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I recently got back into the Shannara series by Terry Brooks and it has me wanting to take a shot at adapting the material into a TTRGP setting. One of the things that I'm hung up on is how to explain how Airships work in the setting since there are some gaps the books never fill in. For those not familiar, Shannara is a fantasy setting with a dash of 'magitech' based around solar powered crystals. In the case of the airships they are literal ships with masts and sails, held aloft by some field generated by these crystals.

The problem is, the crystals are also used for propulsion and steering. From what I can determine the sails are only used as solar collectors, they don't provide any propulsion on their own. That leads to the question then of why you would continue to have masts and sails hundreds of years after the first airships were developed. Having tall masts with a giant piece of material attached is going to create massive drag on a vessel that isn't actually being pushed by the wind. It also makes the vessel topheavy and harder to control.

I would like to keep the setting close to the source material, but I also want to have a reasonable explanation for this feature since it is going to affect a lot of things about how airships are used and maintained. Can anyone suggest a reasonable explanation for why you would want your solar collectors in a mast and rigging configuration rather than something more aerodynamic?

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  • $\begingroup$ Don't you answer the question already? They're solar collectors. That they don't change it for hundreds of years might be the same reason some other things don't progress. The current setup is deemed good enough or perfect to the builders. $\endgroup$
    – Trioxidane
    May 24, 2021 at 14:04
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    $\begingroup$ @Trioxidane in general, the "good enough" argument only lasts for that long when there is no obvious flaw. Square sails for example lasted a long time because common sense said they had more area, and would therefore make you go as fast as possible. Likewise, sticking with solar sails out of tradition only answers this question if you could explain why people with common since would believe this is a good solution. Engineers dating back at least 2500 years have had a decent understanding of air resistance; so, they would not use sails unless there is at least some perceived advantage. $\endgroup$
    – Nosajimiki
    May 24, 2021 at 15:56
  • $\begingroup$ You might want to read The Aeronauts Windlass by Jim Butcher. He solves it quite readily they are not sails but massive web like nets that gather the aether that powers the ships, more web equals more power which means a faster ship. $\endgroup$
    – John
    May 24, 2021 at 20:30
  • $\begingroup$ @Nosajimiki it was more that most of the question, including the title, is why it would have sails. Then suddenly it is answered and a different question is posed. Why is the initial question answered and then a question asked about the answered question? That is why I ask if the question hasn't been answered. $\endgroup$
    – Trioxidane
    May 25, 2021 at 7:08
  • $\begingroup$ @ Trioxidane Perhaps I could have worded it better, but the heart of the question is why would a mast and sail arrangement continue to be used. There are lots of potential ways to add solar collectors to a vessel, so why continue to use one that creates significant drag? $\endgroup$
    – pbuchheit
    May 25, 2021 at 12:37

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So They Can Always Face the Sun

Having tall masts with a giant piece of material attached is going to create massive drag on a vessel

We already do this a lot with real solar panels where we put them on masts so that they can be rotated to maximize surface exposure and angle to the sun. A mast allows the sails to turn axially, and the rigging allows you to tilt the sails up or down. By investing a small amount of energy into keeping a solar panel optimally aligned, you can produce a lot more sum energy.

So, as long as putting a solar sail on a mast produces more energy than you lose to drag, it will be an ideal thing to do. Keep in mind that your ships are basically VTOL aircraft like helicopters. So, a lot of your energy will be needed to just stay in the air. This means that drag, even lots of it, is a relatively small factor compared to optimizing the power needed to maintain leveation.

For Example: Keeping a 100 metric ton airships in the air will require a continuous force of 9,800,000 N. A square-rigged ship of that size will generally have a total frontal cross-section of ~200m^2 and ~1.2 drag coefficient. A sailess ship might only have a ~25m^2 cross sectional area with a ~0.3 drag coefficient. Moving through the air at the speed of a fast age of sail ship (12.5 m/s) with sails means you need to apply a continuous force of 22,900 N to maintain speed against the air resistance, vs 718 N of an aerodynamic sailess design; so, yes, the drag from sails is significant, but still only increases your energy need by about 0.2% while boosting your available surface area for solar collection by over 100%.

Also, due to the square cube law, sails will also become increasingly necessary the larger your ships get. Because a ship's mass increases with size at a cubic scale and their surface area increases at a square scale, this means large ships need to do something extra to increase surface area the larger they get. So if you assume max speed is just a function of lift-to-propulsion then just knowing that the crystals can not make a 100 ton sailless ship move faster than ~260m/s is enough to extrapolate that the ship will move faster with sails. But a larger 800 ton ship of similar proportions would need have a harder time just getting airborne, and would need sails up to crystal efficiencies capable of making it move at any speed of < 360 m/s.

Conversely this also means you can have crystal powered sailess fighters if you so choose because small enough of airships will already have enough surface area to mass to fly.

... So as long as you're setting allows for flying sail ships that don't need to be super fast, you can simply explain that ships of any meaningful mass just can't get off the ground without sails.

Furthermore, you can safely assume that your airships aren't much faster than historical sailing ship if they have chosen to use sails at all. Even the toughest modern sailcloth materials can not endure wind speeds over 50 m/s at the size of a historical square sail. So, if your civilization has chosen to use sails, it can be assumed your airships are not moving so fast that the wind would exceed the breaking strength of the sails.

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Safety and "Parkability"

This leaves the question of why on top vs somewhere else? If you put them under your ship, it would mean that your hull blocks the sun at high noon, plus they get in the way for landing; so, this is a pretty bad idea. You could put them on sides, but this would become an issue of safety and reliability. If you forget to properly align your side masts as you come down to land, you risk hitting the ground with it damaging your sail, it also means you need a much larger landing area. It could also be that your airships need to land during the night and will generally land in the water. Since most of the ship's volume will be under the water line when you do this, it could also be about keeping your sails dry. Basically, by putting them on top of your ship, they stay out of the way when you land making the ship less likely to have any sort of critical failures.

They might also be on top because when you go too far north or south, you may not be able to do much flying due to limited sunlight; so, there is added redundancy if you can use your sails as sails when the need arises.

Furthermore, ports already take up a lot of space, the smaller you making a parking spot, the more money you save on port construction. So, unless there is a distinct advantage to putting sails on the side, top sails would be economically preferable.

enter image description here

It also makes the vessel topheavy and harder to control.

Actually, it would make your ship bottom heavy. The concern with the "top heaviness" of sails on a sailing craft is in relation to the buoyancy effects of water on the hull. In flight, this is not a concern. Your hull with all of your crew and cargo will inevitably be much heavier than the masts protruding out of the top.

The real question, if you are concerned about tipping may be about where you put your crystals. If you put them along your keel, then sure, it might tip your ship, but if you place them inside your masts, then your ship will very stabbly hang from your masts. If they don't fit inside your masts, then I would suggest putting them in the tower decks of a caravel like ship Basically, anywhere you put them that would be above the water line of a stable sailing ship would also be above the center of mass for purposes of an airship, and therefore also stable.

enter image description here

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  • $\begingroup$ One would think that a bottom-sail configuration would be preferable on an airship. Top-mounted sails mean that the force of wind tries to tip the ship over. There have been incidents of this in real life sailing ships in high winds but it's worse for an airship since there's no equivalent interaction between the hull and the water to counteract it despite the weight of the hull, cargo, and crew. In contrast, bottom-mounted sails, to some extent, have this tendency to invert counteracted by the weight of the sail and mast assemblies themselves. $\endgroup$ May 24, 2021 at 15:00
  • $\begingroup$ @GrumpyYoungMan wind will try to tip you regardless of putting sails above or below. The important factor for how well the ship can resist tipping will simply be how far above your center of mass you are applying the levitating force. While it is true that bottom sails move more mass below your hull, it will not make a meaningful difference. As long as your flying crystals are somewhere above the "waterline", an airship will be more stable than a comparable watership. $\endgroup$
    – Nosajimiki
    May 24, 2021 at 15:46
  • $\begingroup$ This is excelent; these are the sort of details I was hoping for. I do have one question though. You mentioned "you need to apply a continuous force of 22,900 N to maintain speed against the air resistance, vs 718 N of an aerodynamic sailess design. ; so, yes, the drag from sails is significant, but still only increases your energy need by about 2% while boosting your available surface area for solar collection by over 100%.". I may be missing something, but how do you get a 2% difference? I ran the numbers and ended up with a 3000% increase. $\endgroup$
    – pbuchheit
    May 24, 2021 at 20:05
  • $\begingroup$ @pbuchheit Opps, that is a typo, it is actually a ~0.2% difference. You need to sum of the energy required for lift: so actual total force needed stay in the air and move forward is 9,822,900 N vs 9,800,718 N $\endgroup$
    – Nosajimiki
    May 24, 2021 at 21:09
  • $\begingroup$ But what happens when you want to go in the opposite direction to the prevailing wind, and the wind is going ONTO the path of the sun? $\endgroup$ May 25, 2021 at 2:29
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  1. Energy collected by a solar collector is a function of area; more area=more power and an airship is massive enough to need quite a lot of power to lift and move around. Granted, sails in close proximity in a normal arrangement on a sailing ship block each other but this hypothetical airship might use something similar to the non-functional arrangement depicted on various fantasy airships that provide more sail area exposed to sun.

  2. Prototype designs exist in the real world for modern commercial sail-assisted cargo ships for reduction in fuel consumption and greenhouse gas emissions, e.g. https://www.forbes.com/sites/nishandegnarain/2020/12/01/a-new-golden-age-of-sailing-is-here-where-is-the-leadership/ , so the use of sails may not yet be obsolete. Depending on how you handwave it, the sails on your airship can be used for wind-assisted propulsion for an increase in speed or a reduction in energy consumption or a reduction of some sort of fictitious wear-and-tear on the crystals in comparison to a sailless airship.

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Added propulsion

You say "from what I can determine", but it suggests you're not sure the crystals are in the sails. Regardless, sails can be prime material for added speed.

Although the crystals give lift, propulsion and speed, it seems undetermined how much speed. The crystals already lift and steer, so it might be that energy isn't sufficient for faster than sailing speed. That means added sails can simply add more speed, like on a normal ship. With the added benefit of being able to choose the layer of air you ride in, you can have the wind in the sails at any time you want. It thus makes sense to add sails, as it'll practically always improve your speed.

If the solar collectors are on the sails you can do the exact same. Turn the sails directly towards the sun for maximum solar gain. Assuming the crystals are on both sides, or the sails can be inverted, you can still always have the wind in the sails and the sun squarely on the collectors.

If you think this won't have much effect, look at new ships with kites or sails. These are moved to the right height for added wind power, severely reducing the energy requirements for the normal engine. For the airships this can mean a greater total power, or more efficiency for stored power(?) If they also need to fly at night.

Conclusion

Sails make a lot of sense in most scenarios. They can practically always improve the speed/duration of the flight. Improving speed and duration is a long term goal of nearly all transport, looking at our current society.

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I know nothing about the series that you mentioned, so without knowing the precise details of the sails, I am going to take the liberty of modifying them to suit.

I am imagining that these crystals might generate considerable heat when converting solar power to electricity. That, of course, is allowable and still leaves a lot of power left over to propel the ship, if they are still able to convert say at 60% efficiency, 40% heat. Imagine that it is the crystals themselves that get hot, the heat is not generated by the process to transfer the energy (as for instance steam generators require). Now, if these crystals operated at maximum efficiency only when cool, and they become very inefficient the higher their temperature, you have the necessity of massive air flow to disperse the heat. That is, the heat is entirely a waste product, not used at all in the generation of electricity.

Therefore, these sails would not just be solar collectors, they would need to be a huge heat radiator. Think of them not as fabric, but as an electrically conductive, heat dispersive net. The air would be free to circulate through the netting, providing for just a very limited drag, but still allowing sufficient surface area to transfer the heat. Thus, the crystals could not be placed around the ship itself, the ship would act as the heat sink.

Of course, since the crystals would have to be widely dispersed to optimize the heat transfer to prevent heat build-up, it becomes necessary to have a huge area of netting to keep them spread out. The material of the netting itself becomes the grid that conducts electricity from every crystal back to the ship.

Now add that the voltages that these crystals produce is extremely large, you do not want your crew near them. Thus, they must start well above the deck. If they extended on BOTH sides of the ship, then you have enclosed the entire ship in one huge electrostatic bubble - think in terms of working inside a huge tesla coil. You would, methinks, want this electrostatic field well outside the ship and off to just one side.

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    $\begingroup$ Electricity might not feature at all with these crystals. They power the fields that propel them, steer them and hold them aloft. That might be anything, including just a magical energy. $\endgroup$
    – Trioxidane
    May 24, 2021 at 14:21
  • $\begingroup$ @Trioxidane As I stated, not knowing how these crystals worked, I took creative licence to modify them to suit, $\endgroup$ May 24, 2021 at 14:24
  • $\begingroup$ While that is an interesting idea, I don't think it is going to work. There are a few instances where we see vessels operating with the sails stowed. Just like in a traditional sailing ship you don't want your sails deployed during a storm. The only issues that ever get mentioned is the potential for the crystals to deplete if the bad weather lasts for too long. There is never a mention of potential danger due to heat build up. $\endgroup$
    – pbuchheit
    May 24, 2021 at 14:26
  • $\begingroup$ I was not intending to describe how they worked in the series, but how one could modify the concept for ones own use in ones own story so that it would be less 'magical'. Only the original author of the series would be able to explain the concept behind the workings in their story. They are the only one in control of and responsible for their plot line and narrative. $\endgroup$ May 24, 2021 at 14:34
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Backup

Suppose your crystals fail for whatever reasons, such as them being solar-powered and you getting too many cloudy days. Since your ship won't be flying, it might at least be sailable on water.

Rule of cool

Enough said. The Flying Dutchman wouldn't be the same without sails.

Those are not sails

Those are flags, which are used for communication. If you spend five minutes studying naval flags, you will see that historically it wasn't uncommon for ships to have more flags than sails on them.

A dressing ship, which is a ship with too many flags

In this particular case the flags are gigantic so as to be more visible while flying, since the distances involved in flight might be greater than the ones possible on the surface of the ocean. The flags are so big they look like sails.

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    $\begingroup$ Speaking of flags, ever wondered why the train whistle, as it approaches a street crossing, is two long, one short, and one very long? Naval history going all the way back to the Thames in England, began with a flag signal. $\endgroup$ May 26, 2021 at 3:22

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