How NASA Reinvented the Rocket Engine

This is not just any

rocket

engine

. Limited by the technology of their time,

engine

ers and physicists have theorized about this method of propulsion for centuries, but time constraints may have been lifted with NASA's groundbreaking test of this rotary detonation engine in January 2023. With the latest advances in materials science and fluid dynamics computing, NASA has made progress by testing a 3D-printed

rocket

engine that could improve rocket fuel efficiency by up to 5%. A 5% improvement in fuel efficiency for a rocket engine is not a marginal gain. Rocket weights are dominated by fuel. The first stage of the Saturn V had a dry mass of 137,000 kilograms, when fully loaded with propellant its weight was 2.2 million kilograms.

The first stage was 94% fuel. A 5% increase in fuel efficiency represents a decrease of 103 tons of fuel. That's not far off the total dry weight of the Falcon 9 rocket. This is not only a cost savings. It represents a drastic change in rocket capabilities. Allow humans to launch larger payloads into the solar system. In this latest test of the technology, the engine ran 18 times, with a maximum ignition duration of 113 seconds per run. This was a test and the engine is not ready to use, but this test represents the culmination of decades of research, made possible by newly developed metals and manufacturing techniques and the latest advances in computational fluid dynamics modeling.

Let's delve into the science and engineering behind rotary detonation engines to see how they work. Detonation-powered engines are not a new concept. The Air Force Research Laboratory developed this highly modified Rutan Long EZ. Driven by 4 tubes in which the detonations occurred. Traditional rocket engines work by mixing fuel and an oxidizer, using a continuous igniter. Since the igniter is usually located at the top of the combustion chamber, a flame front must travel through the fuel mixture to complete combustion. If the speed of this flame front is less than the supersonic speed, the combustion is called a deflagration.

If the "flame front" travels through the mixture at more than Mach 1, then the process is called detonation. Almost all combustion engines are deflagration powered, but as the name implies, rotary detonation engines use a supersonic flame front to produce thrust and, as a result, can produce more thrust with the same volume of fuel. However, this is easier said than done. The energy required to initiate a detonation is very large, and for these pulsed detonation engines used in the Air Force test vehicle, detonations had to occur hundreds of times per second. We can initiate a normal deflagration combustion flame front, which travels along the tube and, with some stimulation, will progress to detonation, but the causes of this are not fully understood and are not always reliably repeatable.

We know that knocking occurs most frequently when the fuel is turbulent, so to create these conditions, pulsed knock engines often include physical barriers. One method to achieve this is by placing a spiral inside the tube. Pulsed detonation has a more obvious problem. They do not provide constant thrust. They press. Rotary detonation engines are the solution to both problems. They consist of a ring-shaped combustion chamber. The process begins by opening a valve at one end of a tube and letting a fuel mixture fill the tube. The valve closes and an ignition source in the closed section is extinguished and detonation begins.

A shock wave begins to travel around the circumference of the ring, raising the pressure and temperature of the gas behind it almost instantaneously. This gas then expands and exits the tube at an extremely high velocity, creating thrust. This cycle is then repeated with the shock wave traveling in an endless circle. This clever arrangement solves the main problems with pulsed detonation engines by having to activate the detonation only once, eliminates the unreliability of activating thousands of detonations per second and its continuous spiral around the combustion chamber results in a thrust constant. Please note that these detonation fronts go FAST and operate at Mach 3-6 depending on fuel mixture.

This means that the waves complete a full circle in one thousandth of a second, with operating ranges between 1 to 10 kHz. This is an incredibly difficult phenomenon to take advantage of. Detonations release energy very quickly, are difficult to control, and are unstable by nature. In fact, over the last century of aerospace advancements, engineers have worked very hard to prevent detonations in the nozzles of their rockets. During the development of the F1 engine in the Apollo program, engineers struggled to control what they called “thermodynamic instabilities.” In reality, these were detonations that were destroying the engine. On June 28, 1962, these combustion instabilities caused one engine to be completely destroyed during testing.

This problem plagued Saturn V engineers and is arguably the biggest challenge they faced in scaling this monster rocket to the moon. A list of 18 potential sources of instability was drawn up. This was an incredibly complex and dynamic problem. One method to reduce instabilities and prevent pressure waves from being amplified on the side walls, such as a sound wave, was to install baffles that radially separated the combustion chamber. Several iterations of deflectors were designed, but with no computational method available, F1 engine engineers resorted to the next best practical method. During operation, they set off small bombs inside the combustion chamber to quickly induce small combustion instabilities.

The combustion chamber pump was fixed to the injector plate with the igniter inside the combustion zone. The igniter was coated with nylon that would wear away from the heat of combustion over time, allowing the engine to reach full operation before detonating. Resulting in a pressure wave that traveled through the engine just as a combustion instability would. Engineers were able to test the effectiveness of the new deflector designs by timing the time it took for normal operation to resume after detonation. These baffles acted effectively as acoustic dampeners, like the foam wedges we placed in the soundproofed rooms.

For decades, engineers have been working to prevent these uncontrollable dynamic detonations. But engineers and scientists are stubborn people. When they hear "uncontrollable," all they hear is "not yet controlled." And rotary detonation engines effectively utilize the exact phenomenon these baffles were intended to prevent, allowing a shock wave to travel tangentially around a ring-shaped combustion chamber. The appeal of using detonations as a combustion and propulsion mechanism is precisely what makes them dangerous, their rapid release of energy. We can visualize the amount of energy we can extract by comparing the combustion cycle of a hypothetical detonation engine and a normal gas turbine engine on a pressure-volume graph.

This graph simply represents the pressure and volume of air as it travels through an engine. In a gas turbine, air enters the engine at this pressure and volume. A compressor decreases its volume at a constant pressure. It then passes to the combustion chamber where heat is added at a constant pressure. The gas then passes through a turbine while it expands until it reaches its initial condition. The energy we extracted can be found by finding the area enclosed by these curves. In a detonation cycle we skip the combustion stage entirely and instead the detonation wave increases the gas pressure almost instantaneously.

Raising it much higher. This creates a much larger area within the curves. The theoretical differences in thermodynamic efficiencies using this method of analysis are immense. Using hydrogen as our gas, a gas turbine engine has a thermodynamic efficiency of 36.9% while a detonation engine has a thermodynamic efficiency of 59.3%. This simply means that more energy is extracted from the same amount of fuel. However, an increase in energy performance is of no use to us if we cannot control it or it simply destroys our engine. To harness its energy we needed to develop new materials and manufacturing techniques. This is where a new material specially developed for metallic 3D printing comes into play.

Technical sheet in table GRCOP-42. An alloy developed by NASA composed of niobium, chromium and copper. A high strength metal alloy specially formulated to not only have a very high melting point, but also high thermal conductivity. Which means it takes more energy to melt it and conducts energy more efficiently to prevent it from melting and more importantly NASA has more recently developed a method to use metallic 3D printing which means that we can create incredibly complex geometries with this new era. material. Maintaining this reaction is incredibly difficult. The shock wave, by nature, travels incredibly fast and causes rapid changes in temperature and pressure.

Take the injectors. They must be designed to withstand not only high flow rates but also drastic changes in pressure and temperature. As the shock wave travels, conditions change from 0.01 megapascals of pressure at 300 kelvin to 2 megapascals of pressure and 3000 kelvin. That cycle occurs a thousand times per second. Our copper-based alloy can help us dissipate that energy, but these inputs must go from injecting gas to resisting the pressure of the shock wave in milliseconds. To manufacture an entrance capable of withstanding these conditions, engineers needed 3D printing. A typically machined injector may look like this, but rotary detonation engine designers need to manage counterflow, and to do so they may turn to fluid diodes, which would be impossible to manufacture with traditional cutting tools.

The shape of this passage allows fuel to flow easily in one direction, but when the flow attempts to travel in the opposite direction, it crashes into itself, resisting the flow. It is effectively a valve with no moving parts. A perfect solution for this application. The use of metal 3D printing is not limited to entryway design. Each part of the rotating detonation engine requires a delicate shape to harness the power of detonation. From the design of the combustion chamber to ensure the shock wave stays spinning to the small cooling channels that run along its walls. The rotary detonation engine NASA developed used a 3D-printed thrust chamber that was actively cooled with deionized water.

It also has an aerospike nozzle. Aerospike nozzles are like inside-out nozzles. Instead of the traditional bell-shaped mouthpiece, it features a cone in the center of the exhaust. Aerospike nozzles allow the rocket to operate more efficiently at a variety of altitudes. To understand why, we need to cover some basics of nozzle design. The bell nozzle works optimally at a certain external atmospheric pressure. To work most efficiently, we want the exhaust pressure at the nozzle outlet to be equal to the external atmospheric pressure. If the external pressure is greater than the exhaust outlet pressure, we are operating in an overexpansion condition.

Separation of the nozzle flow occurs and energy is lost. If the external pressure is less than the exhaust outlet pressure, we are operating in an underexpanded condition, the gas expands out of the nozzle and energy is lost. However, if we equalize the outlet pressure and atmospheric pressure, nothing happens and exhaust velocity is maximized, making our engine more efficient. We could equip rockets with variable geometry nozzles, like fighter planes, but this would add a lot of weight, complexity and cost. These nozzles are much larger than those on fighter aircraft and until very recently were single-use. Aerospikes provide a lighter, simpler alternative to variable geometry exhausts.

In this case, the shape of the exhaust is dictated by the outside atmospheric pressure. At higher pressures, at lower altitudes, the exhaust is compressed against the aerospike, which redirects the exhaust to point directly downward. No flow separation can occur as the pressure pushes it against the aerospike. As the pressure decreases, this expansion continues and the aerospike automatically compensates for external pressure changes. The exact science ofThese aerospike nozzles are much more complicated than this and deserve a video entirely dedicated to them. But they are especially suitable for rotary detonation engines, since an aerospike engine, with this cone design, requires a circular combustion chamber, which the rotary detonation engine also requires.

However, this is a more important reason: to optimize the design of a bell nozzle we need a constant pressure in the chamber to optimize our expansion ratio; With the rotating detonation engine, the chamber pressure constantly changes along the circumference of the combustion chamber. The aerospike's ability to adapt to external atmospheric pressures also helps it adapt to changes in internal chamber pressure. Much more needs to be done to make rotary detonation engines viable. These prototypes, while promising, struggle to operate long enough to produce sustained thrust. In 2021, Japan tested a rotary detonation engine in space, which ran for 6 seconds and produced 500 Newtons of thrust, and while this was a proof of concept, it is a far cry from the millions of Newtons that traditional rocket engines provide. they can reach.

The mechanics of detonations are not yet fully understood. This means that maintaining a stable detonation front is extremely difficult. The system is inherently chaotic. Engineers are fighting entropy to control these powerful turbulent flow regimes. With each revolution of the shock wave more chaos is introduced into the system. A single revolving vortex can cause a domino effect until the rotation becomes completely unstable at rotation 20. NASA even claimed that their tests sometimes range from having 2 to 5 co-rotating detonation waves in the combustion chamber. at once. This NASA test was a huge milestone for technology. They showed that their copper-based 3D printed material could survive long enough in this environment; their longest run lasted almost 2 minutes, and they managed to successfully initiate detonation several times with 17 resets in 600 seconds.

The biggest challenge facing rotary detonation engine engineers is creating flow models to predict these instabilities and developing methods to dampen and control these combustion instabilities, just as the Apollo program engineers did back in the 1960s. Next Real engineering video is now ready. This is the M1 Abrams. We detail its unique turbine engine, a type of engine typically used in aircraft, but used in this heavy armored vehicle for 2 special reasons. We look at how M1 armor has evolved over the past four decades and how it works. It will be available on YouTube in 2 weeks, but you can watch it now, before anyone else on Nebula, as a thank you to the fans of our show.

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