ENGINEERING STUDY:
OPERATIONAL REQUIREMENTS
OF THE
1760 LANYARD RELEASE
UMBILICAL CONNECTOR
Anthony J. Rogers
G&H Technology, Inc.
ENGINEERING STUDY:
OPERATIONAL REQUIREMENTS
OF THE
1760 LANYARD RELEASE
UMBILICAL CONNECTOR
ANTHONY J. ROGERS
G&H Technology, Inc.
750 W. Ventura Blvd.
Camarillo, CA 93010
ABSTRACT
This paper documents the investigation, study,
findings and development of a MIL-DTL-38999/31
Connector, Plug, Lanyard Release, capable of
meeting the operational requirements of present and
future MIL-STD-1760 weapon deployment systems.
The connector forms part of the Mission Store
Interface (MSI) Umbilical as defined by MIL-STD1760, and provides power and signal from the aircraft
to the store. It further provides a means for the
mechanical separation of the umbilical from the store
during deployment.
In the latter portion of this Study possible changes to
MIL-DTL-38999/31 and MIL-HDBK-1760 are
discussed, as they pertain to the findings, and
documented in-services issues. Further applications
and developments outside of the scope of D38999/31,
for installations and store interfaces with unique
separation requirements are also discussed.
INTRODUCTION
A growing concern has arisen in the stores and
weapons loading community regarding the 1760
lanyard release umbilical connector. Numerous
failures to separate in accordance with the
requirements of D38999/31, has prompted
ACC/LGW and others (i.e. US Navy, RAF) to label
this a “high priority issue.”
In January 1998, G&H Technology, Inc. (G&H)
began development of a 63.5mm (2.50 in) barrel
length 1760 lanyard release umbilical connector at
the request of Lockheed Martin Tactical Aircraft
Systems. Intended for use on the F-16 weapons
umbilical, Drawing Number C10987, it was G&H’s
understanding that the basis for this request stemmed
from excessive release forces exhibited by their
present supplier’s connector.
Beginning with its existing 36.8mm (1.45 in) barrel
length connector (qualified in June 1988) G&H
lengthened the connector to 63.5mm (2.50 in),
without any changes to the interface or release
mechanism. The resulting connector exhibited the
same favorable separation results when tested to
D38999/31, including off axis pulls after moving the
separation moment more than 25.4mm (1.00 in)
further away from the actual point of separation.
Samples of the new design were provided to
Lockheed Martin (LM) for test and evaluation.
Again, when tested to D38999/31, LM observed
separation forces similar to those obtained by G&H.
However, when tested at ejection velocities the G&H
samples also exceeded the 400 Newton (90 lb) limit
for straight-pull separations.
What G&H learned from this effort was that the basic
premise for D38999/31 was sound, but the methods
for verification and qualification did not adequately
support the intended application. Most notable of
these were the differences in separation velocities, off
axis pull angles and sensitivity to new mating
receptacles. As a result, G&H concluded that a
connector conforming to the requirements of
D38999/31 did not ensure successful performance
during actual use.
INVESTIGATION: USER REQUIREMENTS
Working with the then available information on the
reported failures (Re: GIDEP Alert No. EA-A-9801), G&H began to develop its “problem statement”
to investigate the causes for high release forces
during separation and the resulting damage it created.
The “Alert” addressed failures on the F-16 that
included separation forces in excess of the
D38999/31 specification limits, loss of umbilical
cables and other pylon mounted hardware due to the
connector’s failure to separate (see Figures 1 & 2).
Using a standard engineering model for problem
solving, G&H developed its study plan. Basic
elements of the plan included:






Determine User Requirements
Establish Design Goals
Conduct Design & Development
Investigation
Consolidate Investigation Findings
Develop a Final Design
Qualify Product to MIL-DTL-38999/31
In order to better understand the various user
requirements and installations, G&H began
participating in the SAE AS-1B2 MIL-STD-1760
User Group. Through this forum G&H learned of
NAS Patuxent River’s efforts to develop a new 1760
umbilical for F-18, Lockheed Martin’s efforts to
select a connector for the F-117, among others.
Already a supplier of its 36.8mm (1.45 in) barrel
connector on the B-1B, G&H began to develop a
model for the current 1760 user requirements and any
connector related issues. What G&H discovered
proved to be very revealing:
Photos courtesy of Eglin AFB
Damage to F-16 pylon resulting from store separation
Figure 1.
1.
Ejected Stores: Many 1760 delivery systems
eject their stores at rates between 3.75 m/s
(12.0 ft/s) to 9.15 m/s (30.0 ft/s). MILDTL-38999/31 is qualified at a rate of 13
cm/s (5.0 in/s).
2.
Common Weapons Umbilical: Though MILDTL-38999/31 provides for eight (8)
different lanyard lengths, most platforms
employ a single umbilical cable, with a
single lanyard length for all store types and
installations (see Figure 3).
Photos courtesy of Eglin AFB
Damage to F-16 pylon resulting from store separation
Figure 2.
At the time, the manufacturer’s offered explanation
for the failures was, “extreme angled pulls when
mated with a new receptacle…” At face value, and
not fully understanding the actual installation, this
seemed a reasonable response. However, the results
of our preliminary investigation seemed to indicate
that velocity is also a factor in the release force
exhibited during separation.
Photo courtesy of Lockheed Martin
An F-117 installation exhibiting excessive slack
in the lanyard
Figure 3.
3.
New Store Receptacles: A typical store
interface is a new (un-used) receptacle.
However, MIL-DTL-38999/31 provides for
the use of a conditioned plug and receptacle
that undergoes 500 cycles of mating and
unmating, including 100 high velocity
separations, prior to the straight and off-axis
“Pull-separations” during qualification.
From this, G&H developed its own design goals and
objectives for a new 1760 connector design. They
included:
4.
Excessive Off-Axis Angles: Both by design
and using the single umbilical approach, it
was determined that many installations
exceeded the 15° max angle limitation.
With clearly defined goals and objectives, G&H
could now proceed with its engineering study to
identify a solution for the separation issues exhibited
by the then available 1760 umbilical connectors.
5.
Pre-Loads Prior to Separation: Created by
racks with minimal clearance, stiff umbilical
cables resulting in exaggerated bend radius
for installation. Furthermore, racks with
non-linear separations, may also influence
the forces applied to the plug’s shell,
segments and coupling ring (see Figure 4).




Low-force separations at ejection velocities
Increased off-axis capability
Equal separation forces in all four quadrants
Insensitive to new receptacles
ENGINEERING STUDY
For the purpose of this study, G&H conducted all of
its analysis and testing on Class “W” finished
connectors in accordance with D38999 (It should be
noted that no comparison was made to any other class
of connector during this study). This decision was
based on G&H historical selection and success in the
use of metal alloys for applications requiring multiple
mates, severe environments and dynamic separations.
Before attempting to solve for the problem, it was
necessary to fully understand the mechanics and
dynamics of the lanyard release connector’s
operation.
Photo courtesy of Eglin AFB
Typical 1760 umbilical installation on F-16
Figure 4.
The investigation provided G&H with a better
understanding of existing installations and 1760
user’s requirements. It further exposed deficiencies
between the specification and qualification
requirements of D38999/31 versus the operational
requirements. With this understanding and seeking
to solve the high release force problem, G&H chose
the following problems statement:
Theory of Operation
As prescribed by D38999/31, the 1760 connector is a
circular, threaded, plug, with lanyard release, failsafe. Its primary use or application is as a store
interface and weapons umbilical disconnect. Mating
and demating are accomplished in the same manner
as a typical straight plug, but the snatch release
involves the added dimensions of both physics and
mechanical interactions.
Mating
To engage or mate the lanyard plug and receptacle,
the assembler must:
1.
Identify the operational requirements and any factors
that affect the release and separation characteristics
of a MIL-DTL-38999/31 connector and provide a
design solution that conforms to the specification
requirements for straight and off-axis pulls.
2.
3.
Pre-align the plug and receptacle so that the
keyways are clocked correctly relative to
each other (master key and keyway aligned).
Push the connector halves together until they
bottom out on the plug and receptacle
threads (soft mate).
Rotate the plug’s “coupling ring” clockwise
to engage the tripe start ACME threads.
Continue
to
rotate
coupling
ring
approximately 360º to engage contacts and
fully mate the connector halves (insure red
indicator band on the receptacle is covered
to verify full mate).
Demating
To disengage or demate the lanyard plug and
receptacle, the assembler must:
1. Rotate the plug’s coupling ring counter
clockwise, until plug and receptacle threads
disengage.
2. Pull connector halves apart until keys have
fully cleared the slots, (demate is complete).
Snatch Release
After the plug has been properly mated with the
receptacle, the lanyard is attached to the bail bar. At
this point, the lanyard release mechanism is now
operational. Snatch release occurs as follows:
1.
2.
3.
As the store and receptacle are ejected away
from the rack, the plug’s coupling ring is
restrained by the lanyard.
As the load increases on the coupling ring it
compresses a wave washer that holds the
connector’s “threaded segments” in place
during mating.
When the wave washer is fully compressed
the plug’s threaded segments are free to
move clear of the receptacle’s threads
enabling separation (see Figure 5 & 6).
Plug and receptacle in mated condition
Figure 5.
Plug and receptacle in release condition
Figure 6.
Note: the total separation force shall not exceed 400
Newton (90 lbs) for a straight pull and 445 Newton
(100 lbs) for a 15º pull per D38999/31.
Based on this understanding of the connector’s
operation, G&H began its study of the physical
interaction between the two connector halves during
separation. Using 3-D modeling, G&H first studied
connector separation with a keyless, segment-less
model and found that the connector’s circular
geometry had an inherent barrel lock situation. We
refined our barrel features as allowable by the
specification until we eliminated barrel lock. We
then created and tested prototypes that added keys
but no segments, as well as segments but no keys, to
study the additive behaviors of each feature.
Confident in our test methodology, we began
individual configuration refinements to measure their
incremental effects and recorded the behavior of our
basic connector geometry. After each series of pulls
for a given configuration, the hardware was visually
and dimensionally inspected for wear. The root
cause for fouling was hypothesized, and then the
designer attempted a fouling recreation with the solid
model. When confirmed, corrective action was taken
to eliminate the cause, the test plan would then be
modified to include this new change, and the whole
sequence would start again. In all, this iterative
process was performed eighty-one (81) times before
the established design goals, objectives and
performances were achieved.
FINDINGS AND STUDY RESULTS
When considering the original problem statement:
Identify the operational requirements and any factors
that affect the release and separation characteristics
of a MIL-DTL-38999/31 connector and provide a
design solution that conforms to the specification
requirements for straight and off-axis pulls.
After completing both the “Investigation” and
“Engineering Study,” it became apparent to G&H
that meeting this requirement on multiple platforms,
using a variety of stores with varying deployment
systems would be a considerable challenge. The
most notable of these were differences in carriage
configuration, internal bay versus external
suspension, ejection velocities, off-axis separation
angles and connector pre-load caused by cabling.
However, the Study also provided evidence of the
connector’s ability to support these differences and
the operational requirements of MIL-STD-1760
could also be achieved.
Enhanced Performance and Reliability
The research and information used in developing the
design goals and objectives for the development of a
new D38999/31 connector, revealed several major
factors that contributed to the high separation forces
experienced by users. They included:
1.
2.
3.
4.
Fundamental principles of force, created by
mass and acceleration (F=ma) during
ejection
Shear stresses induced by pre-loads and offaxis separations
Impulse loads created by slack in the
lanyard
Tolerance stack up and interference
conditions created by new hardware and/or
different manufacturers
The Study in turn showed that incremental changes
and tighter tolerance control of the geometry between
the interacting components of the two connector
halves during separation would eliminate the barrel
lock situation and enhance both keyway and threaded
segment clearance during separation, thus reducing
the force.
Separation forces of 65 - 75 lb. (311 Newton) at
ambient in all four (4) quadrants at 15 deg. off-axis
Figure 7.
Almost no change in separation forces at -65° C in all
four (4) quadrants at 0 & 15 deg. off-axis
Figure 8.
Once proven repeatable in the lab, G&H then
considered the issues of variability in hardware (i.e.
fabrication, tolerance stack and finish) and
installation. To ensure repeatability and reliability of
the connector in service, G&H created its own
performance requirements in excess of D38999/31.
G&H product range up to 22
deg. off-axis
MIL-DTL-38999/31 range
15 deg. off-axis
Expanded off-axis separation capability
Figure 9.
Qualification Requirements
The following section describes the apparent
inadequacies identified during the Study and
proposes changes to eliminate them.
Separation force of 91 lb. max (400 Newton) at
ambient in all four (4) quadrants at 22 deg. off-axis
Figure 10.
G&H rationalized that if the connector’s performance
exceeded the specification requirements, it could
offset many of the variables seen in existing
applications.
By designing beyond the specification requirements,
G&H was able to achieve low-force ejection
separations at both ambient and low temperatures,
increased off-axis separation capability in all four (4)
quadrants, and insensitivity to new receptacles.
Furthermore, its aluminum alloy construction,
combined with its anti-binding and roll-off features
provides added durability and extended life.
In March 1999, G&H was granted qualification on its
new 1760 Umbilical Connector to D38999/31 Type
1, by the Defense Logistics Agency (DLA) Defense
Supply Center, Columbus. G&H has since received
qualification for a Type 4 shell length as well.
RECOMMENDED CHANGES
AND ADDITIONS TO MIL-DTL-38999/31 AND
THE MIL-HDBK-1760
As earlier presented, connectors conforming to the
present requirements of MIL-DTL-38999/31 do not
ensure similar performance during actual use. These
inadequacies combined with the present variability in
installations will only lead to further separation issues
and/or users seeking non-standard proprietary
solutions for their problems. In view of this, G&H
offers the following recommended changes and
additions to both MIL-DTL-38999/31 and MILHDBK-1760.
Durability:
It is generally regarded that D38999/31 connectors
separate at 445 Newton or less for a minimum of 100
pull-separations.
However, when you examine
Durability requirements of D38999/31, no
measurement of force is required and the resulting
forces may be in excess of the 445 Newton.
Furthermore, these separations are straight pulls
occurring after the threads of the plug and receptacle
have been conditioned by “normal mating and
unmating” and they do not consider any pre-loads or
off-axis separations that one might expect installed
on an umbilical.
Durability testing is intended to validate the
robustness of the connector and not a test of the
maximum pull separation force. In view of this,
Durability testing should occur after the “Pullseparation” and “Fail-safe” tests are conducted. This
would ensure that a connector meeting the separation
requirements was also robust enough for the intended
application.
Pull-separation:
As earlier presented, the Investigation showed that
ejected stores could be separated at velocities
between 3.75 m/s (12.0 ft/s) to 9.15 m/s (30.0 ft/s).
However, D38999/31 specifies a pull rate “not to
exceed 13 cm/s (5.0 in/s).” Furthermore, this testing
is conducted after all test specimens (plug and
receptacle) have been worn-in or conditioned by the
500 mates and demates (including 100 pullseparations) performed during Durability testing.
Pull-separation testing is intended to validate the
performance of the connector during store separation.
However, the present qualification test requirement is
representative of a “gravity” release installation and
not an ejected store delivery system. In view of this,
Pull-separation testing should precede Durability
testing and the separation rate (velocity) should be
increased to levels consistent with an ejected store.
In addition to the pull rate, it is suggested that Pullseparation testing be conducted on new (un-used)
receptacles, and that pulls “15° from straight” be
conducted in all four quadrants, relative to the Master
Keyway.
MIL-HDBK-1760
In Section 14.5 “Umbilicals and buffers” the
Handbook addresses connector types, signal set,
contacts, shielding, cabling and the like for a typical
1760 umbilical. It describes differences between
platform specific umbilicals and the related services
issues observed. However, neither this section nor
the remainder of the text addresses the impact of
excessive slack in the lanyard, pre-loads created by
bend radius and clearance when routing cables, or the
relative position of the bail lug to the lanyard plug
during installation. Based on the findings of this
Study, G&H suggests the creation of some basic
umbilical installation guidelines. Such guidelines
would assist stores, rack and airframe builders in the
development and integration of current and future
weapon systems, and avoid fielding installations that
result in separation problems (see Figure 11).
Various perspectives on the 15 deg off-axis angle
Figure 11.
Example: Figure 11. shows a common practice used
by rack and airframe builders for determining offaxis angles. By using the bail rod and MSI as their
fixed points, one perspective is achieved. However,
connector manufacturers measure this same angle
from the pulling moment (lanyard attachment point)
to the bail rod, resulting in a different perspective and
greater angle. This difference in pull angles can be
further complicated by the increase or decrease in
lanyard length and the connector shell height.
The findings of this Study and the reported problems
in the field suggest the need for change, in order to
support the operational requirements of the 1760 user
community. Failure to address these issues, as
previously stated, would result in recurring and future
problems.
Furthermore, it might lead to the
development of proprietary solutions from one
platform to the next.
CONTINUED DEVELOPMENT BEYOND
THE SCOPE OF MIL-DTL-38999/31
During its study, G&H became aware of several
customers whose application and/or installation
requirements exceed the scope of D38999/31, though
a MIL-SPEC solution was clearly desired. In
response, G&H developed two hybrid designs of its
present 1760 connector.
Low-Force Lanyard Release Connector
Originally developed for the canopy release system
on the F-16 Aircraft, the low-force lanyard release
connector
provides
the
performance
and
dependability of MIL-DTL-D38999, with the user’s
ability to select its desired release force within 2.27
kg (5 lbs) increments. Employing a patented ejector
mechanism this connector has been tested at
separation velocities between 30 - 50 ft/s. The
connector has been proven and qualified on
applications for stores with masses near or less than
113.60 kg (250 lbs) and extreme off axis separations
(in excess of 28° from straight). The low-force
feature is currently available in shell size 17 (E)
through 25 (J) for stores release and launch
umbilicals.
Redundant Release Lanyard Connector
Developed for gravity release and internal weapons
bay applications, this connector offers the same
features of the low-force lanyard, plus a patented
redundant fail-safe tied directly into the umbilical
cable assembly. The fail-safe system ensures release
of the connector in the event of a lanyard failure, or
failure to attach the lanyard on the bail during
loading. Furthermore, its cabling and installation
requirements are the same as for a standard
D38999/31 connector when used with a straight
backshell. This feature is presently offered in a shell
size 25 (J) only.
Adjustable Length Lanyard
In addition to the aforementioned hybrid connectors,
G&H has also developed an adjustable length lanyard
in response to the 1760 user community’s desire to
inventory a single umbilical cable for multiple
applications. This patented mechanism is available
as an option on all G&H MIL-DTL-38999 lanyard
release connectors and provides positive incremental
adjustments in the following lanyard length ranges:
Dash #
1
2
3
Length Codes
E, F, G
G, H, I
I, J, K, L
Table A.
The predetermine adjustments prevent the possibility
of cinching the lanyard too tight on the bail bar
resulting in a premature separation due to vibration or
movement between the store and rack.
CONCLUSION
The Investigation and Study conducted by G&H
Technology on the Operational Requirements of the
1760 Lanyard Release Umbilical Connector, exposed
disparities between the qualification requirements of
MIL-DTL-38999/31, and its intended application.
Furthermore, the diversity observed between
platforms and installations seemed to indicate a need
for greater clarity and definition concerning the
installation umbilicals.
The root causes for the high separation forces and
damage observed include:





Ejection velocity rates
Excessive off-axis pull angles
Shear stresses induced by pre-loads
Impulse loads created by lanyard slack
Tolerance stack and interference between
mating hardware
The Study revealed that with proper design and
control of the manufacturing tolerances for the intermating components, a connector conforming to the
performance requirements of MIL-DTL-38999/31
could be achieved. Furthermore, by clearly defining
the criteria for optimum installation of the umbilical
connector, you greatly reduce the chance for jams
and damage caused by high separation forces.
ACKNOWLEDGEMENTS
G&H Technology would like to express its gratitude
to Lockheed Martin Aeronautics Company - Fort
Worth and Lockheed Martin Aeronautics Company Palmdale for their assistance and cooperation during
the development of this connector.
We would also like to acknowledge The Society of
Automotive Engineers (SAE) Avionics Systems
Division (ASD) for providing a forum in which
industry and government representatives can freely
exchange information, leading to an understanding
and betterment of the products, services and industry
as a whole.