Inverse of a solar sail...

[Carl]

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

 

[Sjouke Burry]

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.

 

[John Larkin]

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

 

[whit3rd]

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>

 

[Carl]

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

 

[Phil Hobbs]

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

 

[server]

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

 

[Martin Brown]

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

 

[Anthony William Sloman]

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

 

[bitrex]

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

 

[bitrex]

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/>

 

[Anthony William Sloman]

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