The wood heat shield is an elegant if simple solution to the problem of dealing with reentry heating. Which isn’t entirely surprising when you think that China’s space program grew out of the Soviets’, another program whose beginnings are marked with the elegant simplicity of just making things work with the materials you have on hand. (Think the N-1, the rocket that needed to be more powerful when it was turned into a lunar launch vehicle, so engineers just added more engines.)
China’s space program was an offshoot of sorts from the Sino-Soviet Treaty of Friendship, Alliance, and Mutual Assistance. In place between the end of the Second World War and 1960, it had the Soviet Union offer support to China in military endeavours, including the Korean War, which allowed China to develop military capability based on Soviet technology; the Soviet R-2 missiles begat the basis of China’s first Long March rockets.
But the affiliation with the Soviet Union didn’t solve all of China’s space problems, perhaps because by the time China started developing its own satellite system in the mid-1960s the Sino-Soviet treaty had ben dissolved. Regardless, and inspired by the American Discoverer satellites, China began developing its own series of recoverable Earth orbiting satellites in 1964. The idea was formally approved in 1965 and the design took shape in 1966 as project 911.
The satellites were given the Chinese name of Fanhui Shi Weixing, which translates to “recoverable experimental satellite.” As they took shape, the earliest incarnation emerged as a nearly 4,000 pound with room for a 330 pound payload. It was designed to orbit between 107 and 306 miles, which would see it circle the Earth about once every 91 minutes. And like the earlier American and Soviet reconnaissance satellites, the FSW satellites were designed to use an onboard film camera; the film would have to be recovered and developed after landing on Earth.
Which meant the satellites — or at least the recovered portion — would need some protection from the searing heat of reentry. Friction between the spacecraft and atoms that make up the atmosphere generate heat, and that heat can be as higher than 2,000ºF. Not to mention China would need to develop retrorockets to start that fall back to Earth, the attitude control system to ensure its reentry was at the right orientation, and all the tracking and recovery systems to find and collect the that film.
When it came to the heat shield, Chinese scientists weren’t keen on using ablatives like the Americans and Soviets were using at the time. Ablative heat shields use a chemical material that burns away with reentry, dissipating the heat. They’re effective but heavy, and China wasn’t quite in the right place technologically at the time to develop the low-density foam tiles the space shuttle would eventually use.
So when it came to reentry, Chinese scientists looks for the simple solution, and that turned out to be wood. Specifically, white oak wood. It’s light, doesn’t catch fire too easily, and can effectively protect a payload from heat.
The FSW satellites used a 5.9-inch thick heat shield made of oak. During reentry, friction caused the wood to burn and char, leaving behind a layer of charcoal. That charcoal was blown away in the wind as the satellite fell, exposing more oak, which burned and turned into charcoal. It’s a repeating process that ultimately allowed very little heat to get through to the spacecraft; both the wood and the charcoal are great insulators. It was a light and elegant solution to the reentry problem.
Though the FSW series began in the mid 1960s, it only made its first flight a decade later. Between 1975 and 2005, 23 successful FSW satellites flew as the program evolved. Originally intended to work out the details of recovering satellites from orbit before taking on manned missions, the program focussed on reconnaissance photography when the manned flight goal was cancelled. In the 1980s, the FSW missions started looking into biological and life sciences as well as improved materials and designs for the nation’s ever evolving space program.
So a little note, guys: China being a closed system and a space program I’m less familiar with, this was bit of a tough story to track down. Among all the sources I found describing the FSW satellites and the wood heat shield, I found two from one author that contradict each other. In his 2013 book “China in Space,” Brian Harvey backtracks on his previous title “China’s Space Program” saying the heat shield was made of a non-ablative material called XF, but say anything about where the oak wood rumours (if that’s what they are) came from or where the XF story was hidden all these years. I’m honestly not sure on this one. It’ll take more digging… or ideally finding someone who’s an expert in China’s space program to confirm this one!
Sources: “China in Space: The Great Leap Forward” by Brian Harvey; “China’s Space Program: From Conception to Manned Spaceflight” by Brian Harvey; “Space Oddities” byKarl Kruszelnicki; FSW on Astronautix.