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While rockets are basically minimal fuel tanks*, engines are steel monsters that aught to both weight much and be expensive to manufacture**. Has there ever been tried discarding the depleted fuel tank == first stage while reusing the engine? If no - why?

* somewhere around here there was a reference that Musk's rockets are 2mm(perhaps 4, memory is failing me) steel tubes
** - another reference that I could probably find upon request: WW2 Nazi aerospace engineers designed fighter planes that are entirely discared except the engine; was too late to make the news however

A detail @ErinAnne just informed me of: I do not know the difference between a rocket engine and a nozzle. This is an entirely separate question but something to hold in mind if answering.

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    $\begingroup$ Since tanks are minimal and engines are heavy, the Atlas rockets did the reverse of what you are asking: ditching some of the engines during ascent and retaining the (at staging time still partially filled) tank. $\endgroup$
    – Dohn Joe
    Commented Jul 12 at 8:53
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    $\begingroup$ Do you mean "reuse in same flight" for a different stage of the ascent, or "separate, recover, reattach on the ground to a new fuel tank" ? $\endgroup$
    – MSalters
    Commented Jul 12 at 12:39
  • $\begingroup$ @MSalters As per the accepted answer, the answer is positive. Now come the questions of cooling and whatnot. Those must not be addressed in comments as per the website's excellent policies. I would obviously never mind other great answers. $\endgroup$
    – Vorac
    Commented Jul 12 at 20:18

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Historically, the Space Shuttle's architecture comes the closest to what you're describing. Propellant from its external tank was fed into its orbiter to power the three main engines. When this propellant was used up, the shuttle discarded the external propellant tank. The main engines, installed on the orbiter, were returned to Earth and were reused. Since the engines on the orbiter made it all the way to orbit and were designed to work well in a vacuum, they are similar to the engines on the second or third stage of a multi-stage rocket.

The orbital maneuvering engines on the Soviet Union's Buran shuttle were arguably even more "second stage" engines since the Energia rocket that helped to launch the Buran had more in common with a proper "first stage" than the US Space Shuttle's solid rocket boosters. Unfortunately, the Buran only flew once, so it never demonstrated the reusability of its orbiter's engines.

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  • $\begingroup$ Please provide more information on "It discarded its depleted external propellant tank and recovered the main engines which were installed on the orbiter." and it will constitute a complete answer to my question. Was the external tank fuel pumped into the shuttle(s) to burn? Or was/were the first stage engine/s discarded because of ablation or heat? $\endgroup$
    – Vorac
    Commented Jul 12 at 5:35
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    $\begingroup$ @Vorac Wikipedia article on the shuttle (link is to the subsection about the external tank) is well worth a read - it answers all your questions and the ones you're likely to have next. And yes, the external tank was just a tank, supplying the main engines which were part of the orbiter and used both at launch and in orbit, and in subsequent launches. There were additional boosters at launch as well $\endgroup$
    – Chris H
    Commented Jul 12 at 7:44
  • $\begingroup$ Fair enough. There's even a photo showing showing a total of zero nozzles on it's bottom. $\endgroup$
    – Vorac
    Commented Jul 12 at 9:53
  • $\begingroup$ I think this misses the (implicit) assumption about the in-flight reuse of the engines themselves. The 3 Space Shuttle Main Engines (SSME) aren't used as a "second stage", nor can they be used, because the external fuel tank has been discarded, and there's no internal fuel tank for the SSME's. However, there is an internal fuel tank for the 2 Orbital Maneuvering System (OMS) engines. $\endgroup$
    – MSalters
    Commented Jul 12 at 12:44
  • $\begingroup$ @MSalters STS was a SSTO with strapon boosters and a drop tank. No 2nd stage. $\endgroup$
    – Organic Marble
    Commented Jul 12 at 13:29
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There have been a few proposals (from ULA, for the Vulcan, and Arianespace with Adeline) to recover and reuse the engines for a first stage, but nobody's done it in practice.

Recovering the second stage engines is a lot more difficult: for most rockets, the second stage reaches orbit, so the engines would have to reenter the atmosphere from orbital speed, which requires a heavy heat shield.

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  • $\begingroup$ Rocket nozzles, heat load, heat shock, vacuum efficiency, pressure gradient - too much for me to unpack here. Try talking like to a kid. My main concern is "how do you re-attach the heavy expensive thing to the next 'gasoline' cell? And why would you chose not to?" $\endgroup$
    – Vorac
    Commented Jul 12 at 5:43
  • $\begingroup$ Without landing the heavy expensive thing undamaged within a predefined landing zone on solid ground, reuse is impossible. If the reliability of a reused engine is much smaller than a new engine, reuse does not make sense. $\endgroup$
    – Uwe
    Commented Jul 12 at 13:17
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    $\begingroup$ What do you mean "nobody's done it in practice"? Are you just going to pretend that Falcon doesn't exist? $\endgroup$
    – Lawnmower Man
    Commented Jul 12 at 14:43
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    $\begingroup$ Yes, nobody has built a system to use just the engines from a first stage by detaching them from the propellant tanks in flight. $\endgroup$
    – Hobbes
    Commented Jul 12 at 14:52
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    $\begingroup$ @LawnmowerMan considering that F9 lands the whole first stage, then yes, this answer is accurate. The question explicitly asks about discarding the tank, something Falcon definitely doesn’t do. $\endgroup$
    – fyrepenguin
    Commented Jul 12 at 19:06

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