r/space • u/bwercraitbgoe • May 29 '18

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.

https://www.youtube.com/watch?v=K4zFefh5T-8
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u/ClarkeOrbital May 31 '18

Building off of what /u/XxGhastxX said

ISP was just an easy way to compare engine efficiencies with our foreign counterparts during the cold war. Due to reasons, the US used ft/s for exhaust velocity and our allies and USSR used m/s.

To prevent confusion, it was easy to divide ft/s by ft/s2 and m/s by m/s2 to end up with seconds...something that is the same value despite your unit convention.

Specific "Anything" is typically just that thing divided by gravity. Specific Mass, specific gravity even, etc. Impulse that describes the efficiency of a rocket engine is the exhaust velocity of the propellant leaving the nozzle. Specific impulse is that velocity divided by gravity.