Pump mass is the mass of the rocket engine pump. Pump mass is usually quite small, about a percent of the empty mass. The pump power density of rocket pumps is so high that from the overall performance standpoint, a powerful heavy pump is worth it. The reason rockets don't have heavier pumps and some don't have a pump at all, is because of the enormous cost of pumps. Given pump power and pump power density the pump mass can be calculated which is in turn used to calculate engine mass.

pump mass = pump power / pump power density

engine mass = combustion mass + nozzle mass + pump mass
 
 

This is used in pumped rocket and rocket cost.
 
 

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