HD1967MM3AD
Introduction to this Topic
A while ago I came across this
letter written over 32 years ago. At
first I didn’t recognize it, then it came flooding back – for reasons not obvious
for what the letter says.
This is an example of one minor
problem, of which there were many every day – an example of what it takes to
develop and make a complex missile system reliable. I wrote this memo during a lunch period after fact finding in the
am -- then moved on to the next problem.
At the time I was Supervisor of Systems Requirements and Integration for
the PBPS and was also serving as Project Engineer. What caused me to recall this issue is that this memo completely
exceeded my authority to be handing out assignments – yet all responded – which
was not always the case.
This issue spanned multiple
companies and multiple organizations within Autonetics – there was no clear
definition of the “chain of command”.
Knowing this I moved out with this memo. The PBPS contract was issued via the Guidance and Control part of
AF/TWR then responsibility transferred to Propulsion part of AF/TRW. Guidance and Control and Propulsion, on both
sides of the house, guarded their domain – neither “took orders” from the
other. This letter crossed boundaries;
however, almost all responded to my verbal request based on this letter. Other papers found with this memo show follow
up studies and meetings held to resolve the problem. The Post Boost Propulsion System fit on the back of the Guidance
section and on top of stage 3 to deploy multiple H-Bombs – it was a secret
program at the time. The Arm/Disarm
switch was accessible via a key so ground crews could Safe the missile when
working on it. It was electronically
commanded to Arm.
First the players: BSD = Balistic Systems Division of the AF,
Maj Hilten was the propulsion system representative. TRW = Thomson Ramo Woolridge, technical advisors to the AF,
represented by Deavers. BAC = Bell
Aerosystems Corp Autonetics sub contractor for the PBPS. AN = Autonetics Division of Rockwell,
Minuteman III G&C and PBPS contractor.
MMD = Minuteman Division of Autonetics, of which the PBPS organization
was a part.
P92 is a Flight Control Electronics
box provided by Data Systems Division (DSD) of Autonetics. P-107 is the PBPS electrical harness
provided by DSD. A/D is the Arm/Disarm
switch provided as GFE government furnished equipment by Hercules Powder Co
(HPC). The pyroharness is wiring
provided by Boeing to go with their stage III to PBPS separation joint provided
to Columbus Div of Rockwell who made the PBPS structure. Ordnance devices are explosive valves that
release propellants for use by the PBPS.
ICDR = Interface Control Document Review. AID = Autonetics Interface Document. Personnel are:
Simkins Systems Engineer for PBPS, Triay Project Engineer for Guidance System,
Slauter Project Engineer for DSD, Powers Group Leader PBPS, Evans Project
Engineer for PBPS, Bellamy Mgr PBPS, Buxton Chief Engineer Minuteman.
To many this would seem like a minor
problem, and in some ways it is – however it is diligence on issues such as
this that make for a very reliable and successful missile system. D Landau 09-12-99
****O****
To: Those Listed Date:
Feb __ 1967
From: D. W. Landau
Subj: P-92 to PBPS Ordnance Interface
Statement
of Problem:
It
has been found that the P92 to P-107 to A/D switch t0 pyroharness to ordnance
device circuits do not satisfy the requirements of BSD Exhibit 62-64.
This
issue is an Autonetics Action Item due 10 February 1967 (Ref: ICDR Meeting held
with BSD/TRW/BAC/AN at BAC).
The
solution is not simple or obvious and may require changes to the P-92 or PBPS
equipment or BSD Exhibit or combination of the above.
Based
on pre-Minuteman III information and a preliminary circuit rational, the P-107
plus A/D switch plus pyroharness loads were treated as nil with respect to the
P-92 and ordnance loads, reference Figure 1.
It
has since been found that although an average A/D switch has a resistance of
.04 ohms at acceptance, the model specification for this switch permits a .20
ohm resistance at acceptance and up to a .6 ohm resistance before rejection in the
field. (This is the resistance of two contacts when input is measured with
output shorted, ref: S-133-1003-3).
The
load side of the A/D switch is not the same for each segment of an A/D
switch. The elements shown in Figure 1
represent the maximum number of bridge wires in parallel (minimum resistance
condition) of any PBPS applications.
The bridge wires may range in resistance from .1 to 1.0 ohms each.
(Analysis provided by BAC 1-17-67 is based
on .31 ohms each. Ref: Figure 1).
The
P-92, in a simplified sense, consists of two paralleled transistors which feed
through resistors to the A/D switch and accommodate two circuits of the A/D
switch in parallel (one A/D switch contact is common and handles double the
current of the other contacts). The
P-92 resistors shown in Figure 1 serve a monitor function and as load limiters
to prevent burnout of the transistors.
The
minimum voltage used for design of the P-92 is presently unknown to the
writer. It would be some value less
than 28 volts determined by the dependable battery voltage at Stage III
termination and the voltage drops from the battery to the point in the P-92
circuit shown in Figure 1.
If
one discounts the .6 ohm A/D switch load and uses .2 ohms and if one uses a
nominal load down stream of the A/D switch of .142 ohms per leg then the
effective load seen by the P92 at the input to the A/D switch is .222
ohms. The requirements for the P-92
expressed in IL 66-644-20-CE-56 dated 29 December 1966, indicate a minimum
resistance design load of .17 ohms. This indicates that the actual resistance is
greater and the current is less than-required by BSD Exhibit 62-64 for the
ordnance device.
It
should also be remembered that there is a maximum "off' current from the
P-92 which is set by the "no fire" limit of the ordnance device and
it is typical of transistors that the larger their -power carrying capacity,
the larger their "off" current flow.
This statement is made to indicate that the P-92 design is boxed in on several sides. Over a range of power supply voltages, it
must provide not more than but not less than a specified amount of current into
a variable load which is aggravated
by a broad resistance band on the A/D switch and bridge wires plus a variation
in the number of bridge wires across a P-92 switch, each of which is of a given
(commonality) design.
Possible
Solutions:
1. Take deviations to BSD Exhibit 62-64
and base design on a maximum A/D switch resistance of .2 ohms.
2. Modify the P-92 (it is not known that a
solution is possible even if the A/D switch is treated as .2 ohms).
3. Remove P-92 limiting resistors and add
"trim" resistors to P-92 ordnance loads (This is not obviously
-practicable).
4. Use single bridge wires
5. Add additional P-92 switches and an additional
A/D switch.
(There
are possibly others not immediately apparent to the writer)
It
is the unsubstantiated understanding of the writer that this type circuit and
load is typical of existing Minuteman circuits for which there have been no
known failures attributable to the present P-92 design and circuit
loading. If this can be substantiated,
it would favor solution Number 1.
Required
Action:
1.
MMD perform a load analysis for various
conditions; i.e., .2 and .6 ohm A/D switch, .1 and 1.0 ohm bridge wires, wire
resistance and r.s.s. combinations of the same and provide this data to DSD.
Assigned
to: Landau (Simkins support)
IL
67-644-20-AvE-43 dated 27 January 1967, from A. Triay to M. Stone and G.
Sargent requests that Logistics modify T.O.'s to reflect a .2 ohm requirement
for the A/D switch and for DSD to determine the impact on the P-92 of a .25 ohm
load. This IL is a good first step but
perhaps not technically correct.
What
is lacking is a complete loads analysis of each PBPS ordnance circuits so that
the total range of load seen by the P-92 standardized switch can be accurately
expressed.
A
quick analysis performed by the writer indicates that the variation in
bridge-wire loads only as seen by the P-92 in the circuit of Figure 1 may range
from .073 ohms to .127 ohms. This
calculation was based on a bridge-wire resistance range of .1 to 1.0 ohms or .6
+ - .4 ohms, then using an r.s.s. averaging of the .4 ohm deviation, this
becomes .6 +- .164 ohms or .436 to .764 ohms (.31 ohms were used in BAC analysis)
for each bridge wire which when treated as 6 bridge wires in parallel gives the
values of .073 to .127,ohms as seen by the-P-92. The pyroharness resistance, etc., would need to be added to this.
It
is recommended that D. Griswald, who has briefed the writer in order to prepare
this IL, perform this analysis as he is well versed on the issue. (644-40 does
not currently have anyone available to make a detailed loads analysis). The results of this analysis to be
incorporated in the P-92 to PBPS AID.
2. DSD perform investigative studies and
analysis of possible modifications to the P-92 based on the load conditions
defined by Item 1.
Assigned
to: A. Triay/ V. Slaughter/ Landau
3- Identify
to BSD/TRW (Propulsion) that the solution to the action item due 10 February is
not simply a matter of correcting the PBPS to P-92 AID.
Assigned
to: R. Powers
4.
A best
solution be resolved by @/DSD for presentation to BSD/TRW as soon as
possible. This function involves both
propulsion and G&C sections of BSD/TRW and the solution may require changes
to the A/D switch model spec., changes to ordnance criteria and modification of
the P-92.
Assigned
to: Triay/Evans
It
should also be noted that the present arm disarm switch has a doubtful hardness
and mechanical environmental capability.
An investigation may reveal that a new am disarm switch may be required
and such a need should be clarified as soon as possible.
_____________
D. W. Landau
Those Listed:
P. R. Evans,
R. G. Powers, E. D. Simkins, A. Triay, V. Slaughter
cc:
C. Ballamy,
W. M. Smith, R. Eide, E. R. Buxton, R. C. Hill
