The first rockets
ever built, the fire-arrows of the Chinese, were not very reliable. Many
just exploded on launching. Others flew on erratic courses and landed
in the wrong place. Being a rocketeer in the days of the fire-arrows must
have been an exciting, but also a highly dangerous activity.
rockets are much more reliable. They fly on precise courses and are capable
of going fast enough to escape the gravitational pull of Earth. Modern
rockets are also more efficient today because we have an understanding
of the scientific principles behind rocketry. Our understanding has led
us to develop a wide variety of advanced rocket hardware and devise new
propellants that can be used for longer trips and more powerful takeoffs.
Rocket Engines and
Most rockets today operate
with either solid or liquid propellants. The word propellant does not mean
simply fuel, as you might think; it means both fuel and oxidizer. The fuel
is the chemical rockets burn but, for burning to take place, an oxidizer
(oxygen) must be present. Jet engines draw oxygen into their engines from
the surrounding air. Rockets do not have the luxury that jet planes have;
they must carry oxygen with them into space, where there is no air.
Solid rocket propellants,
which are dry to the touch, contain both the fuel and oxidizer combined
together in the chemical itself. Usually the fuel is a mixture of hydrogen
compounds and carbon and the oxidizer is made up of oxygen compounds.
Liquid propellants, which are often gases that have been chilled until
they turn into liquids, are kept in separate containers, one for the fuel
and the other for the oxidizer. Then, when the engine fires, the fuel
and oxidizer are mixed together in the engine.
rocket has the simplest form of engine. It has a nozzle, a case, insulation,
propellant, and an igniter. The case of the engine is usually a relatively
thin metal that is lined with insulation to keep the propellant from burning
through. The propellant itself is packed inside the insulation layer.
rocket engines feature a hollow core that runs through the propellant.
Rockets that do not have the hollow core must be ignited at the lower
end of the propellants and burning proceeds gradually from one end of
the rocket to the other. In all cases, only the surface of the propellant
burns. However, to get higher thrust, the hollow core is used. This increases
the surface of the propellants available for burning. The propellants
burn from the inside out at a much higher rate, and the gases produced
escape the engine at much higher speeds. This gives a greater thrust.
Some propellant cores are star shaped to increase the burning surface
To fire solid propellants,
many kinds of igniters can be used. Fire-arrows were ignited by fuses,
but sometimes these ignited too quickly and burned the rocketeer. A far
safer and more reliable form of ignition used today is one that employs
electricity. An electric current, coming through wires from some distance
away, heats up a special wire inside the rocket. The wire raises the temperature
of the propellant it is in contact with to the combustion point.
Other igniters are
more advanced than the hot wire device. Some are encased in a chemical
that ignites first, which then ignites the propellants. Still other igniters,
especially those for large rockets, are rocket engines themselves. The
small engine inside the hollow core blasts a stream of flames and hot
gas down from the top of the core and ignites the entire surface area
of the propellants in a fraction of a second.
The nozzle in a
solid-propellant engine is an opening at the back of the rocket that permits
the hot expanding gases to escape. The narrow part of the nozzle is the
throat. Just beyond the throat is the exit cone.
The purpose of the
nozzle is to increase the acceleration of the gases as they leave the
rocket and thereby maximize the thrust. It does this by cutting down the
opening through which the gases can escape. To see how this works, you
can experiment with a garden hose that has a spray nozzle attachment.
This kind of nozzle does not have an exit cone, but that does not matter
in the experiment. The important point about the nozzle is that the size
of the opening can be varied.
Start with the opening
at its widest point. Watch how far the water squirts and feel the thrust
produced by the departing water. Now reduce the diameter of the opening,
and again note the distance the water squirts and feel the thrust. Rocket
nozzles work the same way.
As with the inside
of the rocket case, insulation is needed to protect the nozzle from the
hot gases. The usual insulation is one that gradually erodes as the gas
passes through. Small pieces of the insulation get very hot and break
away from the nozzle. As they are blown away, heat is carried away with
The other main kind
of rocket engine is one that uses liquid propellants. This is a much more
complicated engine, as is evidenced by the fact that solid rocket engines
were used for at least seven hundred years before the first successful
liquid engine was tested. Liquid propellants have separate storage tanks
- one for the fuel and one for the oxidizer. They also have pumps, a combustion
chamber, and a nozzle.
The fuel of a liquid-propellant
rocket is usually kerosene or liquid hydrogen; the oxidizer is usually
liquid oxygen. They are combined inside a cavity called the combustion
chamber. Here the propellants burn and build up high temperatures and
pressures, and the expanding gas escapes through the nozzle at the lower
end. To get the most power from the propellants, they must be mixed as
completely as possible. Small injectors (nozzles) on the roof of the chamber
spray and mix the propellants at the same time. Because the chamber operates
under high pressures, the propellants need to be forced inside. Powerful,
lightweight turbine pumps between the propellant tanks and combustion
chambers take care of this job.
With any rocket,
and especially with liquid-propellant rockets, weight is an important
factor. In general, the heavier the rocket, the more the thrust needed
to get it off the ground. Because of the pumps and fuel lines, liquid
engines are much heavier than solid engines.
One especially good
method of reducing the weight of liquid engines is to make the exit cone
of the nozzle out of very lightweight metals. However, the extremely hot,
fast-moving gases that pass through the cone would quickly melt thin metal.
Therefore, a cooling system is needed. A highly effective though complex
cooling system that is used with some liquid engines takes advantage of
the low temperature of liquid hydrogen. Hydrogen becomes a liquid when
it is chilled to -253C. Before injecting the hydrogen into the combustion
chamber, it is first circulated through small tubes that lace the walls
of the exit cone. In a cutaway view, the exit cone wall looks like the
edge of corrugated cardboard. The hydrogen in the tubes absorbs the excess
heat entering the cone walls and prevents it from melting the walls away.
It also makes the hydrogen more energetic because of the heat it picks
up. We call this kind of cooling system regenerative cooling.
Engine Thrust Control
Controlling the thrust
of an engine is very important to launching payloads (cargoes) into orbit.
Too much thrust or thrust at the wrong time can cause a satellite to be
placed in the wrong orbit or set too far out into space to be useful. Too
little thrust can cause the satellite to fall back to Earth.
Liquid-propellant engines control the thrust by varying the amount of
propellant that enters the combustion chamber. A computer in the rocket's
guidance system determines the amount of thrust that is needed and controls
the propellant flow rate. On more complicated flights, such as going to
the Moon, the engines must be started and stopped several times. Liquid
engines do this by simply starting or stopping the flow of propellants
into the combustion chamber.
rockets are not as easy to control as liquid rockets. Once started, the
propellants burn until they are gone. They are very difficult to stop
or slow down part way into the burn. Sometimes fire extinguishers are
built into the engine to stop the rocket in flight. But using them is
a tricky procedure and doesn't always work. Some solid-fuel engines have
hatches on their sides that can be cut loose by remote control to release
the chamber pressure and terminate thrust.
The burn rate of
solid propellants is carefully planned in advance. The hollow core running
the length of the propellants can be made into a star shape. At first,
there is a very large surface available for burning, but as the points
of the star burn away, the surface area is reduced. For a time, less of
the propellant burns, and this reduces thrust. The Space Shuttle uses
this technique to reduce vibrations early in its flight into orbit.
Although most rockets
used by governments and research organizations are very reliable, there
is still great danger associated with the building and firing of rocket
engines. Individuals interested in rocketry should never attempt to build
their own engines. Even the simplest-looking rocket engines are very complex.
Case-wall bursting strength, propellant packing density, nozzle design,
and propellant chemistry are all design problems beyond the scope of most
amateurs. Many home-built rocket engines have exploded in the faces of their
builders with tragic consequences.
and Control Systems
Building an efficient
rocket engine is only part of the problem in producing a successful rocket.
The rocket must also be stable in flight. A stable rocket is one that flies
in a smooth, uniform direction. An unstable rocket flies along an erratic
path, sometimes tumbling or changing direction. Unstable rockets are dangerous
because it is not possible to predict where they will go. They may even
turn upside down and suddenly head back directly to the launch pad.
Making a rocket
stable requires some form of control system. Controls can be either active
or passive. The difference between these and how they work will be explained
later. It is first important to understand what makes a rocket stable
All matter, regardless
of size, mass, or shape, has a point inside called the center of mass
(CM). The center of mass is the exact spot where all of the mass of that
object is perfectly balanced. You can easily find the center of mass of
an object such as a ruler by balancing the object on your finger. If the
material used to make the ruler is of uniform thickness and density, the
center of mass should be at the halfway point between one end of the stick
and the other. If the ruler were made of wood, and a heavy nail were driven
into one of its ends, the center of mass would no longer be in the middle.
The balance point would then be nearer the end with the nail.
The center of mass
is important in rocket flight because it is around this point that an
unstable rocket tumbles. As a matter of fact, any object in flight tends
to tumble. Throw a stick, and it tumbles end over end. Throw a ball, and
it spins in flight. The act of spinning or tumbling is a way of becoming
stabilized in flight. A Frisbee will go where you want it to only if you
throw it with a deliberate spin. Try throwing a Frisbee without spinning
it. If you succeed, you will see that the Frisbee flies in an erratic
path and falls far short of its mark.
In flight, spinning
or tumbling takes place around one or more of three axes. They are called
roll, pitch, and yaw. The point where all three of these axes intersect
is the center of mass. For rocket flight, the pitch and yaw axes are the
most important because any movement in either of these two directions
can cause the rocket to go off course. The roll axis is the least important
because movement along this axis will not affect the flight path. In fact,
a rolling motion will help stabilize the rocket in the same way a properly
passed football is stabilized by rolling (spiraling) it in flight. Although
a poorly passed football may still fly to its mark even if it tumbles
rather than rolls, a rocket will not. The action-reaction energy of a
football pass will be completely expended by the thrower the moment the
ball leaves the hand. With rockets, thrust from the engine is still being
produced while the rocket is in flight. Unstable motions about the pitch
and yaw axes will cause the rocket to leave the planned course. To prevent
this, a control system is needed to prevent or at least minimize unstable
In addition to center
of mass, there is another important center inside the rocket that affects
its flight. This is the center of pressure (CP). The center of pressure
exists only when air is flowing past the moving rocket. This flowing air,
rubbing and pushing against the outer surface of the rocket, can cause
it to begin moving around one of its three axes. Think for a moment of
a weather vane. A weather vane is an arrow-like stick that is mounted
on a rooftop and used for telling wind direction. The arrow is attached
to a vertical rod that acts as a pivot point. The arrow is balanced so
that the center of mass is right at the pivot point. When the wind blows,
the arrow turns, and the head of the arrow points into the on-coming wind.
The tail of the arrow points in the downwind direction.
The reason that
the weather vane arrow points into the wind is that the tail of the arrow
has a much larger surface area than the arrowhead. The flowing air imparts
a greater force to the tail than the head, and therefore the tail is pushed
away. There is a point on the arrow where the surface area is the same
on one side as the other. This spot is called the center of pressure.
The center of pressure is not in the same place as the center of mass.
If it were, then neither end of the arrow would be favored by the wind
and the arrow would not point. The center of pressure is between the center
of mass and the tail end of the arrow. This means that the tail end has
more surface area than the head end.
It is extremely
important that the center of pressure in a rocket be located toward the
tail and the center of mass be located toward the nose. If they are in
the same place or very near each other, then the rocket will be unstable
in flight. The rocket will then try to rotate about the center of mass
in the pitch and yaw axes, producing a dangerous situation. With the center
of pressure located in the right place, the rocket will remain stable.
for rockets are intended to keep a rocket stable in flight and to steer
it. Small rockets usually require only a stabilizing control system. Large
rockets, such as the ones that launch satellites into orbit, require a
system that not only stabilizes the rocket, but also enable it to change
course while in flight.
Controls on rockets
can either be active or passive. Passive controls are fixed devices that
keep rockets stabilized by their very presence on the rocket's exterior.
Active controls can be moved while the rocket is in flight to stabilize
and steer the craft.
The simplest of
all passive controls is a stick. The Chinese fire-arrows were simple rockets
mounted on the ends of sticks. The stick kept the center of pressure behind
the center of mass. In spite of this, fire-arrows were notoriously inaccurate.
Before the center of pressure could take effect, air had to be flowing
past the rocket. While still on the ground and immobile, the arrow might
lurch and fire the wrong way.
Years later, the
accuracy of fire-arrows was improved considerably by mounting them in
a trough aimed in the proper direction. The trough guided the arrow in
the right direction until it was moving fast enough to be stable on its
As will be explained
in the next section, the weight of the rocket is a critical factor in
performance and range. The fire-arrow stick added too much dead weight
to the rocket, and therefore limited its range considerably.
An important improvement
in rocketry came with the replacement of sticks by clusters of lightweight
fins mounted around the lower end near the nozzle. Fins could be made
out of lightweight materials and be streamlined in shape. They gave rockets
a dartlike appearance. The large surface area of the fins easily kept
the center of pressure behind the center of mass. Some experimenters even
bent the lower tips of the fins in a pinwheel fashion to promote rapid
spinning in flight. With these "spin fins," rockets become much more stable
in flight. But this design also produces more drag and limits the rocket's
With the start of
modern rocketry in the 20th century, new ways were sought to improve rocket
stability and at the same time reduce overall rocket weight. The answer
to this was the development of active controls. Active control systems
included vanes, movable fins, canards, gimbaled nozzles, vernier rockets,
fuel injection, and attitude-control rockets. Tilting fins and canards
are quite similar to each other in appearance. The only real difference
between them is their location on the rockets. Canards are mounted on
the front end of the rocket while the tilting fins are at the rear. In
flight, the fins and canards tilt like rudders to deflect the air flow
and cause the rocket to change course. Motion sensors on the rocket detect
unplanned directional changes, and corrections can be made by slight tilting
of the fins and canards. The advantage of these two devices is size and
weight. They are smaller and lighter and produce less drag than the large
Other active control
systems can eliminate fins and canards altogether. By tilting the angle
at which the exhaust gas leaves the rocket engine, course changes can
be made in flight. Several techniques can be used for changing exhaust
Vanes are small
finlike devices that are placed inside the exhaust of the rocket engine.
Tilting the vanes deflects the exhaust, and by action-reaction the rocket
responds by pointing the opposite way.
Another method for
changing the exhaust direction is to gimbal the nozzle. A gimbaled nozzle
is one that is able to sway while exhaust gases are passing through it.
By tilting the engine nozzle in the proper direction, the rocket responds
by changing course
can also be used to change direction. These are small rockets mounted
on the outside of the large engine. When needed they fire, producing the
desired course change.
In space, only by
spinning the rocket along the roll axis or by using active controls involving
the engine exhaust can the rocket be stabilized or have its direction
changed. Without air, fins and canards have nothing to work upon. (Science
fiction movies showing rockets in space with wings and fins are long on
fiction and short on science.) The most common kinds of active control
used in space are attitude-control rockets. Small clusters of engines
are mounted all around the vehicle. By firing the right combination of
these small rockets, the vehicle can be turned in any direction. As soon
as they are aimed properly, the main engines fire, sending the rocket
off in the new direction.
There is another important
factor affecting the performance of a rocket. 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 engine must produce a thrust that
is greater than the weight of the vehicle. The weight is equal to the mass
of the vehicle times the gravitational acceleration. 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:
Payloads may be satellites,
astronauts, or spacecraft that will travel to other planets or moons.
- Of the total mass,
91 percent should be propellants; 3 percent should be tanks, engines,
fins, etc.; and 6 percent can be the payload.
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
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.