Electric Drives Power Rocket Launcher

A retrofit of the Army's MLRS vehicle, a rocket launcher which carries up to 12 rocket pods, involves the insertion of electric drive technology into an existing hydraulic application that requires high torque and fast dynamic response. The new electric system also provides a strategic advantage by offering silent operation and eliminating the need to run the vehicle's diesel engine to operate the launcher.

"What we were able to demonstrate is that electric drives, historically considered small and limited in load carrying capacity, could drive a system which has to move approximately 12,000 pounds," says Kevin Eschner, senior project engineer for Moog Inc.

The MLRS is a high-mobility automatic system based on the M270 weapons platform manufactured by Lockheed Martin Missiles and Fire Control. Without leaving the cab, a crew of three is able to fire up to 12 GMLRS (Guided Multiple Launch Rocket System) rockets in less than 60 sec from a distance of 42 miles or two ATACMS (Army Tactical Missile System) missiles from a distance of 180 miles.

Moog took the lead on the systems integration of the MLRS retrofit project, converting the launcher drive system from hydraulic to electric over the course of six months. The new system eliminates existing problems on the MLRS including leaks, complex troubleshooting, repair and maintenance work.

"The new system uses two motor controllers and three electric drive motors to control the rocket launcher that sits over the back of the vehicle," says Eschner. One drive controls the Azimuth to rotate the launcher, and two motors in series control elevation or lift to generate the required loads."

Redesign with Few Noticeable Changes

In the Army's training, war fighters under fire are taught not to think about what they're doing. The goal is reacting and letting the training take over, so the soldiers function at optimum speed and efficiency. Since the crew had experience with the existing MLRS, few changes to the interface were desired. Therefore, the design constraints for the retrofit project included using existing interfaces and minimizing maintenance once the system was switched over to the electric drive system.

"We kept exactly the same controls and exactly the same manner of operating the system," says Eschner. "Effectively, the user doesn't know there is an electric drive system except that the system has the significant advantage of silent operation."

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The existing hydraulic design required running the large diesel engines on the vehicle, at all times, to operate the launcher. Because the crew sits over the top of the engine, heat becomes a major factor, especially if the crew is in Iraq or Afghanistan in the middle of summer, with no built-in air conditioning in the vehicle. Studies show that crew performance drops off with higher temperatures, especially when they are in the vehicle anywhere from 10 to 12 hours a day.

Silent operation is also very useful from a strategic standpoint. With the ability to move the launcher electrically, the Army can move the vehicle without running the diesel engine and creating a heat signature. But once they fire a rocket, they expose their position and the launcher becomes a target. So the mode of operation becomes "shoot and scoot."

"What they liked about our design is that they could actually deploy the vehicles months ahead of any initiative and never fire the diesel up, so they had no heat signature on the vehicle," says Eschner. "We drive completely off a set of batteries, and the fire control computers also run on a secondary set of batteries. Once in place, the system can become totally silent and blend into whatever background with no heat signatures."

Motion Control Design

The major challenge in designing the motor/drive system is the size of the load to lift. In addition, the application was locked into a 28V motor performance system. Unable to take a white sheet approach to the design, the engineering team had to work within the confines of a restricted space for the motors, drives and mechanical linkages. The goal was to keep most of the existing mechanical linkages and substitute the electric drives for the hydraulic drives.

"If we had designed the system from the ground up, we would have integrated most of the transmissions and gear boxes within the actuators and delivered an overall smaller package. But this would have been a more costly deliverable from a retrofit standpoint," says Eschner.

In the final solution, two electric drives used in series generate the required elevation torque. To achieve that within the Moog product line of both single- and dual-axis motor controllers, they modified one of their dual-channel commercial boxes. By linking channels together in software, two units drive together to achieve the higher torque requirements. The Azimuth axis uses a single-axis controller.

The system also required redundant braking beyond what would be normal for an electric drive system. Extra electric brakes engineered by Moog could be controlled with the same motor controllers. The result was a compact motor controller using parallel channels and coordination between the two control boxes.

An added advantage to the use of electric drives is eliminating the need to transport, use and store hydraulic liquids. Leakage is always an issue with hydraulic systems, and contamination is a major concern especially when maintenance on vehicles is done out in the field. Electric drives, on the other hand, are sealed and a line replaceable approach means that if components fail there isn't a huge amount of troubleshooting needed.

Even with those advantages, the primary goal of the retrofit was to make the operation of the electric system invisible to the crew. Eschner says that when they brought in a colonel and a number of soldiers at the end of the demonstration project, the gunners who drive and operate the vehicle provided the best compliment. They told Eschner, "If you didn't tell me this wasn't one of my hydraulic systems, I wouldn't have known."

With the motion system itself, engineers worked to meet the required velocities, while contouring the acceleration curves and reducing some of the stresses on the elevation cage. The existing hydraulic system had provided a power dense solution, but there was less flexibility in controlling the motion profiles using the hydraulic system.

"It's more difficult to adjust velocity curves and accelerations, and there are some very violent moves within the operational constraints of the vehicle," says Eschner. "We were able to mimic those moves to get the desired rates and velocities but, near the end of the accelerations, we had to lower some of the g levels to help with reliability and uptime."

Moog created simulations using MatLab Simulink to initially characterize the cage motions, and downloaded optimized tuning parameters from the simulations into the controllers. This provided a good start for the final tuning which was done with a group of engineers. All of the interfaces were also designed to integrate with the hydraulic launcher drive's interlocks, including weapons ready for firing, and also locking in GPS signals. All of the interfaces were developed using a modified off-the-shelf version that plugged directly into the existing system.