Anna and I actually did a Fermi estimate of the fuel for the Apollo 11 mission over dinner last week, and we were off by a factor of two. Some of the available inputs:
A crude estimate of the potential energy of mass lifted from Earth’s surface to a distance of many times its radius
The heights reached by jet aircraft using fuel amounting to only a very small portion of their mass
A crude estimate of the energy content of gasoline (one approximation is to energy content of food, and/or the energy output of humans), with adjustment for the need to carry oxygen into space
Images of rockets launching, which show that the fuel tanks are much bigger than payload, but not thousands or millions of times bigger
Knowledge of the price of consumer gasoline, or the price of oil
The existence of science fiction writers with physics backgrounds, SpaceX, the L-5 societies, and other groups seriously pushing for advancements to slash cost-to-orbit
Rough knowledge of NASA’s budget, either directly or by bounding it relative to known US budget items
Knowledge of the enormous cost of producing military aircraft and naval vessels, which can be in the hundreds of millions to billions of dollars
The existence of an ecology of NASA contractors condemned for their enormous costs (these would be trivial if fuel was the major cost)
But those aren’t the main reasons for the costs. The main reasons are that rockets have not benefited from reduced costs through mass production—something that would be very hard to estimate if you didn’t know or guess that.
Right, and attempting to do a Fermi estimate and then checking it would help you see that that was something you didn’t know. (Maybe I’m not getting my point across well here. My point wasn’t that such a Fermi estimate would have been in any way accurate. My point is that identifying what made it inaccurate would have helped you update more.)
I don’t see how I could have Fermi-estimated the relative costs of the rocket and the fuel...
Anna and I actually did a Fermi estimate of the fuel for the Apollo 11 mission over dinner last week, and we were off by a factor of two. Some of the available inputs:
A crude estimate of the potential energy of mass lifted from Earth’s surface to a distance of many times its radius
The heights reached by jet aircraft using fuel amounting to only a very small portion of their mass
A crude estimate of the energy content of gasoline (one approximation is to energy content of food, and/or the energy output of humans), with adjustment for the need to carry oxygen into space
Images of rockets launching, which show that the fuel tanks are much bigger than payload, but not thousands or millions of times bigger
Knowledge of the price of consumer gasoline, or the price of oil
The existence of science fiction writers with physics backgrounds, SpaceX, the L-5 societies, and other groups seriously pushing for advancements to slash cost-to-orbit
Rough knowledge of NASA’s budget, either directly or by bounding it relative to known US budget items
Knowledge of the enormous cost of producing military aircraft and naval vessels, which can be in the hundreds of millions to billions of dollars
The existence of an ecology of NASA contractors condemned for their enormous costs (these would be trivial if fuel was the major cost)
Estimate how many people you would have needed to employ and what their salaries might be, estimate the cost of materials…
But those aren’t the main reasons for the costs. The main reasons are that rockets have not benefited from reduced costs through mass production—something that would be very hard to estimate if you didn’t know or guess that.
Right, and attempting to do a Fermi estimate and then checking it would help you see that that was something you didn’t know. (Maybe I’m not getting my point across well here. My point wasn’t that such a Fermi estimate would have been in any way accurate. My point is that identifying what made it inaccurate would have helped you update more.)
Ah, I see. I’ll try and bear that in mind next time I go looking for info like that!