The technical problems encountered in the development of an advanced separation nut design are described. The nut is capable of sustaining a large preload and releasing that load with a low level of induced pyrotechnic shock, while demonstrating a tolerance for extremely high shock imposed by other pyrotechnic devices. The analysis of the separation nut was performed to acquire additional understanding of the phenomena affecting operation of the nut and to provide quantitative evaluation of design modification for aerospace applications

Matteo, D. N.

Williams, V. D.

Woebkenberg, W.

English

NASA Technical Reports Server

DEVELOPMENT OF AN ULTRA-LOW-SHOCK SEPARATION NUT 
William Woebkenberg*, Donald N. Matteo*, and Vaughn D. Williams** 
ABSTRACT 
This paper describes the technical problems encountered in the develop- 
ment of an advanced separation nut design which is capable of sustaining 
large preload and releasing that load with low level of induced pyrotechnic 
shock, while demonstrating a tolerance for extremely high shock imposed 
by other pyrotechnic devices. 
INTRODUCTION 
Separation nuts have been used by the aerospace community for over 
twenty (20) years to achieve remote, rapid and/or simultaneous disassembly 
of spacecraft and missile components. Separation nuts differ from so-called 
"frangible" nuts in that the latter depend on a detonation to structurally 
fail the nut and allow the stud to be released, while in separation nuts, 
no structural failure is required. 
- The typical separation nut utilizes a set of threaded collets to engage 
the threads of the attachment bolt. Those collets are held in position by 
the relative position of the internal parts of the nut. When separation is 
desired, the pyrotechnic charge is ignited and the gas thus generated is 
released into the working volume of the nut and acts upon piston surfaces 
to cause motion of the internal parts of the nut. This action releases 
the retaining feature of the collets and allows the separation bolt to be 
released. Figure 1 shows a standard separation nut before and after actua- 
tion. 
Increasing sophistication of typical modern spacecraft has brought about 
the utilization of payloads/instruments' which are increasingly sensitive 
to dynamic environments such as the shock induced by pyrotechnically actuated 
devices. Standard shock isolation techniques are, in many cases, not 
feasible due to functional constraints such as the required strength and 
stiffness of the load path from the separation nut mounting surface to the 
vehicle structure and precise alignment requirements between the sensitive 
instruments and vehicle structure. Accordingly, virtually the entire burden 
of limiting the shock level input to sensitive instruments has been placed 
on the source of shock, the separation nut. 
*Space Systems Operations, General Electric Company 
**Ad-Tech Division (Ordnance), Hi-Shear Corporation 
87 
https://ntrs.nasa.gov/search.jsp?R=19820015475 2020-03-21T16:27:58+00:00Z
POWER CARTRIDGE 
BUFFERZONE 
\ 
BEFORE ACTUATION 
SEPARAT”o- kvi 
L [ I 
I 
?RER ACTUATION 
Figure 1. Standard Separation Nut 
88 
CONSTRAINTS 
A combination of three (3) mutually conflicting requirements has 
placed the designer of pyro-actuated separation nuts in a rather tightly 
bounded situation. His design must perform satisfactorily within a 
requirements '(triangle": bounded on one side by the requirement to 
reliably actuate under the worst combination of loads and environments 
(including the requirement to demonstrate margin by functioning with 
underloaded cartridges); bounded on the second side by the requirement to 
severely limit the pyro shock pulse induced by the device to levels more 
than an order of magnitude lower than those generated by previously produced 
devices (such as the separation nut of Figure 1); and bounded on the third 
side by the requirement that the device must not prematurely actuate at 
shock levels similar to those which it itself produces upon actuation. 
DESIGN 
One such device, designed to the mutually conflicting requirements 
of the above-mentioned triangle, is the separation nut developed by Hi- 
Shear Corporation for General Electric for use in satellite-to-satellite 
separation and in boost-vehicle separation on the Air Force's DSCS-III 
Communications Satellite. 
Early in the development program, the Hi-Shear 9400 Series Standard 
low-shock separation nut was selected for DSCS-III Spacecraft separation 
function to retain and release a l/2-inch bolt. The cartridge charge was 
sized such that the minimum output charge which would reliably actuate the 
nut (under worst-case temperature and preload conditions) was defined as the 
SO% output charge, and 100% charge was determined accordingly. This is due 
to the requirement that, during qualification, the nut must demonstrate 
release at maximum preload and worst-case temperature utilizing a single car- 
tridge loaded to only 80% of nominal output charge. 
As a schedule expedient, induced pyro shock testing was performed on 
a vehicle simulator using engineering separation nuts and cartridges to 
determine response at critical component locations. These test data defined 
the shock qualification requirements for critical components. 
Subsequently, separation nuts and cartridges were fabricated and sub- 
jected to component qualification testing. A failure to release with single 
80% output charge cartridge was incurred. Two approaches were available to 
cause release at specified conditions: 
0 Increase output charge of cartridge 
o Cause separation nut to release at lower applied pressure 
89 
Because the component qualification shock environment for panel-mounted 
components had been established at the low energy level, additional output 
charge could not be added without providing for energy absorption in the 
separation nut itself. Therefore, effort was directed towards reducing 
required actuation pressure in the separation nut. 
Actuation pressure was approximately 34.47 megapascals (5OOOpsi) 
in the as-produced separation nut. Some reduction in actuation pressure 
was realized by burnishing the dry film lubricant which is applied to segments, 
key seat and retainer ring. Burnishing was accomplished by cold gas release 
of the nut while at maximum preload. The retaining ring was undercut 
slightly to provide easier release. 
An energy-absorbing honeycomb cushion was added tr‘ the base to reduce 
shock induced by impact of piston on the housing. 
The segments were "slotted" at the retaining ring interface area to 
reduce area of contact and permit better compliance at that interface. 
Figure 2 shows the separation nut configuration with changes incorporated. 
The combination of changes reduced actuation pressure to approximately 
3.45 - 4.83 megapascals (500-700 psi), and reduced the shock-generating 
capability of the nut. 
Separation nuts were built to the modified configuration for component 
qualification testing. Again the separation nut failed. Mode of failure 
was inadvertent release in pyro shock environment, the second leg of the 
requirements triangle. Qualification test shock requirement was 2300 g at 
1300 Hz. 
All effort was now concentrated on construction of a dynamic model of 
the separation nut. 
DESIGN ANALYSIS 
An analysis of the separation nut was performed to acquire additional 
understanding of the phenomena affecting operation of the nut and to provide 
quantitative evaluation of design modification. 
Physical evidence indicated that rotation of segments under preload 
may provide a component of the preload acting in a direction to "push" 
the retaining ring off the segments. Therefore, the separation nut was 
modelled to investigate segment rotation. Additionally, a theory that 
deflection of housing under shock load may permit contact between the 
housing and piston, causing the piston retaining ring to "walk" off the 
segment, was considered in the analysis. 
90 
FLANGE 
BASE 
BEFORE ACTUATION 
EJECTOR 
LOCKING 
PISTON 
SEPARATOR 
HOUSING PRESSURE 
/ CHAMBER 
-1 
-4 
I 
I L__ 
-1 
I 
-1 
-4 
,:- 
-1 
I 
+ 
I 
. - J 
/ 
CUSHION 
AFTER ACTUATION 
Figure 2. Initial Separation Nut Design 
91 
The analysis used the NASTRAN finite element method. A 3-D model of 
the separation nut was required to account for the fact that the nut is 
segmented. Symmetry permitted modelling of only half of the nut. Careful 
selection of boundary conditions for the many elements of the nut was 
required since in normal functioning, parts slip with respect to one another. 
Results of the analysis showed that the segments did indeed rotate under 
preload and predispose the nut to premature release. The analysis also 
confirmed that the retaining ring slips 0.025mm (.OOl inches) when the-top of the 
case moves laterally 0.229mm (.009 inches) as was measured in shock testing of th: 
original design. A design fix, increasing the length of engagement between 
segments and retaining ring, was prepared. This was done by increasing the 
length of ring. The increased interface length stabilized the segments 
and inhibited rotation under load. Indeed measurements taken later in the 
program showed segment cocking of 0.19 degrees average compared to a predicted 
value of 0.18 degrees. 
FINAL DESIGN 
Increasing the length of interface between segments and ring increased 
pressure required to actuate the separation. Consequently, more output 
charge was required for reliable function. It was therefore necessary to 
absorb more energy within the separation nut to avoid increasing the baseline 
vehicle pyro shock environment which had been established early in the program. 
The aluminum honeycomb energy absorber was redesigned for the nut case 
to perform two functions. The primary function was to limit shock induced 
by actuation of the nut. The second function was to bear against the piston 
in the assembled state to provide added insurance that the piston would not 
move during preloading and environmental exposure. Figure 3 shows the final 
configuration with extended honeycomb. 
Engineering tests of this configuration in dynamic environments were 
successful. Engineering tests were also performed to verify that shock 
induced into the spacecraft by this design was within the program baseline. 
Again, the tests showed the design was good. 
Figures 4 and 5 show comparative induced shock data at 2 vehicle 
locations for the standard low-shock separation nut and the ultra-low-shock 
separation nut. A new lot of hardware was procured and 27 separation nuts 
were subjected to qualification test environments including a shock test 
of 2300 g at 1300 Hz. Each unit was X-rayed after each dynamic environment. 
There was no relative motion between segments and ring in any of the 
test units. All units survived environmental exposure and functioned 
normally. 
92 
SEPARATOR 
HOUSING PRESSURE 
I CHAMBER 
FLANGE 
BASE 
. 
\ 
THREADED 
SEGMENTS 
. 
6EfOREACIUAllON 
RING EJECTOR 
LOCKING 
PISTON 
CUSHION 
AJTER ACTUATlOeY 
I- : 
F.igure 3. Final Separation Nut Design 
93 
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VALIDATION OF DESIGN AT HIGHER SHOCK LEVEL .-___c 
Subsequent to the qualification of the separation nut to the 
2300-g environment, a new and much more severe shock environment of 
4500 g peak, as defined in Figure 6, was identified for a second candidate 
booster combination. A new lot of hardware was prepared and tested to the 
increased environmental level. Qualification testing was completed 
successfully without problems. 
ACKNOWLEDGEMENT 
The authors thank Dr. E. R. Jones and Mr. R. D. Page of General 
Electric Company, Space Systems Division, for their support in creating 
the analytical model which pointed the way to resolution of the inadvertent 
release problem. 
96 
300 
100 
100 
TOLERANCES 
1. Lateral Axes 
100 - 500 Hz +_3 dB 
500 - 3000 Hz +6 dB 
-0 
3000 - 1000 Hz +6 dB 
2. Longitudinal AxiS 
100 - 3000 Hz +3 dB 
3000 - 1000 Hz +6 dB 
3. Below 3000 Hz a total 
of three noh-contiguous 
peaks as great +6 d0 
(+6 dB in the frequency 
dge of 500-3000 Hz in 
the lateral axes) and a 
width of l/3 octave are 
allowed. 
Figure 6. DSCS-III l/2-in Separation Nut Shock Response Spectrum (Q = 10) 
97 
William Woebkenberg 
General Electric Company 
Space Systems Operation 
P.O. Box 8555 
Philadelphia, Pennsylvania 19101 
Mr. Woebkenberg has 22 years' experience in the design, development, and test 
of aerospace components and subsystems, including satellite recovery systems, 
spacecraft separation subsystems, solar array drive mechanisms, and ordnance- 
actuated devices. He received his B.S. degree in Aeronautical Engineering 
from Purdue University in 1959. 
Co-authors of this paper are Mr. Donald N. Matte0 who is also with General 
Electric's Space Systems Operation in Philadelphia and Mr. Vaughn D. Williams 
who is with the Hi-Shear Corporation in Torrance, California. 
98 


Development of an ultra-low-shock separation nut

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