Propelling Space and Defense Missions

Solid Rocket Motors Defined

According to Fuller, a solid rocket motor at its core is a relatively simple device with no moving parts. It includes an outer cylindrical casing, solid propellant with a hole — often star-shaped — down the center, called “the grain”, an igniter to light the propellant and a nozzle to exhaust the combustion gases.

"We mix fuel and oxidizer in liquid form, then cast it inside the composite or steel casing," he said. "The igniter is essentially a mini-rocket motor whose flame ignites the inner surface of the propellant. The gas created by this combustion exits the casing through the nozzle, creating the thrust that drives the rocket forward."

The Balancing Act of Meeting Requirements

Most solid rocket motors originate from new mission requirements.

"We usually start off with a performance spec — deliver X amount of mass to a certain range — then design the rocket around it," remarked Fuller. "Customers usually provide the initial requirements, but then we might suggest ways to improve the motor's performance or reduce its cost."

These initial calculations drive the rocket motor's conceptual design.

"Every solid rocket motor design is a balancing act among performance, cost and complexity," Fuller said. "We look for that sweet spot between casing material, propellant types and nozzle design to give the customer the performance they're looking for at a cost that's acceptable in a package we can produce."

There are times Northrop Grumman is simply asked to build a replacement motor for an existing rocket system, he added. In that case, the main goal is solely to reduce the unit's manufacturing costs, not enhance its performance.

Engineering the Model Rocket

After the conceptual design is established, the rocket motor development team moves on to the detailed design of the motor's major components, calculating, for example, the exact dimensions of its case, the composition of its propellant, the shape of the propellant grain and the critical performance characteristics of its nozzle.

At the heart of this design process is computer simulation, Fuller emphasized. This allows the team to build, modify and virtually test an electronic model of the solid rocket motor under multiple potential operational scenarios.

"We end up with a high-fidelity engineering model of the solid rocket motor that describes the physical characteristics of all of its individual components," he said. "It lays out, in effect, how we plan to build that motor."

3, 2, 1, Ready for ...Testing Subsystems

Before assembling the first article of the motor, the team tests each of its major components against a performance standard required for that part.

"We make most of our rocket motor cases at a Northrop Grumman manufacturing facility in Clearfield, Utah," said Fuller. "Sometimes, we'll pressurize a case to verify it can withstand its design limits — or even pressurize it until it fails, just to make sure it can withstand its expected operating environment."

The team also mixes the proposed propellant in small, sub-scale quantities to ensure it mixes properly, tests it to make sure it burns properly, then scales it up to a production-sized mix, he added.

For a rocket motor's igniter, Fuller explained that the design team often adapts a previously produced, previously characterized igniter to the current application. And regardless of whether the team makes or buys the rocket motor's nozzle, they always conduct a separate test of the vector control system that moves the nozzle during flight.

Even More Testing for… Takeoff

The real proof of a solid rocket motor design comes during a static test of the fully assembled motor. According to Fuller, Propulsion Systems tests its rocket motors in Promontory, and preparations for a test can vary from a few days to several months depending on the requirements, the customer and the program.

"During rocket motor testing, we strap the motor onto a test stand, put it up against a block that can withstand the force of the rocket, wire it up with measuring equipment, then fire the motor to see if it does what we think it's going to do," he said.

Fuller added that the key measurement tool for this test is called a thrust trace. This is a data plot of the amount of thrust produced versus time over the duration of the burn. By analyzing the thrust trace, the test team can determine if the propellant is burning evenly.

The test team also uses high-speed video to analyze the visual characteristics of the nozzle and exhaust during rocket motor testing, he added. And a post-test examination of key components helps to determine whether those components responded as expected to propellant burning at 6,000 degrees Fahrenheit.