Rocket Launch

in Space

Why Do Rockets Use So Much Fuel?

Repost of a Quora answer found here.


How much fuel do rockets use?

The short answer is a lot.

However the picture gets more complicated and that’s what this article explores. Read on to enter the wild world of rocket science and fuel consumption rates.

To make this simple I’ll use a graphic taken from here (shown below).

If you’re curious about the elephants a YouTuber Maxim Sachs made a video showing the appearance of fuel consumption rates in African Elephants (shown below).

https://www.youtube.com/watch?v=X4iMeKif488

Now each of those rockets was launched with different intentions and went to different places and did different things. Each of those changes the amount of fuel needed.

The easy answer then is that for SpaceX to dock with the ISS it takes about 150 elephants. But why that many?

Factors Involved In Fuel Consumption

There are other factors at play in fuel consumption rates. Quite a few in fact. To fully understand this problem lets explore those factors.

  1. Destination. The farther you’re going out the more fuel you need. Anything beyond Earths atmosphere ie the Moon, Mars would require similar amounts of fuel. That amount depends on further factors.
  2. Launch Point. The trajectory of a rocket will depend upon the launch point. That’s why Cape Canaveral is such a popular launch point. Its proximity to the equator and other factors makes it ideal for fuel consumption as the trajectory can be lower to still reach escape velocity and leave Earth.
  3. Weight. Heavier rockets require more thrust to reach escape velocity. Assuming everything else is the same you could identify an efficiency formula for fuel consumption to added weight. But whatever it is it would take more fuel.

    Always More Fuel

  4. The Rockets Design. Different rocket shapes will lead to different aerodynamic characteristics. If its more or less efficient at moving through the air it will require more or less thrust accordingly. This again leads to effect our fuel consumption scenario.
  5. Type of Fuel. Different types of fuel produce different thrust-to-weight ratios. Fuel can be used from different states and from different chemical combinations. More on fuel here. What’s important is the fuel consumption rates will vary based off the characteristics of the fuel.
  6. The Efficiency of Burnt Fuel. Different engine layouts and efficiency ratings will produce different amounts of thrust based off the same weight of fuel. However given every other condition being the same if you had less efficient fuel burning rates you’d need more to move the same rocket through the same air.
  7. Launch Conditions. The air density, any weather systems. Factors like these will effect the aerodynamic tendencies of the rocket and thus its requirement for thrust to reach escape velocity. More thrust needed means more fuel burnt all else being equal.

I’m not a rocket scientist or aerospace engineer so I’ve almost definitely missed some important points. However I think this is a good overview of the factors affecting fuel consumption.

The Fuel Consumption Paradox

It’s quite complex and it pays to be accurate because of the Tsiolkovsky rocket equation. This equation basically means that as speed increases, the mass required (in fuel) increases exponentially. However because we need to reach a certain speed no matter what, called escape velocity. It means that any extra weight must be compensated for by more fuel burnt.

This means that ultimately 94% of the mass of launch at launch is fuel! The math of that is explained a little further here.

Think about that. I want an extra bottle of water. That’s 16 ounces or so. That requires fuel to move. That added fuel now require more fuel to move and so on. This goes on such that those 16 ounces only makes up 6% of the mass. If the other 94% is fuel it means we need just over 250 ounces of fuel to move that water bottle.

Based off this it seems NASA would pay about $7 for fuel to launch that bottle of water.

$7 just for fuel for a bottle of water. NASA launches rockets. Those rockets weigh quite a bit. They launched much of the ISS. They launched people. Those people need food, exercise equipment and more.

It’s easy to see given all that why fuel consumption rates are so exorbitant for rocket launches.

….

TL;DR

For each ounce launched roughly 16 ounces of fuel is needed. This effect snowballs. Rockets must reach 11.186 km/s or ‘escape velocity’ to leave Earth. This means all else constant more weight (like the 165,000 pounds Space Shuttle pre-fuel and cargo) means exponentially more fuel.

The exact amount is complex and based off the factors listed above. Not carrying extra is important because of the fuel paradox listed above. Optimizing fuel then is hard and is one of many reasons it’s called ‘rocket science’.

-Andrew

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Written by Andrew Walls

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