The Brainteaser Elon Musk Asks New SpaceX Engineers

Nerval's Lobster writes: The latest biography of Elon Musk, by technology journalist Ashlee Vance, provides an in-depth look into how the entrepreneur and tech titan built Tesla Motors and SpaceX from the ground up. For developers and engineers, getting a job at SpaceX is difficult, with a long interviewing/testing process... and for some candidates, there's a rather unique final step: an interview with Musk himself. During that interview, Musk reportedly likes to ask candidates a particular brainteaser: "You're standing on the surface of the Earth. You walk one mile south, one mile west, and one mile north. You end up exactly where you started. Where are you?" If you can answer that riddle successfully, and pass all of SpaceX's other stringent tests, you may have a shot at launching rockets into orbit.

Comments

North Pole

[605dave]

I am guessing the answer is the north pole...

Re:North Pole

[Penguinisto]

The answer is indeed the North Pole, and that brain teaser has been around for what, eons now?

I think I'd quickly answer it, then ask him one that I made up and tested long before that final interview.

Re:North Pole

[softarch]

Or another ring of points closer to the south pole such that you go completely around the world twice, and another ring closer such that you go around the world 3 times....

Re:North Pole

[IcyWolfy]

That's the asymptote that needs to be addressed separate due to a division by 0.

The correct answer is an infinite number of points around the south pole, with the exception of the south pole itself, and the north pole.

Or in words:

Given C = 1 mile, there is an longitudinal (East-West) circle north around the south pole with a circumference of 1 mile.
Any point on this line is an answer.
As does any whole divisor of this (1/3 mile circumference is traversed three times in one mile, but back at the same starting point.)

So C(1/1) + C(1/2) + C(1/3) + C(1/4) + C(1/n)
And more generally
= C(1/n) where n != 0 is a circle around the south pole, and n==0 is the north pole solution, whose division by 0 needs resolution by analysis (which is more obvious)

Re:North Pole

[Roger W Moore]

The north pole and a circle of lat 1 + 1 / (2 * PI) north of the south pole.

Actually the answer is the north pole and a circles of lat 1 + 1 / (2*pi*n) north of the south pole where n=1,2,3,4... etc. plus there is a slight correction because the surface of the earth is not entirely flat and so the circumference of a line of latitude is actually less than 2*pi*s where s is the arc length from the line to the south pole for the distances involved it would probably be negligible compared to surface defects.

Re:Either of the poles woulc cause this effect

[just_common_sense]

It doesn't work because you cannot walk East or West from the South pole.

One obvious answer and infinite other answers.

[LawnBoy]

The obvious answer is the North Pole, but there are others. First, find the circle around the South Pole with a circumference of one mile, and then select all the points on the circle with a radius one mile larger around the South Pole. Then, find the circle around the South Pole with a circumference of one half mile, and then select all the points on the circle with a radius one mile larger around the South Pole. Then, find the circle around the South Pole with a circumference of one third mile, and then select all the points on the circle with a radius one mile larger around the South Pole. Then, find the circle around the South Pole with a circumference of one quarter mile, and then select all the points on the circle with a radius one mile larger around the South Pole. Continue ad nauseum.

No he doesn't...

[Anonymous Coward]

I interviewed with SpaceX for a senior-level software position last year, and was offered the job but turned it down on logistical grounds.

I did indeed have to take the tests mentioned here, and did have to interview with Musk himself as the final step. However, he did not ask me this brain teaser question. In fact, he specifically said he doesn't ask brain teaser questions because they are dumb.

Nor would he likely ask such a well-known and old brain teaser anyway. This seems like one of those things erroneously attributed to "Bill Gates" over the past 20 years because he is famous and smart, and fits people's preconceptions.

Re:Harder: self-stabilizing parachute, or balance

[steveha]

Which of these strategies do you choose:
a) Attach a parachute to the nose and let basic physics work.
b) Try to balance it atop rocket engines firing from the bottom.

I realize you were going for humor (and got it; congratulations on being moderated +5 Funny). But here's a serious answer.

It depends on what you are trying to accomplish:

If your top priority is to save the rocket stage, then you pursue an engineering strategy that has the best chance of saving the rocket booster. Maybe that means a parachute system; I don't know.

But a parachute system adds mass and complexity. It becomes another critical system ("if the parachute fails, we lose the rocket stage"). The rocket stage needs functioning rocket engines, so landing on the rocket engines is another use for those engines rather than a new system with a single purpose. All else being equal, the simpler design with fewer systems is more likely to succeed in its tasks.

If you add a few hundred kilograms of parachute system mass, that's going to mean the booster can push less mass to orbit. I'd guess that the loss factor is higher than 1... that each additional kilogram of non-fuel mass on the booster reduces the to-orbit capacity by more than one kilogram. But ask a physics expert for the actual numbers.

Note that new software to make the booster land on its engines does not add mass to the booster.

So I'd say that if your top priority is to efficiently deliver stuff to orbit, the parachute system is right out and the clear engineering decision is (b).

Wrong

[gerald.edward.butler]

The only correct answer is the North Pole. South means follow a line of longitude in the South direction, West means follow a line of latitude West, and North means follows a line of Longitude North. The only place this gets you back to where you started is at the North pole.