Page 70 - Intro to Space Sciences Spacecraft Applications

Propulsion
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forces will be opposite the direction of motion, such as atmospheric drag,
but some may have positive components in the direction of intended
motion as can happen with solar pressure. Another external force, due to
a difference between the propellant exhaust pressure and the external
ambient pressure, may also exist as we will soon see.
The remaining term represents the change in momentum of the exhaust
gases. This exchange of momentum is the main contributor to the accelera-
tion of the rocket and this last quantity is known as the thrust term (T). Note
that the (vector) quantity Vo - 5 would be negative since the exhaust gasses
are ejected opposite the direction of the original velocity. Rewritten as B - V,,,
this quantity describes the velocity of the exhaust gasses relative to the rock-
et itself. We can define this term as the exhaust velocity %.
With the definition of terms given above, equation 3- 1 can be rewritten as:

dij -
m- = F,,, + ?; (3 - 3)
dt
which, in this simple form, is known as the rocket equation.

Rocket Thrust

Looking more closely at the results of the last section, it can be seen
that the thrust term of the rocket equation is proportional to both the pro-
pellant exhaust velocity (f - To = V,,) and the mass flow rate of propel-
lant (dddt = m). We can rewrite the thrust term to better show this
dependency:

To increase the thrust of a rocket then, one could try to increase either
the exhaust velocity of the propellant or the mass flow rate of propellant
through the rocket. To see how these quantities can be changed we must
consider the characteristics of a typical rocket system such as that shown
in Figure 3-2.

Mass Flow Rate. A rocket differs from a jet engine in that a rocket must
carry its own oxidizer as well as fuel supply, although there are some sys-
tems which simply use a single (mono) propellant. In many propulsion
systems, liquid propellants are used and are delivered to the combustion
chamber by mechanical pumps as depicted in the figure. The pumps con-