elwoodblues

Laser propulsion for launch doesn't make a whole lot of sense. You need a whole lot of thrust in a short amount of time to get free of the earth's gravity well, which a laser system doesn't typically provide. Instead, it'd be good for boosting from one planet to another. Provide a small amount of thrust over a long period of time, and it adds up. Of course, then you have to focus the thing over thousands and thousands of miles.

Mageth

Laser propulsion for launch doesn't make a whole lot of sense. You need a whole lot of thrust in a short amount of time to get free of the earth's gravity well, which a laser system doesn't typically provide.

According to the page I linked to and the other article that page linked to (Laser rockets ), the researchers seem pretty optimistic that it has excellent potential, at least for very small loads. I'd agree that much larger loads are perhaps a bit pie-in-the-sky right now, but who knows?

One thing about such a craft lifting a small craft with one or two 'nauts in it is that the system would be much safer than the conventional method.

Instead, it'd be good for boosting from one planet to another.

That sounds like it might be more of a job for the laser sail.

elwoodblues

Ooh, the laser continually detonates a propellant laminated on the back dish of the 'ship'. Okay, that's entirely different from what I was thinking (ie, pushing the damned thing with photons). Yeah, we're on the same page now.

Mageth

The two pages I posted describe the laser propulsion method a bit differently than I originally heard it described. I distinctly remember plasma bursts generated by the laser propelling the "lightcraft."

Here's a page that describes it the way I remember:

The laser-propelled vehicle, called "Lightcraft" because it flies on a beam of laser light, is designed to harness the energy of a laser beam and convert it into propulsive thrust. The Lightcraft receives the kilojoule pulses from the PLVTS laser at a rate of 10 times per second upon the concentrating mirror that forms its rear section. The function of this parabolic mirror is to focus the pulsed laser energy into a ring-shaped "absorption/propulsion" chamber. Here the laser beam is concentrated to extremely high intensities, sufficient to momentarily burst the inlet air into a highly luminous plasma (10-30,000 K), with instantaneous pressures reaching tens of atmospheres providing thrust....

...The spacecraft lifts-off in a laser propelled airbreathing engine mode, and as it nears Mach 5 speed and 30 km altitude, shifts into a laser propelled rocket mode. The airbreathing engine mode would develop quasi-steady thrust by pulsing at hundreds to thousands of times a second -- depending on the mach number and altitude flown along the boost trajectory into orbit. The rocket mode would use on-board propellant, in the form of liquid hydrogen or nitrogen, to convert and expand the laser energy for propulsion once the Lightcraft had climbed above the atmosphere. Unlike Goddard’s rocket engine, no oxydizer is required.

Greg2003

Right. That was my understanding of how the laser propulsion works, that is, superheated plasma causing thrust. But still, the laser itself would have to be extremely powerful to lift any kind of significant payload. And it sounds like we don't have any laser that can do that at this point. This would, however, be the safest and among the most efficient propulsion systems (very little energy loss, and no dirty exhaust).

In any event, the point is that NASA is working on a whole lot of propulsion systems that do not involve nuclear fission or fusion. The nuclear projects that they are working on don't necessarily scare me. But Bush's interest in them does. He must have alterior motives for this besides "bold" initiatives (the popular conservative characterization of every thing Bush does now).

managalar

BOTH of these 'alternatives' require nuclear power (or another unknown power source which can produce hundreds of kilowatts). There is no easy ride, or risk-less rewards (though many people are looking for ways to beat basic physics...because there may well be ways).

I recomend
http://www.nuclearspace.com/
for decent information, or spacedaily (which I read daily) if you want to keep up to date on what's going on. Hello Project Prometheus...how could they resist such a perfect god analogy, and how can we resist using our Nuclear 'fire'?


BTW, I'm really glad someone mentioned NERVA
http://www.f104g.demon.co.uk/space/nerva1.htm
not considering power sources like RTGs, NERVA was the only major nuclear power rocket project actually test fired, and was our only chance at reaching Mars if we were going to race the Soviets there too.

Loren Pechtel

Lightsail doesn't burn up propellant but is far less efficient than using the laser to heat some reaction medium. If you need a lot of thrust (ie, planetary liftoff) you'll have to use propellant.

Mageth

Exactly, and the lasersail is not considered for use as a lift mechanism. But in theory it may be able to achieve @.10c on interstellar flights (where a lot of thrust is not necessary), with the obvious advantage that you don't have to tote along propellant.

Loren Pechtel

Robert Forward proposed using a microwave beam to push a probe to 20% c.

Mageth

Well, that's even better, and the astronauts would have an easy way to heat up their meals!