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A comment was offered in this question, how technically soft landing works without air on the moon?, asking "what do you mean by 'rocket'." This brought to mind the question of what is a rocket. Does NASA or the spaceflight community in general have a specific definition for the term "rocket"?"

Cambridge Dictionary says

a cylindrical device containing material that explodes, sending the device through the air

Oxford says

1 A cylindrical projectile that can be propelled to a great height or distance by the combustion of its contents, used typically as a firework or signal. 1.2 An elongated rocket-propelled missile or spacecraft.

So the LM was not a rocket because it wasn't cylindrical. I understand the Descent Module had a retrorocket because it "fires in the opposite direction to the direction in which the vehicle is traveling, in order to slow it down." Was the Ascent Stage a rocket?

Was the Apollo service module a rocket? The CSM seems to fit the Oxford definition.

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    $\begingroup$ A question about definition of terms! It's what the internet thrives on. I'm tempted to vote to close as opinion based, but I just made this popcorn... $\endgroup$
    – Organic Marble
    Dec 13, 2019 at 14:47
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    $\begingroup$ Stephenson's Rocket was an early steam locomotive without a rocket engine unable to liftoff but called rocket. $\endgroup$
    – Uwe
    Dec 13, 2019 at 15:57
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    $\begingroup$ I missed that part! You have several questions in there. The answer, then, is No. NASA does not use such an imprecise term in technical material. $\endgroup$
    – Organic Marble
    Dec 13, 2019 at 19:40
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    $\begingroup$ The word "cylindrical" probably comes from the older days' usage of the rocket. This webpage describes etymology: originating from Italian ‘rocca’ "and the diminutive form of this, ‘rocchetta’, came to be used for a self-propelling cylinder in various mechanical devices. In French, this became ‘roquette’, and the word finally entered English as ‘rocket’ in the 17th century." Maybe the rocket-word has become too old and we should stop using it? $\endgroup$
    – Sergiy Lenzion
    Dec 13, 2019 at 20:39
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    $\begingroup$ This might be of low relevance, but as a "fun fact" side note, in Russian language there are no separate terms for a rocket, a launch vehicle and a missile (whether guided or not), they are all essentially defined as "ракета", which is the Russian word for "rocket". I suspect the same might be true for German "Rakete" (I.e. same word used for rocket, launch vehicle and missile). $\endgroup$
    – Sergiy Lenzion
    Dec 13, 2019 at 21:35

3 Answers 3

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The problem with the word "rocket" is that it is a colloquialism, subject to use by non-experts and not an exactly-defined term.

The term "Rocket" is used for any vehicle that's propelled by one or more rocket engines.

The Cambridge Dictionary is wrong. Rockets are typically cylindrical, but not universally. Famous counterexamples: N-1, V-2.

Merriam-Webster's definition:

1a : a firework consisting of a case partly filled with a combustible composition fastened to a guiding stick and propelled through the air by the rearward discharge of the gases liberated by combustion
b : a similar device used as an incendiary weapon or as a propelling unit (as for a lifesaving line)
2 : a jet engine that operates on the same principle as the firework rocket, consists essentially of a combustion chamber and an exhaust nozzle, carries either liquid or solid propellants which provide the fuel and oxygen needed for combustion and thus make the engine independent of the oxygen of the air, and is used especially for the propulsion of a missile (such as a bomb or shell) or a vehicle (such as an airplane)
3 : a rocket-propelled bomb, missile, projectile, or vehicle

You can see this is very broad.

NASA uses this definition too, depending on the audience for a particular article. Note that the linked article is aimed at children.

The word "rocket" can mean different things. Most people think of a tall, thin, round vehicle. They think of a rocket that launches into space. "Rocket" can mean a type of engine. The word also can mean a vehicle that uses that engine.

and yes, according to the linked article, NASA considers the Space Shuttle to be a rocket.

Wikipedia also agrees: a rocket is any vehicle powered by a rocket engine, and it includes things like rocket cars.

In the industry, the terms "launcher/launch vehicle" and "spacecraft" are used instead. These more accurate terms avoid the ambiguity of the word 'rocket'.

The important part of the definition of a 'rocket engine' is that it does not need to draw its oxidiser from the surrounding air, which means it can function in a vacuum. This is usually done by carrying the oxidiser in a tank, in addition to the propellant. There are edge cases, but that's a subject for another question.

M-W definition of a vehicle:

1 : a means of carrying or transporting something planes, trains, and other vehicles : such as
a : motor vehicle
b : a piece of mechanized equipment

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    $\begingroup$ Is every piece of hardware using a rocket engine a rocket? A rocket car is not a rocket, only a car on wheels propelled by a rocket engine. Should a vehicle propelled by a rocket engine able to fly to be called a rocket? $\endgroup$
    – Uwe
    Dec 13, 2019 at 14:43
  • $\begingroup$ @Hobbes How would you define a vehicle then? Is it a structurally connected pieces of hardware that are moving at the same speed? I mean, for example Saturn V vehicle at lift off was a rocket, which contained 5 more rockets within itself, like Matryoshka doll? I.e. every time a stage is dropped, the new vehicle (which is not exactly the same as it was before staging) is becoming the new rocket. Also where do you draw the line? Technically RCS thrusters are 'rocket engines'. Would you call CM reentry capsule a rocket then (technically it was slightly propelled by RSC thrusters during reentry) $\endgroup$
    – Sergiy Lenzion
    Dec 13, 2019 at 14:54
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    $\begingroup$ @Uwe Why don't you consider a rocket car to be a rocket? If you pointed it up, it wouldn't be a car, it would be a rocket (with an oddly shaped payload). I think Hobbes nails the definition that a rocket is anything powered by a rocket engine. $\endgroup$
    – Nuclear Hoagie
    Dec 13, 2019 at 15:12
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    $\begingroup$ The problem with the word "rocket" is that it is a COLLOQUIALISM, subject to use by non-experts and not an exactly-defined term. So you can either argue about it until the cows come home, or you can accept that it's an imprecise term. $\endgroup$
    – Hobbes
    Dec 13, 2019 at 15:50
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    $\begingroup$ Given that there can be such thing as a nuclear rocket, it isn't really correct to include stored oxidizer in the definition. I would think that a more accurate definition of a rocket engine would be along the lines of "a device which produces thrust as the reaction force resulting from the emission of stored propellants", regardless of the the nature of the propellants. $\endgroup$
    – Anthony X
    Dec 13, 2019 at 16:22
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Rocket is not a well defined term, but I'd say that

A rocket is a device whose primary purpose is to create thrust using the rocket principle (eject stored propellant to the rear).

A spacecraft has a different primary purpose and is usually not considered a rocket, even if equipped with retrorockets or soft landing rockets. Similarily, an aircraft equipped for rocket-assisted take-off is still an aircraft.

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Where I think people get hung up on the term "rocket" is that most rockets that people are familiar with are actually launch vehicles, designed to put payloads into orbit.

Rockets, however, are better defined as a

A vehicle with a self-oxidized engine burning fuel to produce direct thrust in a given direction.

By noting the engine is self-oxidized, we can exclude jet engines (which we can all agree are not rockets). Limiting it as these dictionaries did excludes other uses for rockets. For instance a rocket car uses a rocket to travel sideways.

As such, things like the Lunar Module qualify as rockets. It would be silly to classify them otherwise, as what else would you call the engine producing the thrust?

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    $\begingroup$ The LM was a spacecraft. No one in the biz would call it a rocket. $\endgroup$
    – Organic Marble
    Dec 13, 2019 at 19:42
  • $\begingroup$ Every engine producing a thrust from its chamber by the exhaust of a medium is rocket engine, but not every spacecraft with a 'rocket engine' would be typically referred to as a rocket. P.S. It is interesting to note that the mentioned launch vehicles wikipedia webpage in the 'general information section' is itself describing some launch vehicles as rockets: "United Launch Alliance manufactures and launches the Delta IV and Atlas V rockets." $\endgroup$
    – Sergiy Lenzion
    Dec 13, 2019 at 19:45
  • $\begingroup$ Using your definition a rocket car used for speed records on salt lakes is a rocket too. Sideways or horizontal is just a given direction, vertikal is another one. $\endgroup$
    – Uwe
    Dec 13, 2019 at 23:09

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