Can you hover by pulling your bootstraps?

Can you hover by pulling your bootstraps?

I'm watching a show called Alien Encounters on the science channel. They posit an interstellar conveyance which is a space ship/capsule/city anchored to a solar sail.

They show the capsule emitting laser beams to fill and power the sail, in addition to the photons the sail uses from stars to propel it. I don't see that this is any different than having giant fans on board a regular oceangoing sailing ship being used to fill the sails and propel this ship.

In other words, it won't work because the equal and opposite force the fan blades exert exactly matches the force they supply to the sails with a net gain in propulsion of zero.

Since this is a science channel and the program has professional astronomers and the like appearing on it, I'm surprised they would present us with so silly a propulsion source as lasers on a platform pushing against a sail attached to the platform.

Or is it just me?

You'd get thrust from the reflected photons because you get 2x momentum change in a reflection -- 1x cancelling the 1x out of the original laser for a net 1x. The same net effect as shooting the laser out the back rather than bothering to reflect it.

You don't however get much momentum out of photons. It's not an efficent propulsion source. Maybe they are heating the sails hoping that the hot side heats the occasional molecule.

http://www.youtube.com/watch?v=uKXMTzMQWjo

Diktor did pretty well doing just that.

You'd get thrust from the reflected photons because you get 2x momentum change in a reflection -- 1x cancelling the 1x out of the original laser for a net 1x. The same net effect as shooting the laser out the back rather than bothering to reflect it.

You don't however get much momentum out of photons. It's not an efficent propulsion source. Maybe they are heating the sails hoping that the hot side heats the occasional molecule.

I seem to recall that they were studying a curiosity in the velocity of the Voyager spacecraft; that it was traveling somewhat faster than its momentum accounted for.

And that the anomaly could be explained by considering the infra-red photon emissions from the radioactive energy source, which were by coincidence pointing back along the direction traveled.

If an accidental and anomalous configuration can boost a spacecraft along its journey, it would seem that an intentional design could take advantage of it.

Admittedly, the efficiency would likely be poor, but if a push can be effected without a loss of mass, that seems worth investigating and possibly developing.

...Admittedly, the efficiency would likely be poor, but if a push can be effected without a loss of mass, that seems worth investigating and possibly developing.

do a few calculations, NnB. Energy efficiency is going to be not merely "poor" but much worse than even "piss-poor".

...Admittedly, the efficiency would likely be poor, but if a push can be effected without a loss of mass, that seems worth investigating and possibly developing.

do a few calculations, NnB. Energy efficiency is going to be not merely "poor" but much worse than even "piss-poor".

This seems a lot more promising than I would have expected at this level of development.

Can you hover by pulling your bootstraps?

I'm watching a show called Alien Encounters on the science channel. They posit an interstellar conveyance which is a space ship/capsule/city anchored to a solar sail.

They show the capsule emitting laser beams to fill and power the sail, in addition to the photons the sail uses from stars to propel it. I don't see that this is any different than having giant fans on board a regular oceangoing sailing ship being used to fill the sails and propel this ship.

In other words, it won't work because the equal and opposite force the fan blades exert exactly matches the force they supply to the sails with a net gain in propulsion of zero.

Since this is a science channel and the program has professional astronomers and the like appearing on it, I'm surprised they would present us with so silly a propulsion source as lasers on a platform pushing against a sail attached to the platform.

Or is it just me?

I both agree and disagree with you.

Disagree: Your objection would be correct if all the photons emitted by the laser returned to the spacecraft. They do not. Instead, they stream away from the craft with a total net momentum that must, by Newton's Third Law, be balanced. That balancing is achieved by a corresponding and opposite change to the momentum of the spacecraft.

Agree: This would, therefore, work to propel the spacecraft, but would be inefficient and rather silly, due to the loss of energy incurred by heating of the sail by those photons which were absorbed rather than reflected. Since the purpose of the laser is to propel the ship , it would be better to turn the laser around and simply point it in the direction from which the ship is coming.

...Admittedly, the efficiency would likely be poor, but if a push can be effected without a loss of mass, that seems worth investigating and possibly developing.

do a few calculations, NnB. Energy efficiency is going to be not merely "poor" but much worse than even "piss-poor".

This seems a lot more promising than I would have expected at this level of development.

With that kind of starry eyed optimism I would love to sell you a slightly used [but still in good working condition] Grand Canyon. Besides, you probably could afford it now, as that winning lottery ticket was sold near Baltimore in MD. "micronewtons" of force after thousand-fold reflection. At the same energy densities photon pressure is millions of times lower than that developed in the engines of our primitive chemical rockets. Which means that we better use something more massive than photons as the reaction material.

This seems a lot more promising than I would have expected at this level of development.

With that kind of starry eyed optimism I would love to sell you a slightly used [but still in good working condition] Grand Canyon. Besides, you probably could afford it now, as that winning lottery ticket was sold near Baltimore in MD. "micronewtons" of force after thousand-fold reflection. At the same energy densities photon pressure is millions of times lower than that developed in the engines of our primitive chemical rockets. Which means that we better use something more massive than photons as the reaction material.

I'm all for using physical substances as reaction material, especially in travel within our solar system. You may recall, I want to use planet Venus as the gas station for this enterprise, consuming its atmospheric carbon dioxide with complete abandon and utter disregard for its potential depletion.

But chemical and nuclear rockets will not suffice for interstellar travel, because they run out of gas too quickly, regardless of what the gas may be.

A fusion-powered photonic propulsion system should be able to provide thrust throughout the journey of a star ship, thus leaving surviving the length of the journey as a problem for the passengers and crew.

Interstellar ram jets. Although there isn't a lot of matter in the vacuum of space, if you travel long distances you'll encounter more than you could carry on your ship.

Can you hover by pulling your bootstraps?

I don't know about that, but I've read that you can fly by throwing yourself at the ground and missing.

Interstellar ram jets. Although there isn't a lot of matter in the vacuum of space, if you travel long distances you'll encounter more than you could carry on your ship.

Doctor Bussard concluded that the problems of collecting the material, and its consequent cost in loss of momentum, would likely limit the possibility that this would be a workable means of high-speed propulsion.

It's possible that such a technique might make an effective brake, which is one of the problems of interstellar travel; assuming that you could get up to speed to get to another star, how do you slow down again to stop there?

This seems a lot more promising than I would have expected at this level of development.

With that kind of starry eyed optimism I would love to sell you a slightly used [but still in good working condition] Grand Canyon. Besides, you probably could afford it now, as that winning lottery ticket was sold near Baltimore in MD. "micronewtons" of force after thousand-fold reflection. At the same energy densities photon pressure is millions of times lower than that developed in the engines of our primitive chemical rockets. Which means that we better use something more massive than photons as the reaction material.

I'm all for using physical substances as reaction material, especially in travel within our solar system. You may recall, I want to use planet Venus as the gas station for this enterprise, consuming its atmospheric carbon dioxide with complete abandon and utter disregard for its potential depletion.

But chemical and nuclear rockets will not suffice for interstellar travel, because they run out of gas too quickly, regardless of what the gas may be.

A fusion-powered photonic propulsion system should be able to provide thrust throughout the journey of a star ship, thus leaving surviving the length of the journey as a problem for the passengers and crew.

if you could use relativistic jets, you would not need much reaction material. But the distant space travel would probably be done [if at all] without much acceleration or deceleration, simply by playing with the space itself. More fun that way. Especially because running into anything, even into a diffuse gas, at the speeds necessary for such travel, would atomize the traveler and his/her/its conveyance.

Can you hover by pulling your bootstraps?

I'm watching a show called Alien Encounters on the science channel. They posit an interstellar conveyance which is a space ship/capsule/city anchored to a solar sail.

They show the capsule emitting laser beams to fill and power the sail, in addition to the photons the sail uses from stars to propel it. I don't see that this is any different than having giant fans on board a regular oceangoing sailing ship being used to fill the sails and propel this ship.

In other words, it won't work because the equal and opposite force the fan blades exert exactly matches the force they supply to the sails with a net gain in propulsion of zero.

Since this is a science channel and the program has professional astronomers and the like appearing on it, I'm surprised they would present us with so silly a propulsion source as lasers on a platform pushing against a sail attached to the platform.

Or is it just me?

Assuming you are describing what they showed accurately, this sounds to me like complete 100% shit science. You'd do better pointing the laser backward into space, and riding the recoil. Even that would be horribly inefficient, given the nature of lasers.

Last edited by Quark2005 on Sat Mar 31, 2012 2:07 pm, edited 1 time in total.

...Admittedly, the efficiency would likely be poor, but if a push can be effected without a loss of mass, that seems worth investigating and possibly developing.

do a few calculations, NnB. Energy efficiency is going to be not merely "poor" but much worse than even "piss-poor".

Even worse than "green-piss-poor."

Or is it just me?

derlaurer is right - it's not QUITE like a fan blowing into a sail - because the sail re-radiates the photons backwards. Appropriate, since this sounds like an ass-backwards plan to move a spacecraft.

Can you hover by pulling your bootstraps?

I don't know about that, but I've read that you can fly by throwing yourself at the ground and missing.

Most certainly.
In fact, NASA (and the old USSR) used to do this nearly every other week, -- only they called it 'orbiting'.