Lesson 02 · 3 min · 5 things to do
Sideways is the whole problem
Tell apart the energy needed to go up and the energy needed to stay up.
Which takes more energy: lifting a satellite 400 km up, or giving it the sideways speed to stay there?
- Yes.Getting to 400 km is a few per cent of the job. Reaching around 7.8 km/s sideways is most of the fuel, most of the rocket, and most of the difficulty.
- Not quite.Height is expensive when you think of a ladder, and cheap compared with the speed required to stay up once you get there.
- Not quite.The energy in the sideways motion is many times the energy of the climb, which is why rockets tip over so early in a launch.
Space is not far away. Orbit is fast. A rocket spends a minute going up and eight minutes turning sideways.
Slide the altitude and watch how fast an orbit has to be, and how long it takes.
400 km20,000 km36,000 kmOrbital speed7.7 km/sA rifle bullet, for comparison0.9 km/s400 km7.7 km/s, once round in about 90 minutes. Low orbit: fast, close, and still touched by traces of atmosphere.
Orbital speed3.9 km/sA rifle bullet, for comparison0.9 km/s20,000 km3.9 km/s, twelve hours per orbit. Where navigation satellites sit.
Orbital speed3.1 km/sA rifle bullet, for comparison0.9 km/s36,000 km3.1 km/s, exactly 24 hours. It hangs over one spot on the equator.
Higher orbits are SLOWER. Why does it still take more energy to reach them?
- Yes.The total energy — height plus motion — rises with altitude even though the speed falls. It is one of the least intuitive facts in the subject and it explains the whole shape of a mission plan.
- Not quite.Fuel is spent changing speed, not covering distance. Coasting is free.
- Not quite.A satellite at 36,000 km costs far more to place than one at 400 km, despite orbiting more slowly.
Move the control to see what changes.
Orbital speed at 400 km is about 7.7 km/s. A passenger jet cruises at about 0.25 km/s. How many times faster is the orbit, to the nearest whole number?
timesYes.About thirty times. The energy in motion grows with the square of speed, so it is not thirty times harder — it is closer to nine hundred times, per kilogram.Which of these are about getting UP, and which about staying up?
Clearing the thickest part of the atmosphere.
Reaching 7.8 km/s sideways.
The rocket tipping over a minute after launch.
Fighting drag in the first 60 seconds.
The long, shallow burn most of the way to orbit.
Yes.Watch any launch and you will see the switch: a short vertical climb to get out of the thick air, then a long tilt that spends most of the fuel going sideways.A tourist flight reaches 100 km and comes straight back down. Is that spaceflight?
- Yes.The altitude is genuine and the speed is a small fraction of orbital. Both facts belong in any honest description, and headlines usually carry only the first.
- Not quite.True about altitude, and it hides a difference of roughly thirty times in speed and far more in energy.
- Not quite.At 100 km it is above essentially all of it. The missing ingredient is speed, not height.
Lesson complete
Space is not far. Orbit is fast, and the speed is nearly all of the cost.
Next: To catch up, slow down →