There
is another important factor affecting the performance of a rocket engine.
The mass of a rocket can make the difference between a successful flight
and just wallowing around on the launch pad. As a basic principle of rocket
flight, it can be said that for a rocket to leave the ground, the rocket
engine must produce a thrust that is greater than the total mass of the
vehicle. It is obvious that a rocket with a lot of unnecessary mass will
not be as efficient as one that is trimmed to just the bare essentials.
For an ideal rocket, the total mass of the vehicle should be distributed
following this general formula:
-
Of the total mass, 91 percent should
be propellants; 3 percent should be tanks, engines, fins, etc.; and 6 percent
can be the payload.
Payloads may be satellites, astronauts,
or spacecraft that will travel to other planets or moons.
In determining the effectiveness
of a rocket design, rocketeers speak in terms of mass fraction (MF). The
mass of the propellants of the rocket divided by the total mass of the
rocket gives mass fraction:
MF = (Mass of Propellants)/(Total
Mass)
The mass fraction of the ideal rocket
given above is 0.91. From the mass fraction formula one might think that
an MF of 1.0 is perfect, but then the entire rocket would be nothing more
than a lump of propellants that would simply ignite into a fireball. The
larger the MF number, the less payload the rocket can carry; the smaller
the MF number, the less its range becomes. An MF number of 0.91 is a good
balance between payload-carrying capability and range. The Space Shuttle
has an MF of approximately 0.82. The MF varies between the different orbiters
in the Space Shuttle fleet and with the different payload weights of each
mission.
Large rockets, able to carry a spacecraft
into space have serious weight problems. To reach space find proper orbital
velocities, a great deal of propellant is needed; therefore, the tanks,
engines, and associated hardware become larger. Up to a point, bigger rockets
fly farther than smaller rockets, but when they become too large their
structures weigh them down too much, and the mass fraction is reduced to
an impossible number.
A solution to the problem of giant
rockets weighing too much can be credited to the 16th-century fireworks
maker Johann Schmidlap. Schmidlap
attached small rockets to the top of big ones. When the large rocket was
exhausted, the rocket casing was dropped behind and the remaining rocket
fired. Much higher altitudes were achieved by this method. (The Space Shuttle
follows the step rocket principle by dropping off its solid rocket boosters
and external tank when they are exhausted of propellants.) The rockets
used by Schmidlap were called step rockets. Today this technique of building
a rocket is called staging. Thanks to staging, it has become possible not
only to reach outer space but the Moon and other planets too.
Information and Images Provided
by NASA
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