Orbit — A Rocket Does Not Go Up, It Goes Sideways

Reaching space is the easy part

Space is usually taken to begin about a hundred kilometres up — a shorter distance than Seoul to Daejeon. If going up were all, it would not be hard.

Rockets that go up and come down again have existed for a long time. The trouble is that what goes up comes back a few minutes later. That is visiting space, not being in it.

How not to fall

Newton imagined a cannon on a very high mountain, fired sideways. Fire it weakly and the ball travels a little way and lands. Fire it harder and it lands further off.

But the Earth is round. While the ball falls, the ground beneath it also curves away. Fire hard enough and the ground runs away as fast as the ball drops, and it never lands.

Which is why a rocket lies down

Watch a launch again and the climb is straight for only a moment. Soon it tips, and later it is nearly horizontal. That is not a mistake; it was the plan.

What a rocket is buying is speed, not height. Going up at first is only to get out of the thick air quickly; after that every bit of thrust goes sideways.

Why it is nearly all fuel tank

That explains the shape. Speed means burning fuel; more fuel means bigger tanks; bigger tanks weigh more, which needs more fuel again.

So a rocket on the pad is almost entirely propellant and the vessels holding it. The satellite it exists for sits tiny at the very top — a few parts in a hundred of the whole weight.

A little further in

How fast is fast enough

A low orbit needs something close to eight kilometres per second — per second. Seoul to Daejeon in twenty seconds, several times faster than a rifle bullet.

That single number explains most of the difficulty of spaceflight. Reaching a hundred kilometres costs far less than reaching eight kilometres per second.

Twice the fuel, twice the speed?

No. And this is the cruellest fact in rocketry.

Extra fuel is extra weight that the fuel itself must carry. Doubling it does not double your speed: the gain grows well at first, then less and less, and past some point more fuel is a loss.

The classical mechanics bench in the physics branch has a rocket handle: add fuel and watch what speed does. Kepler's equal areas is on the same bench.

Direction matters too

The Earth turns west to east, so firing eastward hands you the planet's own speed for free — the more so nearer the equator. That is why the famous launch sites cluster there.

Korea cannot fire east. Spent pieces would come down where neighbours live. That story continues in the next article.

The people who handle orbits

Building the rocket and drawing the orbit are different trades.

1. Someone who works out when, which way and how hard to push to arrive at a given orbit — flight dynamics and mission design.
2. Someone who knows, all the way up, where it is and where it is going — guidance and navigation.
3. Someone who keeps a satellite in the orbit once it is there — satellite operations.
4. Someone who steers it clear of everything already up there — tracking and conjunction analysis.
5. Someone who turns arithmetic no hand could finish into software — mathematics and computing.

Notice that nobody on that list touches the rocket. Drawing an orbit is separate work from building a machine, done on paper and on computers. If you like mathematics and thought space was for other people, this is the door.

The question that remainsIf it is not that they do not fall but that they keep missing — what else that we call stable is in fact still moving?