Any mission launch involves a spacecraft plus the rocket needed to get that spacecraft launched off the Earth. If we think about how much rocket fuel we need to launch spacecraft into space, we will see that rocket science is a very complicated thing after all.
The Three Components
When figuring out how to send a rocket into space, there are three components we need to consider. First, there is the payload. That’s the spacecraft with all its instruments. Basically, it’s the thing we want to get into space.
Then there is the rocket itself, which has all the infrastructure to get the payload into space and store the fuel. And finally there is the fuel, which is usually the largest component. In fact, the mass of the fuel is usually several times larger than the mass of the payload and rocket combined. So, all three components have mass: mass of the payload, mass of the rocket, and mass of the fuel.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, on The Great Courses Plus.
The Delta-v Calculations
To figure out how much fuel we need, we have to calculate the maximum change in velocity—often called delta-v—that we need to get where we’re going.
Starting from the surface of the Earth, we need a change in velocity—a delta-v—of 11.2 kilometers per second to escape Earth’s gravity well. Overcoming atmospheric drag on the rocket only adds another 10%. If you just want to get into low Earth orbit, say at 250 kilometers altitude, then you need a delta-v of around 9 kilometers per second. Although that puts you in orbit around Earth, you are still gravitationally bound to Earth.
But once you are in low Earth orbit, you could escape Earth and get all the way to the surface of the Moon with an added delta-v of about 6 kilometers per second. Then if you wanted to orbit the Moon, you would need another delta-v, aimed in the opposite direction, this time slowing you down so you can be captured into the Moon’s gravity well.
If we launch from low Earth orbit to the surface of Mars, we need 10 kilometers per second of delta-v compared to the 6 kilometers per second to get to the Moon.
Learn more about orbiting Earth.
To figure out how much fuel we need, we also need to know how much thrust we can get from the fuel. This is usually expressed in terms of an exhaust velocity, which engineers call the Ve of the fuel.
Different types of fuel have different exhaust velocities. The best fuel in use is a hydrogen/oxygen mixture that attains an exhaust velocity of 3.4 kilometers per second. It is used in Europe’s Ariane 5 rockets that launch satellite payloads into low Earth orbit.
Another workhorse fuel type is a kerosene/oxygen fuel mixture. It was used in the Saturn V rockets of the Apollo moon missions. Its exhaust velocity was a little lower, at 3.1 kilometers per second. Then there’s the Space Shuttle that used a solid propellant with an exhaust velocity of 3.0 kilometers per second.
The amount of fuel we need to get to a given destination with a given type of fuel is determined from the rocket equation.
A Simple Launch Calculation
Let’s say we want to launch a spacecraft to reach low Earth orbit. We’ll need a total delta-v of about 9.0 kilometers per second. And let’s use the most efficient fuel we have, a hydrogen/oxygen mixture, so our exhaust velocity is 3.4 kilometers per second. The rocket equation then tells us that we’ll need the mass of fuel to be 13 times the mass of the rocket and payload.
But the rocket, to be able to house such a massive amount of fuel, along with withstanding all the forces involved with accelerating through the atmosphere and getting into orbit, can weigh thousands of tons! That’s why we try so hard to make spacecraft payloads as light as possible.
Learn more about exploring Mars from space and the ground.
Here’s something interesting about Earth’s escape velocity and the rocket fuels we use. If Earth were about 5 times more massive, then the delta-v would be about 20 kilometers per second, too large to launch rockets into space with any of the types of fuel used so far. So, we would have to use some other type of propellant or technology, perhaps a nuclear propulsion source.
Would it be possible to have much larger or heavier spacecraft using our existing fuels? Yes. As technologies improve, we are already building bigger and more efficient rockets. The NASA Space Launch System (or SLS for short) is being developed with the goal of being able to send larger payloads and even astronaut crews to deep space. It’s intended to replace the Space Shuttle Program and may be the vehicle that first sends astronauts to Mars.
Launching from the Moon
If we could use less massive bodies as our base to build and launch spacecraft, then we would need far less fuel. What about building spacecraft on the Moon? With an escape velocity of 2.4 kilometers per second—that’s almost a factor of 5 times smaller than Earth’s escape velocity—we’d need far less fuel to escape the Moon’s gravity.
Could we do it? Could we use resources on the Moon to build spacecraft? Could we use resources on the Moon to generate fuel? Well, elements like silicon, titanium, iron, and oxygen have been found in the lunar regolith. Perhaps we could mine these to build our rockets and create rocket fuel.
We also know the Moon has water ice in permanently shadowed polar regions. In principle, water could be broken down into its hydrogen and oxygen components using an energy source like electricity from solar panels. Then the components could be used as fuel, since the hydrogen and oxygen, when they recombine, ignite and burn.
There are so many possibilities that we have still to explore when it comes to powering rockets.
Common Questions about Achieving Escape Velocity: Launching Rockets into Space
Starting from the surface of the Earth, we need a change in velocity—a delta-v—of 11.2 kilometers per second to escape Earth’s gravity well.
The best rocket fuel in use currently is a hydrogen/oxygen mixture that attains an exhaust velocity of 3.4 kilometers per second. It’s used in Europe’s Ariane 5 rockets that launch satellite payloads into low Earth orbit.
If we could build and launch spacecraft, from the Moon, then we would need far less fuel. With an escape velocity of 2.4 kilometers per second—that’s almost a factor of 5 times smaller than Earth’s escape velocity—we’d need far less fuel to escape the Moon’s gravity.