Hi all,
Just got this link from a friend about a new photonic laser thruster (PLT), which in scale up may be able to reach speeds greater than 100 km/s! (>360,000 kph!) and it seems to be the real deal. Anyone know more about this technology and how exactly it works?

Read it here: http://www.photonics.com/content/new...r/7/88894.aspx

It looks like it's just a high output light source. Photon rockets are a pretty old idea, but the issue is: Where does the power come from? For any significant thrust, there needs to be a lot of power. I don't see any mention of that here. Also, photon rockets (aside from light sails using an external energy source) don't make a lot of sense for interplanetary flight.

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The trick is to use several times the same photon , each time it strike the resonant cavity in the right direction , it give a little impulsion to the rocket.

In a conventional photonic rocket photons are used only once and dumped outside. So there is a huge wasting of energy and it why photonic propulsion is so inefficient.
That also why they speak of amplification.

Err... how do you re-use the same photon? (And how do you tell it's the same photon and not a different one? They all look the same to me )
Surely you'll need to expend energy to bring it back for a second go.

If the photon is transferring its energy to drive the spaceship forwards, that energy is gone and you need to use more energy in the system to do the same trick again.

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Not at all , a photon lose only a very small part of its energy when it collide an object. So it can theorically do it a number of time and then transfering a more important part of its energy.

If I understand this contraption correctly, there are photons bouncing back and forth between the vehicle and the platform that is launching it. This can only work for a fairly short distance. Whatever acceleration will be applied must be done in the first few kilometers, or else huge configurable mirrors will be needed to keep the photons where they are supposed to go.

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I didn't see the reflector scheme explained, but yes, that wouldn't go very far without problems, and the photons would redshift (lose energy) as the spacecraft/sail/reflector accelerated away, complicating things further.

I did see some mention of nanoprobes (low mass probes) which makes a bit more sense. And there's always the older idea of laserlaunch (where lasers are used to heat hydrogen or some other reaction mass) that probably is worth investigation.

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But as antoniseb says:
so the photon will transfer energy at both ends of its travel so if the first speeds up the spaceship, the second will slow it down by a similar amount. The only way this would work is if the photons are coming from a stationary launch platform and, as has been pointed out, this can't operate when the spaceship is a long way from its launch platform, nor can it be used to slow the ship down at the other end of its journey.

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Spike
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What you write make sense . And it seems that it is a limited system.

But as they spoke of "resonant cavity" , I ask myself a question : is it possible to curb the photon like in an optical fiber , instead of a mirror and to use it again to move the rocket ? In this case the impulse is only one way and the photon is "milked" for it a great number of time , and so its energy is much better used .

I dont know if it is possible , may be wild speculation. Need more details about the contraption.

The way this thruster works is pretty simple to understand if you're familiar with laser technology. A traditional laser is a lasing medium between two mirrors. With this photonic laser thruster, the lasing medium and one endcap mirror are on board a "laser station". The other endcap mirror is on board the spacecraft. Both the laser station and the spacecraft need to keep the mirrors aimed precisely at each other.

Today's dielectric laser mirrors can efficiently reflect photons thousands of times, so the thrust is thousands of times greater than a laser sail which only uses each photon once. Note that if the relative velocity between the laser station and the spacecraft is great, then accumulated red-shift or blue-shift may be a problem.

So this isn't a self-contained system? It relies on a base station that is in line-of-sight? How far could the ship get before it was too small a target for the laser?

And, more importantly, how does it slow down when it gets to its destination?

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Exactly, it relies on a base station. Ideally, there's a base station in orbit around both the source and the destination. Otherwise, some other method must be used to slow down at the destination.

The maximum range depends on the wavelength of light used and the diameter of the mirrors. Probably, the base station uses a much larger mirror since it doesn't have to go around being accelerated. So, let's assume a 10m mirror for the base station, a 2m mirror at the spacecraft, and a solar pumped NDd:YAG lasing medium at around 1 micron wavelength.

The diffraction limit is d=1.22wavelength f/a, so 2m = 1.22 * 1E-6m * f/10m. Thus, 20 * 1E6 /1.22 = f. Thus, the maximum range is around 16,000 kilometers.

So you can't use this approach for the first trip as there won't be a base station at the destination.

And 16,000k isn't all that far in galactic terms. Guess we haven't found the new route to the stars yet

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This drive is not suitable for interstellar journeys. At relativistic speeds, there's so much red-shifting on the first bounce that a second bounce isn't worth the trouble.

Thank you for these explanations. It is more clearer now.

OK call me simple minded but this sounds a bit like one of those old cartoons where a guy sits in a sailboat with an electric fan mounted in the stern blowing the air forwards to fill the boat's sails which in turn drives the boat forwards.

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So it's a cool idea, but not worth the effort in practical terms. A cheaper lightsail has more versatility.

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If you put the electric fan on a fixed island instead of the stern, and you have two sails--one on the island and one on the sailboard, and if the wind is able to magically bounce back and forth between those sails as if the wind were laser light, then yeah...that's about it.

There are two types of lightsail. One is a solar sail, which is limited to low thrust (although you can achieve some interesting possibilities with advanced sails if you do a Sun flyby). Another is a laser sail, which can be good for interstellar journeys but is extremely inefficient for interplanetary use.

This photonic laser thruster is thousands of times more efficient than a laser sail up to fast interplanetary travel speeds. It's less efficient than laser thermal for slow interplanetary travel speeds, but the same laser system can be used for both. Spacecraft could use both a laser thermal rocket as well as a photonic laser thruster mirror to get the advantages of both.

I like the idea of that second bounce.

I wonder if a series of annular (ring shaped) disks passing off such a light craft like a subway would be possible within a starsystem. Sunlight, beamed light, and the light from such a thruster--it could all add up.

Such disks are located in certain paths and the vessel passes thru the hole in the center of each--building up speed.

And that's a big problem in itself; if it's in orbit it won't be in the line of sight of the ship for half the time!

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the solar system

Only if the station is in LEO. But you're in a higher orbit, the Earth will never block more than a fraction of the time, and you can easily arrange things so that it's never blocking. Consider how rarely the Earth blocks the Sun's view of the Moon.