# Size of a world with airships

I'm making a world with magic and most if not all tech will run on mana. I have airships in this world but I'm having a hard time deciding how large across the world should be. I want - even with airships - for long distance travel to take months (cross a continent or from one continent to another). Should I set a slower airspeed than airplanes now, and have the world the same size as Earth, or increase the size of the world, or both?

• Airships are very vulnerable to wind. have a planet that often has strong winds, storms even, and you get many days where you cannot use the airship, and others where you have to work against a head wind. – Burki Dec 1 '15 at 15:33

Using LZ 127 Graf Zepplin as an example of a transatlantic airship flight from 1928, it took 111 hours to travel from Friedrichshafen, Tübingen, Germany to Lakehurst, New Jersey, United States of America, about 9,926km in length.

Faster:

In a similar airship that is capable of achieving the same speed of 89km/hr, on a similar journey across a Continental divide, the distance would need to be 150,000km to achieve a two month journey taking 10 weeks.

This means your theoretical planet would need to be on a scale of 15x the size of the Earth to achieve a similar journey over a longer time.

This massive world would probably increase the likely-hood of transition to air-travel as the speed to cross long distances would be a must for world spanning empires.

Slower:

If you wish for your airship to just travel slower to make the journey longer, to achieve a 2 month long journey taking 10 weeks,you would need to slow your airship to travel 6km/h to travel roughly 10,000km across an earth sized ocean.

In terms of world building, the slower option makes less sense as most people can walk faster than 6km/hr, and any naval vessel could easily outspeed this airship. Unless of course the ocean in your world is so acidic that the only way to traverse it is to fly above it!

Between:

Below is a quick table of some compromises between the above suggestions:

| Distance  | Scale | Speed  | Likeness to other modes of travel|
| 150,000km | 15x   | 90km/h | Speed of Car                     |
| 100,000km | 10x   | 60km/h | Speed of Early Model Steam Train |
| 50,000km  | 5x    | 30km/h | Speed of Horse Gallop            |
| 20,000km  | 2x    | 12km/h | Speed of Runner                  |
| 10,000km  | 1x    | 6km/h  | Speed of Fast Walker             |

• Have you considered the increased gravity of a planet 15 times larger than Earth? – Samuel Dec 1 '15 at 3:51
• @Samuel ofcourse, I was not implying that such a world is possible, but merely answering OPs question as it isn't science based I thought they would be okay with a massive planet that somehow still has Earth's gravitational pull. – FiringSquadWitness Dec 1 '15 at 5:28
• I would argue that positing a planet 15x times the size of Earth but with the same gravitational pull is very wrong. For comparison, Jupiter is 11x the radius of Earth, made of much lighter materials than Earth, and still has enough gravity to function as the vacuum cleaner of our solar system. Keep in mind that the OP is a newcomer to the site and network, so probably don't know the intricacies of our site-specific tags. – user Dec 1 '15 at 8:34
• Another useful data point: The Oregon Trail took roughly 150 days (5 months) at the grueling slow pace a covered wagon could travel at. That's 21km/day, or about 0.88km/hr over the course of the 3,200km trip across the US – Cort Ammon Dec 1 '15 at 16:44
• How about providing 15 worlds, all connected by magical portals, gaps, canyons, equatorial portal rings, or other permanent phenomena? This solves the travel/size problem without creating a gravity problem. Alternately, you could do a "culture orbital" or "ring world" to get an appropriate size. Another solution is to hand-wave away gravity issues by using mana to create a semi-hollow world. You could even have multiple levels of the planet's surface, accessed via airship. Something like the ice world in the movie Interstellar, yet habitable. – Kimball Robinson Dec 1 '15 at 18:44

Basing this off of JustAnotherDotNetDev's calculations:

The big problem with making the planet much larger is of course gravity. As Michael pointed out in the comments, Jupiter is 11x the radius of earth, is mostly hydrogen and we wouldn't survive for very long under the huge crushing weight of our own bodies. So find reasons to slow things down.

The one thing I can see that might help slow things down is that the people only navigate visually. so when the sun goes down they try to find a place to 'anchor' for the night. This would mostly double the time it takes to fly anywhere, and it would still be significantly faster than walking.

One more thing might be that large merchant ships and large troop transports are large enough to just wallow along at 100-200 feet above the ground, and they might need to partially empty to navigate mountain passes, slowing things down more.

Really high passes might need to actually completely empty the ship and 'carry' it over the pass to the other side, this could take weeks all by itself, and make the seasons have a huge impact on long distance travel.

• Visual navigation would cause much more than twice the travel times. Consider the transatlantic route: if you can travel 12 hrs average, you need to choose a route where you hit land before nightfall. guaranteed, that is, even if the wind is infavourable. That forbids the direct route. – Burki Dec 1 '15 at 15:32
• And one more thing: Daylight only could be a result of a dustcloud surrounding the planet, thus blocking starlight. – Burki Dec 1 '15 at 15:34
• @Burki yes over seas travel would be much more difficult, doubling was a guess at overland travel. – bowlturner Dec 1 '15 at 15:37
• I wonder. For high pass travel in established routes, would it make more sense to have an airship on each side? Just dock, unload, haul the stuff, and load up. Kind of like layovers at an airport. – AndyD273 Dec 1 '15 at 15:46
• @bowlturner but the species evolved on that planet, so surely they would have adapted to higher gravity – womp Dec 2 '15 at 10:30

Since all the tech (including the airship engines I assume) runs on mana, then in addition to bowlturners ideas, you could limit speed by the need to "refuel".

I'll have to make a few assumptions since you didn't specify how mana works, but if mana isn't a constant, unlimited resource, then it'll have to be replentished.

So the airship could be able to travel at 89km/hr, but only be able to travel for 6 hours before having to stop and recharge for a few hours. This could limit the ship to 1000km a day. Less if the engines aren't as efficient.

Add to that high mountain ranges that need to be navigated around so straight line travel is impossible. So a straight "as the crow flies" distance of 10,000km could end up being 30,000km of distance needed to travel.

Another thing that could slow it down is scheduled stops.
Say the world is heavily forested, and mountainous, so ground travel is difficult. You still have settlements and towns that need supplies and people that want to move around. The airships are the obvious choice for this, meaning that airships would have routes and have to make stops at each location along the way. Docking, unloading, and loading could take a while depending on what's being moved around. This would slow down the travel time too. You could still have express airships that only make a few stops.

One other thing would be weather. Airships have trouble in high winds, so if your world has frequent storms, or even something like the Santa Ana Winds or Chinook winds where the wind will blow hard for days/weeks at a time. The airships would have to anchor and wait for things to die down. Either that or travel really slowly, fighting the wind the whole way, and making frequent stops.

Edit
Another weather issue could be fog or low clouds full of things like tall trees and rocks. It could even be kind of like river navigation, where you had to have a navigator that knew where all the sand banks were in order not to run-aground. If this happened frequently, they would be forced to travel very slowly or risk crashing until the clouds lifted.

• Another possibility, if your planet doesn't have a magnetic field then there are no compasses, which may introduce the need for constant course corrections while ships blindly feel their way across the ocean – colmde Dec 1 '15 at 17:09
• @colmde Possible, though even with a compass, a lot of early navigation was handled using other navigational instruments, going off of the sun and stars. The people who traveled from the southern Pacific islands to colonize Hawaii didn't even have the north star to guide them below the equator, and instead had to use the sun by day and different stars at different times of the night to navigate by. That's not to say that its a bad idea. – AndyD273 Dec 1 '15 at 18:15

You can increase the radius/mass of the planet, as well as the density of the atmosphere.

The bigger the radius, the bigger the circumference, and thus the further apart places will be.

The bigger the mass, the higher the gravity thus you need more lift per pound to fly.

The denser the atmosphere the more drag you'll get. Denser atmosphere provides more lift, but the higher gravity easily makes up for that.

At the end you end up with slower airships (due to the higher gravity you'll have weight restriction, won't be able to carry enough fuel etc), more drag and greater distances to travel.

On top of that add finicky airships that require ground service every few days and inability to navigate at night most nights and it could easily take months to go transcontinental.

• "finicky airships that require ground service every few days" would, in any reasonable world, lead to people seeking ways to reduce the need for servicing the airships. You'd need quite a bit of handwavium for people, especially engineers, to not put much thought into that problem for any significant length of time, or businesspeople to not have their engineers looking into the problems on paid time. (It would, however, be a reasonable state of affairs when the technology is very new to the inhabitants, but at that point it wouldn't be used for mass transit anyway, so it's a tie.) – user Dec 1 '15 at 19:27
• After the fall of the Roman Empire, Roman roads and aqueducts felt in disrepair but were used for hundreds of years. There are many examples where things are not maintained properly but continue to be used, the result being ever worsening quality of service. – ventsyv Dec 1 '15 at 20:55
• @MichaelKjörling A possible way to have a need for regular maintenance where engineers might not address the root cause quickly is if it's an inherent issue with the materials themselves. For instance, if there is a problem with acid rain eating the enclosure, and there are no acid proof materials available, then they might work on ways to repair or clean the canopy faster, but it would still require regular maintenance. – AndyD273 Dec 1 '15 at 21:02