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To be most effective, a windmill must face into the wind. Some windmills achieve this with a passive yaw system, where the wind automatically adjusts the direction the windmill is facing, often with a tail fin.

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For my story, space fairing people use mirrors to focus sunlight onto water boilers, forges, and other places where intense heat is needed. Could some type of solar sail tail fin be used as a passive yaw system for directing these mirrors into their optimal orientation? The mirrors are attached to physical objects (space stations, ships, asteroids), not free floating, and they are typically in the vacuum of space with microgravity comparable to the ISS. A mirror is a 3 meter square sheet of silver curved into parabola.

Concentrated solar facilities on Earth seem to always use active controls (electric motors) to move solar mirrors into position. I assume a passive yaw system would not be practical on earth, where air resistance (not to mention wind!) and gravity create significant resistance, and solar wind is deflected by our planet's magnetic shield. I wonder if a passive system would be easier to achieve in space, where there is vacuum, low gravity, and the presence of solar wind.

Is passive yaw control possible for directing space mirrors into the sun? If so, what would such a system look like?

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  • $\begingroup$ "focus sunlight onto water boilers, forges, and other places" - where are those located? Are you talking of focal points on the same "space stations, ships [and] asteriods" that the mirrors are attached to? $\endgroup$
    – Bergi
    Sep 2, 2022 at 10:09

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Sure. And one pattern might well be much like the windmill you show in your question. Especially since one configuration is a spinning sail. The sail is spun to keep it stretched out. A small additional sail could certainly be constructed to provide an orienting force as in the windmill case. You might need to do some clever designing to keep the various torques lined up and prevent the system from tumbling. This is especially true since you would need to control in 3-D, not just rotation on one axis as the yaw control in a windmill. But, yes, it is very likely possible.

However, the main reason you want a passive yaw control for a windmill is because it is difficult to predict the direction of the wind. Additionally, in the example of the windmill, the goal of the system is to provide maximum output of the turbine. Other than "feathering" to slow or stop the turbine, there isn't much in the way of maneuvers that you would want to do with a windmill. So turning the blades to face the wind is preferred. An automated system that does this without active control machinery is very useful.

In the case of a solar sail, the direction of sunlight and solar wind is relatively easy to predict. And, while the angle of the mirror is often chosen to maximize thrust, it is also often chosen to perform maneuvers. So it is necessary to include in the design the ability to actively control the sail. Not just the angle, but the general shape of the sail must be controlled. That means passive control is of far less value. Probably it will be considered to be unrequired and redundant weight that could be removed.

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  • $\begingroup$ I expect that there would be more force on the "tailfin" from the solar wind than from sunlight anyway. $\endgroup$
    – Spencer
    Sep 1, 2022 at 22:32
  • $\begingroup$ Likely, if such were to be built, you would leave a gap in the main sail to allow the tail fin to see the light. I would have to do research to find the relative force of solar wind and light. $\endgroup$
    – BillOnne
    Sep 2, 2022 at 3:00
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A "mirror used for focusing sunlight" is by definition:

  1. A section of a parabola, that has a focal point.
  2. Reflective.
  3. Has mass.

NASA paper on Solar Sail Propulsion

The technology uses solar photons—sunlight—which are reflected off giant, mirror-like sails

Your mirror - IS a solar sail. How you design the mirror, will determine how it will self-orient. Ultimately, your design needs to be such, that at a specific orientation w.r.t. the sun, that you wish to maintain, forces acting on the sail are equal to all sides of the center of mass. If it is disturbed, then your design has to be such, that more light becomes reflected off of the opposing side, which will force the mirror/solar-sail to self-orient back to the desired position.

A windmill design is not the best implementation. The tail will be behind the mirror, so it will be in a shadow - it won't be reflecting photons. If the windmill is disturbed, then it will be some time, until the angle of misalignment will be enough, for the tail to emerge from the shadow and start reflecting light. By then, you may have already built up quite a bit of angular velocity, and the tail might be too small to stop the spin.

You could make several "tails" along the outer edge of the mirror - orient them such, that they are usually parallel to the sunlight. When the mirror gets misaligned, they will start reflecting photons immediately, causing it to start self-alignment instantly. If you continue along this path of thought, and wrap it all up with actual calculations of force created by the reflection of photons, etc... you will eventually come to some of the passively stabilized designs depicted in the linked paper... or maybe something new, and better - then you can publish your own paper =).

One thing to note about the designs in the paper - they are designed to self-orient directly at the sun. If you are trying to focus the light at... say... a planet, that is off to the side, then you will have an asymmetric mirror/parabola/solar-sail, and your stabilizing portion or sails will likewise have to be asymmetric.

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There are some passive solar tracking methods that work on earth. Some use thermally active materials or thermal expansion. Others heat water and shift the center of mass. The first type could work in space; the second where gravity is part of how the forces are balanced wouldn’t work.

Once you're a reasonable distance from the sun, the sun's rays are reasonably parallel, so shadows can be sharp. By controlling what is in shadow or what is getting hot, you can have a thermal mechanical system move the mirror.

The problem in space is that you have to radiate heat away. This would make the design more difficult.

You could also use the shadows or spots to shine on a screen and have your pilot manually steer the mirror, kind of like a helmsman. Or perhaps one person to steer the mirror to keep the optical alignment on the boiler and another person who steers the exhaust nozzle.

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  • $\begingroup$ All of the listed methods actually do work in space. Gravity is not a requirement. In the absence of gravity, photons hitting the sail become the primary external force on the sail, but you can still "balance" the sail, by shifting the center of mass. Also, in space... there is no atmosphere, you have 0K in the shadow, and a huge amount of heat in direct sunlight (depending on distance from the sun). It's as difficult or easy to radiate heat away, as it is to heat something up. If you have a radiator on the dark side - you'll radiate heat away in a snap, perhaps even more than you had planned. $\endgroup$
    – MishaP
    Sep 5, 2022 at 7:50
  • $\begingroup$ I meant since the earths gravity wasn’t present you couldn’t balance against the earths gravity. The ability to get rid of heat can be limited by the size of the radiators as well as their emissivity so it could be tricky if you want to keep one part hot and one part cooler to use the thermal properties of materials and temperature differences. $\endgroup$
    – UVphoton
    Sep 5, 2022 at 10:28
  • $\begingroup$ A very small radiator goes a very long way in space. 0 Kelvin = -273 Celcius - that's REALLY cold! Cooling really won't be a problem. The balancing mechanism relies just that - something to balance against. In space, you might not have gravity, but you can balance against other forces. $\endgroup$
    – MishaP
    Sep 6, 2022 at 8:16
  • $\begingroup$ Vacuum is also insulating so there isn’t conduction or convection so only radiation. Doing the thermal engineering for satellites is hard. The ISS has very large radiators for example because it is hard to get the heat out. If not in the sun or with an internal power source it can also be a problem of being too cold because you radiate the heat away. $\endgroup$
    – UVphoton
    Sep 6, 2022 at 12:05
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If you bend a 3 meter wide mirror into parabola, the apperture may be 1 meter or less. If you put a boiler 1 meter wide, it may block all the sunlight. Instead, put a thin 3 meter long copper pipe at the focus line of parabola and let the water run through it.

enter image description here

Passive Solar Tracking Syatems

As explained here

Zomeworks Track Racks They use liquid freon or another highly responsive, volatile liquid as the thermally active component. The liquid expands or condences, causing a shift in the center of gravity that tilts the array towards the sun throughout the day.

Bimetal Strips Bimetal Strips bend when exposed to direct sunlight.

Bimetal Coils They rotate when heated with sunlight.

Thermally Active Plastic Composite Polyethylene has a very high thermal expansion rate, while Acrylic packing tape is non-thermally active plastic. Stick transparent Acrylic tape to black sheet of Polyethylene giving the bending effect of bimetal surface on heating.

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