Many people call them a necessary evil.
Solid-fuel rocket boosters essentially give you more bang for your buck than rockets powered only by liquid-fueled engines. They pack more energy into a smaller space and cost less to manufacture than the liquid-fueled variety, yet are capable of generating the same thrust.
Thats why nearly every major rocket company flying today relies on a combination of the two types of boosters for their stable of launch vehicles.
If an Atlas rocket doesnt quite have enough power to lift a certain satellite into orbit, just strap on a couple of small solid-fuel boosters and youre in business a lot faster and less expensively than redesigning the whole rocket to use different liquid-fueled engines.
Historys solid compromise
The same choices were presented to NASA during the 1970s when the final design of the space shuttle was resolved. The earliest ideas included using nothing but liquid-fueled engines, like those used on the rockets that launched the Mercury, Gemini and Apollo astronauts.
But in order to lift the shuttle into orbit without spending too much money on development or flight operations, NASA was forced to compromise on a design that included a combination of two reusable sold-fuel boosters with the three main liquid-fueled engines on the new reusable orbiter.
The compromise also was strongly influenced by political considerations. The company ultimately selected to design and build the boosters, Morton Thiokol -- now known as Cordant Technologies -- was headquartered in the state of Utah, where then-NASA Administrator James Fletcher had strong ties.
Fletcher was able to leverage political support for a shuttle program that included solid-fuel boosters by bringing coveted aerospace jobs to the sparsely populated state. The ultimate payoff came a few years later when the first U.S. politician in space turned out to be Utahs U.S. Senator Jake Garn, who flew aboard the space shuttle Discovery in April 1985.
Less than a year after Garns junket, the right-hand solid-fuel booster used on NASAs 25th shuttle mission leaked its hot gases and triggered the series of events that destroyed Challenger and killed seven astronauts on board.
Faults in the boosters design, as well as in NASAs launch decision process, were identified and fixed, leading to the shuttles return to flight in 1988. The twin motors have since worked perfectly during flight, with only minor troubles discovered and corrected on the ground.
NASAs biggest booster
The solid boosters used by the space shuttle are the worlds largest, and the only ones certified for use to carry humans into space.
How big is big?
- Two of the boosters stacked on top of each other stretch the length of a football field. The boosters diameter, or width, is big enough to allow a city bus to drive in.
- It would take 305 full-size Cadillac cars to balance the scale with a single solid rocket booster ready for flight. Combined, the two boosters contribute 2.6 million pounds (1.2 million kilograms) of the shuttles total 4.5 million-pound (2.1 million-kilogram) launch weight.
- In terms of lifting power, the two boosters firing at launch produce 6.6 million pounds of thrust. It would take 32 Boeing 747 jumbo jets, all applying take-off thrust, to get the same power yield.
The most amazing thing about these boosters is that they can be reused many times, saving NASA big money during the course of the shuttle program.
Take a rocket ride
Each booster fires for about two minutes. When they are almost totally empty of fuel which has a rubber-like consistency, kind of like a pencil eraser they are jettisoned to fall into the Atlantic Ocean. At this point the vehicle is usually about 28 miles (45 kilometers) above ground.
As the shuttle continues its climb into orbit, the boosters continue tracing a giant arc in the sky, rising on sheer momentum to an altitude of about 41 miles (66 kilometers) before they begin their splashdown descent about 140 miles (225 kilometers) from the launch pad. Parachutes stored in the each rocket's nosecone are deployed to cushion the water impact.
Two specially outfitted ships wait in the area where the boosters will fall to recover the spent motors. As they float -- nose up -- divers jump in, plug the open nozzle hole and pump air into the body so it floats on its side. The ships then tow the spent rockets back to the Kennedy Space Center.
Once there, the boosters are cleaned, disassembled and shipped to various buildings around the center to be prepared for another flight. The empty casings that once held the booster fuel are shipped by train back to the Cordant factory in Utah to be refilled and then are railed back to Florida.
The boosters are stacked one piece at a time inside the Vehicle Assembly Building. Next comes the bright orange external tank, which holds the liquid propellant for the shuttles main engines. Finally the orbiter is attached.
As the complete shuttle is moved around, the solid-fuel boosters are the two legs that hold the entire stacks weight and keeps the vehicle from falling over. To ensure the shuttle assembly remains upright, each booster is bolted to the launch platform with four giant fasteners, each measuring 28 inches (71 centimeters) in length and 3.5 inches (8.9 centimeters) in diameter.
These bolts are explosive and are detonated to release the shuttle a split second after the solid boosters are fired. This final sequence commences only after the countdown clock hits zero and only if the shuttles three main liquid-fuel engines have already ignited and are up to at least 90 percent power.
NASA managers say that because of the way the computers control the final moments of the countdown, it is physically impossible for the boosters to ignite before they are supposed to.
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