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

ond
/db =" 8. 3/5
7 D
/CDt;
Appra,sal of selection of criteria for deploymen: at
Mach 1. 5 . indicates :hat 1he Hemisflo drogue affords
best infjatior stabilitv and supersonic, non-reefed
drag coef"ieient. However, when reefed, its super
sonic performance is indisti1guishable from that
the flat or conical ribbon types (References 215
and 555) which afford e h: gher subsonic drag coefficient.
A II three are ribbon parachutes and satisfv
other selection criteria ecual!y. The conical ri;:bon
canopy with a constructed an;)le of 15 cO 25 degrees
is a good candida:e , and variable porosity may be
considered optional
Calculation of Wake Dynamic Pressure. The estimated
wake effect factor of the foregoing examQle
may be verified by the method given in Chapter 7
using Equati on 7- 126 with the empirical ooefficie1ts
and exponents given. For a particular drogue system
with body and drogue tmil ing dista:lGe known, the
equation expresses the wake c elative velocity (Vl.vv)
as i: function of the relative distance of 8 pairt from
the centerline of the wake (r/db Recognizing the
approx imate nature of the solution, ratrer than integrate
the resultant dynamic pressure distribution
across the canopy, oS suggested by Equation 7- 129 a
satisfactory solution can be obtained bV a simpler
procedure in which onlv vw'w on the wake centerl ine
(r 0) and at rldb /2d are calculated.
Fro'1 the example
Body
=: "'
Drogue x/db = 7
/db 1.70
('"
'" )' ",
/db)
The coefficients for EquatiQn 7- 126 are:
o. 9B ( 21
O. 42e
The exponef1s
54e
84 21
85; n 0.
'"
512
639
By substitJtion, Equation 7-126 reduces to
!Jvvl 097ge 90J (2
and the wake boundarv is at
/db 639(7)0. ", 1.
Thus, the drogue canopv is essentially equal in diameter
to the wa , B1d the cor-esponding ratios are
2r/db !Jv
ft)
424
1.0
"" '" ;:
0979
0072
The shape of the non-dimensional velocity distribu.
tion curve shown in Figure 6.4.2 jJstifies use of a
simple linear average , so that:
D.lIwiV (aI/g) 0. 0562
and
vwfv(a vg) 0.944
(qw
9447 = 9 891
which corresponds to the 8stimated drag coeffcient
9).
ratio used /CD
For other values of D /db it wOLid not be difficult
with the aid Qf F i9. 6.4! to strike a graphical average
for !:v lv com'nensurate in accuracy with the accuracy
at the empirical coefficient given. For this purpose,
enter Fig. 6.42 with w D cClh;ulaled for
x/d of the body drogue system to looa:e the
canopy radius on the velocity distribution curve.
Drogue Reefing. Since the drogue drag area is
frequentlv deternined by the maximum allowable
dvnamic preSSLJrG for deploymert of the main canopv,
tr, e predicted drogue openin9 force at the maximurr;
dynamic presSU:e cordition on the recQvery system
initiation envelope Tlay exceed the system desi;)n
load factor (Equatior 8 5J. In this case the drag of
the vehicle is usually a significant fraction of the :;otal
and should be mcluded in the ca' culation. T'le maxi.
mum allowable reefed drag al-es may be estimated
with I:quDtion 8
COCltinuing the nUllerical example for System B
the maximum dynamic pressure at Mach 1. 008
feet is
Where
7 PM 1532 psf
=: 912. psf
Given a vehicle drag area at Mach 1 5 of CoA
0 ft the all;:wable drogue opening load fron
Equation 8- 5 is
F X 4000 (6-0) 5 (1532) 800lbs
A trial calculalion is made wi th to establish
the order of rr,agnitude of (from EqLation 8-
(CDS)r
'"
14, 800/1532
Then
312 30.
2, and the mass ra,io at 20,000 feet
altitude is approximatelv
00127(30.2)/124;; 3. 1(10-
Reading Fig. 6. 25, it appears that the opening load
factor could have a maximum '/alue in the order of
= 1. 3, but the limited amount of data availa;:le
on reefed sur,ersonic ribbon parachutes . e.
-=