Plough beam and liquid-propellant rocket engine: what they have in common

1. Step 1. Plough beam — is part of — Plough. The beam to which the traces are attached.
2. Step 2. Plough — is used in — Crop rotation. The three-field system with a heavy plough.
3. Step 3. Crop rotation — evolved into — Mineral fertilizer. Fertilizers solved the problem of fertility that crop rotation used to solve.
4. Step 4. Mineral fertilizer — is made of — Ammonia. Nitrogen fertilizers.
5. Step 5. Ammonia — is made of — Hydrogen. Hydrogen.
6. Step 6. Hydrogen — is used in — Liquid-propellant rocket engine. Hydrogen is an efficient fuel.

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