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What is the lowest thrust for a first stage rocket for a rocket that reached Earth orbit?

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    $\begingroup$ The shuttle's 6000 lbf OMS engines circularized the orbit. Does that count as "pushing into orbit"? How about the little verniers on the first stage of old Atlases? $\endgroup$ – Organic Marble Oct 28 at 14:47
  • $\begingroup$ I think the question is now what the OP intended, although I could be wrong here. There has only been one first stage engine to make it to orbit that I am aware of, the Space Shuttle main engines, but I think the first stage thrust for an orbital rocket is what is actually desired here. $\endgroup$ – PearsonArtPhoto Oct 28 at 15:09
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    $\begingroup$ Now I realized first stage engines do not actually reach orbit. $\endgroup$ – Joe Jobs Oct 28 at 15:34
  • $\begingroup$ Thrust is the force what the drive creates, measured in Newton (N). The question is surely not VLQ (imho it is a pretty okay one, at least now). $\endgroup$ – peterh - Reinstate Monica Oct 28 at 15:53
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    $\begingroup$ Is the intention of the question thrust in absolute numbers or trust to weight ratio? $\endgroup$ – lijat Oct 28 at 22:32
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The U.S. Vanguard rocket reached orbit three times with a first stage thrust of only 125 kN.

The first stage of the three-stage Vanguard Test vehicle was powered by a GE X-405 28,000 pound (~125,000 N) thrust liquid rocket engine.

Vanguard TV3 — NASA NSSDCA

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@2012campion's answer shows that this was not the lowest thrust

The smallest rocket to reach orbit is the Japanese SS520-5. It had a peak thrust of $185 kN$ according to the same web page:

Firing up its first stage, SS-520-5 shot up from its launch rail at 2:03:00 p.m. local time on Saturday with its aft fins sending the climbing rocket into a spin to provide stabilization as it climbed with a thrust outweighing the vehicle’s mass by a factor of seven. The first stage, standing 6.1 meters tall and holding 1,587 Kilograms of propellant, pushed the vehicle skyward with a peak thrust of 185 Kilonewtons (18,900 Kilogram-force), averaging at 143kN (14,600kgf) over the course of a 31.7-second burn.

I can't rule out that a slightly larger rocket nevertheless had a slightly lower thrust, but this must be close to the lowest.

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    $\begingroup$ That might be the smallest absolute thrust, but a launchpad TWR of 7 is far higher than most rockets have. $\endgroup$ – Mark Oct 29 at 3:08
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    $\begingroup$ @JoeJobs This answer came before the one about Vanguard $\endgroup$ – Steve Linton Oct 29 at 18:54
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Note: When this answer was written, the question read as follows:

What is the lowest first stage rocket engine thrust for a rocket that reached Earth orbit?

The first stage vernier engines on early Atlas boosters were 526 lbf (2.3 kN).

Reference: https://space.stackexchange.com/a/33998/6944

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    $\begingroup$ Given OP's recent history of questions, I assume they're asking for the lowest total-first-stage thrust rather than the lowest thrust of an individual engine. $\endgroup$ – Russell Borogove Oct 28 at 16:56
  • $\begingroup$ @RussellBorogove you could, of course, be right. The question has had an extensive edit history and the current version was not written by the OP. When I wrote the answer, the body said "What is the lowest first stage rocket engine thrust for a rocket that reached Earth orbit?" I'll edit for clarity. $\endgroup$ – Organic Marble Oct 28 at 17:24
  • $\begingroup$ Were they used for thrust or just for attitude adjustment? $\endgroup$ – Joe Jobs Oct 28 at 22:04
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    $\begingroup$ Rockets are always used for thrust! There's certainly a vertical component to the force from these verniers. $\endgroup$ – Organic Marble Oct 28 at 22:49
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    $\begingroup$ @JoeJobs: As you can see from the link, these engines fire at an angle, so some of their thrust contributes to upward / forward momentum. But the only reason they're there is for attitude control during ascent, not because their extra thrust is needed. (They are low down on the first stage, so don't make it to orbit, and thus weren't involved in docking. Also of course "early Atlas" means it predates the ISS by a long time, but other things have docked in Earth orbit, including test-runs for how Apollo's 2 modules would reconnect.) $\endgroup$ – Peter Cordes Oct 29 at 2:09

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