challenges and emerging trends in store separation

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Challenges and Emerging Trends in Store Separation Engineering – an Air Force SEEK EAGLE Office

Perspective Stephen R. Perillo1 and Donald J. Atkins2

Air Force SEEK EAGLE Office, Eglin AFB, FL 42542

Two key trends in store separation engineering have emerged in recent years which have dramatically affected the manner and pace in which certification projects are being conducted. These trends influence the gamut of aspects influencing store separation engineering: from the warfighter required configurations and platforms to be addressed, to the designs of the stores themselves, to the racks that carry them, to the tools used in trajectory prediction, to the ground and flight test techniques used to test them, and finally, to the programmatic environment that projects are conducted under – all are changing in meaningful ways from these trends and pre-existing ones. Stores are typically smaller and lighter than past designs, more complicated, and often less stable. Multi-carry racks are introducing the need to account for multi-axis flexibility effects while simultaneously increasing the number of configurations that must be analyzed since practically any release sequence is demanded by the warfighter. Immediate warfighter needs and the budget environment are driving program timelines ever shorter. These two key trends have come to define the fourth facet of distinct themes in warfighter requirements since September 11, 2001 and the advent of the global war on terror (GWOT). The Air Force SEEK EAGLE Office has undertaken a wide range of efforts to be prepared to meet the future needs of the Air Force. We are close to finishing the complete rebuilding of our entire tool suite with an emphasis on productivity – not new capabilities although we have added those, too. We have undertaken paradigm shifting efforts in the way we conduct ejector rack, wind tunnel, and flight testing. The sum of these changes has, and will continue to result in, a number of lessons learned. The efforts we have undertaken to address these new challenges will be addressed.

I. Introduction

The store separation requirements to fight the Global War on Terror (GWOT) sparked by the terrorist attacks of September 11, 2001 have evolved rapidly through at least four stages where distinct themes in warfighter requirements can be discerned. Each of these stages represents a significant, and often pre-dominant, concern that has driven warfighter requirements to the store compatibility community. The first stage was typified by accelerated schedules for in-progress projects and new combinations of previously certified stores on existing aircraft platforms. The second stage was typified by a quest for improved capability targeting pods and the certification of these pods on a wide variety aircraft, including those that did not previously have a targeting pod capability. The third stage was typified by great concern for collateral damage and a powerful desire to be able to attack moving targets. The fourth stage is typified by the requirement to fight a low intensity, counter-insurgency (COIN) conflict with aircraft platforms better suited to the combat currently being conducted in Iraq and Afghanistan. This last theme has been lingering in the background for a number of years but has only recently emerged as a distinct, identifiable stage. It is likely that this stage will be longstanding since counterinsurgency conflicts are protracted (often multi-decade). These stages do not represent time periods where only a single type of compatibility work was performed. Instead, they represent periods where the emphasis between project types changed or even the emergence of a new project type. Indeed, it can be argued that all four of the themes characterizing each of these stages still exist today as well 1 Lead Store Separation Engineer, Air Force SEEK EAGLE Office, 205 West D Avenue, Suite 348, FL 32542, AIAA Senior Member. 2 Store Separation Engineer, TYBRIN Corporation ACSES Contract, 205 West D Avenue, Suite 348, FL 32542, AIAA Senior Member.

47th AIAA Aerospace Sciences Meeting Including The New Horizons Forum and Aerospace Exposition5 - 8 January 2009, Orlando, Florida

AIAA 2009-101

This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.


as other programs associated with peer-to-peer conflicts. They are just fewer in number and of lower priority. All of the stages have shaped the challenges faced by the store separation engineers from the Air Force SEEK EAGLE Office (AFSEO). This paper will detail two key recently emerged trends which have accelerated and come to define the counterinsurgency stage. It will then outline some of the challenges associated with all four stages, and the AFSEO response in today’s environment of shrinking budgets and programmatic timelines.

II. Two Key Recently Emerged Trends There are two key recently emerged trends that have synergistically come to define the fourth stage in post 9/11

aircraft/store compatibility requirements - counterinsurgency warfare. The first key trend is the swift reduction in size and weight, and to a lesser degree, the increasing complexity of the stores and the store separation problems being tested and certified today and in the near future. The second key trend is the re-introduction of slow speed combat aircraft which had largely been absent from the store compatibility workload for about the last 20-25 years.

A. Trend 1: Rapidly diminishing store size and weight but increased store complexity and difficulty of the store separation problem. 1. Smaller size and weight The first key trend that defines the counterinsurgency stage is that store size and weight are down dramatically while the overall complexity of stores and indeed the overall store separation problem have increased. It was less than a decade ago when any munition weighing less than 500 lb was considered a ‘small’ munition and approached with much caution. Now stores an order of magnitude less in weight are in test and it is likely that stores another order of magnitude smaller will be developed in the next few years. The trend for lighter stores began with the Small Smart Bomb Range Extension (SSBREX) demonstration program in the late 1990’s which lead to the Small Diameter Bomb (SDB or GBU-39/B) production program. These first ‘light’ weapons weighed in at approximately 270 lbs. The next major small store program was the Miniature Air Launched Decoy (MALD or ADM-160B) which was slightly larger at approximately 280 lbs. In addition to these large acquisition programs, a number of much smaller acquisition programs have produced munitions as light as 25-30 lbs. Examples are the AGM-114 Hellfire missile (105 lbs) which was adapted from use on helicopters, the Griffin-B (approximately 30 lbs), and the GBU-44/B Viper Strike (approximately 25 lbs). This trend for lighter weapons is expected to continue. Already, the Air Force Special Operations Command (AFSOC) has stated publicly that they have interest in stores in the 1-5 lb range. In addition, micro UAVs are expected to be flying in the next 20 years. Micro stores probably won’t be far behind. While we at the AFSEO knew the prevailing trend was toward lighter stores, we are stunned by how far store weights have dropped and how fast they’ve gotten there. There are two primary reasons why this trend has behaved as it has. The first was the compelling operational need to limit the possibility of collateral damage. All other things being equal, small stores have smaller lethal radii than larger stores. The second reason for the trend was the advent of armed Intelligence, Surveillance and Reconnaissance (ISR) aircraft platforms with relatively light store carriage capability. Smaller stores have less detrimental effects on the host platform and also open up the possibility of carrying a higher total number of stores. It is quite likely that the advent of smaller stores will lead to a greater use of multi-carry racks in the future. This will be particularly true when really small stores get integrated on big aircraft. The only notable exception to this trend, at least in relation to store mass and size, involves the Massive Ordinance Penetrator which is a 25,000 lb class munition Small size and weight can also impact the tools available to the store separation engineer in flight test. For example, the commonly used Summit Industries telemetry (TM) kit is too large and heavy for many of these munitions. This has on occasion led to the use of TM data coming from the store guidance computer. Often the data rate for these types of TM kits are significantly slower than those typically used in store separation applications. In addition, they are usually not functioning when the store is jettisoned versus employed.

2. Increased complexity

Current generation stores are not only smaller in size and weight to older generation CBUs, GBUs, AGMs, and Mk-8X series weapons; they are also typically less stable and more complex. This complexity has manifested itself in a number of areas: increased geometric complexity, greater release order complexity, the use of multi-carry racks, and increased complexity in determining the acceptability of the store separation transient.


a. Geometric complexity A perfect example of the increased geometric complexity is the MALD as depicted in Figs. 1 and 2. The vast

majority of the complexity increase is associated with the addition of a stability augmentation device (SAD) on the rear of the store as shown in Figure 2 that is comprised of essentially three fixed grid fins. The grid fins represent large challenges to both wind tunnel testing (WTT) and computational fluid dynamics (CFD) analysis. It is currently impossible to produce geometrically accurate, yet sufficiently strong, WTT models at the 5 or 10% scales typically used in store separation WTT. This has lead to the development of a pseudo-SAD that had similar overall porous area but a different geometry as shown in Fig. 3.

To support the integration of the MALD on the B-52, CFD was used to generate the data for the simulations. The

CFD model of the SAD also presented a number of challenges. First, the complex geometry of the grid SAD required the use of 10.5 million grid points alone in the AFSEO CFD model while the overall MALD model had only 16.5 million grid points. This limited the number of machines the MALD CFD runs could be submitted to and extended the solution times appreciably. This in turn, limited the number of CFD solutions which could be run in support of the program. This problem was notably worse when the MALD was mounted on the B-52 Heavy Store Adapter Beam (HSAB) which can carry up to eight MALDs. In this case, a compromise had to be made to model the captive MALDs with a pseudo SAD in order to make the size of the problem tractable on the available computer hardware.

b. Release order complexity The pre-existing trend of increasing release order complexity has been aggravated by the advent of smaller stores

because they are typically carried on multi-carry racks to maximize the aircraft’s carriage capability. Smart

Figure 3. Pseudo-SAD on WTT Model.

Figure 2. View of MALD SAD.

Figure 1. MALD on F-16.


weapons, or MIL-STD-1760 capable munitions, started the trend of increasing the release order complexity many years ago. This is because munitions are built-in test (BIT) checked in flight prior to release. If the munition fails BIT, the pilot wants to be able to step-over, or skip, that munition in the release order sequence since it is not functional. This is a perfectly reasonable thing to do from an aircrew perspective, but it has essentially ended the use of the normal release sequence as a means to control the scope of the store separation problem. In the past, store separation engineers could limit the scope of the store separation problem by forcing the user to only release munitions in the default or standard release order sequence. This is no longer acceptable to the user. As a result, the store separation engineer needs to analyze practically any release order possible. This can quickly expand the scope of the analysis and testing effort if the number of stores carried on a multi-carry rack is greater than two or if it is likely that the store trajectories are appreciably influenced by the effects of adjacent stores such as external fuel tanks. A prime example of this are releases from the F-15E conformal fuel tank (CFT).

To illustrate the rapid growth in scope that can be experienced when using a multi-carry rack, where the problem is most acute, consider the case of trying to analyze the SDB from a single weapon station on an aircraft. If the SDB is carried on the parent pylon, there is only a single release store configuration to worry about plus the effects of adjacent stores. If the SDB is carried on the 4-place BRU-61 (Fig. 4) but only released in the default release sequence, there are now four release store configurations to worry about for the SDB plus five for the BRU-61 itself. All of these must consider the effects of adjacent stores. When the release sequence rules are relaxed to allow only a single step-over per BRU-61, the number of SDB and BRU-61 release store configurations grows to ten and eleven respectively. Again, this is in addition to the effects of any adjacent stores. When there are no restrictions at all on the release sequence, the number of SDB and BRU-61 release store configurations that must be considered rise to 15 and 16 respectively. Very few warfighters and program managers have any appreciation for the increased scope induced by utilizing multi-carry racks. This problem can be further compounded if there is a wide range of mass properties associated with the various download configurations of the multi-carry rack as is the case with the BRU-61. In the case of unlimited step-overs, the longitudinal center of gravity (cg) of the BRU-61/SDB combination can vary from about 1 inch in front of the forward lug to 37 inches aft of the forward lug. The lateral cg can vary about 2.5 inches to either side of the BRU-61 centerline. The vertical cg can vary from about 6.5 to 9.8 inches below the forward lug.

One result of the wide range of mass properties is that it is exceedingly difficult to choose a single ejector rack combination that will produce safe trajectories for all possible BRU-61 configurations. A second result of the large mass property range is that it is likely that the mass properties of the BRU-61/SDB combination are well outside the database of historical ejector performance for the rack that is suspending the BRU-61. This can result in the unusual situation where additional parent aircraft ejector rack (e.g. MAU-12, BRU-47) suspension data needs to be collected to support certification of another multi-carry rack (with its own ejector racks) and stores. As if the situation were not already bad enough, the user and the program manager are not usually interested in doing the appropriate amount of carriage system jettison analysis and flight test work. This is true for a few reasons. First, carriage system jettison work does not provide the warfighter a true combat capability. Second, the analysis and testing are time consuming and costly. In addition, you may not like the ‘safety’ answer you get after the analysis and flight test is complete. In other words, some of the trajectories could be deemed to be unsafe. The frustration with this situation ratchets up quickly when one realizes that typically the configurations of greatest concern are associated with step-over configurations which probably will not occur often out in actual operational use. This can lead the warfighter to have the attitude that they do not care about the engineering results. They want the capability to jettison the carriage system regardless of the risks and they are willing to take their chances on needing to jettison a carriage system in a potentially dangerous configuration. One manifestation of the COIN stage in release order complexity is the war fighters desire to carry a mix of store types on a given mission. This is an operational necessity because the aircrew does not always know their target(s) prior to takeoff. It is far more likely that they will carry a mixed load of munitions and loiter until targets are located. At that point, the aircrew will select the most appropriate weapon type on the aircraft to employ. This paradigm increases the scope of the problem the store separation engineer must solve with regard not only to the ‘new’ store

Figure 4. BRU-61 with SDB's.


being certified but also with the regard to releases of the ‘old’ stores in the presence of the ‘new’ store. For example, when the SDB was certified on the F-15E, it was necessary to consider the effect of adjacent stores on SDB trajectories but also to consider the effect of SDBs (and its multi-carry rack, the BRU-61) on the trajectories of all the adjacent stores certified in the flight manual.

c. Complexity induced by multi-carry racks

Multi-carry racks can also increase the release order complexity through flexibility in the carriage system during the ejection stroke. This flexibility can induce multi-axis rate and positional effects into the initial conditions for a store separation simulation which, unfortunately, can be a function of the position from which the store is ejected on the carriage system and the number of adjacent stores. As a result, collecting ejector ground test data is considerably more complex because a greater number of configurations must be tested and because a great deal more instrumentation (e.g. rate gyros and accelerometers) are necessary than was typically used during historical testing. In fact, the complexity and scope of this type of ejector calls out for advanced test methodologies such as design of experiments (DOE) that can efficiently test a large test space with a high degree of engineering rigor. The final question that flexibility raises is whether store separation simulation codes will have to be adapted to model flexibility or its effects frequently. Traditionally, this level of complexity has been ignored.

d. Complexity due to acceptability determination Another area where the complexity of the store separation problem has been growing rapidly is in the area of

acceptability determination. Store releases must be considered both safe and acceptable in order to support certification of the store on a particular aircraft. For the purposes of this discussion, a release is considered acceptable if the store separation transient does not induce any motion or violation of criteria considered to be unacceptable by the store manufacturer. Some examples of typical acceptability criteria include maximum allowable body rates, body attitudes, or wing loads. In recent years, almost every program the AFSEO has been involved with has experienced greater difficulty in determining acceptability than in determining safety. This is because of tighter constraints being imposed to guarantee store warranty and launch acceptability region (LAR) performance than was historically necessary. Traditionally, store separation engineers did not have much to do with the determination of acceptability. They ensured the separation was safe and then left it up to the ballistician to determine coefficients that would allow the weapon to hit its target. Acceptability determination has put new and much more stringent demands on trajectory prediction accuracy. Store separation simulations frequently have to be accurate for a much longer period of time (about 0.65 – 1.00 seconds for acceptability vs. about 0.10 – 0.35 seconds for safety) in order to feed contractor guidance and control simulations with realistic data for determination of an acceptable flight envelope. In response, the manner in which flight test is being conducted has evolved. Store telemetry kits are now typically considered essential for determining acceptability. One area of acceptability determination that has not been done particularly well in the past is associated with whether uncertainty in items such as typical ejector or mass property variations have been accounted for in pre- or post-flight analysis. Frequently, the susceptibility to these variations has not been analyzed. Unfortunately, they can have an impact on the determination of an acceptable trajectory. If they do have an impact on the acceptability of certain regions of the flight envelope, should the item be certified if there is ever a less than 100% chance of having an acceptable separation or does that impose an unnecessary level of conservatism on the warfighter? This same type of thought process can be applied to the determination of the minimum allowable ripple interval. The authors believe that at some point in the future, some type of statistical criteria will be set that will determine the percentage of how many trajectories will be allowed to be unacceptable or how many stores can collide during a ripple release. The final challenging aspect of acceptability determination involves the question of whether store-to-store contact is ever considered acceptable even if the contact appears to be incidental. It is not easy to determine what is and is not an acceptable level of contact and it is extremely difficult to get a store contractor to say whether store-to-store contact is ever acceptable. This leaves the government in a bit of bind when some store-to-store contact truly appears to incidental. Should the government go ahead and certify the configuration even though the store developer does not concur? If it is determined that incidental contact is acceptable, what type of criteria should be used to make the determination and how do you test it? For example, should guided munitions ever be allowed to have contact? Should control surfaces ever be allowed to incidentally contact another item? Should flight testing be constructed such that it attempts to generate a ‘worst-case’ store-to-store contact? Resolution of store-to-store contact issues is even more important for multi-carry racks that are carrying stores with small installed miss distances. The AFSEO will have to address many of questions posed above as it executes the Enhanced Smart Triple Ejector Rack (ESTER) program on the F-16.


B. Trend 2: The return of the slow moving, combat aircraft The second key trend that defines the counterinsurgency stage is the return of slow moving aircraft to a role

where they are being used to release stores in combat. This is very reminiscent to the COIN aircraft used during the Vietnam War in the 1960-70’s. Historical examples of slow moving combat aircraft that released weapons are the O-1 Bird Dog, O-2 Skymaster, OV-10 Bronco, OA-37, A-1 Skyraider, OV-1 Mohawk, and later, the Piper Enforcer. Many of these aircraft are depicted in Fig. 5.

Modern examples of aircraft that would fit in this category are the MQ-1 Predator, the MQ-9 Reaper, the Armed Caravan (a modified Cessna 208B), the AT-6, and the AC-27J Stinger II (proposed program). Two significant differences exist between these two eras. First, the aircraft used in the historical examples were predominantly used in the role of forward air control. Most of the modern examples are better described as intelligence, surveillance, and reconnaissance (ISR) with teeth with the MQ-1 and MQ-9 being the most prominent examples. The second difference is that the modern weapons being released are guided weapons that are far more complex than the weapons being used by these aircraft in the Vietnam War. These types of aircraft have some advantages that are more appropriate for the current stage of the GWOT conflict than the predominant F-16s, F-15Es, B-52s, and B-1s used in the past few years. These slower speed aircraft are cheaper and easier to maintain, operate, and sustain over the wide geographic regions of most concern today. This is especially true because there is no viable air-to-air threat to be concerned with and only a minimal surface-to-air threat to be concerned with. These new aircraft tend to be modifications of existing commercial designs and have very aggressive schedules to get a capability into service as fast as possible.

Figure 5. Vietnam Era COIN Aircraft.


Figure 6. Some Modern COIN Aircraft.

The key challenge associated with these types of aircraft for the modern store separation engineer is determining

the right level of analysis that should be conducted. Most of these platforms fly at really slow speeds (100-200 knots). The need for wind tunnel testing would be a difficult sell, at best, and often seems downright silly. Testing these platforms would typically require different wind tunnels which are not as well equipped to perform store separation style wind tunnel tests efficiently - not to mention that wind tunnel tests do not fit into current programmatic timelines. CFD based analysis is feasible, but typically the AFSEO does not possess the aircraft geometry, let alone a validated grid model. Again, programmatic timelines are too short to measure an aircraft, construct a geometric model, and build a CFD grid. Even if a contractor were to deliver a geometry model, the time necessary to build grid models and conduct the actual analysis can still be too long. As a result, flight testing has been conducted on the basis of brute force test progression, free-stream only analysis, or analysis with only a very crude aircraft flow field interference model. Surprisingly, these reduced order models can work surprisingly well at very slow speeds which, in turn, challenges one of the core principles of store separation engineering; that to do the problem right you need to model the aircraft interference flow-field. At some point one would think that the mass will become too low or that the shape will become too sensitive to ignore the aircraft flow-field. Finding out at what point, in terms of store weight and aerodynamics, does modeling the aircraft flow field become imperative is one of the outstanding challenges for the store separation engineering community. For example, does ‘slow speed’ flight become a problem worthy of in-depth analysis at 300 knots? At 400 knots? Conversely, what types of shapes will be problematic at these airspeeds and is there an airspeed where the shape is no longer problematic?

Another challenge associated with the trend of slow speed platforms is the general lack of tools. This was already mentioned above because of the lack of a trajectory analysis capability, but this problem also manifests itself in other areas of the store separation engineer’s toolbox. For example, there is a lack of geometry models for visualizing a trajectory, computing the miss distance during the course of a trajectory, or for use in photogrammetric techniques. Finally, most of the platforms in this flight regime use gravity release systems. This leaves the separation engineer precious few ‘knobs to turn’ to ensure safe and acceptable separations. This is especially true when the release envelopes are considered extremely small already. There is very little envelope to trade away if you encounter a significant problem.

C. Expected Future Trends All of the trends discussed above are expected to continue in the future. Some are likely to accelerate due to

coupling with other trends. The trends fall into the categories of user, programmatic, and technical requirements. In the area of user requirements, we expect the trend for ever smaller stores to continue because of collateral

damage fears and the emerging ability of ever smaller UAVs to find, track, and (eventually) attack targets. As stated earlier, AFSOC has already stated that they are interested in munitions as small as 1-5 lbs and that micro UAVs are expected within the next 20 years. Based on current trends, it would be surprising not to see 1-5 lb stores into development within the next three years. Similarly, if micro UAVs are developed, we would expect to see micro stores shortly thereafter. COIN will be a major facet of the workload but it will not supplant all other portions of the separation engineer’s workload. We foresee continued growth in the number of platforms flown and a steady stream


of small new stores. The continued growth in the number of platforms supported will continue to pressure compatibility engineering to become more responsive to meeting the user’s needs.

Certain aspects of the current conflict will continue to drive user requirements. For example, targeting pods will continue to evolve rapidly and need to be certified back against everything else that already exists in the flight manual. This will put pressure on the documentation of previously completed work to make the certification process as rapid as possible. Not knowing your target prior to takeoff will continue to be the norm rather than an exception. This will drive the acquisition community to either develop munitions that can attack a wide variety of targets or a wide variety of munitions that can be mixed on the aircraft. Either approach will give the warfighter the flexibility they need to attack any target that pops up. Eventually, multi-carry racks will become more prevalent as store sizes continue to diminish and successful stores developed for slow speed aircraft get integrated onto larger, faster platforms. This could pose a couple of problems for the separation engineer. First, many of these stores were not developed for this type of flight environment. How will they respond in this new flight environment? Second, release order complexity would continue to get worse if different store types were put on the same multi-carry rack. The appeal for the aircrew is obvious, but requirements of this nature would definitely be painful for a store separation engineer.

Programmatic timelines will continue to get compressed by urgent warfighter needs and a tight fiscal environment. This trend is expected to accelerate when both aircraft and stores have been made universal armament interface (UAI) compliant. The successful advent of UAI will sharply reduce the amount of time available to perform and conduct store separation analysis/testing. Integration of a new store onto multiple aircraft simultaneously will become a serious manpower drain to a separations team that supports lots of aircraft platforms. We also expect that system program offices (SPOs) will continue pushing back on the need to conduct carriage system jettison analysis and flight test because the testing produces very little bang for the warfighter dollar and the amount of time and resources invested.

The technical trends of growing analytical scope and complexity are expected to increase because of growing release order complexity, a greater reliance on multi-carry racks, the need to account for flexibility effects, and growing acceptability complexity. Store separation engineers will be challenged to conduct analysis faster and faster, but with greater technical rigor and documentation. The demand for better and better simulation results will continue to push everyone to CFD. For case of low speed platforms, CFD is probably the only reasonable long term approach. Another key challenge for low speed platforms will be to do the proper amount of analysis for the problem at hand. This will demand that the geometry processes that feed CFD will have to be much faster than they are today as well as the time it takes to generate a CFD solution. Statistical guidelines will have to be developed for determining acceptability and ripple release intervals to avoid undue conservatism in the certification limits. Similarly, guidelines will have to be developed to determine when and if store-to-store contact is acceptable with modern guided weapons.

III. The AFSEO’s Vision of Managing Trends and Challenges The following section details the AFSEO store separation team’s approach of handling the aforementioned

trends and challenges as well as pre-existing trends. While the separations group at the AFSEO has grown significantly in the last decade, additional manpower alone is not enough, we require advances in our tools and the way we plan, conduct, and use our ground and flight testing. In the area of our tools, AFSEO is focusing on better integration, usability, flexibility, and sharing. In addition, we are adding capability in the areas of data mining, statistics, and archival while working on bringing CFD to our everyday efforts. For ground and flight testing, we are pushing for remote capability, quick turnaround, and a totally digital environment.

A. Getting the Most from Our Tools Historically, the tools the AFSEO used for store separation analysis were not well integrated and had rather steep

learning curves. To meet our current and future work tasks, the AFSEO has been actively pursuing a major paradigm shift in our tool sets to be more tightly integrated with each other and with the databases that will store our data. In addition, we are focusing on the tool usability for setting up and performing analysis more rigorously and faster. 1. Tool Integration, Flexibility and Usability

The cornerstone of our analysis capability, the 6-DOF trajectory generation program is an excellent example of how we are addressing usability and flexibility. In 2002, the AFSEO and the Arnold Engineering Development Center (AEDC), who develop or jointly develop some of our tools, decided to improve our existing trajectory


generation capability. Our mainstay simulation in use at the time, the AEDC Toolkit Trajectory Generation Prediction (TGP) had been in use for almost a decade. The Toolkit TGP used human editable input decks, had utilities to run large analysis efforts, and worked well as a production tool. However, it did require extensive user training, provided no GUI to the user, conducted minimal error checking, and had limited modularity. For example, it was common to have a specialized code version for a given store autopilot. When the given effort was completed, these versions or ’forks‘, were not maintained when updates/fixes where made to the baseline code. As a result, if a previously integrated autopilot was needed, a re-integration effort needed to be performed. The AEDC had another 6-DOF called the Flowfield Loads Influence Prediction (FLIP) TGP which was in limited use at the AFSEO and the AEDC. The code had additional capabilities such as semi-empirical store load generation, ability to compensate for non-uniform flows, and more flexible data interpolation. However, while much more capable, it was even less user-friendly and had little supporting utilities to assist with large efforts.

During the requirement generation for the new joint tool, the AFSEO and the AEDC decided to combine the best features and capabilities of both tools. The FLIP TGP was used as a starting point on which we added several new technical capabilities such as multiple store configurations during the trajectory (no post-launch), generalized forcing functions, and plug-and-play autopilots. However, the user experience was the focus of the effort. While the ability to run from the command line was retained, more readable inputs, input error checking, and a separate, standalone GUI were added. The new tool, the FLIP4 TGP, brought several new aspects to the end user: the eXtensible Markup Language (XML), the use of the Python scripting language, and a cross platform GUI, Nokia's QtTM. The FLIP4 TGP represented the first use of XML in our tools. XML is easily human or machine modified, is naturally hierarchical, and can allow for very descriptive input decks. Within FLIP4, XML is also used as a “data dictionary” which informs the simulation what inputs are needed, how to validate them, their relationship to one another, and how to provide the value to the FORTRAN kernel. An excerpt of the XML data dictionary for store properties is shown in Fig. 7.

. Figure 7. XML Data Dictionary Representation of a Parameter.

While the number crunching still takes place in a FORTRAN kernel, the Python scripting language is used to interact with the data dictionary, parse the inputs, validate the inputs, control the execution of the code, and write the output. In addition, to minimize the previously mentioned maintenance issues, the concept of a plug-in (e.g. external force, autopilot) has been developed. The plug-in, written in Python, can be self-contained, reference external compiled code, run external codes, etc. Using this approach, once a plug-in is developed, it can be easily used with future versions of the code.

The last key aspect for flexibility of the new tool was the use of a powerful cross platform GUI library, Nokia's Qt. When used in conjunction with Python (pyQt), Qt easily allowed us to create what we call ‘GUIs on the fly.’ Using the inherent flexibility of a scripting language like Python and Qt's easy to use widget layout managers, the FLIP4 GUI did not evolve into a crowded monolithic tool but one that adapts to the user's problem state. Figure 8 presents the GUI rendering of the Store Properties interface determined directly from the XML data dictionary (Fig. 7).

<parameter> <name>XCG</name> <units> ft</units> <description> Axial position of the store reference point with respect to the store cg. positive in the positive

XB direction (store reference point forward of the cg).</description> <requirement>Always</requirement> <default/> <array/> <flip_var>xbrbc_tr</flip_var> <flip_comblk>trc_massprop</flip_comblk>

</parameter>


Figure 8. Rendering of Store Properties GUI as Data is Entered.

As the user enters valid inputs, the red background, which corresponds to invalid input, changes to green. If a future need requires an additional parameter, the XML data dictionary is modified, and the GUI automatically uses it. The use of these three technologies has allowed the FLIP4 TGP to become the easiest 6-DOF to setup and execute. An excellent example of how this capability is paying off for us is in WTT execution. It is now our paradigm to take the 6-DOF with us into a WTT and populate with grid data as it comes in. By doing some we can evaluate the quality of our simulations trajectory match to the WTT trajectory rapidly enough to influence our taking of the grid data. If our simulation is not matching particularly well, we can elect to take grid data at additional Mach numbers or at different store or aircraft orientations. Conversely, if we are getting quality matches with only a limited amount of data, we may elect to discontinue grid data collection. However, it may be prudent to perform variation analysis while at the tunnel to make sure store motion does not exceed the limits of the data. This capability allows us to complete a WTT with great confidence that our simulation is accurate.

An example of how we are pursuing faster setup of our tools is with the AEDC Trajectory Visualization (Tvis) program of which the AFSEO provides requirements, funding, and a limited amount of development. Tvis takes full advantage of the hierarchical XML to describe the aircraft configuration. The Virtual Reality Model Language (VRML), which can be generated from most Computer Aided Design (CAD) packages, is used to describe the geometry of aircraft and stores. Once the user has the VRML representations, a complete, re-usable aircraft configuration is only a few minutes away. Trajectories, stored in the simple commas separated value (CSV) file format can be easily associated through files or through the Qt GUI.

Another vital aspect of our vision is better integration among the different tools. Integration means the engineer spends less time moving data from one tool to another thereby reducing the chance of error when describing the same inputs to each tool. The design of our miss distance tool is an exceptional example of the integration we are pursuing. At the core of the aforementioned Tvis is a C++ database that maintains the scene hierarchy, geometry, trajectory information, and procedures for manipulating, querying, and calculating miss distances. Tvis has an efficient user interface but was not designed to perform large amounts (hundreds or thousands) of miss distance calculations. To answer that need, the AFSEO added a small amount additional C++ code and "wrapped" the entire database in Python. With only a few lines of Python, a scene can be loaded and miss distance calculations generated. Now, without using the Tvis GUI, but a specialized one for the miss distance tool, thousands of miss distances calculations can be submitted to a computational cluster while using the original input decks. In addition, as new capabilities are added to Tvis, little or no changes should have to occur to the miss distance code.

2. Training

The rapid growth of the AFSEO separations team, along with the rebuilding of our toolset, has exposed major shortcomings of our training. Traditionally, new team members were brought into the fold purely by on the job training. Nuances of the tools were passed along verbally and the capabilities were sometimes only known by the code writers themselves, While it is expected to take years to become a well rounded store separation engineer, too much time is being spent learning the tools. As a result, the AFSEO separations team is currently writing training material including reference material, training presentations, and examples for all our tools. Upon completion of these materials, the new team member will then be given case studies (essentially homework problems) to demonstrate proficiency with the tools and concepts used in store separation engineering. After which, the focus will shift to our processes and aircraft/store knowledge. Again, XML is being utilized by writing the material in a format called DocBook. The DocBook format is independent of presentation and allows easy reuse of content in different documents. An eXtensible Stylesheet Language Transformations (XSLT) file, which itself is XML, is used to


directly convert the DocBook into html, or through some additional steps, into an AdobeTM Portable Document Format (PDF).

3. Tool Sharing

Another way the AFSEO plans to expand our toolset is through sharing and developing them among intra- and inter-service offices. As has been noted, the AFSEO and the AEDC have a strong, synergistic tool development association. By broadening the use of the FLIP4 TGP and our other tools among a wide range of users, the tools will become better tested and eventually be able to address a larger set of use cases. In addition, it will become easier to secure funding for upgrades as well as promote lessons learned among the different groups. Currently, the FLIP4 TGP has been shared with the Army AMRDEC and Navy NAVAIR for their store separation use.

4. Data Mining and Knowledge Discovery Tools

The new tools have given AFSEO a tremendous ability to generate large sets of trajectory data. For parametric variation, Monte Carlo techniques are typically used while genetic algorithms are used for worst case optimizations such as minimizing miss distance. To properly examine the statistical aspects of the results, which can be on the order of one-thousand trajectories, the AFSEO developed, under a Small Business Innovative Research (SBIR) contract, a MatlabTM toolbox called 'Separations Trajectory Analysis’ (SEPTRAN). SEPTRAN allows the user to perform interactive visualization, locate problem areas, identify causes by studying the relationships between input and output parameters, and estimate the probability. Techniques such as clustering and probability binning allow the engineer to determine which parameters are key contributors to the store's performance.

In the future, statistical guidelines for acceptability, likelihood of contact, etc. will probably need to be generated. The large data sets techniques can easily create situations that can drive unnecessary conservatism or even mislead the engineer. Using tools like SEPTRAN to better understand a store's behavior, the AFSEO can provide its customers capability while managing safety and program risk.

5. ‘Production-izing’ of CFD

Another critical part of the AFSEO's vision is the ‘production-izing’ of CFD. While many engineers only think of using CFD for generating trajectories, the AFSEO expects to build our typical wind tunnel aerodynamic databases such as free-stream and interference grid data to feed FLIP4 TGP. A typical store separation database can be on the order of 50,000 points. While the capabilities of CFD have increased dramatically in the last decade, there is still much work to do. CFD codes and hardware have become increasing faster, but our use cases (e.g. non-axisymmetric stores, maneuvering aircraft, deploying surfaces, autopilots) have become more complicated as well.

To support desired timelines, geometries need to be measured, built, and gridded in a fraction of the time they are today. As previously noted, CFD will be the only means of addressing some small stores unless larger scale aircraft wind tunnel models are built. As the industry relies less on wind tunnel based data, the need for validation of the CFD models is becoming more prevalent. Recently, the AFSEO took a big step in this direction by gathering flow-field probe data of the F-16 aircraft during a recent store separation test (Fig. 9). While the flow-field data can be used in the FLIP4 TGP, the primary purpose was to gather data for CFD to validate against. The data has already resulted in improvements to our ability to model the F-16 aircraft. As new CFD tools and techniques become available; the flow-field dataset will be a valuable aid. For future validation efforts, the AFSEO is currently planning the acquisition of flow-fields for the B-1B aircraft. With bays, spoilers, and door position dependencies, the B-1B presents a challenging environment to measure in the wind tunnel and for CFD to validate against. Another important area for CFD validation will be store models. In the rush to meet deadlines, the validation of free-stream models is often either skipped or done notionally. Future AFSEO efforts will endeavor to build a library of store grids that have been validated against large scale free stream data for a wide range of store attitudes and Mach numbers. The

Figure 9. F-16 with Flow Field Probe.


documentation of the individual efforts should capture comparisons against the truth models that were available and discuss mesh quality.

The AFSEO is involved in several efforts to address the challenges of using CFD in a production environment. The Institute for HPC Application to Air Armament (IHAAA), of which the AFSEO belongs and supports, sponsors several programs for making CFD more accessible to the store integration process. One such project, N6E, which stands for New 6-DOF Environment, is a catalyst for looking at how unified XML inputs, common Application Programming Interfaces (API), and common coordinate systems can accelerate the use of CFD. Typical issues the AFSEO has seen have been the misunderstanding of the different coordinates systems, units, and reference points used by separation and CFD engineers.

The Kestrel framework, which is part of a 12 year long Office of the Secretary of Defense (OSD) program called Computational Research and Engineering for Acquisition Tools and Environments (CREATE), is an additional effort the AFSEO is associated with. The Kestrel environment is being written from scratch to address the needs of air armament on fixed-wing aircraft and improve the acquisition process. Like the FLIP4 TGP, it is driven by a Python core with plug-ins for flow solvers, domain connectivity, force accounting, 6-DOF, etc. The AFSEO has provided requirements and inputs early in the design of the new tool. The Kestrel effort also uses a concept called "shadow ops" wherein real life engineering problems are submitted by end users to help define requirements and desired capabilities of the new tools. The AFSEO separations team has currently contributed several projects involving bay flow, autopilots, and maneuvering aircraft to the Kestrel team.

6. Data Archival

Finally, the AFSEO is also addressing the archival of our data. In particular, we are finishing up the first phase of the Flight Test Database (FTDB). The new tool will allow us to plan, create proper documentation, store results (e.g. flight conditions, store properties), and search our extensive history (30+ years) of flight testing. Historically, the AFSEO has used Microsoft PowerPoint for flight test documentation and relied on file system directories to maintain the data. As with the FLIP4 TGP, the FTDB makes extensive use of Python, pyQt, and XML to give us the flexibility to handle the varied aircraft we support. In particular, the aircraft configurations are built using a GUI and stored within the database as XML. As a result, a future version of the FTDB will be to directly launch Tvis with the correct configuration and appropriate trajectory. Another data archival task the AFSEO has undertaken is the digitization of our extensive film library of over three thousand films. By having digital access to our historical film, engineers will be able to quickly examine and compare this important resource from within the FTDB.

B. Getting the Most from Every Ground and Flight Test Due to increasing costs, weapon complexity, and compressed timelines, it is imperative that we capture as much

data as possible from all of our tests whether it is a flight test, ground ejector rack test, or a wind tunnel test. The AFSEO is increasing the amount of instrumentation, switching to digital video, and utilizing new techniques to meet these challenges. 1. No-RTB

For the last 5 years, AFSEO has been working on the concept of ‘New Control Room Techniques to Reduce Time Between Missions’ (No-RTB). Under No-RTB, the AFSEO is striving to get all of our tools, as well as appropriate flight test data, into the control room. The goal is to be able to fly multiple build-up test points for a given configuration in a single mission without returning to base for data reduction and analysis. A key component that will allow the AFSEO to accomplish this is the use of airborne separation video (ASV), a system of high speed digital cameras that can be relayed to the ground. Currently the 46th Test Wing at Eglin AFB has modified the F-15E and F-16 jets used for separation efforts to handle the ASV system. In addition, the 412th Test Wing at Edward AFB has modified the two B-1B bombers that the AFSEO uses for store separation. However, while the aircraft at Eglin can allow a certain amount of adjustment in flight to ensure quality images, the ASV implementation on the B-1B aircraft were only partially completed. The lack of adjustability is even more problematic with a bomber whose lighting conditions in the bay are also affected by store load out and door positions. With the additional crew members on the B-1B, it is hoped a much more robust and functional solution can be implemented at some point. The conversion of the B-2 aircraft to ASV is currently in the design phase with the intent to do a full integration including telemetry. The B-2 aircraft is a prime candidate for the No-RTB concept because of the difficulty of securing aircraft time as well as the relatively high cost of missions.

Our shift to using a digital chase camera has been much slower than using digital on board cameras. The small confines of a fighter cockpit introduce many challenges with respect to battery life, weight, and usability of controls.


Currently, one F-16 at Eglin is wired to allow downloading of chase video to the ground with the modification of others scheduled. For stores that undergo configuration changes near the aircraft, digital chase in the control room will be an important part of the No-RTB concept. However, photo chase is currently not allowed with UAV work. To overcome this limitation, the use of ground tracking cameras, self-contained video pods, or maybe even area chase with specialized cameras would be necessary.

Ejector racks is another instrumentation area the AFSEO has invested in for the Eglin AFB jets. The B-1B and B-2 aircraft at Edwards AFB are instrumented as well. The ejector rack data is very beneficial when looking evaluating a trajectory. The results contribute towards comparing simulations to flight test and in the determination of whether ejection anomalies occurred. However, not all Eglin AFB separation jets and associated auxiliary suspension hardware are modified with instrumented racks. As a result, the requirement for having an instrument rack can present scheduling constraints which, in some cases, forces the AFSEO to forgo the capture of the data.

Lastly, the AFSEO intends to use store separation TM kits (or TM provided by the store itself) wherever possible. Currently, the additional cost involved has limited their use within the AFSEO. When used, TM can be an excellent source of motion data to compare simulations to, including the determination of ejector performance. Telemetry does not need to be constrained to the released store. An extra unit could be mounted within the pylon or, if using a multi-carry rack, in an adjacent store to help determine the flexure that occurred. Several kits now allow for the transmission of external signals. For example, instrumentation data from store away switches sway brace loads, ejectors, and video could be relayed to the ground. The concept could allow the use of the No-RTB concept for aircraft that are not yet modified. In addition, TM kits can be useful for ground testing such as ejector testing or wind tunnel drop models3. In fact, during an upcoming BRU-61 SDB ejector rack test, the AFSEO will be using TM kits in a direct connection mode (non-radiating) along with additional rate gyros. The AFSEO believes that the traditional string potentiometers and load cells used alone will be insufficient to capture the off-axis motion of a BRU-61 ejection.

2. Condensation

One issue that has hampered execution of flight tests in the Eglin AFB Gulf of Mexico range space has been the occurrence of condensation at high transonic airspeeds. In 2005, the AFSEO undertook an effort to better predict and mitigate the affect of condensation on our flight tests. A suite of tools and methodologies1 were developed in conjunction with new policies and procedures. As a result, the AFSEO now builds a condensation potential model for upcoming flight test events based upon the flight conditions, aircraft configuration, and expectant weather. The model can be used to determine locations for release within the range, change release altitudes (within test tolerances) to find an acceptable level of condensation, or even ground abort a mission if deemed necessary. The previously discussed ASV cameras, developed for documenting the store release, can be observed in the control room to assist in the evaluation of the amount of condensation present. The condensation toolset has been successfully used on many separation flights.

3. WICS

Another way the AFSEO is changing the way it conducts ground tests is through a program called Weapon Integration / Compatibility Support2 (WICS). The program, initiated in April of 2001, has the goal of establishing data connections between the control rooms of Eglin AFB and other Air Force test centers. Currently, the effort has established a link between Eglin and the Arnold Engineering Development Center in Tennessee (AEDC). The AEDC Data Display Room (ADDR) at Eglin AFB allows the AFSEO engineers to remotely monitor and participate in wind tunnel tests conducted at the Propulsion Wind Tunnels (PWT) at the AEDC. A virtual private network was established to allow Eglin AFB computers to receive and transmit data and video between the two sites. The facility provides valuable training for new engineers and allows seasoned engineers to support tunnel operations from afar. Some limitations have become apparent with using the ADDR facility. One such issue is with situational awareness. It is difficult using remote cameras and microphones alone to determine the current status of the tunnel test. To currently accomplish this, an individual at AEDC needs to be tasked with relaying test status, current issues, and other such information. One possible remediation is though using a concept like the popular internet service Twitter. The Twitter service is basically a micro-blog, notification service allowing participants to be able to subscribe to each other’s activities. The concept could be applied to WICS, by modifying tunnel software in the WICS network to report on status changes such as tunnel functions, aircraft configuration changes, and current test points being captured.


4. Design of Experiments The AFSEO has recently begun exploring and implementing the use of Design of Experiments (DOE)

methodology. DOE is primarily an analytical tool that can be used to improve test methodology and streamline analysis for characterizing system performance. DOE consists of both test planning and statistical analysis. DOE provides a rigorous framework for test planning that applies standard guidelines and intuitive principles to arrive at a robust test matrix. Statistical analysis is used to determine the significance of factors influencing one or more system measures of performance, assess repeatability, and estimate the performance at intermittent untested settings. During an upcoming ejector rack ground test, the AFSEO will fully utilize DOE to characterize the performance of the BRU-61 SDB carriage system over a wide range of configurations that would be difficult to test in a conventional fashion. Future efforts to involve DOE in the trajectory analysis phase are also being investigated.

IV. Conclusion Two recently emerging key trends in warfighter requirements have combined to form a fourth distinct stage in warfighter requirements since the advent of the GWOT. The first stage accelerated schedules for in-progress projects and new combinations of previously certified items. The second stage sought improved capability targeting pods for a wide variety aircraft. The third stage was concerned with collateral damage and moving targets. The fourth stage is typified by a low intensity, COIN conflict. As a minimum, the second, third, and fourth stages all still exist and influence the work environment of store separation engineers. The challenges and trends of the COIN stage have been presented along with expected future trends in the categories of user, programmatic, and technical requirements. The AFSEO’s response to these challenges and trends is though major tool and instrumentation upgrades to get the most from our analysis of ground and flight testing in the most rapid manner possible.

References 1 Harding, Gregory C., and Barton Kyle. M., “Predicting Aircraft Flowfield Induced Water Vapor Condensation for Store

Separation Flight Tests,” paper presented at the 25th AIAA Applied Aerodynamics Conference, 25 - 28 June 2007, Miami, FL. 2Muerle, Douglas C., “The Development and Evolution of a Remote Wind Tunnel Control Room at Eglin AFB,” paper

presented at the 2006 Aircraft-Store Compatibility Symposium XIV sponsored by the International Test and Evaluation Association, April 12-14, 2006, Fort Walton Beach, FL.

3Marquart, E. J., Davis, K. I., Walker, G. P. and Dix, R. E. “Kinematic Telemetry from Small-Scale Wind Tunnel Models.” AEDC-TR-94-13, January 1995.

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