# What orbit-raising propulsion system will Tiangong use, and how does it differ from that of the ISS in terms of performance and technology?

The new Wall Street Journal video China's Answer to the Aging International Space Station: The Tech Behind Tiangong | WSJ emphasizes after 03:26 that Tiangong's orbit-boosting propulsion system is more efficient and powerful than the ISS', and while it "still uses regular fuel propellants like the international space station", even though it's "not using a liquid or solid propellant".

This confuses me, so I thought I would ask:

Question: What orbit-raising propulsion system with Tiangong use, and how does it differ from that of the ISS in terms of performance and technology?

And one of them is a system that helps power Tiangong with solar panels.

“Our energy system can convert more than 30% [of the solar power] which is much higher than what we previously saw in other space stations.”

The electricity generated from the panel wings essentially helps keep the lights on.

But one unique part about China’s setup is that it also powers the spacecraft’s propulsion technology. So even though Tiangong still uses regular fuel propellants like the international space station, China says it doesn’t have to use as much to stay afloat in orbit.

If you’re not using a liquid or solid propellant for your propulsion, then you’re not carrying that weight, then that’s a saving.

China Space Agency says its propulsion system is about five times more efficient and powerful than the International Space Station’s, which is a gas guzzler. Consuming about nine tons of fuel annually to stay in orbit.

• – uhoh
Aug 26 '21 at 15:57

If it's not "a solid or liquid propellant", and more fuel-efficient, and not magical, it's presumably some sort of electric thruster.

This Interesting Engineering article (CW: journalism) says Tiangong uses ion thrusters:

The space station’s core Tianhe module, which will welcome its first astronauts later this month if all goes to plan, is propelled by four ion thrusters, which utilize electricity to accelerate ions as a type of propulsion.

The Chinese scientists put the thrusters through rigorous testing to make sure the engines could resist the damage caused by the particles. By putting a magnetic field over the engine's inner wall to repel damaging particles, they were able to protect the engine from erosion. They also developed a unique ceramic material designed to withstand severe heat or radiation for an extended period of time. Adoption has been hampered by the fact that the thrust produced isn’t very significant.

Their ion thruster has reportedly run non-stop for more than 11 months without a hitch.

A Yahoo News article (CW: journalism) says specifically that they're Hall-effect thrusters.

The ISS uses hypergolic liquid propellants with specific impulse of about 300 sec. "5 times more efficient" than ISS puts it in the ballpark of 1500 sec specific impulse, which is typical for Hall effect. The thrust of ion engines is much lower than that of chemical ones, so I assume that instead of periodic brief reboosts, the thrust will be closer to continuous -- maybe even one or two of the four thrusters at a time, running at 100% duty cycle. Ion thrusters have demonstrated month-long continuous fire in space and over five-year continuous fire in the lab.

• A continuous boost to maintain orbit would actually provide for a superior quality of microgravity inside the station, it removes external drag as an upsetting factor. Aug 27 '21 at 6:29
• The fastest altitude drop rate I can find is 400 meters per day, corresponding to about 1 N of force or 2.6 $\times 10^{-7}$ gees of microgravity on average. Of course there will be significant day/night + random variations Diurnal (day/night) effect with 250 %
– uhoh
Aug 27 '21 at 23:40
• @uhoh yep. But if the station has zero longterm average acceleration, just short-term vibrations, then it allows for the decoupling of a microgravity experiment from the structural frame of the station. At the rotational center of the station (so centrifugal force is also not applicable), the only remaining disturbances should be those conducted by sound through the air. It is definately not perfect, but it does potentially remove a couple of sources of microgravity disruption. Aug 28 '21 at 6:38