in order to create the propulsion, the laser has to *hit* the craft, not be directed away from it. If I read this correctly, the heat questioned in the grandparent post comes not from powering the laser but from the laser beam smacking against the drive plate.
And given the lack of atmosphere, a heat sink wouldn't help much. The only way to dissipate the heat would be through radiation, and that's slow compared to convection.
The question is, of course, is this really an issue? How much heat is generated from the laser blasting against the drive plate? How quickly will the heat be dissipated?
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highly insulating structure? what about vacuum? the engine section does all the work and pushes the living section forward using magnetic repulsion.
Yeah, scaled up a rocket engine could also get them there in a week. What does it take to scale up one of these laser engines to make it suitable for the perceived task?
I think we are missing something here.
If the ship is heating, then it is absorbing energy. The point of this (if I understood correctly) is for it NOT to absorb the energy, so it can move.
Yes, it can't be 100% efficient, so there will always be heat absorption. But I wonder if it will be enough to be a problem.
And yes, I don't have any idea what I'm talking about. This is just gut feeling.
morcego
Of course, since he's talking about a laser, it's possible he means to have the equipment on the ground (or moon, or earth orbit) and propel a much smaller craft. With sufficiently focused optics, you could propel a small probe the whole way to mars (in a week? My envelope just ran out of space...), though it would require some pretty heat-resistant mirrors. Fortunately, the energy requirements for that Newton drop by half when you factor reflection into the equation.
I highly recommend the book Accelerando by Charles Stross, which has an extended story arc which deals with exactly this idea. They're trying to get a coke-can sized space shuttle with a solar sail to a brown start about three light years away (which has an intergalatic router nearby), and they power the shuttle with a laser beam powered by a cable dragged through the jupiter atmosphere/magnetic field. I highly recommend the book. Amazing concepts throughout.
Aren't all lasers "photonic" by definition? Was this thing named by the Redundant Department of Redundancy Department?
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How to scale up.
The original demo was from 10 watt lasers and 3,000 reflections (it is good to actually research original papers to know what is being discussed.)
It is theoretically possible to achieve 100,000 reflections (you may have to go outside the atmosphere to ensure less losses of energy (ie like from a lunar launch system
We will soon be making 100 Kilowatt solid state lasers. (US military made 67 kw earlier this year and will have 100 kw system done later this year or early next year.
We can use arrays of lasers
(ie more than one). Power is provided in electrical form to the lasers. Say from nuclear power (3.2 GW twin reactors, and can have more reactors) or hydro power (Three gorges dam generates 18 GW). So wattage can go up say 100 million times to 1GW. (reduced the nuclear plant power by inefficiencies for the lasers.
the reflections can increase by 33 times.
Therefore, 3.3 billion times more power.
Thus you can send several ton vehicle to Mars at high speed
http://advancednano.blogspot.com/2007/02/use-67-kilowatt-solid-state-lasers-for.html