Photo: Deep Blue Aerospace

The first high-altitude vertical recovery flight test of China's privately run carrier rocket Nebula-1 has smoothly nailed more than 90 percent of its verification tasks, despite the anomaly at the last stage that led to the crashing of the vehicle. The test signaled a "milestone" step forward as it represented the country's first ever high-altitude recovery flight by an orbital rocket rather than an experimental one, Executive President Zhao Ya of Deep Blue Aerospace, developer of Nebula-1, told the Global Times in an exclusive interview on Monday. 

The key technical points verified during this test process totaled 11 items, including four items in the rocket's acceleration phase during takeoff, two in the deceleration ascent phase, one in the descent phase, two in the guided maneuver deceleration phase, and another two in the final landing deceleration phase, according to the company. 

Except for the last node of the landing deceleration phase, which was not completed, the other 10 verification tasks proceeded smoothly and steadily, Zhao told the Global Times. "From the perspective of flying time, the Nebula-1 flew for 179 seconds, with issues arising only in the final 3.8 seconds," he said. 

Zhao said the overall performance of this flight test has already exceeded their expectations, as the majority of the preset tasks was completed quite well. 

The test was considered "a milestone event" for China's progress in the development of reusable rocket, as it is the first time that an orbital rocket, rather than experimental ones, is used. 

Zhao explained that the design complexity of an orbital rocket's recovery is far greater than that of an experimental rocket, as experimental rockets normally weigh about 50 to 70 percent of the total, allowing for designs of redundancies and structural elements to improve the vehicle's stability, while the Nebula-1 orbital rocket's weight only accounted for 10 percent of the total.

The reduced structural weight allows more propellant to be fueled, so that it can travel farther into the preset orbit, a distance that experimental rockets cannot achieve, Zhao said.

Elaborating on the failure of the last stage, Zhao said that the Nebula-1 consumes approximately 60 kilograms of propellant each second, and as a result, the weight of the rocket was continuously changing over the 180 seconds of flight time, especially during the landing phase when the propellant is nearly depleted, leading to significant differences in weight and speed.

In the final 3.8 seconds, the thrust control mechanism failed to accurately track the decrease in thrust, and the actual thrust turned out higher than expected. This caused the rocket's speed to drop to zero when it was about 8 meters above the ground. If it continued, the rocket could have potentially reversed and taken off again, potentially bringing safety risks. Therefore, a shutdown command was triggered to ensure safety, Zhao said. 

"Our current assessments suggest that the cause of this failure lie in the aging of engine. It has already been used for over 1,000 seconds," Zhao noted. 

The company said in a statement on Monday that plans are already in place for another high-altitude vertical recovery test in November, building on the lessons learned from this initial attempt.

The Nebula-1 rocket is Deep Blue Aerospace's first commercial liquid rocket capable of reaching orbit and being reused. It serves as a crucial platform for breaking through and validating vertical recovery and reusability technologies for rockets. The Nebula-1 has a body diameter of 3.35 meters and a first stage height of approximately 21 meters. It is equipped with the Thunder-R liquid oxygen and kerosene engine, a reusable liquid rocket engine developed entirely in-house by Deep Blue Aerospace, with over 90 percent of its main structure made using high-temperature alloy 3D printing technology.