02 · The equation

Staging

Empty tanks are dead weight. Drop them and the log starts over. Same propellant, more Δv. SSTO is the same physics with worse ratios.

In English

Once a tank is empty it is just metal you are still accelerating. Drop it, and the next burn only has to push what is left. Same propellant, more Δv. That is staging.

Try this: Compare “one fat stage” with “drop the tank.” Same payload. Watch which one clears 9.4 km/s.
Keep this: Staging is the rocket equation applied twice. SSTO is stubborn.

Lab · drop the dry mass

Same payload (2 t). Same kind of propellant. One vehicle keeps the empty tank. The others throw it away. The log only cares about mass you are still carrying.

Stage 1 · 80 t prop + 8 t dry · Isp 300 s · 3.69 km/s
Stage 2 · 20 t prop + 2 t dry · Isp 350 s · 6.15 km/s
Payload 2 t

Liftoff mass: 112 t
Total Δv: 9.84 km/s · LEO

Each stage is a fresh mass ratio on a lighter stack. Dry mass you have already emptied is the enemy. Fairings and interstages are the same idea in miniature.

Reusable first stages spend some Δv to land. That is a tax on the log, paid because the dry mass is a vehicle you want back.