What is SLS?
Space Launch System — it’s really a Shuttle-derived vehicle, and despite thirty years of success with the Shuttle engines and solid rocket boosters, no one seems to be able to warm it over and make something that works. The Shuttle was technically reusable (apart from the 40% of the vehicles which were destroyed, one at launch, one during reentry) but cost north of $500 million per launch.
Meanwhile, the Falcon 9 boosters have been throwing mass into orbit and beyond, nearly 400 of those launches have been recovered by landing on unmanned drone ships or back near the launch sites, and SpaceX has a number that have done more than 20 launches each. Just lately I heard somewhere that the company expects to get 40 launches out of each one built.
space launch System. To replace shuttles that last flew in 2011. NASA hasn’t had much success since.