Effect Of Propulsion System On Vehicle Performance

This section gives several methods for improving flight vehicle performance. Most of these enhancements, listed below, are directly influenced by the selection or design of the propulsion system. A few of the flight vehicle performance improvements do not depend on the propulsion system. Most of those listed below apply to all missions, but some are peculiar to some missions only.

1. The effective exhaust velocity c or the specific impulse Is usually have a direct effect on the vehicle's flight performance. For example the vehicle final velocity increment Au can be inceased by a higher Is. This can be done by using a more energetic propellant (see Chapter 7 and 12), by a higher chamber pressure and, for upper stages operating at high altitudes, also by a larger nozzle area ratio.

2. The mass ratio m0/my has a logarithmic effect. It can be increased in several ways. One way is by reducing the final mass mf, which consists of the inert hardware plus the nonusable, residual propellant mass. Reducing the inert mass implies lighter structures, smaller payloads, lighter guidance/control devices, or less unavailable residual propellant; this means going to stronger structural materials at higher stresses, more efficient power supplies, or smaller electronic packages. During design there is always great emphasis to reduce all hardware masses and the residual propellants to their practical minima. Another way is to increase the initial mass, namely by increasing the thrust and adding more propellant, but with a minimum increase in the structure or propulsion system masses. It is possible to improve the effective mass ratio greatly by using two or more stages, as will be explained in Section 4.7.

3. Reducing the burning time (i.e., increasing the thrust level) will reduce the gravitational loss. However, the higher acceleration usually requires more structural and propulsion system mass, which in turn causes the mass ratio to be less favorable.

4. The drag, which can be considered as a negative thrust, can be reduced in at least four ways. The drag has several components: (a) The form drag depends on the aerodynamic shape. A slender pointed nose or sharp, thin leading edges of fins or wings have less drag than a stubby, blunt shape, (b) A vehicle with a small cross-sectional area has less drag. A propulsion design that can be packaged in a long, thin shape will be preferred, (c) The drag is proportional to the cross-sectional or frontal vehicle area. A higher propellant density will decrease the propellant volume and therefore will allow a smaller cross section, (d) The skin drag is caused by the friction of the air flowing over all the vehicle's outer surfaces. A smooth contour and a polished surface are usually better. The skin drag is also influenced by the propellant density, because it gives a smaller volume and thus a lower surface area, (e) The base drag is the fourth component; it is a function of the local ambient air pressure acting over the surface of the vehicle's base or bottom plate. It is influenced by the nozzle exit design (exit pressure) and the geometry of the vehicle base design. It is discussed further in Chapter 18.

5. The length of the propulsion nozzle often is a significant part of the overall vehicle or stage length. As was described in Chapter 3, there is an optimum nozzle contour and length, which can be determined by trade-off analysis. A shorter nozzle length allows a somewhat shorter vehicle; on many designs this implies a somewhat lighter vehicle structure and a slightly better vehicle mass ratio.

6. The final vehicle velocity at propulsion termination can be increased by increasing the initial velocity u0. By launching a satellite in an eastward direction the rotational speed of the earth is added to the final satellite orbital velocity. This tangential velocity of the earth is about 464 m/sec or 1523 ft/sec at the equator and about 408 m/sec or 1340 ft/sec for an easterly launch at Kennedy Space Center (latitude of 28.5° north). Conversely, a westerly satellite launch has a negative initial velocity and thus requires a higher-velocity increment. Another way to increase u is to launch a spacecraft from a satellite or an aircraft, which increases the initial vehicle velocity and allows launching in the desired direction, or to launch an air-to-surface missile from an airplane.

7. For vehicles that fly in the atmosphere it is possible to increase the range when aerodynamic lift is used to counteract gravity and reduce gravity losses. Using a set of wings or flying at an angle of attack increases the lift, but is also increases the drag. This lift can also be used to increase the maneuverability and trajectory flexibility.

8. When the flight velocity u is close to the rocket's effective exhaust velocity c, the propulsive efficiency is the highest (Eq. 2-23) and more of the rocket exhaust gas energy is transformed into the vehicle's flight energy. Trajectories where u is close in value to c for a major portion of the flight therefore need less propellant.

Several of these influencing parameters can be optimized. Therefore, for every mission of flight application there is an optimum propulsion system design and the propulsion parameters that define the optimum condition are dependent on vehicle or flight parameters.

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