If we took all of the farts by every human to ever live, could we get a spaceship to the moon and back?
Short answer: Yes. Not only could our flatulence fuel thousands of rockets to the moon, but to Mars as well.
Long answer: People have an absurd number of questions about their body’s waste which fall on a spectrum of interesting to insane. This question is squarely in the middle. The premise is pretty simple – humans have been around producing flatulence for a long time, and every school boy knows that you can light a fart on fire. But could it get us to the moon?
Farts are rocket fuel. Or at least, they contain the major ingredients for rocket fuel. This is because flatulence contains two combustible gasses: diatomic hydrogen (H2) and methane (CH4). Reacting them with oxygen (O2) is exothermic, producing carbon dioxide, water vapor, and heat. The heat is the important part of this recipe – it causes the gasses to expand, which can be expelled at high speeds to generate thrust.
It’s that last point that matters most – what’s your thrust? The thrust that your rocket can generate is really the thing determining the load you can launch to orbit, not the volume of fuel you carry. You actually hit diminishing returns when adding fuel quite quickly. Adding fuel increases your launch mass, requiring more fuel in order to launch that extra fuel, etc. In fact, you could build a rocket that just had tons and tons of fuel, but if you lit the burners it would just stay put on the launch pad. The thrust won’t overcome gravity until it’s shed enough weight.
The question now becomes: “What real rockets are there that use hydrogen and methane as fuel, and how many of them can we fill?”
Hydrogen has been used as rocket fuel for as long as there have been rockets. The second and third stages of the Saturn V rocket burned liquid hydrogen (LH2) and liquid oxygen (LOX) in order to propel the rocket to the moon (the first stage used kerosene fuel). By liquefying the gasses they could store a much larger amount of fuel, though hydrogen and oxygen are only liquids at temperatures near absolute zero. For every liter of LH2 you carry, you can make 845 L of gaseous H2  at standard pressure, or 625 for CH4 [4,5]. How ironic is it that one of the hottest things on earth is powered by one of the coldest things?
The average person produces about 476 to 1491 ml of flatulence a day, which is a mixture of gasses . Let’s take this number to be 1 L per person per day. If 108 billion humans have ever been born , each with an average lifespan of about 20 years (due to high infant mortality for most of history), then all-time human flatulence output totals
(108 billion people)x(20 years/person)x(365 days/year)x(1 L/day) = 7.9 x 1014 L
This volume is the same as a gaseous cube 10 km on a side. If you could get this gas all in once place, you could replace the entire column of atmosphere above Philadelphia with flatulence. The composition of flatulence varies considerably , so I’ll take average flatulence to be about 5% methane and 35% hydrogen by volume. Separating and condensing these gasses out yields 3.2 x 1011 L of LH2 and 6.3 x 1010 L of LCH4.
The Saturn Vs fuel tanks held 984,000 L and 229,000 L [4,5] of LH2, for a total of 1.2 x 106 L. This means we could launch 270,000 missions to the Moon using just the hydrogen from human flatulence. Considering that there were only ever 13 Saturn V launches (12 Apollo missions + Skylab), we could run the entire Apollo program over 20,000 times on this fuel.
But what about the methane? The Saturn V didn’t use methane so it won’t get us to the moon, but it will some day get us to Mars. SpaceX is currently developing the Raptor engine, which will be the main engine of their Mars Colonial Transport launch vehicle. No hard numbers for fuel tank volumes are available at the moment, but a back of the envelope calculation indicates that the number of rockets we can send to Mars with CH4 is comparable to the number we can send to the Moon with H2.
image credit: Wikimedia Commons
asked by /u/FeedMeLobster
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