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## Flying a Cessna On Other Worlds: xkcd Gets Noticed By a Physics Professor148

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."
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## Flying a Cessna On Other Worlds: xkcd Gets Noticed By a Physics Professor

• #### Mars plane by Boeing (Score:5, Funny)

by Anonymous Coward on Saturday February 02, 2013 @06:03PM (#42773665)

"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 Saturday February 02, 2013 @08:21PM (#42774519) Homepage

"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)

on Saturday February 02, 2013 @06:11PM (#42773705) Journal

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.

It would be an X-Plane!

• #### Re: (Score:2, Informative)

by Anonymous Coward

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)

on Saturday February 02, 2013 @06:13PM (#42773711) Homepage

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.

• #### Re: (Score:2)

What's that based off of? Seems familiar.

• #### Re: (Score:2)

look for the xkcd comic on up goer 5
• #### Re: (Score:2)

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"

• #### Re: (Score:3)

In case anyone misses your reference, here is XKCD's dumbed downed explanation for flying a Saturn V [xkcd.com]

This end should point toward ground if you want to go to space.

If it starts pointing toward space you are having a bad problem and you will not go to space today.

• #### Re: (Score:2)

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.

• #### Re: (Score:2)

What if it was made of sodium?

• #### Might be possible on Titan (Score:4, Insightful)

on Saturday February 02, 2013 @06:39PM (#42773843) Homepage Journal

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.

• #### Re: (Score:2)

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.

• #### Re: (Score:2)

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.

• #### Re: (Score:2)

This idea (Methane from the atmosphere, stored oxygen in the aircraft) was used for aircraft flight in the 1954 Winston Series science fiction novel, "Trouble on Titan", by Alan E. Nourse. It's a pretty good read, for a novel aimed at the juvenile market.
• #### Re: (Score:2)

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)

by Anonymous Coward

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/

• #### Re: (Score:2)

Hmm... maybe we should do a headline FTFY:

Physics Professor Gets Hits from XKCD Readers

• #### Picking nits (Score:4, Interesting)

on Saturday February 02, 2013 @09:25PM (#42774821)

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.

• #### Re: (Score:1)

Are you sure this is right? It has been a while since I took physics, but I tried to derive this. It would seem x = t, y = C - kt^2 which is definitely a parabola... What am I missing? I'm genuinely curious...
• #### Re: (Score:1)

by Anonymous Coward

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.

• #### Re: (Score:2)

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.

• #### Re: (Score:2)

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

• #### Re: (Score:2)

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?). However, your summary is correct, the plane would behave just like a rock - and remain stationary.
• #### Re: (Score:2)

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...

• #### Re: (Score:2)

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.

• #### Re: (Score:1)

by Anonymous Coward

...typically referred to as a crash.

The scientific term is "lithobraking maneuver".

• #### Steak Drop (Score:2)

For those who didn't RTFSteak blog entry, here's a summary of the analysis:

Drop the steak from orbit, it's the only way to be sure.

I love xkcd :)

• #### Re: (Score:2)

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?

• #### Re: (Score:3)

This other what-if actually addresses this pretty well: http://what-if.xkcd.com/20/ [xkcd.com]

• #### Re: (Score:1)

by Anonymous Coward

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.

• #### Re: (Score:1)

Thank you both. I am in appreciation of your taking time to answer.

• #### What If? (Score:1)

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 "physics professor" commentary is inane (Score:2)

The two body problem [wikipedia.org] 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.
• #### 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!!!

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