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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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  • $\begingroup$ Theoretically with enough thrust and fuel you can get to space with a very low ISP, the reason ISP is so important for rockets is due to cost. The bigger the rocket and the more fuel it has the more expensive it is. $\endgroup$
    – Dakota Wharton
    Commented May 24 at 16:14
  • $\begingroup$ You can technically have an Isp of 100 $\endgroup$
    – Lawn Hollander Lawn
    Commented May 25 at 8:11
  • $\begingroup$ Wikipedia isn't all the engines that have been in space $\endgroup$
    – Lawn Hollander Lawn
    Commented May 25 at 8:11

2 Answers 2

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The only French orbital rocket ever flown was the Diamant.

In the first configuration (Diamant A), the first stage only had, according to Wikipedia and a document produced by the engine manufacturer, an Isp of 203s, which is the worst engine Isp I could find.

I think it's mainly because of the use of nitric acid and turpentine as liquid propellants and the use of a sort of Pressure-fed engine cycle that led to a low pressure in the combustion chamber (around 2.2mPa) and therefore low efficiency. It's also the only rocket to my knowledge to reach orbit with this kind of propellant.

The following up versions (Diamant B & BP4) Change the first stage propellants for a more common mix of UDMH and nitrogen tetroxide but still had only about 221s of Isp.

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  • $\begingroup$ That's amazing. Turpentine! $\endgroup$
    – Organic Marble
    Commented May 24 at 16:28
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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
    Commented Feb 4, 2021 at 21:14
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    $\begingroup$ 1958 was another country. $\endgroup$
    – Russell Borogove
    Commented Feb 4, 2021 at 21:15

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