Everything space sails

[Spaceception]

Hey guys, I'm planning on writing an in depth blog post about solar sails, and their derivatives (That includes sails pushed by an energy source, i.e. lasers, microwaves, solar wind etc, not antimatter sails or anything like that). So give me your pdf's, articles, YouTube videos, and everything else that's accurate, I want it to be as good as possible, and I have a lot of sources already, but I don't want to get anything wrong.

I'll begin to write it later this fall if I graduate high school, so I can focus on it.

Otherwise, what do you guys think? Anything I should focus about most? Power efficiency of lasers, how reflective the sails have to be so they don't vaporize, misconceptions, etc.

Edited August 10, 2017 by Spaceception

[MaverickSawyer]

You might want to look into Robert L. Forward's Rocheworld series. He uses a laser sail to push a spacecraft to Barnard's Star in 20 years.

[monstah]

Atomic Rockets, naturally.

@nyrath used to appear sporadically on the forum, but is sadly gone.

(oh, you follow him on the forum already! XD)

[Spaceception]

9 minutes ago, MaverickSawyer said:

You might want to look into Robert L. Forward's Rocheworld series. He uses a laser sail to push a spacecraft to Barnard's Star in 20 years.

I've heard of it, (Planning a mod based around the Rocheworld part of it), but I was talking more about news articles, and scientific documents

Just now, monstah said:

Atomic Rockets, naturally.

@nyrath used to appear sporadically on the forum, but is sadly gone.

Well duh that's where I got most of my sources.

Yeah, I wonder if he'll return someday, I think he still references the game.

[MaverickSawyer]

1 minute ago, Spaceception said:

I've heard of it, (Planning a mod based around the Rocheworld part of it), but I was talking more about news articles, and scientific documents

Ah. That... could be tricky. Much of it will probably be in academic journals.

[kerbiloid]

Some calculators
http://www.deepspace.ucsb.edu/projects/directed-energy-interstellar-precursors

[wumpus]

16 hours ago, MaverickSawyer said:

You might want to look into Robert L. Forward's Rocheworld series. He uses a laser sail to push a spacecraft to Barnard's Star in 20 years.

You might want to look at this thread http://forum.kerbalspaceprogram.com/index.php?/topic/163773-great-news-for-breakthrough-starshot/

"Starshot" is a proposal for sending a nanosat to another star.  It got a bit savaged in the forum as the only thing it is sending at relativistic speeds is "recondensed plasma" (actually don't expect the plasma to maintain enough of a heading to gravitate back together, much less hit the right star).  This idea probably wouldn't work even if we had the solar system sufficiently colonized to blast power from the orbits of the outer planets.  "The golden age of science fiction is 10 years old", now with the exception of 10-year-old KSP players who see all the mistakes...

Solar sails are fine for interplanetary travel, provided you aren't in a hurry (read probes/cargo).  I imagine it all comes down to how thin/large you can make the sails and how you have to compete with solar powered ions (oddly enough, I'd expect solar sails to scale up better than ions, probably because of ion scaling issues more than sail scaling).  Also expect things to really only work well within Mars orbit (although you might bounce between inner planets increasing your velocity).

Don't underestimate the value of hauling fuel as cargo.  You can avoid much of the tyranny of the rocket equation by supplying the fuel as needed, and get around the costs of "using fuel to move fuel" by using something like a solar sail.

[MaverickSawyer]

2 hours ago, wumpus said:

Don't underestimate the value of hauling fuel as cargo.  You can avoid much of the tyranny of the rocket equation by supplying the fuel as needed, and get around the costs of "using fuel to move fuel" by using something like a solar sail.

Which is precisely why I love the Venture Star class ships from Avatar. (Don't judge me... I love the hardware. The plot? Not so much.) The official description of the ship is that it uses a laser-pumped sail to accelerate towards Alpha Centauri (something like 1.5g for 180 days), folds up the sail and coasts at .7c for several years, then uses proton-antiproton rockets to slow down at Alpha Centauri (again, something like 1.5g for 180 days). After that, it's out of fuel and spends several months to a year at Pandora refuelling and taking on cargo. Once refuelled and loaded with unobtainium (and whatever personnel have completed their rotation on Pandora safely tucked in cryosleep), they use the proton-antiproton rockets to accelerate outsystem and towards Earth, then use the sail to slow down at Earth. Using this strategy dramatically simplifies the ship, as it only has to carry .7c of dV, rather than 2.8c.

[insert_name]

Only solar sail to go interplanetary

https://en.m.wikipedia.org/wiki/IKAROS

[Spaceception]

8 minutes ago, insert_name said:

Only solar sail to go interplanetary

https://en.m.wikipedia.org/wiki/IKAROS

Of course I'm going to include that, why wouldn't I?

I'll also include lightsail, starship etc.

[magnemoe]

10 hours ago, wumpus said:

You might want to look at this thread http://forum.kerbalspaceprogram.com/index.php?/topic/163773-great-news-for-breakthrough-starshot/

"Starshot" is a proposal for sending a nanosat to another star.  It got a bit savaged in the forum as the only thing it is sending at relativistic speeds is "recondensed plasma" (actually don't expect the plasma to maintain enough of a heading to gravitate back together, much less hit the right star).  This idea probably wouldn't work even if we had the solar system sufficiently colonized to blast power from the orbits of the outer planets.  "The golden age of science fiction is 10 years old", now with the exception of 10-year-old KSP players who see all the mistakes...

Solar sails are fine for interplanetary travel, provided you aren't in a hurry (read probes/cargo).  I imagine it all comes down to how thin/large you can make the sails and how you have to compete with solar powered ions (oddly enough, I'd expect solar sails to scale up better than ions, probably because of ion scaling issues more than sail scaling).  Also expect things to really only work well within Mars orbit (although you might bounce between inner planets increasing your velocity).

Don't underestimate the value of hauling fuel as cargo.  You can avoid much of the tyranny of the rocket equation by supplying the fuel as needed, and get around the costs of "using fuel to move fuel" by using something like a solar sail.

Starshot is based on the idea of reflecting laser light between two mirrors lots of times. 
This probably work, reaching 10% of c with an 50k g acceleration over time has some issues
Using it to send small cubesat around in the solar system would make sense. 

 

[kerbiloid]

Just because somebody would mention.
https://en.wikipedia.org/wiki/Sunjammer

[wumpus]

12 hours ago, magnemoe said:

Starshot is based on the idea of reflecting laser light between two mirrors lots of times. 
This probably work, reaching 10% of c with an 50k g acceleration over time has some issues
Using it to send small cubesat around in the solar system would make sense. 

Too bad nobody brought it up in its own discussion.  That feels like it violates a few conservation laws, but perhaps the efficiency of "bouncing a laser off things to impart momentum" is so low that the asymptote is 100% efficiency.  Nobody understood the idea of a "train of nanosats", but if they were staging (and controlling) reflecting mirrors as it sped out on its way, that would be pretty cool (no idea what the minimum mass needed to adjust a mirror with the required precision would be).

It doesn't change the "turn into plasma*" calculations one bit (or the g forces).  Each bounce of the photons still contain that amount of energy, and any inefficiency of the mirror will quickly turn the thing into plasma.  While this idea might need less energy/power from the lasers, it still needs the same amount of efficiency from the mirrors as if they only had one reflection.  See "why you can't protect things from a laser gun with mirrors" for a better explanation.

* obviously, you aren't going to bounce lasers off a plasma, but other than that things aren't changed.

[Steel]

There's a book: Space Sailing by Jerome L. Wright that might be worth checking out on the subject. It is very much in a textbook style, so plenty of calculations and things.

[MatterBeam]

I did some research! It is part of a write-up on interstellar propulsion designs here. The relevant section:

Quote

Beamed propulsion 

Leave the power plant at home, but take the power with you! 

Beamed propulsion allows dozens of gigawatts of laser power to be produced by lasing stations of several thousands of tons, but can be used by laser sails massing only a few grams. This means extreme power-to-weight ratios on the spacecraft's end.

Light sails use the momentum of reflected photons to accelerate. The acceleration depends on reflectivity and photon pressure.

• Acceleration = (Photon Pressure / Mass) * (1+Reflectivity)

Photon pressure depends on the flux of the laser beam, measured in watts per square meter. Mass is dominated by the laser sail's surface area. Reflectivity determines how many photons are bounced back for extra thrust, and how much of the laser beam is absorbed and turned to heat. It is a percentage.

• Total Photon Thrust: Laser Power * 6.67 * 10^-9 * (1+Reflectivity)

Laser Power is in watts, and the Thrust in Newtons. We can calculate that a gigawatt laser beam, perfectly reflected, gives the sail 13.34 Newtons of thrust. 

However, no material is perfectly reflective. If even a fraction of that gigawatt beam is absorbed, it will vaporize the laser sail! The solution is to spread the beam over a large area. This increases mass, and lowers acceleration. Therefore, the laser sail will be built to the maximum thermal limits of its materials. It is not usually the melting point of the material, but the temperature at which the delicate reflective material starts degrading.

The choice of laser wavelength is another factor. Microwaves are easy to reflect with a lightweight metal mesh. Huge laser sails can be built to high reflectivity and lightweight masses. However, microwave beams suffer from terrible divergence. As the laser sails accelerates away from the laser generator, the 'spot size' of the beam expands greatly, which requires large sails to catch the full width of the beam, so might negate the advantages of the lightweight construction. 

Microwave and infrared sails are best suited to sharp accelerations by powerful lasers. The sail reaches several percent of lightspeed in a short distance. 

Visible wavelength lasers have better range and allow for smaller sails to catch the beam over longer distances. However, high reflectivity can only be achieved by dielectric mirrors. These are fragile and mass a lot per square meter. 

Even shorter wavelengths, such as ultraviolet or x-ray, can focus the beam tightly enough to continue accelerating a laser sail over millions of kilometers. The downside is that maintaining good reflectivity becomes more and more difficult. A significant fraction of x-rays might even pass right through their target!

A major factor for selecting this mode of propulsion is the existing infrastructure.

The cheapest interstellar operation is one that relies on systems already built and operated by someone else. In Laser Weapon Webs, I noted that lasers could be used to power interplanetary rockets. To move thousands of tons at rapid rates, gigawatts of laser power had to be available across the Solar System. It is best for a laser sail to be designed to use these very same beams to reach high velocities.

Therefore, the design objective would not be which combination of laser wavelength, sail area and heat tolerance would make for a perfect propulsion system, but to design a laser sail that best uses existing infrastructure.

First, we must ask what kind of lasers are best for interplanetary travel, as those would be funded and built long before any interstellar operation is conceived. If they are military in origin, they are likely to based on short-wavelength technology that can double as a destructive weapon. If it developed under civilian patrons, it will be built for the highest efficiency. This is because increasing laser energy is more expensive than building larger focusing mirrors. 

The interstellar operation would only have to 'rent' a laser beam instead of asking investors to build their own gigantic laser. A mature Laser Web that handles thousands of tons of traffic daily across interplanetary distance can reasonably track a laser sail over millions of kilometers. Such a web could extend over is 8.8AU, the distance between Mercury and Saturn. 

Let us suppose for an example that the web operates on microwaves. It is easy to achieve high reflectivity with a lightweight metallic mesh.

Let us target a trip to Alpha Centauri within 10 years. Travel velocity has to be 0.42C. Payload and braking engine are assumed to be 1kg. Acceleration must reach an average 6km/s^2.

These accelerations make it difficult to construct a probe that doesn't melt.

One option is to boost a section of the laser web out of the solar system. This would allow the laser to be focused over much longer distances, which reduces the stress of the sail itself. 

Laser sails might resemble white-hot disks shooting through space.

Another option is to do away with thermal limits of delicate reflective surfaces entirely and use melting points of materials instead. While the sail would lose the x2 factor in thrust, it could gain a much better resistance to high laser intensities. For example, an aluminium-based coating for a microwave mesh could achieve 99% reflectivity of 5 cm microwave light. It would melt at 933K, but would probably start losing reflectivity at much lower temperatures. A graphite microwave-absorbing mesh can survive 3800K, easily allowing five times higher laser intensities. A reflective laser sail would need a reflectivity of 99.6% or better to match the performance of an absorbing sail in this case. This is possible with dielectrics, but then it would become extremely difficult to get the low masses per square meter necessary. 

The best laser sail is one that manages high reflectivity at high temperatures. 

Carbon-carbon tip exposed to 3.9MW/m^2

 

 

 

The acceleration of a laser sail is limited by the photon pressure divided by the mass per square meter. If the photon pressure is temperature limited, the sail cannot accelerate faster by using a larger sail. The mass per square meter is therefore an important variable. Using larger wavelengths allows for lighter sails, as larger and larger empty holes can be built into the sail material (like the grid in a microwave door), but normally the beam divergence negates this advantage. In an Interplanetary Transport Web, re-focusing stations or mirrors are evenly spaced and make beam divergence a non-issue for our purposes. Advances in materials technology allow for the creation of paper-thin meshes that can survive extreme accelerations without bending. Carbon nanotubes, for example, have been tested to survive 6 tons of strain or more using strands on a millimetre thick. 

Particle beams can be used as alternative means of beamed propulsion. They provide better thrust and better thrust-to-weight ratios than lasers. The spaceship would need to extend a magnetic field around itself to catch the particle beams, which means having to install electromagnets on-board. Powering the magnets can be done by extracting a fraction of energy from the particle beam. They have similar ranges as infrared lasers.

An combination of particle beams and lasers can be used to boost a hybrid sail. Using electromagnetic fields to both deflect particles and hold in place conductive wires can allow the sail to extract the energy of a high-momentum beam and a long wavelength laser simultaneously. This concept will be explored in a future post.

One major downside to beamed propulsion for interstellar travel is that has to be a separate propulsion system for stopping at the destination. This imposes a minimum mass on the payload being carried by the laser sail, to be used as a second stage.

 

[wumpus]

14 hours ago, MatterBeam said:

I did some research! It is part of a write-up on interstellar propulsion designs here. The relevant section:

Any idea on the reflectivity of gold-leaf?  Would Na/Li/Be work as "metal-leaf" in space and have the reflectivity/g needed?  Also make sure your sail is "reflective" on each side (or at least non black/absorptive), that "white hot" energy preferentially radiates from the darker side.

You still aren't getting anywhere near relativistic speed (the W/g calculations won't change), but you might get to the "we might even know we launched a spacecraft hundreds of years ago" stage when it gets there (or more likely the probe that *was* sent via relativistic speed a few century's later detects it and they have to re-write history not knowing how advanced [or not] 21st century tech actually was).

[MatterBeam]

4 hours ago, wumpus said:

Any idea on the reflectivity of gold-leaf?  Would Na/Li/Be work as "metal-leaf" in space and have the reflectivity/g needed?  Also make sure your sail is "reflective" on each side (or at least non black/absorptive), that "white hot" energy preferentially radiates from the darker side.

You still aren't getting anywhere near relativistic speed (the W/g calculations won't change), but you might get to the "we might even know we launched a spacecraft hundreds of years ago" stage when it gets there (or more likely the probe that *was* sent via relativistic speed a few century's later detects it and they have to re-write history not knowing how advanced [or not] 21st century tech actually was).

As far as I know, gold has very poor reflectivity under 700nm (red light) and quite good reflectivity (95 to 98%) in the 1000 to 5000nm wavelength range. This makes it decently useful for use in a near-infrared laser, but its low melting point (1337K) and very high density (19300kg/m^3) make it a bad choice for a laser sail.

What you should be looking it as an optical coating that works best with the laser's wavelength. Like, selecting aluminium for blue and ultraviolet light, or a carbon mesh for microwaves. 

Laser sails are generally highly reflective on one side and pitch black on the opposite side. 

Achieving relativistic speeds is just a matter of shining your laser on the sail for long enough. Everything else just determines the maximum acceleration.