Upmass is the maximum allowable mass that a rocket or spacecraft can carry from the Earth's surface into orbit. Mission planners track this metric strictly to balance fuel, crew supplies, scientific instruments, and structural components. Exceeding the upmass limit means the launch vehicle will lack the delta-v required to reach its target trajectory.
Heard on the job
We're restricted to 4,000 kilograms of upmass for this resupply run to the station.
Insider tip
Upmass is extremely expensive, often costing thousands of dollars per kilogram, which drives engineers to shave grams off every component.