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

ed. (Df course the reverse is true in the case of Gn
ascending system, a condition seldom encountered
in practice. Being a function of 9 as well as the
inter-stage ballistic cceffkierH of the system and the
staging interval , 6q tends to be a maximum for steep
descents when the flight path angle is approaching 90
degrees. Prior to the steging operation, ,he system
usuallv has decelerated to a near-equilibrium velocitY.
Drogue Sizing and Type Selection. The dreg area
A) of the (body) or payload . and the maximum
reefed main :lecelera-
dynamic vessure at which the
basis for estima-
tor can be safely deployed, prcvide a
ting the required drogue drag area. The maximum
allowable dynamic pressure at the end of the drogue
working interval can be defined with reasonable
precision for most recovery systems because one of
the advantages afforded by the drogue chute is the
important one of enabling the main decelerator to be
deployed at essentially the same speed and altitude
in every operation initiated above a predetermined
minimum altitude. Typically, droglle disconnect is
initieted by a ba' oswitch, and 'the systerr is approaching
its equilibrium descent velocity at the t:me. When
the critical maxi'Thjm dynamic pressure (qsm occurs
under non-equilibrium conditions, suitable trajectocy
computations must be performed, and the following
simplified method of making a preliminary estimate
of S) is not appl icable.
1 8.
The equilibrium dynamic pressure with full open
d rogue is
WI1/CD S)
qe
This is somewhat less than the dvnamic pressure felt
at main canopy line.stretch because of the re-accelera:ion
following drogue disconnect. A reasonable
allowance is made by let:in9 qSm
15 for which
qe
the drogue drag area required, using Equation 8- 13.
is a:.proxilTatel Y
S)
'"
(U5W/qsm A 8-
The body CD or is usually gl'Jen as a function
Mac' ! number Drogue drag coefficie'lts vary widely
with both body and free s:ream Mach number as
shown in Fig, 6. 55, However, the drogue disconnecT
condi tion is lsuslly subsonic and Fig. 6.43 provides
data for estimation of ICDq. for a given drogue
trailing distance. Relative trailing distances frequent-
IV used are /db"" 750 that conservative drag coefficients
will be cbtained as follows:
ldb Co/Co
-( 3 1 0.
Since Co /CDoo is the same as the wake dynamic
the alternative method given
pressure ratio
/q.,,
423
'"
'"
in Chapter 7 'nay be used Instead, to predict :he average
dynarric ore5sure across the drogue canopy in the
body wake, This method is based on the drag coeffcient
of the towing body, and db may be calculated
as the hydraulic diameter of the orojected frontal
area for other than circular shapes.
The drogue drag coefficients given in Table 2,
were measured under conditions ces igned to mini.
mize wake effects so that a reasonable value for
S/C may be obtained with the assumption
that COco cBo
Select on cnteria for the type of drogue chute best
suited for the application include
Maximum flight Mach number at deploYI'1ent.
SLpersonic inflat' on stability and C
Subsonic drag eHic8ncy or
SIW
Installation characteristics and serviceability.
Reefing characteristics.
Of course. the development status of each drogue
type m\Jst be su ficieltlv well.advanced to suppor!
evaluation :Jf the criteria with firm data,
I\umerical Example
Prior calculations for System B (Table 8. 1) indicate
that a reasonable maximum allowable dynamic pressure
for deplovment of the main puachute with one
reefed stage is := 77 psf.
Given:
Recoverable weight
Trajectory angle
Body ubsonic drag area
Body diameter
AI!owable load factor
Drogue requ remen:s
Trailing distance
Deployment at.
Velocity
Altitude
Flight path
Calculations.
Drogue drag area required
EquDtion 8 14
Size estimate for trial
order of magnitude:
Then
'"
Mach
;: '" ;: .. '" ;:
4000 Ibs
0 ft
db 5. 0 ft
" 6.
(J .. 0
Odb
000 ft
(subsonic) from
(1. 15 (4000)/77)- 55, 1 ft
55. 7/. 111.4
2 D
to identify
11.