AEDC team puts new small diameter bomb to the test

  • Published
  • By Philip Lorenz III
  • AEDC/PA
ARNOLD AIR FORCE BASE, Tenn. --  Imagine a lightweight and powerful precision-guided bomb that would enable an F-15E Strike Eagle fighter pilot to find and destroy a moving enemy target under challenging conditions - like during a powerful dust storm at night with anti-aircraft rounds being launched.

And in case that first bomb fails to take out the target, several more of these 250-pound class destructors are available on the aircraft to finish the job.

AEDC engineers are helping to ensure Raytheon's Small Diameter Bomb (SDB) II is just what the warfighter ordered. Store separation and aerodynamic testing of a 1/20th scale model of the weapon and F-15E is ongoing in Arnold's 4-foot transonic wind tunnel.

Dr. Andrew Frits, Raytheon project engineer, said AEDC is the logical choice when his company wants to conduct complex store separation testing on products like the SDB II.

"There are advantages of coming to AEDC, most of it is the experience-base and the fact that they've done so much validation on the F-15E with other rounds [stores]," he said. "We consider AEDC to be the Cadillac of wind tunnel testing. You go there if you have something that needs to be done right; testing that carries a lot of complexity. Another thing, too, is AEDC actually has the
F-15E parent model as well."

Ensuring the effective and efficient ejection and trajectory of a weapon or other store from an aircraft in flight to an enemy target is imperative to the safety of the pilot, aircraft and the success of the mission.

According to Dr. Frits, wind tunnel testing is critical to the success of the Small Diameter Bomb II program and paves the way for a safe, effective and less costly flight test campaign.

"[The] SDB II is the next generation air-to-ground weapon," he said. "It is designed to hit vehicles, trucks, tanks [and] those types of things, either moving or not moving in adverse weather conditions.

"It has a very advanced tri-mode seeker. It's a fully networked weapon with a full data link capability so it will be able to communicate with launch platforms and off-platform targeting groups."

Dr. Frits said work on the system has provided a weapon that is currently in the midst of the engineering, manufacturing and development (EMD) phase. He said a considerable effort went into preparing the weapon system for the current phase of wind tunnel testing at AEDC.

"We designed a weapon that safely separates from the aircraft," he said. "You won't have any trouble of it accidently coming back up and hitting the airplane.

"This wind tunnel test supports a flight clearance recommendation for the full operational weapon envelope on the F-15E. The data from this test will help determine what additional flight test points need to be gathered, then the flight test data along with wind tunnel and computational fluid dynamics data will be used to determine the separation envelope."

Adam Plondke, the ATA test project engineer, said the project's first phase was to conduct free stream testing of the bomb which is still in EMD phase.

"This was just the SDB II model by itself in the tunnel, there was no aircraft present," he explained. "We go through a whole array of store attitudes in the tunnel, which gives us a
database of forces and moments the store will see at these various orientations in the pure free stream flow field by itself."

The next phase involved the use of a captive trajectory system (CTS) to put the bomb, mounted on a sting, through a computer-generated series of attitudes simulating the store deploying from the aircraft.

"With a computer, we simulate the ejector pistons pushing on the store and the forces and moments that our internal balance measures."

Plondke said, "This data will then be used to calculate where the store would move next."
The CTS allows the testers to put the SDB II model through a full range of simulated release conditions, including ejector and control forces as well as G-forces due to pull-up or push-down maneuvers of both the store and the plane.

The system also simulates how the airflow interactions between the aircraft and other airborne stores, including conformal fuel tanks, other weapons and sensors affect the SDB II as it drops away from the aircraft. This includes subjecting the weapon and aircraft models to a variety of attitudes of pitch, roll and yaw configurations.

"We do the first part of the trajectory with the fins stowed," Dr. Frits explained. "Then, in a tactical trajectory - at some point shortly after the weapon deploys - the fins will deploy, changing the aerodynamic characteristics of our weapon and then we can begin steering it if we need to."

The third phase of the test at AEDC involves a grid survey approach, in which SDB II aerodynamic loads are measured at a pre-determined array of store positions and attitudes. The information from this testing is used to create a database of the spatial variation of the loads in proximity to the F-15E.

"Our primary goal there is we want to just collect enough data that we can build a model of the aerodynamics of the system," Dr. Frits said. "And from there we get nice sets of clean data at various different orientations and we can build a nice computer model of the aerodynamics at any given angular orientation relative to the aircraft."