You have things like: The vacuum of space Heat managementproblems The difficulty of re-entry Orbital mechanics Micrometeorites and spacedebris Cosmic and solar radiation Restroom facilities in a weightlessenvironment And so on. . . But the biggest problem of all is harnessing enoughenergy simply to get a spaceship off the ground. That is where rocket enginescome in.
Rocket engines are on the one hand so simple that you can build and flyyour own model rockets very inexpensively (see the links at the bottom of thepage for details). On the other hand, rocket engines (and their fuel systems)are so complicated that only two countries have actually ever put people inorbit. In this edition of How Stuff Works we will look at rocket engines tounderstand how they work, as well as to understand some of the complexity. TheBasics When most people think about motors or engines, they think aboutrotation. For example, a reciprocating gasoline engine in a car producesrotational energy to drive the wheels.
An electric motor produces rotationalenergy to drive a fan or spin a disk. A steam engine is used to do the samething, as is a steam turbine and most gas turbines. Rocket engines arefundamentally different. Rocket engines are reaction engines.
The basicprinciple driving a rocket engine is the famous Newtonian principle that”to every action there is an equal and opposite reaction”. A rocketengine is throwing mass in one direction and benefiting from the reaction thatoccurs in the other direction as a result. This concept of “throwing massand benefiting from the reaction” can be hard to grasp at first, becausethat does not seem to be what is happening. Rocket engines seem to be aboutflames and noise and pressure, not “throwing things”. So let’s look ata few examples to get a better picture of reality: If you have ever shot ashotgun, 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” yourshoulder back with a great deal of force. That kick is a reaction. A shotgun isshooting 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 rollerskates or standing on a skate board when you shot the gun, then the gun would beacting like a rocket engine and you would react by rolling in the oppositedirection. If you have ever seen a big fire hose spraying water, you may havenoticed that it takes a lot of strength to hold the hose (sometimes you will seetwo 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 theirstrength and weight to counteract the reaction. If they were to let go of thehose, it would thrash around with tremendous force. If the firemen were allstanding 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 beforerunning out of air, you have created a rocket engine. In this case, what isbeing thrown is the air molecules inside the balloon.
Many people believe thatair molecules don’t weigh anything, but they do (see the page on helium to get abetter picture of the weight of air). When you throw them out the nozzle of aballoon the rest of the balloon reacts in the opposite direction. Imagine thefollowing situation. Let’s say that you are wearing a space suit and you arefloating in space beside the space shuttle.
You happen to have in your hand abaseball. If you throw the baseball, your body will react by moving away in theopposite direction. The thing that controls the speed at which your body movesaway is the weight of the baseball that you throw and the amount of accelerationthat 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 identicalreaction force applied to your body (m * a = m * a). So let’s say that thebaseball weighs 1 pound and your body plus the space suit weighs 100 pounds. Youthrow the baseball away at a speed of 32 feet per second (21 MPH). That is tosay, you accelerate the baseball with your arm so that it obtains a velocity of21 MPH. What you had to do is accelerate the one pound baseball to 21 MPH.
Yourbody reacts, but it weights 100 times more than the baseball. Therefore it movesaway at 1/100th the velocity, or 0. 32 feet per second (0. 21 MPH). If you want togenerate more thrust from your baseball, you have two options.
You can eitherthrow a heavier baseball (increase the mass), or you can throw the baseballfaster (increasing the acceleration on it), or you can throw a number ofbaseballs 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 inthe form of a high-pressure gas. The engine throws the mass of gas out in onedirection in order to get a reaction in the opposite direction.
The mass comesfrom the weight of the fuel that the rocket engine burns. The burning processaccelerates the mass of fuel so that it comes out of the rocket nozzle at highspeed. The fact that the fuel turns from a solid or liquid into a gas when itburns does not change its mass. If you burn a pound of rocket fuel, a pound ofexhaust comes out the nozzle in the form of a high-temperature, high-velocitygas.
The form changes, but the mass does not. The burning process acceleratesthe mass. The “strength” of a rocket engine is called its thrust. Thrust is measured in “pounds of thrust” in the U.
S. and in newtonsunder the metric system (4. 45 newtons of thrust equals 1 pound of thrust). Apound of thrust is the amount of thrust it would take to keep a one pound objectstationary against the force of gravity on earth. So on earth the accelerationof gravity is 32 feet per second per second (21 MPH per second). So if you werefloating in space with a bag of baseballs and you threw 1 baseball per secondaway from you at 21 MPH, your baseballs would be generating the equivalent of 1pound of thrust.
If you were to throw the baseballs instead at 42 MPH, then youwould be generating 2 pounds of thrust. If you throw them at 2,100 MPH (perhapsby shooting them out of some sort of baseball gun), then you are generating 100pounds of thrust, and so on. One of the funny problems rockets have is that theobjects that the engine wants to throw actually weigh something, and the rockethas to carry that weight around. So let’s say that you want to generate 100pounds of thrust for an hour by throwing 1 baseball every second at a speed of2,100 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 youonly 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 fuelweights 36 times more than the payload. And that is very common. That is why youhave to have a huge rocket to get a tiny person into space right now – you haveto carry a lot of fuel.
You can see this weight equation very clearly on theSpace Shuttle. If you have ever seen the Space Shuttle launch, you know thatthere are three parts: the shuttle itself the big external tank the two solidrocket boosters (SRBs). The shuttle weighs 165,000 pounds empty. The externaltank weighs 78,100 pounds empty. The two solid rocket boosters weigh 185,000pounds empty each.
But then you have to load in the fuel. Each SRB holds 1. 1million 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). Thewhole vehicle – shuttle, external tank, solid rocket booster casings and all thefuel – has a total weight of 4.
4 million pounds at launch. 4. 4 million pounds toget 165,000 pounds in orbit is a pretty big difference! To be fair, the shuttlecan also carry a 65,000 pound payload (up to 15 x 60 feet in size), but it isstill 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 beingthrown 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 and10,000 MPH). The SRBs burn for about 2 minutes and generate about 3. 3 millionpounds of thrust each at launch (2. 65 million pounds average over the burn).
The3 main engines (which use the fuel in the external tank) burn for about 8minutes, generating 375,000 pounds of thrust each during the burn. Solid-fuelRocket 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 widelysince then. The line about “the rocket’s red glare” in the NationalAnthem (written in the early 1800’s) is talking about small military solid-fuelrockets used to deliver bombs or incendiary devices. So you can see that rocketshave been in use quite awhile. The idea behind a simple solid-fuel rocket isstraightforward.
What you want to do is create something that burns very quicklybut does not explode. As you are probably aware, gunpowder explodes. Gunpowderis made up 75% nitrate, 15% carbon and 10% sulfur. In a rocket engine you don’twant an explosion – you would like the power released more evenly over a periodof 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. Thissort 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 afterignition On the left you see the rocket before ignition. The solid fuel is shownin green. It is cylindrical, with a tube drilled down the middle.
When you lightthe fuel, it burns along the wall of the tube. As it burns, it burns outwardtoward the casing until all the fuel has burned. In a small model rocket engineor in a tiny bottle rocket the burn might last a second or less. In a SpaceShuttle SRB containing over a million pounds of fuel, the burn lasts about 2minutes. When you read about advanced solid-fuel rockets like the Shuttle’sSolid Rocket Boosters, you often read things like: The propellant mixture ineach SRB motor consists of an ammonium perchlorate (oxidizer, 69. 6 percent byweight), aluminum (fuel, 16 percent), iron oxide (a catalyst, 0.
4 percent), apolymer (a binder that holds the mixture together, 12. 04 percent), and an epoxycuring agent (1. 96 percent). The propellant is an 11-point star-shapedperforation in the forward motor segment and a double- truncated- coneperforation in each of the aft segments and aft closure. This configurationprovides high thrust at ignition and then reduces the thrust by approximately athird 50 seconds after lift-off to prevent overstressing the vehicle duringmaximum dynamic pressure. This paragraph discusses not only the fuel mixture butalso the configuration of the channel drilled in the center of the fuel.
An”11-point star-shaped perforation” might look like this: The idea isto increase the surface area of the channel, thereby increasing the burn areaand 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 lowerthrust in the middle of the flight. Solid-fuel rocket engines have threeimportant advantages: Simplicity Low cost Safety They also have twodisadvantages: Thrust cannot be controlled Once ignited, the engine cannot bestopped or restarted The disadvantages mean that solid-fuel rockets are usefulfor short-lifetime tasks (like missiles), or for booster systems.
When you needto be able to control the engine, you must use a liquid propellant system. Liquid Propellant Rockets In 1926, Robert Goddard tested the first liquidpropellant rocket engine. His engine used gasoline and liquid oxygen. He alsoworked 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 basicidea 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 to10,000 MPH exit velocities being typical), and then leave the engine.
Thefollowing highly simplified diagram shows you the basic components. This diagramdoes not show the actual complexities of a typical engine (see some of the linksat the bottom of the page for good images and descriptions of real engines). Forexample, it is normal for either the fuel of the oxidizer to be a cold liquefiedgas like liquid hydrogen or liquid oxygen. One of the big problems in a liquidpropellant rocket engine is cooling the combustion chamber and nozzle, so thecryogenic liquids are first circulated around the super-heated parts to coolthem. The pumps have to generate extremely high pressures in order to overcomethe pressure that the burning fuel creates in the combustion chamber. The mainengines in the Space Shuttle actually use two pumping stages and burn fuel todrive the second stage pumps.
All of this pumping and cooling makes a typicalliquid propellant engine look more like a plumbing project gone haywire thananything else – look at the engines on this page to see what I mean. All kindsof fuel combinations get used in liquid propellant rocket engines. For example:Liquid hydrogen and liquid oxygen – used in the Space Shuttle main enginesGasoline and liquid oxygen – used in Goddard’s early rockets Kerosene and liquidoxygen – used on the first stage of the large Saturn V boosters in the Apolloprogram Alcohol and Liquid Oxygen – used in the German V2 rockets Nitrogentetroxide (NTO)/monomethyl hydrazine (MMH) – used in the Cassini engines OtherPossibilities We are accustomed to seeing chemical rocket engines that burntheir fuel to generate thrust. There are many other ways to generate thrusthowever. Any system that throws mass would do. If you could figure out a way toaccelerate baseballs to extremely high speeds, you would have a viable rocketengine.
The only problem with such an approach would be the baseball”exhaust” (high-speed baseballs at that. . . ) left streaming throughspace. This small problem causes rocket engine designers to favor gases for theexhaust product.
Many rocket engines are very small. For example, attitudethrusters on satellites don’t need to produce much thrust. One common enginedesign found on satellites uses no “fuel” at all – pressurizednitrogen 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’smanned maneuvering system.
New engine designs are trying to find ways toaccelerate ions or atomic particles to extremely high speeds to create thrustmore efficiently. NASA’s Deep Space-1 spacecraft will be the first to use ionengines for propulsion. See this page for additional discussion of plasma andion engines. This article discusses a number of other alternatives.Science