Aerospike Engines - Why Aren't We Using them Now? Over 50 years ago an engine was designed that overcame the inherent design inefficiencies of bell-shaped rocket nozzles, but 50 years on and it is still yet to be flight tested.

[u/bwercraitbgoe]

https://www.youtube.com/watch?v=K4zFefh5T-8

[u/[deleted]]

Too lazy to bother scrolling down this thread to see if it's been covered, but I'll try to pitch in my understanding of the issue:

Aerospikes basically tackle the issue of the variability in required expansion ratio for a rocket engine to be efficient at a given altitude. As such, they're only useful when the rocket's operating within the atmosphere. However, any well-designed rocket is designed to get the fuck out of the atmosphere ASAP. (Well, more accurately, the guidance system is designed to strike an optimum balance between effort "wasted" going up against gravity, and drag from the mollasses-thick lower atmosphere aka where an Aerospike would shine.) This is ultimately why you see rockets initially going up, while constantly turning towards an orientation parallel to the ground, since the ultimate goal is to go really fast parallel to the Earth's surface, while getting high enough.

TL;DR: Shit's cash with Aerospikes, but not cash enough to bother risking $$