It's delightful to be able to bring the newsletter to you all knowing that we have returned to in-person events with our Saturday lectures. Our big thanks go to Sandra Adams, RDML USN (ret), Jim Sandberg, Col USMC (ret) and Jim Brown for their engaging presentations that have inspired us all. Some of these lectures were postponed from the very beginning of the pandemic. We are also thrilled this month to include a piece by one of our youngest volunteers, Joshua, about the Radio Calibration Sphere. It's always great to see interest in the aerospace industry amongst those who can lead it into the future! Our docents have been hard at work supporting our 4-day a week schedule now that the museum is open to the public again. We have been sad to lose some of our volunteers but grateful to the many who have stepped in to fill the gaps these past 18 months.

The primary role of military combat aircraft is to deliver the mission payload to the desired target area, safely and accurately deploy the payload, and return to base. The mission payload can consist of a mix of sensors or other electronics, offensive and defensive weapons and auxiliary fuel tanks. The individual components of the mission payload, commonly referred to as “stores”, may be carried on external pylons mounted on the wings and/or fuselage of the carrier aircraft or inside enclosed weapons bays. The integration of the carrier aircraft and stores is an important consideration during the design, development and fielding of combat aircraft and the associated stores.

When stores are carried on external pylons, they interact with the aerodynamics of the carrier aircraft, affecting the aircraft flight performance and loads on the aircraft structure and individual stores. Aircraft performance impacts can include mission range and allowable speed/altitude flight envelope limitations, as well as changes in handling characteristics. The complex aircraft/store aerodynamics produce loads that influence the design of the aircraft structure, store ejection subsystems and the structural design of the individual stores. Variations in the number and mix of external stores needed to meet mission requirements must be evaluated to validate aircraft operational performance, structural loads and store separation characteristics, preferably before they are flown for the first time.

Carrying stores inside internal bays presents a different set of design challenges. When internal bays are opened, the resulting airflow turbulence, known as aeroacoustics, causes significant loads on the bay doors, the aircraft structure and the stores being carried. This flow turbulence also strongly influences the flight characteristics of the store when it is released. Rapid fluctuations in the flow turbulence make it very difficult to characterize through computer simulations, requiring the use of a combination of controlled wind tunnel tests and in-flight measurements to map the internal and external flow field and evaluate aeroacoustic impacts over a variety of flight conditions and payload configurations.

It is critically important that stores behave in a safe, predictable manner when released to avoid potential damage to the carrier aircraft or collisions between stores, and to ensure accurate delivery to the target. The complex interaction between stores and the airflow around the aircraft strongly influences store separation characteristics. Heavy, aerodynamically stable stores such as most bombs and many electronics/weapons pods tend to have benign separation characteristics under most release conditions. Lower density stores such as empty fuel tanks or aerodynamically unstable stores can be much more problematic. Large stores such as cruise missiles with wings and/or tail surfaces that are extended after launch can be unstable when first launched, making their separation characteristics sensitive to variations in payload configuration and aircraft flight conditions. Validating the ability to safely and accurately release stores is a major part of any combat aircraft or store development and operational assessment program.

Store Carriage and Separation Testing Methods: The methods used to validate the safe separation of stores from aircraft have evolved significantly since the early 20th century. At the beginning of World War 1 small bombs were dropped by hand from the cockpit of airplanes flying at low speed, with gravity and a stable bomb design ensuring safe separation of the bombs. As the war progressed, specially designed bomber aircraft mounted their bombs on trays under the wings/fuselage, allowing for heavier payloads and the design of larger bombs. Any study of the carriage and separation characteristics of aircraft/store combinations in this era was done by trial-and-error flight testing or gained through operational experience.

The ever-increasing performance of military aircraft after WW1 was a direct result of the widespread use of wind tunnels to develop new aerodynamic technologies and efficiently evaluate the flight performance of proposed aircraft designs. The typical biplane designs of the 1920’s gave way to all-metal monoplanes that were designed to minimize drag and increase top speed, range and maneuverability. During this period the emphasis of the wind tunnel test programs was on the characteristics of the aircraft in a “clean” configuration, with little study of the impacts of adding external payloads. The technology of the time did not allow detailed study of store carriage loads or separation characteristics in the wind tunnel, so any testing was performed during flight tests that slowly expanded flight conditions until problems arose. Test pilots were not thrilled with the inherent dangers of this approach.

The test methods used to evaluate store carriage and separation characteristics in the wind tunnel significantly evolved after World War II. Advances in force measurement instrumentation allowed the loads on individual stores and their mounting pylons to be measured during tests to evaluate aircraft performance impacts, giving valuable data to structural designers and aerodynamicists. Early store separation wind tunnel testing involved the use of specially designed store models that reflected not only the shape of the store but also its mass and inertial properties. These models were literally dropped from an aircraft model mounted to the roof of the wind tunnel and cameras were used to document the store trajectory after release. As computers became more powerful and instrumentation technology evolved, specialized wind tunnels incorporating independently controlled support systems for the carrier aircraft and the store were developed in the 1960’s, allowing more detailed mapping of the interactions between the carrier aircraft flowfield and the store over a wide range of conditions. The data gathered during these tests were used to build computer models of the influence of the carrier aircraft flowfield on the store flight characteristics, reducing the number of flight tests required to certify an aircraft/store combination as safe and effective. Eventually it became possible to simulate a release trajectory in the wind tunnel in near real time using advanced computer software and live test data using a technique known as captive trajectory simulation (CTS).

My Job: After completing my bachelor’s degree in aerospace engineering at Cal Poly San Luis Obispo, I went to work in the wind tunnel group at Rockwell International in El Segundo, CA in 1982. While there I was involved in several wind tunnel tests to evaluate the integration of externally and internally carried payloads with the B-1B bomber. These tests included measuring impact of external store carriage on B-1 performance, loads on the external stores and their associated mounting pylons and measuring the aerodynamics of externally and internally carried stores as they moved through the B-1 flowfield after launch.

I took a new job as a test engineer in the wind tunnel group at Northrop Aircraft Division in Hawthorne two years later. After working on a variety of advanced aerodynamic technology and initial configuration development tests for the YF-23 and other Northrop programs, I became the lead engineer responsible for the weapons carriage and separation wind tunnel testing on the AGM-137 Tri-Service Standoff Attack Missile (TSSAM) initial full-scale development (IFSD) and full-scale development (FSD) programs. The air-launched variant of the TSSAM was to be integrated with a total of six Air Force and Navy aircraft (including the A-6E, B-1B, B-2, B-52H, F-16 C/D, and F/A-18 C/D), each requiring a parallel series of carriage and separation tests. As a result, the planned integration wind tunnel test program was a significant portion of the overall of the TSSAM vehicle development wind tunnel test program. A combination of technical and programmatic issues caused significant growth in the scope of the carriage and separation test program, which eventually consisted of over 50 wind tunnel tests over the course of 7 years. During this period, it was normal for the carriage and separation test team to spend several months each year traveling to wind tunnel test sites and the various aircraft contractors. The lessons I learned during the TSSAM effort proved invaluable throughout the rest of my engineering career.

TSSAM Carriage and Separation WT Test Program: The AGM-137 Tri-Service Stand-Off Attack Missile (TSSAM) was a stealth cruise missile developed by Northrop Corporation at their facilities in Hawthorne, CA beginning in 1984. The large size of the air-launched variants of the TSSAM required that the wings and tail surfaces be stowed during carriage and launch, unfolding after the missile was clear of the carrier aircraft. Unfortunately, the TSSAM fuselage shape made the vehicle aerodynamically unstable with the wings and tails stowed. Northrop conducted initial store carriage and separations tests on the B-52 and F/A-18 during the initial full-scale development (IFSD) program prior to award of the full-scale development (FSD) contract in 1986.

The complexity of integrating the TSSAM on multiple different aircraft became apparent early in FSD. The original carriage and separation wind tunnel test plan called for a limited number of tests on each of the six intended carrier aircraft (about 15 tests total) over the course of 2 years. During extensive meetings with the Air Force, Navy and the various aircraft contractors Northrop learned that each group had their own definition of safe store separation and that TSSAM must meet them all with a common configuration. While developing the detailed wind tunnel test plans, the number of test configurations for each aircraft grew to account for more mission loadouts and more detailed flowfield mapping requirements– resulting in longer test durations. Tests that were originally proposed to last about 100 hours in the wind tunnel grew to 300 hours or more and a single separation flowfield wind tunnel test could last up to 6 weeks.

The results of the first round of separation wind tunnel tests drove changes to the aerodynamics of the TSSAM in the stowed configuration to ensure safe and reliable separation from all the carrier aircraft. These configuration changes were significant enough to require several more rounds of wind tunnel tests of longer duration to achieve the desired launch characteristics for each carrier aircraft. We also learned that certain aircraft required testing of a wider array of aircraft configurations to fully model operational characteristics, further increasing test durations. During this period there could be 2-3 TSSAM wind tunnel tests being conducted concurrently in facilities spread across the country, posing many management and staffing challenges. By the time the TSSAM program was cancelled in 1994, more than 50 carriage and separation wind tunnel tests had been conducted.

The lessons learned on the TSSAM program informed significant improvements in the store certification process that have occurred over the past 30 years. The advances in computer computational fluid dynamics (CFD) technologies now allow detailed modeling of the flowfield around complex external store configurations. In-depth analysis of the correlation between analytical models, wind tunnel and flight test data have allowed modern store certification efforts to be conducted far more efficiently than was possible during the TSSAM program. The role of separation wind tunnel testing changed from the primary data gathering method into a valuable tool for validating CFD modeling results, significantly reducing the scope of the store certification test effort and the associated costs. Wind tunnel tests that once consisted of thousands of runs and lasted 4 to 6 weeks now consist of several hundred runs and can be completed in about 2 weeks. Similar savings in flight tests costs have also been possible as fewer test points are required to validate analytical results. The ability to model critical flight conditions has greatly improved the safety and cost of modern store certification flight test programs.

Photos (from top): Boeing EA-18G Growler with mixed external stores (Boeing/DoD-USN photo); Hand-launched bombs in WW1 (www.gettyimages.com); Captive Trajectory Simulation (CTS) in AEDC 4T wind tunnel (AEDC brochure); TSSAM characteristics (www.fas.org).

It has been 63 years since the beginning of the jet age. New technology is arising from many companies including Northrop (Later to be Northrop Grumman), Boeing, and so many more in the competition for passenger air travel, military aircraft, and government spacecraft. Many incredible aircraft and spacecraft achievements came out during and before this 63 year mark including the F-5A, the F-14 Tomcat, the F-20 Tigershark, all of the Apollo missions to the moon, as well as many other memorable missions/spacecraft/aircraft. But then one aircraft came out that changed the game for good. It was the B-2 stealth bomber that can’t be traced by radar. It was was revolutionary technology for 1989. One of the major factors that led to the creation of the B-2 bomber and its technology was the Radio Calibration Sphere (or 'RCS'). The RCS is an aluminum ball that reflects radar signals in many directions. The museum has one available for viewing near the entrance, across from the Northrop Grumman timeline/history in the Unmanned Aerial Vehicle (UAV) exhibit.

The Beginning of the Radio Calibration Sphere: The RCS was made in 1982 as a part of the Tacit Blue Technology Demonstration Program. The program was “created to demonstrate that a low observable surveillance aircraft with a low probability of intercept radar and other sensors could operate close to the forward line of battle with a high degree of survivability“ (1). The Tacit Blue Technology Demonstration Program’s main product was the Tacit Blue Aircraft (Picture 2). The Calibration Sphere helped measure the distance between the radar and the target, otherwise known as a radar cross section. First the sphere was dropped from an airplane and then radar signals were reflected from it in a number of ways. A sphere was the perfect shape since it would always reflect a signal in all directions. The radar cross section was calculated and if it was the correct calculation (the cross-section of the sphere's dropping point) than the system had been calibrated successfully.

The Radio Calibration Sphere at Work: The RCS has been used for many projects from radar dishes to missile detection. During the cold war both the Soviet Union and the United States feared the possibility of nuclear attacks and so both developed missile warning systems. The RCS helped calibrate the United States’ System. One of the most visible places where this system operates is Hawaii. The ballistic missile alarm towers are still up today and are tested monthly (Picture 3).

The Calibration Sphere was also used to calibrate the Haystack Long Range Imaging Radar facility. The Haystack Long Range Imaging Radar is a radar dish in Boston (Picture 4). The facility is currently being run by MIT but was originally run by the United States Air Force. It’s the only US radar capable of imaging satellites out to geosynchronous orbit range.

The RCS and the B-2 Bomber: The RCS heavily influenced the B-2 Bomber given its history in the development of the Tacit Blue Stealth Plane and that the B-2 utilized many of the same technologies.

In conclusion, the Radio Calibration Sphere shows the ingenuity that US aerospace companies have, as well as our nation's great efforts to build amazing feats of technology that change the world forever.

Don't forget that you can check out our most recent presentations (and many older ones too) right on our YouTube channel.

The Western Museum of Flight is recruiting volunteer docents! If you live in the South Bay and would like to volunteer at the museum on a regular basis, please email info@wmof.com. We will get right back to you.

We know that many of you reading this are former members of Southern California's aerospace industry and may have some interesting stories and experiences from your careers. We ask you to consider sharing some of these stories with us, whether about a particular company's aircraft project/program or during the course of military service associated with one of the many aircraft types built in SoCal. We look to preserve these stories before they are lost to time.

Our mission is to preserve the histories of the aircraft built here in Southern California, primarily airframes in earlier times, for the defense of our nation, experimental research air vehicles, spacecraft and commercial airliners. Lockheed (Burbank, Palmdale); Douglas/McDonnell Douglas (Santa Monica, El Segundo, Long Beach), Hughes (Culver City, El Segundo), North American Aviation/Rockwell/Boeing (LAX, Downey, Palmdale), Boeing (Long Beach), Northrop Grumman (Hawthorne, El Segundo, Palmdale), Vultee (Downey), Consolidated/Convair/General Dynamics (San Diego), TRW/Northrop Grumman (Redondo Beach), SpaceX (Hawthorne), Robinson Helicopters (Torrance), these amongst all the larger companies.

We look to the future as well, as history continues to be written with new initiatives and opportunities for further space exploration. We'll help to polish up the words if you are not a professional writer. Or it may simply be an interesting photo(s) with a caption added to tell the story.

Please contact us directly via email: edit@wmof.com, with your thoughts and comments.