One of the greatest challenges with designing solid rockets and control systems for missiles and space launch vehicles is protecting the hardware and payloads from the intense heat and high pressures generated by burning solid propellant. Flame temperatures can range from 2000 – 6000 degrees Fahrenheit and motor operating pressures can be thousands of pounds per square inch. In this extreme environment, the material used to insulate key components of rocket propulsion control systems begins to erode, decompose or change states, often within tens of seconds. Without insulation, these components can rapidly overheat and fail structurally, limiting a rocket’s performance and operational range.
Cool Under Pressure
Enter Tim Dominick, senior principal engineer for propulsion, Northrop Grumman. Since 2013, he’s been leading the company’s efforts to develop a new type of thermal insulator that remains stable and effective in high-temperature, high-pressure environments.
Known as JT-700, the new insulator is a carbon-fiber-reinforced silicon carbide material. It is part of a class of materials known as ceramic matrix composites (CMC). JT-700 promises to improve the performance and extend the mission duration of solid propulsion control systems by an order of magnitude.
“With JT-700, we can meet our customers’ desire for increasingly compact, high-performance rockets that can fly for extended periods of time,” said Dominick, who received AIAA‘s coveted 2019 Engineer of the Year award for his leadership of the JT-700 effort.
High Strength, Low Conductivity
JT-700, Dominick explains, is an aerospace technology called a structural insulator; it performs well as a thermal insulator (i.e., low in heat conductivity), yet is strong enough to serve as a load-bearing part of a structure.
“Carbon-fiber reinforced silicon carbide CMCs have been around for about 25 years, but they’ve exhibited both high strength and high heat conductivity,” said Dominick. “We’ve figured out a way to make JT-700 in a way that retains its strength while driving down its conductivity.”
Fabricating the Future
Northrop Grumman is using JT-700 to fabricate complete nozzles for rocket propulsion control systems, he adds. Each nozzle is machined from a single block of JT-700 CMC material, which simplifies the nozzle assembly process. In the past, these parts were produced from refractory (high-temperature) metals to which phenolic insulation had been bonded.
During rocket flight, explain NASA scientists, combustion of a rocket’s solid propellant produces large amounts of gas at high temperature and high pressure. This exhaust gas flows through the rocket motor’s nozzles, creating thrust. A propulsion control system “steers” the rocket by diverting exhaust gas selectively through each nozzle for discrete periods of time.
To date, reports Dominick, Northrop Grumman has validated the JT-700 technology in both laboratory and prototype tests as a zero-erosion, low conductivity nozzle material suitable for use in solid propulsion control systems.
Dominick believes that JT-700’s insulating and structural properties also lend themselves potentially to another high-temperature, high-pressure defense application called hypersonic weapons.
The Economist describes hypersonic weapons as “those that can travel more than five times the speed of sound, or about one mile (1.6km) per second.” Hypersonic boost-glide weapons are so named because they are launched into the upper atmosphere atop ballistic missiles, then release hypersonic glide vehicles to carry a weapon to its target.
According to Arun Bhattacharya, Ph.D., a senior engineer and subject matter expert in hypersonic structures and high temperature materials for Northrop Grumman, the company is exploring the possible use of JT-700 in this application.
“Hypersonic air and space vehicles experience extremely high temperatures and high rates of heating during flight,” he said. “This extreme environment demands that we select materials that can best meet the critical temperature, structural and weight requirements of these evolving air and space missions.”
Higher Temps, Higher Efficiency
The commercial aircraft industry could also benefit potentially from JT-700, Dominick believes.
“In a jet aircraft, a lot of incoming air is diverted and used for cooling metal parts in the engine,” he explains. “Those metal parts can’t survive at higher operating temperatures, which artificially limits the engines to a combustion temperature less than the service temperature of those parts.”
By replacing key metal turbine parts with a structural insulator such as JT-700, Dominick proposes, the need to cool those metal parts with “bleed air” could be eliminated. These “new” turbines would be able to operate at much higher temperatures, which would allow them to operate more efficiently and burn less fuel, an important economic consideration for any airline fleet.
A Very Cool Future
Although JT-700 has not entered production, it is an aerospace technology that promises to broaden, enhance and disrupt the material capabilities of the aerospace industry. Its arrival has not only inspired the development of other types of novel CMC materials, but also encouraged industry suppliers to enhance their CMC manufacturing capabilities to support the nation’s most critical air and space missions.
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