Typical Performance Values

Typical values of representative performance parameters for different types of rocket propulsion are given in Table 2-1 and in Fig. 2-5.

Chemical rockets have relatively low values of specific impulse, relatively light machinery (i.e., low engine weight), a very high thrust capability, and therefore high acceleration and high specific power. At the other extreme, the ion propulsion devices have a very high specific impulse, but they must carry a heavy electrical power source with them to deliver the power necessary for high ejection velocities. The very low acceleration potential for the electrical propulsion units and those using solar radiation energy usually requires a long period for accelerating and thus these systems are best used for missions where the flight time is long. The low thrust values of electrical systems imply that they are not useful in fields of strong gravitational gradients (for takeoff or landing) but are best used in a true space flight mission.

The chemical systems (solid and liquid propellant rockets) are fully developed and widely used for many different vehicle applications. They are described in Chapters 5 to 15. Electrical propulsion has been in operation in many space flight applications (see Chapter 19). Some of the other types are still in their exploratory or development phase, but may become useful.

Example 2-3. As a comparison of different propulsion systems, compute the energy input and the propellant flow required for 100 N thrust with several types of propulsion systems.

SOLUTION. From Equations 2-13 and 2-19, m = F/{Isg0) power input = Pjet///int = \rhv\/r}-ml

From Table 2-1 typical values of Is and from experience typical internal efficiencies were selected. Depending on the propellant and the design, these values may vary somewhat. The equations above were solved for m and the power input as indicated in the table below.

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