Rocket Engines

.. n 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.

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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.