Delta-v: the total change in velocity a mission must produce. Every burn adds to the delta-v total.
- Propellant consumed is tied to delta-v. A mission that needs less total delta-v needs less propellant — which means less mass, smaller tanks, and more margin for the burns that matter.
- Coasting mission: TLI plus one small midcourse correction. Total delta-v is dominated by TLI alone.
- Continuous-burn mission: the engine runs throughout the transit to hold the trajectory. Delta-v accumulates every second, and the fuel gauge empties long before arrival.
- The Apollo strategy is therefore not clever engine technology — it is a budget decision: spend delta-v only where physics cannot do the job for free.