XRS-2200 Linear Aerospike Engine Test fire at NASA Stennis Space Center (SSC)

A reusable lower stage with powered landing also had never been flown to orbit at that time. And in contrast to aerospike engines, which had been tested before on the ground [1], you can't do ground testing with rocket stage landings. I think SpaceX didn't try to develop aerospike nozzles because the advantages probably aren't that large compared to the mixed nozzle design they are currently using. They also reused the same ceramic heat shield material developed for the space shuttle instead of developing something new. Compare that to the cancelled "VentureStar": It would have used both linear aerospike engines and a new metallic thermal protection system (TPS) instead of a ceramic one. I remember an interview where Musk answered the question of why they aren't doing aerospikes or metallic heat shields etc, that there are many ways to skin a cat. They are only doing one thing that they think will work, which is not necessarily the best possible solution, but potentially faster or cheaper to develop. [1] ▶ this video

I love SpaceX, but I don't find them quite as miraculously impressive as some people. SpaceX is "just" doing what NASA did in their glory days: rapidly iterating, innovating, and then carrying those innovations all the way through to real world flight. That last part is key. In industry we call that "shipping." NASA didn't stop innovating per se. In the 1980s and 1990s they worked out their own version of vertical take-off and landing and actually test flew it: link Linear aerospike engines are pretty neat too: ▶ this video Problem is absolutely none of this stuff ever got close to operational. It kept getting cancelled in the prototype stage. If the DC-X had been developed into a fully reusable first stage, we'd have had something like Falcon 9 by the year 2000 and might be on Mars by now.

Lots of work done on aerospike engines. Google IYF. FYI, here's a video of a linear aerospike test: ▶ this video