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a1call
2015-Jun-15, 03:21 PM
Hi,
From the Law of Conservation of momentum I would expect a high powered laser machine to experience a reactive force similar to a force observed when holding a fire hose though much more subtle.
Would such a reactive force exist?
Has it ever been observed?
Thanks in advance.

Noclevername
2015-Jun-15, 04:11 PM
Yes. It's called photon pressure or radiation pressure (https://en.wikipedia.org/wiki/Radiation_pressure). But the cross section of a laser is very small, and photon pressure is very weak. That's why solar sails have to be extremely large to get even small accelerations.

cjameshuff
2015-Jun-15, 04:17 PM
Hi,
From the Law of Conservation of momentum I would expect a high powered laser machine to experience a reactive force similar to a force observed when holding a fire hose though much more subtle.
Would such a reactive force exist?
Has it ever been observed?
Thanks in advance.

Yes, this is the principle behind a photon rocket. Photon pressure has been observed, though I don't know that it's ever been observed for a macroscopic light source. Since the radiation is only emitted, the force would be half of that applied to a reflective surface, and equal to that applied to an absorbing surface.

a1call
2015-Jun-15, 04:24 PM
Thank you both.
I will post a follow-up question once I have the chance.
BTW didn't know the concept of photon rocket existed. Will have to look out up.
Thanks James.

a1call
2015-Jun-15, 07:07 PM
Hi James,
Do you recall the post you made about the solar sail propulsion system where gravity and photon pressure would cancel out since both where inversely proportional to square of the distance.
In that system when the vessel would freefall by Sun's gravity, one of the advantages would be that the vessel would experience no g forces despite accelerating substantially.

Do you know of any other propulsion system concept that involves freefall I.E. no g forces experienced?

Thanks in advance.

P.S. This is not off topic to this thread. I will post relations when I have a chance.

ETA HERE is the post I am referring to:

http://cosmoquest.org/forum/showthread.php?88555-Propulsion-using-Solar-Sails&p=1494149#post1494149

cjameshuff
2015-Jun-15, 07:45 PM
Hi James,
Do you recall the post you made about the solar sail propulsion system where gravity and photon pressure would cancel out since both where inversely proportional to square of the distance.
In that system when the vessel would freefall by Sun's gravity, one of the advantages would be that the vessel would experience no g forces despite accelerating substantially.

Do you know of any other propulsion system concept that involves freefall I.E. no g forces experienced?

Thanks in advance.

P.S. This is not off topic to this thread. I will post relations when I have a chance.

ETA HERE is the post I am referring to:

http://cosmoquest.org/forum/showthread.php?88555-Propulsion-using-Solar-Sails&p=1494149#post1494149

That was largely to provide a simple and clear illustration of how fast solar sailing would work, the only thing special about the point where photon pressure exactly cancels gravity is that it makes the math simple. A real sailcraft could have a smaller, heavier sail that doesn't quite cancel gravity and still get a benefit, or a bigger/lighter one that gets it going faster, and it could use the sail to accelerate to some degree on the way in as well, instead of just falling in ballistically.

The craft does experience forces while accelerating due to light pressure on the sail. The simplified version described there involves a fully ballistic fall in toward the sun (again to simplify the math), but the craft experiences the acceleration on the way out.

a1call
2015-Jun-15, 08:32 PM
Yes I see what you mean and apologize for putting you on the spot liked this. But you are the most knowledge member I know on different propulsion systems.
Regardless I was very impressed by the concept mostly because of providing acceleration without significant g force experience.
This part is deduction on my part but seems logical to not feel g forces when falling.
So the follow up question to the board members is
What other propulsion system concept exist that would provide acceleration without feeling g forces.
The only other system I vaguely recall would be something called bubble drive or something like that.
ETA my keyboard typed prison instead of propulsion. Apologies for that.

Reality Check
2015-Jun-15, 10:19 PM
Easily answered: No known propulsion system concept (including light sails) provide acceleration without g forces because g forces are caused by acceleration (or gravity).
There are speculations such as the Alcubierre drive (https://en.wikipedia.org/wiki/Alcubierre_drive) but we should not detail the thread - I suspect there will be an existing thread about that topic.

a1call
2015-Jun-15, 10:48 PM
Hi Reality Check,
Thank you for the reply.I suspect you are correct.However a clarification is required with what I mean about no g forces experienced.In the above example the gravity of the sun is a force but essentially deorbiting from our current orbit and falling towards (hopefully missing it of course) will provide great acceleration that would be otherwise not survivable. To quickly describe my understanding of the system. Corrections are welcome.
Basically you would use Sun's gravity to accelerate on a path towards it missing it at a safe distance where you open your closed solar sales which will cancel/reduce/overtake Sun's gravity on your way out of the gravity well.The size of sales won't be prohibitively large since both gravity and light are proportional to the inverse of the square off the distance.So any sale which will overcome Sun's gravity at earth's orbit will overcome Sun's gravity at any other distance.The significant part for me is that no g forces are felt by the occupants while falling towards the sun.

a1call
2015-Jun-15, 11:02 PM
I am having a lot of issues with the board software today.
As explained in the other thread gravity is a function of distance from the center of the mass of the Sun while light intensity is a function of the distance from where it's produced close to the surface.

WayneFrancis
2015-Jun-16, 12:23 AM
The discussion of the science of Interstellar got me thinking about this. Yes orbiting another body doesn't induce any acceleration that you can detect either in the fall in or the climb out of the gravity well.
I finally pin pointed the issue I had with the situation with interstellar which is the landing. Once the ship started stopped its free fall into the black hole to then land on the planet all that energy become an acceleration force that the astronauts would feel. Like wise the subsequent take of would also cause them to feel the the same amount of total force to get back into that same free fall orbit.
With the solar sail, or any ship orbiting another body, when they stop just being in free fall and use propulsion they will feel the force of that component of acceleration.
If they use a gravitational assist they will also feel an acceleration force. All that is happening there is that they are transferring some of the momentum of body they are orbiting to accelerate them relative to the sun.

So if my understanding is correct you can not use the sun as a gravitational assist to leave the solar system. So I'm not sure why, beside a more efficient trajectory, you would actually want to free fall into the sun to then open your sails on the way out. Why not just open your sales when you are already far out?
disclaimer: My understanding could still be deeply flawed and I welcome discussion to clarify it.

a1call
2015-Jun-16, 01:14 AM
This is not a typical gravity assist, but let's call it that.
You would use Sun's gravity assistance rather than deploy sail at earth's orbit because you could accelerate up to 274 .1 m/s^2 i.e. 27.9 times 1g rather than without it when you deploy sails. And do so with no onboard destructive g forces.


Link (https://en.m.wikipedia.org/wiki/Gravity_of_Earth#Comparative_gravities_in_various_ cities_around_the_world)


This is different from the scenario you describe because we just happen to have a stored relative potential energy where we are, relative to the sun that we can use as free boost. We are not taking off from the surface of the sun.


The only theoretical g forces in this scenario is while deorbiting from earth's orbit. Achieve escape velocity at any acceleration rate you wish, travel at 30 km/s at opposite direction to earth's orbit and you will fall to the center of the sun. So travel slower and miss the sun at a safe distance. Accelerate at up to 274.1 m/s^2 and feel no g forces onboard while doing so.
Then deploy your sail and cancel out Sun's gravity and continue at the high rate of velocity you have achieved by inertia again feeling no g forces from the sun onboard.

ETA After thinking some more there would be g forces after deploying the sail felt onboard. The photon pressure will changed the freefall path. However it probably will be worthwhile if compensated for.

a1call
2015-Jun-17, 02:37 AM
So the follow up question to the board members is
What other propulsion system concept exist that would provide acceleration without feeling g forces.



There are speculations such as the Alcubierre drive (https://en.wikipedia.org/wiki/Alcubierre_drive) but we should not detail the thread - I suspect there will be an existing thread about that topic.
There is of course typical Gravity Assist (https://en.wikipedia.org/wiki/Gravity_assist)
These will suffice for now but should there be more please let me know..

I know I can be stubborn at times so for the next followup question I will keep from arguing about any posts/input.
Follow up question:
Please point-out / describe any short comings of the system discussed above and why it is not feasible.

Again, there will be no arguments coming from me. I am only interested in a compilation of pointers.
Thanks in advance.

WayneFrancis
2015-Jun-17, 07:28 AM
This is not a typical gravity assist, but let's call it that.
You would use Sun's gravity assistance rather than deploy sail at earth's orbit because you could accelerate up to 274 .1 m/s^2 i.e. 27.9 times 1g rather than without it when you deploy sails. And do so with no onboard destructive g forces.


Link (https://en.m.wikipedia.org/wiki/Gravity_of_Earth#Comparative_gravities_in_various_ cities_around_the_world)


This is different from the scenario you describe because we just happen to have a stored relative potential energy where we are, relative to the sun that we can use as free boost. We are not taking off from the surface of the sun.


The only theoretical g forces in this scenario is while deorbiting from earth's orbit. Achieve escape velocity at any acceleration rate you wish, travel at 30 km/s at opposite direction to earth's orbit and you will fall to the center of the sun. So travel slower and miss the sun at a safe distance. Accelerate at up to 274.1 m/s^2 and feel no g forces onboard while doing so.
Then deploy your sail and cancel out Sun's gravity and continue at the high rate of velocity you have achieved by inertia again feeling no g forces from the sun onboard.

ETA After thinking some more there would be g forces after deploying the sail felt onboard. The photon pressure will changed the freefall path. However it probably will be worthwhile if compensated for.

Perhaps I'm missing something. You have to accelerate to 30km/s in the opposite direction, 30,000m/s. You'll experience the forces during that acceleration. You then free fall towards the sun, at which time you now feel no g forces. When you reach periapsis you deploy your sale. You then experience any forces you gain from the solar sail. The most at the beginning and then it tapper off at inverse square root rate.

So I guess I ask why not just use boost we would have used, and felt, to start to free fall towards the sun to just add to our current orbital speed around the sun and also open the sail right away?

cjameshuff
2015-Jun-17, 11:37 AM
Perhaps I'm missing something. You have to accelerate to 30km/s in the opposite direction, 30,000m/s. You'll experience the forces during that acceleration. You then free fall towards the sun, at which time you now feel no g forces. When you reach periapsis you deploy your sale. You then experience any forces you gain from the solar sail. The most at the beginning and then it tapper off at inverse square root rate.

So I guess I ask why not just use boost we would have used, and felt, to start to free fall towards the sun to just add to our current orbital speed around the sun and also open the sail right away?

You open the sail right away and use it to move into a trajectory that makes the close flyby of the sun. If you just use it to accelerate, your ability to accelerate only decreases as your distance from the sun increases.

http://copernico.dm.unipi.it/simca/preprints/2002/July02/vulpetti_23_07_2002.pdf

In a real world application, you use the sail to accelerate constantly and always experience acceleration, the the ballistic fall in towards the sun in my example is purely to simplify the math.

IsaacKuo
2015-Jun-18, 02:05 PM
Perhaps I'm missing something. You have to accelerate to 30km/s in the opposite direction, 30,000m/s. You'll experience the forces during that acceleration. You then free fall towards the sun, at which time you now feel no g forces. When you reach periapsis you deploy your sale. You then experience any forces you gain from the solar sail. The most at the beginning and then it tapper off at inverse square root rate.

So I guess I ask why not just use boost we would have used, and felt, to start to free fall towards the sun to just add to our current orbital speed around the sun and also open the sail right away?

Two reasons. The main one is the Oberth effect, which applies to any propulsion system. Basically, the Sun's force of gravity conserves orbital energy, and the amount of orbital energy added by a given amount of impulse is proportional to the speed of the vehicle when applying the thrust. The faster you're going, the more energy is added for a given amount of impulse.

The other reason, which is solar sail specific, is the fact that you get more thrust when you're closer to the Sun. Even if the Sun had no gravitational pull, the simple fact that the sail gets a lot more thrust when it's near the Sun is enough reason to first go "inward" before unfurling the sail. It's like pulling a bowstring all the way back, vs just bulling it part of the way.

cjameshuff
2015-Jun-18, 10:28 PM
Take a sail that exactly balances the sun's gravity, to make the math simple. When you turn it to face the sun, you effectively turn gravity off. If you do that from a 1 AU heliocentric orbit, you cruise away from the sun at 30 km/s. Tilt it to accelerate and you gain some, but quickly move too far from the sun for solar sailing to be effective.

Cancel out about 17.5 km/s of your orbital velocity so your perihelion dips down to 0.1 AU, and turn to face the sun at perihelion. You're now departing the system at something like 130 km/s...provided your sail can survive.

Initially accelerate forward and outward a moderate distance, and you can dip down close to the sun with less delta-v. Tilt your sail to accelerate forward as you pass the sun, and you benefit from both more intense sunlight and the Oberth effect, and you wind up exiting the system at quite considerable speeds.

WayneFrancis
2015-Jun-19, 04:13 AM
ta

a1call
2015-Jun-25, 03:23 AM
I have a follow up question regarding the discussed system.
It appears to me that there can even higher speeds achieved if the solar freefall is repeated once the vessel reaches greater distances from the sun say from around Martian orbit or even further. This way the freefall acceleration phase will be lengthened.
Am I correct?
Thanks in advance.