My Standard Caution About Examples applies to this page.
Start with real input1:
3.2 General Design Requirements.
3.2.2 Deployables.
3.2.2.2 Retention and Release Devices.
3.2.2.2.1 General. Positive retention provisions shall be provided for deployables in the stowed and in the deployed positions. The effects of deflections such as those induced by centrifugal forces or differential thermal growth of any deployable with respect to its space vehicle attachments shall be considered in the design of the attachments. Devices that may be subject to binding due to misalignment, adverse tolerances, or contamination shall not be used. Slip joints shall be avoided, where practicable. Flexures, four bar linkages, or other types of pivotal linkage are preferred. Self-aligning features, such as self-aligning bearings and rod ends, shall be used, where practicable, to preclude binding of pivoting elements. Continuous hinges, such as piano hinges, shall not be used for large deployable panels. Pyrotechically actuated devices, motor driven devices, or other suitable techniques may be used to retain the deployable in the stowed position. Release mechanisms which permit ejection of parts away from the space vehicle shall be avoided, where practicable. The design of latching devices shall be such that peaking of resistance near the end of travel of the deployables in minimized. Leaf spring latches which become the primary element for reacting deployment or deployment rebound loads shall be avoided, where practicable. Catches using a permanent magnet as the holding element shall be avoided, where practicable. The design and materials used for the retention devices shall be such that the stresses are maintained sufficiently below the fatigue endurance limit to avoid fatigue failures due to cyclic design load levels and environmental exposure. Retention and release devices shall be designed preclude cold welding and friction welding (see 6.2.2). For surfaces that slide or separate during operational use, the contact pressures at the interfaces shall be minimized consistent with providing adequate ascent stiffness. These surfaces shall be fabricated from appropriate materials and lubricated so as to prevent galling or seizure. Where contact areas may be reduced from the nominal as a results of tolerance build-up, the minimum area which could occur shall be used in determining contact pressure.
Discussion
Object/Process: Retention/Release Devices
Normally, I wouldn’t want to use any manner of concatenation in naming objects or processes. However, the author’s technical intent clearly recognizes two (2) basic functions of the devices being considered. These devices both retain the deployable mechanism for launch and release it for deployment once on orbit. I could have selected either of the two & let the Functional Allocations fall where they may, but chose to keep both in order to more explicitly recognize the original mind set.
Topic: Retention
Positive retention provisions shall be provided for deployables in the stowed and in the deployed positions.
The use of “provisions” in this context refers to identifiable features of the design, which may be either physical or functional. Use of the plural implies that the subsequent requirements are to be individually associated with each deployable that is in-scope to the contractual allocation, essentially telling System Engineering how to structure their requirements as the Conceptual Design matures into a Preliminary Design.
Parameterization:
“Positive” can be interpreted in at least two ways: with regard to intent or with regard to some parameter’s value. Because these are requirements, an interpretation of “intent” is not supportable: “intent” is impossible to objectively verify 2.
Therefore, this requirement should be interpreted as technical, in reference to a measure, and the technical effort would be better off if that measure were explicit. As an example of that idea, I’ll form a measure as a margin3.
Retention Margin – Defined herein as:
where
LR≡ Load supportable by the retention stop (or mechanism)
LA≡ Limit applied load during service
If desired, this margin could be defined as a “vector of vectors” to simplify traceability. In that case, each element establishes margin with respect to the limit load for one retained position in one direction of retention. Note that is possible for the limit capability to have different direction for a retained mechanism as compared to that same mechanism in the deployed configuration, which creates the “inner” vectors. That statement applies equally to each intermediate retained position, which is what makes the “outer” vector. It is not necessarily true that each retained position will have the same number of limit load directions, so the Retention Margin as defined here cannot necessarily be formed as a rectangular array.
Note that the margin specified above measures the ability of the retention device to maintain a state of deployment for the Deployable Mechanism itself. The detailed topic is, therefore, a Function (“Retain”), and the Retention Margin is a Measure of Performance (which is a class of parameter). The two classes of Load are also parameters, hierarchically subordinate to the margin.
The original statement provides an acceptable range for the margin as defined above: it must be positive (> 0)4.
Topic: Deflections at the Mounting Interface
The effects of deflections such as those induced by centrifugal forces or differential thermal growth of any deployable with respect to its space vehicle attachments shall be considered in the design of the attachments.
We rely on normal stress analysis (which also considers “the effect of deflections”) to verify static and quasi-static load-carrying capability; that is, a completely different set of requirements exists for that subject, and it won’t be replicated as part of this exercise. The topic of this requirement concerns the propagation of deflections at the mounting interface that might have arisen between acceptance (at the end of manufacturing) and a restraint actuation event. It is also clear that the subject issues are due to factors of circumstance: the provision discusses the desire for the features of the retention mechanism to be able to accommodate them, but those features themselves (whatever they turn out to be) are actually different topics in their own right.
Use of “such as” implies that the developer is expected to derive such a list. Because the provision is mandatory (an unmodified “shall”), it is clear that the authors intend for disinterested expertise to audit the execution before approving the design. Heritage specification practices will properly insist on negotiating that list of conditions to eliminate the use of the unverifiable “such as”, which means that this provision has to be at least partly treated as Statement of Work (because the customer and the developer will have to develop and agree on that list).
This arbitration between SoW and Development Specification at the start of a project often causes a hangfire in the SE process. Management will often fail to understand the effort required to resolve issues before they become crises…which they will, down the road. To make matters worse, many SE practices don’t allow a traceability relationship to exist between SoW and specification. Here, I’m explicitly advocating exactly that relationship: in many cases, we create requirements because the SoW told us to do so.
Parameterization:
Deferred
These interfaces can be quite complex, and include both properties and functions5. The detailed parameterization is far more clear if we await concrete definition of the interface (e.g., materials, geometries, loads). It should not be generalized in an attempt to make detailed requirements more mature than they really are.
To be of available as input to the detailed design, any such list must be developed and agreed to no later than PDR (that is, before the detailed design has progressed very far). Depending on the contract structure, that definitization might (or might not) take place during negotiation of a Part I ICD in response to direction in the SoW.
Topic: Binding
Devices that may be subject to binding due to misalignment, adverse tolerances, or contamination shall not be used.
Binding is a dynamic condition where a mechanism fails to actuate smoothly. The topic is surprisingly difficult to address without resorting to some appropriate technical insight. Its treatment is a useful example of why (and how) System Engineering works better when that insight is available (as noted below).
Parameterization:
Binding Figure of Merit as defined here 6.
From a technical perspective, the three causes addressed in the original requirement (misalignment, adverse tolerance, and contamination) fall into two groups, with contamination being qualitatively distinct7. Although their traceability is superficially subordinate to the single parent clause, they’re actually independent topics and are treated that way here.
Topic: Misalignment and Adverse Tolerances
For the purposes of this exercise, “binding” due to misalignment or adverse tolerances are “envelope corner cases” of jamming and wedging as they are formally defined in McKerrow8. Neither jamming nor wedging introduce new characteristics (performance, physical characteristics, etc.) to the retention device itself; they address regions in the mechanical configuration space of the design (combinations of positions, tolerances, and misalignments across interfaces) where each retention and its associated actuation(s) requires one or more explicit points of verification due to the enhanced probability of failure.
Note that “mechanical configuration space” is different than the Design Configuration category of the abstract attribute tree. Deconfliction of these two terms might, for certain projects, necessitate renaming one (or both) terms in order to avoid confusion, or to accommodate strongly held preferences by some portion of the Engineering staff or customer base.
It is important to recognize the impracticality of explicitly representing the full combinatorial range of these variables in an affordable number of testable units. This reality usually precludes their verification by test, although good testing at the combinatorial extremes can help validate models. This topic more reasonably addresses a set of associated Analytical Methods, which are Developmental Processes.
My preference would be to assemble the appropriate project expertise (with, if necessary, acquisition customer personnel) and negotiate a specific methodology for verification of the mechanisms in scope to the approved conceptual design. That might be quite a long list! Alternatively, the developer may have a standard set of published practices for certain design prototypes that can be reviewed by the customer in advance.
Because the underlying technical notion is that at least the boundary of the designs’ “mechanical configuration space” must be verified, I’ve left these two items as a single topic. I’d be willing to consider moving the entire topic to the verification section and renaming it as something like “Verification of the Mechanical Configuration Space”. Legacy (detail) specification practices would have done exactly that, allocating all applicable design requirements to be verified by a related standard practice (along with other verification techniques). Other (detail) specification practices that do not admit to n:m allocation between design and verification requirements might well try to treat them as “Design and Construction” requirements, or omit them altogether.
Parameterization
Following the aforementioned reference (McKerrow):
Friction coefficient – a dynamic property; a value for each point where the moving parts meet during actuation; depending on the dynamic conditions, this could be either a static or dynamic coefficient.
Resultant Force – a dynamic property; a vector for each point where the moving parts meet during actuation. Note that not all points necessarily meet at the same time and, for any given point, there can be more than one direction.
Plastic Deformation – a dynamic property; a vector for each point where the moving parts meet during actuation.
The first two items address jamming: a transient condition where the resultant angle is within the friction cone. The half-angle of the cone is equal to the friction coefficient at that point. No elastic deformation takes place; relief of the external load allows the parts to move. The third item addresses wedging: a static condition where permanent deformation takes place such that the parts cannot move after external loads are relieved (i.e., there is a residual stress locking them in place).
It should be noted that this is not the only suitable parameterization for this issue. It is adopted here due to its relative simplicity.
Topic: Contamination
Relative to mechanisms, the topic of contamination can be divided into two broad areas: particulates and tribological agents. In both cases, the agents require explicit identification and characterization.
As a broad generalization, particulates decrease the clearances between retention device surfaces that undergo relative motion. They are characterized by the statistics of distribution for density, composition, size, shape, and hardness9. In addition to decreasing the clearance, particulates can also degrade the smoothness of the faying surfaces.
Unlike particulates, most tribological agents do not immediately infringe on the tolerance between surfaces undergoing relative motion. Their immediate effect is usually to change the coefficient of friction between the faying surfaces. That change can be assessed only if the materials and surface condition of the design are known; the change itself cannot be abstractly parameterized. We can parameterize only the nature of the contamination itself.
Parameterization
This is a surprisingly subtle subject to parameterize, with some interesting implications for classification of topics in the environments of complicated Systems. It needs its own category…eventually!
Topic: Design Constraints
By “Design Constraints”, I mean design approaches that are placed under restriction up to and including outright prohibition. In all cases, the parameterization has a trivial “count the number of places where the designers did what we told them not to do”, so I have not bothered with explicit repetition thereof. There are (however) a few exceptions to that generalization in the subject document section. I have extracted them for consideration at the end of this section.
Slip joints shall be avoided, where practicable. Flexures, four bar linkages, or other types of pivotal linkage are preferred. Self-aligning features, such as self-aligning bearings and rod ends, shall be used, where practicable, to preclude binding of pivoting elements.
Use of “where practicable” means that these statements are merely guidance requiring explanation when not observed. That’s actually SoW rather than technical requirement.
Continuous hinges, such as piano hinges, shall not be used for large deployable panels.
A “piano hinge” is intended to spread loads all along the line, allowing (sometimes) the piece parts to be lighter for a given load. For most spacecraft applications, this comes at an unacceptable cost: thermally-induced deflections across the hinge line are often non-commensurate, and any thermal gradients10 on each side can provide opportunities for the elements of the hinge to experience contact beyond that intended by the design. Such gradients can result in non-affine geometric transformation of the surfaces on either side of the hinge line and, therefore, non-uniformity in alignment and contact stresses along the hinge line (from one knuckle to the next). As a result, the two hinge leaves, and/or the rod(s) between them, can jam (or even wedge).
Both the gradients and the resulting deflections can be difficult to predict and, due to the size of the panels, expensive to conduct the full-scale tests that would be necessary to either verify directly by test, or to validate models of the non-uniformity induced by thermal gradients. Some of the thermal deflection issues can be mitigated by careful selection of materials for thermal expansion coefficients, and by passive thermal controls.
Having many potentially faying surfaces, the long hinge line having many knuckles also tends to be more susceptible to contamination than some of the competing design approaches.
These issues not only make the design approach problematic, but difficult and expensive to verify. The authors of MIL-A-83577 seem to wish that they could absolutely forbid the use of piano hinges…but appear to implicitly recognize that the design might be acceptable some applications. The meaning of “large” is, however, very difficult to generically characterize (let alone assess) in this context. The potential for the adverse consequences of using piano hinges depends strongly on environmental conditions, material choices, and optical properties of the mechanism’s parts in addition to mere sizing details. Rather than weakly wording an attempt to discourage their use, it would be better to impose explicit requirements on verification when they are used. It is interesting to note that the document takes this approach in other areas.
Under normal circumstances, I would topically identify the design concept as a matter of “Configuration”, but would not normally attempt to parameterize the internal workings of a design concept in the detail alluded to above. With few exceptions, that level of detail is (and should be) the province of an Engineering staff that is tightly focused on such issues. Other than operationally-relevant issues such as mechanical stroke, jamming/wedging, initiation, etc., the staff should be allowed to select and implement the parameters consistent with the conceptual design. The sole exception to that notion is in the parameterization of environmental conditions (both natural and induced), which will likely be shared with other facets of the project.
Pyrotechnically actuated devices, motor driven devices, or other suitable techniques may be used to retain the deployable in the stowed position.
The first sentence is simple permission, and has no effect as a requirement. Use of “where practicable” (further weakened by “avoided”) and “may” means that these statements are mere guidance requiring explanation when not observed. Unless an Acquisition Customer felt strongly to the contrary, I would go no further than to identify them as topics under “Configuration/Common Designs”.
Leaf spring latches which become the primary element for reacting deployment or deployment rebound loads shall be avoided, where practicable. Catches using a permanent magnet as the holding element shall be avoided, where practicable.
“Avoidance” is actually a SoW term. Use of “where practicable” means that these statements are merely guidance requiring explanation when not observed.
Topic: Ejected Parts
Release mechanisms which permit ejection of parts away from the space vehicle shall be avoided, where practicable.
It may seem obvious that “…ejection of parts…” refers to self-induced particulate contamination. The phrase probably includes that. It likely attempts to restrict the inducement of particulate contamination for other spacecraft parts, as well and, in that sense, is actually a Measure of Performance.
It might be tempting to treat this as a Physical Characteristic, since the statistical distribution of ejected parts can be developed by physical examination around the retention mechanism after it is operated. However, the phrase “…after it is operated” means that it is inseparably associated with the functioning of the mechanism, and that makes it a Measure of Performance.
Parameterization
Ejecta Distribution – The relationship between size and quantity (count) of material ejected from the retention mechanism as a result of actuation. This can be explicit (the count of particles within each defined size bin) or reduced (the “parameters” of some relevant distribution shape). Either way, at least a vector; if species needs to be considered, it might be a vector of vectors.
Topic: Travel Resistance Peaking
The design of latching devices shall be such that peaking of resistance near the end of travel of the deployables is minimized.
See the spin-off page here, which discusses why my preference would be to have no unique parameters sourced to this provision11.
Topic: Contact Optimization
The design and materials used for the retention devices shall be such that the stresses are maintained sufficiently below the fatigue endurance limit to avoid fatigue failures due to cyclic design load levels and environmental exposure. Retention and release devices shall be designed to preclude cold welding and friction welding (see 6.2.2). For surfaces that slide or separate during operational use, the contact pressures at the interfaces shall be minimized consistent with providing adequate ascent stiffness. These surfaces shall be fabricated from appropriate materials and lubricated so as to prevent galling or seizure. Where contact areas may be reduced from the nominal as a results of tolerance build-up, the minimum area which could occur shall be used in determining contact pressure.
Taken as a whole, this paragraph is very nearly pure SoW. It deals with a critical underlying trade-off between stiffness during launch and certain failure mechanisms.
Typically, we’d like a deployable mechanism to be very stiff when packaged for launch; much of that stiffness is driven by the resistive load of the retention mechanism. The contact stresses between faying surfaces12 are usually in direct proportion to that load. Those surfaces typically undergo slight relative motion during launch. Such motion, although of low amplitude, can be of considerable frequency. Allowing those contact stresses to rise can raise the deployable mechanism’s resonant frequency, reducing the potential for dynamic coupling with those of the launch vehicle. This is typically developed as a general requirement for the payloads on each specific launch vehicle.
Unfortunately, letting the contact stresses rise too high can have serious adverse consequences. The subject paragraph introduces several of those issues in a manner so abstract as to be of limited utility in the verification phase. It serves to do little more than to alert the (possibly inexperienced) developer or contract monitor to the relationship between these factors.
My normal practice when encountering a general requirement of this type would be to survey other project requirements documentation, to determine whether each of the potential subordinate topics and parameters are addressed elsewhere…and I usually find that they are typically addressed with more explicitly written requirements. Since I’d not normally want to write more than a single requirement on any one topic parameter, I’d not normally make a formal definition of parameters in response to this particular paragraph. I might (however) add the subject material to the traceability for those elsewhere-found parameters and their values. That list of provisional parameters might look something like the following. In most cases, the details of their calculation should be verifiably mandated by Developer standard practices available for customer review before any development contracts are let.
Parameterization (provisional)
Contact Stress – That stress (force/area) resulting from total external loads. It is calculated at each mechanical joint for the minimum contact area given variations in size (e.g., thermal, tolerances). This is at least a “vector of vectors”13: for all points in the loaded configuration space, under all conditions, for each pair of elements in each joint.
Fatigue Limit – The stochastic relationship between load and number of cycles showing how many times a specific material can be exercised before failure can reasonably be expected. Nominally a curve, it is sometimes expressed in terms of a stochastic Power Spectral Density. The load cycle statistics are often combined with temperature variation into a three-dimensional relationship.
Limit Contact Stress – That value of contact stress causing permanent deformation of the parent material.
Galling Stress – That value of contact stress which, when experienced between a pair of materials, cause an adhesive failure mode in which both surfaces are deformed in excess of their manufactured surface roughness (“galling”). Note: this is a joint property of the materials as a pair, not an independent property of either one.
Vacuum Welding Stress – That value of contact stress between two surfaces of the same material causing the sharing or transfer of molecules between them. The resulting condition is a “locked” joint capable of carrying loads independent of externally applied forces.
Footnotes- “Assemblies, Moving Mechanical, for Space and Launch Vehicles, General Specification for (MIL-A-83577B)”; USAF Space Division, SD/ALM, 01 Feb 1988[↩]
- It could, however, have been treated as Statement of Work, in which case mere contract verification practices would suffice.[↩]
- Because consideration of a factor, which might go negative would be absurd in this context.[↩]
- In this framework, the requirement would be re-written as “the Retention Margin for each means of deployable mechanism retention shall be greater than 0”.[↩]
- Yes, we can consider a function to be a type of property. It doesn’t make any philosophical difference in this context.[↩]
- Moved to a separate page due to its lengthy derivation.[↩]
- formally, most contamination is “exogenous” to the design: a property of the environment rather than a property of the part itself. The exception is “self-generated debris” which, when causing binding to occur, is generally considered to be an inherent failure rather than “caused by contamination”. This issue is in scope to the other performance requirements because it will naturally occur during properly conducted life-cycle testing. Relevant provisions are found elsewhere in MIL-A-83577, so need not be addressed here.[↩]
- McKerrow, Phillip John “Introduction to Robotics”; Addison-Wesley Publishers Ltd, 1991, ISBN 0-201-18240-8[↩]
- For some particulates, hardness is redundant with composition. For others, it is not.[↩]
- Which can be either transient or transient-steady-state.[↩]
- Subject to customer concurrence, or course.[↩]
- which (later) experience relative motion during actuation[↩]
- possibly a vector of arrays[↩]