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Which engine with the lowest possible specific impulse has reached orbit propelling a given rocket? There is a page at Wikipedia that compares orbital engines but some engines are missing Isp value and the list may be incomplete. The lowest Isp listed so far is the Indian SLV-1 on Polar Satellite Launch Vehicle with an impulse of 253 seconds.

I am primarily interested in first stage engines but mentioning other stages is possible. I guess flying a low Isp engine is difficult since your rocket have to be bigger (needs more fuel because of inefficient engine).

Related question: What is the lowest first stage thrust for a launch reaching orbit?

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There may be some cases where something like the Atlas vernier rockets contribute a little bit to thrust and have a significantly lower Isp than the primary first-stage engines.

As for primary first-stage engines, the US's earliest orbital rockets in the years 1958-1960 used relatively inefficient engines.

Vanguard used a liquid rocket engine (kerosene/LOX) on the first stage with 248 s specific impulse. The third stage was a solid with 230 s Isp.

Juno I was an even poorer performer, at 235 s on the first stage (Hydyne/LOX).

Scout X-1 and its sibling Blue Scout II, which, although designed as sounding rockets, did manage orbital launches, used 236 s solid rocket first stages.

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    $\begingroup$ Wow! great numbers. $\endgroup$
    – Organic Marble
    Feb 4, 2021 at 21:14
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    $\begingroup$ 1958 was another country. $\endgroup$
    – Russell Borogove
    Feb 4, 2021 at 21:15

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