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Laser Launch to Orbit


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Laser Launch to Orbit

In this post, we will look at laser launch systems, how they would look like and perform, and how they might be applied to reach orbit and beyond.
The advantages of laser launch
 
fig4.png
140GHz is microwave.
A typical rocket engine does two things: deliver propellant and heat it up using an energy source.

In a chemical-fuel rocket, the propellant is combustible and is burned in a combustion chamber. The resulting heat and gasses serve both as propellant and an energy source. In a nuclear thermal rocket, the propellant is inert and nuclear material is used to heat it up. An electric rocket uses internal power, derived from a nuclear reactor or solar panels, to accelerate inert propellant using electrostatic or electromagnetic effects. 
Nozzle.jpg
color-temp-chart.png
What temperature do you think the gasses inside the nozzle are at?
The performance of a rocket is limited by how much energy can be delivered to the propellant. This is how much energy is released by the combustion of fuels such as liquid hydrogen or kerosene, how much heat is released by a nuclear core of how much electricity is delivered to an electric rocket. 

However, rocket fuel is only so energetic, and there are strict limits on how hot a nuclear core can get before it starts melting down or has to be designed larger and heavier. Electric engines are rather low on specific power, and the more the rocket consumes, the more mass has to be dedicated to producing energy. 

Liquid fuelled rockets are a mature technology that have pretty much reached the limits of chemical performance. For example, the SSME Plus was designed for an Isp of 467 seconds (4580m/s exhaust velocity), and the Rocketdyne AEC engine with 481 seconds (4718m/s exhaust velocity). This is very near the theoretical maximum for liquid hydrogen and liquid oxygen (about 500s). Going slightly further requires impractical fluoride oxidizers.

Nuclear rockets can push the envelope, but testing of solid-core designs delivered low Isp at high thrust levels, or high isp but low thrust in vacuum. Gaseous core rockets can provide both high thrust and high isp, but they require decades of research.

Electric rockets have similar problems. Nuclear reactors in space are very heavy, and solar panels do not provide enough energy to lift off the Earth. 

The solution is to separate the energy source from the spaceship.

 
skylonlaserboost.jpg
A Skylon variant where energy for heating the hydrogen propellant is provided by laser beam.
Laser beams can deliver the output of a several-thousand-ton power plants on the ground to the engine, at no extra cost. Although the specifics depend on the designs being used, the performance of laser-powered rockets ranges from 700 to 10000 seconds, with no upper limit except for laser power levels.  

A combination of high Isp and powerful engines that do not require on-board reactors, nuclear materials or volatile chemicals makes for small, cheap and safe rockets. The price per kilo in orbit can be made manageable at $1 to $100 per kg, therefore opening up access to space.

 
The problems to solve

Naturally, a rocket going into space cannot carry along an electrical wire to the ground to deliver energy. A power plant on the ground generates electricity, which is used to power a laser generator. The beam is then focused onto the spaceship, where an engine uses the laser energy directly, or absorbs it as heat indirectly. 
4Approaches.png
Four rough approaches to using laser power in a rocket

The intermediary steps between the power plant's electrical energy and the spaceship's engine create efficiency losses. The biggest loss is in the laser itself. Laser generators are quite an inefficient piece of technology. Conventional lasers, such as solid-state lasers, have an efficiency of about 25%. Pure diode lasers can reach over 60% efficiency, but cannot generate intense pulses. Pulsed lasers have to rely on flash-pumping technology, which gives mediocre 0.1% to 5% efficiency. 
fibre%2Blaser.jpg
Fibre laser configuration from a cutting machine.
Fibre lasers, where hundreds of tiny beams are joined through optical fibres into a larger beam, have both high efficiency, high resistance to heat and high pulsed power output, so are the best solution for a laser launch system. 

Another source of losses is from the laser beam travelling through the atmosphere. Some of it is absorbed. The best wavelengths for focusing a laser on a spaceship, such as ultraviolet (<400nm wavelength) do not travel far through the atmosphere. Optical wavelengths (700 to 400nm) and infrared to microwave wavelengths (700nm to 1cm) have narrow 'atmospheric windows' where they can travel through air without being quickly absorbed. Even so, a few percent is lost when the laser beam traverse dozens of kilometers of water vapour and various gasses.
atmospheric-windows.jpg
The 'atmospheric window' wavelengths
If there is a heavy cloud cover, optical wavelengths will not go through. Microwave beams are the only solution, as they are the least affected by clouds and water vapour. 

A lot of power is needed to launch rockets on laser. Generating a multi-gigawatt laser beam requires expensive hardware, and a lot of it. You'd also need to build ground facilities such as custom power storage, a miniature electrical grid, a large laser focusing array and so on before the first rocket is even launched... this is a level of up-front investment that may be difficult to find people or organizations willing to pay for. 

In comparison, conventional rockets only need to built one booster per mission. The costs are specific to the task they need to complete. Ground installations are minimal in comparison to those of a laser launch facility. 

Some laser-powered rocket designs require that a laser pulse strike a small target in precisely the right time and location, with the correct amount of joules. While the fine targeting can be done using on-board mirrors, the ground focusing array is still required to track the spaceship across a wide range of altitudes and velocities. The biggest problem is that lasers do not strictly travel in straight lines through the air due to atmospheric distortions. Adaptive optics and some sort of guide laser and feedback loops are required, which are complicated to set up and might fail to achieve the desired accuracy.

 
Laser%2BLaunch.jpeg
A depiction of a laser launch facility.

To summarize, a laser launch capability must compensate for the various losses from equipment inefficiencies and atmospheric absorption. It must produce a high quality beam that strikes the target in all situations, through atmospheric distortions and weather effects. During this time, electrical power supply and cooling must be managed.

These imply enormous up-front costs due to the quantity of expensive equipment required.

Reference design and comparisons

Reaching space is hard. Depending on the flight profile, a deltaV capacity between 9.5 and 11km/s is required to reach Low Earth Orbit. Due to the extreme variety in ways to achieve this amount of deltaV, we will use a reference design that we can compare the laser launch methods to.

Our objective will be 10 tons in orbit. 

Estimating engine mass is hard, as more thrust means a heavier engine with required more propellant which leads more thrust. Estimating tank masses is even more complicated, as they scale with volume. To remove the need for hundreds of hours of iterative calculations, we will retro-actively convert the necessary amount of payload mass into structural, tank and engine masses once the propellant requirements have been calculated.   

A chemical-fuels rocket using Kerosene-Oxygen at 300s Isp in the lower stage and Liquid Hydrogen-Oxygen at 420s Isp in the upper stage can reach orbit using 23.6 tons of LH2/LOx and 121 tons of Kerolox. Total mass is 155 tons. Upper stage deltaV is 5000m/s, lower stage deltaV is 4500m/s for a total of 9500m/s. Overall mass-ratio is 15.5. 

Laser-powered rockets must achieve orbit using a much lower mass ratio to be competitive. 

The designs

[...]

Continued here.

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Note that an engine heated by an laser beam will have the same constraint as an an nuclear one in that its limits on how much you can heat the engine, you might be able to push that a bit higher as tungsten or cheramic can take higher temperatures than the reactor but it would be hard / dangerous with the high fuel flow.
So say 1000 m/s, in atmosphere you can heat air who is free however as speed goes up the intake air become hotter making the engine less efficient. 

In space for reaching GEO or moon, an laser powered vasimr engine makes some sense, an solar panel designed for just the laser frequency will be very efficient so you can deliver plenty with power without giant panels. 

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A company called Escape Dynamics tried to do this in recent years. They had to shut their doors in 2015 after five years of work due to being unable to raise enough money for the monumental R&D and infrastructure effort involved. They built and tested some hardware, notably a ground station that could keep a laser pointed reliably at a quadcopter drone regardless of maneuvers flown, and a small beamed microwave thermal hydrogen thruster measured at 500s Isp on a test stand. But they never got anywhere near an actual vehicle or launch site.

Edited by Streetwind
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2 hours ago, Streetwind said:

A company called Escape Dynamics tried to do this in recent years. They had to shut their doors in 2015 after five years of work due to being unable to raise enough money for the monumental R&D and infrastructure effort involved. They built and tested some hardware, notably a ground station that could keep a laser pointed reliably at a quadcopter drone regardless of maneuvers flown, and a small beamed microwave thermal hydrogen thruster measured at 500s Isp on a test stand. But they never got anywhere near an actual vehicle or launch site.

That's my point: it's tested, readiness 1 technology that just needs more money for more powerful lasers. I noted that powerful lasers were being dragged alongside smaller microchip scales, and that the tech industry might be the indirect benefactor for laser launch technology that might be more reliable or faster than military research.

5 hours ago, magnemoe said:

Note that an engine heated by an laser beam will have the same constraint as an an nuclear one in that its limits on how much you can heat the engine, you might be able to push that a bit higher as tungsten or cheramic can take higher temperatures than the reactor but it would be hard / dangerous with the high fuel flow.
So say 1000 m/s, in atmosphere you can heat air who is free however as speed goes up the intake air become hotter making the engine less efficient. 

In space for reaching GEO or moon, an laser powered vasimr engine makes some sense, an solar panel designed for just the laser frequency will be very efficient so you can deliver plenty with power without giant panels. 

That is true for the Heat Exchanger laser thermal rocket design. There are ways to slightly push past this limit, such as using liquid hydrogen active cooling. The higher Isp designs directly heat an independently floating piece of propellant. In ablative or 'pulsed plasma' designs, the propellant never interacts with the nozzle or any other parts of the engine while being heated. This allows for 10000K+ temperatures that do not damage anything. After the heating is completed, which takes between a nanosecond and a millisecond, you end up with a rapidly expanding ball of propellant. Gasses cool down as they expand (PV=nRT). If it starts out as a 1cm sphere, and expands to a 10cm sphere before bouncing off the nozzle walls, it would have cooled by a factor 1000.

Suppose we take a 1 gram squirt of liquid hydrogen. We heat it up to 280000K using about 12kJ. It starts out as a blog 3cm in diameter. After heating, it explodes at about 25.5km/s. Within 2.6 microseconds, it has expanded to a ball of plasma about 13cm wide. By this time, it has cooled down to less than 1000K. This is easily handled even by small, metallic nozzles.  

In other words, there are ways to generate incredible exhaust velocities.

The lightcraft operates similarly. Air is superheated to tens of thousands of kelvin by a pulse of laser, thrust is generated by its expansion, then the air is replenished and the next pulse arrives. At higher velocities, airflow is faster and the pulses can cycle quicker... but there is less air. The maximum quaoted in one of my referenced studies was Mach 10. Beyond that point, it became very difficult to generate enough thrust to match the drag. 

If the air is superheated to 10000K, then its initial temperature accounts for 0 to 10% of the total energy. At 100000K, it is 1%. At the several million Kelvin quoted as possible, the initial temperature of the air can simply be ignored. 

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7 hours ago, magnemoe said:

Note that an engine heated by an laser beam will have the same constraint as an an nuclear one in that its limits on how much you can heat the engine, you might be able to push that a bit higher as tungsten or cheramic can take higher temperatures than the reactor but it would be hard / dangerous with the high fuel flow.
So say 1000 m/s, in atmosphere you can heat air who is free however as speed goes up the intake air become hotter making the engine less efficient. 

In space for reaching GEO or moon, an laser powered vasimr engine makes some sense, an solar panel designed for just the laser frequency will be very efficient so you can deliver plenty with power without giant panels. 

And that's why I think we're ought to go straight to pulse-laser ablation.

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1 hour ago, DDE said:

And that's why I think we're ought to go straight to pulse-laser ablation.

 

That and the US Navy is testing pulse-lasers capable of achieving the necessary power (although almost certainly with duty cycles far too low to work for this application).  The power source seems to be one of the big ???s in the plan, the other is where the money comes from to build all the infrastructure.

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2 hours ago, wumpus said:

 

That and the US Navy is testing pulse-lasers capable of achieving the necessary power (although almost certainly with duty cycles far too low to work for this application).  The power source seems to be one of the big ???s in the plan, the other is where the money comes from to build all the infrastructure.

If it's the USN it's likely to be atomic. They're supposed to produce a Fast Electron prototype this year.

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3 hours ago, DDE said:

If it's the USN it's likely to be atomic. They're supposed to produce a Fast Electron prototype this year.

 

This is very interesting relative to the space warfare posts on my blog, where it is stated that 'The Laser Problem' becomes significant at shorter laser wavelengths. Free Electron Lasers can able to produce such wavelengths. 

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The real kicker isn't so much power, which will determine the cost of the laser system, but the energy required, and its associated cost, which is quite low. The general rule of thumb is about a megawatt per kilogram of payload into LEO, so we'd need gigawatts to get tonnes into orbit. But it's gigawatts applied for a matter of minutes, and the power output of conventional rocket engines is comparable. The advantage provided by laser launch is separating the power from the rocket, increasing thrust to weight.

20 tonnes is 20 thousand kilograms, which, according to the rule of thumb, would need 20 gigawatts. Let's say that our time to orbit is 8 minutes, or 480 seconds.

20 gigawatts * 480 seconds == 9.6 terajoules

Divide by 3.6 million joules per kilowatt-hour, and you get roughly 2.67 million kilowatt-hours. Assuming average price per kilowatt-hour is 12 cents, then it costs roughly 320,000 USD for electricity alone. This is going to be the smallest cost. The real problem will be the cost of the lasers, which will need to be spread out over a large number of launches. Maintenance will be an issue, but it shouldn't be a big one. Another problem is that if we use LH2, the tank will be huge. As in, huge.

But pulsed laser propulsion looks promising, as it can use the atmosphere for propellant initially, and almost anything else afterwards, since the temperatures involved are so freaking huge. It's almost like an Orion sans nukes...

Edited by Bill Phil
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1 hour ago, Bill Phil said:

The real kicker isn't so much power, which will determine the cost of the laser system, but the energy required, and its associated cost, which is quite low. The general rule of thumb is about a megawatt per kilogram of payload into LEO, so we'd need gigawatts to get tonnes into orbit. But it's gigawatts applied for a matter of minutes, and the power output of conventional rocket engines is comparable. The advantage provided by laser launch is separating the power from the rocket, increasing thrust to weight.

20 tonnes is 20 thousand kilograms, which, according to the rule of thumb, would need 20 gigawatts. Let's say that our time to orbit is 8 minutes, or 480 seconds.

20 gigawatts * 480 seconds == 9.6 terajoules

Divide by 3.6 million joules per kilowatt-hour, and you get roughly 2.67 million kilowatt-hours. Assuming average price per kilowatt-hour is 12 cents, then it costs roughly 320,000 USD for electricity alone. This is going to be the smallest cost. The real problem will be the cost of the lasers, which will need to be spread out over a large number of launches. Maintenance will be an issue, but it shouldn't be a big one. Another problem is that if we use LH2, the tank will be huge. As in, huge.

But pulsed laser propulsion looks promising, as it can use the atmosphere for propellant initially, and almost anything else afterwards, since the temperatures involved are so freaking huge. It's almost like an Orion sans nukes...

Quite right! 

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On 3/3/2017 at 5:40 AM, MatterBeam said:

If the air is superheated to 10000K, then its initial temperature accounts for 0 to 10% of the total energy. At 100000K, it is 1%. At the several million Kelvin quoted as possible, the initial temperature of the air can simply be ignored. 

The catch is this type of thing typically involves heating the entire vehicle.  ISPs quote by escape dynamics were similar to 1970s NTRs in that they used hydrogen propellant at temperatures less than chemical engines.  On the other hand, if you are using air as a propellant the whole concept of ISP goes away (you still have to pay for the energy for the laser, but you are moving a much smaller vehicle.  Get the thing up to mach 4-8 and then switch to hydrogen and you have an efficient SSTO.

But you aren't heating anything to 10kK with a laser.  The laser might be originally designed for that (as a weapon), but doing so will turn any receiver into plasma.  Expect to limit things into something a fuel tank can withstand (and transmit to the reaction mass inside).

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4 hours ago, wumpus said:

The catch is this type of thing typically involves heating the entire vehicle.  ISPs quote by escape dynamics were similar to 1970s NTRs in that they used hydrogen propellant at temperatures less than chemical engines.  On the other hand, if you are using air as a propellant the whole concept of ISP goes away (you still have to pay for the energy for the laser, but you are moving a much smaller vehicle.  Get the thing up to mach 4-8 and then switch to hydrogen and you have an efficient SSTO.

But you aren't heating anything to 10kK with a laser.  The laser might be originally designed for that (as a weapon), but doing so will turn any receiver into plasma.  Expect to limit things into something a fuel tank can withstand (and transmit to the reaction mass inside).

I think you are confusing the Heat Exchanger Laser Thermal Rocket with the Lightcraft/Pulsed Laser Plasma Rocket.

 

The HX-LTR has to have a hot piece of metal that acts an an intermediary between the laser's energy and the propellant. Temperatures are limited to the melting point of the heat exchanger. Like a nuclear thermal rocket, this is about 3500K maximum.

A PLPR does not have a laser receiver or any element that heats up. It is a mirror surface that focuses a laser into a small torus under the vehicle. Air caught in that focal point is heated to plasma, then the plasma quickly absorbs the laser energy and reaches thousands of degrees Kelvin. There is no upper limit. The PLPR's mirror surface actually only reflects a relatively weak beam, of about a few dozen solar intensities (10kW/m^2 or more) and extremely low total joules per square meter. 

light-propulsion-test.jpg

It has been tested, and it works. Blue-violet flashes indicate 10000K+ temperatures. 

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  • 5 months later...
On 2-3-2017 at 7:12 PM, MatterBeam said:
Nozzle.jpg
color-temp-chart.png
What temperature do you think the gasses inside the nozzle are at?
 

I don't think the colour of this exhaust represents the temperature of a gas, chemicals also emit light when heatup to high temperates which emit colors not matching the black body heat temperature

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On 2-3-2017 at 7:12 PM, MatterBeam said:
 
 
 
 
 
 
Another source of losses is from the laser beam travelling through the atmosphere. Some of it is absorbed. The best wavelengths for focusing a laser on a spaceship, such as ultraviolet (<400nm wavelength) do not travel far through the atmosphere. Optical wavelengths (700 to 400nm) and infrared to microwave wavelengths (700nm to 1cm) have narrow 'atmospheric windows' where they can travel through air without being quickly absorbed. Even so, a few percent is lost when the laser beam traverse dozens of kilometers of water vapour and various gasses.
atmospheric-windows.jpg
The 'atmospheric window' wavelengths
 
 
 
 

Continued h

4

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?

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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.

 

Edited by KSK
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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.

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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.

Of course, the longer 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.

Edited by FreeThinker
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@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.

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26 minutes 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.

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

hubble_distortion.png

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

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37 minutes ago, YNM said:

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

hubble_distortion.png

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

Of course, Anything powerful that can damage something, can be weaponized.

Edited by FreeThinker
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1 hour ago, FreeThinker said:

Of course, Anything powerful that can damage something, can be weaponized.

So, how does that fare under *that* treaty...

I know that even blank rockets can be used as "a weapon", but nowhere near the lethal forces (or at least perceived lethality) of laser guns and such. Seriously, the big problem with this system is that it's not mature enough.

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54 minutes ago, YNM said:

So, how does that fare under *that* treaty...

I know that even blank rockets can be used as "a weapon", but nowhere near the lethal forces (or at least perceived lethality) of laser guns and such. Seriously, the big problem with this system is that it's not mature enough.

I can't imagine it matters, unless plans slipped out that mentioned that weaponized targeting systems were already on board.  Certainly satellites such as Skylab and Kosmos 954 could be considered weapons when they came crashing down, but that hardly violated the ABM treaty (I'm less sure of the general treaty that is even signed by the North Koreans).

Ok, I don't think *those* satellites could have their landing zones altered, but I think other treaties *demand* such satellites produced now have controlled reentering.  I'm happier with controlled attacks on the Indian Ocean than dealing with Kessel syndrome.

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