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Have there been any strap-on boosters that are attached to the main body of a rocket with actual straps (like bands of metal or something similar), as the name implies? If not, what's the origin of the term "strap-on booster"?

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The Soyuz line all the way back to the R-7 has straps or cables connecting the boosters to the core. The straps, plus a "socket" on the core catching the front tip of the booster, are apparently how they're held on. One of the straps, near the base of the booster, is shown highlighted in blue here:

enter image description here

This RussianSpaceWeb page has a rendering of the boosters from the R-7 configuration showing the cables more clearly -- note that on the clockwise-from-below side of each booster the cable is split into a sort of "ring" letting the single cable from the neighboring booster pass through:

enter image description here

Soyuz aside, for any side-mounted booster on a large rocket, you want a secure method of transmitting upward force to the core. Merely strapping the booster to the core would allow slippage, which you definitely wouldn't want.

Small hobby rockets, of course, could probably get by with purely strapped-on boosters very easily.

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Titan 34D boosters had thin, strappy looking attachments (but they were probably actually struts vs. straps).

I always thought the upper attachments looked odd. They must have only been to resist lateral forces, with all the thrust force being transmitted through the aft ones.

enter image description here

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