Lesson 04 · 3 min · 5 things to do
The tyranny of the fuel
Explain why rockets are almost entirely propellant.
- Propellant90% of mass at launch
- Structure and engines7% of mass at launch
- Payload3% of mass at launch
The whole point of the machine is the smallest slice. A rocket on the pad is about 90% propellant by mass. Why so much?
- Yes.Every kilogram of propellant near the end had to be lifted and accelerated by the propellant burned before it. The demand compounds, which is why the number is 90% and not 50%.
- Not quite.Rocket engines are among the most efficient machines ever built. The mass problem would exist with perfect engines.
- Not quite.Space is 100 km up. The propellant is for the speed, and for accelerating the rest of the propellant.
A rocket has to accelerate its own fuel. That single fact sets the shape of every launch vehicle ever built.
Slide the speed you need and watch how much of the rocket must be propellant.
1 km/s4 km/s9 km/s13 km/s- Propellant25% of mass
- Everything else75% of mass
1 km/sAbout a quarter propellant. Comfortable — this is roughly a large missile.
- Propellant66% of mass
- Everything else34% of mass
4 km/sAbout two thirds. The structure is getting thin.
- Propellant90% of mass
- Everything else10% of mass
9 km/sAbout 90%. Almost nothing left for tanks, engines and payload.
- Propellant97% of mass
- Everything else3% of mass
13 km/sAround 97%. A single-stage vehicle cannot be built this light.
The required speed roughly doubled from 4 to 9 km/s. Why did the propellant share rise so much more than double?
- Yes.That multiplication is the rocket equation, and it is why the difference between reaching space and reaching orbit is a completely different class of machine.
- Not quite.Better engines shift the curve a little. The shape stays exponential whatever the engine.
- Not quite.Drag costs a few per cent. The curve is there in the vacuum too.
Move the control to see what changes.
Why do rockets throw away their lower stages?
- Yes.Once a tank is empty it is dead weight that everything above must keep accelerating. Dropping it is the cheapest way to escape the exponential.
- Not quite.Engines can be reused, as several vehicles now demonstrate. Staging exists for the mass, not the wear.
- Not quite.Landing is an added difficulty. The reason for separating them in the first place is the fuel cost of carrying them.
Which of these meaningfully help a rocket reach orbit?
Dropping empty stages as soon as they are empty.
Launching eastward, with Earth's rotation.
Launching from near the equator.
Launching from a high mountain.
Launching on a clear day.
Yes.Altitude at launch saves a tiny share of the energy, because the problem is speed, not height. Earth's own rotation at the equator gives about 0.46 km/s for free — a real and useful head start.A rocket masses 500 tonnes at launch and carries a 15 tonne payload. What percentage of the launch mass is payload?
%Yes.15 ÷ 500. Everything else is fuel, tanks and engines to move those three per cent fast enough sideways.
Lesson complete
A rocket must accelerate its own fuel, which is why it is almost entirely fuel.
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