Chris Combs Profile picture
@deehowardorg Endowed Asst. Prof. @UTSA Hypersonics, aerospace engineering, fluids, aerodynamics, propulsion, energy, lasers, imaging, sports science 🚀✈️💥🔥
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

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…
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
23 Nov 20
It’s November which means we’re getting into GRAD SCHOOL APPLICATION SEASON so I thought I’d take a break from my usually #AvGeek/rocket threads & talk some #AcademicChatter with my personal advice for those considering grad school. A thread...

#AcademicTwitter @AcademicChatter
All the usual caveats apply: I’m in engineering and while there are many different paths & perspectives I’ll be giving advice based on my personal experience. With that said hopefully it’s helpful to many of you w/ broad applicability outside of engineering
First: don’t pay to go to grad school. Many faculty have paid research assistantships (GRAs) and will provide tuition + stipend. You’ll still be underpaid but it’s really difficult to economically justify the benefit of grad school vs lost income if you pay your own way
Read 33 tweets
1 Sep 20

Okay folks, it’s been long enough. Time to finally address the biggest point of confusion on #AvGeek twitter: what exactly is going on with vapor cones?

Let’s just say much of what you’ve read is (probably) wrong. Wonder no more: a thread... 1/n
2/n We’ve all seen the photos & videos—whether it’s a jet flying over the beach or a rocket during its ascent—with the beautiful vapor cone (you also hear shock collar or even shock egg). Inevitably the exclamations follow: “sonic boom!” “Mach 1” & “breaking the sound barrier”
3/n And I totally understand. It’s a nuanced concept & a quick Google search brings up all kinds of misinformation. I could not find a single article that seemed to truly “get it”. So no shaming here—it’s debunking time
Read 25 tweets
11 Aug 20

It’s been too long since a good propulsion thread. With enticing potential for supersonic & hypersonic flight, pulsed & rotating detonation engines are an exciting possibility for future high-speed vehicles. Let’s discuss 1/n

🎥 credit @PurdueEngineers
So the top-line big number is the potential 25% theoretical efficiency improvement from pulsed & rotating detonation engine concepts (PDEs & RDEs, respectively) & ISP > 8000s for hydrogen fuel up to hypersonic Mach numbers, far exceeding other air-breathing and rocket engines 2/n
And this is in no way a new concept—detonation engines were theorized way back in the 19th century! With initial RDE experiments performed in the 1950s. But recent advances in experimental and computational methods have started to make this complex problem more tractable. 3/n
Read 26 tweets
25 Jul 20
A quick and funny story from the lab this week that I think does a nice job encapsulating the day-to-day in experimental work/#phdlife...

#AcademicChatter #AcademicTwitter @AcademicChatter #SaturdayThoughts
So we had a subsystem test planned for our hypersonic wind tunnel and as we’re going through the checklist our high-pressure air compressor for some reason won’t turn on. Grad students are baffled. Nothing has changed since the last test!
I ask (seriously) “is everything plugged in? Is everything turned on?” Because trust me I’ve spent hours debugging a problem in the lab only to find out something wasn’t plugged in. So frustrating when it happens. This is the type of insight you get from a PhD 😂
Read 5 tweets
22 Jul 20

With the recent visit from Comet NEOWISE I thought it would be fun to dive into the fascinating world of fluid dynamics relating to comets. Beware: some of these concepts are REALLY weird but I certainly enjoyed researching the topic. Here we go…(a thread)
DISCLAIMERS: I researched this post & while I study high-speed aero (including gas dynamics and some statistical mechanics), I don’t specialize in rarefied interplanetary flows.

Also, we’re dealing with some ridiculous numbers approximated to many orders of magnitude in spots.
First, what is a comet?

These are “cosmic snowballs” (HT to @NASA for that one), composed mostly of rock, dust, ice, frozen CO2, and a mix of other things. They orbit the sun in highly eccentric (long) elliptical orbits and range from < 1 km to 10s of km in diameter. Source:
Read 39 tweets
20 Jul 20
I noticed some chatter last week abt aerospace engineering careers (unfortunately quite politicized) & thought it might be a good idea for a short thread w some real talk about aero career paths.

This isn’t meant to be comprehensive but just some insight from my perspective...
First, jobs in the aero sector ARE in high demand. This is especially true here in San Antonio (really much of Texas) and I have regular conversations with local industry where they make it clear they can’t find enough qualified applicants.
These jobs are available at many different skill levels BUT it is of course true that technical training is required for any aero-related technical position. You unfortunately can’t just roll out of bed one day and start a job in aero.

But how much training?
Read 14 tweets
4 Jun 20
Ever heard of a Turbine-Based Combined Cycle (TBCC) engine design? It’s a pretty fun concept that can conceivably provide air-breathing propulsion from take-off to hypersonic speeds. Basically you combine a turbojet and a ramjet/scramjet—read this thread for more info... #AvGeek
First, why can’t you just use a turbojet to get to hypersonic speeds? There are many complications that limit the top speed and efficiency of a jet engine, most notably the elevated temperatures of high speed flight. But turbojets/fans are generally limited to Mach numbers ~2.5
What about rockets? Rockets will certainly do the trick, but there are the obvious issues that rockets are (usually) expendable, they are less efficient than air-breathing engines, you generally won’t use a rocket to fly subsonic, and there is probably more of a safety concern.
Read 11 tweets
26 May 20
Happy 10th Anniversary to the @Boeing X-51A Waverider! This collab. led by @AFResearchLab also included @DARPA & @NASA, producing a scramjet-powered #hypersonic test platform that first flew #OTD 10 yrs ago, flying at Mach 5 for over 2 minutes! That is 1 mile per second! #AvGeek
The X-51 was air-dropped from a B-52 & propelled by a rocket booster to Mach 4.5, then the booster would separate and the scramjet would kick in.

This is the classic scramjet figure you always see. If you’re moving fast enough you can compress with shocks not a turbine!
The program saw four unmanned test flights, two of which (1 & 4) were successful. The X-51 program concluded on a high note as the 4th flight set a record for the longest air-breathing hypersonic flight of 210 seconds at Mach 5.1! Flight ended when fuel was exhausted.
Read 6 tweets
27 Dec 19
And there it is—Russia is now officially commissioning their new hypersonic weapon for service. They claim a top speed of Mach 27. If anyone had doubts, this likely solidifies that hypersonics will be a key defense issue for the foreseeable future.…
This is what’s called a “boost-glide” system. The vehicle is “boosted” to high altitude and velocity on top of a rocket, where it then separates from the booster and performs a controlled, high-speed, low-altitude “glide” to the target. It’s like riding a roller coaster.
The problem for the US right now is that the glide portion is very difficult to track with long range radar (too low) and extremely difficult to intercept (too fast and maneuverable).
Read 8 tweets