With the successful recoveries of the Long March 10B and Zhuque-3, rocket recovery and reuse have become one of the hottest topics in the space industry. Yet the real value lies not in the dramatic moment of recovery, but in the underlying economics: reducing the cost per launch by reusing high-value components.
As the most expensive and technically complex core component of a launch vehicle, the reusable engine directly determines the rocket’s economic viability and technological ceiling. It represents one of the most important competitive frontiers in the global commercial space industry.
Compared with expendable engines, reusable engines must support multiple ignitions, operate across a wide range of conditions, and incorporate lifecycle health management and fault diagnosis. At the same time, they must meet engineering requirements for low cost, high reliability and rapid turnaround. These tightly coupled demands make reusable engine development exponentially more difficult. The principal technical challenges can be divided into the following areas.
1. Multiple Ignitions at Low Inlet Pressure
During the powered deceleration phase, the engine must restart under low-g conditions and extremely low inlet pressure while flying through shock-wave conditions in the rarefied upper atmosphere. For a newly developed rocket, problems are particularly likely to occur during engine restart.
There are two main challenges. First, propellant injection and ignition behave very differently from those in ground conditions. Second, the low acceleration before ignition results in extremely low engine inlet pressure, making self-starting difficult.
During the landing burn, the rocket is only several hundred meters above the ground, and the engine must ignite against the airflow under high back-pressure conditions. Pumps, valves and pipelines simultaneously experience high-pressure, high-flow transients, creating extremely demanding operating conditions.
In addition, the pressure at the nozzle exit can be higher than the ambient atmospheric pressure encountered during a ground test. Controlling propellant delivery during ignition against the oncoming airflow is therefore a major challenge.
2. Wide-Range Throttling
To support a soft landing, a vertical-takeoff, vertical-landing rocket must operate when its mass has fallen to less than one-tenth—or even one-fifteenth—of its liftoff mass. Its engines therefore need a wide throttling range, typically down to 20–40% of rated thrust, to meet return-flight control requirements while maintaining a favorable reentry thermal environment.
This requires continuous variable-thrust operation as well as effective strategies and sufficient control authority for regulating the propellant mixture ratio.
As the engine transitions from rated conditions to low-power operation, its disturbance-rejection capability deteriorates significantly and delays in the engine’s dynamic response become longer. Under certain frequency and phase relationships, disturbances in system parameters and the resulting responses can form a positive-feedback loop, potentially triggering unstable, low-frequency coupled oscillations across the entire propulsion system.
3. Fault Diagnosis and Testing
Fault diagnosis presents three primary technical challenges.
First, engines have a wide variety of potential failure modes. Turbopumps may experience abnormal axial forces, turbine erosion or blade fractures. Thrust chambers can suffer weld cracking or tearing, throat erosion, leakage and combustion instability. During final assembly, problems may include system instability as well as leakage or fractures in assembled pipelines.
Second, faults can develop extremely quickly. In a high-pressure staged-combustion engine, a serious turbopump fault can escalate into catastrophic damage within approximately 10 milliseconds.
Third, some critical parameters are difficult to monitor. Accurately measuring combustion-chamber temperatures, high-temperature gas pulsations and internal structural damage remains challenging. Manufacturing constraints and severe vibration can also distort measurements and undermine sensor reliability.
4. Inspection and Maintenance
Rapid inspection is one of the first problems that must be solved before high-frequency launches can become possible.
Reusable engines create a new requirement for in-situ, nondestructive inspection without disassembly. Critical areas must be examined quickly and accurately to identify damage and quantitatively assess defects. This requires continued improvement in ultrasonic testing, eddy-current testing, penetrant testing, optical inspection, borescope inspection and image recognition.
Maintenance and repair can be divided into on-vehicle maintenance and off-vehicle repair. To make an engine ready to fly again shortly after returning with the rocket, operators must develop rapid on-vehicle inspection and maintenance capabilities. These include internal cavity treatment, visual inspection, structural inspection, borescope inspection, leak testing, electrical checks and gimbal-angle verification.
Off-vehicle work includes both minor and major repairs. Achieving rapid reflight also requires a comprehensive operational support system for inspection and maintenance. This is no longer purely a technical challenge: it demands close coordination with the launch site. Ideally, inspection and repair facilities should be located nearby, which in turn requires the surrounding area to have a strong industrial base.
5. Reuse-Life Assessment
Reuse-life assessment is essential. It provides the foundation for safely and reliably reusing an engine, reducing launch costs and enabling multiple flight cycles.
Engine life must be evaluated under high temperatures, high pressures, severe vibration, large temperature gradients, thermal and mechanical shocks during startup and shutdown, and repeated temperature and pressure cycles. This requires the development of accurate load-prediction methods.
Engine life must also be investigated through exploratory endurance firing tests to verify its ability to operate repeatedly and for extended periods.
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