|The Falcon Heavy uses a |
cluster of 27 engines
In the long run, though, the rockets that will become most common will simply be the most cost effective ones. Many future payloads—such as crew, fuel, and supplies—will not need to be launched on a single launch vehicle. If a smaller (or larger) rocket can deliver mass more efficiently than Falcon Heavy, then the former will prevail. Part of SpaceX's strategy to make this rocket cost effective, though, is to launch a dozen or so times per year—allowing the engine, which is also used on the smaller Falcon 9 rocket, to be mass-produced on an industrial scale.
Falcon Heavy's engine configuration has also been a source for skepticism. The challenge with using so many engines is igniting them at the same time and keeping them from failing during flight. Soviet engineers toiled without success to manage a cluster of 30 engines on their ill-fated moon rocket, the N1. I'm more confident that SpaceX can operate these engines, though, because they are properly testing testing them, and they plan to build tolerance for multiple engine failures into Falcon Heavy.