N1 Rocket Hot Staging

Hot staging refers to a method of rocket propulsion in which the rocket's stages are ignited while still attached to each other. This allows for the rocket to achieve a higher specific impulse (a measure of the efficiency of a rocket's propulsion system) by using the exhaust from the lower stages to ignite the upper stages.
The N1 Rocket used a hot-staging system in its design, with multiple stages igniting while still attached to each other. This allowed for the rocket to achieve a higher specific impulse and more efficient propulsion, but unfortunately, this system was one of the reasons for the rocket's failure as it turned out to be quite complex and not reliable.

The N1 super heavy-lift launch vehicle intended to deliver payloads beyond low Earth orbit. The N1 was the Soviet counterpart to the US Saturn V and was intended to enable crewed travel to Earth's Moon and beyond. Its first stage remains the most powerful rocket stage ever built, but all of the four flown N1 Block A first stages failed. The N1 was designed to compete with the United States Apollo program to land a person on the Moon, using a similar lunar orbit rendezvous method.