Uploaded March 2018 | Updated September 2026, 2 weeks ago

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I came across a weird sentence in an "RS-25 Incredible Facts" document while searching for some information on the engines. In this video, we will see why I thought it was weird, and why the fact is phrased the way it is.

===== TOO LONG DIDN'T WATCH =====
► What they do:
The document divides the exit velocity of the gases coming out of the nozzle by the ambient speed of sound at sea level.
► Why it's misleading:
When you compare a velocity to a speed of sound, it is generally taken as a Mach number, which relates the local velocity to the local speed of sound. The local speed of sound in this case would be the speed of sound at the exit plane of the nozzle, which is much higher than the speed of sound at sea level standard.
► Why it's not incorrect:
They don't actually say anything about a Mach number in the document. They also don't mention what speed of sound they're talking about. It's vague enough to be correct.
► Why did I make a video about this?
I'm not quite sure.

===== NOTES =====
→ I actually emailed Aerojet Rocketdyne asking where this number came from when I first read this document. To their credit, they emailed back very quickly and asked for more information about my question, but by that time I had already figured out where it came from. I ended up forgetting to email them back, but it was nice that they responded so quickly.

===== RELEVANT LINKS =====
► RS-25 Incredible Facts Document
rocket.com/files/aerojet/documents/Capabilities/PDFs/RS-25_incred_facts_update_3-25-16_cropped.pdf

► VT Calculator
http://www.dept.aoe.vt.edu/~devenpor/aoe3114/calc.html

► CEA Online
cearun.grc.nasa.gov

===== THUMBNAIL CREDIT =====
NASA Image Gallery
images-assets.nasa.gov/image/9400164/9400164~orig.jpg
NASA ID: 9400164
"Space Shuttle Project"