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  1. Hi! From this post. 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 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 [...] Continued here.
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