Slashdot Mirror
Flying a Cessna On Other Worlds: xkcd Gets Noticed By a Physics Professor
djl4570 writes "xkcd's 'What If' series consists of humorous takes on highly implausible but oddly interesting hypothetical physics questions, like how to cook a steak with heat from atmospheric re-entry. The most recent entry dealt with flying a Cessna on other planets and moons in the solar system. Mars: 'The tricky thing is that with so little atmosphere, to get any lift, you have to go fast. You need to approach Mach 1 just to get off the ground, and once you get moving, you have so much inertia that it’s hard to change course—if you turn, your plane rotates, but keeps moving in the original direction.' Venus: 'Unfortunately, X-Plane is not capable of simulating the hellish environment near the surface of Venus. But physics calculations give us an idea of what flight there would be like. The upshot is: Your plane would fly pretty well, except it would be on fire the whole time, and then it would stop flying, and then stop being a plane.' There are also a bunch of illustrations for flightpaths on various moons (crashpaths might be more apt), which drew the attention of physics professor Rhett Allain, who explained the math in further detail and provided more accurate paths."
It is a cessna engine, it doesn't run on air but on money.
i thought once I was found, but it was only a dream.
"plane would fly pretty well, except it would be on fire the whole time"
I think Boeing has a plane that meets part of the criteria already.
... but I think it went over his head.
In the What-if it's explicitly stated that the gas tanks have been replaced with batteries and had an electric engine installed.
It would be an X-Plane!
Any sufficiently unpopular but cohesive argument is indistinguishable from trolling.
If your plane is on fire and not a plane anymore then you are having a bad problem. You will not fly on Venus today.
In the What-if it's explicitly stated that the gas tanks have been replaced with batteries and had an electric engine installed.
And also that this means that the plane won't fly for very long anyway, around 10 minutes. Not that this is a particular issue on many of the worlds of the solar system (unless you can also make the Cessna acid-proof, which would help a lot for Venus).
It's just what-iffery.
"Little does he know, but there is no 'I' in 'Idiot'!"
This is covered in the simulations as well. Is there something in particular preventing you from reading it?
Yeah, that sentence was for Venus.
And what is good, Phaedrus, And what is not good... Need we ask anyone to tell us these things?
This is covered in the simulations as well. Is there something in particular preventing you from reading it?
Although I am not the poster you asked this question of, I have to admit not ever reading xkcd, having more important things on my Kindle.
Having left my e-ink display in the car, I read through what-if and if nothing else, the penny exercise had me laughing out loud. Tough to force on a rocket scientist with humor less moist than a block of dry ice, but it happens.
Thanks to / for not posting a slashvertisement and giving me the giggles.
When the foot seeks the place of the head, the line is crossed. Know your place. Keep your place. Be a shoe.
Well, not without a little hacking. Just pipe oxygen into the intake. But with that little atmosphere you have more problems, like how to get lift.
Free Martian Whores!
Load liquid oxygen into the fuel tanks. Methane comes into the engine from the atmosphere. An engine with minor modifications might be made to operate.
http://michaelsmith.id.au
Mach 1 is the speed of sound - in that medium.
I want to delete my account but Slashdot doesn't allow it.
> I have to admit not ever reading xkcd, having more important things on my Kindle.
It publishes 3 strips a week, plus a what-if from time to time. It's not a book, or anything else which would compete with whatever's on your kindle for your attention, unless you're a very, very slow reader.
The xkcd page said that, not the professor. I don't know if Randall meant Mach 1 on earth at some altitude, or Mach 1 on Mars. I just took it to mean "too fast for a Cessna."
Very first tile in illustration: rip out engine, install batteries and electric motor.
RTFA
Although I am not the poster you asked this question of, I have to admit not ever reading xkcd, having more important things on my Kindle
Like slashdot?
no taxation without representation!
As a rocket scientist, perhaps you might get a chuckle out of this xkcd: http://xkcd.com/1133/
This is helped somewhat by the higher density of the martian atmosphere, in relation to its pressure. The density is pushed up by the low temperature and the higher density of carbon dioxide. OTH Mars is quite windy so your vehicle will get blown around quite a bit which could create hazards.
http://michaelsmith.id.au
> I have to admit not ever reading xkcd, having more important things on my Kindle.
It publishes 3 strips a week, plus a what-if from time to time. It's not a book, or anything else which would compete with whatever's on your kindle for your attention, unless you're a very, very slow reader.
Or unless you bought the Humble eBook Bundle back in October.
Correct. Also, it's important to point out that the Mach number of a vehicle is a local measure of vehicle speed. As the speed of sound varies with temperature, and thus altitude, you'll find that two vehicles having the same trace ground speed but that are flying different altitudes will be at different Mach numbers. Acoustics and aerodynamics are fun.
Name: Mr. Anon E Mouse; SSN: 555-55-5555
He used a simulator for Mars that accounts for things like the effects of density differences on prop thrust, son.
What exactly are you trying to prove by refusing to read the article?
Not to mention when he says that Venus' upper atmosphere is "room temperature" - duh! rooms on Venus would have a very different temperature from Earth's rooms! What and idiot.
sic transit gloria mundi
You can't hand-waive away physics.
Sense of humor, on the other hand, is commonly waived.
FTFA: The motor is electric, and the fuel tanks are replaced with Li-Ion batteries. But I'll give you style points attempting to stifle scientific hypothetical inquiry and outside-of-the-box thinking with cynical non-imaginativism. Keep it up and you might win the scientific curmudgeon of the year award!
Icebike is proving what I have previously pointed out about him. It is not important to him that he knows what he is talking about. Knowing what you are talking about is hard.
"His name was James Damore."
What's that based off of? Seems familiar.
For large sets, this will be our guide even unto death, for the LORD will work for each type of data it is applied to...
They also didn't point out that if attempting to fly in the Sun's atmosphere, you may last longer if you do it at night. :P
This space unintentionally left blank.
Um, did you read the what-if it takes atmosphere into account and on each planet the plane is magically launched from a reasonable height (it does not take off)
null
This physicist has been reading xkcd for quite some time, actually. He has written at least one other article about it, namely the click-and-drag world.
http://www.wired.com/wiredscience/2012/09/how-big-is-the-xkcd-click-drag-world/
Letter To Iran
The temperature and composition of the gas are entirely sufficient to calculate the speed of sound. The previous poster is entirely correct.
Yes, density has a bearing on velocity -- not of sound, but of the vehicle -- because it affects drag.
You must be great at parties.
Dude, read the full article. Seriously.
> I have to admit not ever reading xkcd, having more important things on my Kindle.
It publishes 3 strips a week, plus a what-if from time to time. It's not a book, or anything else which would compete with whatever's on your kindle for your attention, unless you're a very, very slow reader.
The bigger problem is that Friday's comic was number 1168, so if you've only just started reading now you have a lot of catching up to do. Then half way through you'll realise that if you hover the mouse over the picture some additional text pops up so you'll have to go all the way back and start again[1]. Then you need to read the blag to figure out what all the references to cancer are about.
Most of the comics can be fully enjoyed in 30 seconds or less, but some require a bit more effort...
The What-If's come out once a week and also require a bit more attention but there's only a handful of them so far.
[1] I don't know how to get hover text on my Samsung Galaxy S2... maybe kindle's can't get to it either?
Yes, you would need a rocket Cesna! Load up with LOX and rocket fuel and away you go, may need looong runway and perhaps a ski-jump to get airborne!
One of Einstein's what-ifs when something like this. Light moves at the same speed for all observers. So imagine a space ship travelling at near the speed of light relative to Observer A. Observer B is in the space ship and shines a laser straight up and back down off of a mirror. To Observer A the laser appears to move along the top of a triangle as the ship goes by at near light speed. Observer A sees the light travel a greater distance than Observer B. Both observers see light traveling at 299 792 458 m / s. So the only way this can be true is if time is moving slower for Observer B than A.
Feel free however to point that we don't know how to make a space ship go that fast. Point out the difficulty in actually observing such an event. Deride imagination.
look for the xkcd comic on up goer 5
"Cursed is he who rises early in the morning..." Isiah 5:11
Congratulations!
You seemed to forget the entire point of XKCD's what-if series is, in fact, taking childish daydreams and running with it. It's a bit odd, anyways, that a person who (begin rant) thinks a COTS laptop, in a shielded cabin in a magnetosphere-shielded environment using a tiny node size is every bit as radiation-hardened as a RAD750 with a 150nm node size to reduce susceptibility to smaller particles, with latchup-proof logic, parity-checked memory, etc etc. (end rant) is behaving as a physics expert to begin with.
Woosh!!!!! This low flying humor clearly gets no propulsion in all that hot air your blowing.
A fool throws a stone into a well and a thousand sages can not remove it.
Flight does not require propulsion when gravity is pulling you to the center of a planetary body. It only requires lift. The examples all clearly state that the plane is dropped from a great height.
A fool throws a stone into a well and a thousand sages can not remove it.
I really want that on a t-shirt... I'm a big guy, so it should work well on a 4X
Michael J. Ryan - tracker1.info
http://m.xkcd.org/ is a better version for mobile. The title below the comic has a clickable superscript (alt text) link that will display the alt text underneath.
You will not fly on Venus today.
I don't think your plane would actually catch fire. Melt, yes. But combust? Not enough oxygen in Venus' atmosphere.
Have gnu, will travel.
If that professor wants to pick nits with xkcd, the path an object follows while falling in a vacuum isn't a parabola. Its an ellipse. In most cases, the ellipse intersects the surface of the body being orbited in what is typically referred to as a crash. But if one is considering dropping the object (with some forward velocity) above a small enough body, the distinction becomes important.
Have gnu, will travel.
It is explained that on one world, you burn then crash - as opposed to crash and burn - and why it would happen in that order. And, on another world, you would crash, but not burn, and why.
This little "what if" is a reasonable explanation of conditions on other worlds, as we understand them, and how they would affect flight in a particular type and model of aircraft.
If the story teller were addressing an international physics conference, he might sound a bit stupid with this presentation. As he is addressing an audience of nerds, with the intent of amusing and possibly educating them - he's done an excellent job.
"Windows is like the faint smell of piss in a subway: it's there, and there's nothing you can do about it." - Charlie Br
I was thinking more just ORBIT. You first have to ask yourself what is the height required for the mass to orbit? (this is assuming you can start the plane out at any given direction, and you know the mass of the plane)
After that, anything starting slower, lower, or weighing more will need to do some sort of powered flight to stay up.
If you start a little bit below orbital requirements, the demand will be very minor. The farther down you go, the slower you start, or the heavier you are, the more demand there will be. It's not a yes/no thing. It could be a system just off equilibrium.
Unless you have the scenario they described with the sun, where you don't hit atmosphere until you are well inside orbit (unless you have some impressive speed)
So I guess what I'm saying is that the starting conditions (speed, heading, elevation) are at least as important as the planet you are trying to fly at. Since they somewhat arbitrarily picked them, the resulting comparisons are equally arbitrary. And not by an insignificant amount.
That being said, revisit the starting conditions. If we say you must start with your speed, heading, and elevation, so that you are say, 10% below orbital speed, it becomes a question of atmosphere - "can you sustain flight?", rather than "can you pull out in time?". Can you generate enough lift to achieve... not sure what to call it... stable sub-orbital trajectory? I suppose that's the best definition of "powered flight". aka "straight and level", with an "at stable speed" thrown in for good measure.
Numbers become more critical as your ability to generate lift is lowered, or as that 10% below orbit is raised. At some point it will become a question of whether or not you have time to pull out of the dive. (or whether it's even possible, assuming there's no ground) And then we get into what your "ceiling" would be - the highest altitude you can climb to, where you finally level out while trying to climb, equilibrium of climb. It's interesting to ponder that most craft have TWO ceilings... one is their orbit, and the other is somewhere below that. So what we may really be asking here is, "does the craft have a ceiling other than orbit?"
Although that article does dig a bit deeper than xkcd did, it's still quite a long way from the goal.
I work for the Department of Redundancy Department.
Wow, you mean he's wrong and the Cessna would fly awesome and not just fall to the ground?
Glad we had you here to set things right. I'm going to get started on my plan to fly to Mars!
If I have been able to see further than others, it is because I bought a pair of binoculars.
"And get propulsion from a prop in an atmosphere of .6 that on earth?"
There're two things to consider:
1) Of course you get prop: it's a rotating wing, isn't it? So as long as there's any atmosphere, you'll get propulsion. Maybe your question was not about "propulsion" but about "enough propulsion", which gets us into point two.
2) Who said that "enough propulsion" needs to be produced exclusively by the main rotor? In the experiment another quite porwerful prop source is included: gravity. You just take even a pig at 30 Km over the Martian surface and you'll see how it gains speed even without revolving its pig tail.
The thougth experiment was not about flying a Cessna in Mars (and other objects) but about *how* it would *try* to fly over there. See, for instance, in Jupiter it would crush, but it wouldn't crash.
Drop the steak from orbit, it's the only way to be sure.
I love xkcd :)
Great idea! I'm also a big guy, but it might fit me best if I were to lay it out horizontally...):
Wow-- I just noticed this-- I got linked!
(at the pdf report linked at the words "...The acid's no fun, but it turns out the area right above the clouds is a great environment for an airplane" in the Venus section)
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20030003716_2002108457.pdf
http://www.geoffreylandis.com
Like I said - in that medium. Except you wrote something wordier so you gathered more mods.
I want to delete my account but Slashdot doesn't allow it.
First, if you are going to say that multiple times, maybe you should get it right, as it is 0.6%, not 0.6 of the atmosphere.
Second, propellers still work in such an atmosphere, just not well. When NASA was considering propulsion methods for a powered aircraft on Mars, it came down to a choice between propeller based or rocket based. The former expected to give 3-5 times the range of using rockets if powered by an internal combustion engine (requiring both fuel and oxidizer to be carried). In one particular possible case, it was a range of about 2000 km.
Of course, this is of no relevance to what was in the article being discussed, where gravity was described as more important, but I guess the article wasn't relevant to what you said either.
He is not a reader, he has a kindle.
In a non related issue:
Did anyone notice US teachers have begun 'boycotting' proficiency testing?
No brain, no pain.
http://m.xkcd.org/ is a better version for mobile. The title below the comic has a clickable superscript (alt text) link that will display the alt text underneath.
Awesome. Tanks for the tip. I've now changed my bookmark :)
On a drunken beerday many many years ago, i postulated that a cessna flung at .99 c (just under the speed of light) striking the earth would probably destroy it.
I was ridiculed, maybe rightly, but if we're running physics and math here, what would the effect be?
_ _ _ Go for the eyes Boo! GO FOR THE EYES!
Our Cessna 172 isn’t up to the challenge. Launched from 1 km, it doesn’t build up enough speed to pull out of a dive, and plows into the Martian terrain at over 60 m/s (135 mph). If dropped from four or five kilometers, it could gain enough speed to pull up into a glide—at over half the speed of sound.
At no point does he claim the plane achieves propulsion, in fact he says exactly the opposite. Remember that the word glide means that the cesna does not achieve powered flight, and "launched from 1 km" means that it is already in the air when it falls and hits the ground, completely realistic given the terms of the scenario and detailed enough for the context.
null
Tanks for the tip.
Another tip: xkcd comic usually get a transcript (which helps as an explainer) after a day or two.
Can I get a Tank too? I am not picky, though I wouldn't mind a nice little Sherman :D
I am an ACCA student. Got a query on Accountancy/Finance? Maybe I can help!
I love this series. The scenarios that he works out are so absurd it's hard not to be laughing the whole time while reading (and visualizing) Randall's explanation. I had a hard time keeping it together while imaging a giant rain drop dropping down on one house or imaging someone dropping a steak from space for the purpose of cooking.
The speed of light is constant and cannot be exceeded, therefore
By implication, we must have time dialation depending on frame of reference
We can work out how much we would expect that time dialation to be
We have a testable hypothesis that could potentially be disproven by experiment on board Concorde or another fast aircraft.
I consider it a bad trend to make separate mobile web pages. The same link will be used by desktop and mobile users, but no matter what page you link to, it will be the wrong page for someone. Instead the page should display correctly for both desktop and mobile; if this is not possible with a common HTML file, just serve different files depending on whether it was accessed from a desktop or mobile browser.
The Tao of math: The numbers you can count are not the real numbers.
Nope, we could just fly it there. A C172, fully laden with fuel (or equivalent mass in batteries) weighs less than the Curiosity probe plus landing system, so we could just fly it there in a conventional rocket and release it into the atmosphere (possibly with a little entry shielding). You're taking this far too seriously ;)
Please consider this account deleted, I just can't be bothered with the spam anymore.
The two body problem has been solved for hundreds of years and it is one of the foundational results in physics. A lack of familiarity with it is damning. Being pedantic and obnoxious while proving you have no idea what you are talking about is unforgivable.
On Mars the Cessna wouldn't have enough lift, so you'd make a plane with a much better power-to-weight ratio by using thin carbon fiber delta wings to increase the effective area of the lift surfaces.
Or use a digirible.
Tubby or not tubby. Fat is the question
What if it was made of sodium?
Confucius say, "Find worm in apple - bad. Find half a worm - worse."
What?
And that's not what I meant. It was in a flight manual or something. Various conditions that ended with "you will not be X today"
For large sets, this will be our guide even unto death, for the LORD will work for each type of data it is applied to...
No!!!! I don't want ANY page to EVER display something different on my mobile browser from on the PC. The biggest frustration I have surfing the web on mobile devices is convincing web sites that I'm not surfing on a 10 year old feature phone. Mobile displays these days have just as good resolution as laptop displays (sometimes better) I'm tired of missing 3/4 of the features of the page just because my user agent string says I'm on a mobile device. (Slashdot is bad this way, but at least it honours the "request desktop version" flag, if only I didn't have to set it every single time I visited the site, Unfortunately many sites completely ignore that flag and shovel their garbage mobile version on you anyway.)
Mobile sites were useful 10 years ago, but today they're outdated and I wish they would just die.
Of course that drawback is also specifically stated in the xkcd link.
I know people have an aversion to reading the articles, but if a comic is too much to read, maybe you shouldn't be commenting.
I have to admit not ever reading xkcd, having more important things on my Kindle.
If you've never read it, then how do you know? A little bit of faith based knowledge there?
I have to admit not ever reading xkcd, having more important things on my Kindle.
If you've never read it, then how do you know? A little bit of faith based knowledge there?
No, just snobbery, like those people who refused to waste their time with a TV until the mid-1970's, then loudly proclaimed that they only watched PBS because there was nothing worth watching on any other channels.
I did not know that, very interesting. Thank you.
http://xkcd.com/1053/
> Mars is quite windy so your vehicle will get blown around
I don't think the atmosphere on Mars has enough molecules in it to blow around anything of substance. Atmospheric density on Mars at ground level is lower than anything a basic physics-classroom vacuum pump can produce.
Cut that out, or I will ship you to Norilsk in a box.
> Mars is quite windy so your vehicle will get blown around
I don't think the atmosphere on Mars has enough molecules in it to blow around anything of substance. Atmospheric density on Mars at ground level is lower than anything a basic physics-classroom vacuum pump can produce.
If your vehicle is off the ground and in the air it is going to move at the speed of the air until terrain gets in the way, regardless of how thin the air is.
http://michaelsmith.id.au
Childish daydreams of youth, maybe, but not science.
Einstein once asked himself 'What would a light wave look like if you caught up with it?' - and lo, general relativity was discovered.
No colour or religion ever stopped the bullet from a gun
Venus: 'Unfortunately, X-Plane is not capable of simulating the hellish environment near the surface of Venus. But physics calculations give us an idea of what flight there would be like. The upshot is: Your plane would fly pretty well, except it would be on fire the whole time, and then it would stop flying, and then stop being a plane.'
Exactly like my first wife. The sex was great at first, and she was totally on fire, then the sex stopped and she stopped being a human being!!!
Sure enough, the cow costume was hanging up next to the superhero outfit and sailors uniform. (S,Spud)
That's okay, so did the shuttle.
sub f{($f)=@_;print"$f(q{$f});";}f(q{sub f{($f)=@_;print"$f(q{$f});";}f});
> If your vehicle is off the ground and in the air
> it is going to move at the speed of the air until
> terrain gets in the way, regardless of how thin
If your vehicle were freely floating in the air, like a soap bubble, that would be true. If your vehicle has any significant propulsion mechanism, however, then no. (Then again, nothing with any really significant propulsion could be made lighter than the "air" on Mars, so perhaps you're right.)
Cut that out, or I will ship you to Norilsk in a box.