Pages

February 26, 2007

Use 67 kilowatt solid state lasers for Mars in 10 days

A proof of concept demonstration of a photonic laser propulsion system using mirrors to bounce laser light and multiply the effectiveness of lasers was made



UPDATE: New article on how to stop the vehicle at the destination

A 67 kilowatt solid state laser has been achieved



The Solid State Heat Capacity Laser (SSHCL) has achieved 67 kilowatts (kW) of average power in the laboratory.

It could take only a further six to eight months to break the "magic" 100kW mark required for the battlefield, the project's chief scientist told the BBC. Hitting 67kW, said SSHCL programme manager Bob Yamamoto, meant 100kW was now within reach.

SSHCL uses an array of many diodes - not dissimilar to the LEDs used in bicycle lights and remote controls - to generate a beam. SSHCL generates a pulsed beam which fires 200 times a second at a wavelength of one micron. However, other experts place more stock in a continuous wave (CW), or "always-on", beam format.

One of the biggest hurdles to surmount for solid state lasers is achieving a sufficient beam quality. This is a measure of how tightly a laser beam can be focused under certain conditions.

Dr Yamamoto said improving the beam quality was one of the current goals for his team. The Livermore group is one of a number working on solid state lasers and is looking for further funding.

In 2005, Massachusetts-based Textron Systems won a $40m grant from the US Department of Defense to build a 100kW laser by 2009.

I am less interested in the weapon aspect as the potential for laser launching systems for space applications.

The proof of concept photonic laser propulsion system using mirrors to bounce laser light and multiply the effectiveness of lasers generate 35 micronewtons of thrust using low wattage lasers and 3000 bounces.

It has been proposed that extremely small payloads (10 kg) could be delivered to Mars in only 10 days of travel time using laser-based lightsail caft (Meyer, 1984), but in order to do so, would require a 47 GW laser system.

One thousand 100 kilowatt laser modules and 2000 bounces would be equal to a 200 Gigawatt laser. This would be 4 times the 10 kg system and could deliver 40kg payloads to Mars in ten days. Ten thousand modules would allow for 400 kg payloads to Mars in ten days.

A twenty ton vehicle could be sent to Mars in 96 days using two 1 GW laser source and 1000 reflections. The sail would have an areal density of 10 gm/m**2. For a sail with a 1000 m diameter the resulting total weight would be 7850 kg, within the weight budget of the 20 tonne lightsail craft. The example has a lightsail with a mass of 10 tonne, carrying 10 tonne of cargo.

An equivalent modular system would need ten thousand 100 kilowatt laser modules and 2000 reflections.

The deployment of large sails can be done using magnetically inflated cables

Modular laser launch systems are described here

The infrastructure for many thousand laser modules is substantial but not impossible in the range of several billion dollars. The cost of the electricity for each launch to Mars is:
Where P is the laser power of 1x10**9 W, the total time the lasers run, t, is 20 hours, and h is the wall-plug efficiency of the laser, which for purposes of this example will be assumed to be 25%. Under these conditions the total energy requirement becomes 8x10**7 kW-hr. The cost of producing electricity in the US is currently on the order of $0.03/kW-hr, which would result in a total power cost of $2.4 million, or only $240/kg for the delivered cargo.

7 comments:

Anonymous said...

Niven's Law "any sufficiently powerful propulsion system is also a very powerful weapon"?

Brian Dunbar said...

Question: The laser you want to power the craft with - ground based or in orbit?

bw said...

I think that both ground based arrays and space based arrays would be needed depending upon the mission.

The space based system for station keeping is the easiest, lowest power and simplest. Nasa looks like they are proceeding to develop it.

The mirrors bouncing lasers would also have less issues for energy losses and scattering if they were in or close to a vacuum. A lunar base with gas core nuclear reactors powering laser arrays would also work well. If one were launching a vehicle to Mars or a distant probe that seems like a good place to setup for launch.

An orbital system should have more mass than what it is launching and would also need engines to counter the thrust of launching a craft.

A ground based system would have challenges preventing the loss of laser energy. The wavelength would need to be one that is not absorbed by the atmosphere. There is also issues getting a properly designed vehicle that would have the right mirrored surface to reflect the lasers and be stable for launching through the atmosphere.

Brian Dunbar said...

The wavelength would need to be one that is not absorbed by the atmosphere.

Adaptive optics.

My ears pricked up because I do some work for Liftport. One likely candidate for providing power to the lifters ascending the elevator ribbon are ground-based free-electron lasers using adaptive optics to beam power to receptors on the lifters.

It's always good to see that there might be other revenue streams for the laser gangs.

bw said...

So free electron lasers like the
http://www.physorg.com/news82217403.html

14 kw system U.S. Department of Energy's Thomas Jefferson National Accelerator Facility

http://en.wikipedia.org/wiki/Adaptive_optics

I think that could work for the ground launch system. Although once it gets 1-60 miles up the thousands of relections would be like going through many thousands of miles of atmosphere. Although even getting a a launch system that still had a 100 to a few hundred fold gain from the reflections would definitely be worth it.

bw said...

Brian Dunbar

Since you work on the space elevator, would it make sense to adapt the mirror laser reflection system for the space elevator?

If the climber was a donut around the ribbon and its base was mirrored. the donut could hold some number (6,8,12) of properly shaped mirrored targets as well. Then lasers would bounce between the mirrors and mirrors at the base. The efficiency could be quite good because it could be easier to keep maximum number of reflections. The system requirements would be reduced and the efficiency of the climber propulsion could be quite high. Also there would be less need to drive the climber at a very fast rate. The climber could also retract some of attachments to the ribbon if it was being driven very fast and would only need to stabilize and center itself. So the climb could be faster than a system that had beamed energy to drive a climbing mechanism. It would be like a controlled upward fall.

This would only be necessary if the ground launch with lasers and mirrors only had trouble with stability, performance and sufficient reflections without a space elevator.

Brian Dunbar said...

Since you work on the space elevator, would it make sense to adapt the mirror laser reflection system for the space elevator?

Up front let me point out that I am merely the bloke who takes care of the computers. That said "qualified maybe".

The important thing is to deliver energy to the lifter. How it gets done is immaterial except that the power delivery has to be reliable and cheap.

We've been going with the assumption that the laser only needs to have line of sight to the climber. Obviously the higher the lifter the wider the geographic spread for the laser gangs.

Mirrors imply the lasers could be anywhere. Hooray for flexibility.

But for at least the first space elevator getting anything into orbit is going to add enormously to the cost, which is always a huge deal killer. A mirror system based anywhere but GEO implies a constellation of mirror sats ..

On the other hand orbiting mirrors mean we can put the laser gangs where energy production is cheap. Would the cost of orbital mirrors be more than the cost of laser gangs in South, Central and North America?

Makes me want to go back to school for my Operational Research degree.