There’s been a lot of confusion centered around this (very cool) video of yesterday’s Falcon 9 launch so I wanted to jump in and clear some things up

Short thread…
#AvGeek #Falcon9 #SpaceX @elonmusk
For starters, I recommend checking out this earlier thread I wrote on vapor cones and expansion fans as a lot of relevant points are covered there, so I won’t circle back to everything

This should explain the *transonic* vapor collar close to the rocket
Okay, so the audio in the original video is misleading. This event happened at about 9km altitude, so any wave generated at that instant would reach the ground roughly 30sec later. Not sure how the audio was synced but if it’s real time from the ground, it’s coincidental
Notice you don’t hear this “boom” in the original SpaceX audio
Now, the flash of white you see *is* associated w the vehicle reaching supersonic speed & the resulting shock wave that forms at the nose

NASA & @DJSnM have noted this is related to upper cloud layer ice crystals interacting with the passing shock wave
➡️ nasa.gov/mission_pages/…
Note that if you look closely you can even see the shadow of the rocket in the thin layer of clouds

I think there’s still some speculation as to the exact mechanism at play here but the NASA explanation tracks IMO
It’s worth clarifying that the shock wave & sonic boom are not transient processes—i.e. it’s not some instantaneous event that happens when you reach Mach 1 like an explosion/blast that then goes away

It’s a wave system that remains attached to the vehicle & moves w/ it
📷NASA
As a stationary observer you just hear a single boom when the wave moves past you
Hope that helps, please share and drop this in the replies to some of the posts going around!
Oh and if you want to see something a little similar to a shock wave system forming on a vehicle accelerating beyond Mach 1, here’s a schlieren video we recently recorded of a wedge during startup of our Mach 7 facility

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More from @DrChrisCombs

22 Apr
It's been a tough semester for everybody, so to lighten the mood and provide some extra credit points I posted a meme challenge to my fluid mechanics class

The submissions were BRILLIANT

Sit back and enjoy this nerdy fluids meme thread. Our @UTSA students rock
#AcademicChatter
There were so many hilarious submissions (over 100!!) so I unfortunately can't highlight them all, but there are some that literally made me LOL
This one is a bit niche perhaps, but it was the perfect mix of educational, weird, and original that it made me laugh harder than perhaps all the rest
Read 18 tweets
20 Apr
Came across a ram/scramjet review paper from the Stanford Center for Turbulence Research & it is an absolute treasure trove of wonderful illustrations related to hypersonics

A quick thread of my favorites, but first I got distracted w this simulation of scramjet combustion
Here’s a closer look at that animation, with links to the paper and YouTube source

What you’re seeing is an overlay of temperature, density gradient (analogous to schlieren), and velocity in a scramjet cavity at Mach 6.5

🎥
📄 web.stanford.edu/group/ctr/ResB…
So back to the review paper, a bunch of these figures caught my attention

First up since we’re already on scramjets, a great illustration of a combined-cycle turboramjet + dual-mode scramjet

Gets you from takeoff to Mach 10!

📄 annualreviews.org/doi/abs/10.114…
Read 8 tweets
2 Apr
NASA just released some interesting preliminary data from their instrumentation onboard the #Mars2020 lander that delivered #Perseverance

Here’s a quick thread w some thoughts

[TLDR: Mars entry is HOT, but we made the heat shield too big (again)]

So to start, let’s turn back the clock to 2012. While you were watching The Walking Dead and the London Olympics, the good folks at NASA were measuring the aerothermal environment of Mars entry for the Mars Science Laboratory (MSL Curiosity) mission
They did this with an instrumentation suite called “MEDLI” which stands for Mars Entry, Descent, & Landing Instrumentation

This contained, among other things, a bunch of heat transfer, pressure, and temperature measurement devices in and around the MSL heat shield
Read 13 tweets
20 Feb
Oh my goodness, this is video of the turbofan on a Boeing 777-200–flight #UA328 out of Denver—which experienced a serious engine failure after take-off.

Remarkably sounds like no injuries in the air or on the ground #AvGeek
I mean, just look at these photos...
#AvGeek #ua328
Worth noting, twin-engine aircraft are designed to be able to fly safely with only one engine and pilots are trained for this (seems they did a phenomenal job here!)...but people and homes are NOT designed to withstand falling turbofan cowlings. Miraculous no one was hurt
Read 12 tweets
19 Feb
WOOWWWWW

Check out these images of #PerseveranceRover being dropped from the sky crane (left) and descent with chutes deployed via Mars Reconnaissance Orbiter! Incredible photos!

#Mars2020 #Perseverance #NASAPerseverance #NASA #Mars ImageImage
Folks, we just snapped of a photo of a spacecraft landing (possibly still at supersonic speed) on ANOTHER PLANET from a satellite also orbiting that planet. That is WILD

#NASAPerseverance #NASA #PerseveranceRover #Perseverance #Mars2020 #ScienceTwitter Image
I just can’t believe this image—am I right that we’re seeing some of the dust plume from the retro rockets? This is like a baroque painting Image
Read 5 tweets
18 Feb
NASA is LIVE with coverage of the Perseverance landing, happening around 2:55p US CT, this is the place to be

Percy about to hit the atmosphere at 3.5 miles/sec (5.5 km/s) and create some serious heat
#CountdownToMars #MarsPerseverance #Perseverance

pscp.tv/w/cv11hDFsWkVw…
The extreme velocity of atmospheric entry will heat the surface of the heat shield to nearly 2,400 F (1,300 C) due to intense shock heating and aerodynamic forces
It’s a tricky part of the mission for many reasons, but our ability to predict/model the physics of these atmospheric entry events is currently limited—it’s a “multi-physics” problem which makes it tough even for our best super-computers to handle
Read 5 tweets

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