Info

Contingency

2-10% of total load"

Source: Adapted from data supplied by TRW, Inc. "Total load is sum of items 1, 2, and 7.

Source: Adapted from data supplied by TRW, Inc. "Total load is sum of items 1, 2, and 7.

3. In a rocket propulsion system with a thrust vector control (TVC) system, such as a swiveling thrust chamber or nozzle, the thrust vector will be rotated by a few degrees. Thrust vector control systems are described in Chapter 16. There is a slight decrease in the axial thrust and that reduces the vehicle velocity increment in item 1. The extra propellant needed to compensate for the small velocity reduction can be determined from the mission requirements and TVC duty cycle. It could be between 0.1 and 4% of the total propellant.

4. In some engines a small portion of cryogenic propellants is heated, vaporized, and used to pressurize cryogenic propellant tanks. A heat exchanger is used to heat liquid oxygen from the pump discharge and pressurize the oxygen tank, as shown schematically in Fig. 1-4. This method is used in the hydrogen and oxygen tanks of the Space Shuttle external tank (see Ref. 6-6).

5. Auxiliary rocket engines that provide for trajectory corrections, station keeping, maneuvers, or attitude control usually have a series of small restartable thrusters (see Chapter 4). The propellants for these auxiliary thrusters have to be included in the propellant budget if they are supplied from the same feed system and tanks as the larger rocket engine. Depending on the mission and the propulsion system concept, this auxiliary propulsion system can consume a significant portion of the available propellants.

6. The residual propellant that clings to tank walls or remains trapped in valves, pipes, injector passages, or cooling passages is unavailable for producing thrust. It is typically 0.5 to 2% of the total propellant load. It increases the final vehicle mass at thrust termination and reduces the final vehicle velocity slightly.

7. A loading uncertainty exists due to variations in tank volume or changes in propellant density or liquid level in the tank. This is typically 0.25 to 0.75% of the total propellant. It depends, in part, on the accuracy of the method of measuring the propellant mass during loading (weighing the vehicle, flow meters, level gages, etc.).

8. The off-nominal rocket performance is due to variations in the manufacture of hardware from one engine to another (such as slightly different pressure losses in a cooling jacket, in injectors and valves, or somewhat different pump characteristics); these cause slight changes in combustion behavior, mixture ratio, or specific impulse. If there are slight variations in mixture ratio, one of the two liquid propellants will be consumed fully and an unusable residue will remain in the other pro-pellant's tank. If a minimum total impulse requirement has to be met, extra propellant has to be tanked to allow for these mixture ratio variations. This can amount up to perhaps 2.0% of one of the propellants.

9. Operational factors can result in additional propellant requirements, such as filling more propellant than needed into a tank or incorrectly, adjusting regulators or control valves. It can also include the effect of changes in flight acceleration from the nominal value. For an engine that has been carefully calibrated and tested, this factor can be small, usually betwen 0.1 and 1.0%.

10. When using cryogenic propellants an allowance for evaporation and cooling down has to be included. It is the mass of extra propellant that is allowed to evaporate (and be vented overboard while the vehicle is waiting to be launched) or that is fed through the engine to cool it down, before the remaining propellant in the tank becomes less than the minimum needed for the flight mission. Its quantity depends on the amount of time between topping off (partial refilling) of the tank.

11. Finally, an overall contingency or ignorance factor is needed to allow for unforeseen propellant needs or inadequate or uncertain estimates of any of the items above. This can also include allowances for vehicle drag uncertainties, variations in the guidance and control system, wind, or leaks.

Only some of the items above provide axial thrust (items 1, 2, and sometimes also 3 and 5), but all the items need to be considered in determining the total propellant mass and volume.

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