Mach 31 Speed, Double Skip Re-Entry, and 90-Degree Turn at 5000 km

The cutting-edge technology of Chang’e-6 has been unveiled, showcasing a 90-degree turn at Mach 31 speed at the edge of the atmosphere, demonstrating extraordinary space maneuvers. Ouyang Ziyuan, known as the “Father of Chang’e,” once said, “Whoever controls outer space controls the Earth.” The Chang’e-6 lunar probe, traveling at Mach 31, returned to Earth in perfect form, highlighting breakthroughs in China’s space technology.

The return of Chang’e-6’s re-entry module was remarkably smooth, landing precisely in the designated area of Siziwang Banner, Inner Mongolia. The instrument compartment operated normally, marking the complete success of the Chang’e-6 lunar exploration mission, achieving the world’s first sample return from the far side of the moon.

In recent days, Chang’e-6’s return speed has captured global attention. Upon re-entering Earth, Chang’e-6 reached a maximum speed of Mach 31, surpassing the speed of “Ultraman” and becoming the fastest re-entry spacecraft to date.

Mach number is the ratio of speed to the speed of sound, which varies with altitude, temperature, and atmospheric density, making Mach number a relative value. At standard atmospheric pressure and 15°C, the speed of sound in the air is approximately 340 m/s (or 1224 km/h) at sea level. As altitude increases and atmospheric temperature decreases, the speed of sound decreases, such as 290 m/s at 20 km altitude, and even lower at the Karman line (about 100 km altitude), the boundary between the atmosphere and outer space.

Ignoring altitude and temperature changes, Chang’e-6’s Mach 31 speed roughly translates to 10.5 km/s, exceeding the first cosmic velocity of 7.9 km/s (approximately Mach 25). The maximum shutdown speed of the longest-range intercontinental ballistic missiles is about Mach 25, while Chang’e-6’s speed surpasses this.

Additionally, Chang’e-6’s trajectory is quite unusual, with a re-entry point altitude of 5000 km, resembling a “skipping across water” return to Earth. The latter part of its trajectory is similar to a wave-rider hypersonic missile.

Chang’e-6’s return to Earth followed the “Qian Xuesen-Sanger trajectory,” performing a space “skip maneuver” with two exits and re-entries into the atmosphere, akin to skipping twice across the atmosphere.

The “Qian Xuesen-Sanger trajectory” combines concepts from two different ideas. The Sanger trajectory, proposed by German scientist Professor Sanger in the 1930s, envisioned a rocket-powered vehicle that ascends to 100-120 km altitude and then bounces along the edge of the atmosphere. Meanwhile, American scientist Qian Xuesen proposed a revolutionary trajectory where a rocket boosts the vehicle into space, reaching an orbit height of 200-300 km, before diving into the atmosphere at 100 km altitude and gliding to achieve longer distances with less fuel.

Qian Xuesen’s trajectory involves a rocket-boosted ascent into space followed by gliding along the atmospheric edge, while the Sanger trajectory involves bouncing within the atmosphere. The two concepts were later collectively termed the “Qian Xuesen-Sanger trajectory.”

At an altitude of 5000 km above the South Atlantic, Chang’e-6’s orbiter and re-entry module set the return trajectory parameters, similar to inputting targeting data for an intercontinental ballistic missile in space. The orbiter and re-entry module separated, akin to an ICBM releasing multiple warheads. The re-entry module descended at Mach 31, entering the Earth’s atmosphere at around 120 km altitude for the first aerodynamic deceleration.

After the first deceleration, the re-entry module descended to a predetermined altitude (approximately 80 km), adjusted its aerodynamic attitude, and bounced out of the atmosphere. After reaching a certain height, it turned downward for a second atmospheric re-entry and aerodynamic deceleration.

This “skipping across water” trajectory, formally known as “Lunar-Earth Free Return Trajectory with Semi-Ballistic Jump Landing Technique,” involves a re-entry module entering the atmosphere thousands of kilometers from the landing zone, bouncing back into space, and re-entering for a normal parachute-assisted landing. This process is akin to a wave-rider hypersonic warhead “skipping across water” penetration maneuver of an intercontinental ballistic missile.

Chang’e-6 used this unique double “skipping across water” return trajectory, or “semi-ballistic jump return technique,” to manage the high re-entry speed of Mach 31. Direct re-entry at such speeds would cause extreme aerodynamic heating, necessitating excessive heat protection measures that would significantly increase the re-entry module’s weight.

Given Chang’e-6’s small size (1.25 meters in height and diameter, weighing only 300 kg), it couldn’t use extensive heat shielding like the multi-ton Shenzhou spacecraft. Instead, the engineers employed a skipping trajectory to decelerate and mitigate aerodynamic heating.

To achieve atmospheric deceleration for Chang’e-6, Chinese aerospace engineers designed a gentler two-skip trajectory based on the “Qian Xuesen-Sanger” gliding trajectory. This controlled the re-entry module’s heating within ideal limits.

Chang’e-6’s entire maneuver resembled two “skipping across water” actions, or “semi-ballistic jump return” technique. This technique, while not unique to China, has been refined from its crude applications by the Soviet Union and the United States during lunar missions (e.g., Zond-6, Zond-7, Zond-8 by the Soviet Union, and unmanned Apollo missions by the US). The Chinese version achieved higher jump heights, longer distances, and more precise landing accuracy, as demonstrated by the Chang’e-6 module landing just 1.85 km from the Chang’e-5 landing site, despite parachute drift. Without the parachute, terminal precision within hundreds of meters is achievable, indicating near-perfect control and attitude during re-entry.

This advanced technology is now used in the most sophisticated intercontinental ballistic missiles and hypersonic weapons. China’s DF-17 hypersonic missile, which uses the “skipping across water” technique to evade enemy defenses, illustrates this. If the Chang’e-6 re-entry module carried a nuclear warhead, it would function as an advanced intercontinental ballistic missile with a hypersonic wave-rider warhead. (Feng Huo)

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