"lfk f; \"A Lt. - Airborne Systems

Aeria! retrieval malfunctions may be caused by deployment
malfunctiQn , collapse or inadequate stability
of engagement car'Opy. and insufficient structural
rein forcernent of para-:hute assembly for engagement
impact.
Causes of Un reliabi I ity
l'Aajor causes of unreliebillty In deceleratcr operation
may be identified as either inadequate design.
materials failure due to accident, or hurran error in
decelerator assembly. j:8cking, and use, In designing
for reliable performance. and in assessing the reliability
of a given design. the possibility of failures from
,hese three causes must be considered.
From trle viewpoint of operat onal parach.;t8s and
other decelerator systems, inadequate desi!;n is generally
not a majorfai II. ra factor in actual field use. Since
virtually every decelerator systerrl goes thruugh a de.
sign, a development, and a shake-down te51'ng period,
design errors are ger,erallv eliminated in development.
The exceptions a"e those cases in whic!' the design
error can be said to be marginal , and in which the failure
rate d.J€ to the design error's so law as to be undetectable
even with an adequate test program, and
Indistinguishable from accidental causes.
Mater ials failures may
e divided intD two classes
failures of t'le fabric and static hardware portions of
decelerator., a'ld failures of the mechanical devices
wh ich ace necessary to deceleration system operation,
Failures in the fabric ponions of canooies are proba
bly the most difficLlt o assess on a theoretica basis,
First. experience indicates that fabric failures must be
considered "from the viewpoint of critical oc non.c itieal
applc8cions of the fabric in the parachute. Thus,
for example, in many missions the blowing out of a
panel in a canopy does not necessarily mean a failure
of the mission. If the decelerator functior) is merely
to land the IQad without damage in a general area,
and the decelerator is des gned with a normal margin
of safety the loss of a panel from t18 canopy may
not affect the reliability of mission performance at
all. On the other hand, in such a mission the failure
of a riser or a suspension line could very well result in
major damage or destruction of the load, and thus
failLre of ,he rrission.
In cases ih which both t'18 deceleration of the loao
and the achie'./ement of a reasonably precise touchdown
point are vital , e. g. delivery of OJ special weap-
. failure of a single panel mighT so change the tra-
jectory of the decelerator syster' l that the mission
ld be a failure even though the deceleration func-
tion had been accomplished successfully. Thus, mission
analysis is a vital factor in dete"rnining which
specific types of fabric failure cause decelerator sys.
tem failure,
327
To pinpoint tre exact reason for tre failure of a
canopy is a rather difficult er,gineering task, Thp.
tensile strength of a ;arge number of samples of the
fabric used for any given member of the canopy, test.
ed lInoer conditions simulating those of actual use,
will be found to vary according to a normal distri bu.
tion curve, with the actual strength values generally
tending to group around a central average,
A similar study of stresses which are placed on the
various canopy Members during parachute deploy.
ment and descent will indicate that for each given
phase of decelerator deployment and operation. a
given range of stresses will act on each component.
Again , a series of measurements wil tend o show a
range of such stresses a1 any given point, deperding
on the specific manner in which The canopy unfolds,
etc, As in the s1rength case, these values usually will
group around a cen:ral average with I:oth extremes
considerably less common than the mean,
:rc understand the reasons for many accidental
failures of canopies, it is recessary to examine the
relationship between distribution of strength of ti,e
material in each portion of the canopy and stress or
each portion oi the canQ\Y in its operation. In every
application af decelerators there is some probability
of very low or very high stress , but the probability of
the extremes is less tnan the probabi lity of a stress
closer to the average. Similarly. in every choice of a
specific piece of fabric for the constructioll of a cano.
py the'e is a possiblity of getting a lawer.strength
piece or a higher.strength piece. Thus. it can be seen
that the probDbility of accidental failure wil depe'ld
largely on the probability of the specific portion of
th2 canopy which fails being constructed of a low.
strength piece of material trat encounter. an acciden-
he operation. Many of trl8
tally high stress during
structural fail ures wh ich do DeCli r are caused by this
factor , i.e" excessive ovsrlap of the strength and
stress distribution curves.
Failures of mechanical devices used in varicus
decelerator systems present 8 more straight-forward
probleM than failures in fabric portions. With static
hardware, items whose functions are primarily p3Ssive,
the cause of failure may be attributed to tre distribution
of strength and stresses of the components
as in the case of fabrics. However, "'here is another
class of mec;"snical devices wrich mllst pe-form
active functions during the deployment or operation
of the system. This c, sss consists of reefing line
cutters, interstage disconnects, deployment. init' aticn
devices, etc. Here the relial)ilitv problem is one of
mechanical functioning in an environment which may
include low temperature, shock, vibration , accelera.
tion, end possibly other interfering factors.