US Air Force successfully tests advanced rocket preburner

The United States Air Force Research Laboratory (AFRL) has successfully tested a component that promises to usher in a new generation of high-performance American rocket engines. Called a preburner, the unit allows rockets to operate at a higher pressure and achieve greater levels of thrust.

In much the same way that there's more than one way to skin a cat, there's more than one way to build a rocket engine. Ever since the likes of Rober Goddard and Hermann Oberth started playing around with crude liquid-fueled rockets, engineers have been trying to come up with more powerful, more efficient engines, resulting in whole families of surprisingly diverse designs.

One key feature of a liquid-fueled rocket is that it has to be able to feed in large amounts of fuel and oxidizer in order to go. In the smallest and simplest of engines, this is done by the force of gravity but for anything practical, it needs some sort of turbine pump to force the fuel in at the desired rate. The question then becomes, how to run the pump?

For most rockets, and especially those built in the United States, this involves what is known as a gas generator. To describe this in very simple terms, part of the stream of fuel and oxidizer going to the combustion chamber is diverted into the gas generator, which is a small chamber where a high ratio of fuel to oxygen is fed in. When this is ignited, there isn't enough oxygen to burn more than a small amount of the fuel, so the balance turns into a gas, which then powers the turbine pumps. Afterward, the gas is dumped overboard.

It's a system that works and was good enough to send the first men to the Moon during the Apollo missions but the gas generator can only manage very low pressures and very low fuel flows, so the rocket's performance isn't all that it could be.

However, there is an alternative called the staged combustion cycle, where the gas generator is replaced by a preburner. As the name suggests, the preburner actually burns the fuel in a very high oxygen environment, producing a hot, high-pressure gas for the turbines. This produces much higher fuel pressures and flow rates for better performance. And as an added bonus, the unburned fuel is fed into the combustion chamber rather than dumped, for better efficiency.

The staged combustion cycle engine was first developed in Russia and the AFRL has been very keen to adopt the technology for its own version, called the Hydrocarbon Boost program (HCB), through the efforts of the prime contractor Aerojet Rocketdyne, the Air Force Space and Missile Systems Center, and NASA's Stennis Space Center rocket testing facilities.

According to the Air Force, the program has been going on for over 20 years as part of the goal to produce American-made rockets capable of powering reusable spacecraft. However, staged combustion cycle engines are extremely complicated and there have been obstacles.

One of the biggest is that the preburner has to operate in the very hot, oxygen-rich environment of the highest pressure rocket combustor ever tested in the United States, which can cause most metals to melt and corrode at the same time. To prevent this, the AFRL has turned to super-nickel alloys that are notable for their high-strength and burn resistance. Having worked well in small-scale tests, they have now been successfully tested in a full-size preburner as the engineers try to gain a better understanding of the basic issues of the technology, including combustion stability and thermal management.

"The HCB preburner pushed the boundaries of these extreme conditions and redefined state-of-the-art for these challenging systems," says Dr. Shawn Phillips, Chief of the Rocket Propulsion Division, which is part of AFRL’s Aerospace Systems Directorate.

Source: AFRL