Talk-Polywell.org

Now THIS seems like a pretty good idea.

http://www.space.com/businesstechnology ... craft.html

David

Beamed energy propulsion has been arround - at least as a concept - for a while. I am somewhat sceptical, but willing to be convinced. It does sound good on the surface, but the devil is in the details and there are lots of those that need to be taken care of. Well, we will see.

Skipjack wrote:Beamed energy propulsion has been arround - at least as a concept - for a while. I am somewhat sceptical, but willing to be convinced. It does sound good on the surface, but the devil is in the details and there are lots of those that need to be taken care of. Well, we will see.

I didn't read the link closely, but I seemed to recall it mentioning the idea of twin hypersonic shockwave tunnels fed by infrared laser beams, and the vision it conjured in my mind was a scram jet that was fed by onboard laser energy.

One of the big problems with scram jets is that the air is moving so fast it's hard to initiate and maintain combustion with fuel.

Using a laser beam which the air in a tunnel can absorb ought to work at heating the air rapidly enough to produce thrust.

In other words, your using laser energy to heat the airstream rather than fuel. Of course, given the efficiency of generating laser light, the idea is probably too lossy to be practical, but it does sound like out of the box thinking to me.


David

There was some video (PBS maybe) of this crew some years back launching test projectiles with a laser. Test projectile: think in terms of a styrofoam cup. Something about that size and weight.

They could hit it with a laser and it would really shoot up a few meters like a bat out of heck. The early experiment had no guidance and they couldn't keep the laser on the target much further out, but it was obvious this method could produce meaningful thrust. It also did not appear to hurt the projectile.

Evidently they've gotten more sophisticated now.

I think you could probably launch almost to orbit using this trick, my main worry being that I would not want to be a private pilot and blunder into the beam. The big power source stays on the ground, and all you would need to launch would be the equivalent of an upper stage, with enough fuel to circularize the orbit.

Considering the size of the laser, I'm not sure I'd want to launch people this way. Hitting the wrong part of the ship would almost certainly destroy it instantly.

I believe I posted the idea on Simon's blog, of using microwaves and water.

The big difference was that I was looking at putting the emitter/maser on the ground. I don't know how far you'd get before it'd crap out, but it's appealing. Be even more with cheaper electricity.

Anyone read the Millennial Project ?
(other than me)

http://en.wikipedia.org/wiki/The_Millen ... Easy_Steps


Aquarius is a cybergenically grown floating island - a space colony at sea.



Powerful laser beams of the Bifrost Bridge converge to propel a wave-rider skyward.


The Lunar crater Copernicus, domed over and terraformed to create an ecosphere


David

There is a whole beamed energy comunity, not just for LV- propulsion but also for "climbers" that would climb up a space elevator (after all, you would have to get the power to them somehow too).
Then there are those that want to beam power down to earth from space.
I remember that the beamed energy propulsion topic has come up many times on Clarks RLV- news page in the last... gee... 6 years at least, dont even know anymore how long I have been reading that page, hehehe.
I am pretty sure that it was a topic in the alt.space newsgroups before that as well.
Sigh, how quickly time goes by.

There seem to be two versions of this laser launch concept. One, called lightcraft, uses the laser to heat the air below and behind the vehicle to propel it upward and forward. The other, called ALP, uses the laser to heat a material (usually a metal) on the back end of the vehicle, which then ablates to provide a propulsive force. There appear to be trade-offs between the two approaches. Lightcraft does not carry its own propellant, which makes for a lighter vehicle. However, it can only be powered while in the atmosphere. ALP, having onboard propellant, can be powered in space, but requires a heavier vehicle (it is carrying propellant).

It seems to me that a workable laser launch would have to be a hybrid of these two schemes. Use the lightcraft technique while in the atmosphere, then use the ablative on-board propellant once outside the atmosphere. Can the lauch vehicle itself be engineered for both mechanisms?

Any laser launch concept would have to use very lightweight launch vehicles where many of them are launched "in-line" on a continuous basis.

One question is how far to orbit can a laser launch system get towards orbit before having to switch over to using the ablative on-board propellant? Obviously the less on-board propellant required the lower the cost to orbit this thing will deliver.

I suspect the only way to answer these questions is more experiments.

kurt9 wrote:There seem to be two versions of this laser launch concept. One, called lightcraft, uses the laser to heat the air below and behind the vehicle to propel it upward and forward. The other, called ALP, uses the laser to heat a material (usually a metal) on the back end of the vehicle, which then ablates to provide a propulsive force. There appear to be trade-offs between the two approaches. Lightcraft does not carry its own propellant, which makes for a lighter vehicle. However, it can only be powered while in the atmosphere. ALP, having onboard propellant, can be powered in space, but requires a heavier vehicle (it is carrying propellant).

There's also Jordin Kare's HX Laser Steam Rocket.

kurt9 wrote:It seems to me that a workable laser launch would have to be a hybrid of these two schemes. Use the lightcraft technique while in the atmosphere, then use the ablative on-board propellant once outside the atmosphere. Can the launch vehicle itself be engineered for both mechanisms?

You'd need to shield the propellant from the laser during the air breathing phase. A liquid ablative might be better - spray it on the reflective lightcraft underbody.

kurt9 wrote:Any laser launch concept would have to use very lightweight launch vehicles where many of them are launched "in-line" on a continuous basis.

Not necessarily.

kurt9 wrote:One question is how far to orbit can a laser launch system get towards orbit before having to switch over to using the ablative on-board propellant? Obviously the less on-board propellant required the lower the cost to orbit this thing will deliver.

Decide on acceleration, guesstimate switchover @ 50km up, do the distance equation. Isp for ALP is approx 5000 seconds per wiki. Also, progress on ALP has ground to a halt for the last few years since the lead researcher at UAH has been on trial for his wife's murder.

The SF collection "High Justice" by jerry Pournelle explored this back in the 70s http://en.wikipedia.org/wiki/High_Justice
The Wikipedia article gives some more information on the concepts.

Dan

djolds1 wrote:You'd need to shield the propellant from the laser during the air breathing phase. A liquid ablative might be better - spray it on the reflective lightcraft underbody.

I would think hydrogen would be perfect for this. Light, well known, high volume gas. Modern rocket engines do not work the way most people think. Most of the thrust is generated by unburnt hydrogen. The heat of the O2+2H2 -> 2H2O reaction heats extra H2 into atomic hydrogen. This provides most of the thrust. Think about it. 3 moles of liquid turn into 2 moles of hot gas. Add two moles of H2 and you get 6 moles of not as hot, but still very hot gas. You don't run the engine burn lean at all. Cools the exhaust down nicely as well. With a laser drive, just vent the H2 out the back and let the heat of the laser turn it into atomic hydrogen (or even a plasma) and you would get a lot of kick. You would need to pick a frequency of light that is very well absorbed by both air and hydrogen. That would be a problem with any fuel you would pick. Makes me think about how the efficient the laser could be at a distance. Wouldn't the air between the craft's engine and laser absorb more and more of the laser's energy?

Emission spectrum of hydrogen nm

I always wondered what happened to Leik! Looks like he's done well for himself w/ that new contract. I like this concept and I hope it pans out.

pfrit wrote:

djolds1 wrote:You'd need to shield the propellant from the laser during the air breathing phase. A liquid ablative might be better - spray it on the reflective lightcraft underbody.

I would think hydrogen would be perfect for this. Light, well known, high volume gas. Modern rocket engines do not work the way most people think. Most of the thrust is generated by unburnt hydrogen. The heat of the O2+2H2 -> 2H2O reaction heats extra H2 into atomic hydrogen. This provides most of the thrust. Think about it. 3 moles of liquid turn into 2 moles of hot gas. Add two moles of H2 and you get 6 moles of not as hot, but still very hot gas. You don't run the engine burn lean at all. Cools the exhaust down nicely as well. With a laser drive, just vent the H2 out the back and let the heat of the laser turn it into atomic hydrogen (or even a plasma) and you would get a lot of kick. You would need to pick a frequency of light that is very well absorbed by both air and hydrogen. That would be a problem with any fuel you would pick. Makes me think about how the efficient the laser could be at a distance. Wouldn't the air between the craft's engine and laser absorb more and more of the laser's energy?

I'm not sure that excess hydrogen produces most of the thrust. Certainly providing more weight in the exaust stream at similar temperatures and pressures increases thrust, but unreacted propellent would cool the exaust, so that more thrust is obtained, but at decreased efficiency (ISP). I understand that some military rockets use tungsten to increase the aviable thrust (at the expense of decreased efficiency). Why you would use a light element like hydrogen, that would need much more structural weight (tanks) to carry the same weight of nonreacting propellent is unknown. Atomic hydrogen certainly would improve the weight per particle which is in the denominator of the ISP formula (ISP= square root ((temp * pressure)/molecular weight of exaust), but at the expense of the temperature and pressure obtained. I suspect that the excess hydrogen, if pressent, is a compromise to keep the engine cooler without harming efficieny excessively.
A link that discusses hydrogen excess and efficiency in rocket engines is below-
http://www.aerowebspace.com/AdvancedPer ... ngines.pdf


Dan Tibbets

Dan,

If the engine melts the thrust goes to zero. That would be most unfortunate.

The flame temperature of hydrogen and Oxygen is 3200 C.

Stainless melts at 2,781 F roughly 1,550 C.

H being lower mass will give the highest ISP at temperatures the engine can reasonably attain (about 2,400 C for the shuttle IIRC).