Dealing with Stress

5-8 Lesson Plan

How 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 flying to space.  NASA has to be sure an astronaut can do his or her job even with all the stresses that affect the crew.  That's one thing that astronauts and fuel tanks have in common.

During launch, a rocket’s fuel tanks are under a lot of pressure.  And just like the astronauts, NASA has to make sure the tanks can handle the stresses.  When a rocket takes off, different forces act on the fuel tank of the rocket.  Some of these forces are internal, and some of are external.  Rocket fuel is burned at an incredibly rapid rate during a launch.  This means that the fuel tank is emptied very quickly.  If you've ever taken a drink 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.  To keep the fuel tank from collapsing, the tank and the engine must be specially designed.

The fuel tank faces stresses during the launch from external sources as well.  The thrust from the engine pushes upward on the rocket, including the fuel tank.  At the same time, though, the Earth's atmosphere is creating drag on the rocket.  This pushes downward on the rocket.  The result is compression from both ends.  This compression increases during the early part of the launch as the rocket begins to go faster.  It then decreases as the rocket soars higher.  This decrease occurs at higher altitudes, where the thinner atmosphere produces less drag.  In the middle is the point known as "Max Q."   This is where 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 about 1 minute after launch.

To deal with the extra pressure of Max Q, the Shuttle's engines are throttled down.  This keeps the Shuttle from reaching too great a speed before this point.  If it were going to fast at Max Q, the pressures could tear the Shuttle 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.  The can 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 disastrous for the rocket.   In fact, fuel tanks are often used as the main building blocks of the launch vehicle in which they are used.  That means they have to be strong enough to support themselves and the entire rocket.  This is like how your skeleton supports the weight of your body.  If the tank is strong enough, no other support is needed.  This can save thousands of pounds of vehicle weight.  Since the fuel tank is such an important part, you can see why fuel tank testing on the ground is so important to NASA.

But, how do you figure out if a fuel tank is strong enough to withstand the pressures of a launch without actually launching it?  One way NASA does this is at a place that was first used to test rocket engines.  Equipment that was designed to provide fuel to engines during test firings is used to fill the tank with rocket fuel.  The fuel is then pressurized to match launch conditions.  This creates 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.  This simulates the compression forces it would experience as a rocket takes off.  The fuel tank is then emptied.  This copies the conditions of fuel leaving the tank to be burned in the rocket engine.

If 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

 
It's Crush Time!

Subject:   Science, Technology

To learn about simple machines, and then use that knowledge to design and construct a machine to crush aluminum cans.

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Signs Of Stress

Subject:   Health, Science
To research the signs of being overstressed and how to reduce stress levels. Students will then create a poster to share this information with classmates.

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The Thinning Atmosphere

Subject:   Science, Mathematics, Graphing

To graph the atmospheric pressure at different altitudes to show that pressure decreases as altitude increases.

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Dunked Napkin

Subject:   Science

The students will experiment to determine if air occupies space.

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