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In all liquid propellant rocket engines some of the propellants are used for purposes other than producing thrust or increasing the velocity of the vehicle. This propellant must also be included in the propellant tanks. A propellant budget can include the eleven items listed below, but very few engines have allowances for all these items. Table l(M shows an example of a budget for a spacecraft pressure-fed engine system with several small thrusters and one larger thrust chamber.

1. Enough propellant has to be a available for achieving the required vehicle velocity increase of the particular application and the particular flight vehicle or stage. The nominal velocity increment is usually defined by systems analysis and mission optimization using an iterative calculation based on Eqs. 4-19 or 4-35. If there are alternative flight paths or missions for the same vehicle, the mission with an unfavorable flight path and the highest total impulse should be selected. This mission-required propellant is the largest portion of the total propellants loaded into the vehicle tanks.

2. In a turbopump system using a gas generator cycle, a small portion of the overall propellant is burned in a separate gas generator. It has a lower flame temperature than the thrust chamber and operates at a different mixture ratio; this causes a slight change in the overall mixture ratio of propellants flowing from the tanks, as shown by Eqs. 10-14 and 10-16.

TABLE 10-4. Example of a Propellant Budget for a Spacecraft Propulsion System with a Pressurized Monopropellant Feed System

Budget Element Typical Value

TABLE 10-4. Example of a Propellant Budget for a Spacecraft Propulsion System with a Pressurized Monopropellant Feed System

Budget Element Typical Value

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