Design improvements and detailed functional analyses are reviewed to trace the development of a pyroactuated release device with segmented thread design from its intermediate design into one that reduces the levels of shock spectra generated during its operation by 50%. Comparisons of shock output and internal load distribution are presented, along with descriptions of mechanical operation for both designs. Results also show the potential areas where design development activity can gain further progress in lowering actuation shock levels

Gordon, W.

Otth, D. H.

English

NASA Technical Reports Server

N76-28289
DESIGN EVOLUTION OF A LOW SHOCK RELEASE NUT*
By David H. Otth - Jet Propulsion Laboratory
and
William Gordon - Hi-Shear Corpuration
ABSTRACT
Design improvements and detailed functional analyses are reviewed to
trace the development of a pyro-actuated release device with segmented
tbread design from its intermediate design into one that reduces the levels of
shoe]- spectra generated during its operation by 50%. Comparisons of shock
output ,_nd internal load distribution are presented, along with descriptions
. of mechanical operation for both designs. Results also show the potential
areas where design development activity can gain further progress in lower-
ing actuation shock levels.
• INTRODUCTION
Pyro-actuated release device configurations with launch load carrying _;
capability for spacecraft or expended-stage separation plane designs charac-
: teristically reduce their installed preloads to zero within millisecond func- _
tion times to complete interface separations. This design feature provides )
high mechanical and dynamic efficiency for separation but generates high fre- :_
quency and magnitude shock levels which are transmitted through the inter-
face to the adjacent structure. If electronic packaging, attitude control gyros,
or science instruments sensitive to shock are nearby, this side effect may be
_ undesirable depending upon the magnitude of shock spectra received.
_ Development of the release device for the Viking Orbiter 1975 (VOt75) ] ¢?
separation interfaces brought about an understanding of the release forces "
: _ and internal dynamic action related to shock generation. Subsequent design
i,nprovements that significantly reduced shock levels were then incorporated
: _ into the release device for the Mariner lupiter/Saturn 1977 (MJS'77) space-
craft interface. Both designs involve a segmented nut that releases I/Z-Z0
bolts and, for this report, are individually presented in two sections. Dis-
cussions are limited, however, to findings and design improveme s that
resulted in lowering shock during operation.
: RELEASE DEVICE FOR VO'75
Description of Operation and Development Program
The Viking Orbiter 1975 release mechanism was a dual squib segment-
ed nut design that mated with a I,/2-20 strain gaged bolt and was utilised on
both Orbiter separation interfaces (Viking Lander adapte r and spacec raft
adapter). The release nut assembly provided an ll.800-1b tension launch pre-
load at four interface hardpoints coincident with each VO'75 separation plane.
Upon simultaneous commands to eight squibs, all four mechanisms functioned
within 6 milliseconds to reduce hardpoint preloads to sero and eject bolts,
thereby completing interface separation.
This paper presents the results of one phase of research carried out at the
Jet Propulsion Laboratory, California Institute of Technology, under Contract
No. NAS 7-100, sponsored by the National Aeronautics and Space
Administration,
e •
J*PL Technical Memorandum 33-777 ! 75
1976021184-182
https://ntrs.nasa.gov/search.jsp?R=19760021201 2020-03-20T02:33:22+00:00Z
The Viking device was a modified version of the segmented nut design
used on the Surveyor Program with improvements in areas of materials,
configuration, and lubrication, These improvements were made during the
development program ann were initiated to improve internal load distribu-
tion, low-temperature performance, and reusability as related to repeated
pneumatic testing. A significant portion of the development program was
devoted to understanding the source and distribution of the 100,000-g peak
accelerometer shock response generated during its operation and to reducing
levels as electronic bays were installed adjacent to the separation plane ..
hardpoints. Reducing the shock output required extensive modification, , :
which was not feasible for VOt75, but an understanding of the source of
shock and the distribution of loads was obtained. Design changes that would
result in reduced shock levels were implemented on the release nut for
MJSt77 and are discussed in detail in the next section.
Referring to Figure 1, the VO_75 release nut basically consists of
three threaded segments positioned by the base key seat, locking piston,
and separator. When the retease nut is preloaded, the bolt load is reacted
out within the assembly into axial loads, which are parallel to the bolt axis,
and lateral loads, generated by the 60-degree thread angle, which are
perpendicular to the bolt axis. The axial portion of the bolt load is trans- :-
ferred to the base key seat and the boltts lateral load is transferred through
the segmert lands to mating lands in the locking piston. The bolt ejector
is contained in the separator and can add velocity to the bolt only after
the bolt has been released. The release nut is operated by pneumatic or
squib press_ re, which drives the locking piston forward, thereby atlowing
the segments to move radially outward and release the bolt. The pressur-
ized separato- keeps the segments out after :elease by applying a radial i"
load to the segments through the angled interfaces of the separator and ;
base key seat. The true-arc ring is used for initial positioning of the sep-
arator and segr,_ents relative to the piston lands o,,ring assembly and the :
O-ring's limit o_tgassing of squib contam_.nants.
Distribution of Loads
The significant design changes that resulted in lowerin_ actuation _,
shock levels were based upon an understanding of the preload distribution i "'
and stresses on the segments and locking piston within the releace nut.
For a given bolt preload, the load distribution on a threaded segment is _,
1
described with the aid of Figure 2. The axial portion of the load is corn- i
_ pressive in nature and is reacted out into the bane key seat. This compo-
nent of the load acts along a well defined path and therefore requires little i
description. However, the 6D-degree included angle on the bolt and segment
threads generates a large lateral component whose load path is not precisely
known, i "-
The exact position and shape of the segment load distribution are only
generally known; that ic the first few threads carry the majority of the
preload. In addition, the reactive load to the piston is also a distribution, _.
but again shape and exact location are unknown. The end result is the I
: inability to determine a precise value for {1} the maximum bending stress _ ",
on the segments, which occurs at the minimum thread diameter adjacent to ! ;.
the front, lands, shown as point A in the close-up portion of Figure 2, and
176 JPL Te¢ cat M.morandum 33-777
_o t =._._j ,_"
1976021184-183
(2) the piston hoop and bending stress, as shown in Figure 3. To circumvent
the problem of inexact load description, a comparative analysis was con-
ducted using point loading rather than distributions and determining the
segment bending stress at point A. The bending stress established was given
a value of 1.0 and was compared to recalculated stresses when (1) landareas
were increased and moved forward toward the base, (2) the first few threads
were moved in line with the center of land areas, and (3) segment thickness
was increased in the area of highest bending stress (see Table 1). The main
design improvement made for VO_75 was increasing the segment thickness
behind the first land to lower bending stresses. Aligning the first few threads
with the center of land areas and moving the lands forward required extensive
redesign, and both changes were incorporated into the MJSt77 configuration,
which also eliminated the lightly loaded rear land. These changes not only
• reduced internal stresses by 60% but lowered piston deflection and changed
the segment configuration, which were key factors _n reducing actuation
shock, as will be discussed in the second section.
Table 1 also shows that although bending stresses were reduced for
VOW75, deflection was not, at least from segment changes. On the other
hand, the piston's first land area could be easily increased and was, by
0.050", to decrease piston deflection by 20%. The objective was to lower
the open end "belling" of the piston, as shown in Figure 3, and thereby the i
"ramp effect" configuration between the piston and segment lands. This in
turn lowered the threshold of "release pressure" and increased functional
margins. Moreover, further reductions in piston deflections were possible _
and were implemented later for the MJS device to lower the squib energy
_ needed for operation and lower shock.
Another hardware change that resulted in lowering actuation pressures
was a combination of materials selected to withstand repeated pneumatic _i
actuation without dimensional changes and an improved moly-disulfide coat-
i_g. The coating was used on the threaded segments and piston to reduce the
coefficient of friction between mating surfaces. The friction coefficient was
reduced further by burnishing the land areas prior to assembly and then
during two pneumatic actuations prior to flight. A 1 5% reduction in threshold _pressure was obtained by using this process, which again increased functional
margin or allowed lower squib energy to be used for operation.
Sources of Shock
g
The impact of the piston on the base, the strain energy release of the
_ bolt from the segments, and squib firing contribute to the high-level shock
generated during operation. The VO'75 development program identified the
individual contribution of each source in order to determine those areas
where redesign or modification could reduce shock levels. The release
device was mounted to a flight-type spacecraft structure with three electronic
bays and was instrumented with shock accelerometers located next to the
device.
A series of shock signatures were generated by using a number of
release nuts actuated with pneumatic, hydraulic, or single and dual squib
pressures. The test results have been summari--ed in Table 2, which gives
JPL Technical Memorandum 33-777 177
1976021184-184
the individual contribution of shock sources that result in a 100,000-g peak
shock load next to the device. Varying squib loads changcd the shock levels
by 15%, while hydraulic actuations of the nut which drastically slowed down
the piston velocity (compared to squib actuations) indicated that the major
shock source was piston impact and represented 60% of the total. This was
verified by using Fastax camera coverage that determined the piston velocity
iu excess of 250 feet per second prior to impact. Using the initial piston
velocity and the measured penetration of the piston into a steel base, the ..
deceleration was calculated to be on the order of 60 CO0 g.
Lowering Shock Levels
' Figure 2 shows that shock output can be readily lowered by reducing
squib loading and damping piston impact, assuming that methods to reduce :"
the strain energy release are not convenient. By using the load distribution
analyses, a new segment configuration was designed having the same preload
capability but requiring a more compact and lightweight piston. The new
configuration also provided room within the release nut to design a method of
reducing piston impact. Damping as well as reversing the direction of piston
impact, along with lowering the threshold release forces to permit operation
with VO'75 squibs, were the design goals oftbe low shock release nut.
RELEASE DEVICE FOR MJS'77
Design Featu_'es
_'_Concurrent with the effort described for the VO'75 program, the need
for an improved release nut design that inherently provided reduc,-d shock
levels was recognized. Development work that ensued included design, fab-
rication,and testingof three differentnut configurations that followed the
basic bolt retentionmethod used on Surveyor and VO'75. Tb.ese were tested _,
and evaluated on the basis of several relevant factors, including simplicity,
cost, weight, producibility, and the reduction of shock output. The resulting
low shock design, depicted iv Figure 4, was selected for the MJS'77 program.
The load distribution analyses of the Viking development program led to
a further revis;.on of the VO'75 segment, moving the lands toward the base to _i
react against the thread radial loading induced in the first few threads. The
secc.:ld set of lands, located where little cr no radial loading exists, ,.'ere
eliminated. The balance of the changes involved those parts which retain the
segments and a lighter two-piece piston assembly to ma_e with the new
segment design.
With the primary shock-generating event in the operation of previous _.
relea.-e nuts being the impact of the piston with the base, two methods of
i eliminating this collision were employed. First, tlle piston was driven away :_
i from the base and secondly, it was brought to rest through mechanical and
! squib gas damping. Reversing the piston direction also aided in lowering :_
threshold release forces as less energy was required to back off the ramp "I
configuration between segment and piston lands caused by ' open end" belling.
i Thus, the same release pressure was _btained with a smaller piston.
178 $PL Technical Memorandum 33-777 _
?
f
1976021184-185
Description of Operation
As can be seen in the installed view of Figure 4, the segments are
retained by the piston, base key seat, and separator. The separator, similar
to that previously used, bears on the upper ends of the segments, helping to
stabilize and align them as well as forcing them apart when squib pressure
is introduced above the separator. An optional ejector pin is shown that
pushes the bolt out of the nut, again when propelled by squib pressure.
Unlike the VO'75 design, the pressurized gases are introduced between the
separator and piston rather than on top of the piston. This results in the
piston being driven away from the base of the nut and allows the segments to
move radially outwards. At the same time, gas pressure acts on the separa-
tor and ejector, forcing them against the segment andbolt respectively.
° After the piston has moved sufficiently to release the segments, the lock
piston ring contacts the separator. The collision occurs in such a way •s to
avoid transmitting shock into the adjacent structure as before.
A comparison of the effective pressure area of the piston and separa-
tors shows that the separator has more effective area than the piston. This
additional area provides a force which acts to decelerate the piston and sep-
arator after the piston strikes the sevarator. The separator is momentarily
unseated from the top of the segments due to the inertia of the piston. The
major portion of tbe shock energy in the piston is absorbed during upward
motion of piston and separator. The space above the piston is ideally sized
such that the two components stop their upward motion due to the pressure
area differential, then are forced downward until the separator again seats
on the top of the segments. This final seating does generate some shock but
_' significantly less than the direct collision of the piston and b. ;e.
,1
Comparison of Shock Levels i
The VOt75 device and M3SW77 low shock design were evaluated compar-
atively on a full-scale flight-type spacecraft structure with electronic bays.
Shock signatures were obtained from accelerometers mounted within the
: electronic bays. The results have been presented as shock spectra, which
show peak structural response (G's) versus frequency (Hz). The structural
response is derived from the accelerometer peak g time trace, frequency,
: and a structural amplification factor (Q) of Z0. The end result is a conve-
- : nient representation of total shock content, transferred to the structure in ,
, terms of peak (3 shock and frequency.
Referring to Figure 5, the peak G level of the low shock device is 50%
less than that of the VO'75 design •s recorded within the electronic bay. If
:_ the lviJS device was not hard-mounted to the structure but allowed to rebound
off the structure after functioning, • 70% reduction in shock spectra was
obtained. The same mounting scheme was used for the VO'75 device, but no
appreciable reductions were recorded.
JPL Technical Memorandum 33-777 179 :
i .
1976021184-186
CONCLUDING REMARKS
Significant reductions in shock response have been achieved; however,
further reductions may be possible by lowering the shock contributions
caused by preload release and squib actuation. Although lowering squib
energy was not a design goal for MJSt77, reduction of the release force was
achieved. Future configurations which are not limited to a specific device
diameter can incorporate larger piston diameters and capitalize on the lower ":
release forces. Large areas will allow release pressures to be obtained
with less squib energy, As further improvements lower actuation shock,
more compact packaging of separation plane hardpoints with electronic bays
or science can occur.
Table 1, Segment stresses and piston deflection vs. segment configurations
Configuration Segment Bending Open End PistonStress Factors _' Deflection, inches
1) Surveyor segment design 1. 0 0.00252
2) Increase segment thickness b 0.46 0.00252
3) Align first threads with center 0.39 0.00194
of land plus (2) above
4) Mo_e land forward plus (2) 0.32 0.00173
and (3) above c "
aRepresents maximum bending stress comparisons.
bBetweenlands, as was done for VOW75 segment design.
CThis represents MJSW77 segment design.
Table 2. VCt75 release device shock sources and contribution
i i 3'
Accelerometer Response Next to Device
Source Item
i g*s Contribution, % :
Squib firing 15, 000 15
Piston impact 60, 000 60
Strain energy release 25, 000 25 o
of preload ",
• _ 1
: Total assembly 100, 000 100
$ I
' 180 3PL Technical Memorandum 33-777
1976021184-187
tJ
LOCKING PISTON i-i-i-_ r- SEGMENTS(3 PIECES)O-RING SEALS
11,800-,b BOLT _ -"
PRELOADDISTRIIIUTION(l I, 800 ib) :',_,r_,_ _;
AXIAL COMPONENT _- -!t IJ_,
_:d_--_"'- aoLtuEctO_ _."
BAsEKEYSEAT
SECTION A-A
Figure I, VO'75 release nut assembly cross section i
± F
3PL Technical Memorandum 33-??? 181
1976021184-188
! t l 'tm"
|
_ MAXIMUM
SURVEYOR _EGMENT L6,NDS _DING
t PRELOA'_D--D__v_ - -- '
. MJS '77
Figure. Z. Cross section of segment designs showing load distribution
LHEAVILY LOADE ___I LIGHTLY LOADED i °
FRONT LANDS REARLANDS !
LOADING FROM
SEGMENT LANDS EXAGGERATED"OPEN END !
BELLING" FROM SEGMENT
LOADS i
Figure 3. Piston cross sections showing loading and belling
182 3PL Technical Memorandum 33-777
1976021184-189
T
i
.. ,__/-,_oc,,: P,sTo__,_G
z ,
"'" . ?--I ._
i , _- -,--- SEPARATOR
// / ,.
/ / , x ,_
-,, \
SEPARATED
I. LOCK PISTON MOVES AWAY FROM STRUCTURETO
UNLOCK THREADED SEGMENTS.
2. SEGMENTS DISPLACE RADIALLY AWAY FROM I_OLT.
3. SEPARATOR PISTON LOCKS SEGMENTS IN CPEN POSITION.
Figure 4. MJS'77 low shock release nut
JPL Technical Memorandum 33-777 183 .
1976021184-190
10,000
MJS' 77
VO' 75 DEVICE (FIXED MOUNT)
o (3,ooo) _
(1,500) ........ "_
i 1,000(8,50)
m J MJS' 77
(FREE MOUNT)100
100 1,000 (2,500) 10,000 20,000 ,'_
FREQUENCY, Hz
Figure 5. Comparison of peak G shock spectra generated
by VO'75 and MJS'77 low shock device vs.
frequency
/
184 3PL Technical Memorandum _3-777
1976021184-191


Design evolution of a low shock release nut

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