Modern Engineering Thermodynamics

Summary 25
FIGURE 1.19
Case study 1.2, Z accelerator cross-section.
Case study 1.2. Sandia’s hypervelocity gun
The high-velocity impact of even a small particle having a mass of
only 1 g can have a disastrous effect on a spacecraft. To develop
shields against such an eventuality, engineers at the Sandia
National Laboratories developed a high-velocity launcher (gun)
that allows the testing of materials and equipment here on Earth.
Sandia’s hypervelocity launcher, known as the Z accelerator, is capable
of accelerating dime-sized projectiles a few centimeters to gain
information that can be used to simulate the effect of meteoroid
impact on spacecraft (Figure 1.19). The propulsion technique uses
the Z machine’s 20 million amps to produce a huge magnetic field
that expands in approximately 200 nanoseconds. The smooth acceleration
produced by the expanding magnetic field produces a
smooth projectile acceleration rather than that produced by shock
of an explosion. When accelerated to a velocity of 20 km/sec, an
aluminum projectile is liquefied but not vaporized.
Hypervelocity impact testing is also an accurate method of determiningamaterial’s
“equation of state,” which predicts how a
material will react when the pressure and temperature are changed
by specific amounts.
The energy required to launch a small projectile to 20 km/s is
about 15 times the energy required to melt and vaporize the projectile.
Therefore, the energy must be imparted in a well-controlled
manner to prevent this from happening. This is achieved by using a
variable density assembly to impact a stationary projectile to propel
it to very high velocity without melting or fracturing.
The kinetic energy contained in a 1.00 g projectile launched at a
velocity of 20.0 km/s is
ðKEÞ launch
= ðmV 2 Þ/2 = ð1:00×10 –3 kgÞð20:0×10 3 m/sÞ 2 /2
= 200:×10 3 kgðm/sÞ 2 = 200:×10 3 N.m = 200:×10 3 J = 200:kJ
which is about the same kinetic energy as contained in a 1000 kg
(2200 lb) automobile traveling at 20 m/s (45 mph). The impact of
a 1.00 g object traveling at 20.0 km/s is spread over a very small
area, and the material damage produced is enormous.
SUMMARY
At the beginning of this chapter we saw the significance of understanding basic thermodynamics in a wellrounded
engineering education. A working definition of thermodynamics is presented and the value of thermodynamics
to all engineering fields discussed. A basic problem solving technique is presented that is used throughout
the text and expanded on in later chapters.
Engineers must have a sound understanding of how units systems are constructed and how the various popular
units systems relate to each other, because engineering units are not trivial. An accurate computation depends as
much on correct units management as it does on correct numerical calculation. In this chapter, the concepts of
units, dimensions, and metrology are also discussed. We see that ancient units of measurement evolved from a growing
need to expand and quantify the elements of commerce and are undeniably woven into the history of civilizations.
The historical evolution of these units often involved the binary doubling of size between successive
units. It is pointed out that temperature units came into use quite recently, and they have their origin in the common
medical practice of sensing fever in the human body.
By the turn of the 20th century, classical mechanical and electrical units systems had been developed and were in
common use by engineers. Other units, such as chemical units, are also often used in engineering analysis.
The development of modern unit systems began in 1870 and is still going on. The United States is currently in the
process of converting all its commerce and technology into the SI system. Since it is not known exactly how
long this will take, textbooks such as this one present material in both the traditional Engineering English units
system and the SI units system so that you, the next generation of engineers, will be able to work with both