While walking around my neighborhood in Austin. I saw this advertisement on the back of a U-Haul.
Now, I don’t know about you, but when I think of U-Haul,
I don’t think of aerodynamics. So I’m going to put their claims to the
test, but I’m not going to test this u-haul. I’m going to make this u-haul go hypersonic.
Dr Combs depending on how things are going, we've got
a mechanism we might even be able to just cut this loose and an empty test section and really see how
dynamically stable this guy is at MO seven Brian
This is the stupidest video I've ever made.
So, I am on my way to the University of Texas San Antonio, about 2 hours south of here.
They have an extremely rare piece of equipment, a hypersonic wind tunnel, managed by none other
than Dr. Chris Combs. Who you may know as the laser eyed hypersonics professor on twitter.
Dr. Combs is currently using it to test control mechanisms
for hypersonic vehicles traveling in the upper atmosphere. I’ll let the man himself
explain the difficulties in controlling a hypersonic vehicle in the upper atmosphere.
Dr. Combs: You get high
enough up in the altitude where, uh, control surfaces, like flap flaps and fins don't work
anymore. There's not enough drag, there's not enough aero force for that to happen.
Um, but you still need to be able to move around. And so usually we use control jets. And so like
the space shuttle, a lot of space vehicles, you see those using the little rcs that fire.
And when you're out in space, like properly in orbit,
you're not worried about the interaction of aerodynamics that's happening with that.
But there's some middle ground where you're high enough up in altitude where the flaps
still aren't effective, but you're low enough where you have arrow forces happening
and then suddenly you have this hypersonic airflow hitting your jet and it rips the jet apart.
and you lose effectiveness. And NASA has even seen that for certain tests. Um,
you, you know, you imagine from a Newton's laws perspective,
you fire a jet one way, you move the other way kind of thing.
That interaction gets so complex that sometimes you fire the jet, you move the same direction.
That’s a major problem for the new wave of hypersonic aircraft being developed. You may be
surprised to learn that ICBMS travel MUCH faster than any hypersonic vehicles being developed.
Intercontinental ballistic missiles are designed to exit earth's atmosphere, travel extremely
quickly at speeds of up to 7.8 km/s through space and then suddenly dive on their targets.
They move quickly, but their lofted trajectory gives their enemies more
time to detect and aim anti-missile defenses.
Hypersonic missiles are designed to cruise within the earth's atmosphere
at about half the speed of an ICBM. The atmosphere slows them down,
but their low trajectory means they suddenly appear on the horizon, previously being hidden
by the earth’s curve, giving their targets minimal time to detect and intercept them.
So, this hypersonic wind tunnel is an extremely useful tool for companies
and government agencies looking to develop hypersonic vehicles.
An extremely useful tool that we are going to use to test a vehicle,
designed to move your old furniture across the country, at mach 7.
Dr Combs: So it will go, they'll face this way cuz air flow
is this direction. I'm assuming you don't want this going Box seven in reverse, right? .
Um, so it'll get mounted facing like that. Nothing somewhere in the middle, right? Yeah. Hey,
maybe, you know, maybe that's a, that's a stretch goal if, depending on how much time we have,
Brian We have multiple of them.
Dr Combs: Yeah. So it's
gonna sit right here like that. Should we go ahead and unbox this guy? Yeah. Is it? Yeah.
Brian: Someone's gonna get a Yeah,
Dr Combs : We're gonna upset a collector
somewhere. I'm sorry. Whoever's out there watching this, that's a u-Haul model collector.
Brian : Um, it has the original price
on it of $7. I got it for 40. Dr Combs):
Yeah. I mean with inflation, who knows how much that's worth . Okay. So yeah, we're, it's
gonna sit, we're gonna take the flat plate out. Um, it's gonna be mounted like that on the sting.
Eli, you got a demo for us? For what kind of sting mount? We're probably talking about .
Eli : So this
is our variable angle of attack strut that, uh, still has a cow on it from a couple weeks
ago. Yeah. Um, but yeah, we'll need to mount cuz this has a quarter 20, uh, thread in the
back. So we need Dr Combs:
To Yeah. Eli ):
Figure out how to mount the U-haul. Dr Combs (02:07):
Yeah. So we'll, uh, gingerly remove that cow. I think
Brian:
Poor thing. Dr Combs:
Yeah, exactly. Brian:
That's nothing that the vets don't do to . Man. That's when I decided not
to be a vet that left a Mark House call and they're like, this is what your job is now
Dr Combs (02:23): Yeah .
The results for that cow are excellent. Ground breaking bovine technology.
So how does this wind tunnel get air to travel at mach 7? On one end is a big old vacuum chamber,
and on the other is a pressurized tube, the two sides are separated by a metal diaphragm
that is designed to pop at a target pressure differential. You would think that increasing
the pressure would increase the speed of air traveling between the sections, but it doesn’t.
Dr. Combs:
one other little nuance is people think that you can, if you increase the pressure,
that should increase the mach number. Right? That kind of conceptually makes
sense. Our Mach number is actually fixed by the geometry. It's just all about expansion ratio.
Brian: Mm-hmm.
Dr. Combs: Uh, so changing pressure just changes the reynolds
number and the, the mass flux and the equivalent altitude that you have. But the Mach number is 7.2
basically, regardless of what diaphragm that we use. Um, and that's fixed by the shape.
That shape is the shape of the nozzle, which works exactly like a rocket nozzle. Rocket nozzles work
by speeding exhaust gasses up from subsonic speeds to supersonic speeds in the converging section,
hitting mach 1 at the throat. At which point the physics do a 180 turn in rules.
Dr. Combs: And so what happens is, it's just
like traffic on the interstate. If you put I 35 down to one lane, everything stops. All right.
Everything's going slow. You add lanes 3, 4, 5, 6, everybody's able to speed up and spread out.
Air molecules do the same thing once they're going faster than mach one,
Brian my friend.Not,
not just bikes might have, uh, some arguments for them. Yeah.
Dr. Combs: .
Um, and so, uh, you're, and you're really, you could expand this out more and go higher mock
number, but then you get limited on the expansion end because the pressure's getting lower,
the temperature's getting lower, and we're actually already pushing the limits of like
reification with this. So the reason that we have insulation there on that piece of pipe
behind you is cuz we actually heat up the air. So these are big, uh, cables to that power electric
heaters that are wrapped around this pipe. And then we have, uh, insulation on it so you don't
burn yourself. And it also keeps the heat in, all right, we can heat this pipe up to 700 K, 800 f
Brian What's the purpose of heating it,
Dr. Combs: But so that the air doesn't freeze in the free
stream. Um, because even when we heat, even when we heat the free stream air basically equivalent
temperature, like if it was the, if you Brian:
The upper the expansion, it's, Dr. Combs:
Yeah, it's the expansion. So the energy has to come from somewhere you can't
cheat. Right. And so that energy is all going into the momentum and the velocity of the gas.
Um, but you lose the pressure, you lose the temperature. So it all gets sapped out.
Brian: What effect would that have on the test with?
Dr. Combs: Well, it wouldn't
Brian Literally freeze, I assume.
Dr. Combs:: Yeah. So what happens is,
um, really you scale it out with like Reynolds number, right? Basically. And so it's on the one
end, the pressure being low is more of an altitude consideration, but then you basically, you have
temperatures that aren't realistic, right? Right. And this is a problem in a lot of hypersonic
facilities and that we have too, because this is still a cold tunnel. Mm-hmm. , we
heat up to 700 K and that's still cold because the free stream equivalent air temperature that
we get is 60 K. Right. So minus hundreds of degrees, um, it's cryogenically cold really.
And that's just above the liquifaction point. All right. Um, so you're really just heating so
Brian): That, so you're actually
keeping it above the liquefaction. Dr. Combs:
Yeah, exactly. That's all you're trying to do liquid. Yeah. So it won't turn to,
it won't basically, you know, you won't get condensation, you won't start to, uh, have things
turned into liquid. And that's really all we're going for because to get a true flight enthalpy,
we'd have to heat up to 2000 K or so, uh, which you hit material limits for
Dr. Combs: stainless in your
driver tube pipe, things like that. So there are very few facilities that actually operate that
way. You have to go into a completely different kind of mechanism to run those types of tunnels
So, in summary, this machine essentially builds a pressure differential between the vacuum on one
side and a long pressurized tank on the other, to drive air through a converging-diverging
nozzle. This pressurized side is designed to be as long as possible because a longer tube
equates to a longer test. The test may occur in the blink of an eye, but with a high speed camera
we can capture a lot of data when a long column of air is traveling by, instead of a short one.
So, onto the test itself, but it wasn’t all smooth sailing. When we started to
pressurize the system some pressure gauge readings started to glitch out,
and this was a big problem, because those pressure gauges were needed to trip the camera trigger box.
Dr. Combs:
Without the triggering box we have to manually get the camera to record when
it’s supposed to. And we have basically a 1 second window of camera buffer,
so Eli is either going to fix the trigger box while in the like 40 minutes that the vacuum is
getting pulled down, or if he can’t fix that short, then uh, we are going to sit here and
then when we hear the diaphragm burst, smack the trigger button, and that SHOULD catch it.
Brian: Okay *doubt*
Dr. Combs: So that’s the status right now.
And so began a mad scramble by Eli to find the source of
the problem. We knew that the problem was being caused by a short circuit,
so it was a matter of unplugging everything until we could track down the faulty part.
With only 40 minutes before the test was due to start, I was clearly anxious.
BUT, with all hands on deck, and Eli’s understanding of the machine,
we did manage to find the culprit, and proceeded to smash it.
Eli: There we go we got some shrapnel.
Brian: That’s for the last hour and a half.
Eli: Freaking micro controller
Brian: So it was just the arduino board that was broken
Eli: Yeah,
it was running the code, but was not outputting a voltage
Brian: Okay, so you just replaced…
Eli: Yeah just popped in a new board. Worked fine!
With that we could finally get the tunnel up to pressure,
but first we had to lock the place down for safety.
Eli: Last thing
I am going to do is to let the surrounding labs know that we are going to pressurize,
so we have a laser bell that will ring to let the other labs
Brian: A laser bell?!
Now, it may not be obvious there, but when I heard laser bell. I assumed it was some
sort of over complicated, sci-fi, laser actuated bell, and was clearly confused.
“A laser bell?!”
Only to slowly realize that it was in fact just a normal bell to tell people a laser was in use.
“Ohhhhh it’s a to tell people there is a laser in..okay
And that a test is about to happen
“It’s a laser in use bell, cause lasers will blind the shit out of ya. Enough?”
I’m not sure doing this on camera stuff was the right call.
At this point, we just had to wait for the
pressure differential to get high enough to burst the diaphragm.
Dr. Combs “Now we just hope that it doesn’t
get to 300 and not burst” Eli:
“These diaphrams with this score depth can burst,
safely, to like 320 psi. I don’t know if I trust the model to survive 320 psi though”
*diaphram bursts” Brian:
“That was not the sound I was expecting,
I was expecting a bit more of a boom than like a prolonged fart”
Dr. Combs “It’s like a big metallic fart yeah”
Eli: “So I turned off the buffer tanks so we aren’t
still pushing air through there, uh if you want to turn off the right to file just so we don’t
save excess data” Brian:
“Wait wait, will you do this bit, I don’t want to lose the data”
Eli: “So I will stop the write to file and turn off the
vacuum pump, and then before we all go out there I am gonna poke my head out and do a safety check”
So the group of over excited nerds headed outside to review the high speed footage.
Okay there goes the release. So it’s flying now.
Oh yeah this is great we have the bow shock set up. This is perfect.
Oh what? Laugh
Yes!
Did we get it stable?!
Dr: Combs: We got it! Laugh. The u-haul is stable.
Eli: Oh my god!
Brian: Oh it’s starting to tip down now.
Oh That’s incredible.
Eli: Oh my god. It’s almost stable in flight
Brian: Yeah the
more it tips down the more it’s catching the hood.
Unfortunately the viewing window for the test is pretty small,
but in the words of Dr. Combs. I don’t know what we learned, but I’m excited.
The two bow shock waves result in some funky shock wave interaction. On our static test,
where we fixed the truck in place, the cab vibrated wildly, which honestly,
the fact this little plastic model held up to a hypersonic blast of air is impressive.
So, with all this data, we do get an idea of how a hypersonic u-haul may behave,
so I had our wonderful animator Eli create an artistic depiction of what
a full scale hypersonic u-haul test may look like.
U-Haul dropping from B-52. Rocket
engines firing in the back. Immediately nose diving and turning end over end. Exploding.
Science. It’s a wonderful thing. So, what did we learn? That Dr. Chris Combs is a fun guy. That
hypersonic wind tunnels are pretty neat. That I need to learn to speak more clearly on camera.
And that U-Hauls are absolutely not designed for hypersonic flight. But the thing I am most
curious about, what became of the U-Haul model? We may need to wait a year or two to find that out.
Dr Combs And then, like I said, hopefully, you know,
maybe we try a free flight test. Um, Brian:
What will actually happen? So you just let it go and it just flies
back. Is there, oh, is there like a mesh back here or something that will catch it?
Dr Combs (28:44): No, it's gonna go
Brian: (28:46): Into the
Dr Combs (28:46): Flying, flying into the vacuum tank. There's
a graveyard of models down in the bottom of that right now. And periodically we get an undergrad to
go fish stuff out. . It was funny when I, so this vacuum tank was custom. Uh, there's this
company in Ohio that makes vacuum tanks like this. And when, I called him to one of the conversations
we had where we were, you know, kind of specking this thing out. and I said, Hey, could we put some
extra access flanges on the side? And they're, oh yeah, sure, no problem. How big. And then I'm kind
of just thinking out loud on the phone. I was like, about how big do you think it would need
to be for a student to fit their arm inside? And they're like, what are you making man, ?
Brian:(30:16): I was like, who's our smallest grad
student? Yeah. Right into this thing. Brian:(30:19):
Yeah. I, I mean, I do, I do joke that it's big enough to fit grad students in
there that have misbehaved, , uhm, you know, sort of as a threat.
So, if we have learned anything you don’t want to misbehave in Dr. Combs program,
but you can still learn from him in the safety of your own home. I have uploaded our uncut tour of
the hypersonic wind tunnel to Nebula. It’s over 27 minutes long and full of fun moments that I
would have loved to have included in this video, but got cut down to package a video more suitable
to the YouTube algorithm. Relatively higher paced and not dwelling too long on any singular point.
You can get access to that now with a huge discounted price of just 2.50 a month,
using the link in the description. This money goes directly to our team. We do our best
to give back by uploading early and ad free, this one has been on Nebula for over 2 weeks,
and the next episode of Real Engineering is now on Nebula too, detailing the incredible engineering
behind Japan's new magnetic levitation trains. We also upload exclusive bonus videos like this uncut
interview, and upload original documentaries, like the Logistics of D-Day and Battle of Britain
series. But it’s not just my exclusive videos. It’s your other favorite YouTubers too. Like
Mustard, Practical Engineering, Neo and Wendover Productions, and for the price of 2.50 a month.
I have done my best to push the envelope of what YouTube can be with in-depth engineering videos,
but there are some things I need to sacrifice in order to succeed here. I used to explain
equations completely in older videos, variable by variable, but I could see in our analytics that I
would lose 50% of the audience during those moments. Now I just show you the equations
and allow viewers to pause if they have interest. Now we have spikes of attention in those moments.
On Nebula I don’t need to worry about things like that. It’s where
our biggest fans watch our videos. The ones that aren’t bored away by slower,
more in depth videos. That’s why I upload uncut interviews like this there,
because people actually want to watch them on Nebula. So if that sounds like you,
please sign up and support our channel for a massively discounted rate of just 2.50 a month.