Why Rockets Haven't Replaced Jet Engines in Commercial Aviation

While rockets offer a promising alternative for some applications, such as Elon Musk's Starship that could theoretically fly from Seattle to Paris in just one hour, current commercial aviation still relies on jet engines. This article explores why rockets haven't replaced jet engines and the reasons behind the inefficiency of using rockets for commercial aircraft.

Rocket and Jet Engines: A Comparative Overview

Both rocket engines and jet engines belong to the category of reaction engines. They both operate by converting the energy from burning fuel into thrust through the expulsion of matter in the opposite direction. Key similarities include:

Reaction engines generate forward thrust by throwing mass backward at high velocity.

Their energy source comes from the combustion of fuel, combining with an oxidizer in an exothermic reaction.

Their thrust is derived from the hot gases produced during the burning process.

The primary difference lies in how they acquire the oxidizer needed for combustion. Jet engines extract air from the atmosphere, whereas rockets carry their own oxidizer, leading to distinct advantages and disadvantages.

The Role of Air in Jet Engines

A key advantage of jet engines is their ability to take in air from the atmosphere as part of their oxidizer. This air constitutes over 90% of the total reaction mass. Consequently, jet engines have a higher fuel-to-thrust ratio because they don’t require as much oxidizer. Moreover, the additional air in jet engines increases thrust, as seen in turbofan engines. Despite this, increasing the air intake also adds bulk and weight to the engine, and the exhaust gases are too cool for afterburners to be used effectively.

Advantages and Drawbacks of Rocket Engines

Rocket engines, while simpler and cheaper to build for single-use applications, have significant drawbacks when it comes to commercial aviation. Some of these include:

Their oxidizer requirements are a significant payload weight constraint. Every ounce of oxidizer is an ounce of payload not carried.

Rocket oxidizers are highly dangerous and difficult to handle and store, presenting a safety risk.

Absence of atmospheric oxygen means rockets must carry all their oxidizer, weighing much more than jet engines.

The higher thrust-to-weight ratio of rockets isn't feasible for reusable aircraft.

Applications of Rockets in Aviation

While rockets may not be suitable for commercial aviation, they excel in specialized applications. For instance, rocket engines are used in:

Von Neumann probes that travel to distant parts of the solar system.

Satellite launch vehicles, like the Falcon 9 rocket.

Aircraft emergency boost systems (JATO/RATO bottles) to assist takeoffs from short runways.

Aerospace propulsion systems for high-speed and high-altitude flights, such as during takeoff and low Earth orbit missions.

Conclusion

In summary, while rocket engines hold impressive potential for certain applications, their current limitations make them unsuitable for widespread use in commercial aviation. Advances in technology may one day change this dynamic, but for now, jet engines remain the most efficient and practical choice for commercial aircraft.