Impact Physics
Cratering and Perforation
| Spall
| Hydrodynamic Flow
Shockwaves in Materials | The Debris Cloud | Ballistic Limits
Shockwaves in Materials | The Debris Cloud | Ballistic Limits
Cratering and Perforation
Just like those impacts on the moon, hypervelocity impacts often leave craters on the
surface of the impacted object. Deep craters in a thin plate can often cause perforations.
A hypervelocity impact usually manifests itself through cratering and perforation. A typical
crater will have a frozen raised lip around its perimeter.
The extreme energies generated during the hypervelocity impact event cause the material
to melt momentarily. This melted material is ejected out the front surface by the force
of the impact, where it quickly cools to solid and is "frozen" in time. If the impacted
material is thin enough, the crater will perforate the rear surface.
Spall
Reflected shock waves can cause internal cracking, or they can propel detached material
from the back of an impacted object at potentially lethal speeds to any astronaut in
harm's way. When a target is impacted by a projectile traveling at hypervelocity, a
compressive (pushing) shock wave is generated. When it reaches the free unsupported
surface at the back of the plate, it is reflected as a tensile (pulling) wave. The
tensile wave is often strong enough to pull some of the material away from the back
of the plate, and produce internal cracking.
In some cases, material is thrown off the back of the plate (at very high velocity) without
generating a complete perforation of the plate. This is referred to as detached spall, and
can be just as lethal as if the projectile had passed all the way through the plate. While
there is no perforation or hole on the back surface of the aluminum slab, material has been
ejected out the rear by the shockwaves induced.
Hydrodynamic Flow
Impacts at hypervelocities between metal objects usually cause the metals to behave like fluids
for a short period of time. This phenomena is called hydrodynamic flow. Hydrodynamic flow, is
not a result of temperature melting the metal, but rather is caused by extremely high stress
concentrations generated within the metal during impact.
Impacts with sufficient velocity and energy can induce shock waves in the impacted metals which
are accompanied by extremely high stresses (millions of pounds per square inch). The stresses
are so high, in fact, that they cause the metal to flow-like a fluid.
The result of hydrodynamic flow is seen in the photograph of the crater in the thick aluminum
target. Note the lip surrounding the crater. Impact into this thick, semi-infinite target is
very much like a drop of fluid hitting a liquid surface.
Shockwaves in Materials
One reason metals behave like fluids is that stress waves move through the metal at supersonic
speeds. Hypervelocity impacts occur with enough energy to generate shock waves within metals.
This means that the stress waves generated during impact are traveling faster than the speed
of sound in that metal.
The generation of shock waves in a spacecraft hull can be very damaging, because spall can
be formed. But, shock waves can also be used in designing shields that destroy an impacting
projectile before it reaches the spacecraft.
The Debris Cloud
When a projectile impacts and perforates a thin plate, a debris cloud is propelled out the
rear of the plate, while an ejecta cloud is propelled back out the front surface. Both clouds
contain fragments of both projectile and plate material.
When a projectile traveling at hypervelocity (greater than 3 km/s) impacts a thin plate, like
those found in many orbital debris shields, it fragments into smaller particles. As previously
explained, shock waves from the impact will cause fragmentation, leading to possible melting
and vaporization. If the plate is too thin to completely stop the projectile, a cloud of debris
will be expelled out the rear of the thin plate.
Likewise, an ejecta cloud may be ejected out the front surface of the plate. Both clouds are
composed of material from both the thin plate and the projectile. Clouds consist of various
combinations of solid, liquid, and gaseous materials, depending on impact parameters like
projectile density, shape, impact angle, and impact velocity.
The resulting debris cloud is less dense than the original projectile, and the force of the
debris cloud impact on any structure downstream is spread out over a larger area. More insight
on debris clouds can be found in the high-speed camera section.
Ballistic Limits
Meteoroid and debris shields are rated based on the size projectile they can stop at a given
velocity. Ballistic limit curves functionally describe the shield efficiency based on its
ability to withstand the physical phenomena described above.
The ballistic limit describes the diameter-velocity threshold at which point a specific meteoroid
and orbital shield defeats a given projectile. Any slight damage increase beyond this threshold
would result in shield failure. Pass and failure of a particular shield is generally a predefined
spacecraft requirement and is dependent on the criticality of the component it is protecting.
Generally, shield failure is deemed to occur when the shield rear wall is visibly perforated or
spalled. The ballistic limit for a shield is a function of many parameters including projectile
diameter, velocity, impact angle, density, target areal and volume density, and more.
The ballistic limit may not always behave as one may expect. For example, you would expect that
shield damage would consistently increase with projectile velocity (the bigger the impact, the
greater the damage). In fact, low velocity projectiles (~3 km/s) can cause more damage than faster
moving projectile at 7 km/s. This is because low velocity projectiles do not break up and fragment
as higher energy projectiles do. Thus, lower velocity projectiles are often capable of penetrating
deeper into the shield.
The graphs provided illustrate a generic ballistic limit curve for a single-wall monolithic shield
and for a multi-wall shield. The ballistic limit curves show projectile diameter on the vertical
axis and impact velocity on the horizontal axis. Ballistic limit equations are important because
they functionally describe a shield's performance, and they are combined with the space meteoroid
and orbital debris environment model to produce an overall risk assessment for actual spacecraft.