Flying a Cessna On Other Worlds: xkcd Gets Noticed By a Physics Professor 148
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."
Re:Not going anywhere... (Score:5, Funny)
It is a cessna engine, it doesn't run on air but on money.
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That's okay, so did the shuttle.
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In the What-if it's explicitly stated that the gas tanks have been replaced with batteries and had an electric engine installed.
Re:Not going anywhere... (Score:5, Insightful)
Re:Not going anywhere... (Score:4, Insightful)
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.
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> 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.
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> 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 [humblebundle.com] back in October.
Re:Not going anywhere... (Score:4, Informative)
> 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?
Re:Not going anywhere... (Score:5, Informative)
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http://m.xkcd.org/ [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 :)
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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
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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.
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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 "re
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He is not a reader, he has a kindle.
In a non related issue:
Did anyone notice US teachers have begun 'boycotting' proficiency testing?
Re:Not going anywhere... (Score:5, Insightful)
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?
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As a rocket scientist, perhaps you might get a chuckle out of this xkcd: http://xkcd.com/1133/ [xkcd.com]
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Great idea! I'm also a big guy, but it might fit me best if I were to lay it out horizontally...):
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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?
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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.
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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.
I got linked! (Score:5, Interesting)
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 [nasa.gov]
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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.
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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.
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Re:Not going anywhere... (Score:5, Insightful)
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.
Re:Not going anywhere... (Score:4, Funny)
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!
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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.
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Very first tile in illustration: rip out engine, install batteries and electric motor.
RTFA
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What exactly are you trying to prove by refusing to read the article?
Re:Not going anywhere... (Score:5, Informative)
Re:Not going anywhere... (Score:4, Funny)
You can't hand-waive away physics.
Sense of humor, on the other hand, is commonly waived.
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Woosh!!!!! This low flying humor clearly gets no propulsion in all that hot air your blowing.
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"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
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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 partic
Re:Not going anywhere... (Score:5, Funny)
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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.
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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 p
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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.
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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.
Re:Not going anywhere...s (Score:2)
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.
Mars plane by Boeing (Score:5, Funny)
"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.
Re:Mars plane by Boeing (Score:4, Funny)
"on fire the whole time"
Typical. You go to all the trouble of flying a plane on Venus and all you get is petty criticism of minor teething troubles. There's no pleasing some people.
It's a nice analysis of a joke... (Score:2, Insightful)
... but I think it went over his head.
X-Plane (Score:5, Funny)
It would be an X-Plane!
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X-plane needs your help.
http://www.x-plane.com
https://petitions.whitehouse.gov/petition/make-patent-trolls-pay-all-costs-associated-their-frivolous-lawsuits-if-they-lose/gWPpVYMt
Ed
If your plane is on fire and not a plane anymore (Score:3, Funny)
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.
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What's that based off of? Seems familiar.
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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"
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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.
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What if it was made of sodium?
Might be possible on Titan (Score:4, Insightful)
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.
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Would that work at 5% methane? Now for a boat cruising on a methane lake or river would work though getting your engine started at less than 100 K might be hard.
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One problem is that you have all 1.6 atmospheres of nitrogen sucking the heat out of your combustion chambers but I suppose you could pre-heat it with your exhaust and heat the engine and air to get the cycle going.
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Ah thats interesting. I got it mainly from Arthur C Clarke's novel Imperial Earth in 1975.
Not the first time he's commented on xkcd (Score:1)
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/
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Picking nits (Score:4, Interesting)
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.
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If you want to nitpick, the curve of a free falling projectile is a conic section. Depending on initial conditions it may be a circle, an ellipse, a parabola or hyperbola.
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Yes, that's the general solution. But the parabolic and hyperbolic trajectories can be eliminated if we assume an initial vertical velocity of zero at release above the planet or moon.
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Actually I would like to nit picks with the professor in that the starting conditions of the flight are specifically not stated. As the professor himself says, "Randall doesn’t explicitly state the starting conditions for the Cessna, so let me guess that it starts off 1 km above the surface with a speed of 60 m/s." With different values for the starting speed, different results will be obtained. In the graph where he shows Randall's and his calculated trajectories in one, he's specifically not "provid
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However, I propose that the starting altitude should be 0m from the surface too (how did the thing take off?).
Good luck even defining what the surface is on Jupiter and the other gas giants. You'll need some kind of platform to launch from first there, so you can postulate any altitude you want. You'll want to pick one with atmospheric density similar to that of Earth...
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I agree with the starting speed. However, I propose that the starting altitude should be 0m from the surface too (how did the thing take off?).
Dropped from a passing spacecraft. Or just sent there on a long slow path given an initial push. How you survived the trip is an interesting question, however.
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...typically referred to as a crash.
The scientific term is "lithobraking maneuver".
Steak Drop (Score:2)
Drop the steak from orbit, it's the only way to be sure.
I love xkcd :)
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You know, what I got from that is that steak might be a good material for ablative cooling. First one to the patent office wins.
I'm hoping the prof is watching (Score:1)
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?
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This other what-if actually addresses this pretty well: http://what-if.xkcd.com/20/ [xkcd.com]
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Gamma at 0.99c is 7, and a cessna weight pretty close to 1 tonne. So we're talking about 7 tonnes of energy or 6*10^20 Joules.
This is roughly equivalent (in energy) to a magnitude 8.7 earthquake,3 Tsar Bombas going off at once, or a or a meteor a few hundred meters hitting the earth.
There would be widespread destruction but it would not destroy the earth.
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Thank you both. I am in appreciation of your taking time to answer.
What If? (Score:1)
The "physics professor" commentary is inane (Score:2)
So, women really are from Venus! (Score:1)
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!!!
Re:Wrong Professor is Wrong (Score:5, Informative)
Mach 1 is the speed of sound - in that medium.
Re:Wrong Professor is Wrong (Score:5, Informative)
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Like I said - in that medium. Except you wrote something wordier so you gathered more mods.
I'm one of the 10,000! (Score:2)
http://xkcd.com/1053/
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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."
Re:Wrong Professor is Wrong (Score:5, Funny)
Re:Wrong Professor is Wrong (Score:5, Funny)
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
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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.
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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.
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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.
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> 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.
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> 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.)
Re:Orbit (Score:2)
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 th
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