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

TABLE 8. LEAD CANOPY WORST CASE OPENING LOADS
Configuration F(max) CyF' Ibs
Reefed 033 1.24 26.000
Full Open 174 5BO 112 000
equivalent systems such as tf at identified
with Referenc!: 431.
Fu!1 open max/l1eai force rat ios after leaa
canopy disreefing are never likel)' to approach
the worst case indicated by y (max) because
of the low probabiUy of raving all but one of
the canopies carry no load throughout lead
cano?y inflation.
Equalization of Cluster O;)ening Load Peaks. The
test data of Table 6. 2 justify use of diff9rent cluster
oPEning load factors reefed and on disreef in the
present nunerical examp e as fOI lows:
Reefed Cy 1.
Fu!lopen C
y '" ==
The two opening load peaks may be equalized for the
design limit case by rev sing the reefing ratio derived
from he synchronous opening calculations
08). Without repeating the calculatio'1s , it will be
seen tha1 the original reeftng rstio of
comes close 10 satisfying ,he requirement and the
following design limit loads are indicated.
Ree ed
Full open
F;
900 (1. 3/ =33 670 fbs
15,300(2.0)=30 600lbs
These may be rQunded off for each stage to
F x
::
000 Ibs
In other words , a reefing I ins diameter of 10 percent
is cbs': tu optiilum for the free cluster , while as
indicated above. 8 percent would be used with the
controlled cluster, and the design limit load would be
F x 000 Ibs.
reduced to
..
Strength of Materials. Thp. strength of materials
required in the deceleretor strJcture is detormined,
first. 'by apprOXi'T8te preliminary internal loads
analysis based on predicted design limit opening
loads. Later . the structure may be r",tinea , as desired,
through applicaticn of one of the more rigorous
computerized methods of structural analysis (see
Chapter 7). From the design limit loads predIcted for
each opening stage when the conopy is reefed, the
critical unit load 10 each structural member, Fe
413
calculated and multiplied by a design factor to deter.
mine the minimum acceptable strength of material.
Equation 7. 72 states
PR""D
where the design factor p, S is the
saf'3tv factor and is the allowable strength factor.
For parachutes , recommended components of the
allowable strength factor are given in Table 8.6, with
recommended safety factors and corresponding
design factors. When actual minimum joints and
seam efficiencies are kr,own , e. g., the results of labor.
atory tes'"s, these values of should be w:ed. For
asymmetry of ioading,
is used when no quantitative
evaluation can be made from system or decelerator
geometry and test experience. Factors i
(v.scuum) and (teMperature) should be evaluated for
the conditions expected to prevail at the time of the
deceleration operation, because the recovery of losses
caused by the on.board mission environment is rapid
and can usually be quartified.
The main parachute of System B, introduced in
Table 8. 1, provides the Dasis for a numerical example
which illlls"'rates the short method of calculating the
ap;:Joximate strength of materials requi"ed in a poly.
symme::ric parac;hute struct..re.
Given: Urmanned vehi::e
== ""
Design limit loae (reefed and after disreefingl
Main parachute
Reefed
20,OOOlbs
98
= 7545
7017 ft
351
A.pproximate projected diameters
FL;!I (2/3 ) D 65.
Reefed
D,.D OS7 or D 53
Pr
23. 1
Effective suspension line length
108 ft (-1.
4000lbs