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Many families of rockets use booster rockets, most notably the R-7 rockets that the Soviet and later Russians use. In comparison the American space-race era rockets mostly didn't.

For example, the Saturn V was an enormous rocket that might have been easier to make smaller with boosters instead.

What were the reasons they went with this approach?

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    $\begingroup$ Space Shuttle and SRB's. Titan and SRBs, Delta and Atlas families and SRBs. Unless you mean specifically liquid boosters, evidence seems contrary to your assertion. Sure Saturn V, but most other US Boosters did use side booster SRBs. $\endgroup$ – geoffc Jun 8 at 0:52
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    $\begingroup$ @geoffc The first sentence specifies "the American space-race era rockets". Did you read that far before commenting? For those that don't I'll add it to the title as well. $\endgroup$ – uhoh Jun 8 at 1:26
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    $\begingroup$ @geoffc Yes, those are included in "many families of rocets", but thanks for assuming I didn't know the Space Shuttle used SRBs. I did not say that NASA exclusively doesn't use boosters, but I specifically mentioned space-race era rockets. Such as the Saturn, Vanguard, Thor variations, early Atlas, Delta, and Titan. The fact that they started to use boosters later on is also a reason why I'm curious they didn't use them earlier on, especially since the Soviets were successfully using them already. $\endgroup$ – Sarke Jun 8 at 2:26
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    $\begingroup$ The Titan IIIC was first launched on 18 June 1965. There were a lot of thoughts about slapping those big UA120Xs on Saturn Vs too. $\endgroup$ – Anton Hengst Jun 8 at 5:10
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    $\begingroup$ @TooTea Yes, not the main first stage but strap-on SRBs or LRBs that assist the first stage. Quoting wikipedia: "extra boost at take-off, and/or increase the total payload that can be carried. It is attached to the side of a rocket." $\endgroup$ – Sarke Jun 8 at 8:01
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There isn't that much of a difference after all.

It's mostly a matter of perception because the R-7 family evolved into many different variants, so it feels like most Soviet launch systems use liquid boosters. Proton, for example, doesn't use any strap-on boosters.

The following "early ICBM" designs of the early 60's are actually pretty similar:

  • R-7 with a liquid booster/sustainer layout, where the first stage (liquid strap-ons) is ignited together with the second stage (sustainer core).
  • The original Atlas with its stage-and-a-half layout, where booster engines drop off and the core sustainer keeps burning.

Essentially the only difference here is that Atlas doesn't drop first-stage tankage, and that's because the light balloon tanks made that unnecessary.

Both designs were motivated mainly by concerns about reliably air-starting a bunch of big kerolox engines. It's way simpler to ignite everything on pad and then just drop stuff off, even though it's strictly less efficient as part of the second-stage tankage is already empty by the time the first stage burns out.

However, in the end, the R-7 family ended up spawning most of the Soviet space program, while the Atlas was just one of many competing designs among US launch systems.

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