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

c Q
w u
\\
\ , '
Composite Date From Different Sources
100 Ft (D ) G- 71A (l
:"" . () ", "" " "" ""
0) (Ref. 382)
100 Ft (D ) Flat (V '25 FTS! (Ref. 217)
100 F Fl - 25 - 30 FTS' (Rof. 183 217J
88. 1 Ft (D ) Ringsail (l
85.6 Ft (D l Modii. RS (I
128.9 Ft (D ) Ringsail (1/0
Thf10ry (S88 Table
.70
\ '. "
\
r. ":"1
I
/))
J 392. 394
1.40/ (Ref. 217)
Number of Parachutes
1.44) (Ref. 2171
1.15) fRef. 217)
Figure 6.30 Effer:t of Clustering on Drag Coefficient
Graph ieal analysis of the contact geometry of differen
numbers of circular canopies in compact roti:tionally
symmetrical clusters yields the following
approximation relationship for the average riser angle.
ifc K*(lc
) radians 6-
where K" ,/D and is the mean radius of the canopy
centers from the cluster axis at the level of the
skirtS, as developed on a flat layout.
It is instructive to compare measured with calcul,,-
ted cl uster drag coefficients on the
basis of the as-
sumptions made. and these purely geometrical considerations
with
COdCDQ cos (K*/n/'J 6-
for di fferent number of canopies and correspondi ng
values of K* in Table 6.5. As shown in Figure 6.
the agreement for 2 and '" 3 is good. bwt
divergence increases rapidly for larger clusters. Tlte
implication is that if elustered canopies could be pre-
267
;:
ventEd from spreadi ng fal- afJarL thalr drag performaree
would be greatly improved when =4 or more.
PartiBI confirmation of this i5 found in Ref€nm::c 3
for clusters of three 48 ft conical ribbon parachutes
tested both with and without canopy skirt
connections at the tangency point . The average drag
coefficient of the bound cluster was approxirrately
11 percent greater than of the free cluster.
Another implication cf the comparison IS that the
best 3-canopy clusters ere performing close to the
theoretical maximurr with whatever mutual Interference
may exist. while the best 2-canopy clusters even
though free, clearl'( are benefitting from effects not
accounted for by the 'theory. For example, two side.
by-side ca'1opies may experience tV'o-dilrensione:
flow augmentation oi morrentum drag analogous to
the effect 0 r aspect ra ti Q.
A cluster of three 4.78 ft ribbon drogues with
o t:
8 were tested on a supersonic rocket sled376
wiih the results shown in Figure 6.32. Of the five
tests performed. two were supersonic. Supersonic
operarion was characterized by partial squidcing witI'
canopy area ra:ios ofS 2 5 to .35, compared to
.435subs01ic , and average angular excursions
greater by a factor of 3. 5 relative to the subsonic
average. The traill1g distance,
was roughly 1Odb'
T'
Use of ribbon drogLles in pairs is fairly common.
The Apollo drogue systern . consisting of " pair of
16. 5 ft
" conica, ribbon parachutes. scarcely
o.
cual ifies as a c: uster because the riscrs wero attachod
to separate points on the command rrodule giving
effective rl ggln g leng:h considerabl y g-eater than
e =
o.
The canopy trailing distance.
r.
was approxi-
mately 6db' Consequently, there was no detectable
interference between canopies and their drag per.
f:rmance was the same as tWQ independent parachutes
with a rigging length of 200.
TABLE 6. PARAMETERS OF SYMMETRICAL
PARACHUTE CLUSTERS
318 .369 .461 538 .637 530
IjC (radians) 225 .213 230 .241 .260 203
COC/CO 975 .977 974 .971 966 979
'"