The Schmidding SG 34 was a solid-fuelled rocket motor which was used by the Bachem Natter to provide extra thrust for launching.
Such "Starthilfe" (literally takeoff-help) rocket-based booster systems, both solid and liquid-fueled were already in use on some horizontal-takeoff German warplanes such as the Arado Ar 234B jet-powered recon-bomber to assist takeoff
The Natter had a single liquid-fuel engine, but this was not powerful enough to accelerate the Bachem Natter to the necessary 1000 km/h, so it needed two or four of the Schmidding SG-34 as an auxiliary power source.