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.