Fire control predictor and fuel cell: what they have in common

1. Step 1. Fire control predictor — is part of — Anti-aircraft gun. It calculates the lead.
2. Step 2. Anti-aircraft gun — uses — Gunpowder. Gunpowder propels the projectile.
3. Step 3. Gunpowder — is used in — Rocket. The first rockets were powered by gunpowder.
4. Step 4. Rocket — evolved into — Liquid-propellant rocket engine. Goddard switched to liquid fuel.
5. Step 5. Liquid-propellant rocket engine — uses — Hydrogen. Hydrogen is an efficient fuel.
6. Step 6. Hydrogen — is used in — Fuel cell. The fuel is hydrogen.

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