Hypervelocity Impacts
Hypervelocity impacts have been studied for a number
of years both on the ground through such systems as light gas guns, particle
accelerators, plasma drag guns, rail guns and also hydrocode computer simulations.
In addition the hypervelocity impact phenomena is readily seen as a natural
event on orbiting spacecraft and interplanetary spacecraft where particles
can impact a spacecraft with velocities ranging from 300m/s to 70km/s.
When studying the impact sites located on the
SFU SSM (second surface mirrors) it was clear that they could defined into
4 categories dependant upon morphology. These categories were designated:
To understand the reasons for this classification
the SSM structure needs to be understood.
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Figure 1 Cross section of SSM
The structure for PLU SSM is shown in figure 1,
not to scale. The various layers, as labelled in the figure, are:
-
A = FEP Teflon tape, thickness, T = 127 µm
( = 5 mil.)
-
B = Silver, T = 1,500 Å
-
C = Inconel, T = 275 Å
-
D = 3M 966 acrylic pressure sensitive adhesive, T
= 51 µm
-
E = 2024-T81 aluminium plate, T = 381 µm
-
F = 5056 aluminium honeycomb, T = 20 mm
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Figure 2 Impact parameters
A typical impact is shown in Fig. 2 with the definitions
of the various parameters labeled as follows:
-
Dm Delamination ring
-
Dp Central aluminium
crater
-
Dc Inner Teflon lip (diameter
of Teflon hole)
-
Dr Outer ring diameter
Type A
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Figure 3: Example of Type A impact
Type A is characterised as:
-
No visible delamination ring
-
One Teflon inner lip
-
Central pit usually seen
-
Typical Dc size range 30-90 µm
Type B
 |
Figure 4: Example of Type B impact
Type B is characterised as:
-
Delamination ring
-
One Teflon inner lip
-
Central pit usually seen
-
Typical Dc size range 90-180 µm
This looks very much like Type A but with a delamination
ring present.
Type C
 |
Figure 5: Example of Type C impact
Type C is characterised as:
-
Delamination ring
-
Two Teflon inner lips
-
In many cases extensive damage is seen to the underlying
silver layer
-
Typical Dc size range 180-420 µm
Type D
 |
Figure 6: Example of Type D impact
Type D is characterised as:
-
Delamination ring
-
No Teflon inner lip
-
Smooth Teflon upper lip
-
Typical Dc size range 450µm and above
These impacts have typically perforated the upper Teflon layer, and blown
away the silver coating, revealing the aluminium structure beneath, as
Fig. 6 shows.
| PLU-1 | PLU-2
| PLU-4 | SPLU-1 | SPLU-2
| BSU-1 |
| BSU-2 | Scuff
plates | IRTS | SEM
| EFFU | 2DHV | Solar
arrays |