Laser Launch to Orbit

[KSK]

4 hours ago, shynung said:

@KSK The key advantage to a laser launch is that the energy source need not be carried by the vehicle. This means vehicle's thrust power is effectively how much the laser station is able to transmit at a given time, which enables the vehicle itself to have e.g. NTR-like performance without carrying a heavy reactor.

True, the vehicle still needs to carry its own propellant, nozzle assembly, and the associated plumbing, in addition to the laser receiver/heat exchanger system needed to make it work. But the ability to detach the power source from the vehicle grants that vehicle access to power sources that are much more powerful than what it can afford to carry by itself.

Yup - but as somebody already pointed out, a heat exchanger system will have similar thermal limitations to an NTR, so you're not gaining that much of a power advantage. I'm also curious as to how much the reactor for an NTR would actually weigh? Some of the later designs were pretty light as I recall. Mind you that does mean that you're left with a choice of powering your spacecraft by death rays or nukular reactors, so either way the popular press is going to have a field day. 

The way around those thermal limitations of course, is to go for some kind of external pulsed propulsion but as already mentioned, I think those have some problems, or at the least, some very interesting engineering trade-offs to consider.

Edited August 28, 2017 by KSK

[DDE]

2 hours ago, FreeThinker said:

If you aim a gigawatt long infrared at a full tourist beach, I guarantee you it will become classified as a WDM

Depends on the text of the document being argued over. E.g. UNSC resolution 687 defines them as

Quote

• "Nuclear weapons or nuclear-weapons-usable material or any sub-systems or components or any research, development, support or manufacturing facilities relating to [nuclear weapons].
• Chemical and biological weapons and all stocks of agents and all related subsystems and components and all research, development, support and manufacturing facilities.
• Ballistic missiles with a range greater than 150 kilometres and related major parts, and repair and production facilities."

 

[FreeThinker]

1 minute ago, DDE said:

Depends on the text of the document being argued over. E.g. UNSC resolution 687 defines them as

 

What about Gigawatt laser beam powered by nulcear reactors. You could ague the Laser is a Nuclear weapon by proxy

[DDE]

18 minutes ago, FreeThinker said:

What about Gigawatt laser beam powered by nulcear reactors. You could ague the Laser is a Nuclear weapon by proxy

No supercriticality, no WMD.

[MatterBeam]

3 hours ago, FreeThinker said:

What I wonder what happens on with this picture on planets without oxygen, like on Venus/ Mars. Will the Ultraviolet and X-rays still be blocked?

For Mars, use the absorption spectrum of CO2... but it won't really matter when air pressure at the ground starts at 0.01 atm. For Venus, the atmosphere is too thick for anything shorter than radio wavelengths.

1 hour ago, KSK said:

I love the cute pictures, especially the one of mini-Skylon being zapped by an off-screen Death Star. Where exactly are those lasers coming from?

I have many, many questions about the technical viability of laser launch, let alone whether its ever going to be economically or politically viable.

A laser launched spacecraft is still going to need a lot of propellant. A PLPR arrangement only works in atmosphere and only gets you moving so fast. Once you've left the atmosphere you still need a lot of delta V to get to orbit and you've just run out of air to use as propellant.

A heat exchange engine is a conceptually simple solution but then you have two engine types on the same vehicle, not to mention the fact that a launch laser optimized for PLPR may not be optimized for running a heat exchanger. I guess you could keep your PLPR arrangement and use it to vaporize ejected propellant but that's going to be horribly inefficient.

Assuming that you can reliably vaporize all your propellant, you've got no nozzle to speak of to efficiently transfer the momentum of your vaporizing propellant to the spacecraft. [Orion was (apparently) going to get around this by using shaped nuclear charges and by wrapping those charges in a good thick layer of propellant.]. Then there's the question of propellant choice. The article asserts that laser launch won't require volatile liquid propellants but it will almost certainly be using liquids of some sort. Solid propellant is going to be a nightmare as any chemical engineer who's had to move powders around will tell you. (Not to mention the fact that your powder dispenser has just had a good hard shaking) Make the 'powder' grains big enough to handle easily and you're left with the problem of reliably focusing enough laser energy on them to vaporize them quickly enough.

All in all, it seems to me that laser launch is much like air launch in many ways. Superficially attractive but as soon as you leave the atmosphere you hit all the same mass problems that rocket designers have struggled with since the start of the Space Age, and which the article conveniently hand-waves away.

 

Hi!

I can answer some of these questions.

The skylon's lasers seem to be beamed down from a higher orbit.

A PLPR is a high ISP drive that can utilize gigawatts from the ground for an overall high performance. A lightcraft here means an air-breathing PLPR that can take off an reach Mach 10 without expending any propellant. Reaching orbit needs onboard propellant, up to an additional 7km/s. Due to the high ISP, low mass fractions are acceptable. For example, a 10000s ISP PLPR switching to onboard hydrogen at Mach 10 only needs a mass ratio of 1.074. 

A ten ton payload would need only 740kg of propellant.

A PLPR uses a reflective half-torus opening to an aerospike nozzle. The torus is where a laser pulse is concentrated into a thin ring, which coincides to where the propellant is injected. Air or hydrogen can both be used by the same rocket by varying pulse duration, pattern and shape. No heat exchanger involved. No reason why this would be horribly inefficient.

The propellant expands and bounces off the nozzle, providing thrust.

By volatile, I meant explosive or chemically energetic, such as NTO+UDMH. Inert is something like liquid oxygen or hydrogen.

Laser launch requires a large initial investment but unlocks rockets with very low mass ratios to orbit.

1 hour ago, YNM said:

One question : how far downrange can the laser goes before it can't support the whole flight ? I'm specifically asking how close to the horizon one can go with the lasers and whether under some limit the lasers will not go focused enough to be able to support the combustion. Atmosphere still have airmass you know.

You would probably want to avoid shallow angles through the thick and turbulent lower atmosphere. Building the laser launch complex high up in the dry air of a mountain would be best, but a floating mirror to focus the beam after the initial boost is also convenient. Or a horizontal line of ground relay bouncing the beam down the lightcraft's atmospheric trajectory.

Alternatively, use lower frequency wavelengths that are less affected by the optical properties of air, such as radio waves.

1 hour ago, FreeThinker said:

Of course, the longe the beam of light travels the atmosphere, the more it will be absorbed or scattered by the atmosphere.

Therefore Instead of trying to shoot just over the horizon, it makes more sense to first fire as laser beam straight up to a Geo stationary orbit where you reflect the beam of light down to the vessel you attempt to get into orbit.

The problem with geostationary is that it's 35000km+ away. A lower orbit series of relays will cut thrust for a few seconds between transitions but allows for much smaller mirrors and more lenient accuracy.

1 hour ago, shynung said:

@KSK The key advantage to a laser launch is that the energy source need not be carried by the vehicle. This means vehicle's thrust power is effectively how much the laser station is able to transmit at a given time, which enables the vehicle itself to have e.g. NTR-like performance without carrying a heavy reactor.

True, the vehicle still needs to carry its own propellant, nozzle assembly, and the associated plumbing, in addition to the laser receiver/heat exchanger system needed to make it work. But the ability to detach the power source from the vehicle grants that vehicle access to power sources that are much more powerful than what it can afford to carry by itself.

Exactly this. A 5GW lasing station needs several hundreds of tons of power generating equipment, but would allow a 10ton 10000s ISP craft to accelerate at 1G comfortably.

38 minutes ago, YNM said:

While that's going to look nifty, you should realize that it doesn't remove the problem.

Also, wouldn't capabilities like this be classified into space weaponry ?

Adaptive optics, guide beams and at worst, a pre-pulse lasing channel can solve thia problem. Of course, it is the same solution for low orbit laser defenses.

20 minutes ago, FreeThinker said:

Of course, Anything powerful can be weaponized.

It might be a civilian contracted development of a service that doubles as anti-ballistic missile defense. Cheap for government but incredibly effective against nuclear powers that cannot replace their stock of ICBMs.

[YNM]

3 hours ago, MatterBeam said:

Adaptive optics, guide beams and at worst, a pre-pulse lasing channel can solve thia problem. Of course, it is the same solution for low orbit laser defenses.

AOs are not magic. They only work if they come from where you want to see them ie. if you're seeing it from space then you want it to come from Earth, but as explained this one is not needed. What's needed is AO onboard the rocket. Well yeah, AO at a few km/s within the ionosphere sounds fun 

Edited August 29, 2017 by YNM

[MatterBeam]

9 hours ago, YNM said:

AOs are not magic. They only work if they come from where you want to see them ie. if you're seeing it from space then you want it to come from Earth, but as explained this one is not needed. What's needed is AO onboard the rocket. Well yeah, AO at a few km/s within the ionosphere sounds fun 

AFAIK, you can shoot a guide beam through the atmosphere, watch its distortions and model your adaptive optics to cancel out those distortions. This is what telescopes use, and they certainly don't have anything on the target's end. 

[YNM]

25 minutes ago, MatterBeam said:

AFAIK, you can shoot a guide beam through the atmosphere, watch its distortions and model your adaptive optics to cancel out those distortions. This is what telescopes use, and they certainly don't have anything on the target's end.

They do. It's called the ionosphere.

Edited August 29, 2017 by YNM

[Green Baron]

Yeah, adaptive optics use lasers to paint artificial stars ("guide stars") in the sky ("on the other end"). One wants a guide star close to the object that's being watched, and such guides aren't always there when needed. That's why :-)

The flickering of this guide star is used to adjust the optics mirrors in almost real time to account for the distortions, assuming that real stars would experience the same distortions. This is mainly to correct effects of the lower atmosphere where the weather takes place. The technology though is still partially experimental, especially for the planned super large multi mirror telescopes TMT and E-ELT. Without such a thing they would be virtually useless.

Edit: how can this serve a fantastic laser driven spaceship ?

Edited August 29, 2017 by Green Baron

[MatterBeam]

6 hours ago, YNM said:

They do. It's called the ionosphere.

The ionosphere is good enough. 80-105km altitude is far above what a laser-driven spaceship needs for in-atmosphere propulsion, and I suppose that a relay orbiting at 200km can be targeted by an adaptive optics laser beam guided this way. The relay will power the lasership during its circularization burn by bouncing the laser from vertical to horizontal. 

6 hours ago, Green Baron said:

Yeah, adaptive optics use lasers to paint artificial stars ("guide stars") in the sky ("on the other end"). One wants a guide star close to the object that's being watched, and such guides aren't always there when needed. That's why :-)

The flickering of this guide star is used to adjust the optics mirrors in almost real time to account for the distortions, assuming that real stars would experience the same distortions. This is mainly to correct effects of the lower atmosphere where the weather takes place. The technology though is still partially experimental, especially for the planned super large multi mirror telescopes TMT and E-ELT. Without such a thing they would be virtually useless.

Edit: how can this serve a fantastic laser driven spaceship ?

However, this laser guide star technique is pretty specific to visual wavelengths. Microwaves might need a different technique for guidance. 

It serves mostly to reduce the beam power being wasted to compensate for errors.

If we did not have access to adaptive optics, we'd be forced to use a very wide beam from the ground, blooming perhaps to 10 meters wide by the time it reaches the spaceship. If the nozzle assembly is 4 meters wide, it has a 3 meter margin on each side for beam deviations and distortions by the atmosphere but will suffer an 84% efficiency penalty. 
A feedback loop between the spaceship and the beam station can help steer the beam towards the center of the nozzle and narrow the margins required, but it is a primitive solution.