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Dealing with Stress

9-12 Lesson Plan

fueltank1.jpgHow well do you handle pressure?  Can you keep going when you're under a lot of stress? 

These are important attributes for an astronaut to have.  There is a lot of pressure involved in a spaceflight.  NASA has to be sure that an astronaut will be able to do his or her job despite all the stresses affecting the crew.  That's one thing that astronauts and fuel tanks have in common.

The fuel tanks of a rocket are also under a lot of pressure during the launch.  And just as with the astronauts, NASA has to be sure the tanks can handle the stresses.  When a rocket takes off, different forces act on the fuel tank of the rocket, some of which are internal, and some of which are external.  Rocket fuel is burned at an incredibly rapid rate during a launch, which means that the fuel tank is rapidly emptied.  If you've ever drunk from a drink pouch, you know what happens when this occurs.  With the drink pouch, as the liquid inside is emptied through the straw, the pressure inside the pouch decreases.  The sides of the pouch collapse in on themselves, and the pouch flattens.  A rocket tank being emptied of fuel faces a similar situation.  Therefore, the fuel tank and the engine must be designed in a way to prevent this from happening.

fueltank2.jpgThe fuel tank faces stresses during the launch from external sources as well.  The thrust from the engine is pushing upward on the rocket, including the fuel tank.  At the same time, though, the Earth's atmosphere is creating drag on the rocket, essentially pushing downward on it.  The result is compression from both ends.  The compression increases during the early part of the launch as the rocket's speed increases.  It then decreases as the rocket soars higher, where the thinner atmosphere produces less drag.  In the middle, though, is the point known as "Max Q," at which the pressure on the rocket is greatest (Q is a quantity used to represent dynamic pressure).  During a Space Shuttle launch, for example, Max Q occurs roughly 1 minute after launch.  The Shuttle's engines are throttled down during launch so that it does not reach too great a speed before this point, creating pressures that could tear it apart.  Again, a drink container can serve as an excellent analogy for these stresses.  Picture what happens to a soft drink can when strong enough forces are placed on the top and bottom at the same time—it crumbles up to a tiny fraction of its original height.  This is definitely not something engineers want to happen during a launch!

Fuel tanks have to be designed to withstand these forces to prevent them from collapsing during launch.  If that were to happen, the results would be catastrophic for the rocket.  In fact, fuel tanks can be used as structural elements of the launch vehicle in which they are used.  That means they have to be strong enough to support not only themselves, but the entire booster, just like your skeleton supports the weight of your body.  If the tank is strong enough, then additional structural support is not needed, saving thousands of pounds of vehicle weight.  When one understands how important the strength of a fuel tank is, it's easy to understand why fuel tank testing on the ground is so important to NASA and other rocket designers.  But, how do you determine if a fuel tank is strong enough to withstand the pressures of a launch without actually launching it?  One way NASA tests fuel tanks is at a facility originally used to test rocket engines.  Equipment initially designed to provide fuel to engines during test firings is used to fill the tank with rocket fuel, such as liquid hydrogen.  The fuel is then pressurized to match launch conditions, creating pressure for the tank to stretch, or increase in length.  Then, the launch “crush can‿ loads are applied on the top and bottom of the tank, simulating the compression forces it would experience as a rocket takes off.  The fuel tank is then emptied, replicating the conditions of fuel leaving the tank to be burned in the rocket engine.

fueltank3.jpgIf a fuel tank is able to repeatedly withstand the pressures it experiences during this testing, NASA engineers can be pretty confident that it's got what it takes to be used in a launch.  And, that can take a lot of stress off their minds.

 
 
Courtesy of NASA's Aeronautics Mission Directorate
Published by NASAexplores: September 16, 2004

 


A Crushing Blow To Cans

Subject:   Physical Science, Physics
To measure the amount of force needed to crush aluminum cans.

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Max Q Graphing

Subject:   Algebra, Geometry, Physical Science, Physics, Technology
To determine the Max Q value on the Space Shuttle during launch.

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Air Power

Subject:   Science, Physical Science
Students will observe how unequal pressure creates power, and demonstrate a source of thrust found in rocket engines.

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Wind Tunnel Calculations


Subject:   Mathematics, Geometry, Science, Chemistry, Physics, Technology
To calculate the area, velocity, and pressure in a wind tunnel.

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