A blog dedicated to helping writers and worldbuilders create consistent, plausible Science Fiction.

Thursday, 2 March 2017

Laser Launch into 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

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. 

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.

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. 

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 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.

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.

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


Laser Lightsail



In this design, the beam of light produced by a laser is used as the 'propellant'. Specifically, the momentum of photons is used as thrust. We must disregard this design for our purposes as the thrust produced by a lightsail will never reasonably overcome the force of gravity and launch a rocket from the ground. It might be useful if the spacecraft is already in orbit. 

Ablative laser propulsion


Test rig configuration for an Ablative Laser Propulsion Study.
Pulsed lasers concentrate their power into short bursts. When a pulsed laser strikes a target's surface, they can heat it up quickly enough to make it explode. 

In a controlled manner, ablative laser propulsion uses the force of explosively heated material as thrust.


The simplest configuration is a slab of metallic or plastic propellant which has one face exposed to the laser. A powerful laser blast 'ablates' this surface, creating thrust. It does so by exposing the material to ten terawatt per square meter pulses that instantly transform the solids into 10000K plasma. 


Isp is low, about 200 seconds or so. This is because the propellant has to be 'tough' enough to support its own weight and the thrust force that hits it like a hammer. Metallic propellants are used in this case. Another issue is that ultra-short (nanosecond-scale) pulses affect the air the laser travels through by ionizing it, creating issues.

Experimental performance. Note how Isp changes with laser intensity.
In vacuum, with no atmosphere in the way, much higher Isp is possible. Experimental tests have revealed 3660 second Isp using gold-based propellants. 

Reaching orbit with this design is quite difficult. The low Isp version can survive strong accelerations, but requires that the spaceship be a pyramid of metallic propellant with the payload on top. The design is greatly improved by using a chemical-powered second stage to handle circularization.

Advantages are the extreme simplicity and robustness of the design. It is well suited to high-G launches (such as rapid-response ICBM interceptors) and small-scale applications such as manoeuvring thrusters. Their usefulness increases in vacuum. 

Disadvantages are their are sensitive to how the laser is distributed over the target surface, and how evenly the propellant is ablated. External control, such as air flaps, are needed to smooth out imperfections in the material, and to turn the rocket. If these are not corrected and one side of the target is burned through faster than the rest, an imbalance in the thrust delivered could easily flip the rocket. 


Two-pulse laser ablation

A slightly more complex solution is to ablate a solid propellant using a two-staged pulse


From the High Frontier board game.
The first pulse 'scrapes off' some of the ablative material at low power by heating it to vaporization temperatures. For a plastic propellant, this can be as low as 400K. Most metals quoted vaporize at 1000 to 1500K. 

The second pulse heats up this material to incredible temperatures. It can do so because the laser energy is delivered too quickly for the material to radiate, conduct or dissipate it in any way. Tens of thousands of Kelvin are possible, which is hotter than the surface of the sun.

Water, ethanol and hexane struck by 8GW/m^2 pulses.
Ablative material heated by a two-stage pulse can deliver performance of up to 5000s Isp or more.

This allows for very small rockets that efficiently reach orbit riding on a single stack of metal disks. Since the plasma heating is done at a distance from the metal surface, there is less risk of the propellant interfering with the incoming laser beam. 

However, in addition to the single-pulse version's limitations, the two-pulse design requires even more accurate laser delivery. The second pulse has to arrive just as the debris from the first pulse starts expanding, which requires precise timing. Efficiency also suffers compared to single-pulse designs. 


Pellet ablation propulsion

Inspired by fusion research attempts and nuclear pulse propulsion, an innovate solution by ToughSF would be to use small solid coated with graphite. 


The propellant pellets are ejected mechanically into a thrust chamber. A powerful laser pulse is focused on this pellet, causing it to heat up, ionize and explode. Thrust is generated by the rapidly expanding propellant inside the nozzle. 
It is similar to an  inertial confinement fusion engine design... without the fusion. 
Unlike its inspiration, power is delivered externally instead of from nuclear reactions. It has no minimum pellet size and the only criterion is that the laser be powerful enough to heat the pellet quickly. This allows for smooth ad easily variable thrust. The laser can arrive from any angle, and does not have an upper limit on the pulse rate.

The same level of performance as two-stage pulsed ablative laser propulsion is expected, or higher. Instead of plastic polymers or metals, frozen hydrogen can be used as propellant. Lower atomic mass allows for squared increases in exhaust velocity. The graphite coating solves the laser absorption issues with hydrogen, which is relatively transparent to light. If 3600 second Isp is achievable using the experimental set-up mentioned here, using gold (atomic number 79) then 5.6 million second Isp is achievable using hydrogen (atomic number 2).   


The advantages include better energy efficiency compared to two-stage ablation, a simpler design that is less demanding of laser accuracy and timing and the flexibility to change propellant materials on the fly! Technically, any heating mechanism can be used. Particle beams, if built to higher efficiency, can be used instead of lasers.

One disadvantage is that smaller pellet sizes have a disproportionate percentage of their mass contained in the graphite shell. It works better with large pulse energies and large pellets.

Laser-heated plasma propulsion


These designs do not use solid targets for the laser. Gaseous propellant is simply heated to thousands of degrees, becoming plasma. 


An interesting version uses the atmospheric air surrounding the rocket as propellant. A mirror chamber, also acting as a nozzle, focuses an incoming laser beam into narrow ring under the rocket. Air heats and ionizes, becoming plasma. As a plasma, it starts absorbing laser energy very quickly. This generates a superheated wave of dissociated oxygen and nitrogen travelling rapidly away from the laser's focal point. When this wave hits the nozzle, thrust is generated.

A continuous wave laser would require fresh air to be delivered into the chamber by a turbine or ram-air scoop. A pulsed version can simply wait for cold air to drift into position. 

The main interest of such a design is that it requires no on-board propellant while the rocket is in the atmosphere. Experimental designs have shown that Mach 10 velocities are achievable for 'free'. This is twice as much deltaV as is produced by the first stage of a Falcon 9 rocket (3000m/s vs 1666m/s before engine cutoff).


It would make for a very economical rocket. It would be most effective in the lower atmosphere, and efficiency quickly falls off with altitude.
In the upper atmosphere and in space, the rocket would switch from external air to internal propellants. Liquid hydrogen provides the highest Isp, but requires tiny flakes of carbon to be mixed into the propellant flow, as hydrogen is quite transparent to optical and microwave lasers. Even so, heating hydrogen gas using a laser directly is difficult. 

One concern is that large liquid hydrogen propellant tanks interfere with designing a streamlined rocket that can reach Mach 10. It might be more interesting to use denser propellants to achieve a smaller cross-section. The reduction in exhaust velocity would be compensated for by more 'free' deltaV while in the atmosphere.

Another concern is laser accuracy. In the atmosphere, the laser would have to point directly up the nozzle, and it becomes more difficulty to do so as the spacecraft tilts and starts accelerating horizontally. In a vacuum, the laser would have to come up the same way the propellant exits, which is a concerning point. 

Laser-thermal rocket


This is the second simplest laser-powered rocket design. 

This shape maximizes the target area for a ground laser. It is possibly a lifting body.
In this design, the laser's energy is applied indirectly to the propellant. An intermediary heat exchanger is used. 

As shown, the laser targets a receptor that absorbs the beam's power as heat. This heat is transferred to the propellant through a heat exchanger inside a conventional-looking rocket chamber. 

The main advantage is that pointing a beam at a large surface on the rocket is quite easy. Lower accuracy requirements means a smaller focusing mirror can be used, and it is less dangerous if the less tightly focused beam wanders off-target. The design is also lenient on how the propellant flow is configured, as the laser receptor is independent from the heat exchanger and heating chamber.

However, there are limitations. The first is the temperatures that can be achieved. While pulsed direct-energy designs easily achieve tens to hundreds of thousands of Kelvin, an indirect design cannot get hotter than what the heat exchanger can survive. If the heat exchanger is made of steel, it cannot be heated beyond 1800K, if it is carbon-based, the limit is 3800K. The practical limits are much lower, as solids quickly lose strength as they approach melting point. 


Such temperatures are not much higher than those generated by a conventional liquid-hydrogen/liquid-oxygen rocket (3500K) and might actually be lower. Using lighter propellants, such as pure liquid hydrogen, allows for a maximum Isp of about 1000 seconds. Usually, it is between 600 and 800s Isp.

Another limit is the overall efficiency. Instead of just heating up the propellant, a laser-thermal rocket has to keep the receptor, coolant loop and heat exchanger at optimal temperatures despite radiation and conductive losses. Moving coolant consumes energy. A large receptor surface is not very aerodynamic either. 


A laser-thermal rocket is not the most efficient or the simplest laser-powered rocket design. It cannot achieve incredible exhaust velocities or provide 'free' deltaV. It is, however, the best understood design and the cheapest to build. It is the best at generating high thrust for rocket launches and imposes the lowest constraints on the launch facility's accuracy, laser type or power output.   

What can we do?


A laser-launch facility can easily reduce the cost of reaching space a thousand-fold. To make the construction and use of such infrastructure easier, cheaper or more useful, we can:


-Use laser-thermal rockets


They are the most practical design and can generate huge amounts of thrust. Many examples have already been built and flown, although at smaller scales. Verified technology is the best way to secure investments in the project. 


-Use a two stage design


A single-stage-to-orbit, fully recoverable laser-thermal rocket is cheaper in the long term than a two-stage-to-orbit rocket. However, an SSTO requires higher Isp, more megawatts per kilonewton of thrust and a flimsier construction that costs a lot to mass-optimize.


In other words, the advantages of a SSTO, such as lower propellant load, better use of engines and full recoverability, do not outweigh the benefits of a TSTO. This is especially highlighted by the recent successes of SpaceX's partially-recoverable TSTO rockets.


An SSTO laser-thermal rocket would have to be halfway optimized for both high and low pressure environments, which is not efficient. The laser beam would have to track it for much longer distances, meaning a bigger focusing mirror and larger atmospheric losses. Worse, the rocket would have a very high horizontal velocity at the end of its run, so the laser would have to follow it at non-negligible rotation rates of 1 degree per second or more. These two requirements lead to a large, expensive and motorized focusing array. 

Laser tracking of a target in orbit, in this case, a piece of debris.
In any case, the rocket would have to stabilize its orbit with a circularization burn on the other side of the planet. It would have to do so under internal power, likely a chemical rocket. 

So why not use a fully chemically-powered second stage? The first stage is a laser-thermal booster that stays close to the laser launch facility to reduce beam losses and accuracy or tracking requirements. The second stage is a conventional rocket that reaches orbit independently.


-Use modular lasers


Examples of all-in-one laser units that can be added one by one to the laser array, as needed.
Building a single large laser with enough capacity to remain useful in the future is extremely expensive. It is much easier to use small, modular lasers that can be added to a laser array only when extra capacity is required. This spreads the costs from an up-front investment into gradual increments over time. 

Modular lasers exist today, and focusing dozens or even hundreds of beams on a single spaceship is not much harder than focusing a single beam. They also offer a degree of safety through redundancy: if one modular laser fails, the spaceship still receives 99% or more of the required laser power. 


Using hundreds of lasers, a launch facility can deploy cheaper or newer lasers that have a rate of failure that would be unacceptable in a single, large laser. There are also significant large-scale savings that come from producing one small item hundreds of times instead of one item once. 


-Use efficient fibre lasers


Fibre lasers have been designed to 50% efficiencies and gigawatt-level pulsed power levels. They are less expensive and save enormously on power and cooling requirements. They fit perfectly into the modular requirements of a practical laser launch facility.


-Use phased arrays

The more individual emitters, the closer to a conventional laser it becomes.
A focusing mirror that can focus a beam onto a rocket tens of kilometers distant is quite large. An 11 micron beam needs a 3m wide mirror to be focused on a 1m wide target. A microwave beam requires an even larger mirror. Several of these are needed to handle the dozens of individual beams from a modular laser array. For comparison, the 4.3m mirror in the Discovery Channel Telescope cost $3 million alone, before actuators, sensors and other essential components are included. 

A cheaper solution would be to construct a phased array. These do not need focusing mirrors. However, they are restricted to lower wavelengths, such as microwaves, and are unlikely to generate pulsed lasers. 


-Store energy in flywheels


Using a 700 Isp laser-thermal engine on the lower stage of a 50 ton rocket requires more than 2 GW of laser power to lift off the launch pad. Accounting for inefficiencies, between 5.5 and 18GW of electrical power has to be fed to the launch facility.


Generating this level of electrical power on-site is an enormous task. For comparison, the Hoover Dam power plant produces 2GW, and the world's largest nuclear power plant produces 8GW. 


Building a power-generating infrastructure nearby is not always convenient. It would be economical to use some form of power storage instead. 

Flywheels are well suited to the task. They have both high energy density and the ability to release their energy in quick bursts, which is useful for charging capacitors for pulsed lasers. Hundreds of slightly upgraded versions of these would cover any storage requirements. 


-Add retro-propulsion capability


Aero-braking is the main method of returning to Earth for orbital vehicles. This is because it requires no propellant, only a heatshield. 


A pulsed plasma laser thruster that uses air as propellant could be an alternative option that is equally cheap. Once a laser launch facility is operational, each launch costs pennies on the dollar in terms of electricity. This means that using lasers for both launch and re-entry maximizes the utility of the installation. 

The plasma retro-thruster offers a way to safely and slowly return a payload from orbit using a much smaller (if any) heatshield. This means lower g-loads and lighter structural support. A soft landing is possible. 


The main advantage is that a plasma retro-thruster removes the weight of a heatshield and parachutes from the payload, which leads to a lighter rocket. Every ton of equipment removed from the rocket leads to several tons less propellant required on the launchpad, and terajoules less of energy to be delivered during ascent.  


-Put a laser or mirror in orbit


A ground based laser might have to travel through a lot of atmosphere, diagonally, to keep the beam on the target as it accelerates to orbital velocity sideways. With relatively long wavelengths such as microwaves, it becomes increasingly difficult to focus the laser on the target. Larger, heavier and more expensive focusing arrays will be needed to achieve longer beam lengths.


At a certain point, and certainly before the spacecraft travels over the horizon, it might be more economical to install a mirror in orbit. This mirror arrives over the horizon before launch and passes over the launch facility. 


When the beam reaches a certain length, the launch facility switches targets to the orbital mirror. This mirror re-focuses and re-targets the beam onto the spaceship. However, it does so from above, going through only the thin upper atmosphere instead of punching through the thick and humid troposphere. When the spaceship rises above the atmosphere entirely to complete its circularization burn, nothing will interfere with the beam. The spaceship speeds up and lowers its relative velocity with the orbital mirror, allowing the two to remain close to each other. This is especially useful of laser-powered SSTOs that need beam energy for the entire launch.


When the cost of access to space is lower, an orbital laser before affordable. It would be powered by solar panels spinning up flywheels, or by nuclear reactors. This space-based laser can accelerate spacecraft from space, with the ground-based laser only handling the initial acceleration. Shorter wavelengths can be used, creating more tightly focused beams using smaller and lighter focusing arrays. 


Worked examples



Here is a fully worked out example using cost estimates, but the figures used are quite outdated.

Let's try for both pessimistic and optimistic alternatives. The pessimistic alternative uses lower efficiencies, while and optimistic alternative assumes better efficiencies. 


We are attempting to put 10 tons into orbit.


Pessimistic alternative:


The rocket is a two-stage to orbit craft. The upper stage is a liquid hydrogen/liquid oxygen 450s Isp rocket delivering 5000m/s of deltaV using 21 tons of propellant. The lower stage is a 700s Isp liquid hydrogen laser-thermal rocket delivering 5000m/s of deltaV using 56 tons of propellant. Total launch mass is 87 tons and initial deltaV is 10000m/s. 


Launch TWR is 1.2, increasing to 3.4 at Main Engine CutOff. 


The Laser-Thermal rocket produces 1.024MN of thrust. It is 90% efficient, requiring 3.9GW of laser power. It burns for about 223 seconds. 


We use a continuous wave diode laser at 30% efficiency, operating at 900nm wavelength (near infrared). It requires 13GW of power for a total of 2.89TJ. At 5kg/kW,  19500 tons of modular diode lasers are required, about 3900x1MW units. If 1000 beams can be handled by a single mirror, then four mirrors are required.


A 1mx1m target at 100km on the spacecraft requires a focusing array with 30cm wide mirrors. The beam can wander by 1 radii and safely remain on target.  


A large assembly of flywheels is required to provide this energy. 136 kiloTonnes of 1GJ flywheels might be required... but at this scale, it might be cheaper to build a gravity dam. The water would be pumped back into the reservoirs between launches. 10 MW consumption by 50% efficient water pumps could recover the energy lost in 3 days 9 hours. This energy could be delivered on-site by solar panels.


The biggest costs are the lasers, followed by the flywheels. At $10 per watt, the diode lasers would cost $39 billion, or about 3 years' NASA budget. 


Optimistic alternative:


The rocket is a 1.5 stage rocket. The first stage uses a laser-pulsed plasma rocket to accelerate using air as propellant. At 30km altitude and 3000m/s horizontal velocity, the spacecraft switches to internal liquid hydrogen reserves for propellant. The 1.5 stage is an 2600s Isp pulsed plasma thruster that delivers 7000m/s of deltaV from 3.2 tons of propellant. Total deltaV is 10000m/s. 


0.1MW/kg is required to lift off, so launch power is is about 1.31GW. To produce 1g of thrust for entering orbit, 1.32GW is required in the upper atmosphere.


The pulsed thruster is nearly 100% efficient, however, as much as 10% can be lost to atmospheric absorption over long laser distances through the atmosphere. 1.46GW in laser power is required.


The laser beam is 12 micron infrared produced by 50% efficient fibre lasers. Electrical power requirements are 2.93GW. At $10/W, the fibre lasers would cost $14.6 billion. 


This can be delivered by a nuclear reactor on-site. A 3GW reactor costs about $16 to $23 billion to build, but these would be absorbed by the electrical sector rather than over a NASA budget. Such projects would actually become profitable over a short number of years, since 2.93GW translates into $5.6 billion of electricity per year at $0.22/kWh prices. 


The pessimitic alternative allows for a rocket just over half the size of a chemical rocket (56%). The optimistic alternative allows for a rocket that is much, much smaller than a chemical rocket (13.2 tons vs 155 tons).

Conclusion

Laser-powered rockets are a very interesting solution for producing high-thrust AND high Isp. Pulsed designs can achieve exhaust velocities that rival and surpass electric rockets, and even the simple laser-thermal rocket is competitive compared to chemical or solid-core nuclear propulsion. 

Only high-energy nuclear rockets compare in performance. 

As shown in the optimistic worked example, based entirely on existing technology and verified experiments, the size of rockets can be drastically reduced and the cost of reaching space becomes nearly trivial. 

However, beyond technical issues such as tracking a rocket through the atmosphere or dealing with beam attenuation due to atmospheric effects, the biggest hurdle to this method of launching rockets is the initial cost.

Generating gigawatts of electricity or storing terajoules of energy is very expensive. Converting this energy into a laser beam is simply not affordable. Laser launches become interesting only if space traffic justifies the long-term investment in a field of lasers and reactors, or if laser technology drops significantly in price.

Considering that laser costs are tied to the microprocessor industry, and that is following Moore's law, this might not be too far off...

The alternative is to start small. Only 1 MW of laser power is needed to put a 6kg micro-satellite in orbit.

22 comments:

  1. Laserlaunch, and especially lightcraft, have an additional use: ballistic intercontinental travel, faster than any aircraft. The lasersystem can also be used for powering a lasersail or as the starting point of a laser weapon web. So, while the laser installation is very expensive, it's quite versatile and much more economical than an orbital massdriver.

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    1. You're quite right.

      If you string together several laser launch facilities, you can start delivering people and warheads across continents at Mach 10. They can manoeuvre in the upper atmosphere, and aren't necessarily fragile like the flying fuel tanks that modern rockets are.

      Of course, the laser has many uses. You can switch the beam through a high quality, large, polished mirror and focus it down to weapon strength. It can slice through ICBMs and Low Earth Orbit targets in between launching payloads to space.

      A mass driver has never been practical on Earth...

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  2. Beamed energy may actually take a very indirect path. Currently military research throughout the world is focused on 100kW lasers for point defense of ships, aircraft and ground targets like bases. Since these weapons will necessarily have very high degrees of pointing and accuracy they could evolve into power sources for long duration UAV's (emitting low power to the drones. When the drone detects a target, the laser is cued and switches to high power to engage).

    This sort of technology is small enough to be placed on pods on a modern fighter like an F-22 or F-35, so repurposing this to space to defend high value targets like communications or spy satellites might be the next evolution. From there, it becomes an easy sell to use these as "laser brooms" to clear orbital debris, or to provide energy to move cooperative satellites into different orbits.

    Since individual units are small and relatively cheap (due to mass production), it becomes feasible to consider ganging hundreds or thousands of units together on the ground to build a laser launcher to orbit unit. with the bulk of the expense being the energy generation/storage system.

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    1. That's an interesting point!

      Extending the flight time of UAVs is very useful, and doing so with ground-based lasers instead of larger fuel loads even more so.

      I doubt that lasers will be mounted on F-22s for this purpose, as they could simply go check out the area themselves, and would have to stay up in the air just as long as the drone they're powering. Instead, they'd have lasers first as an electronic countermeasure, then they'll be powerful enough to spoof missiles and dazzle opponents.

      Pulsed laser lightcraft could theoretically allow tiny warheads to reach the accelerations of solid-rocket-booster Patriots and Sprint missiles. This is good for defense (less cost per ICBM intercepted) and great for offense (low radar cross-section and no kilometer-long exhaust plume).

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    2. While I don' exactly see F-22s mounting lasers for this purpose, I disagree somewhat with your reasoning. First, you appear to be assuming that there would only be 1 drone assigned to each F-22. More likely, there would be dozens of drones assigned to a host, in order to cover more territory. Second, goin in directly puts the MUCH more expensive F-22 at considerable risk; so, no, you won't want them going in themselves.
      However, F-22s simply aren't suited to this task, as they really don't like loitering. Instead, it is much more likely that a Cobra or Apache would be assigned to the task, or perhaps an A-10. These are craft that are specifically designed to loiter in a given area for extended times.

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    3. Agreed. I was simply picturing an F-22 stuck having to maintain line of sight with its drone(s) while the they slowly spread out and take film the area. It would be vastly more efficient if the F-22 gave up the laser, zoomed past and dropped hundreds of battery-powered drones instead.

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  3. I was actually considering laser powered drones orbiting warships and bases, but a large "arsenal aircraft" might mount a laser for this purpose. The current picture of F-22 and F-35's is to use the laser as a weapon in its own right, so sorry if I wasn't clear.

    The progressions then is laser point defense weapons, then using the laser to power a drone to extend the range of sensors (the drone is at a high enough altitude to provide a much longer line of sight), then lasers to provide thrust.

    Using smaller lasers to power SAM type interceptors is an idea I wish I had thought of, and also provides a one-two punch provided the launcher has the energy system (boost an interceptor then use the laser for terminal defense should the interceptor miss). Laser weapons are surprisingly versatile, given a creative user base and a large number of lasers to experiment with.

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    1. Yeah, there could be a 'natural evolution' for both aircraft-based continuous lasers and ground-based pulsed lasers.

      Aircraft lasers are offensive tools, meant to attack directly or to increase the effectiveness of other weapons. Ground lasers are defensive, meant to shoot down missiles and aircraft. The latter could evolve into interceptor-missile launch systems, then become powerful enough to push human-rated spacecraft into orbit!

      I really enjoy it when these concept come together in a consistent manner.

      However, I have noticed lately that the blog is taking on a preference for my own views of what is realistic or practical. I haven't considered a far-future version of laser launch systems, perhaps using Terawatt lasers dotted across the globe. These could point straight up into an ring of laser-pumped laser generators (like big frequency multipliers), thereby converting the microwave ground beam into an ultraviolet vacuum beam. This vacuum beam could be effective well beyond the Earth-Moon system, and become the principal method of transport to and away from Earth.

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    2. If this (http://faculty.ecnu.edu.cn/picture/article/2535/2a/87/ffa54e374c5e8f285957d70bf3e8/ed83c8b8-ac61-44f5-9828-a1bb99c7458d.pdf) is to be believed, laser frequency dividers exist too. This means you can use a lowish frequency to get trough atmosphere, convert it in orbit with frequency doublers to high frequency beams for long range transportation and convert it in orbit around it's destination back to lowish frequencies with a frequency divider so it can easily get through atmosphere.

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    3. I will have to look this up to see how bad this might be, but frequency changes in laser systems would seem to be a big area for losses to occur. You would have a massive laser or microwave beam on the ground turning into a puny laser pointer once all the conversion steps are done (and glowing red hot radiators as well).

      We may have to accept specialization; a microwave powered craft to exit the atmosphere and a laser generator in orbit or on the Moon to power spacecraft outside the atmosphere. Leyk Mirabo, who developed the "Lightcraft" concept of using lasers to heat the atmosphere for orbital launches (and atmospheric flight as well, BTW) had also worked on using MHD systems embedded in the skin of the lightcraft to manipulate the atmosphere and power the craft that way was well. His vision, as I understand it, is orbiting solar power stations project microwave beams down to the craft, rather than ground based systems beaming up.

      This has a certain elegance as well; the power station is in orbit and can service ground targets when not powering launches, or can beam microwave energy to spacecraft in view in Earth orbit, reducing the need for on board power systems and making spacecraft lighter and simpler overall (ion or plasma engines use a microwave antenna to draw power rather than a heavy on board nuclear reactor).

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  4. Hm... In theory, for many designs the orbital laser facility would be most effective - lost of solar energy, no horizon or atmospheric problems...

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    1. Quite true. However, it will be the most expensive to set up.

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    2. True... Hm. I'm not laser expert by any means, but I could recall the concept of solar-pumped laser on iodine. I wonder: would such kind of laser be more cost-effective as orbital launch facility than more "conventional" solar-to-electricity lasers?

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    3. http://ieeexplore.ieee.org/document/1073096/

      An old concept with maximum theoretical efficiency of 0.2%.... it won't ever be cost effective if electrical power stations are sitting on the ground, just 100km away.

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  5. How possible is it for a laser to produce an atmospheric distortion? Diffraction for anti- laser work, for example.

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    1. Took some searching, but I found an article on the concept: http://www.nextbigfuture.com/2017/01/bae-wants-to-enhance-reflective.html

      BAE trusts that they can do it, so I think it is possible.

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  6. This conversation reminds me of Starshot. The concept seems sound, but yet very power consuming, and would require almost microscopic accuracy. Can we be so accurate to have something moving with Earth, blast a ray at an object in space moving differently? Then have it launch said object in a desired direction at a desired speed, compensating for fluctuations and weather in the atmosphere. Then where to locate the ground based lasers. I read that starshot will do it from the Equator, but if you're going celestial south (to Alpha C.) that's a lot more atmosphere to penetrate, compared to shooting it from New Zealand. Wouldn't putting that laser on a rocket be far better, and fire from much closer? How nice it would be if it worked.

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    1. You're missing a critical point: the probe reaches its final velocity in two minutes. 60000G-force applied by a 100GW laser to a 1 gram micro-chip.

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  7. I thought it was more like 2 seconds. I can't imagine a laser can stay in that sail for two minutes. Yet still one zap is all we get. If it's not exact, then hello Banard's Star, or wind up in the middle of lost in space.

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  8. Issac Arthur takes the idea to "11": https://www.youtube.com/watch?v=oDR4AHYRmlk

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    1. A version extended into interstellar space? Excellent!
      I've been watching his videos recently. Slowly, because each one is quite long, but soon I'll be able to comment directly on a recent video.

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