Reading an article about multistage rockets (not educated at all on the topic), and from the get go it seems to assume they are used/need to be used.
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$\begingroup$ You may find rather a plethora of information on any site dedicated to rocketry or space exploration. Also en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation $\endgroup$– Carl WitthoftCommented Aug 23, 2018 at 18:44
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2 Answers
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As the rocket is propelled upwards, it expends fuel. So there is no need to carry half empty fuel tanks. By splitting it up into separate stages, you can simply drop off unneeded mass.
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$\begingroup$ Searched specifically for this and it seems to be the case. Thanks. $\endgroup$ Commented Aug 23, 2018 at 1:16
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$\begingroup$ ...especially that the mass that reaches the orbit is of order of 5% or less of the launch mass. It's pretty hard to make a rocket that would weigh less than 5% of the fuel it carries, never mind fitting any meaningful payload in that! $\endgroup$– SF.Commented Aug 23, 2018 at 12:17
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1$\begingroup$ True, but short on details. You should at least link to the en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation $\endgroup$ Commented Aug 23, 2018 at 18:44
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$\begingroup$ Agree, this would get my +1 if it contained a short description of the rocket equation. $\endgroup$– MarkCommented Aug 23, 2018 at 19:51
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Another reason is that each motor stage can be designed for different purposes to match the flights characteristics.
So, a relatively short high thrust burn to escape gravity
And a longer or continuous burn to accelerate over a course / trajectory
Can save weight and complexity so adjustable nozzles are not always needed.
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$\begingroup$ Thrust to weight ratio does play a role, but I think another big factor would be the surrounding air pressure. Since a long nozzle at sea level would lead to an underexpanded exhaust and a short nozzle in vacuum would lead to an overexpanded exhaust, both of which would make the engine less efficient. $\endgroup$ Commented Aug 23, 2018 at 12:09
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$\begingroup$ It is easy to see where Solar Mike's answer can directly apply to motor selection for a LV designer -- many chemical engines will either be categorized into [at least] the two bins of "atmospheric operation" or "vacuum operation" $\endgroup$ Commented Aug 23, 2018 at 19:10