Not directly electronics design, but todays YouTube video by Anton Petrov
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was on solar sails and it made me wonder if you could turn the idea around. Instead of using a sail, why not mount a laser and create the light onboard instead of depending on incoming light from the Sun or an external laser. Like an ion drive except using photons. I'm sure I'm not the first to think of this, so why won't it work? Anton mentioned that using just a 100 watt laser to irradiate a solar sail would be enough to push a very light satellite/sail. If you kept the sail and added a laser you could mount a retroreflector somewhere like the Moon and aim the laser there to get a second push from the same photos, if you were trying to get to an outer planet. Anyway, would it be practical?
If you had free energy, maybe; the momentum of light, though, is very small compared to its energy, and an ion gun (with reaction mass) is still very stingy on its fuel consumption, but gets high specific impulse from each watt-second spent. Laser drive IS a serious proposition when there's a nearby (ground-based) laser aimed at the 'combustion' chamber, ablating layer after layer of the projectile's fuel with ground-based laser energy.
I was just really struck by his comment about an external 100 watt laser shining on a sail providing enough of a push for orbital maneuvering for a cubesat. The push from a photon hitting the sail should be the same as a similar photon emitted by an on board laser, right? Does the wavelength of the photon matter (do you get more push from an ultraviolet photon than from an infrared one)?
What is the lowest power consumption light source per photon? I know it depends on the wavelength, but in the visible my guess would be an LED or diode laser.
For a photon, E = p c. Force is the time derivative of momentum, so the power is
dE/dt = P = F c.
Thus you to get 1 newton of force (about 1/4 pound) you need 300 MW of light. Your nuke plant isn't going to navigate the solar system very fast at that rate, even if you could cool it.
The idea is rather old and is called "photon engine". The problem is low energy efficiency.
One could try multiple reflections, but getting say 100 bounces looks tricky. Note that sail reverses momentum, so you get twice bigger thurst from reflection than from having laser on board (with multiple reflections external light source would give very similar effect to on-board laser). Anyway, at low speeds essentially only advantage of light is that you can use external energy source. With that advantage gone ion drive looks much better than light.
Photons do carry away a tiny amount of momentum so in principle you could accelerate by firing photons out of the back of a rocket.
The sail can do you no good at all. Emitting the photon causes a recoil in the opposite direction and it bouncing off the sail gives you no more back than just firing the photon away in the opposite direction.
OTOH reflecting a photon that has been fired at the vehicle from an external source gives you double the momentum.
In practice you can do a whole lot better by firing heavy ions out of the back by using an ion drive rocket motor. The more momentum per particle leaving and the faster the exhaust speed the better.
Once outside the Earth's atmosphere they probably offer the best option for a vehicle carrying its own power. You still have the problem of making a power supply light enough and strong enough to work well. (not to mention sufficient propellant to last)
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Solar sail or laser powered sail only works provided that you can make the vehicle incredibly lightweight and with a huge sail capture area.
It is worth pointing out that the momentum that a photon imparts is so tiny that all the toy Crooke's radiometers sold spin the wrong way!
They are not powered by by photon pressure but by excess pressure of the recoil of warm air from the black side of the vane. You have to push the to a much higher hard vacuum before they first stop and then spin more feebly in the opposite direction truly driven by photon pressure.
Your problem is the weight of the laser and its huge power supply would lead to an acceleration that would be measured in mm/year^2 if that.
A laser-driven interstellar craft is one of the plot devices. Not a human-crewed spaceship - human beings have discovered an even better (if somewhat less plausible) plot device for getting around in interstellar space - but Larry Niven does spell out the physics. Jerry Pournelle did the right-wing politics, which is as dumb as you'd expect in American science fiction, but the book was a good read (back in 1974) and sold well.
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Something like this driving a conventional ion thruster is probably much more efficient on paper:
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The materials science issues in building a durable pressure vessel and efficient photovoltaic receptor to give it an advantage over a thermionic fission-type reactor seem large, though
They also wrote a book called "Lucifer's Hammer" that I read a number of year back, it was 1/3rd about a comet impact and 2/3rds about a race war. It seemed dumb even for the 1970s.
It was one more end-of-the-world science fiction novel. The politics was straight Pournelle and even dumber than usual. The "race war" was incidental. It was more about preserving as much of a technological civilisation as possible after a catastrophe. Apparently it started off throwing in an alien invasion but Jim Baen got them to chuck that out, and that part of the story got recycled in the novel "Footfall" which was even more thick-eared.
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