Most "rocket trains" are tandem multi-stage rockets, because the separation between stages of this type of rocket is easy to achieve, making it the preferred structure for launch vehicles. The strap-on rocket is to bundle a number of booster rockets evenly in pairs around the core-stage rocket. After the rocket is launched, the booster rocket works first, and then separates from the core-stage rocket. The biggest advantage of the strap-on rocket is that it can significantly shorten the length of the entire rocket, because the booster rocket does not occupy the length of the rocket alone, thus avoiding various difficulties in structural manufacturing and flight caused by the large slenderness ratio of the rocket. Since the bundled rocket does not increase the total length of the rocket, we also call this part of the rocket a half-stage rocket. For example, a two-stage rocket plus bundles is called a two-stage and a half rocket.
However, strap-on rockets are technically more difficult. Because the rocket separates between mid-flight stages, it must be absolutely safe and reliable, and the separation must not affect the work and attitude of the core-stage rocket. The bundled rocket uses lateral separation, which is much more technically complex than the longitudinal separation of the tandem rocket. my country's "Long March 2E" and "Long March 3B" carrier rockets are based on the original two-stage and three-stage rockets, with four booster rockets tied to the core stage respectively. Compared with unbundled rockets, their carrying capacity has increased by more than three times.
The first person to apply bundling technology to a rocket was Korolev, the famous aerospace chief designer of the former Soviet Union. In 1957, he used an intercontinental missile as the core stage, bundled four booster rockets around it, and successfully launched the world's first artificial earth satellite.
In addition to being widely used on launch vehicles, bundling technology is also used in some missile weapons.