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peteshimmon
2005-Nov-14, 07:28 PM
The threads on the anomalous movements of the
Pioneer probes makes me wonder if physicists
have tried measuring any possible thrust from
radio energy emmiters. Its been a little
project I mean to try when I clear my working
area sometime. A balanced 12 inch wooden
ruler pivoted in the middle with a battery
powered transistorised transmitter feeding two
directional aerials pointing in opposite
directions at the ends of the ruler. Any
thrust should rotate it. But someone must have
tried something like this sometime! If it is
part of the explanation then I imagine the
radio emmision "pulls" the probes back! ie not
really a thrust.

swansont
2005-Nov-14, 10:53 PM
It's easily calculable. P = dE/dt, and p = E/c, so dp/dt, which is the force, is P/c (P is power, p is momentum)

A 1 Watt transmitter gives you 3x10-8 N of thrust, assuming it's directed along a single line. A symmetric transmission gives you nothing, of course.

Enzp
2005-Nov-15, 01:16 AM
You would need for your little antennae to be highly directional. Just a piece of wire sticking in a direction does not prevent the radiation from going in all directions. not sure what it would be thrusting with though. Nothing is moving or thrown out.

peteshimmon
2005-Nov-15, 01:39 AM
Which means there are endless oportunities for
experimentation and fiddliing:) Aerials can
be made directional with reflectors but getting
a decent amount of efficiency to get the thing
moving at all I guess might be difficult. But
surely its been tried this last 100 years just
to prove the physics! And there is still the
point about any thrust opposing this extra pull
on the Pioneers.

peteshimmon
2005-Nov-16, 07:11 PM
Just thought, the radiation from a laser pen
is pretty directional. But a one milliwatt beam
supposedly is 1000 times less than Swansonts
calculation. Might make a good Science Fair
project to show up this somehow!

trinitree88
2005-Nov-22, 01:43 AM
The change in momentum, delta (mv) and hence the force (F)is twice as much for a photon reflected, as opposed to one of equal value that is emitted. Freeman Dyson of Princeton proposed long ago a solar sail, that would use the impulse from reflected sunlight to sail between the planets. Doable, but pokey...real pokey, and it required assembling the large surface area sail in orbit. Tricky. Not impossible, but there has been little interest to date.

peteshimmon
2005-Nov-22, 04:49 PM
This reminds me of a duff explanation of the Crookes Radiometer I read
in my radio mag almost 40 years ago. It stated the vanes rotated because
light was reflected from the white side while absorbed by the black sides.
Buying one a few years later I found it rotated the other way! Another
explanation was found. The little bit of gas in the bulb was more excited
by the warmth of the black sides and pushed against them. OK. Now
someone tells me there is another explanation out there, The molecues
swirl past the edges of the vanes and cause rotation. Rats! The thing
works! I have wondered if a well balance rotor from a busted unit would
spin to destruction if placed in solar orbit. Thats thinking of the original
explanation.

swansont
2005-Nov-27, 03:11 PM
This reminds me of a duff explanation of the Crookes Radiometer I read
in my radio mag almost 40 years ago. It stated the vanes rotated because
light was reflected from the white side while absorbed by the black sides.
Buying one a few years later I found it rotated the other way! Another
explanation was found. The little bit of gas in the bulb was more excited
by the warmth of the black sides and pushed against them. OK. Now
someone tells me there is another explanation out there, The molecues
swirl past the edges of the vanes and cause rotation. Rats! The thing
works! I have wondered if a well balance rotor from a busted unit would
spin to destruction if placed in solar orbit. Thats thinking of the original
explanation.


IIRC the photon recoil has been observed on a similar device, but it was done in a really good vacuum.

peteshimmon
2005-Nov-29, 04:29 PM
I remember an illustration showing two mirrors at each end of a suspended
beam in an evacuated glass container. A bright light made the beam move.
Also reminds me that the early Mariner spaceprobes had adjustable panels
at the ends of the solar panels which were described as attitude control
using solar radiation pressure. Wonder if they worked?