AirBooster

The most promising propulsion concepts often rely on the combination of two different types of propulsion methods within the same vehicle. It is a statement that I frequently reiterate, and it largely stems from the fact that no one system is ideal for all applications. One such exam

ple is the AirBooster – Air-Breathing Booster Stage for launch vehicles. The concept of the AirBooster is easy to understand - it involves a two-stage launch vehicle whose first stage (the AirBooster) consists of jet-engines while the upper stage consists of chemical rocket engines. The problem with modern chemical launch vehicles is that the mass penalty of carrying a bunch of oxidizer liquid is very high. So as the rocket ascends through the atmosphere it makes sense to extract the oxygen directly from the atmosphere - this makes for a much lighter rocket. This is exactly what jet-engines are designed to do. The idea is to use rocket engines in vacuum and jet-engines in the atmosphere – that is what the AirBooster concept is all about.

To be fair, the idea of augmenting rockets with air-breathing stages in not new. Some examples are the Startolaunch, Scaled Composites White Knight, and the now bankrupt Virgin Orbit. All these vehicles have one thing in common, a large airplane (commonly referred to as the "carrier aircraft") that ascends to an altitude before dropping off the rocket (the "payload"). These are all horizontal drops so by the time the rocket descends low enough to clear the airplane, it loses virtually all boost velocity given to it by the airplane. As a result, the performance gains are negligible with this type of system. The only real benefit of airplane launched rockets is the ability to launch from airports. If you want a real performance gain that actually makes the launch vehicle more efficient as a whole, a vertical boost using the AirBooster followed by a vertical burn of the upper stage is the way to go.

Back in 2008, I embarked on a voyage to design and better understand the viability of building and operating the AirBooster. The first of such designs was the AB400, it consisted of four PW F100 Afterburning Turbofan engines. This particular engine was chosen as it has excellent thrust-to-weight ratio with full afterburner and is capable of operating at supersonic speeds. The thrust-vectoring systems were used in conjunction with cold-gas thrusters (fed by the compressors) to perform attitude stabilization. The AB400 was designed to boost the upper stage to an ascent velocity of Mach 3 at an altitude of 30km before disengaging, throttling down the jet-engines and returning for a vertical landing back at the launch-pad. A similar vehicle to the AB400 was successfully tested by Blue Origin in 2009 - the vehicle is known as Charon. However, it was discontinued soon after the test as it merely served as a test article for navigation software.

As my research into AB400 progressed, SpaceX came along and blew the AirBooster concept out of the water (no pun intended). SpaceX has successfully demonstrated that ordinary rocket stages are capable of vertical landing back at the pad with minimal performance losses. It became clear that the added complexity of operating turbine jet-engines was just no longer needed, especially considering that the cost of extra oxidizer is nothing when compared to the cost of the actual hardware.

But AirBooster lives on - it just assumed a different form. Ignoring all setbacks, I continued working on the AirBooster and constructed a small-scale prototype of which utilized four microturbine jet-engines. Over time, this device became known as the JetQuad. The JetQuad is a quad jet-engine VTOL drone that combines the top speed of a jetfighter with the vertical lift capability of a helicopter. It's interesting to consider that an obsolete rocket technology like the AirBooster can morph into a next generation unmanned aerial system (UAS). But this is exactly what pushing technology boundaries is all about, one dead-end is merely a steppingstone to something new and exciting.

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