[The Polymath]

I was searching for the latest news on sloar sails, when I thought of something. If a device that generates electromagnetic radiation were pointed behind a spaceship, opposite the direction the space ship was supposed to go, wouldn't the spaceship accerlerate? What I was wondering, is what equation should I use to find the thrust provided by, say, a 40 mW LED?

[CraigD]

The Polymath, on 31 January 2011 - 05:57 PM, said:

I was searching for the latest news on sloar sails, when I thought of something. If a device that generates electromagnetic radiation were pointed behind a spaceship, opposite the direction the space ship was supposed to go, wouldn't the spaceship accerlerate? What I was wondering, is what equation should I use to find the thrust provided by, say, a 40 mW LED?

Thrust is in units of momentum/time (eg: kg m/s²).

Power is in units of energy/time (eg: km m²/s³).

The momentum of light is given by

So, a light rocket of power has thrust .

For example, assuming 100% efficiency, 40 mW light source would have a thrust of about .

Such rockets are usually called "photonic rockets". They're attractive because they don't need any reaction mass, but unattactive because they have very low thrusts to power ratios.

[The Polymath]

Thanks!

[SextonBlake]

Solar sails are still beyond us but we could do a low tech equivalent. We can make very efficient solar panels, and make them so thin that they roll up. These could provide electricity, so propulsion to an electric rocket, maybe with permanent magnets to accelerate the particles? They would have to build up much of their speed in inner solar system fly-bys to get the best they can out of strong sunlight before heading outwards.

[arKane]

SextonBlake, on 04 August 2011 - 07:16 AM, said:

Solar sails are still beyond us but we could do a low tech equivalent. We can make very efficient solar panels, and make them so thin that they roll up. These could provide electricity, so propulsion to an electric rocket, maybe with permanent magnets to accelerate the particles? They would have to build up much of their speed in inner solar system fly-bys to get the best they can out of strong sunlight before heading outwards.

I'm going to have to pass on that ride. I just can't get behind heading outward with only solar sails as my only power source. I hope we don't put to much effort into developing that technology.

[Moontanman]

Magnetic sails would be a viable alternative, they could, in theory, be made as big as you want, really large accelerations should be possible with huge sails and even large space craft. They would require fission power plants at least, high temp superconductors and large heat sinks which could be part of the sail. A torus shaped craft would be good for this arrangement since the magnetic field would be torus shaped as well. The torus could be spun for artificial gravity inside and wouldn't have to stop spinning to maneuver. Small scout type ships could be launched and recovered. It would be a good craft for exploring the solar system, even star travel if you could live inside for generations.

[CraigD]

SextonBlake, on 04 August 2011 - 07:16 AM, said:

Solar sails are still beyond us but we could do a low tech equivalent. We can make very efficient solar panels, and make them so thin that they roll up. These could provide electricity, so propulsion to an electric rocket, ...

Though not as established a technology as electric rockets (by which I assume we mean ion thrusters), I wouldn’t describe solar sails as “beyond us”. JAXA’s small (20 m diameter) IKAROS spacecraft has been flying under solar sail thrust since June 2010.

IMHO, the most ambitious ion thruster propelled spacecraft to date was JAXA’s Hayabusa, which successfully rendezvoused with and returned a sample of a near-Earth asteroid to Earth from 2003 to 2010.

IKAROS’s sail include an array of thin (0.000025 m thick, vs. the main part of the sail’s 0.0000075 m) solar cells that power its control, instruments, and radios. The much more massive (510 kg) Hayabusa used more conventional folding solar panel “paddles”.

Although both technologies are promising, I think it’s important to note a key difference between light-pressure sails and ion thrusters: light sails don’t require a spacecraft to carry any reaction mass, while ion thrusters do. Ion thrusters like those on Hayabusa, or those used for attitude control and station keeping maneuvering on several Earth orbiting satellites, eventually exhaust their supply of reaction mass (usually stored as a compressed gas of some heavy atom, such as xenon) and stop working. Light sails can work as long as they have a supply of light.

arKane, on 04 August 2011 - 05:26 PM, said:

I'm going to have to pass on that ride. I just can't get behind heading outward with only solar sails as my only power source. I hope we don't put to much effort into developing that technology.

There’s not requirement that a light sail be pushed only by natural sun/star light.

To accelerate at more than modest rates, a light sail propelled spacecraft would almost certainly need to be pushed by an artificial light beam, such as an optical or microwave laser. See Forward's Light-Sail Propulsion System and Starwisp for examples of proposed systems of these. Note that these systems, proposed by the late Robert Forward in the 1980s and ‘90s, take no advantage of the more recent (2002) “multi-bounce mode” idea (see Multi-Bounce Laser-Based Sails, by Robert A. Metzger and Geoffrey Landis).

Moontanman, on 04 August 2011 - 06:14 PM, said:

Magnetic sails would be a viable alternative ...

I agree – magsails, which are pushed by charged particles rather than photons, are promising. However, they suffer from the same problem as natural-light pushed sails – at all but very close distances from a star, they don’t produce much thrust. So I expect that, to accelerate at more than modest rates, like lightsails by lasers, magsail spacecraft would have to be pushed by artificially generated particle beams.

[arKane]

None of these technologies look very promising for anything but in solar system, in which case the ion thrusters look to be the best choice IMHO. For anything traveling from 20 to 100 light years away, we are looking at some very serious travel time, which most likely will be only a one way trip for anybody along for the ride.

Reminds me of a SciFi short story I read a long time ago, where people started out on a 100 year trip to a promising planet, and 50 years into the trip new technology came along allowing much faster space travel. The new ship passed the old one still on the way and when they arrived there was already a good sized colony well established.

[Moontanman]

CraigD, on 05 August 2011 - 05:06 AM, said:

Though not as established a technology as electric rockets (by which I assume we mean ion thrusters), I wouldn’t describe solar sails as “beyond us”. JAXA’s small (20 m diameter) IKAROS spacecraft has been flying under solar sail thrust since June 2010.

IMHO, the most ambitious ion thruster propelled spacecraft to date was JAXA’s Hayabusa, which successfully rendezvoused with and returned a sample of a near-Earth asteroid to Earth from 2003 to 2010.

IKAROS’s sail include an array of thin (0.000025 m thick, vs. the main part of the sail’s 0.0000075 m) solar cells that power its control, instruments, and radios. The much more massive (510 kg) Hayabusa used more conventional folding solar panel “paddles”.

Although both technologies are promising, I think it’s important to note a key difference between light-pressure sails and ion thrusters: light sails don’t require a spacecraft to carry any reaction mass, while ion thrusters do. Ion thrusters like those on Hayabusa, or those used for attitude control and station keeping maneuvering on several Earth orbiting satellites, eventually exhaust their supply of reaction mass (usually stored as a compressed gas of some heavy atom, such as xenon) and stop working. Light sails can work as long as they have a supply of light.


There’s not requirement that a light sail be pushed only by natural sun/star light.

To accelerate at more than modest rates, a light sail propelled spacecraft would almost certainly need to be pushed by an artificial light beam, such as an optical or microwave laser. See Forward's Light-Sail Propulsion System and Starwisp for examples of proposed systems of these. Note that these systems, proposed by the late Robert Forward in the 1980s and ‘90s, take no advantage of the more recent (2002) “multi-bounce mode” idea (see Multi-Bounce Laser-Based Sails, by Robert A. Metzger and Geoffrey Landis).


I agree – magsails, which are pushed by charged particles rather than photons, are promising. However, they suffer from the same problem as natural-light pushed sails – at all but very close distances from a star, they don’t produce much thrust. So I expect that, to accelerate at more than modest rates, like lightsails by lasers, magsail spacecraft would have to be pushed by artificially generated particle beams.

One type of mag sail would increase in size as it ventered further from the sun thus keeping it's acceleration potential more constant.

Mini-magnetospheric plasma propulsion

Quote

In order to reduce the size and weight of the magnet of the magnetic sail, it may be possible to inflate the magnetic field using a plasma in the same way that the plasma around the Earth stretches out the Earth's magnetic field in the magnetosphere. In this approach, called mini-magnetospheric plasma propulsion (M2P2), currents running through the plasma augment and partially replace the currents in the coil. This is expected to be especially useful far from the Sun, where the increased effective size of a M2P2 sail compensates for the reduced dynamic pressure of the solar wind. The original NASA design[4] proposes a spacecraft containing a can-shaped electromagnet into which a plasma is injected. The plasma pressure stretches the magnetic field and inflates a bubble of plasma around the spacecraft. The current in the plasma in this case augments and partially replaces current in the coils. The plasma then generates a kind of miniaturized magnetosphere around the spacecraft, analogous to the magnetosphere that surrounds the Earth. The protons and electrons which make up the solar wind are deflected by this magnetosphere and the reaction accelerates the spacecraft. The thrust of the M2P2 device would be steerable to some extent, potentially allowing the spacecraft to 'tack' into the solar wind and allowing efficient changes of orbit.
In the case of the (M2P2) system the spacecraft releases gas to create the plasma needed to maintain the somewhat leaky plasma bubble. The M2P2 system therefore has an effective specific impulse which is the amount of gas consumed per newton of thrust. This is a figure of merit usually used for rockets, where the fuel is actually reaction mass. Robert Winglee, who originally proposed the M2P2 technique, calculates a specific impulse of 200 kN·s/kg (roughly 50 times better than the space shuttle main engine). These calculations suggest that the system requires on the order of a kilowatt of power per newton of thrust, considerably lower than electric thrusters, and that the system generates the same thrust anywhere within the heliopause because the sail spreads automatically as the solar wind becomes less dense. However, this technique is less well understood than the simpler magnetic sail and issues of how large and heavy the magnetic coil would have to be[5][6] or whether the momentum from the solar wind can be efficiently transferred to the spacecraft[7] are under dispute.
The expansion of the magnetic field using plasma injected has been successfully tested in a large vacuum chamber on Earth, but the development of thrust was not part of the experiment. A beam-powered variant, MagBeam,[8] is also under development.

[CraigD]

arKane, on 05 August 2011 - 10:11 AM, said:

None of these technologies look very promising for anything but in solar system, in which case the ion thrusters look to be the best choice IMHO. For anything traveling from 20 to 100 light years away, we are looking at some very serious travel time, which most likely will be only a one way trip for anybody along for the ride.

The laser-pushed lightsail ship Forward described in his hard SF novel Rocheworld that I linked to above, the “Prometheus”, was designed to travel to and explore Barnard’s star’s system, traveling 5.9 ly in about 40 years, reaching a max speed of about 0.2 c, accelerating at about 0.01 g for 20 years, coasting for 18, then decelerating at about 0.1 g for 2 years. The reason it decelerated at 10 times the rate it accelerates is because most of its mass is in the large outer sail, which separates from the smaller inner one and the crew-containing vehicle at the beginning of its 2 years of deceleration. The outer sail continues traveling and is lost, so Prometheus can only travel locally around the Barnard’s Star system after its arrival.

The most far-fetched part of the Prometheus’s system isn’t the lighsail ship itself, which is essentially an airtight metal cylinder with a self-contained, highly recycled life support system capable of supporting a dozen people for 50+ years, attached to a sail much bigger (1000 km in diameter) but not much different in capabilities from the nonfictional IKAROS’s, but the stay-at-home laser that pushes it. This is described as a thousand generators in close orbit around the sun, with a total output power of 1.5 petawatts (about 100 times as much at the artificial power consumed by present day humankind) collimated and aimed so precisely it illuminates the Promethius at a distance of 5.9 ly away – the equivalent of a 7 mm target at Earth-Moon distance.

Far-fetched as this may sound, it’s more plausible IMHO than the idea of a interstellar spacecraft that carries its own fuel and reaction mass – that is, a rocket ship. The necessary thrust and specific impulse pair of such a rocket is orders of magnitude from plausibility.

Quote

Reminds me of a SciFi short story I read a long time ago, where people started out on a 100 year trip to a promising planet, and 50 years into the trip new technology came along allowing much faster space travel. The new ship passed the old one still on the way and when they arrived there was already a good sized colony well established.

I’ve read a few like that myself. One of my favorite – can’t recall the name at the moment – had the slow-moving generation ships being evacuated and made into history tourist destinations for wide-spread, fast-moving humankind.

As we’ve discussed a lot at hypography, though, any spacecraft traveling faster than a few tenths of the speed of light faces engineering difficulties that makes Forward’s giant laser powered lightsail ship Prometheus look like an engineering freebee, so I’m not overly worried that we’re wasting our time talking about realistic near-term spaceflight engineering. Maybe what’s ultimately possible isn’t much beyond what we can reasonably imagine now.

[arKane]

CraigD, on 05 August 2011 - 01:25 PM, said:

As we’ve discussed a lot at hypography, though, any spacecraft traveling faster than a few tenths of the speed of light faces engineering difficulties that makes Forward’s giant laser powered lightsail ship Prometheus look like an engineering freebee, so I’m not overly worried that we’re wasting our time talking about realistic near-term spaceflight engineering. Maybe what’s ultimately possible isn’t much beyond what we can reasonably imagine now.

Basically were not going very far without a major breakthrough. But I wouldn't expect such a discovery until we can get around our own solar system better than we are now.

[arKane]

This NASA article about Dawn (ion propelled spacecraft) is very interesting.

NASA's Dawn Spacecraft Begins Science Orbits of Vesta

[SextonBlake]

arKane, on 04 August 2011 - 05:26 PM, said:

I'm going to have to pass on that ride. I just can't get behind heading outward with only solar sails as my only power source. I hope we don't put to much effort into developing that technology.

Solar cells would get the high initial speed. When they were of little use (somewhere past Mars), they could be discarded and another form of propulsion used.

[SextonBlake]

CraigD. At present a laser beam 2mm in diameter is 1.6 km diameter at the distance of the Moon. How can such diffusion be stopped when we are working over light years?

Also, how does the lightsail ship slow down?

[CraigD]

http://scienceforums...tic-propulsion/

arKane, on 05 August 2011 - 07:22 PM, said:

This NASA article about Dawn (ion propelled spacecraft) is very interesting.

NASA's Dawn Spacecraft Begins Science Orbits of Vesta

Dawn is very cool, similar to Hayabusa. It’s empty mass is 815 kg, carries 425 kg of xenon propellant (reaction mass), and can change its velocity by (has a v of) about 10000 m/s, though very slowly, with a max acceleration of about 0.0001 m/s/s.

Like any rocket, however, these spacecraft are limited by the amount of reaction mass they carry.

Moontanman, on 05 August 2011 - 11:59 AM, said:

One type of mag sail would increase in size as it ventered further from the sun thus keeping it's acceleration potential more constant.

Mini-magnetospheric plasma propulsion

Pretty cool, even the math-pun acronym M2P2, but by the ionized gas sprayed into space to make the “mini magnetosphere” that allows a larger magnetic field to be generated is predicted to “leak” from the confinement of the magnetic field induced by the current through it, escaping into space, so like a rocket, it’d eventually runs out of stored mass. Its “effective specific impulse”, however, is predicted to be high – about 200000 m/s, about 6.5 times that of Dawn’s xenon ion thrusters’s actual specific impulse of 30400 m/s, but still, when it runs out of plasma, it’ll stop working.

Another few of disadvantage of charged particle sails (eg magsails) vs. light sails are:

• They must be powered to generate their magnetic field “sails”. Light sails don’t need much power, just a little for guidance and instruments, which they can get from solar cells integrated into them.
  
• Because the charged particles emitted by the sun – the solar wind - are gradually slowed by the interstellar medium, charged particle sails stop being useable for propulsion at the heliopause, about 100 AU from the Sun. Like from the solar wind, the force of light from the sun per unit area decrease with the square of the distance from the Sun, but isn’t significantly decreased by the interstellar medium

and, as I think artificially generated light and particle beams are ultimately the most feasible way to push light and charged particle sails, IMHO the big disadvantage:

• Charged particle beams (the major natural ones in the solar wind being protons) must have the same charge, or they’d combine into neutral composite particles (eg: protons and electrons forming hydrogen). Particle with the same charge repel one another. Charge particle beams can’t remain collimated the way light beams can – at great distances, they disperse. So the distance that a beam-pushed magsail, like the proposed MagBeam system, can operate from its beam emitters is many times shorter than a beam-pushed lightsail, like the Forward system – good for Earth-to-Mars, perhaps, but not for interstellar trips.

arKane, on 05 August 2011 - 03:13 PM, said:

Basically were not going very far without a major breakthrough. But I wouldn't expect such a discovery until we can get around our own solar system better than we are now.

Over the years of discussion of spacecraft propulsion here at hypography, I’ve come to be firmly convinced that “major breakthrough” in this subject has been made, conceptually, for the past 30 years or more

SextonBlake, on 08 August 2011 - 02:52 AM, said:

CraigD. At present a laser beam 2mm in diameter is 1.6 km diameter at the distance of the Moon. How can such diffusion be stopped when we are working over light years?

Such diffusion – known as beam divergence – can’t be completely eliminated, because electromagnetic radiation is inherently subject to diffraction. However, it can be minimized by making the aperture of the light emitter (what Forward and most others conclude must be the open end of a laser cavity, because no other approach seems likely to be capable of the necessary power) very large.

Formulas for the definition of beam divergence and its theoretical minimum are given in the linked wikipedia article.

An few important concept to grasp before considering the precise details, with some specific examples calculated:

• Beam divergence is an angle a difference, not a ratio, so an emitter with the same divergence as the example you give, Sexton (, ) with an aperture would have a beam diameter at Earth-Moon distance of , not 8,000,000,000 m.
  
• Minimum beam divergence – the Gaussian diffraction limit - is inversely proportional to , so increasing the aperture decreases its angle. The beam dispersion angle in your example, about , is too low for it’s , which it must be at least 0.04 m. For , . Increasing the distance from the Earth-Moon to Barnard’s star’s 5.9 ly distance, , which gives an attenuation of about . So Forward’s fictional system isn’t in principle too implausible – if it could be aimed accurately enought

Quote

Also, how does the lightsail ship slow down?

That’s the clever part of the system, and by all accounts I’ve heard, though conceptually simple, not thought of before Forward imagined it in the 1980s.

The sail consists of an inner disk and an outer ring (annulus). To slow down, the outer ring is released from the inner disk. Having a greater area to mass ratio, it is pushed ahead of the inner ring. The outer ring focuses the light from reflected by its greater area on the far side of the inner disk, so that the light pressure on the far side of the disk is greater than that on the near side, slowing the inner ring.

Here’s a sketch, from the wikipedia’s commons: