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Typing "rocket motor" into google returns a page full of links mostly about rocket engines. It seems that The Google has AI-synonymized them.

Question: Can "engine" and "motor" be used interchangeably in spaceflight? Are there any cases where they can't be (apart from proper nouns; the names of specific engines or motors).

Evidence of research

Asking The Google's ngram viewer instead of The Google's search:

British English:

google ngram "rocket engine" vs "rocket motor" British English

American English

google ngram "rocket engine" vs "rocket motor" American English

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    $\begingroup$ In Spanish, "motor" and "engine" translate to the same word: "motor" $\endgroup$
    – Aaron F
    Jun 12 at 20:22
  • $\begingroup$ @AaronF is that true in the context of spaceflight specifically? $\endgroup$
    – uhoh
    Jun 12 at 23:05
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    $\begingroup$ it is! compare, for example, the Spanish Wikipedia entry Apolo 11 with the English Apollo 11 $\endgroup$
    – Aaron F
    Jun 13 at 0:47
  • $\begingroup$ @AaronF Are you seriously suggesting no technically educated spanish speakers recognise a difference between "motor" and "engine"? If not, where does 'in Spanish, "motor" and "engine" translate to the same word: "motor", ' come from? Which language pairs are you even considering, and which way; to Spanish from what, or from Spanish to what? On my screen, even Google thinks you missed a trick… $\endgroup$ Jun 13 at 20:19
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    $\begingroup$ @RobbieGoodwin Aaron is not "suggesting" anything, nor making a statement about educated Spanish speakers' abilities. Aaron is just stating the fact that the Spanish language uses the same word (motor) for both "motor" and "engine" —just like it uses the same word (dedo) for "finger" and "toe"; just like English uses the same word ("leg") for pata (animal leg) and pierna (human leg)—. They are virtually synonyms in English too engineering.mit.edu/engage/ask-an-engineer/…, so what's your point anyway? $\endgroup$
    – walen
    Jun 14 at 9:00
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"Motor" by convention refers to a solid rocket, "engine" by convention to a liquid rocket. There can be exceptions.

...the word "motor" is as common to solid rockets as the word "engine" is to liquid rockets...

Rocket Propulsion Elements, Sutton, 4th edition, p. 354

Anecdotally, at least on shuttle you could get away with calling a liquid engine a "motor" more than calling a solid motor an "engine". Anyone referring to the "solid rocket engines" would have gotten funny looks.

And then there are "jets" and "thrusters". It all depends on the cultural jargon of the program you are working on.

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    $\begingroup$ Traditionally, solid rocket motors use a solid propellant and don't have moving parts while rocket engines use a liquid or gaseous propellant and do have moving parts. The distinction has gotten fuzzy with the recent invention of a restartable solid rocket motor, which does have moving parts so as to shut down and restart the motor. $\endgroup$ Jun 11 at 12:26
  • $\begingroup$ I also forgot to mention hybrids. $\endgroup$ Jun 11 at 12:27
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    $\begingroup$ You also didn't mention solar sails or Bussard ramjets, both of which escape the tyranny of the rocket equation by not carrying propellant. $\endgroup$ Jun 11 at 12:38
  • $\begingroup$ Would "motor" ever refer to an electric motor (as used to control some part of the spacecraft, e.g., opening or closing doors), or is another term preferred? $\endgroup$
    – Cadence
    Jun 11 at 13:50
  • $\begingroup$ @Cadence sure, but that's not what the question is about. Shuttle had lots of electric motors. $\endgroup$ Jun 11 at 13:55
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According to Wikipedia, they are effectively interchangable:

Motor and engine are interchangeable in standard English. In some engineering jargons, the two words have different meanings, in which engine is a device that burns or otherwise consumes fuel, changing its chemical composition, and a motor is a device driven by electricity, air, or hydraulic pressure, which does not change the chemical composition of its energy source. However, rocketry uses the term rocket motor, even though they consume fuel.

However, despite the technical interchangability, it seems to be vastly more common to refer to solid rocket motors and liquid rocket engines.

Google ngrams viewer showing "liquid rocket engine" used far more often than "liquid rocket motor"

Google ngrams viewer showing "solid rocket motor" used far more often as "solid rocket engine"

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While the "rocket engine" vs "rocket motor" angle is already adequately covered, I would like to note that the question as asked contains an additional nuance:

Can "engine" and "motor" be used interchangeably in spaceflight?

It's not uncommon to cut the "rocket-" part, using just "engine" and "motor" alone. But this introduces ambiguity with other on-board equipment. While no other propulsion than rockets is used, you can find plenty of other uses for powered equipment on a spacecraft. These are not completely interchangeable.

Consider for instance: "The docking attempt failed due to the failure of a small motor." This "motor" could plausibly be both a little RCS rocket engine, or some electric actuator in a docking mechanism. The second interpretation goes away if "engine" is used.

Non-rocket meanings of "engine" and "motor" still exist in the spaceflight namespace.

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    $\begingroup$ Great point! There are at least 2 kinds of "thrusters" too, one is a pusher-thing the other is a rocket-engine-thing. $\endgroup$ Jun 11 at 18:52
  • $\begingroup$ Nuanced? Moi? $\endgroup$
    – uhoh
    Jun 11 at 23:07
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additional supporting evidence of a distinction dound in CNN's Pentagon tracked failed Iranian satellite launch and new images reveal Tehran is set to try again:

While the US defense officials did not identify the rocket that was used in the launch earlier this month, Jeffrey Lewis, a professor at the Middlebury Institute of International Studies, said it was likely the Simorgh rocket, a two-state space launch vehicle using engines based on a North Korean design.

Nevertheless, Lewis suggested Iran would develop a different rocket if it pursued ICBMs, saying, "The Simorgh is huge and uses engines that are basically super-sized Scud engines, which are pretty inefficient. If Iran wanted to build an ICBM, it would follow North Korea's path and build an ICBM with a better engine or motor and small enough to be transported by a truck."

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