A Shenzhou test team conducted several simulated descents of the Descent Module late last month in the Gobi Desert, the Chinese aerospace publication China Space News reported on Aug. 7.

An unconfirmed report in today's (Aug. 15) Beijing Entertainment News writes that the recent airdrop tests are the last of such exercise. An unnamed official, who is involved in the research and construction of Shenzhou, says that there is no plan for further descent subsystem drop tests before the launch of SZ-5.

The official also says that the launch of SZ-4 is tentatively set for December this year. However, media reports in recent months had hinted that the liftoff might occur as early as September.

SZ-4 will remain unmanned, although the same test flight dummies will occupy the seats on the flight as on the SZ-3 unmanned mission earlier in the year.

"If the launch is successful, SZ-4 will be China's last unmanned [Shenzhou] spacecraft. The launch of SZ-5 will realize the dream of yuhangyuans ("Chinese astronauts") going into space," says the official.

But this official says that there is no concrete timeline on when the momentous SZ-5 mission might happen.

Descent Subsystem Checkout
As part of the ongoing evaluation and testing of the descent subsystem of the Shenzhou spacecraft, a team of engineers and researchers from the No. 508 Research Institute of the Chinese Academy of Space Technology (CAST) conducted five tests in the searing heat of the Gobi Desert at the end of July. The team operated five sorties of a transport plane for the tests.

The 508 Research Institute, also called the Beijing Institute of Space Machine and Electricity, is the prime research unit of China's recoverable satellites; particularly the descent subsystem.

Team members told China Space News that the successful landing of the SZ-3 Descent Module in an upright attitude was an encouraging news and "laid a solid foundation for future safe return of yuhangyuans."

Photos released after the completion of the unmanned SZ-1 mission showed that the Descent Module landed on its side.

The fate of the Descent Module of the second unmanned test mission SZ-2 remains unknown as no photo has ever been released. This was in stark contrast to the many images that space officials released after SZ-1 and SZ-3 completed their primary missions.

The lack of news and photos at the conclusion of the SZ-2 primary mission has led to speculation that the Descent Module might not have landed smoothly.

Unofficial words communicated via the Swedish Space Corporation, which had collaborated with China on several projects, indicated that the module had "a 'hard touchdown' after one of the connections on the single parachute broke during the descent. The damage was minimal and the occupants of the capsule were unharmed."

Chinese space officials have denied such a report but provided no additional information or clarification all along.

But during the SZ-3 primary mission, the Hong Kong-based newspaper Wen Wei Po reported on Mar. 26 that an unidentified space specialist did not deny the allegation when asked about the SZ-2 landing. However, the specialist provided no further comment on the matter.

The descent subsystem test team carried out the simulated descents in the northwestern part of the desert. The team prepared the subsystem and installed it on the mock Descent Module in an old aircraft hangar there. During the drop tests the team observed and collected data under the scorching Sun and the sweltering heat of nearly 50 degrees Celsius.

A transport plane took the module to an altitude of 11 km (36,000 ft).

Upon arriving the planned test site, the transport crew released the mock module. The descent subsystem immediately initiated the descent sequence with the ejection of the parachute capsule cover, then the deployment of the pilot chute, deceleration chute and main chute.

The purpose of the tests was to spot check and batch tests the components, assess the operational capability of the descent subsystem and gather data on its safety and reliability.

The testing scenarios include one-time deployment of the main chute, and breaking of several main chute cords after the chute opening. The test team wanted to assess whether the main chute would be rendered ineffective in case of a mishap, and if the descent subsystem would instantly switch to the spare chute system when the main chute failed.

After each test the team reviewed the collected data and examined the parachutes and their cords. When the engineers and researchers finished the analysis and examination, they cleaned, organized and assembled all the descent subsystem components to prepare for another test.

The team said that spacecraft reentry technology involved integration of the spacecraft overall and structural designs, and technologies of aerodynamic deceleration, control, and explosive. The most direct and effective method to test and evaluate the descent subsystem would be to simulate the spacecraft reentry conditions in airdrop tests.

To ensure perfection of the reentry technology for the Shenzhou spacecraft, the test team has already performed several tens of such drop tests.

World's Largest Descent Parachute
According to a China Space News report on May 17, the descent parachute on Shenzhou is the largest among all the reentry vehicles in the world.

With a canopy area of 1,200 square meters, the main descent parachute of Shenzhou is 200 square meters larger than the chute on the Russian Soyuz capsule.

The Shenzhou main chute canopy is more than 30 meters long. When the length of the cords is included, the parachute measures almost 80 meters long.

The spare chute canopy is 63 percent of the size of the main chute, with an area of 760 square meters.

More than 1,900 small pieces of special fabric are stitched together like scales on a fish to form the canopy of the main parachute. The thin, yet strong fabric has received heat treatment and can withstand a high temperature of 400 degrees Celsius without being burned.

Each of the thin 25-mm diameter parachute cord can bear a load of 300 kg. With almost 100 cords attached to the main descent chute, the entire assembly grasps tightly the Descent Module that weighs more than three tonnes.

The descent subsystem goes into action when the Descent Module decelerates to a subsonic speed at 15 km above ground. Ejection of the parachute capsule cover occurs when the Descent Module slows down to a speed of about 200 m/s. Then the pilot chute deploys for 16 seconds to decelerate the module from 180 m/s to 80 m/s.

Next the pilot chute pulls and deploys the deceleration chute, which then extracts the main chute. The Descent Module drops in speed from 80 m/s to 40 m/s, and eventually to 15 m/s when the main chute is fully opened.

A cutting mechanism severs the main chute at 4 seconds before touchdown.

If the main chute remains attached to the Descent Module after the touchdown, strong wind gust can drag the module on the ground for kilometers.

At 1 meter above ground, a gamma sensor at the base of the module senses the rapidly approaching surface. The sensor activates a retrorocket firing, which results in an immediate deceleration to 1 to 2 m/s and a soft touchdown of the module.

SZ-4 Mission Prep Continues
Meanwhile preparation of the SZ-4 spacecraft and its carrier, the Changzheng-2F (Long March-2F) rocket, is well underway. Zhang Qingwei, General Manager of China Aerospace Science and Technology Corporation (CASC), said on Tuesday (Aug. 13) at the corporate working conference that research and development of SZ-4 and CZ-2F had been proceeding according to plan and had intensified. He estimated that the launch of the fourth unmanned test flight would be in the last few months of the year.

The main focus of the working conference was the strategic planning on the development and reform of the corporation in the next three years.

Zhang also said that CASC would strive to achieve breakthrough in key technologies in spacecraft rendezvous, space laboratory, satellite group network, and deep space exploration.

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