I would say the main element of their success is not really in any specific close kept secrets - its in actually committing to reusable rockets and keep working until they had a working system.
While there are some really nice components and clever ideas (Merlin/Raptor engines & very good guidance tech) this all really has been doable for decades in less efficient form.
But so far no one other than Space X has been able to win against all the naysayers who were so sure only single use rockets are ever going to work, get enough funding to build a partial RLV & then operate it successfully as a business.
I don't think it depends on any single technology or a set of them only they have access to - rather that they have been able to persist and see it through, unlike all the other RLV projects that never got funding to go past the paper stage or very simply not viable (Space shuttle).
While there are some really nice components and clever ideas (Merlin/Raptor engines & very good guidance tech) this all really has been doable for decades in less efficient form.
But so far no one other than Space X has been able to win against all the naysayers who were so sure only single use rockets are ever going to work, get enough funding to build a partial RLV & then operate it successfully as a business.
I don't think it depends on any single technology or a set of them only they have access to - rather that they have been able to persist and see it through, unlike all the other RLV projects that never got funding to go past the paper stage or very simply not viable (Space shuttle).