Rocket Engines
One of the most amazing endeavors man has ever undertaken is the
exploration of
space. A big part of the amazement is the complexity. Space
exploration is
complicated because there are so many interesting problems to
solve and
obstacles to overcome. You have things like: The vacuum of space
Heat management
problems The difficulty of re-entry Orbital mechanics
Micrometeorites and space
debris Cosmic and solar radiation Restroom
facilities in a weightless
environment And so on... But the biggest problem
of all is harnessing enough
energy simply to get a spaceship off the ground.
That is where rocket engines
come in. Rocket engines are on the one hand so
simple that you can build and fly
your own model rockets very inexpensively
(see the links at the bottom of the
page for details). On the other hand,
rocket engines (and their fuel systems)
are so complicated that only two
countries have actually ever put people in
orbit. In this edition of How
Stuff Works we will look at rocket engines to
understand how they work, as
well as to understand some of the complexity. The
Basics When most people
think about motors or engines, they think about
rotation. For example, a
reciprocating gasoline engine in a car produces
rotational energy to drive
the wheels. An electric motor produces rotational
energy to drive a fan or
spin a disk. A steam engine is used to do the same
thing, as is a steam
turbine and most gas turbines. Rocket engines are
fundamentally different.
Rocket engines are reaction engines. The basic
principle driving a rocket
engine is the famous Newtonian principle that
"to every action there is an
equal and opposite reaction". A rocket
engine is throwing mass in one
direction and benefiting from the reaction that
occurs in the other direction
as a result. This concept of "throwing mass
and benefiting from the reaction"
can be hard to grasp at first, because
that does not seem to be what is
happening. Rocket engines seem to be about
flames and noise and pressure, not
"throwing things". So let's look at
a few examples to get a better picture of
reality: If you have ever shot a
shotgun, especially a big 12 guage shot gun,
then you know that it has a lot of
"kick". That is, when you shoot the gun it
"kicks" your
shoulder back with a great deal of force. That kick is a
reaction. A shotgun is
shooting about an ounce of metal in one direction at
about 700 miles per hour.
Therefore your shoulder gets hit with the
reaction. If you were wearing roller
skates or standing on a skate board when
you shot the gun, then the gun would be
acting like a rocket engine and you
would react by rolling in the opposite
direction. If you have ever seen a big
fire hose spraying water, you may have
noticed that it takes a lot of
strength to hold the hose (sometimes you will see
two or three firemen
holding the hose). The hose is acting like a rocket engine.
The hose is
throwing water in one direction, and the firemen are using their
strength and
weight to counteract the reaction. If they were to let go of the
hose, it
would thrash around with tremendous force. If the firemen were all
standing
on skateboards, the hose would propel them backwards at great speed!
When
you blow up a balloon and let it go so it flies all over the room
before
running out of air, you have created a rocket engine. In this case,
what is
being thrown is the air molecules inside the balloon. Many people
believe that
air molecules don't weigh anything, but they do (see the page on
helium to get a
better picture of the weight of air). When you throw them out
the nozzle of a
balloon the rest of the balloon reacts in the opposite
direction. Imagine the
following situation. Let's say that you are wearing a
space suit and you are
floating in space beside the space shuttle. You happen
to have in your hand a
baseball. If you throw the baseball, your body will
react by moving away in the
opposite direction. The thing that controls the
speed at which your body moves
away is the weight of the baseball that you
throw and the amount of acceleration
that you apply to it. Mass multiplied by
acceleration is force (f = m * a).
Whatever force you apply to the
baseball will be equalized by an identical
reaction force applied to your
body (m * a = m * a). So let's say that the
baseball weighs 1 pound and your
body plus the space suit weighs 100 pounds. You
throw the baseball away at a
speed of 32 feet per second (21 MPH). That is to
say, you accelerate the
baseball with your arm so that it obtains a velocity of
21 MPH. What you
had to do is accelerate the one pound baseball to 21 MPH. Your
body reacts,
but it weights 100 times more than the baseball. Therefore it moves
away at
1/100th the velocity, or 0.32 feet per second (0.21 MPH). If you want
to
generate more thrust from your baseball, you have two options. You can
either
throw a heavier baseball (increase the mass), or you can throw the
baseball
faster (increasing the acceleration on it), or you can throw a
number of
baseballs one after another (which is just another way of
increasing the mass).
But that is all that you can do. A rocket engine is
generally throwing mass in
the form of a high-pressure gas. The engine throws
the mass of gas out in one
direction in order to get a reaction in the
opposite direction. The mass comes
from the weight of the fuel that the
rocket engine burns. The burning process
accelerates the mass of fuel so that
it comes out of the rocket nozzle at high
speed. The fact that the fuel turns
from a solid or liquid into a gas when it
burns does not change its mass. If
you burn a pound of rocket fuel, a pound of
exhaust comes out the nozzle in
the form of a high-temperature, high-velocity
gas. The form changes, but the
mass does not. The burning process accelerates
the mass. The "strength" of a
rocket engine is called its thrust.
Thrust is measured in "pounds of
thrust" in the U.S. and in newtons
under the metric system (4.45 newtons of
thrust equals 1 pound of thrust). A
pound of thrust is the amount of thrust
it would take to keep a one pound object
stationary against the force of
gravity on earth. So on earth the acceleration
of gravity is 32 feet per
second per second (21 MPH per second). So if you were
floating in space with
a bag of baseballs and you threw 1 baseball per second
away from you at 21
MPH, your baseballs would be generating the equivalent of 1
pound of thrust.
If you were to throw the baseballs instead at 42 MPH, then you
would be
generating 2 pounds of thrust. If you throw them at 2,100 MPH (perhaps
by
shooting them out of some sort of baseball gun), then you are generating
100
pounds of thrust, and so on. One of the funny problems rockets have is
that the
objects that the engine wants to throw actually weigh something, and
the rocket
has to carry that weight around. So let's say that you want to
generate 100
pounds of thrust for an hour by throwing 1 baseball every second
at a speed of
2,100 MPH. That means that you have to start with 3,600 one
pound baseballs
(there are 3,600 seconds in an hour), or 3,600 pounds of
baseballs. Since you
only weigh 100 pounds in your spacesuit, you can see
that the weight of your
"fuel" dwarfs the weight of the payload (you). In
fact, the fuel
weights 36 times more than the payload. And that is very
common. That is why you
have to have a huge rocket to get a tiny person into
space right now - you have
to carry a lot of fuel. You can see this weight
equation very clearly on the
Space Shuttle. If you have ever seen the
Space Shuttle launch, you know that
there are three parts: the shuttle itself
the big external tank the two solid
rocket boosters (SRBs). The shuttle
weighs 165,000 pounds empty. The external
tank weighs 78,100 pounds empty.
The two solid rocket boosters weigh 185,000
pounds empty each. But then you
have to load in the fuel. Each SRB holds 1.1
million pounds of fuel. The
external tank holds 143,000 gallons of liquid oxygen
(1,359,000 pounds) and
383,000 gallons of liquid hydrogen (226,000 pounds). The
whole vehicle -
shuttle, external tank, solid rocket booster casings and all the
fuel - has a
total weight of 4.4 million pounds at launch. 4.4 million pounds to
get
165,000 pounds in orbit is a pretty big difference! To be fair, the
shuttle
can also carry a 65,000 pound payload (up to 15 x 60 feet in size),
but it is
still a big difference. The fuel weighs almost 20 times more than
the Shuttle.
[Reference: The Space Shuttle Operator's Manual] All of that
fuel is being
thrown out the back of the Space Shuttle at a speed of perhaps
6,000 MPH
(typical rocket exhaust velocities for chemical rockets range
between 5,000 and
10,000 MPH). The SRBs burn for about 2 minutes and
generate about 3.3 million
pounds of thrust each at launch (2.65 million
pounds average over the burn). The
3 main engines (which use the fuel in
the external tank) burn for about 8
minutes, generating 375,000 pounds of
thrust each during the burn. Solid-fuel
Rocket Engines Solid-fuel rocket
engines were the first engines created by man.
They were invented
hundreds of years ago in China and have been used widely
since then. The line
about "the rocket's red glare" in the National
Anthem (written in the
early 1800's) is talking about small military solid-fuel
rockets used to
deliver bombs or incendiary devices. So you can see that rockets
have been in
use quite awhile. The idea behind a simple solid-fuel rocket
is
straightforward. What you want to do is create something that burns very
quickly
but does not explode. As you are probably aware, gunpowder explodes.
Gunpowder
is made up 75% nitrate, 15% carbon and 10% sulfur. In a rocket
engine you don't
want an explosion - you would like the power released more
evenly over a period
of time. Therefore you might change the mix to 72%
nitrate, 24% carbon and 4%
sulfur. In this case, instead of gunpowder, you
get a simple rocket fuel. This
sort of mix will burn very rapidly, but it
does not explode if loaded properly.
Here's a typical cross section: A
solid-fuel rocket immediately before and after
ignition On the left you see
the rocket before ignition. The solid fuel is shown
in green. It is
cylindrical, with a tube drilled down the middle. When you light
the fuel, it
burns along the wall of the tube. As it burns, it burns outward
toward the
casing until all the fuel has burned. In a small model rocket engine
or in a
tiny bottle rocket the burn might last a second or less. In a
Space
Shuttle SRB containing over a million pounds of fuel, the burn
lasts about 2
minutes. When you read about advanced solid-fuel rockets like
the Shuttle's
Solid Rocket Boosters, you often read things like: The
propellant mixture in
each SRB motor consists of an ammonium perchlorate
(oxidizer, 69.6 percent by
weight), aluminum (fuel, 16 percent), iron oxide
(a catalyst, 0.4 percent), a
polymer (a binder that holds the mixture
together, 12.04 percent), and an epoxy
curing agent (1.96 percent). The
propellant is an 11-point star-shaped
perforation in the forward motor
segment and a double- truncated- cone
perforation in each of the aft segments
and aft closure. This configuration
provides high thrust at ignition and then
reduces the thrust by approximately a
third 50 seconds after lift-off to
prevent overstressing the vehicle during
maximum dynamic pressure. This
paragraph discusses not only the fuel mixture but
also the configuration of
the channel drilled in the center of the fuel. An
"11-point star-shaped
perforation" might look like this: The idea is
to increase the surface area
of the channel, thereby increasing the burn area
and therefore the thrust. As
the fuel burns the shape evens out into a circle.
In the case of the
SRBs, it gives the engine high initial thrust and lower
thrust in the middle
of the flight. Solid-fuel rocket engines have three
important advantages:
Simplicity Low cost Safety They also have two
disadvantages: Thrust cannot be
controlled Once ignited, the engine cannot be
stopped or restarted The
disadvantages mean that solid-fuel rockets are useful
for short-lifetime
tasks (like missiles), or for booster systems. When you need
to be able to
control the engine, you must use a liquid propellant system.
Liquid
Propellant Rockets In 1926, Robert Goddard tested the first liquid
propellant
rocket engine. His engine used gasoline and liquid oxygen. He also
worked on
and solved a number of fundamental problems in rocket engine
design,
including pumping mechanisms, cooling strategies and steering
arrangements.
These problems are what make liquid propellant rockets so
complicated. The basic
idea is simple. In most liquid propellant rocket
engines, a fuel and an oxidizer
(for example, gasoline and liquid oxygen) are
pumped into a combustion chamber.
There they burn to create a
high-pressure and high-velocity stream of hot gases.
These gases flow
through a nozzle which accelerates them further (5,000 to
10,000 MPH exit
velocities being typical), and then leave the engine. The
following highly
simplified diagram shows you the basic components. This diagram
does not show
the actual complexities of a typical engine (see some of the links
at the
bottom of the page for good images and descriptions of real engines).
For
example, it is normal for either the fuel of the oxidizer to be a cold
liquefied
gas like liquid hydrogen or liquid oxygen. One of the big problems
in a liquid
propellant rocket engine is cooling the combustion chamber and
nozzle, so the
cryogenic liquids are first circulated around the super-heated
parts to cool
them. The pumps have to generate extremely high pressures in
order to overcome
the pressure that the burning fuel creates in the
combustion chamber. The main
engines in the Space Shuttle actually use two
pumping stages and burn fuel to
drive the second stage pumps. All of this
pumping and cooling makes a typical
liquid propellant engine look more like a
plumbing project gone haywire than
anything else - look at the engines on
this page to see what I mean. All kinds
of fuel combinations get used in
liquid propellant rocket engines. For example:
Liquid hydrogen and liquid
oxygen - used in the Space Shuttle main engines
Gasoline and liquid
oxygen - used in Goddard's early rockets Kerosene and liquid
oxygen - used on
the first stage of the large Saturn V boosters in the Apollo
program Alcohol
and Liquid Oxygen - used in the German V2 rockets Nitrogen
tetroxide
(NTO)/monomethyl hydrazine (MMH) - used in the Cassini engines
Other
Possibilities We are accustomed to seeing chemical rocket engines
that burn
their fuel to generate thrust. There are many other ways to
generate thrust
however. Any system that throws mass would do. If you could
figure out a way to
accelerate baseballs to extremely high speeds, you would
have a viable rocket
engine. The only problem with such an approach would be
the baseball
"exhaust" (high-speed baseballs at that...) left streaming
through
space. This small problem causes rocket engine designers to favor
gases for the
exhaust product. Many rocket engines are very small. For
example, attitude
thrusters on satellites don't need to produce much thrust.
One common engine
design found on satellites uses no "fuel" at all -
pressurized
nitrogen thrusters simply blow nitrogen gas from a tank through a
nozzle.
Thrusters like these kept Skylab in orbit, and are also used on
the shuttle's
manned maneuvering system. New engine designs are trying to
find ways to
accelerate ions or atomic particles to extremely high speeds to
create thrust
more efficiently. NASA's Deep Space-1 spacecraft will be the
first to use ion
engines for propulsion. See this page for additional
discussion of plasma and
ion engines. This article discusses a number of
other alternatives.