Exhibition 4 · Leaving the Ground
This page starts on the launch pad and climbs. As you scroll, the sky behind the words fades from blue to space and the line at the side tracks your height.
Two heights are marked on it: 80 km, where the United States says space begins, and 100 km, the Kármán line used for world records. Where space starts depends on who you ask.
With nothing to push against, how does a rocket move?
A rocket in space has nothing to push against: no ground, no air. NASA says how it moves anyway is the question it is asked most. Try it with a cart floating in empty space and a load of balls to throw.
With nothing to push against, how does a rocket move?
Predict first. Which gets the cart going faster?
How much rocket does it take to reach orbit?
Speed is what a rocket is for. To stay up you need about 7.8 km/s sideways, and the climb through air and gravity costs more on top, so the speed budget to reach low orbit is about 9.4 km/s.
Start from a real rocket, then choose what its upper stages burn and how much it carries. Each engine burns its own propellant, so choosing one chooses the other. A rocket that makes orbit is saved for the next exhibition.
How much rocket does it take to reach orbit?
Predict first. How much of a rocket that reaches orbit is fuel?
Which way does a rocket really go to reach orbit?
Now fly it. On a real rocket a computer drops each empty stage the moment its engines stop. Here you are the flight computer: watch the countdown and separate each stage as it burns out.
Which way does a rocket really go to reach orbit?
Why do rockets tip over so soon after lift-off?
Climbing to 200 km only takes about 2 km/s. Staying there takes 7.8 km/s sideways. So rockets start to tip seconds after they leave the pad; Apollo 11 began at 13 seconds. After one small tip, the nose follows the direction of travel and gravity bends the path over. How far should it tip?
Why do rockets tip over so soon after lift-off?
Predict first. How far does it need to tip?
How gently do two spacecraft have to meet?
In orbit, the hard part becomes meeting something else. Crew Dragon paused 20 metres from the space station to check everything before its last approach. Take it from there, using short pushes from the thrusters.
How gently do two spacecraft have to meet?
Coming back from the Moon at 11 km/s, how do you stop?
Going home, the problem turns round. A capsule back from the Moon hits the top of the air at 11 km/s, and the air, not rockets, has to take nearly all of that away. The angle you come in at decides what happens.
Coming back from the Moon at 11 km/s, how do you stop?
Predict first. What happens if you come in too shallow?
You started on the launch pad, reached orbit and came home. From orbit, at 7.8 km/s, the Moon is about 3.1 km/s more away, in one burn at the right moment. Apollo 11's burn added 3,181 m/s.
Where to from here?