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Why aren't 1.5 stage rockets, like the SM-65 Atlas, used as small sat launchers? It seems to me like that would be simple as you don't need as many tanks and the plumbing to connect the to the engines on each stage. Also recovering the "first stage" engines could be done the S.M.A.R.T. way, which seems easier than also recovering the tanks.

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    $\begingroup$ The disconnects for the engines you drop off are not "simple". $\endgroup$ May 10 '20 at 13:38
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    $\begingroup$ Mostly because, with current fuels, a 2.5 stage rocket is optimal for reaching Earth orbit. $\endgroup$
    – Mark
    May 13 '20 at 0:43
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A non-exhaustive list of reasons:

1.5-stage rockets need more plumbing, as not only do they need to feed propellant to the engines, they need to cut off that flow and separate the engines in flight without interfering with the sustainer engine. There's pipes, valves, couplings, and mechanical separation hardware that wouldn't exist in a staged version.

1.5-stage rockets carry a lot of tank and thrust structure mass into orbit. The SM-65 Atlas used balloon tanks to minimize the former, which had to be kept pressurized all the time, complicating handling. And much like a SSTO, any excess mass outside the engines that get dropped off has to be carried all the way to orbit. It all has to be optimized to a degree you'd only apply to an upper stage of a 2+ stage launcher.

The SM-65 Atlas was only built that way because air-starting engines was a big deal in the 1950s, it's not much of an issue today.

SMART reuse has its own list of problems. You need to add parachutes and reentry hardware, perform hazardous capture operations with large helicopters operating from ships out at sea where the engines come down, requalify the engines, and then build, integrate, and test another whole vehicle using them. ULA hasn't exactly been in a rush to implement it.

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