• Inverse of a solar sail

    From Carl@21:1/5 to All on Fri Apr 8 19:33:45 2022
    Not directly electronics design, but todays YouTube video by Anton
    Petrov
    (https://www.youtube.com/watch?v=JM-J_VZOw60&ab_channel=AntonPetrov) 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?

    --
    Regards,
    Carl

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  • From Sjouke Burry@21:1/5 to Carl on Sat Apr 9 02:08:59 2022
    On 09.04.22 1:33, Carl wrote:
    Not directly electronics design, but todays YouTube video by Anton
    Petrov
    (https://www.youtube.com/watch?v=JM-J_VZOw60&ab_channel=AntonPetrov) 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?

    No, the onboard laser installation would weight several times
    the payload, and is therefore not capable to move itself plus
    the spacecraft.

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  • From John Larkin@21:1/5 to All on Fri Apr 8 17:18:43 2022
    On Fri, 8 Apr 2022 19:33:45 -0400, Carl <carl.ijamesxx@yyverizon.net>
    wrote:

    Not directly electronics design, but todays YouTube video by Anton
    Petrov
    (https://www.youtube.com/watch?v=JM-J_VZOw60&ab_channel=AntonPetrov) 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?

    The energy used to make light is hugely more than the thrust. Photons
    have a lot of energy but very little momentum.

    A solar sail makes a gentle push and the sunlight is free.

    --

    If a man will begin with certainties, he shall end with doubts,
    but if he will be content to begin with doubts he shall end in certainties. Francis Bacon

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  • From whit3rd@21:1/5 to Carl on Fri Apr 8 17:23:38 2022
    On Friday, April 8, 2022 at 4:33:52 PM UTC-7, Carl wrote:
    Not directly electronics design, but todays YouTube video by Anton
    Petrov
    (https://www.youtube.com/watch?v=JM-J_VZOw60&ab_channel=AntonPetrov) 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.

    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.

    <https://www.sciencedirect.com/science/article/abs/pii/S0094576520303817>

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  • From Carl@21:1/5 to John Larkin on Fri Apr 8 21:12:00 2022
    On 4/8/22 20:18, John Larkin wrote:
    On Fri, 8 Apr 2022 19:33:45 -0400, Carl <carl.ijamesxx@yyverizon.net>
    wrote:

    Not directly electronics design, but todays YouTube video by Anton
    Petrov
    (https://www.youtube.com/watch?v=JM-J_VZOw60&ab_channel=AntonPetrov) 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?

    The energy used to make light is hugely more than the thrust. Photons
    have a lot of energy but very little momentum.

    A solar sail makes a gentle push and the sunlight is free.


    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.

    --
    Regards,
    Carl

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  • From Phil Hobbs@21:1/5 to Carl on Fri Apr 8 21:45:07 2022
    Carl wrote:
    On 4/8/22 20:18, John Larkin wrote:
    On Fri, 8 Apr 2022 19:33:45 -0400, Carl <carl.ijamesxx@yyverizon.net>
    wrote:

    Not directly electronics design, but todays YouTube video by Anton
    Petrov
    (https://www.youtube.com/watch?v=JM-J_VZOw60&ab_channel=AntonPetrov) 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?

    The energy used to make light is hugely more than the thrust. Photons
    have a lot of energy but very little momentum.

    A solar sail makes a gentle push and the sunlight is free.


    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.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

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  • From antispam@math.uni.wroc.pl@21:1/5 to Carl on Sat Apr 9 02:01:07 2022
    Carl <carl.ijamesxx@yyverizon.net> wrote:
    Not directly electronics design, but todays YouTube video by Anton
    Petrov
    (https://www.youtube.com/watch?v=JM-J_VZOw60&ab_channel=AntonPetrov) 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?

    The idea is rather old and is called "photon engine". The problem
    is low energy efficiency.

    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?

    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.

    --
    Waldek Hebisch

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  • From Martin Brown@21:1/5 to Carl on Sat Apr 9 13:06:24 2022
    On 09/04/2022 00:33, Carl wrote:
    Not directly electronics design, but todays YouTube video by Anton
    Petrov
    (https://www.youtube.com/watch?v=JM-J_VZOw60&ab_channel=AntonPetrov) 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?

    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)

    https://en.wikipedia.org/wiki/Ion_thruster

    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.

    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?

    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.

    --
    Regards,
    Martin Brown

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  • From Anthony William Sloman@21:1/5 to Martin Brown on Sat Apr 9 07:57:56 2022
    On Saturday, April 9, 2022 at 10:06:32 PM UTC+10, Martin Brown wrote:
    On 09/04/2022 00:33, Carl wrote:

    <snip>

    OTOH reflecting a photon that has been fired at the vehicle from an
    external source gives you double the momentum.

    The idea has made it into hard science fiction.

    https://en.wikipedia.org/wiki/The_Mote_in_God%27s_Eye

    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.

    <snip>

    --
    Bill Sloman, Sydney

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  • From bitrex@21:1/5 to Anthony William Sloman on Sat Apr 9 13:04:08 2022
    On 4/9/2022 10:57 AM, Anthony William Sloman wrote:
    On Saturday, April 9, 2022 at 10:06:32 PM UTC+10, Martin Brown wrote:
    On 09/04/2022 00:33, Carl wrote:

    <snip>

    OTOH reflecting a photon that has been fired at the vehicle from an
    external source gives you double the momentum.

    The idea has made it into hard science fiction.

    https://en.wikipedia.org/wiki/The_Mote_in_God%27s_Eye

    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.

    <snip>


    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.

    <https://snarkypenguin.wordpress.com/2017/09/13/r-i-p-to-a-right-wing-ass-dr-jerry-pournelle/>

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  • From bitrex@21:1/5 to Phil Hobbs on Sat Apr 9 12:52:36 2022
    On 4/8/2022 9:45 PM, Phil Hobbs wrote:
    Carl wrote:
    On 4/8/22 20:18, John Larkin wrote:
    On Fri, 8 Apr 2022 19:33:45 -0400, Carl <carl.ijamesxx@yyverizon.net>
    wrote:

    Not directly electronics design, but todays YouTube video by Anton
    Petrov
    (https://www.youtube.com/watch?v=JM-J_VZOw60&ab_channel=AntonPetrov)
    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?

    The energy used to make light is hugely more than the thrust. Photons
    have a lot of energy but very little momentum.

    A solar sail makes a gentle push and the sunlight is free.


    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.

    Cheers

    Phil Hobbs


    Something like this driving a conventional ion thruster is probably much
    more efficient on paper:

    <https://en.wikipedia.org/wiki/Optoelectric_nuclear_battery>

    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

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  • From Anthony William Sloman@21:1/5 to bitrex on Sat Apr 9 19:03:45 2022
    On Sunday, April 10, 2022 at 3:04:17 AM UTC+10, bitrex wrote:
    On 4/9/2022 10:57 AM, Anthony William Sloman wrote:
    On Saturday, April 9, 2022 at 10:06:32 PM UTC+10, Martin Brown wrote:
    On 09/04/2022 00:33, Carl wrote:

    <snip>

    OTOH reflecting a photon that has been fired at the vehicle from an
    external source gives you double the momentum.

    The idea has made it into hard science fiction.

    https://en.wikipedia.org/wiki/The_Mote_in_God%27s_Eye

    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.

    <snip>

    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.

    <https://snarkypenguin.wordpress.com/2017/09/13/r-i-p-to-a-right-wing-ass-dr-jerry-pournelle/>

    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.

    --
    Bill Sloman, Sydney

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