Secret Star Wars program

[jdbenner]

Unlike Don, I do not believe that laser powered starships are out of the question.  The power requirements are the limiting factor.

The largest currently proposed optical telescope has an aperture of 30m.  And is designed to observe light with a wavelength of between .7x10^-6m to 1.4x10^-6m.

www.tmt.org/observatory

The laser beam may be allowed to spread wider than the collector.  The light will still be more concentrated near the center of the beam so if the beam is three times as wide as the collector, the collector still receives over 1/9 of the laser radiation.

Let our baseline system use a thirty meter diameter aperture.

Let our baseline laser have a wavelength of 10^-6m

Let our maximum acceptable beam diameter be 2,000Km or 2,000,000m

Let us assume 10m/s^2 acceleration

Let us assume that the starship starts at rest (If it starts at motion, performance can be enhanced)

Let us assume that the transmitter is not following the target, but at rest.

The distance the beam can travel given our constraints is r.



r = ((2x10^6m)(30m))/(2.44(10^-6m)) = 2.46x10^13m


V = (2(10m/s^2)(2.46x10^13))^(.5) = 22,181 Km/s = 7.4%c


Except for a collector on the scale of 500+ Km diameter,  and the power to drive the laser, we could do this today.



Robert Zubrin, in his book Entering Space, proposed a laser sail starship with the following parameters:
200 meter diameter aperture laser.
686Km diameter light sail (0.001 micron average thickness).
121 billion kilometer laser range.
9m/s^2 acceleration
15% light speed
1,000 ton Starship
240 Terawatt laser.
Magnetic sail “parachute” for deceleration at destination.

[ijuin]

I don't know about the one-nanometer thickness for the sail, unless it's something like a reflective gold film backed by a single-layer weave of carbon nanotubes. Just about any other material that I can imagine would not have enough tensile strength at that thickness.

[DonPMitchell]

One point in favor is that it is easier to build a large sail than it would be to build a large aperture.

Zubrin makes a living selling science-fiction packaged as science -- books, lecture gigs, and some very questionable grants from NASA.  He was part of that whole cabal of NIAC guys who reviewed each other's grants and doled out $30 million to each other.

He's pushed the parameters well beyond the possible to make the physics come out.  240 terawatts is a crazy amount of power -- the total US generating capacity for electricity is 1 terawatt.  And the best laser is 10% efficient, so you need to generate 2400 terawatts of power to produce that beam.  Even if you could generate that much energy, I seriously doubt that a laser of that power could be built that would not destroy itself.  How much energy would be deposited in the 30m lenses and mirrors?  How much heat would it dissapate overall?

I also doubt if a sail of that size and thinness could be made.

He claims 10 m/s**2 acceleration?  A 240 terawatt beam would produce 800,000 Newtons of radiation force.  With a 1000 ton spacecraft, that gives 0.8 m/s**2.  Am I doing the math wrong?  So we need another factor of 10 to fudge this.

[jdbenner]

Don, you are right about the difficulty in acquiring that amount of power.  Even Robert Zubrin was expecting a century of growth in the power industry before this would be possible.

Multiple lasers could be used but each would need the same aperture.

A laser powered rocket, could receive power coming and going.  The spacecraft could pass the laser.

A laser propelled spacecraft could be accelerated towards the sun, and the same laser could resume acceleration after the spacecraft’s hyperbolic orbit takes it past the laser a second time.

More than one laser could be used, one picking up where the other leaves off.

But none of these solve the power issue.

Don, did you take in to consideration the reflection off the sail?  Elastic collisions impart more momentum than inelastic collisions. 

[jdbenner]

Perhaps Microwaves could be used rather than lasers.

A phased array antenna on earth could in principle be the size of the earth.

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

The timing signal could come from the spacecraft.

Earth’s Diameter is approximately 1.27x10^7m

Let us assume an antenna aperture of 10^7m

Let us assume a microwave wavelength of 10cm = 10^-1 m

Let us assume a maximum acceptable beam diameter of 10^6 m

Then the range of the beam would be:

r = ((10^6m)(10^7m))/(2.44(10^-1m)) = 4x10^13m

And the velocity imparted to a spacecraft accelerating at 10m/s^2  over a distance of 4x10^13m is:

V = (2(10m/s^2)(4x10^13m))^(½) = 28,000,000 m/s > 9%c

The sail or antenna could be made from mesh as coarse as chicken wire.


The distributed nature of the transmitter along with its higher efficiency, would render moot concerns about lasers overheating.

The higher efficiency could somewhat reduce power requirements.

Using an antenna power source  and electrical rockets rather than a sail, could also reduce the power requirements.

The power supply issue would still be a major obstacle.

[DonPMitchell]

There are some good points there.  Microwaves only need a wire mesh instead of foil.  You pay a price for longer wavelength, but you can build a bigger aperture.  I still think generating hundreds of terawatts makes the cost of the project millions of times greater than a nuclear-powered/ion drive spacecraft.

[jdbenner]

Well, The Beamed power spacecraft are hard to scale down.  The Nuclear Rocket could be made much smaller.

[ijuin]

For hundreds of terawatts of energy, you need either fusion power or huge solar collectors--your solar collector area would have to be something like a hundred times the area of the spacecraft's sail.

[LunarOrbit]

Maybe we should build a huge Dyson's Sphere around the Sun with solar collectors on the entire inside surface and a power emitter on the outside.

[DonPMitchell]

Hehe.  BY the way, it's a myth that Dyson proposed building a sphere around a star.  He actually suggested that a stage II civilization would  just gather all the light from a star, with a cloud of orbiting collectors.

Let's move this to a new thread...