Nitpick "The Martian"

[peadar1987]

He uses the fuel plant from the MAV to produce C02, the Hydrazine and catalyst come from the MDV.

Theres no suggestion of any part of the MAV using hydrazine. Unless - the MAV brings hydrogen with it to produce fuel, but hydrogen is worse for boiloff than LOX is. Its possible that there is a descent stage, and excess hydrazine is decomposed to produce the hydrogen the fuel plant needs. However, Mars Direct, which much of the mission in The Martian was based on, claims that hydrogen boiloff can be reduced to 1% per month using insulation and gelling the hydrogen with methane.

Not quite sure what you're getting at here. My understanding is that the descent stage uses hydrazine as a fuel to set the lander down on the surface. Watney specifically says that the reason there is plenty of hydrazine left is that Martinez performed a very fuel-efficient landing, so it's definitely for the engines, not as a hydrogen store or anything.

The fuel for the MAV (which would probably leave the descent engines and tankage behind to save on mass) is never mentioned in the book as far as I remember, but is most likely methane, as most ISRU concepts rely on the Sabatier process to produce methane and water from the Martian atmosphere.

[andrewas]

I thought you were still talking about the descent stage of the MAV?

[AngelLestat]

Ok, I was not happy with the KerikBalm estimations and conversions.. so lets calculate all in mars conditions without correlations with earth.

What is the force needed to drag a person in mars?

Fs= μmg

Fs = force of static friction

μ= coheficient of static friction (0.3 dust terrain)

m=mass = 80kg (with spacesuit)

g= gravity in newtons. (3.7m/s2)

Force of the wind in a person on mars:

Fd=1/2ÃÂv²CdA

Fd= force of the wind

p= density of the atmosphere = 0.0165kg/m3 (half between your value and mine, also we need to consider the dust in the air)

V= wind speed = 111 m/s = 400km/h

Cd*A = drag coheficient * body area = 0.84 m2 (source), it is a bit higher due spacesuit --> 1m2

Friction Force= 88 Newtons

Force of the wind= 101 Newtons.

Conclusion: weeeeeeeee bye bye..

[peadar1987]

I thought you were still talking about the descent stage of the MAV?

You're dead right, I'm having a very bad run of wrongness on this thread! MDV landed by Martinez was carrying stuff like the Hab and the rovers, MAV was landed by remote control by Ares 2 years previously. Possibly still used Hydrazine, possibly Metholox, but it's never stated in the book.

Ok, I was not happy with the KerikBalm estimations and conversions.. so lets calculate all in mars conditions without correlations with earth.

What is the force needed to drag a person in mars?

Fs= μmg

Fs = force of static friction

μ= coheficient of static friction (0.3 dust terrain)

m=mass = 80kg (with spacesuit)

g= gravity in newtons. (3.7m/s2)

Force of the wind in a person on mars:

Fd=1/2ÃÂv²CdA

Fd= force of the wind

p= density of the atmosphere = 0.0165kg/m3 (half between your value and mine, also we need to consider the dust in the air)

V= wind speed = 111 m/s = 400km/h

Cd*A = drag coheficient * body area = 0.84 m2 (source), it is a bit higher due spacesuit --> 1m2

Friction Force= 88 Newtons

Force of the wind= 101 Newtons.

Conclusion: weeeeeeeee bye bye..

Hmm... More likely you would try and huddle to avoid the worst of the wind, reducing your drag coefficient. You only need to reduce your frontal area to the wind to 80%, which is quite easily achieved by just turning 90 degrees to it. You wouldn't blow away, but you would quite likely blow over in the biggest gusts. Once on the ground, both the wind speed and your CoD are reduced, and you'll be able to crawl without too much bother.

[SomeGuy12]

a leaking seal, a line got hit by an object, or just moved around enough for a seal to become loose, or they wer doing something when the storm hit, and due to dust the thing didn't seal right... whatever).... and if they wait around too much (Oxidizer?) will have leaked, and they won't make orbit. So then they have to go then and there while the MAV can still make it... and they think the guy died anyway and because of the storm, they couldn't really tell where he disappeared.

Nitpick of your nitpick : they could replace missing oxidizer because they are cracking oxygen off the CO2 on Mars. They can't replace missing fuel because the methane is formed from liquid hydrogen they brought and CO2 on Mars. Yes, there's water on Mars, but they didn't bring the mining or drilling equipment to collect any of it because that's a lot heavier than bringing tanks of the lightest element in the universe.

[Windspren]

When NASA said he needed to make more fuel for the MAV to reach Hermes, where does it go? Why would there be extra unused tankage on the MAV?

He could overpressurize the tanks

- - - Updated - - -

He uses the fuel plant from the MAV to produce C02, the Hydrazine and catalyst come from the MDV.

Theres no suggestion of any part of the MAV using hydrazine. Unless - the MAV brings hydrogen with it to produce fuel, but hydrogen is worse for boiloff than LOX is. Its possible that there is a descent stage, and excess hydrazine is decomposed to produce the hydrogen the fuel plant needs. However, Mars Direct, which much of the mission in The Martian was based on, claims that hydrogen boiloff can be reduced to 1% per month using insulation and gelling the hydrogen with methane.

No, he uses the fuel from the MDV, not the MAV, to make water

[KerikBalm]

Ok, I was not happy with the KerikBalm estimations and conversions.. so lets calculate all in mars conditions without correlations with earth.

What is the force needed to drag a person in mars?

Fs= μmg

Fs = force of static friction

μ= coheficient of static friction (0.3 dust terrain)

m=mass = 80kg (with spacesuit)

g= gravity in newtons. (3.7m/s2)

Force of the wind in a person on mars:

Fd=1/2ÃÂv²CdA

Fd= force of the wind

p= density of the atmosphere = 0.0165kg/m3 (half between your value and mine, also we need to consider the dust in the air)

V= wind speed = 111 m/s = 400km/h

Cd*A = drag coheficient * body area = 0.84 m2 (source), it is a bit higher due spacesuit --> 1m2

Friction Force= 88 Newtons

Force of the wind= 101 Newtons.

Conclusion: weeeeeeeee bye bye..

Using those figures, 23kg of force* would be enough for a person to go "weeeeee" on earth.

*edit: Yea ok, thats not a good term, its 233 N. But thats the force that is exerted due to gravity by 23 kg on earth, to put things in perspective.

223 = V^2/2 * 1M2*1.2kg/m^3

372= V^2

v= 19 m/s

= 68 km/h

= 42 mph winds

Your chosen values for drag and friction would mean that a person on Earth wouldn't be able to walk around in 42 mph winds.

I can assure you that I can (and have done so) walk around in 40-45 mph winds.... yea.. the force does start to get noticeable, and its fun to see how far forward one can lean before you actually start to fall forward.... but all heck doesn't break loose.

Edited June 28, 2015 by KerikBalm

[AngelLestat]

Hmm... More likely you would try and huddle to avoid the worst of the wind, reducing your drag coefficient. You only need to reduce your frontal area to the wind to 80%, which is quite easily achieved by just turning 90 degrees to it. You wouldn't blow away, but you would quite likely blow over in the biggest gusts. Once on the ground, both the wind speed and your CoD are reduced, and you'll be able to crawl without too much bother.

Is not so easy, wind is not always constant over surface, you have gust and different angles of attack.

I can also make the math to calculate the force needed to turn you, we have only one foot on the ground when we are walking, it does not need much, which may leave us in front to the wind.

Also if we bow forward, does only helps to avoid turn over our back, but in that angle, we get another drag component that points up, that counter our weight which reduce our friction.

And once the static friction force is overcome, then the friction coefficient is reduce by almost half.

Using those figures, 23kg of force* would be enough for a person to go "weeeeee" on earth.

*edit: Yea ok, thats not a good term, its 233 N. But thats the force that is exerted due to gravity by 23 kg on earth, to put things in perspective.

223 = V^2/2 * 1M2*1.2kg/m^3

372= V^2

v= 19 m/s

= 68 km/h

= 42 mph winds

Your chosen values for drag and friction would mean that a person on Earth wouldn't be able to walk around in 42 mph winds.

I can assure you that I can (and have done so) walk around in 40-45 mph winds.... yea.. the force does start to get noticeable, and its fun to see how far forward one can lean before you actually start to fall forward.... but all heck doesn't break loose.

Ok, then tell me what is the right friction coefficient value for a dusty grave ground as mars?

I try to did my best estimation taking into account the examples that I had

http://www.engineeringtoolbox.com/friction-coefficients-d_778.html

For example: car tire and grass is 0.35, leather and metal is 0.4, carbon and steel 0.14, I dont know.

Even if we choose 0.4, we are in the limit.. And in my experience, grass has more friction than dust grounds.

If you use the decimals and you fix the force which is 235 (not 223), the wind speed would be 19.5m/s or 70km/h

Now add the spacesuit and stop imagine you on asphalt.

In fact, that wind speed match the Beaufort scale, which points that at force 8 wind, 17.2–20.7 m/s

"Some twigs broken from trees. Cars veer on road. Progress on foot is seriously impeded."

https://en.wikipedia.org/wiki/Beaufort_scale

Now if we watch

and we add the extra momentum from the flying object, all match fine with reality and calculations.

When people criticism movies, is because something is very far from reality. But at least I dont see much of the point with your number 1) statement from the OP.

[Rakaydos]

So, it takes 24 months for the MRV to fuel itself The Ares 4 MRV was landed on Sol 0 or -1 of Ares 3's mission.

About 570 sols later or so, the Ares 4 MRV is not only topped off but overcharged to get more delta V out of the mission.

That math seems off.

[peadar1987]

Is not so easy, wind is not always constant over surface, you have gust and different angles of attack.

I can also make the math to calculate the force needed to turn you, we have only one foot on the ground when we are walking, it does not need much, which may leave us in front to the wind.

If the wind is trying to turn you like a wind vane, it's going to try and turn you side-on to the wind, minimising drag.

Also if we bow forward, does only helps to avoid turn over our back, but in that angle, we get another drag component that points up, that counter our weight which reduce our friction.

I think you misunderstood what I meant by "huddle". Arms by your sides, bent knees, bowed head, not just lean into the wind.

I've walked in 40kts of breeze before (heck, I've sailed a boat in 40 kts of breeze). It's not easy, but it's not going to send you flying.

Edited June 29, 2015 by peadar1987

[AngelLestat]

If the wind is trying to turn you like a wind vane, it's going to try and turn you side-on to the wind, minimising drag.

But you already said that you are side-on, so if it turn you can be on front again, at least for one moment..

I think you misunderstood what I meant by "huddle". Arms by your sides, bent knees, bowed head, not just lean into the wind.

Arms at side? Spacesuit remember...

I've walked in 40kts of breeze before (heck, I've sailed a boat in 40 kts of breeze). It's not easy, but it's not going to send you flying.

Yeah me too, if you are sit in the sailboat there is not much trouble because only 1/3 of your body is expose, but if you want to remove the genoa, then you are screw.

At least you do it walking in 4 legs holding with your 2 arms.

I dont know what you want me to said.. first the OP said that it would be like a 45km/h wind in earth, I said not, we need to take into account the gravity. I made all calculations and I prove that we are in the limit to be drag by the wind.

But... you two still think that is not real...

In brief: 400km/h wind, taking into account that drag is square... is not joke.. even with much lower density, because the gravity is very low too.

Edited June 29, 2015 by AngelLestat

[KerikBalm]

Gravity may be lower, but things anchored into the ground aren't held in place just from gravity.

Also a large heavy object, like the fully fueled MAV, will have more mass per cross sectional area.

Whether or not you can walk around in the storm is not really relevant. I'm not talking about the scenes in the movie trailer, I'm talking about the events in the book.

In the book they didn't leave because it would be hard to walk around, they aborted because the full fueled Mars Ascent Vehicle would topple over due to the wind...

That seems to be unlikely to me... certainly something that it should have been designed to withstand.

Ok, then tell me what is the right friction coefficient value for a dusty grave ground as mars?

I try to did my best estimation taking into account the examples that I had

Math is good, math is great... but math is a model, and you must test models against reality.

Your model tells me that people wouldn't be able to walk around in 40 mph winds here on earth, and I know that is not the case.

Note that any coefficient of friction of less than 0.5 implies that it will start sliding down a 30 degree slope.

Thus... using your numbers, we'd conclude that the guy can't walk up a 30 degree incline on mars either...

I'm sorry, but your numbers just don't seem right.

Of course shoes have tread, and that equation is assuming two flat surfaces... etc...

we need to take into account the gravity.

I did.

Lower gravity reduces the force of static friction by 2.66x

That means the required wind force to be "equivalent" on earth is 2.66x... thats why I was comparing it to winds on Earth sqrt 2.66x.

So then really, the real problem is the actual atmospheric density of mars during the daytime(? IIRC, it happened during the day) at the equator.

Mars can actually get relatively warm at the equator at midday... and that lowers air density.

It would be much denser at night, when it gets much colder.

Pressure varies 10% daily on mars, and curiosity found temperatures at its landing site in the martian spring to vary between about 200k at night to 280k in the day.

40% absolute temperature variation will have a large effect on the density of the atmosphere. The altitude of the landing site should also be taken into account...

I'm not sure what the atmospheric density would be there (I also forget what season it was supposed to be in the book)... and obviously that is a very important value.

http://www.walkhighlands.co.uk/Forum/viewtopic.php?f=1&t=24814

Here's a topic about the wind speed people seem to be able to walk in. Its not exactly a scientific article, but...

"Wind bothers some more than others. 20-30 is kind of strong breeze and a bit more. 50-60 is hard to hear in but still possible to walk into. You may find yourself leaning into it. It is tiring to walk into and of course excellent to walk behind. Side-on is a psychological nightmare!

As for possible: the worst you can really function in is 100-120 mph. At that point, you must be crawling between gusts and gusts will have you on all four clinging to the ground. If the gusts are especially vicious you'll risk being blown off your feet, and all these other horror stories. "

"personally i would go out in 50mph but that is getting towards the upper limit- its meant to be enjoyable.

you probably could struggle along in 70-80 mph but could you cover the ground you need to in the time you need to?"

Edited June 29, 2015 by KerikBalm

[andrewas]

So, it takes 24 months for the MRV to fuel itself The Ares 4 MRV was landed on Sol 0 or -1 of Ares 3's mission.

About 570 sols later or so, the Ares 4 MRV is not only topped off but overcharged to get more delta V out of the mission.

That math seems off.

The MAV is landed by the previous mission so that there's a human pilot involved, not because it needs all that time to make fuel. Also, hydrogen boiloff means they probably designed it to make methane more quickly than that. The MAV can replace LOX easily but has no source of additional hydrogen.

Also, Watney brings solar cells with him and may have been able to amp up the fuel plant a bit.

[Albert VDS]

So, it takes 24 months for the MRV to fuel itself The Ares 4 MRV was landed on Sol 0 or -1 of Ares 3's mission.

About 570 sols later or so, the Ares 4 MRV is not only topped off but overcharged to get more delta V out of the mission.

That math seems off.

It's based off the ERV from the Mars Direct plan, which (IIRC) would make all it's propellant(and more for the rovers) within 6 months.

[Jesrad]

Not sure why you are all arguing about wind drag possibly toppling the MAV, IIRC the book explains that the problem of the dust storm is not so much the force of the wind, but rather the abrasion from the 400 mph dust.

[KerikBalm]

I seem to recall otherwise, I guess I'll have to go back and look again... but I specifically remember that they had to use RCS to stop it from tipping over, and that the tipping issue is why they couldn't wait any longer

[Albert VDS]

The tipping is exactly why they couldn't stay any longer.

[AngelLestat]

Gravity may be lower, but things anchored into the ground aren't held in place just from gravity.

A 400km/h wind is not normal, I will said that is very rare, when you work all days in calm enviroment you tend to lower the guard.

Maybe things are anchored but as Jesrad said there is a thing called abrasion, which can set free objects, there is always things out of your control.

More in chaotic scenaries like this.

Also a large heavy object, like the fully fueled MAV, will have more mass per cross sectional area.

Whether or not you can walk around in the storm is not really relevant. I'm not talking about the scenes in the movie trailer, I'm talking about the events in the book.

In the book they didn't leave because it would be hard to walk around, they aborted because the full fueled Mars Ascent Vehicle would topple over due to the wind...

That seems to be unlikely to me... certainly something that it should have been designed to withstand.

More mass against what object? A human? no even close.

The human body has a density close to the water.. A space vehicle even if the 100% of the vehicle volume is full of fuel, it would not have the same density than a human, fuels are lighter than water.

And a better estimation for a mars vehicle points to 15% of the volume would be fuel (in case your transfer vehicle is in orbit).

So yeah, that is a very low density vehicle, which nobody would include in the design "safe to launch on 400km/h winds", because had that case into account would make the vehicle very inefficient.

Math is good, math is great... but math is a model, and you must test models against reality.

Your model tells me that people wouldn't be able to walk around in 40 mph winds here on earth, and I know that is not the case.

You was very sure under your own calculations, now that more accurate calculations show you wrong, the results needs to be ignored?

And again, being in asphalt and without spacesuit is not the same.. And the Beaufort scale description match.

Note that any coefficient of friction of less than 0.5 implies that it will start sliding down a 30 degree slope.

Thus... using your numbers, we'd conclude that the guy can't walk up a 30 degree incline on mars either...

30 degrees is a lot.

Let me remind you:

Also not sure if the force distribution between the box and the angle of the person standing is equal.

But I can said for sure that if you are on a dust ground in that inclination you go down.

Of course shoes have tread, and that equation is assuming two flat surfaces... etc...

Grass and tire are 2 flat surfaces?? please....

Also, in mars all water that might be under the dust is freeze.. You have that into account too?

I can give you 0.4 in the coefficient, that stills makes my point. Or even higher if the static friction over a short time is overcome and becomes kinetic friction.

I did.

Lower gravity reduces the force of static friction by 2.66x

That means the required wind force to be "equivalent" on earth is 2.66x... thats why I was comparing it to winds on Earth sqrt 2.66x.

Which is wrong like I point. Call someone good with physsics, he will choose what is the best propper way to solve this.

So then really, the real problem is the actual atmospheric density of mars during the daytime(? IIRC, it happened during the day) at the equator.

Mars can actually get relatively warm at the equator at midday... and that lowers air density.

It would be much denser at night, when it gets much colder.

haha, you are really desperate to change the values more into your favor. Why is so difficult to accept that your first "reality" concern was not so accurate as you thoght in the begining?

http://www.walkhighlands.co.uk/Forum/viewtopic.php?f=1&t=24814

Here's a topic about the wind speed people seem to be able to walk in. Its not exactly a scientific article, but...

"Wind bothers some more than others. 20-30 is kind of strong breeze and a bit more. 50-60 is hard to hear in but still possible to walk into. You may find yourself leaning into it. It is tiring to walk into and of course excellent to walk behind. Side-on is a psychological nightmare!

Yeah, I imagine they use all kind of instruments to measure the wind speed, with asphalt under their feets and without astronaut suit, also with all kind of weird positions. Yeah we should take into account this instead an international wind scale.

As for possible: the worst you can really function in is 100-120 mph. At that point, you must be crawling between gusts and gusts will have you on all four clinging to the ground. If the gusts are especially vicious you'll risk being blown off your feet, and all these other horror stories. "

You dont have any idea what are you talking about.. that wind speed is terminal velocity. So you need to counter your same weight with only your feets on the ground.

[KerikBalm]

And a better estimation for a mars vehicle points to 15% of the volume would be fuel (in case your transfer vehicle is in orbit).

Ridiculous

You was very sure under your own calculations, now that more accurate calculations show you wrong, the results needs to be ignored?

The calculations are no more accurate, they are just different ways of arriving at a number, with guesses as to the coefficients.

I am assuming the similar coefficients on mars and earth to get comparable wind strengths, whereas you are pulling out coefficients that mean one can't walk up a 30 degree incline.

One must check their theory against the real world whenever possible.

It is possible to check your numbers against the real world. Your numbers fail that test.

And the Beaufort scale description match.

A scale meant for people working on a ship with a heaving deck and considerable spray... and it only briefly says walking around would be difficult

30 degrees is a lot....But I can said for sure that if you are on a dust ground in that inclination you go down.

LOL....

Again demonstrating a lack of real world experience.

Also, in mars all water that might be under the dust is freeze.. You have that into account too?

I can give you 0.4 in the coefficient, that stills makes my point. Or even higher if the static friction over a short time is overcome and becomes kinetic friction.

So... you won't give any number that allows a person to stand on a 3 degree slope.... Ok then.... I can see this discussion will go no where

haha, you are really desperate to change the values more into your favor. Why is so difficult to accept that your first "reality" concern was not so accurate as you thoght in the begining?

I'm just taking values that agree with actual experimentation.

People can stand on gravel/sand on a 30 degree slope.

People can walk in 50 mph winds

You are really desperate to get your made up numbers to work...

You dont have any idea what are you talking about.. that wind speed is terminal velocity. So you need to counter your same weight with only your feets on the ground.

Terminal velocity in spread eagle position... nobody was talking about being in that position... its crawling along the ground...

I, and many other people, do carry portable things to measure windspeed, and if you notice, they are talking about walking a trail in the scottish highlands... mountain ridges and such... not asphalt.

Yet more real experience about walking in strong winds - but a bit more realistic with the 75mph limit (granted the 100+ claim was down on all 4s clinging to the ground)

http://www.summitpost.org/phpBB3/judging-wind-speeds-t62760.html

Edited June 29, 2015 by KerikBalm

[peadar1987]

...

On the 40kt day, we were racing in a 4.2m dinghy, sailed against the wind on the way back to the club for ~40 minutes, pulled the boat against the wind up the slipway into the dinghy park with the sails still up, and then derigged it. Was it tough? Sure. Did we get dragged back down into the sea? Nope. A few weeks ago, I was sailing in 35kts in a Sonar. The bowman was able to walk forwards and backwards to the genoa just fine.

I've done some digging into the drag of a human. Average frontal area when standing is about 0.6-0.7m^2. Average drag coefficient is abut 1.1 or 1.2, giving you an upper estimate of 0.84 for area*drag, which is the same figure as your source. However, this is only true for relatively low windspeeds, below about 15 m/s. At higher windspeeds, the flow becomes turbulent, and the flow better follows the contours of the body, reducing drag coefficient from ~1.2 to ~0.3 fairly abruptly. This is the reason dimpled golf balls fly further than smooth ones, higher Reynolds numbers tend to lower drag. 400km/h winds will have huge Reynolds numbers.

This means you only get a fraction of the expected force (25-30%), which means you can withstand approximately double the windspeed you would otherwise expect.

(Turns out Randall Munroe has looked into this as well, and come up with pretty similar conclusions: https://what-if.xkcd.com/66/)

[KerikBalm]

http://onlinelibrary.wiley.com/doi/10.1256/wea.29.02/pdf

Beaufort scale of 8-10... lean into the wind = 2.9 to 5.7 degrees....

This reminds me of that old joke about how they used to calculate that it was impossible for a bumblebee to fly.

[AngelLestat]

Ridiculous

Ok a mental mistake 60% of the ship is fuel.

One must check their theory against the real world whenever possible.

It is possible to check your numbers against the real world. Your numbers fail that test.

No it doesn´t, you are taking examples here on earth that are very different from the ones set in the calculations and conditions.

A scale meant for people working on a ship with a heaving deck and considerable spray... and it only briefly says walking around would be difficult

It said seriously impeded. And it will be similar to the mars ground in a storm where you can not see much with the spacesuit addiction.

Good luck returning to your ship if you are far.

LOL....

Again demonstrating a lack of real world experience.

That is because you can not separate your experiences from real test.

You might remember stand on a ground ramp without fall, but you dont have into account that natural inclined planes are not perfect, they always had small horizontal parts or salients where you can put your foot bending your finguers which would count as an horizontal plane. We do all that instinctively, so your inclined plane example, just exploit from false experiences.

The same that how much you can stand in the wind, without spacesuit or with asphalt under you and in a weird posture (not walking) and without different gusts. Just constant wind.

Terminal velocity in spread eagle position... nobody was talking about being in that position... its crawling along the ground...

Is not what the trailer shows.

I, and many other people, do carry portable things to measure windspeed, and if you notice, they are talking about walking a trail in the scottish highlands... mountain ridges and such... not asphalt.

Yet more real experience about walking in strong winds - but a bit more realistic with the 75mph limit (granted the 100+ claim was down on all 4s clinging to the ground)

http://www.summitpost.org/phpBB3/judging-wind-speeds-t62760.html

We are making calculations.. use the metric system.. Why I need to do conversions? Is not my fault. I am already using a languege that is not easy for me.

Also, those are your sources? some kids with not real measuments or parameters in the wind? I just tell you one thing.. is super hard to walk on winds with more than 100 km/h without using your hands to grab. And I mean walk.. not crawl.

On the 40kt day, we were racing in a 4.2m dinghy, sailed against the wind on the way back to the club for ~40 minutes, pulled the boat against the wind up the slipway into the dinghy park with the sails still up, and then derigged it. Was it tough? Sure. Did we get dragged back down into the sea? Nope. A few weeks ago, I was sailing in 35kts in a Sonar. The bowman was able to walk forwards and backwards to the genoa just fine

There are not the accurate experiences that I am looking for.. in your 4.2m dinghy or even in your Sonar, none of those has an anenometer, I imagine you check the wind speed with an app in your phone connected to a webpage with data transfer from a no so close beacon. And personally I cant believe you did that in just small sail without using your hands (I cant even do it in 10m sailboat).

I've done some digging into the drag of a human. Average frontal area when standing is about 0.6-0.7m^2. Average drag coefficient is abut 1.1 or 1.2, giving you an upper estimate of 0.84 for area*drag, which is the same figure as your source. However, this is only true for relatively low windspeeds, below about 15 m/s. At higher windspeeds, the flow becomes turbulent, and the flow better follows the contours of the body, reducing drag coefficient from ~1.2 to ~0.3 fairly abruptly. This is the reason dimpled golf balls fly further than smooth ones, higher Reynolds numbers tend to lower drag. 400km/h winds will have huge Reynolds numbers.

This means you only get a fraction of the expected force (25-30%), which means you can withstand approximately double the windspeed you would otherwise expect.

mmm, not sure about that. But I will not make judgments yet.

The only thing that I can see that if I try to calculate the terminal velocity with 0.84 of CdA, and pressure equal to 1000 meters (sky diver) I exceed a little bit from the mentioned in books.

But not sooo much to calculate the 30% of that.

I can see that the pdf you post, does not calculate the lift force from the lean angle which reduce the friction coefficient.

You have another source that explain me that change in the Cd?? I did and read many times drag calculations and I never saw it.. I understand that happens, but not with such change.

[peadar1987]

There are not the accurate experiences that I am looking for.. in your 4.2m dinghy or even in your Sonar, none of those has an anenometer, I imagine you check the wind speed with an app in your phone connected to a webpage with data transfer from a no so close beacon. And personally I cant believe you did that in just small sail without using your hands (I cant even do it in 10m sailboat).

The dinghy event was this one: http://www.bbc.co.uk/news/uk-northern-ireland-28745787 . Here's a sequence of me even going for a gybe in it (although the wind was closer to 30kts at that stage): https://www.flickr.com/photos/gp14world/14783002368/in/album-72157646574273156

The 60mph figure (Which is 54 knots) is from a masthead anemometer on the committee boat. 40kts was from a deck-level anemometer on the lifeboat which was called out.

The Sonar had a Windex at the top of the mast, which topped out at 36 knots when we were on the water. The wind was increasing as we moved of the boat onto the shore, but I was still able to carry a sail along the pier back to the clubhouse.

mmm, not sure about that. But I will not make judgments yet.

The only thing that I can see that if I try to calculate the terminal velocity with 0.84 of CdA, and pressure equal to 1000 meters (sky diver) I exceed a little bit from the mentioned in books.

But not sooo much to calculate the 30% of that.

I can see that the pdf you post, does not calculate the lift force from the lean angle which reduce the friction coefficient.

The lift force isn't going to be particularly great. The human body isn't an efficient airfoil. Add to that the fact that if you lean forward, you're reducing your frontal area to the wind, and I doubt it makes much of a difference.

Just to run the maths on it, if you lean forward 45 degrees, your frontal area is now sin(45), or 0.707 what it was before, so Fnew=0.707*F. If you assume that the drag force is all from stagnation pressure on your leading edge, then it will have a lifting component of Fnew*sin(45) and a dragging component of Fnew*cos(45), your weight will be reduced by 29.3%, but the dragging force will be reduced by the same factor, cancelling each other out. You do get slightly more lift at lower angles, but this is a very conservative case. In reality not all drag from a human is bluff body drag, especially at higher windspeeds, and people don't generally lean forward as if on a hinge, they tend to hunker and squat, bending their knees.

You have another source that explain me that change in the Cd?? I did and read many times drag calculations and I never saw it.. I understand that happens, but not with such change.

Here it is for a cylinder:

It drops from ~1.6 to about ~0.3 at a Reynolds number of about 3*10^5.

Since Reynolds number=(D*u)/Mu, for air at atmospheric pressure (Mu=0.00001568), and a D of 0.5 for the trunk diameter of a human, this transition occurs at about 9.4 m/s (or 19.2 kts), sooner for thinner people or on warm days. Obviously humans aren't actually cylinders, but it's a reasonable order of magnitude estimation.

Edited June 30, 2015 by peadar1987

[KerikBalm]

No it doesn´t, you are taking examples here on earth that are very different from the ones set in the calculations and conditions.

Yet some of those are the same... If one takes your calculations, and substitutes Earth gravity and Earth's air density, one gets results that don't agree with observations.

It said seriously impeded. And it will be similar to the mars ground in a storm where you can not see much with the spacesuit addiction.

Good luck returning to your ship if you are far.

But he wasn't far... none of them were. And again, the real problem in the book (stop assuming the trailer is giving you a complete idea of what is going on), was that the ascent vehicle was about to tip over.

That is because you can not separate your experiences from real test.

Its not just my experiences, and its easy to do a real test... fill a box with gravel... put a shoe on top of it, tilt the box to 30 degrees... does the shoe fall off?

No.

You might remember stand on a ground ramp without fall, but you dont have into account that natural inclined planes are not perfect, they always had small horizontal parts or salients where you can put your foot bending your finguers which would count as an horizontal plane. We do all that instinctively, so your inclined plane example, just exploit from false experiences.

Likewise, when walking on mars, its not two flat surfaces where all points of contact are in a plane, as those equations assume... the ground is uneven, and that will increase the traction one can get. You can dig your feet into the sand a bit with each step... etc etc.

Is not what the trailer shows.

How is the trailer even relevant in that case?

We are making calculations.. use the metric system.. Why I need to do conversions?

Fine, no conversion to mph, some people are more familiar with that, so I list both.

Also, those are your sources? some kids with not real measuments or parameters in the wind? I just tell you one thing.. is super hard to walk on winds with more than 100 km/h without using your hands to grab. And I mean walk.. not crawl.

Kids? I think people whose hoppy is to go out hiking in scotland are probably middle aged or old... and I, like many others, do often carry instruments for measuring wind speed.

Also... no you're claiming super hard to walk in winds of 100 km/h... by your equations... it should be impossible using earth's gravity and atmospheric denisty values... and impossible at less than 1/4 the wind strength... something is clearly off there.

Also, I want to come back to your statements:

Fs= μmg

...

μ= coheficient of static friction (0.3 dust terrain)

....

Friction Force= 88 Newtons

Force of the wind= 101 Newtons.

Conclusion: weeeeeeeee bye bye..

and

Even if we choose 0.4, we are in the limit...

I can give you 0.4 in the coefficient, that stills makes my point.

The relationship to static friction is linear with the coefficient of static friction.

0.4/0.3 = 1.33333

1.333 * 88N = 117N

Friction force = 117N

Force of the wind= 101 Newtons.

Conclusion: you're staying put without even having to hunker down and reduct your cross sectional area...

I don't think your numbers are reasonable... a little increase... and one arrives at the conclusion that one can walk around in that wind

[AngelLestat]

The dinghy event was this one: http://www.bbc.co.uk/news/uk-northern-ireland-28745787 . Here's a sequence of me even going for a gybe in it (although the wind was closer to 30kts at that stage): https://www.flickr.com/photos/gp14world/14783002368/in/album-72157646574273156

For one side, you are saying that the wind is nothing and you can walk without problem. For the other side was a event who cause considerable caos and it was in the news.

The 60mph figure (Which is 54 knots) is from a masthead anemometer on the committee boat. 40kts was from a deck-level anemometer on the lifeboat which was called out.

The Sonar had a Windex at the top of the mast, which topped out at 36 knots when we were on the water. The wind was increasing as we moved of the boat onto the shore, but I was still able to carry a sail along the pier back to the clubhouse.

Ok, the windex with anenometer in such small sailboat seems weird, but if you had the committee boat I will shut my mouth.

The lift force isn't going to be particularly great. The human body isn't an efficient airfoil. Add to that the fact that if you lean forward, you're reducing your frontal area to the wind, and I doubt it makes much of a difference.

You have 2 ways of lift, one is due the airfoil shape, and the other is just by angle toward the wind, like a normal kite (there is not airfoil shape there, also if it would, does not help in nothing).

Just to run the maths on it, if you lean forward 45 degrees, your frontal area is now sin(45), or 0.707 what it was before, so Fnew=0.707*F. If you assume that the drag force is all from stagnation pressure on your leading edge, then it will have a lifting component of Fnew*sin(45) and a dragging component of Fnew*cos(45), your weight will be reduced by 29.3%, but the dragging force will be reduced by the same factor, cancelling each other out. You do get slightly more lift at lower angles, but this is a very conservative case. In reality not all drag from a human is bluff body drag, especially at higher windspeeds, and people don't generally lean forward as if on a hinge, they tend to hunker and squat, bending their knees.

45 degrees is an impossible angle to keep. The wind force is not constant and you still need to walk, and as I said, that position just help you to not tilt over.

Bending knees does not help for get equilibrium, something you really need.

Here it is for a cylinder:

http://www.esru.strath.ac.uk/EandE/Web_sites/13-14/Jacket_Substructures/jackets/images/Shape_CoefficientC.jpg

It drops from ~1.6 to about ~0.3 at a Reynolds number of about 3*10^5.

Ok, this is what I am interested, because yeah, it may add considerable errors in my calculations.

That is just a function of CD with respect to Reynold, but I am not sure that is in Air.. I guess that graph is for water...

Since Reynolds number=(D*u)/Mu, for air at atmospheric pressure (Mu=0.00001568), and a D of 0.5 for the trunk diameter of a human, this transition occurs at about 9.4 m/s (or 19.2 kts), sooner for thinner people or on warm days. Obviously humans aren't actually cylinders, but it's a reasonable order of magnitude estimation.

reynolds and cd calculations is a nightmare, the only way to get a rought number is with special software and computer power, that is why the only good parameter is in a wind tunnel.

The only way to prove partially if you are right and in what porcentage, it is knowing real data on terminal velocity (last 300 meters to keep the air density constant) and then correlate those numbers with your new Cd estimation in case is turbulant.

Of course in mars the density and pressure is very low, so not sure the speed needed to become turbulent and how much it will change with our original value.

Is not something that I prentend to investigate, but in case someone does, I will read it with pleasure.

So in this moment I accept that my values may or may not be accurate.

But he wasn't far... none of them were. And again, the real problem in the book (stop assuming the trailer is giving you a complete idea of what is going on), was that the ascent vehicle was about to tip over.

Ok, in this case we need to pretend that is not firmly fixed (nailed) to the ground. In my opinion it can happen, is less dense than a human and it does not do postures to counteract the wind.

Its not just my experiences, and its easy to do a real test... fill a box with gravel... put a shoe on top of it, tilt the box to 30 degrees... does the shoe fall off?

No.

full grave does not work, because your foot sinks to reach horizontal position. Is more like a solid ground with dust (small stones or dirt of different size).

Likewise, when walking on mars, its not two flat surfaces where all points of contact are in a plane, as those equations assume... the ground is uneven, and that will increase the traction one can get. You can dig your feet into the sand a bit with each step... etc etc.

You cant see, and the surface is kinda flat but uneven. How that helps you?

by your equations... it should be impossible using earth's gravity and atmospheric denisty values... and impossible at less than 1/4 the wind strength... something is clearly off there.

But you always forget of the spacesuit estimation in the Cd, also the surface.

The relationship to static friction is linear with the coefficient of static friction.

0.4/0.3 = 1.33333

1.333 * 88N = 117N

Friction force = 117N

Force of the wind= 101 Newtons.

Conclusion: you're staying put without even having to hunker down and reduct your cross sectional area...

I don't think your numbers are reasonable... a little increase... and one arrives at the conclusion that one can walk around in that wind

?? You think that 6 newtons of force will keep you in constant safe of not fall? Where are you walking? In a wind tunnel with a perfect feet cordination and a uniform surface?

As I said to peadar.. Maybe I was wrong with the Cd due turbulant, but we can not know that, is too hard to calculate and we dont have that kind of mars data, not even for its surface.

But the wind effect is considerable.. is not something to said "ohh how could the book being so wrong!!".

I love when books or movies have accurate physsics effects or even try to accomplish those. So not sure why you want so badly that his is wrong with their assumptions.