Why Rocket Engines Aren't Reused Like Aircraft Jet Engines

Contrary to a common misconception, rocket engines are reusable. Companies such as SpaceX have successfully demonstrated this. However, there are several significant challenges that have historically made reusing rocket engines difficult, but with advancements in technology, the potential for widespread reusability is becoming a reality.

Challenges of Reusing Rocket Engines

The process of recovering and refurbishing rocket engines is complex and requires significant effort. After a launch, the engine and its surroundings must be recovered and inspected for damage and wear. This recovery process involves either the entire rocket booster or the separation of engines and their subsequent recovery.

Protection and Weight Considerations

Recovering rocket engines requires a robust recovery mechanism, which adds weight. Additionally, engine components, particularly the nozzle, are fragile and can suffer damage during reentry and recovery. Ensuring that the engines can withstand these conditions is crucial but adds to the complexity of the process.

Extreme Operating Conditions

Rocket engines operate at the very edge of physical capability, subjected to extreme temperature and pressure conditions. For instance, using cryogenic fuels such as liquid methane and oxygen exposes the engine to temperatures from vacuum to over 4000 PSI. Every launch involves running the engine at these extreme conditions, making it challenging to ensure that it can withstand multiple launches without failure or degradation.

Historical Challenges of Reusing Rocket Engines

For a long time, the cost of launching payloads was so high that it was not economically viable to recover and reuse rocket engines. Manufacturers, therefore, assumed that they would be disposed of after a single use. However, with advancements in CAD and simulation technologies, along with advanced materials, it has become feasible to create engines that can be reused multiple times.

Cryogenic Liquid Methane and Reusability

The adoption of cryogenic liquid methane as a fuel has been a game-changer. Unlike traditional fuels like RP-1 (a type of kerosene), methane burns completely without leaving carbon deposits inside the engine. This characteristic makes it easier to clean and refurbish the engine for reuse. SpaceX has successfully demonstrated this with the Raptor engine, which powers their Starship spacecraft. In comparison, engines using RP-1 required extensive manual cleaning to remove carbon deposits before reuse.

Key Examples of Reusable Rocket Engines

Space Shuttle Main Engine (SSME)

One of the most notable examples of a reusable rocket engine is the Space Shuttle Main Engine (SSME), also known as the RS-25. The SSME is a fuel-rich, dual-axis staged combustion engine that provided tremendous thrust during launches. Despite its performance, the SSME required extensive refurbishment after each use due to carbon deposits and other wear and tear.

RL-10 and DC-X Project

The RL-10 engine, which was originally designed for upper stages of rockets, was adapted into a first-stage engine for the DC-X project. This project, while limited in scope, helped to demonstrate many reusable engine concepts. The DC-X and DC-XA, which flew several times with minimal maintenance, showed that reusable engines could be feasible with careful design and maintenance.

SpaceX Merlin Engine

SpaceX's Merlin engines, particularly the Merlin 1D used on the Falcon 9, have made significant strides in reusable technology. These engines can be restarted multiple times, including in flight, for various purposes such as reentry and boostback burns. This capability showcases the potential for more frequent and routine reuse of rocket engines.

Blue Origin BE-3 and BE-4 Engines

Blue Origin's BE-3 engine, which powers their New Shepard suborbital vehicle, is designed for reuse and restart. Their next-generation BE-4 engine will be used in the ULA Vulcan Centaur rocket and Blue Origin's New Glenn launch vehicle. These engines are expected to be more extensively reused in future launches, further pushing the boundaries of rocket reusability.

New Space Companies Pioneering Reusability

Other companies, such as Rocket Lab and Relativity Space, are also developing their own reusable engines. The Archimedes and Aeon R engines are designed to power large reusable launchers, bringing us closer to a future where rockets can be reused more efficiently and cost-effectively.

As technology continues to advance, the reusability of rocket engines is becoming increasingly viable. Companies like SpaceX are leading the way, demonstrating that with careful design, robust recovery mechanisms, and advanced materials, rocket engines can be used multiple times, significantly reducing the cost of space launches.