I understand atmospheric pressure has an impact on the exhaust shape, but why aren't the nozzles that feed onto the aerospike affected just like a traditional engine?
I.e. in their illustration on the page, the two nozzles next to the spike itself?
They are affected by air pressure, but it doesn’t actually hurt, it actually helps! because of the way an aerospike engine works it actually relies on that pressure change behaviour on the “outside” edge. The expansion of the exhaust on the atmospheric side of the aerospike is actually part of how it obtains its efficiency…. That expansion pushes the expanding exhaust plume back towards the nozzle wall in proportion to the normal expansion with outside pressure and that drives the physics that make aerospike nozzles work at all altitudes. It’s a clever feedback loop if you want a computer analogy, but it’s really just a clever arrangement that maximises system efficiency in that lovely “continuous” way that mechanical/physical systems work.
It seems that the nozzles feeding onto the aerospike are sufficiently small to operate safely at sea level air pressure. That kind of nozzle can operate safely at lower air pressure as well, but is inefficient. The aerospike gives you back your efficiency at lower atmospheric pressures, so you end up with something that's both safe and efficient at all altitudes, relative to a traditional bell nozzle.
At least that's my understanding - I just like to learn about rockets.
You may want to check out the video / text from everyday astronaut pointed to elsewhere in this post, which goes through a much deeper research and explanation on the topic.
The TLDR as I remember it, is physical nozzles on the aerospike don't work the same way as they do on a traditional rocket. It's more like like just an exit from the combustion chamber.
The appeal of an aerospike is by having lots of exhaust port aligned around a spike, is it creates a sort of virtual nozzle that is the right size for the atmospheric pressure. And as such acts sort of like a nozzle that changes shape and size as the rocket goes through the thinner and thinner atmosphere.
This in theory gains efficiency over a rocket that has a set shape for it's nozzle, and isn't always operating at optimal efficiency due to the size of the nozzle.
* Not an expert on these things, just going off of memory
I.e. in their illustration on the page, the two nozzles next to the spike itself?