Are Solar Sails Actually Practical?

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[00:00:37]Solar sails pop up now and then in sci-fi thanks to the elegance inherent in any sailcraft, but there is a big gulf between the gorgeous solar sails we see in fiction and their real-life inspirations. Today's video will be looking at how the real ones work as well as the other types of sail not seen in popular fiction that may very well be better.

[00:01:01]First of all, a very quick primer on regular rocket engines, which work via Newton's third law of motion. Throw mass one way and an opposite force moves you the other way. Since space is empty, you need to bring along your own mass to throw called propellant because it propels you. But propellant is finite, so it can only take you so far. On the other hand, sails do not need propellant since they make use of something external instead. The specific thing they use depends on the type of sail and we'll get into the lesser known ones later after we talk about solar sails.

[00:01:33]Well, technically speaking, they're really photon sails relying on all the free light being sent out by the sun to move around. As the photons that make up this light impact any surface, they get reflected or absorbed, and when this happens, they exert mechanical pressure on whatever they hit. It's a really, really, really, really tiny amount of pressure because photons don't have mass, but weirdly, it is still there, and it can be exploited. Because this minuscule force is so tiny, the best way to use it is to maximize surface area to have more photons impact your craft.

[00:02:05]This is the sail part, but it's also very important to ensure the sail size is huge compared to the size of the payload. So, when For All Mankind showed one being deployed to get an edge in a race to Mars, it should have made the sail a lot bigger. But, props at least for getting the look of it right and not having it be billowing out as if in a breeze. The problem here really is that the thrust from photon pressure is so low that along with the sail being big, the spacecraft needs to be exceptionally low mass, which does include the sails.

[00:02:34]They need to be incredibly thin to achieve this, and yet also strong enough to not tear when deployed, and be able to maintain their shape in order to most efficiently transfer the pressure applied to its payload. So, they need masts and rigging, just like a regular sailing vessel. Since those things also add mass, which is bad, some photon sail designs instead opt to spin the craft to use centrifugal force to hold the sails up and out from where they were deployed. These considerations and limitations are the Achilles' heel for solar sails. The thrust may be accurate, free, but it is so low that it places extreme limits on the rest of the craft and what you can do with it, including how it flies. Just like sailing ships sailing on the seven seas, you have to angle the sail against the incoming light to bounce it off opposite the way you want to go, or go edge on to the light if you don't want to change anything at all. At least with real sails, they are far too fragile to attempt to lower them out of the way. At least it is possible to rotate and angle a photon sail for free by changing how reflective parts of it are, like with liquid crystal panels to darken some parts, or make many rectangular sails that can individually turn to reflect the light in a different direction.

[00:03:46]Another downside from the low thrust is how long it takes to do a maneuver. A big roaring rocket engine can change your trajectory a lot in a short space of time, but a photon cell needs weeks or months to match that with a path that ends up looking like a big spiral. Yet another consideration is that the photon pressure that's available changes with distance from the sun. So, if you want to exploit propellant-free acceleration to visit the gas giants, your thrust when you get out there is going to be even lower. The opposite is also true.

[00:04:15]Go down towards the sun and the thrust goes up. And yeah, you can go towards the sun with sail. You just use it to slow down your orbit around it until you're going downwards. In fact, you can slow down so much that you're not orbiting it at all, or really even moving, becoming what is called a statite. You just hover, balancing the downward pull of gravity with the upward force of thrust. You can do this in a normal rocket engine, of course, but only for as long as you have propellant.

[00:04:41]A sail can do this forever. And the neat thing is that if you can do it at one altitude above a star, that craft can do it at any altitude above that star, since both light and gravity fall off with distance at the same rate.

[00:04:54]Something down low might feel gravity more strongly, but there's more light to push back up, and vice versa. As we can see, photon sails using sunlight certainly have their attractions, especially for probes and stuff, but sadly, you're not going to be doing any wild sci-fi things using just sunlight.

[00:05:11]But you can get a bit closer by using laser sails. Firing masses of photons in a big coherent beam means you're no longer limited by the amount of sunlight there is. So, you can more thrust out of the same surface area of a sail. And the highly focused nature of the beam also means the force doesn't drop off over distance anywhere near as much, to the point where you can even use this tech as an interstellar distance. You can learn more about that in this video right here on Space Dock. However, one downside is that you might pour so much power into the beam and onto the sail that, while it may be 99% reflective, the 1% of energy that does get absorbed is enough to vaporize the ultra-thin sail. Luckily, since a laser can be a very specific frequency, you could use a dielectric mirror, which is extraordinarily good at reflecting very specific frequencies, like up to 99.999% or better. That optimization lets you get more out of a sail and put more into it in the first place, which is what makes those insane interstellar concepts actually viable. The last downside of a photon sail is that you really shouldn't use them too close to the atmosphere of a planet. Even the practical vacuum within hundreds of kilometers of Earth might cause too much drag on a sail for sunlight to overcome, causing it to fall out of orbit. Though the greater power from a laser should work. Now we get to the first alternative option, the electric sail, which still uses the sun, but not the photons from it. Instead, e-sails use the solar wind. This constant outpouring of charged particles is packed full of energy waiting to be used. An e-sail craft grabs that by giving itself an electrostatic charge, which it spreads over a much wider area by carrying it along a bunch of really long cables, around which the craft's electric field spreads out into the vacuum of space. When the charged particles in the solar wind meet to the charge surrounding the craft, they get deflected and transfer momentum to the sail. Now some considerations. The pressure from the solar wind is orders of magnitude less than that of photon pressure, but since the wires and their electric fields are much easier to scale up with an a photon sail is, it can sort of even out. Also, the solar wind scales down much less as distance from the sun goes up, so they work better than photon sails when you go further out. And you can turn off the field around some tethers to easily maneuver the craft. As with the photon sails, e-sails are also not suitable to be used anywhere near planets, though only ones that have a magnetic field, which blocks the solar wind. Wait a minute, that sounds exploitable. This is exactly what a magnetic sail does. Generate a big enough magnetic field on your craft and you can catch a ride on that plasma just like with the e-sail. The difference is in craft design, which swaps out the long cables running outward for a great big ring containing a coil from which the magnetic field emanates. The benefit over the e-sail is that one of these can operate within a planet's magnetic field using that for thrust instead of the charged particles of the solar wind. You can also do the laser sail trick with these, swapping out the laser for a plasma beam, which sounds like a crazy exotic sci-fi thing, but is much less exciting. It only works because both the sender and receiver have magnetic fields that connect together, forming a tunnel that the plasma can safely flow down without going out everywhere into space.

[00:08:25]And I thought surely no popular sci-fi thing has used either of these alternative sails, but lo and behold, Gundam has one. It's a bit fanciful, of course, and not technically an e-sail or a magsail, but the ring shape and being able to use a projected beam is close enough to really give magsail vibes, even if it may be coincidental. An evolution of the magsail is one that uses the actual plasma of the solar wind as its magnetic coil instead of a big loop of superconducting material. With one of these, you can really scale up the magnetic field, catching more thrust. And since there's no mass to its sail, you get a much better thrust to weight ratio. These work so well, in fact, they have great potential for use on interstellar craft. An even further evolution of this concept isn't really a sail at all, but does make use of one to generate power to run its engine that uses regular propellant. It's a bit more complex than that summary, so this so-called Q-drive will be the subject of its own video in future. So, that's the various types of real-life solar sail, from the ones you know to the ones you probably didn't. As they all use the sun as an external source of thrust, they can all accelerate forever, albeit very slowly, but that seems like a worthy trade-off, though one that can be mitigated by using an additional external source of energy to boost their acceleration. These real ones are also quite different, so the rare but elegant ones seen in popular sci-fi.

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