# Designing a flag for Mars: drag and proportion

I’ve been thinking about flag designs for colonies on Mars, and something occurred to me.

## The “official” Flag of Mars has a critical flaw

The average wind speed on Mars is 10 m/s (20 mph); the fastest speeds recorded by the Viking lander were 30 m/s (60 mph), barely half the speed of hurricane winds on Earth. More importantly: Mars’ atmosphere is just 1% the density of Earth’s. These factors combined mean one couldn't fly a kite there.

## A flag flying on Mars

According to what I’ve read, winds of about 5 m/s (11 mph) are needed (on Earth) to get a flag flapping. In Martian terms, that’s wind at 50 m/s (111 mph). (Dynamic pressure of the atmosphere needs to be multiplied by about ten for Mars’ wind to be comparable to Earth’s.) That’s certainly above the average-to-high range for wind on Mars. So no, Pascal Lee’s Martian tricolour couldn’t actually fly on Mars.

## Changing the proportions

In flag terms, the wind velocity needed to overcome the drag coefficient increases as the length (fly) increases over the drag (hoist); in other words the longer a flag is, the stronger the wind must be to make it unfurl. Shorten the length/fly and you lower the required velocity. That suggests a flag shape with a much shorter length/fly than typically found on Earth. The actual math for calculating this is a bit above my ability, but it makes sense that if wind pressure on Mars is a tenth of Earth’s, a flag with a tenth the length/fly should flutter on Mars the way its full-length counterpart on Earth would. (Lighter, thinner material would obviously have an easier time too.)

Of course that means you could just have a tiny square for a flag, but that rather reduces its visibility at a distance, and would restrict how complicated the designs could be. So I propose the ideal proportions for flags flown on Mars would look more like nobori than your standard modern flag: 4+:1. Tall but narrow; probably no more than 25 cm (10 in) across, but no less than a metre tall, if I had to guess. (Ideally Martian flags would not need the cross-bar used in nobori, but flags made too large or of materials too heavy wouldn't have a choice.)

## Bonus: What that means for the design of Martian flags

I suspect not much would change. Lee’s tricolour design, if rotated 90°, would definitely work. Horizontal stripes and divisions of the field would be as common as on Earth, but vertical divisions and stripes might be more easily obscured when at rest. Use of charges and emblems may similarly depreciate as the narrowed fly would require proportionately smaller designs. Cantons would likely occupy the full width at the top of the hoist, and chevrons might point down from the top. Bends and saltires would still work, and Nordic crosses could be rotated, but the vertical bar of any cross design could be a problem if it isn’t a significant width of the fly.

# The question(s):

Do the numbers work out, and if so would the suggested change of proportion be the best solution (not requiring a cross-bar)? Bonus: am I overlooking any ramifications for flag design within these constraints?

• And why not to use lighter fabric? It doesn't need to be as resistant as flags you use in Earth atmosphere, right? Commented Nov 15, 2016 at 17:10
• Does the weight (density) of the material matter? Maybe it will be made of ultralight fabric. Commented Nov 15, 2016 at 17:12
• I am not aware of any kind of law demanding that a flag has to be mounted on a vertical pole. Actually, i think if one went counting, the majority (in total specimens) aren't, but are painted or printed on any kind of surface. Commented Nov 15, 2016 at 17:14
• @Burki It may not be a law but it is traditional.
– rek
Commented Nov 15, 2016 at 17:22
• @rek living in caves is traditional, too. We give up traditions every time we notice they no longer fit our needs. Or at least some of us do. Commented Nov 16, 2016 at 7:59

Use a lighter cloth.

The reason we use such heavy cloth on Earth is so it has some longevity in high winds and rain. That's not a problem on Mars so you could use much lighter weight materials.

You're usually looking at materials that are 85-160gsm to make a flag. You probably want something around 10-20gsm, equivalent to tissue paper, which appears to be within standard cloth manufacture ranges.

This would be shredded by normal winds on Earth, but should be about right for Mars.

• More to the point -- a high-tech material rather than traditional cloth. For instance mylar, light enough to fly on Mars and tough enough to fly on Earth. Commented Feb 1, 2017 at 21:17
• This answer wins the prize in my book, especially for the hard numbers and the link to pages describing specific fabrics. Thanks, @Separatrix!
– SRM
Commented Feb 5, 2017 at 14:46
• @Harper, pennants flutter in the breeze, a flag should flap with weight and dignity, we could just use a light ripstop nylon, but that would flutter. Commented Feb 6, 2017 at 9:10
• I appreciate this approach to the answer, but I think it's missing a demonstration that lighter material really is all that would be needed on Mars.
– rek
Commented Mar 15, 2017 at 20:25
• @rek, you are correct, I haven't done so, but that's because vortex shedding becomes a little too much heavy duty calculation. Knock yourself out though, note on page 2 the importance of flag density and bending rigidity. Plugging your atmospheric values into the formulae given should allow you to work backwards to find the ideal flag for your given conditions. Commented Mar 15, 2017 at 20:45

Does a flag have to wave in the wind? Consider the more general case of vacuum or other arbitrary conditions. Will the flag only be displayed on Mars, or will embassies etc. want to display it elsewhere?

So maybe it should be feestanding like a sign.

It may have animation as part of the design! So it is presented in active media, not a dumb cloth. Whatever attention grabbing feature will be designed in directly, not reliant on the wind as a crude driver of visual change.

• The goal is a flag free to flap in the winds of Mars as one would on Earth.
– rek
Commented Nov 15, 2016 at 17:21

You can't fly a kite on Mars but balloons should still work. Just make parts of your flag inflatable and fill them with a very light gas.

If you segment the inflatable part of your flag along its horizontal length, and hinged each segment to those before and after it along that length, then the flag would even wave as the lightweight winds hit it.

• Yes, yes, just so. But should this be called a flagoon or a ballag? I am thinking flagoon because ballag sounds like something you catch in the mens room of a seedy Martian spaceport. Commented Feb 3, 2017 at 19:46

Flags aren't necessarily rectangular, consider pennants or their variants, as in the flag of Nepal.

Or go for a 3D flag by loosely tying the outer end, like drawstrings around the mouth of a bag, to make a windsock. Standard windsocks are fully extended at 17kph, so around 5 m/s, so that should be adequate given the average wind speed on Mars. The best part, the flapping effect is now due to the mass of air moving through the cone.

Domine Satanas beat me to the punch, but I was also going to suggest, using a different flag material. If an Earth flag is made with a certain amount of density and mass, you would need to also scale this down so that it adjusts for the proper wind and density of the air. Instead of using very heavy and dense fabric, what about a lighter cloth one similar to T-shirts? Or make up some synthetic material designed specifically for the air density of Mars. When it comes to the density (Density = Mass/Volume), we also have to think about mass of an object. The less dense an object is, the less mass it has, the less force would be required to move it. force (Force = Mass x Acceleration). So by reducing the mass, you can also account for the reduction in acceleration applied to the flag by the wind.

If you want to keep the heavy fabric that Earth flags are made out of, you will need to shape the flag so that it can gain more surface area to catch the wind. Think of sailing. The bigger the sail, the more surface area it has, the more wind it can catch, and the bigger the boat that can be made to move and or the faster the ship can travel. Since our flags are designed to catch wind with our atmosphere, you may find it that you will need to increase the size of the flag and possibly add pockets to to help the flag catch wind.

The other thing to consider is that gravity on Mars is only that of 38% of what it is on Earth. In other words, if you weigh 100 lbs on Earth, you would weigh 38 lbs on Mars. This means that even though there is less wind and atmosphere, the flag by ratio, would also be significantly lighter compared to the same flag on earth.

Don't use a common flag pole but let it hang down from a horizontal bar.

Your flag will always be displayed properly and it will also wave in the wind. But still, the cloth has to be as light as possible.

• I am thinking of a flagpole with a triangle made of pole, point side down mounted on it. On the top bar of the triangle hangs the flag as Alex describes here. The flag depicts a triangle in honor of the triangular Martian moon. Commented Feb 3, 2017 at 19:48
• This doesn't really allow it to wave in the same way as an Earth flag. It's definitely a novel solution compared to the other answers (unless I've missed something), but it is another way, in my opinion, of dodging the waving question. But I can see it being how practical Martians might handle it. :-)
– SRM
Commented Feb 5, 2017 at 14:45

Even on Earth, there may be need to make flags fly steadily or flap at the right rate instead of merely hoping that they do. Outside when there isn't enough wind, or inside where there probably isn't any wind.

Such a flag would have to be made of two pieces of cloth or plastic, one for the obverse side and one for the reverse side. And there would be stuff between them to make the flag fly and flap. For instance there could be tiny horizontal tubes that run the length of the flag, each with a tiny fan at one end. When the fans were not running, the flag would only fly and flap as much as the current wind made it. When the fans were all turned on to full power they would blow enough air through the tiny horizontal tubes to inflate them and make them stand stiff and horizontal, pulling the flag into a a stiff horizontal display. And when the fans were turned on and off at different rates and combinations, the flag would flap like it was in the wind.

Another possibility would be to put tiny electromagnets in the space between the two sides of the flag. Electromagnets with the same charge repel each other. So if the electromagnets have the same charge and given full power they will repeal each other strongly stretch out the flag to a stiff horizontal display, if they have no charge the flag will hang limply without any wind, and if they are given power levels that vary with time the flag will flap.

Of course such flags will need computer controls and power supplies.

You want the flag flapping free. Intead of having wind do it, maybe have an automated "flag flapping tentacle arm" in the flag to simulate wind? If you don't want a tentacle flag, maybe make it from a super light plastic, like saran wrap, that's been art'ed up?

• As conspiracy theorists are keen to point out: The US flag on the moon waved. As physicists are keen to point out: There was a small bar in the top that wobbled because it was springy. Commented Feb 1, 2017 at 18:27
• Yeah, but springy does not equal freely flapping. Commented Feb 1, 2017 at 20:53

Forget the math entirely. Just give your Astronauts Augmented-Reality HUDs in their helmets, and render the flags digitally.

There's more than one way to skin a cat.

• This also “dodges the question” like my original answer. Read the bounty text. IAC this is inferior to mine since an active panel display would work without soecial viewing equipment. Commented Feb 1, 2017 at 20:04
• @JDługosz I'll agree this is a hack answer, but we're talking about a flag on another planet. To see this flag at all you're going to have hitch a ride on a trillion dollar spaceship, move into a billion-dollar high-tech hab-module, and wear a million-dollar high-tech spacesuit. DARPA has wanted to give soldiers Augmented-Reality equipment for decades. What's to say it's not standard issue on a space-suit? Commented Feb 1, 2017 at 21:56
• I agree, thinking outside the box is normally a good answer, and mine stood for months ’till someone other than the OP changed the rules with the bounty posting, and mine got downvoted all the way back down. Commented Feb 2, 2017 at 0:06