# How do satellite motors keep themselves cool and lubricated?

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 every 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 is the basic funda behind it?

• “It might seem kinda stupid to the astrophysics gurus out there but I'm curious and I can't help it.” You don’t really need to explain yourself like that on here, as long as a question is well written and on topic, we won’t judge. Jan 22 at 17:09
• You have accepted an incorrect answer. Don't be so quick to accept the next time you ask a question. Jan 22 at 17:22
• Proposed piston engines in a rocket upper stage discussed here: space.stackexchange.com/q/44729/6944 Jan 22 at 20:54
• Great question! But it is best to wait for at least 48 hours before accepting an answer, to give more people time to respond. Jan 22 at 21:36
• I strongly recommend that you un-accept the current answer and wait a while for more and better answers to be posted. I usually wait a week.
– uhoh
Jan 22 at 23:43

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:

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.

• Jan 23 at 13:54
• Great answer – though come on, that one car in space obviously doesn't have a piston engine either. It's a Tesla! Jan 23 at 16:58
• @leftaroundabout OMG what a screw-up. I was so proud of that joke! Jan 23 at 19:08
• @Peter Good job. Do they happen to talk about the cooling issue in that paper. Might be a separate question, but I wonder if there are examples of satellite failures from excess heat build up and how to overcome it. I suspect lubricants aren't as useful at cooling in space. Jan 24 at 3:40
• I think the lunar rovers qualify as cars, no? Still no piston engines though. Jan 24 at 8:30

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

• I imagine people are objecting to Satellites never use combustion engines in them which might be more accurately phrased as satellites don't use internal combustion engines, satellites definitely do have thrusters most of which use some form of combustion so would fall within combustion engines, others use inert gasses or even ion engines so aren't combustion engines Jan 24 at 10:20
• @AlanBirtles I thought combustion engine generally mean the engines used in cars. Isnt the thrusters in satellites commonly known as chemical propulsion engines? Jan 24 at 10:23
• they are engines that work by combusting a fuel, in fact according to wikipedia rockets and jets fall into the ICE category too Jan 24 at 10:28
• @AlanBirtles Thats fine then, I will rephrase the line Jan 24 at 10:30
• Related - inline flat6 ICE engine planned for space use by 2045. autoevolution.com/news/… Jan 25 at 2:05

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.

• A rotating motor must have a shaft held by a bearing (ball or friction), so there are parts rubbing against each other, right? Just fewer of them, and different motions and forces. Jan 23 at 12:42
• True, I think @Ross's answer deals with that angle so I think I will point you there Jan 24 at 3:42
• I think reaction wheels might be the most pertinent component to this question, it might be worth considering them explicitly in your answer Jan 24 at 16:36
• @Persistence you are right to point that out. I think Ross' answer does a better job of dealing with reaction wheel motors. They clearly do require some lubrication, but I think the load and number of revolutions in the working life are important factors, as he points out. I also Realised as writting the above that oil changes are mostly due to losses from oil getting combusted and dirtied with soot, 2 factors that are not so relevant to electric motors (although brushed motors do generate soot) Jan 24 at 23:02

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.

• There are many reasons to have motors in satellites aside from generating electricity. Jan 23 at 19:01
• @ScottSeidman any reason for motors with pistons?
– Uwe
Jan 23 at 19:07

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.

• Interesting! Finally an answer: Yes, the lubricant needs to be replaced/replenished, and there is a reservoir for that! So which lubricant is it? Would oil work? Silicone oil? I always thought that a lot of "lubrication" in space uses graphite (which doesn't evaporate (much) and doesn't decay); is that wrong? Jan 23 at 12:46
• @Peter-ReinstateMonica there are perfectly usable vacuum greases commercially available space.stackexchange.com/a/39521/6944 Jan 23 at 13:52
• @OrganicMarble Yes, I just did a bit of research -- I see you have seen my answer. Jan 23 at 13:59

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

• Most of this doesn’t seem like an answer at all. Jan 22 at 19:40
• Follow up questions are better placed in the comments Jan 22 at 19:56
• Sorry kruemi... I'm VERY new here and did not yet have rights to comment. could only answer as guest. wanted to encourage person's curiosity. NOW can comment. Jan 22 at 21:24
• This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review Jan 23 at 10:03
• @GremlinWranger This is a better answer than many others because it addresses, how ever cursorily, the heart of the question (after we got the piston idea out of the way): How on Earth, errr, in space do you lubricate at all? Jan 23 at 12:45

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.

• I was going to ask whether a vehicle on a planet (or moon) other than Earth would still count as "in space" - but not sure if these use pistons or not... Jan 25 at 17:42
• As far as I'm aware, there has never been a rover or lander with an internal combustion engine. Everything's electric on/around other planets. Most are solar, some are radiothermal (powered by the heat from decaying radioactives), nothing burns fuel to pump pistons up and down. Jan 25 at 21:20