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."

Re:Star Trek anyone?

[Short Circuit]

I don't believe it was mentioned in TOS. However, 1970s scifi books used it. (Notably The Mote in God's Eye).

Re:acceleration?

[Harmonious Botch]

About 1/2 G.

Re:Solar system escape velocity!

[AKAImBatman]

So anyone riding the spacecraft better hope that there isn't a malfunction of the "slowing down" laser at the other end, as depending on the angle, that might be enough to exit the solar system altogether!

The article calls this a "Photon Thruster". What that means is that the device would be mounted on the vehicle as a thruster rather than the vehicle "riding" a laser-beam like in Beam-powered propulsion [wikipedia.org]. So as long as the laser restarts after you flip the ship, you're good to go.

Note that this is a separate issue from powering a laser cluster large enough to reach Mars in a week...

Re:acceleration?

[Harmonious Botch]

It includes turnaround at the halfway point.

Re:acceleration?

[Score Whore]

It seems likely that it would be quite a small amount of thrust, it's photons that you are "pushing" against. Even on the big engine.

The important thing is that it'll accelerate all the way there. With continuous acceleration it doesn't take much to get going really fast.

According to the article Mars is 100 Million km away and a big version of this will travel that in a week. We'll assume that you want to stop when you get there so just figure half the trip in half the time (since the second half will be braking):

50,000,000 Km = a * (302400 sec) ^ 2

a = .0005467722 Km/s^2

Acceleration due to gravity is 9.81 m/s^2, or 0.00981 Km/s^2. So he's talking about 1/18th G acceleration. Speed at turnover will be: .0005467722 Km/s^2 * 302400 sec = 165 Km/s.

Whee!

Of course it's more complicated than that since that low of an acceleration won't get you off the ground. So you'll be starting your trip in orbit. Which means you've got to take some time to get to a high enough orbit that you can accelerate away from the earth without having to do lots of high thrust maneuvers. Still, you can probably plan on Mars in a month.

Re: Metric Joke

[Telephone Sanitizer]

A harsh lesson that I have learned here...

If you're going to make a lame joke, at least include a cite so there's a chance of getting modded up as "informative."

The Mars Climate Orbiter:
http://en.wikipedia.org/wiki/Mars_Climate_Orbiter [wikipedia.org]

"The Mars Climate Orbiter was intended to enter orbit at an altitude of 140-150 km above Mars. However, a navigation error caused the spacecraft to reach as low as 57 km. The spacecraft was destroyed by atmospheric stresses and friction at this low altitude. The navigation error arose because a NASA subcontractor (Lockheed Martin) used Imperial units (pound-seconds) instead of the metric units (newton-seconds) as specified by NASA."

Re:I smell bullshit

[s4m7]

In other words, no existing institution would accept the good doctor, so he made his own, and issued a press release written in false third person.

http://www.photonics.com/content/news/2007/September/7/88894.aspx [photonics.com]

Bae founded the institute to develop space technologies and has pursued concepts such as photon, antimatter and fusion propulsion for more than 20 years at SRI International, Brookhaven National Lab and the Air Force Research Lab. He has a PhD in atomic and nuclear physics from UC Berkeley. Several aerospace organizations have expressed interest in collaborating with the institute to further develop and integrate PLT into civilian, military and commercial space systems, Bae said, and he has recently been invited to present his work by NASA, JPL, DARPA and the Air Force Research Laboratory (AFRL).

Re:I guess I don't get...

[arivanov]

No. The momentum gathered from sunlight points in one direction, the laser in another and you are going wherever the vector sum leads you.

Re:acceleration?

[Score Whore]

Half a G will get you way further than Mars in a week. The greatest distance between Earth and Mars is 391 million Km. Assuming you're going to go constant acceleration half way and constant acceleration in the other direction the second half of the trip, 1/2 G acceleration will get you 897 million Km end to end in seven days.

If you don't mind going through the Sun, that 1/2 G will get you Earth to Jupiter, in the worst geometry possible, in seven days and one hour and thirty minutes.

Re:I smell bullshit

[Rie Beam]

His website [baeinstitute.com] doesn't exactly inspire confidence, either

Bae Institute is a unique institute dedicated to creating revolutionary technologies for the next generation space and medical endeavors, yet aiming at facilitating their rapid implementation and commercialization. For that reason, we specialize in applying highly focused proven technologies to innovative solutions, thereby reducing development time while improving the viability of practical applications. An important goal of the Bae Institute is the commercialization of our innovative and revolutionary technologies. By licensing our unique intellectual properties, launching commercially viable companies, or by partnering with existing companies - we hope to quickly bring proven solutions to market.

Re: Minor correction

[Alaria Phrozen]

He didn't. Worst case distance, to get to Mars at 1G constant acceleration takes 3.5 days. What is there to confuse? The calculations are all throughout people's comments. RTF Cs?

Call me when it's 1:1 thrust:weight

[The Master Control P]

Because until then, you're still paying $10000 per kilogram to low orbit where you can engage the photon drive, which means that no meaningful exploration is gonna happen.

Did I mention that 45 years ago the USAF tested a nuclear thruster that almost reached 1:1? And how fifty-five years ago they drew up plans for an 8 million ton nuclear-driven starship as part of Project Orion?

Re:acceleration?

[demonlapin]

No, that only works if you're accelerating in the same direction at 1/2G the whole time. If you want to end up in the right place with zero speed, you need:

s = 0.5at^2

s = 0.5 * 4.9 * (3.5d * 24h/d * 3600s/hr)^2

= 224 042 112 000 m, a bit over 224 million km

Then double it, since you'll go just as far in the deceleration, and you get 448 million km, not 897.

Re:Energy source?

[Yazeran]

Well that is not a big problem. At least a couple of Mars probes have used atmospheric breaking to enter Mars orbit (called 'areocapture' i believe). While it is a tricky maneuver to get right, it can slow the spacecraft enough to enter a stationary orbit (do it wrong and you either burn up or 'skip' off the atmosphere and continue off into interplanetary space...) Fortunately Mars atmosphere is thin and has a higher 'scale height' than Earths atmosphere making the maneuver slightly easier on Mars, but still..

Yours Yazeran

Plan: To go to Mars one day with a hammer.

Re:acceleration?

[Hucko]

Actually the funny part is, I was confused. I thought I had misunderstood my physics education.

Re:How "scaled up" is this?

[StressedEd]

but right now we're generating power using f'ing COAL.

It's worse, much worse. Burning coal releases copious quantities of radioactive isotopes into the air [ornl.gov].

Re:Energy source?

[Arabani]

The concept of external (i.e. explosions are not contained within the ship's structure) nuclear pulse propulsion was actually studied in the late 50s, early 60s as Project Orion [wikipedia.org] (internal NPP, which is like your car analogy but with nuclear explosions instead of fuel-air explosions, places too great of a stress on the ship's structure to be feasible).

They never did get enough funding for a test with a nuke, but they did build 1-meter scale models powered by RDX charges. Powered by I believe 6 explosive charges, one of these reached 100 meters in a controlled test flight, proving that the concept worked (at least with lower energy pulses). As for whether or not it would work with nukes, their numerical modeling strongly indicated that it would.

You mentioned that the blast wave might be moving too fast to be useful, but actually that's the whole point - the impulse of the blast wave impacting against and then rebounding off the back of the spaceship is what provides thrust, so the faster the blast wave is moving, the greater the impulse and thrust.

Of course, the spaceship would have to be stupidly large to survive the instantaneous acceleration, but that was why it was so attractive. A ship around 10000 tons could've made it to Pluto and back within a year. Plus, it had a very high thrust-weight ratio, which meant that the fraction of the weight that was useful payload was stupidly high as well.

So then if NPP is so good, why was the project killed? It wasn't because it didn't work ... it was a combination of quite a few political reasons:
1) NASA had thrown its support behind the competing NERVA rocket.
2) Fallout was problematic.
3) There was no mandate from Congress for missions that would require such performance, and NASA had no desire to dictate policy.
4) Partial Test Ban Treaty of 1963 banned all above-ground nuclear testing.

Re: Minor correction

[Anonymous Coward]

With velocity, the time you need is inversely proportional to the speed. However with acceleration, the time you need is inversely proportional to the square root of the acceleration. In other words, if you want to get there in twice as much time, you only need 1/4 of the acceleration. However Harmonious Botch claimed you'd need half of the acceleration, which is wrong, but would be correct if it were about velocity instead of acceleration. Thus the assumption that he mixed up acceleration and velocity suggests itself.

Re:acceleration?

[Anonymous Coward]

I didn't check your math, but I think you get a good point there. However, as some may have pointed out, human can deal with much more than 1 G. Every day, we ARE exposed to 1 G! It's the earth that suck us in.

So, going up from the earth surface, every force will add 1 G, but once escape the earth, it becomes just the force alone. Assume that 1/2 G is what we can do here, then up there, we can do 1.5 G. Great isn't it? To Mar and back in 4 days!

Does this limit us accelerate faster than any G at all? Not really. If we can device a machine that works on us similarly to the G force, we can go at any acceleration, as long as we can turn that off at the end.

Re:acceleration?

[mykdavies]

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?

Strangely enough, the answer to this could be lasers as well - have a search for Laser Cooling [lanl.gov]

Re:acceleration?

[The Fun Guy]

"The only way to dissipate the heat would be through radiation, and that's slow compared to convection."

It's only slow if there is a small temperature differential between your source and your sink. Pointing the radiating fins out toward dark space would let them dissipate it pretty quickly.

Re:acceleration?

[Clith]

Although his math was a bit off, the grandparent's point remains valid.

At closest approach, Mars is about 56 million km away.Iif we switch the d=½at^2/ equation around, we get t=sqrt(2d/a). 'd' would be ½ the 56 million km distance, to allow for turnover, giving t/2, so..

t/2 = sqrt( 2 * 28*10^9 / 4.9 )
t = 59.4 hours =~ 2½ days

So between 2½ days and a week to get to Mars. Not bad..

Re:acceleration?

[HarvardAce]

Why wouldn't you go in a straight line from Earth to where Mars would be when you got there?

Well, there's a few issues.
1) If you're talking about the point when Mars is farthest from Earth, it's presumably on the other side of the sun. Going in a straight line would lead you through the sun, which probably would cause a few issues.
2) There's this thing called gravity...while you could, for the most part, ignore the gravity of the planets, the sun is another issue. It's going to cause you to travel in an arc, unless you're moving directly to/from the sun (which incidentally you would be doing in the first case).

Re:acceleration?

[tarkas]

Generally, radiatiative cooling will be limited by the Stefan Boltzman law, j (watts/m^2) = (stefan-boltzman constant) * T^4 (kelvins), http://en.wikipedia.org/wiki/Black_body [wikipedia.org]

where = 5.670 400(40)×108 Wm-2K-4. http://en.wikipedia.org/wiki/Stefan-Boltzmann_constant [wikipedia.org] So, the hotter your radiator, it increases output by a power of 4 and since space is very near absolute zero, for emissivity and absorption considerations, it's really dumping energy. You'd be surprised at how fast a simple radiation cooling scheme will operate.

I had to run a themo-vacc qualification test for some ISS hardware (on the mobile transporter). In a chanber with a very hard vaccum, even under a shroud made from a 1/8" skin aluminum box, painted with high emmissivity paint, we had good performance using a cooler lining the chamber, chilled with LN2, aprox -375F IIRC. I forget the cooling rate, but it wasn't bad. We had to modulate the cooler to get our cooling/heating profile, so we could have gone faster.

From TFA, it wasn't clear how they were pumping the photon source, I assume it'll be electric. So it's either batteries(Ha!) or some sort of nuke plant - thermionic orf some sort of (sterling ?) heat engine, either of which will be rejecting a bunch of heat, to generate - what, someone said like 370MWatt? So ya, big radiators of some sort. Plus, the photon source might also be generating it's own heat, aside from the photons, depending on the efficiency.

This'll basically be a big flashlight, just don't stand behind it or you're looking at one heck of sunburn, at least until you're vaporized. But the really cool thing is you don't need to schlep along tons of reaction mass, the photons do it for you, as they have a (very small) momentum. You just need a nice compact high power energy source.

Just turning around does not work.

[nanotrends]

Just turning around does not work. You are being pushed by a laser from a remote source.
You either have to first deploy the receiving laser array and power system.
Or bring an alternative drive for breaking.

Here is my desciption of how to perform this in more detail
http://advancednano.blogspot.com/2007/03/putting-brakes-on-laser-mirror-systems.html [blogspot.com]

Re:acceleration?

[Blain]

3) There's this other thing called momentum, which anything leaving Earth orbit has to account for. The vehicle doesn't start out at the edge of Earth orbit stationary compared to anything -- it starts out orbiting the Sun at the same speed as the Earth. The most efficient path won't be to try to kill that momentum and then push in a straight line at Mars. IANARS, but my guess is that the most efficient path would be a curve between the orbits.

only at ludicrous speeds

[Anonymous Coward]

The effect you describe is a relativistic [stanford.edu] effect. It becomes relevant at high velocities, a significant fraction of the speed of light. Constant acceleration has already been employed by ion thrusters such as those used by Deep Space 1 [wikipedia.org]. The topic of this thread, getting to mars in a week, makes for an interesting press release, but the technology is really more interesting. It's presumably offering a much higher specific impulse [wikipedia.org], allowing a wider range of mass trade offs than possible with engines that have a lower specific impulse. Lower thrusts and slightly longer trip times would still be dramatically shorter than the six months (or longer) trip times discussed for other plans (i.e. Mars Direct [wikipedia.org] plans an eight month transit time to Mars). The photon engine technology could be applied, for example, to a trip plan that featured a six week transit time to Mars, reducing the engine and power plant mass.

Unfortunately it isn't clear how the technology described would be applied to a Mars type of mission (e.g. any non-formation-flying mission where the photon engine would be used to provide significant and arbitrary velocity delta for solo ships. The "thrust" appears to occur between the mirrors, which the good doctor apparently plans to place on different spacecraft for stationkeeping. Perhaps one of the mirrors could be mounted on the moon or something. Dunno. The details seem to be absent. You would think this could be explained in a way that we geeks could understand. "Amplifying" chambers. Yeah. Right. Zero mass. Amplify that all you want and you still get zero thrust.