One track, two stations: A proposal for cooperation on the ISS and the Chinese Space Station
by Chen Lan
|Abandoning the current plan or significantly changing it means not only a great loss of all that has been invested, but also a major shift in the course of the long-term plan. It would cost too much.|
Prof. Wörner’s idea is indeed in the interests of humankind and is based in good intention. Undoubtedly a single station built by all spacefaring countries perfectly represents peace, harmony, and unification of the world and reflects on a long-held dream of humanity. However, there is a large gap between reality and that ideal. We have to be aware of the following situation:
It is too late now. The Chinese government approved the Chinese Space Station (CSS) plan in 2010. Five years later, the design has been frozen, hardware development is in full swing, and supporting projects like new launchers and launch site are near completion. There is less than three years until the 2018 launch of the core module. Also, China has a very good track record in executing its long-term space plan. Abandoning the current plan or significantly changing it means not only a great loss of all that has been invested, but also a major shift in the course of the long-term plan. It would cost too much. If it had happened 15 years, or even 10 years ago when the CSS was still in its conceptual phase and China’s long -term station plan still just an outline, it would have been very welcomed by the Chinese side. In fact, China expressed a willingness to join ISS many times during those days, but all those attempts were rejected by the US side.
China must have concerns about its independence and the planned retirement of the ISS in 2024 or maybe 2028. It would be too risky to bind China’s future human spaceflight activities with the ISS. Russia’s plan to separate its segment from the ISS, and the problems caused by the Ukraine crisis, also reminds the Chinese not to repeat Russia’s path. There are potential conflicts of interest, global or regional, between China and the US, and China may face a similar or more serious situation than Russia is facing today.
The US, specifically the US Congress, is always the main obstacle for China to join the ISS, and there is no sign that the situation will change in the near future. As long as China continues to grow in its current direction, members of Congress will not change their minds. ESA may be able to influence NASA, but not Congress.
Based on these three points, we can draw a conclusion with certainty that China will not give up its space station that is almost ready to go to space. That will never happen, though joining ISS with limited resources is possible: for example, sending a small module to the ISS, if the US finally agreed. But that’s not what Wörner expects.
|The concept is that the two stations fly in the same orbit—the 51.6 degree orbit used by the ISS—maintaining a certain separation, such as 100 kilometers.|
Is there still a chance to cooperate with China on the space station program? I think the answer is yes. This is the proposal I present here: a “coordinated space station formation” (or group, fleet, column, queue, etc.) consisting of two space stations, the ISS and the CSS, and more in the future.
The concept is that the two stations fly in the same orbit—the 51.6 degree orbit used by the ISS—maintaining a certain separation, such as 100 kilometers. Either station runs independently but can also work jointly to support each other. Visiting vehicles, with cargo and crew inside, can move quickly and easily from one station to another. This “multi-station formation” has the following major advantages that a standalone station does not have:
Mutual rescue: in case of an emergency on one station where its rescue vehicles are damaged, as long as one crewed transportation vehicle at any of the stations is available, it would be possible to transfer the crew to another station to wait for later vehicles to bring them back to the Earth. The two stations provide a shelter for each other, which is extremely important for both the aging ISS and the Chinese, who have little experience on long-duration space station operation. It would be a capability never before realized in the history of space flight and will provide a doubled safety guarantee to both crews.
Mutual support: the two stations provide redundancy on many basic capabilities and can also support each other, especially when there is something one station lacks. For example, sharing of orbit re-boosting vehicles, storage space and supplies, ground-station data links, onboard medical support, and back-up docking/berthing ports, etc. The free-flyer telescope planned for the Chinese station can also be serviced by the ISS, if needed. Without such mutual support capability, as currently on ISS, the same job will cost more and even need a new launch to complete.
Joint experiment: it enables experiments relying on two stations, for example, laser communication and long baseline astronomical observation. And also, the dual-station model makes it possible to do the same experiment simultaneously on two stations for comparison. In fact, it opens a door for innovative experiments we have never imagined before.
Crewed formation flying: in the early days of space flight, there have been two crewed spacecraft flying closely, but they all were short-duration flights. Crewed formation flight may be valuable for future human deep-space missions. It provides more redundancy and flexibility for long-duration missions. This proposal provides a chance to experiment with this concept.
Implementing such a space station formation will be much easier for the Chinese than giving up the CSS and joining the ISS. But still, a lot of work needs to be done.
|Implementing such a space station formation will be much easier for the Chinese than giving up the CSS and joining the ISS.|
First, China has to launch the CSS into the same orbit as the ISS. The ISS orbit is a circular orbit at about 400 kilometers with an inclination of 51.6 degrees, instead of the CSS’s planned 42–43 degrees. All visiting cargo and crew vehicles have also to be launched into this orbit. The CZ-5B launcher, which will launch the station modules, is able to send 23 tonnes to a 200 x 400 x 42° orbit. While the planned modules have a mass of 20–22 tonnes each, there is only one to three tonnes of margin left to raise the orbital inclination from 42 to 51.6 degrees. It might not be enough.
One possible way is to launch them into a lower orbit, such as 300–350 km, orbit (Shenzhou and Tiangong orbits) but still in the same orbital plane. Once assembled (in 2022), one or more cargo ships would then lift it to the ISS orbit. This would mean that equipment/ supply and crew transfer between the two stations in the first four years (from 2018 to 2022) will need more fuel. That may be acceptable. If ISS can be lowered to about 350 kilometers, as it was before 2011, it will significantly reduce the gap and remove the four-year transitional period. At the same time, cutting fuel and removable equipment in the modules to reduce launch mass could also help. As the gap of the launch mass is probably minor or does not exist at all, there should be an acceptable solution. For the Tianzhou cargo ship and the Shenzhou crewed vehicle, the problem is less serious. Removing a little cargo (from Tianzhou) and onboard equipment/supply (from Shenzhou, where full supply and even the fully-functional orbital module are not required in station ferry missions) would easily solve the problem.
The inclination change would present another problem, caused by the fact that the locations of current Chinese tracking stations are not suited for a 51.6-degree trajectory, especially during the ascent and re-entry phases. For launches from Wenchang on Hainan Island, reallocating the tracking ships may solve the issue easily. But there will still be Shenzhou launches from Jiuquan because China’s new generation multi-functional crewed spacecraft that could be launched from Wenchang will be put into use after 2020. For the 51.6-degree orbit, a northbound launch from Jiuquan will put its downrange within Mongolia. China may have to launch the Shenzhou in its descending orbit phase (southwards). So, one or more new tracking stations have to be built to the southeast of Jiuquan, in central China.
Similarly, to support re-entry and landing from a 51.6-degree orbit, two new tracking stations would have to be built southwest of the landing zone (probably in Qinghai and Tibet) to replace the Jiuquan and Kashi stations. Tracking ships would have to be deployed in the Indian Ocean to replace the Malindi and Karachi stations. The Tianlian data relay satellites may reduce the number of new stations to be built, and it could be further improved if there was support from the EDRS, or even NASA’s TDRS and Russia’s Luch systems. All these have no technical barriers but need time and investment to build.
For landing from the 51.6-degree orbit, the Inner Mongolian primary landing zone in Siziwang Banner can still be used, but the backup one in Jiuquan would no longer be useful. China would have to establish a new backup landing zone southwest of Siziwang Banner for the case of ballistic reentry. Also, all emergency landing sites worldwide would have to be re-planned and prepared.
Besides, both sides need to make the plan together and do the necessary technical work, such as making docking/berthing mechanisms compatible, integrating the data communication systems, establishing a coordinated mission management and control system, etc. For all this kind of work, experience from the ISS would help a lot.
Optionally, new modules for each other’s station can be developed and launched, if political and financial conditions allow. China may send a module to the ISS, and the ISS partners (most likely ESA) send a module to the CSS, and crew can be exchanged between the two stations.
|ESA may be in the best position to lead this project. However, there is not much time left. It needs strong will and a resolute decision from both sides so that all challenges can be overcome.|
To complete the above work within three years is not an easy job. But if CSS joins ISS, as Wörner suggested, all the above work still exists, and there would be many more additional challenging tasks. This proposal is the most realistic, lowest cost, and easiest to implement. Once completed, it can be seen as an integrated “virtual” space station playing many roles one station normally plays. In future, new stations such as Bigelow Aerospace’s planned commercial stations could also join the formation. It requires not only the two stations running in one physical orbit, but also all participants working on the same track. This is exactly the spirit of cooperation we have seen on the ISS. This can achieve the same objective as the ISS.
ESA may be in the best position to lead this project. However, there is not much time left. It needs strong will and a resolute decision from both sides so that all challenges can be overcome. Hopefully, CNSA, ESA, and NASA take some action quickly to grab this chance and change the history of human space flight for several decades to come.