Comment by WJW
1 day ago
I don't think that's true, at least it wasn't conceptually radical. People have noticed the cost of "throwing away" the lower stages for ages, and many approaches have been thought of how not to do that. Take the (partly) renewable SSRBs of the space shuttle program for example, which came down by parachute. Landing a rocket on its tail is also quite an old idea. NASA had several demonstrators demonstrating the concept in flight.
SpaceX took a lot of ideas which had been individually proven before, and then put in the work to perfect them and integrate them in a production ready spacecraft. That is important work and good engineering, but not radical. An aerospike had literally never been flown to orbit at that time (I think still not), so it would have been a way worse fit for the SpaceX method of developing the Falcon 9.
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] https://youtube.com/watch?v=FcW9kUUTfxY