
The challenges of building a hypersonic airliner - williamhpark
http://www.bbc.com/future/story/20150914-the-challenges-of-building-a-hypersonic-airliner
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gopalv
"the heat that accumulated at Mach 8 was up to 30% less than at Mach 5."

while the original paper says

"Actually the thermal flux during Mach 8 flight is shown to be only 25 to 35%
higher than at Mach 5. On top of this, the reduced flight times at Mach 8
yields a 20 to 25% reduction of the overall integrated heat load to the
fuselage with respect to the Mach 5 cruise."

The combined effect seems to be that the shorter the flight (& faster), the
lower the impact of the heating for people inside.

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Gravityloss
Active cooling: If you put the extra heat into fuel, you have a higher fuel
flow per second at the faster speed so you can indeed cope with the higher
heat flux and enjoy the lower total heat quantity.

Passive cooling: If you have to radiate the heat away , then the surfaces will
reach a steady state relatively quickly and you have to deal with the flux. If
you burn up in two minutes it doesn't help you that the flight would only last
two hours. You can't carry heavy heat sinks in an aircraft.

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matthewmcg
This article is somewhere between "needs significant copyediting" and
"probably generated by a bot." It's almost unreadable.

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bengali3
> (caption) The Soviet Tu-144 was the only only airliner to take passengers
> faster than the speed of sound

I would agree with your bot assessment.

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notahacker
I'm going with drunk/nonexistent copyeditor, but the Beeb did publish this
recently:
[http://www.bbc.co.uk/news/technology-34204052](http://www.bbc.co.uk/news/technology-34204052)

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cstross
Last time I flew trans-Atlantic -- last month -- in-flight wifi _with mid-
ocean coverage_ was available for $30. And we've got Facebook, SpaceX, et al
promising us all clusters of satellites delivering broadband everywhere on the
planet within 5-10 years.

As the energy cost of high speed travel goes up with the square of the
velocity, it'd be a whole lot cheaper to stick to subsonic travel in business
class -- lie-flat beds, in-flight showers, and broadband. How many people are
willing to pay $10,000+ to save 10-15 hours on the flight time from LHR-SYD or
LAX-Tokyo? Especially when the accommodation during hypersonic flight is going
to be cramped and spartan due to weight constraints (and it's _still_ going to
involve security theater at the gate and waiting hours for a scheduled
departure time to roll round)?

I can believe in Aerion's supersonic Mach 1.5 bizjet -- the core market is
CEOs wanting to bounce over to the opposite coast of the USA and back for a
meeting within the same working day -- but I just don't believe there's going
to be a mass market for hypersonic antipodeal travel on a commercial scheduled
service.

~~~
Retric
Energy costs don't necessarily increase as the square of velocity, at higher
speeds you can travel though less dense atmosphere which dramatically reduces
drag. Another major issue is afterburners are not efficient and have been used
for a wide range of high Mach aircraft. On a positive note, a faster aircraft
can do more trips per day which reduces depreciation and crew costs.

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podgib
"This thoroughbred airliner could fly from London to Sydney in 17 hours, three
minutes and 45 seconds; compared to around 22 hours on a Boeing 747."

This sounds wrong - the concorde's cruising speed was more than double that of
a 747. Unless they're taking the refueling time into account (Concorde would
need more refueling stops)

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notahacker
That's apparently the record time it achieved, as a charter flight. I'm sure
there were multiple stopovers built in, and another major factor is likely to
have been speed restrictions when flying overland.

On the grounds of comfort I'd have preferred the even longer journey in the
747...

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scrumper
Yes that would have been a fairly miserable trip. Concorde was pretty noisy,
cramped and, near the end of the flight as the fuel (which was also a coolant)
got used up, really quite hot. I did once fly on her from London to New York
and it was a pretty amazing trip, but for any flight much longer than that I'd
rather take first class on a 747 any day.

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porsupah
I admit, I was a little disappointed to find no mention of Skylon, even if
it's at an earlier stage of development than some of these other contenders.

The SABRE engine appears to be coming along well, with one of the largest
challenges - cooling the incoming air extremely rapidly - seemingly overcome.

[http://www.reactionengines.co.uk/sabre_howworks.html](http://www.reactionengines.co.uk/sabre_howworks.html)

~~~
rst
Lapcat is a project from the same company that's doing Skylon (Reaction
Engines, Ltd.); it's a different application of similar propulsion technology,
with an airframe redesigned to suit.

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douche
Why not go the other direction? Build a large, slow moving blimp/dirigible,
and offer overnight service with relatively comfortable accommodations?

If I could take off in Boston or New York in the evening, sleep through the
night, and wake up in London or Paris or Berlin, I think that would be an
improvement over the transatlantic flights that I have taken.

~~~
rconti
With a 7.5h flight time from JFK to LHR, plus a 5 hour time difference, that's
a 12.5h of effective travel time. So, you already DO take off at, say, 19:30
from JFK and land at 08:00 in Heathrow.

From SFO this is a 19.5h journey when you factor in the time difference, which
means to "wake up" at 08:00 at LHR, you leave at 12:30 from SFO.

In both cases, making the journey longer isn't going to make it particularly
more pleasant, even if the dirigible was more comfortable. And anyway, how
fast do you envision this craft flying? A 200mph dirigible sounds like a feat,
and it still would take AGES (with time difference, basically 24h from JFK to
LHR).

(yes, I'm aware we don't typically factor time change into these travel time
calculations, but I'm doing it for the sake of "go to sleep in X and arrive
rested and refreshed in Y")

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imglorp
The biggest challenge will the same thing that killed Concorde: finding
customers to pay super prices for super speed.

~~~
paulmd
Sonic booms are a not-insignificant problem as well. We've been pouring money
and research into it for _decades_ and we have barely made a dent in it.

The only known shape which produces no sonic boom (Busemann's Biplane) also
produces no lift. There is a known shape called the Sears-Haack body which
produces very little turbulence - basically it's like a very long pointed
football. The best we have been able to do is make airplanes resemble that
shape by having short stubby wings at the widest portion of the airflow body,
with an elongated snout and tail. It does reduce turbulence to a degree, but
again you are hurting your lift.

I think this may actually be an insoluble problem. You need to disturb the air
to produce lift, i.e. create a low-pressure zone over the wing, and a high-
pressure zone under the wing. My fear is that this is inextricably linked to
sonic booms, as we just seem to be exploring various locations within a space
defining the tradeoffs between lift and sonic booms.

We may be able to do it if we get the sonic boom _low enough_ , but this gets
back to the cost tradeoff. A rocket is certainly one possible solution within
this space - a no-lift body that follows a ballistic trajectory could probably
be constructed not to produce a sonic boom. However any solution on this end
runs right back into the cost problem that you noted. Rockets need a lot more
fuel and maintenance than a fixed-wing craft, and they're a lot less safe.

~~~
nether
Reducing the boom is achieved by damping the pressure N-wave from the oblique
shocks near the aircraft forebody. This is done in various ways but at this
time we're exploring blunting the nose to increase the local air temperature
which increases the local acoustic velocity and reduces the pressure jump
across the shock, weakening (quieting) it.

~~~
JorgeGT
That's very interesting! Got any paper? =)

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phkahler
How does water vapor last longer at 25km than at 33km? From 33km it has to
pass through the lower altitude which would mean more time than vapor that
started at 25.

~~~
paulmd
It probably precipitates more rapidly due to temperature or having more nuclei
to form droplets around. Once it's precipitated, it's no longer vapor, so it
doesn't spend any time at 25km.

Not sure which, or what particles there would be more of at 33km than 25km,
but that's my immediate guess.

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euroclydon
If the engine has no moving parts, how do you get a ramjet off the ground?

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paulmd
It needs to be assisted up to something like Mach 0.5 by a conventional engine
or a first-stage.

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Nemant
Want shorter travel time? No need for faster planes, just get rid of the
useless TSA and create efficient check-in systems. Most travel time would
decrease by about 15%-25%.

