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After stage separation, why do the rockets not glide back to Earth with wings?

Is retro propulsion a better idea than gliding rockets back to 'Earth'? Take Energia-II as an example:

Energia-II core stage Energia-II booster in flyback configuration
Source: buran.ru

In Energia-II, all stages (including payload fairing) were planned to be recovered.

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    $\begingroup$ Why not parachute? $\endgroup$ – Voltage Spike Jan 2 at 20:39
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    $\begingroup$ Well, if it doesn't work in KSP... $\endgroup$ – Mazura Jan 3 at 0:27
  • $\begingroup$ I still don't understand why we don't fly most of the way up with a giant cargo plane first and launch from the plane. $\endgroup$ – takintoolong Jan 3 at 3:34
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    $\begingroup$ @takintoolong This has been discussing on Space.SE here and to some extent here. Basically the reason is that to reach orbit you need speed, not altitude. Air launching from a plane provides only a bit of the latter and essentially none of the former. This post summarizes the numbers quite nicely. $\endgroup$ – Alex Hajnal Jan 3 at 3:58
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    $\begingroup$ @takintoolong Obligatory XKCD article $\endgroup$ – Nelson Jan 3 at 6:02
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As with most things space, it all comes down to tradeoffs. The most efficient rocket is one that is purely expendable and has no mass that does not contribute to getting the payload towards orbit.

If the aim is to reuse the rocket, you need a mechanism to achieve control in the upper atmosphere, a method to control descent rate and a method to achieve a landing that does not destroy the hardware (or just tough hardware). And do all of this without increasing the risk of the launch itself failing due to hardware for the return.

Wings are a partial option, they work well for achieving a controllable descent rate, but they do not do much useful at high altitude and need additional supporting hardware (wheels/runway/airbags etc) to achieve a soft touchdown and a fair bit of flight control smarts.

Using the existing rocket engine is less efficient than using wings, but it is something that is already there on the rocket and gives you a system with extra capability in expendable mode.

So the final decision involves a lot of trade offs that often have much to do with seemingly minor details and less on perfection of a single aspect. For example SpaceX has ambitions on the moon and mars, and both of those require mastering powered descent rather than wings.

Related questions/answers SpaceX decision making Wings during ascent Plans to have wings on SRBs

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    $\begingroup$ You probably mean "descent rate" instead of "descent rate" $\endgroup$ – Kakturus Jan 2 at 14:32
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    $\begingroup$ @Kakturus you probably meant "decent rate" the second time. 😆 I had to read that a dozen times before I figured out what happened here. $\endgroup$ – MikeTheLiar Jan 2 at 15:37
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    $\begingroup$ @Kakturus That's how you send a human brain into an infinite loop... $\endgroup$ – Volker Siegel Jan 2 at 17:28
  • $\begingroup$ @Kakturus decent of you to point that out - thank you. $\endgroup$ – Criggie Jan 3 at 19:06
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Wings are heavy. They also add mass to the rocket's structure, because it is loaded horizontally when flying with wings rather than vertically as it is at launch. At the time Energia was developed, control systems were not developed enough for a vertically landing rocket. However, now that we have that ability (as Blue Origin and SpaceX have demonstrated), there is less of a weight penalty to carry a bit of extra fuel for a powered landing than to add wings.

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    $\begingroup$ Instead of carrying perhaps twice as much fuel, those proposed boosters have LOX engines with air intakes (as opposed to an extremely simple SRB - but those you can't turn on and off...), three sets of retractable wheels, and robust armatures for four variable wings, let alone those wings themselves. Their payload would have to be about the size of the zero painted on the side, which would make the boosters not worth their addition to the cross section. And it looks like you'd still need a computer to fly those tiny wings. $\endgroup$ – Mazura Jan 3 at 0:21
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    $\begingroup$ @Mazura thing is though, you don't need twice as much fuel. You're no longer accelerating the Second Stage, and you've already burned most of the weight of the first stage. $\endgroup$ – Zac Faragher Jan 3 at 4:24
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Wings won't work on the moon, and won't work nearly as well on mars. SpaceX is getting some practice in with the landings (note that recovering boosters is still in beta according to SpaceX) on the Falcon 9 before they build the Starship (Formerly known as the BFR) that will need to be able to land with no atmosphere.

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    $\begingroup$ The question is not about the Moon or Mars. $\endgroup$ – Organic Marble Jan 2 at 18:15
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    $\begingroup$ @OrganicMarble, the question is about why existing rockets are designed the way they are. Expections about travel to the moon and Mars are relevant to the design choices that have been made. $\endgroup$ – prl Jan 2 at 20:56
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    $\begingroup$ @OrganicMarble and it's something that has obviously taken a lot of practice to get right on earth, which is why they'd like some practice before attempting it on another planet or moon. $\endgroup$ – Sdarb Jan 2 at 22:44
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    $\begingroup$ @organic SpaceX wants to go to mars. On earth they can either invest in a dead end technology that will never take them to mars or on earth they can invest in a technology that will work both on earth and on mars. An organisation's long term objectives are relevant to what an organisation is doing today. The answerer may be wrong, j don't know, but they are answering the question $\endgroup$ – Richard Tingle Jan 3 at 0:13
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    $\begingroup$ @OrganicMarble considering there is currently only one orbital class rocket in the world capable of recovery and reuse, I think that that particular company's motivations are entirely relevant to the question. If the question was "Why have there never been any rockets that glide back" that would be a very different and much muddier question. $\endgroup$ – Sdarb Jan 3 at 2:10

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