Aerospace & Defense Technology - February 2024 - 30

Hypersonic
Monteverde et al. examined the
mechanical properties of zirconium
diboride-silicon carbide (SiC) matrix
composites. The density of the composites
was found to be between 5.51 and
6.09 g/cm3 dependent on the SiC weight
percent (5 to 20 wt). The modulus E was
found to be between 518 and 477 GPa,
the fracture toughness was between 4.49
and 5.42 MPaM1/2, the hardness was
between 19.3 and 21.7 GPa, and Poisson's
ratio was between 0.103 and 0.117.
It should be noted that at these high
Mach numbers, as a droplet transits the
shock wave, it will tend to dissipate due
to sheer forces and heating. Droplets
will also tend to become elongated perpendicular
to the direction of motion,
becoming disc-like and spreading any
force out over a larger area thus reducing
damage. Additionally, the angle
between the water disc and the projectile's
surface will affect the area the
impacting disc is spread out over as well
as reduce the normal component of the
impact force. This indicates that a leading
edge that is very " sharp " will suffer
less impact damage.
Figure 3: Plot of the Mach cone angle as a function of Mach number.
cule has and consequently the number
of quantum mechanical energy states
available (or the density of states) the gas
molecules can occupy.
The shock-induced temperatures
and frictional forces that gases undergo
during hypersonic rates raise the
kinetic energy high enough to enable
additional degrees of freedom, specifically
the vibrational modes of the
diatomic gas are activated. This means
that the heat capacity of the gas is not
constant but changes due to the additional
energy states available. These
vibrational states are initially activated
in the Mach 3 to 4 range and lead to
measurable deviations from the ideal
gas law at hypersonic speeds. This
increased ability to carry heat consequently
leads to an increased heat
transfer to the flight body.
In the Mach 9 range at altitudes in the
lower stratosphere (~10,000 m) range,
oxygen dissociation begins to take
place. This oxygen dissociation is also
referred to as one of the " thermochemical
effects. " At speeds around Mach
30
16, another thermochemical effect
begins to take place, nitrogen dissociation.
At this point, both oxygen and
nitrogen radicals can land on surfaces
and cause thermochemical-induced
reactions with the surfaces. Finally, at
speeds near Mach 30, plasma will begin
to form a conductive sheath around the
flight object. This plasma sheath has
sufficient electrical conductivity to act
as a Faraday cage, leading to the blocking
of both the transmission and receipt
of electromagnetic communication signals
(i.e., a " blackout " ).
Erosion
At hypersonic speeds, the kinetic energy
in a collision between vehicle surfaces
and dust/raindrops can result in erosion
due to impact damage. A theoretical equation
for the damage threshold velocity
from is represented as: Vthreshold = (K1C 2
cR/ρw2cw2dw)1/3. The fracture toughness
is KIc, the Rayleigh surface wave speed cR,
density of water ρw, the compressive wave
speed of water cw = 1,500 m/s, and the
diameter of the raindrop dw = 1.5 mm.
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Conclusion
The challenges associated with hypersonic
flight are significant in that the
drag forces, temperatures, and heat flux
put large demands on materials. First
and foremost is managing the exposure
to high temperatures that will be
in excess of 1,000 °C. The high heat
flux to the flight body can result in very
high temperatures of the entire vehicle
during flight. Mechanisms to carry
away heat, such as increased emissivity
of leading-edge materials, should be
explored. The use of cooling in the form
of water vaporization may be a route
that can be used to increase rocket fuel
efficiency and provide a mechanism
for throttling a solid fuel rocket. As the
effects of erosion are unavoidable at
hypersonic speeds, the use of materials
with high Young's moduli could mitigate
this effect.
This article was written by Thomas
Parker, Materials Engineer, U.S. Army
Research Lab. It has been edited. For
more information, visit arl.devcom.
army.mil.
Aerospace & Defense Technology, February 2024
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Aerospace & Defense Technology - February 2024

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