Fuel Consumption To The Moon

[Bert45]

There was a question on The Chase recently about the fuel consumption of Apollo 11's journey to the Moon (and back, probably). I can't remember the three possible answers exactly, but they were all 7 something. Like 7 inches per gallon, 7 feet per gallon, or 7 miles per gallon. Buzz Aldrin's name came up as the person who had worked it out (I think). And I can't remember what the correct answer was. I have googled, of course, but I haven't found any relevant information. I have just watched the repeat of "8 Days: To the Moon and Back" and i noticed that the Americans were using nautical miles for the unit of distance, and presumably the American gallon for the amount of fuel they were carrying. So I just wondered if the fuel consumption had been worked out correctly, with the right units.

[sddsddean]

According to this site
https://www.businessinsider.com/how-much-fuel-it-takes-to-get-to-the-moon-2017-8?r=US&IR=T
the 3 stages of the Saturn 5 used 947,459 gallons. We'll ignore the relatively small amount used by the service module and the lunar module.
For arguments sake, its about 248,000 miles to the moon...a round trip of about 496,000 miles. So that means the average fuel consumption was about 0.52 mpg, which doesn't really match any of your figures...unless I've done something wrong!

[nescio]

It is not really valid to state the fuel consumption as x units per gallon, because while the engines are burning fuel at a constant rate, the rocket is accelerating, so the distance travelled per unit time is constantly increasing. The five Rocketdyne F1 engines powering the first stage of the Saturn V burned fuel ( RP1) and oxidiser (LOX) at almost 13 tons per second. I read once that all five engines had to function flawlessly for the first 15 seconds, otherwise the rocket would be too heavy to achieve orbit. During those first 15 seconds, it had got through about 200 tons of propellant.

[ToraToraTora]

it used most of the fuel in the first 10 minutes to get into earth orbit.

[Bert45]

Thanks for the comments. I don't think that the acceleration enters into the problem or its solution. While the mpg is increasing while the rocket is accelerating, the question, as framed on The Chase, concerned the overall mpg not an instantaneous value. If we know the total distance travelled and the quantity of fuel consumed, divide the first by the second to get the answer. In addition to the distance from the Earth to the Moon (which will not be the straight line distance as the Moon is moving while the craft is crossing between the two bodies), there is also the orbit of the Earth and the Moon to add on. I also suspect that the distances were worked out in nautical miles (why on Earth would they use nautical miles?) and the volume of propellants would be in US gallons, whereas we would expect the volume to be in imperial gallons. Even if the consumption worked out to be 7 inches per gallon, they would have calculated the inches from the nautical miles. What I need is the source used by The Chase.

[Mamyalynne]

https://departingearth.com/why-is-space-travel-measured-in-nautical-miles/

[ToraToraTora]

the question was about the first stage and it was 7 inches to the gallon.

[Bert45]

Mamyalynne's answer was almost worth marking as Best Answer, but TTT got my award.

[ToraToraTora]

Bert, thanks for the BA Bert, I do remember the actual question! For the whole journey it was about half a mile per gallon.

[sddsddean]

^^ which is what I worked out above. Would have helped to know it was first stage only in the Chase question!

[Hopkirk]

Not very environmentally friendly!

Perhaps they planted a few trees to offset the emissions.

[Khandro]

All a stupid waste of the Earths fossil resources. the fuel used to launch a rocket isn't liquid (far too dangerous) it is a solid 'cake' something like fire-lighters I believe.

[Bert45]

According to Wiki, the first stage goes to a height of 67km and downrange 93km. Using Pythagoras, I calculated that the rocket had travelled 114.6km. The first stage carried 770,000 litres of fuel. If that figure is used, it gives 0.000148 km/L = 0.0004203 mpg(UK) = 26.63 inches per gallon(UK)(ipg). That's nowhere near 7". So I thought they must have included the LOX, 1,305,000L, i.e.2,075,000L total. 0.0000552 mpg = 9.88 ipg(UK) (close). If I convert km/L to mpg(US) I get 8.22 ipg(US) (closer). But it all depends on the starting figures (obvs). Would you know the figures used to get 7ipg and if the g is UK or US?

[THECORBYLOON]

"Using Pythagoras, I calculated that the rocket had travelled 114.6km." Did it travel in a straight line?

[nescio]

Khandro, the main engines of rockets use liquid fuel. Solid fuel is used in boosters, e.g. the solid rocket boosters on the Space Shuttle. Once solid fuel rockets are ignited, they are essentially uncontrollable and just burn to completion. You may be interested to learn that the principal fuel in the Space Shuttle SRBs was essentially powdered aluminium (or aluminum as it is known in the US), with ammonium perchlorate as oxidiser. It also contained a polymer binder.

[beso]

There are two reasons the fuel consumption is so huge at first. Obviously the weight of the fuel is maximum at takeoff.

Secondly the efficiency of both jet and rocket engines is poor when the vehicle is moving slowly. Efficiency is optimum when the exhaust velocity matches the vehicle speed which is obviously impractical when taking off.

[Khandro]

Propellants can be in the form of a solid, liquid or gas, each with their own advantages and disadvantages. Solid propellants have higher density--and therefore thrust. They also are storable, transportable, reliable, less complex and can also contain their own oxidizer. Once ignited, however, solid propellants burn continuously, limiting the number of applications. Examples of rockets using solid propellants include the first stage of military missiles, commercial rockets and the first stage boosters that are attached to both sides of the liquid-fuel tank on the space shuttle. (Though attached, the two cylindrical boosters are separate units from the tank, which itself supplies the shuttle orbiters own liquid-fuel engines.) Ammonium perchlorate mixed with powdered aluminium that is held together in a rubberlike matrix is the most common solid propellant.


Scientific American

[Bert45]

Interesting stuff about fuels. Thanks to all. Wiki says that the fuel in the first stage is RP-1 (alternatively, Rocket Propellant-1 or Refined Petroleum-1) a highly refined form of kerosene outwardly similar to jet fuel, used as rocket fuel. I had never thought to ask why NASA uses nautical miles, and I didn't know that a nautical mile was first defined as 1 minute of latitude. But that varies since the Earth is not a sphere, so it's now defined as 1852 metres. Something new every day!

[Bert45]

I asked "The Chase" about this question. They replied: We actually asked, 'According to Buzz Aldrin, what was the maximum fuel consumption during the Apollo 11 mission?', with the options being 7 yards to the gallon, 7 feet to the gallon, 7 inches to the gallon. The correct answer being 7 inches to the gallon.Further information on Buzz Aldrin's quote can be found on his website:https://buzzaldrin.com/kennedy-space-center-marks-40th-anniversary-of-apollo-11-launch/
There is no justification for Aldrin's assertion in the link, so we just have to take his word for it. He still alive (aged 91) so I suppose somebody could ask him. So I was wrong in stating that the acceleration had nothing to do with it. I thought they were asking what was the average fuel consumption. Though they didn't mention that it was during the first stage that the question related to, it is clear that the maximum fuel consumption was during the first stage, and, moreover, it was an instantaneous fuel consumption that was being asked, and the maximum fuel consumption is (almost certainly – I haven't done any calculation) at lift-off.
However, what is wrong with the question is that it begins 'According to Buzz Aldrin, ...' Unless you've seen or heard the quote, there is no way that anyone can KNOW the answer to that question. You may say that that doesn't matter, you might be able to work it out – not in 5 seconds, you can't. So it's really a purely guessing game, which I don't think that "The Chase" usually is. Most of the questions they ask have answers that (to me, at least) are well-known facts. I have to admit, they do ask some questions that few people are likely to KNOW the answer to, but you can usually make an "educated" guess. But sometimes you ask yourself, why is that stupid answer there, unless it's the right answer.