[Gravityloss]

I wonder if aerospikes were popular as an idea in the J-2 engine ~Apollo era since USA only had gas generator engines (and expander RL-10).

You can't get to very high chamber pressures with those, and then maybe aerospike was a way to work around the limitations.

Then XLR-129 and SSME came along with staged combustion cycle and you can just have higher pressure engines. They can both run at sea level and still have a decent efficiency in vacuum.

The linear aerospike for X-33 was kind of a neat tech demo and fit in with the whole shape of the vehicle and composites, non-tubular tanks and large base area. Maybe too many new things at once in retrospect.