19

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

uhoh
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    In Spanish, "motor" and "engine" translate to the same word: "motor" – Aaron F Jun 12 '21 at 20:22
  • @AaronF is that true in the context of spaceflight specifically? – uhoh Jun 12 '21 at 23:05
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    it is! compare, for example, the Spanish Wikipedia entry Apolo 11 with the English Apollo 11 – Aaron F Jun 13 '21 at 00:47
  • @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…

     – Robbie Goodwin Jun 13 '21 at 20:19
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    @RobbieGoodwin see https://dle.rae.es/motor https://www.lexico.com/en-es/translate/engine https://www.lexico.com/es-en/traducir/motor . There's also the word locomotora (locomotive), but that word talks about the whole vehicle rather than the motor which drives it. – Aaron F Jun 13 '21 at 21:40
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    @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 https://engineering.mit.edu/engage/ask-an-engineer/whats-the-difference-between-a-motor-and-an-engine/, so what's your point anyway? – walen Jun 14 '21 at 09:00
  • @walen Like it or not, and at risk of extended discussion, Aaron was stating that In Spanish, "motor" and "engine" translate to the same word: "motor". Neither strictly true, not clearly expressed, that's a distraction at best, or a "suggestion" about Spanish in general and Spanish astronautics as used by in "educated Spanish speakers" in particular. "Locomotora" cuts no more mustard. – Robbie Goodwin Jun 14 '21 at 12:13

4 Answers4

35

"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.

Organic Marble
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    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. – David Hammen Jun 11 '21 at 12:26
  • I also forgot to mention hybrids. – Organic Marble Jun 11 '21 at 12:27
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    You also didn't mention solar sails or Bussard ramjets, both of which escape the tyranny of the rocket equation by not carrying propellant. – David Hammen Jun 11 '21 at 12:38
  • 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? – Cadence Jun 11 '21 at 13:50
  • @Cadence sure, but that's not what the question is about. Shuttle had lots of electric motors. – Organic Marble Jun 11 '21 at 13:55
  • Forgot to mention up goer and "fire comes out here". – DrSheldon Jun 11 '21 at 16:05
  • @OrganicMarble For control surfaces and such one would probably call them actuators, right? Were there real electric "motors" e.g. for fans etc.? Edit: I see, Wikipedia uses "motor" for the arm "actuators". – Peter - Reinstate Monica Jun 11 '21 at 16:28
  • @Peter-ReinstateMonica shuttle had lots of electric motors - fans, pumps, latches for payloads, vent doors, joint drives in the robot arm, payload bay doors and latches, ET umbillical doors and latches, you name it. And yes, the hydraulic thingies that moved the control surfaces and the engines were "actuators". – Organic Marble Jun 11 '21 at 16:58
  • @Peter-ReinstateMonica A whole lot of oomph was required to move the Shuttle's aero surfaces when it was moving at very high speeds through a not-so-thin atmosphere. Electronically powered actuators were not up to the task. Many of the control systems were powered by hydraulics, with the hydraulic system being powered by auxiliary power units (APUs). Large jet airplanes oftentimes similarly use hydraulic systems powered by APUs to move their actuators. If you know what to listen for, you can hear the whine of an airliner's single point of failure APU powering up before main engine start. – David Hammen Jun 11 '21 at 21:41
  • The Shuttle's APUs were essential for flight, and because they might fail, the Shuttle had multiple APUs. Some did fail. The catalyst bed blowing out was a typical culprit. Monopropellant hydrazine is one of the nastiest chemicals known to humankind. – David Hammen Jun 11 '21 at 21:43
  • There was a fascinating APU failure on STS-79 caused by by a bad wiring harness plus a bad sensor. Nothing like the dual APU explosion and fire on STS-9 though. – Organic Marble Jun 11 '21 at 21:44
  • I've always/often referred to pressure-fed hypergol "engines" as motors--I guess in my head the lack of moving parts is enough to qualify, say, an Aerobee or its AJ-10 derivatives as motors. Don't know where I picked that up. – Anton Hengst Jun 26 '21 at 04:34
  • @AntonHengst we called the shuttle version of the AJ-10 the OMS engine https://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-oms.html#sts-oms – Organic Marble Jun 26 '21 at 12:28
26

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"

Russell Borogove
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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.

SE - stop firing the good guys
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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."

uhoh
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