How Do Rockets Get to Space?
The short answer
A rocket throws hot gas down, very fast, and the gas pushes the rocket up. The Saturn V's first stage did this with about 33 million newtons (7.5 million pounds) of thrust. Going up is only half of it. To stay in space, a rocket must also go sideways, at about 7.8 km/s (17,500 mph).
At that speed it is still falling toward Earth. It just moves sideways so fast that it keeps missing. That is what an orbit is.
Why fuel is the hard part
Every kilogram of rocket needs fuel to lift it. That fuel weighs something too, so it needs more fuel, which weighs more again.
The Saturn V weighed more than 3,000 metric tons at launch. Its first stage had five F1 engines. Together they burned about 15 metric tons of fuel a second for about two and a half minutes. That comes to about 2,250 metric tons of fuel in the first stage alone. When those engines shut down, the rocket was 66 km up and moving at 9,840 km/h.
Then the empty stage drops away. There is no point carrying an empty tank. A lighter rocket speeds up faster on the fuel it has left. That is staging.
Why orbit is falling
At the height of the International Space Station, about 400 km (248 miles) up, gravity is still about 90% as strong as on the ground. Astronauts float because they and the station are falling together, not because gravity has gone.
The station also moves sideways at about 8 km/s (17,900 mph). As it falls, Earth's curved surface drops away below it, so it never lands. It goes all the way around in about an hour and a half. The film draws this as Newton's cannon: each faster shot lands farther away, until one never comes down.
Gravity's last word, "Eventually," is fair. A little thin air is left at that height. It drags on spacecraft and slowly pulls their orbits down.
Key numbers
| What | Number | Source |
|---|---|---|
| Saturn V first stage thrust | 3.4 million kilograms of thrust, about 33 million newtons (7.5 million pounds) | Our arithmetic from BBC Science Focus |
| F1 engines on the Saturn V first stage | 5 | BBC Science Focus |
| Fuel burned by the first stage | about 15 metric tons a second (the film says 13 tons) | BBC Science Focus |
| How long the first stage engines burned | about 2.5 minutes | BBC Science Focus |
| Height and speed when the first stage shut down | 66 km up, 9,840 km/h | BBC Science Focus |
| Saturn V weight at launch | more than 3,000 metric tons | BBC Science Focus |
| Fuel burned by the first stage alone | about 2,250 metric tons (15 metric tons a second for 150 seconds) | Our arithmetic from BBC Science Focus |
| Sideways speed needed for low Earth orbit | about 7.8 km/s (17,500 mph) | Space.com |
| Height of the International Space Station | about 400 km (248 miles) | NASA STEMonstrations: Orbits |
| Speed of the International Space Station | about 8 km/s (17,900 mph) | NASA STEMonstrations: Orbits |
| Time for one orbit of the Space Station | about 1.5 hours (88.7 or 92.5 minutes, worked out two ways) | NASA STEMonstrations: Orbits |
| Gravity at Space Station height, compared with the ground | about 90% | Space.com |
What the film simplifies
- The film says the rocket burns 13 tons of fuel a second. BBC Science Focus gives about 15 metric tons a second for the Saturn V's first stage, so the film's number is on the low side.
- The drawn rocket, with two side boosters that drop off at two minutes, is a cartoon. The thrust and fuel numbers come from the Saturn V's first stage, which burned for about two and a half minutes.
- "Gas goes down, rocket goes up" is true at liftoff. Once the rocket turns to go sideways, the gas goes out the back, not straight down.
- Newton's cannon is a drawn thought experiment, with a mountain on a small Earth shrunk to fit the screen.
- Gravity has a face and a hand, and the rocket and the satellite talk. That part is cartoon.
For teachers
Runtime 1 minute 30 seconds · Topics: Rockets, Gravity, Orbits · Full script below
Ask your class
What does a rocket push against to go up?
Answer
Its own exhaust. It throws hot gas down very fast, and pushing the gas down pushes the rocket up (Newton's third law).
The Saturn V's first stage burned about 15 metric tons of fuel a second for about two and a half minutes. About how much fuel is that?
Answer
About 15 × 150 = 2,250 metric tons. That is most of a rocket that weighed more than 3,000 metric tons at launch.
At the Space Station's height, gravity is still about 90% as strong as on the ground. So why do astronauts float?
Answer
They are falling, along with the station. The station moves sideways at about 8 km/s, so it keeps missing Earth instead of landing.
The script
Host: Okay. You wanna leave Earth? Cool.
Host: Earth does not want you to leave.
Gravity: Nothing leaves.
Host: Meet gravity. It pulls on everything. All the time.
Rocket: Seven and a half million pounds of thrust. Try and stop me!
Host: Burning fuel blasts hot gas out the bottom. Gas goes down, rocket goes up. That's the whole trick.
Host: Here's the problem. Every pound of rocket needs fuel. And the fuel weighs something too.
Host: So a rocket is basically a giant flying fuel tank. Ninety percent of it is fuel.
Mission Control: T minus ten. Main engine start.
Mission Control: Three. Two. One. Liftoff!
Host: It burns thirteen tons of fuel. Every. Single. Second.
Rocket: Mach one, baby! Nobody can hear me now!
Rocket: Boosters, you've done your job. Get off my back!
Host: Lighter rocket, faster rocket. That's staging.
Host: Now the weird part. To get to orbit, you don't just go up. You go sideways.
Lil Sat: Wait. Are we going sideways? Why are we going sideways?!
Host: Seventeen thousand five hundred miles an hour. Sideways.
Host: Because orbit isn't floating. It's falling. You fall toward Earth, and keep missing it.
Lil Sat: We're falling?! Forever?!
Gravity: Eventually.
Host: Next time: how do you come back down without burning up? Follow, so gravity doesn't win.
Sources
- Saturn V's first stage made about 7.5 million pounds of thrust. Saturn V's first stage burned over 13 tons of propellant a second.BBC Science Focus
- Low Earth orbit takes a speed of about 17,500 mph.Space.com
- Orbiting spacecraft are in free fall around Earth.NASA STEMonstrations: Orbits