How do satellite motors keep themselves cool and lubricated?

Question

I'm a high-schooler and today we were taught about the car engines and we were told that in order to maintain proper lubrication in order to reduce the heating effect of friction in the pistons using several engine oils. However, these oils need to be replaced once in a while, depending upon the usage of engine.

I'm curious about how the satellites and other objects in space maintain lubrication in their engine, since the satellites keep functioning for years and years and there's no chance of replacing the oil in the space, of course.

To sum up, I'm curious about how the heating effect of friction in the pistons is tackled in satellites. What are the basic fundamentals behind it?

Answer 1

There is only one regular car in space, but it doesn't have a piston engine — as leftroundabout pointed out so helpfully, it is a Tesla, an electric vehicle.

Even without piston engines, however, there are moving parts in space that need lubrication, often parts of electric motors. There is a NASA paper from 1994 that explores "the tribology practices used in space".

Tribology is a word I learned a minute ago. According to Merriam-Webster it is the "study that deals with the design, friction, wear, and lubrication of interacting surfaces in relative motion (as in bearings or gears)"¹, which is at the core of your question.

My main takeaway from skimming over the paper is that there is a host of different lubricants and that yes, they all must be replenished for longer missions/more cycles.

The main categories are liquid lubricants, including greases, and solid lubricants; the latter can be divided into film and lamellar lubricants (little particles like graphite or certain metal compounds). Both solids and liquids have their specific (dis)advantages under the extraordinary conditions in space. Quoting from the paper:

Liquid lubricants are replenished from a reservoir; if I understand correctly, some systems are passive, like a liquid-filled sponge-like structure, while others are active.

Solid lubricants can possibly also be replenished, e.g. by letting a ball in a bearing rub against a retainer of a composite material including the lubricant. That technique was "used with limited success to lubricate the ball bearings in the space shuttle turbopumps."

Those turbopumps (to use the original compound word) were probably among the most challenging mechanical engineering problems on the Shuttle, or any other rocket. They needed a few revisions before they were reliable enough under high load or other stress. This webpage provides more detail:

At these temperatures, and particularly in the presence of liquid oxygen, conventional lubricants will not work. In this bearing, bronze-filled Teflon(TM) inserts on the bearing-ball separator provide lubrication through a mechanism called solid- film transfer. As the balls rub against the inserts, they wipe off a thin film of Teflon made by DuPont, which then transfers to the ball-race contact point.

"We have had tremendous success, beyond our expectations, with the silicon nitride balls in this bearing," says Program Manager John Price. "The silicon nitride is harder, lighter, and has higher thermal conductivity. This design reduces all the wear-producing mechanisms and also generates less heat."

Now that I learned how to embed Wikipedia images I'll conclude with a picture of silicon nitride ball bearings:

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_{¹ The word was coined no earlier than 1966 by a single British engineer, Peter Jost, who recognized that what was, until then, approached from different angles was in fact a coherent subject of its own. He wrote a seminal report about the economic impact of bad lubrication, The Jost Report. The concept quickly was recognized, the British government founded several research institutions, he was appointed Commander of the Most Excellent Order of the British Empire, and the rest is history.}

Answer 2

Satellites never use Internal combustion engines like in cars (obviously they do have propulsion engines that require combustion for station-keeping purposes) in them, they mostly use solar-power and even long solar probes either use solar power or nuclear power (like the voyager 1), when the Sunlight is blocked they use batteries that will be charged by the excess power generated by the solar cells. The lubrication that are applied in space-crafts are not like the one we use it in cars and gear boxes, They use lubricators such as Hydrostatic Lubrication and other solid lubricants, their viscosity, pressure factor and all other properties are tested to adhere to the space environment. (A lot of testing will be done on these) Thus there is no need for periodic overhaul. About maintaining temperature of a satellite see here

Answer 3

There are a few elements here that are worth considering. Part of the story is to do with the purpose of the engines you might find in a satellite, or space vehicle for that matter. While the other part has to do with how those function. In short, the engines in space have fewer moving parts that require constant lubrication and tend to be used for a few quick burns now and then. By way of contrast the engine in a land vehicle tends to be constantly working and friction is a major factor that needs to be over come.

The sort of engine you talk about in your question is fundamentally quite different to the sort of engine you might put into a space vehicle.

In cars and most vehicles on Earth we tend to use reciprocating internal combustion engines. These capture the energy of an explosion and convert it into a back and forth linear motion (via movement of pistons, up and down, or side to side). The pistons are connected to a special shaft which cranks the linear motion into rotation. The rotating parts then push against something, such as the ground, or air (in the case of a propeller) to make the thing move. Every moving part in the engine is in some way rubbing up against the parts that they are linked to, and fighting friction with each action. In addition all those explosions are making the whole environment quite hot, expanding all the metals, in turn increasing the amount of rubbing. At a certain point the total friction becomes too high and the whole thing can seize up. As you rightly observe, oil pumped through the system can both cool the engine and reduce the friction, massively reducing the chance of seizure.

In space there is not a whole lot to push against. Rocket thrusters are generally used instead. They burn their fuel and eject the heated contents into space, which in turn creates a thrust force in the opposite direction. A rocket doesn't have that many mechanically moving parts, compared with the internal combustion engine. It isn't fighting against friction in order to produce thrust.

A few people have mentioned electric motors. For rotating a satellite in place, it turns out you can use reaction wheels or control momentum gyroscope in place of thrusters. Much like in this device here. These use exploit the torque generated by apply force to, or changing the speed of a rotating wheel, which is used to change the orientation of the satellite. There are a bunch of different styles of electric motors, but in general these don't require lubrication. Electric motors do still generate heat, but not nearly as much as a combustion engine. And for small occasional movements they may not heat up enough to warrant a dedicated cooling system.

Answer 4

For a satellite you need something that functions in zero gravity for many years without any maintenance or repair. Therefore piston engines are not used.

To produce electrical energy solar cells are much better, they require only solar light. No moving parts, no vibration, no lubrication, no problems with zero gravity, no liquid cooling, no fuel consumption and no exhaust. Reliability proven by hundreds of satellites.

Answer 5

The requirements on engines (really motors in my experience) in space are very different from the requirements on automobile engines on the ground. This leads to very different designs and very different lubrication strategies. Many of the motors are low speed, low load, so lubrication is not an issue. For a geosynchronous satellite the motor that rotates the solar array rotates once per day. For a 15 year mission that is less than 6000 revolutions total, less than your car might do in a minute. Everything is carefully balanced so there is little perturbation to the attitude, so the loads on the motor are very small. The motors that steer the antennas on a low orbit satellite might need to swing it a few times in 90 minutes, but that is still very slow compared to many ground situations.

Many satellites have momentum or reaction wheels for attitude control that spin at thousands of RPM. They have to do this for the lifetime of the satellite without ground intervention. The ones I knew had reservoirs that would slowly feed lubricant to the bearings to replace what is lost. A lot of careful engineering goes into assuring the reliability of the system. The power levels are small so heating is less of an issue. Removing heat is much harder without convection available.

The real answer is that it is a very different problem with very different solutions.

Answer 6

For extreme cold conditions like the WEBB telescope, most lubrication is provided by dry film lubricants since any oil would freeze solid.

Answer 7

Many of the answers here are fantastic at addressing the actual question, but it's worth mentioning that there are sometimes electric motors on satellites and space probes that have a strict runtime limit before they have to sit idle to cool down.

Perhaps the most notable example of this is not in space at all, but on Mars. The Ingenuity drone that the Perseverance rover dropped off can only fly for about three minutes at a time before it has to land to let the motors cool off. In the thin Martian atmosphere, the motors have to run pretty fast to get enough lift to take off, and there's not enough airflow to carry off the heat produced by that, so the motors just keep getting hotter and hotter the longer they run. They have beryllium heatsinks to extend the run-time but they can only do so much before they get saturated.

This problem gets worse in the Martian summer, when the air warms up and gets thinner, requiring higher rotor speeds and faster motor heating:

At that faster spin speed, the helicopter can fly for only 130 seconds at a time instead of the 170 seconds it managed before, without running the risk of the motors overheating.

In space, generally there's no need to run anything at that kind of speed, so it's less of an issue despite empty space being even worse for cooling than a very thin atmosphere.