A Review of Aerospike Nozzles: Current Trends in Aerospace Applications

The abstract brings up SSTOs, but has there been anything in recent invention that will make them anything other than the white whale people have been chasing since forever?

There has been some progress on scramjet propulsion.

There's been progress on scramjets for cruise missions. For acceleration missions, like launchers, scramjets make no sense at all.

That really depends on how fast you can cruise. High speed scramjets above mach 15 will make space missions possible. The craft will be at the sufficient height and just enough speed, so that a rocket engine won't have to add too much deltav. Scramjets are still in their infancy. There are already developments on for variable-geometry multi-mode ramjets for this purpose.

PS: I have seen early-stage (but successfully tested) scramjets being developed for this purpose.

I don't think any of the considerations you mention there change my conclusion.

We have to ask: what exactly is a scramjet vehicle delivering? It's enabling the use of air instead of liquid oxygen. But how valuable is this? LOX is the second cheapest industrial liquid after water. The fuel part of a rocket propellant combination typically dominates the propellant cost. If a scramjet launcher uses more fuel (especially hydrogen) than a rocket vehicle would, it will end up increasing propellant cost per unit payload to orbit. It will also likely increase propellant volume per unit payload to orbit, especially if LH2 is used (LH2 being just 5% of the density of LOX).

All scramjet launchers need a rocket to reach stable orbit (since a scramjet cannot produce thrust at apogee to circularize above the atmosphere. So one can ask, what the tradeoff between the delta-V this rocket provides and that of the scramjets? From what I've heard, all such trade studies end up optimizing to 100% rocket and 0% scramjet.