66 Engine dossier | Wave J-1 Past pulsejets also suffered from undesirable fuel inefficiency. The V1 pulsejet engine (Argus As 014) had a TSFC of 3-4 pounds of fuel per force-poundhour (lb/lbf-hr). Lastly, while not necessarily being high-vibration, it was an acoustically louder engine that was uncomfortable for any human sitting nearby. “Today, however, we have a valveless engine design, which leverages modern innovations such that we don’t need mechanical air valves that wear out quickly, so that’s problem one fixed,” Maqbool says. “My colleagues and I had long believed we could reduce fuel consumption by designing and optimising a pulsejet using modern engineering methods. We’ve gone as low as 1.8 lb/lbf-hr in TSFC, and we’re confident we can get it very close to 1 lb/lbf-hr, which is the realm of gas turbine engine TSFCs. Frankly, we’ve only just started on the TSFC optimisation phase of our r&d.” As for being a loud engine, Maqbool points out that in some applications, noise is irrelevant. High-altitude, highspeed surveillance and urgent deliveries for defence users have often used less-than-quiet aircraft or other vehicles. For applications that need to be quieter, including a range of commercial operations, some forms of noise damping are being developed. Among these, Wave has successfully integrated two of its pulsejets together, and acoustically coupled them to fire alternating pulses (such that when one is firing, the other is ingesting fresh air), finding that this arrangement cancels the other’s pressure pulses and therefore much of the noise emissions. “We believe that all the technology components are there to create a power plant capable of making jet-speed aircraft without any moving parts – other than a couple of external fuel ancillaries – at a fraction of the manufacturing and maintenance costs of gas turbine jet engines,” Maqbool summarises. Pulsejet thrust For those confused by how the pulsejet draws in and expels gas from both of its tubes simultaneously, and wondering why it doesn’t simply stay in place, or stutter terribly in its efforts to produce forward thrust, it is important to understand that negative thrust is not created during the intake phase. When air gets drawn into the two tubes, it does so as a sink flow, moving in from virtually all directions around the outside of their inlets. Sink flows do not generate any significant net momentum, given the concentric effect of the vacuum in the tubes, so no negative thrust is generated. Then, when the burned fuel-air rushes out of the tubes at high pressure, it escapes as two jetstreams pointing in the same direction, creating a highly concentrated force that propels the aircraft forwards. The resulting jet thrust is unaffected by the intake phase’s sink thrust. “This is a classical problem, however,” Maqbool says. “There is something called Feynman’s Sprinkler. Take a garden sprinkler that spins when spraying water and immerse it in a pool of water. Now run it in reverse so it sucks water instead; would it spin in the opposite direction? “That might sound incredibly simple to resolve, but it has actually posed a serious, non-trivial physics problem. There are still papers coming out about it, and it relates to the conundrum of sink thrust versus jet thrust. Fortunately, we’ve been developing and testing this engine for some time, and we have concluded from our results that there is no negative thrust during the intake phase.” October/November 2024 | Uncrewed Systems Technology Wave Engine (formerly North American Wave Engine Corporation, NAWEC) uses a valveless design to avoid the short lifespan and poor fuel efficiency of past pulsejets
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