Chris Combs Profile picture
May 20 20 tweets 9 min read
I've been getting a lot of questions about the aerodynamic skirt and grids on the Boeing Starliner OFT-2 configuration

I wanted to take some time and look into it, but after doing some digging I feel like I have a much better idea of what's going on

THREAD... Image
First, let's all get on the same page about what we are talking about. If it wasn't obvious from the first image, the questions revolve around a couple changes to the OML of the vehicle near Starliner: 1) an aerodynamic skirt; 2) a series of black "grids" Image
Regarding the need for the aero skirt, after some reading I'm pretty confident I can explain this

For the grid structure, it's a bit less clear but I will make some educated guesses

So let's start with the aero skirt...
This story came out in Aug. 2016 (HT @Cosmic_Andrew1 for sharing) describing some modifications made to the Starliner configuration

There were some enlightening quotes from Chris Ferguson referring to "non-linear aerodynamic loads" behind the vehicle

spaceflightnow.com/2016/08/02/boe… Image
My first thought was to try to dig up some wind tunnel data. Alas, it is unsurprisingly not published (at least not that I could find). It's highly likely we never see it.

But words like "non-linear loads" are often code for "shock waves"

NASA Technical Reports Server time...
I say this a lot but it is so true, predictable, and hilarious to me: everything we do in this field is already somewhere on DTIC and/or the NASA Technical Reports Server

So of course, there was a NASA report from *1962* studying this exact problem. Bingo
ntrs.nasa.gov/api/citations/… Image
The issue, which was more-or-less understood 60 years ago, is that if you use a "hammerhead" launch vehicle you can experience some very large unsteady loads due to shock waves at transonic speeds. The geometry makes a huge difference Image
The key word here is "hammerhead" and if you type something like "hammerhead launch vehicle" into Google scholar you can read papers on this all day

The appeal of this configuration is you can mount a payload with a larger diameter than your rocket

scholar.google.com/scholar?hl=en&… Image
This graphic illustrates the problem pretty well

At transonic speeds (M ~0.7-1.3) you get a shock structure that can form just downstream of your capsule. This shock interacts with the growing boundary layer/separated flow & oscillates rapidly. This produces large pressure loads Image
NASA looked at how geometry impacts the pressure loading in a series of tests ~30 years ago

The take-away: if you have a low L/D for your hammerhead (it's too stubby) you can get some very elevated pressure loads right behind your capsule. Original Starliner had L/D ~ 0.4 ImageImageImage
Does that L/D = 1.2 configuration look familiar? Also notice that most "hammerhead" vehicles have a longer fairing section? This is why.

Compare to the original Starliner mockups and revisit the old wind tunnel data, it's not surprising there were acoustic loading issues ImageImage
I'm sure NASA/ULA/Boeing had plenty of super smart people that were familiar w these design considerations. They saw the issue w the pressure loading & I imagine knew right away that the aero skirt would be a quick fix. Not aesthetically pleasing to some but that's why it's there Image
On to part 2, the "grid fin" looking feature

As I said, I'm not quite as confident on the reasoning but I have some guesses... Image
@torybruno answered this a while back but let's try to decipher the meaning

He mentions that the feature is there to manage the location of the shock attach point. That has me thinking...
One thing that happens with these oscillating shock waves on flat surfaces like this is that they have a way of moving around quite a bit and covering a lot of ground. That's not great. So it's possible this feature is there mainly to force the shock to stay in ~the same spot
This would also provide some symmetry to the shock structure, which would otherwise likely oscillate in different ways in the axial direction (as you go around the vehicle)
The feature could also in part be serving to trip the boundary layer from a laminar/transitional state to ensure it is fully turbulent. This would help manage flow separation (keep flow attached longer) and also make the shock interaction a bit more predictable
You also can see shocks that impinge further downstream (on the rocket body) and cause similar problems. The grid structure might be helping to manage how that feature behaves
web.archive.org/web/2018072410… Image
If i'm off-base on any of this, @torybruno please jump in to correct me! A couple of WAGs mixed in here...
Last thing, I found some other cool images that I didn't end up using in the thread, but just wanted to share anyway

Sources:
web.archive.org/web/2018072410…
citeseerx.ist.psu.edu/viewdoc/downlo…
arc.aiaa.org/doi/pdf/10.251… ImageImageImageImage

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More from @DrChrisCombs

May 19
So what’s Elon going to name his new political party?
IMO it's very likely he goes with something bordering on indecipherable that only has meaning to him and like 10 other people...
OTOH he might just name it "420-69-💩🍆" because he would find it hilarious
Read 4 tweets
Jan 15
Wow this is terrifying, the type of sonic boom you hope you never hear. That is a staggering pressure spike. [NSFW language]

Hoping everyone in the vicinity of the Tonga volcanic eruption is staying safe

shared from Facebook by @WhiteHaTterz
This is satellite imagery demonstrating the unbelievable scale of the Tonga wave propagation and impact, shared by @weatherdak

The Tonga volcano shockwave as heard from Fiji, nearly 1000 km away 😧

video from @portiajessene
Read 14 tweets
Jan 14
I've seen this getting traction & wanted to jump in w some thoughts in a brief thread

First, I have to validate Nicole's comments as it's unfortunately true that in many engineering curricula the opportunities to BUILD, EXPLORE, & EXPERIMENT are far too scare

What can we do?...
For starters, I wanted to provide some ideas for students that feel stuck and underwhelmed with their experience so far.

1) The first thing that comes to mind is to get involved in research. Most faculty conduct research, and many are happy to have UGs work in their labs
How do you get involved? Often you just need to ask. Look up the faculty in your department/college and find out what they're doing. If it looks interesting, send them an email or go to office hours. Tell them you want to volunteer in their lab. Cast a wide net
Read 16 tweets
Jan 13
I see this sentiment often in comments & it is entirely incorrect

Our computational models (particularly in hypersonics) simply don't cut it right now. Our ability to predict is poor. To fully resolve a real hypersonic flow with direct numerical simulation would take > 1B years
There are shortcuts you can take (called modelling) but developing models requires data and we don't have enough of it right now. To get the data to develop reliable models you need wind tunnels. And while some pretty good simulations are possible now, they are expensive and slow
In the meantime, while models are developed and computing power continues to increase, we're pretty reliant on wind tunnels and flight test. I don't see that changing anytime soon.
Read 4 tweets
Dec 15, 2021
For anyone wondering: Why do you tweet about @elonmusk so much?

Because like it or not, he’s the literal face of the aero industry right now.

That comes with responsibility, but he is a petty, immature child and it’s such a wasted opportunity. I’m sick of it. #ELON #SpaceX
He was the Time Person of the Year, he hosted SNL, he was in freaking Iron Man 2

He has a larger Twitter following than the President of the US, the NASA Admin, Tory, and Bezos combined

But instead of using that platform to get people excited about rockets he’s just a troll
As someone that works every day to get people excited about aero and diversify our mostly homogenous field, it is so utterly disappointing

I imagine that for every crypto bro that’s a space tech fan because of Elon, there’s a girl that sees confirmation space is a “boys club”
Read 5 tweets
Dec 14, 2021
This is really bad but unfortunately not surprising

And we all know it’s not just SpaceX

Without a massive cultural shift in our field, aero will continue to suffer from a woeful lack of diversity that holds back the entire industry. We need to do better
I’ve got absolutely no clue how this type of culture could be allowed to exist at a company as successful and forward-thinking as SpaceX
The fact that I’m losing followers for this “take” also makes it clear that we have a serious problem

Glad I’m weeding out some creeps today
Read 4 tweets

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