h a l f b a k e r yWhy not imagine it in a way that works?
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I suspect this isn't a great idea, so I'll keep it short to offer a
smaller target.
Man-carrying electric aeroplanes exist, but only as research
tools or for very short hops. An all-electric A380 is out of
the
question with current or foreseen battery technology.
Howevertheless, a hybrid
turbofan aircraft might be
feasible.
Replace the existing electrical engine-starting thingy with a
decent motor; a turbofan gets most of its thrust from the
bypass, so it doesn't matter much whether you spin it by
burning fuel or with an electric motor. Manageable
batteries
will only power the engine for a couple of minutes, but
that's
enough to get off the ground and a few thousand feet in the
air, at which point you open the taps and light the burners.
During the flight, the batteries are recharged by the on-
board
generators, allowing you to do your approach and landing
electrically as well.
Energy density of good batteries is about 1/30th that of jet
fuel, per kg. So, all other things being etc, batteries
equivalent to 1/5th of your fuel-weight would allow you to
fly electrically for 1/150th of the flight, or 4 minutes of a
long-haul flight.
The point? Well, you won't save any fuel and you have to lug
around a few hundred kilos of batteries. But it should allow
quieter take-offs and landings. This, in turn, might make
earlier (or later) operation from airports near houses, which
is
most of them.
Wickety Pink
https://www.ft.com/...9-bf5c-6eeb837566c5
The "hybrid" reference is some way down the article. [pertinax, Jun 22 2019]
[link]
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The space below is reserved for retrospective comments to
pre-empt the scathing critique which will doubtless come
from [8th]. |
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// it should allow quieter take-offs and landings. // |
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No. You still have to accelerate the same amount of reaction mass, by whatever means. High-bypass turbofans emit less noise because they move a bigger mass of air at a lower velocity. If you get rid of the high pitched noise from the fast-rotating core, it will be a bit quieter, but you'll still have the low frequency noise from the fan. |
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//accellerate// ha.... cat purrs contentedly as 8th
skulks into a corner wearing the dunce's cap. |
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I suspect that the //high pitched noise from the fast-rotating
core// has disproportionately high annoyance value. |
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Also please return that cap - it's the Intercalary's. |
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I'm pretty sure I saw hybrid planes described in the Financial
Times last week. WKTE? |
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//Energy density of good batteries is about 1/30th that of
jet fuel, per kg.// |
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We don't get all the energy of the fuel out as useful work.
Put motor efficiency at 90% Vs. Jet engine at 40% and the
sums look a little kinder, although getting motor output
to thrust is going to have losses that are included in the
jet figure. What might help things further is powering the
aircraft as a serial hybrid. A jet turbine generator tuned
for efficiency might outperform conventional jet
propulsion, only a small battery bank would be needed to
smooth the supply-demand mismatches in take-off, cruise
and descent. |
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//a turbofan gets most of its thrust from the bypass, so it
doesn't matter much whether you spin it by burning fuel
or with an electric motor.// |
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There is a slight snag. As far as I can work out, 1 747
engine at cruise is about 18MW of propulsive power.
25,000 horsepower or so. Even a 90% efficient electric
motor is going to generate 10% of that as heat. With the
best will in the world, it's going to be impossible to reject
that heat from something as dense as a motor at the low
temperatures the magnets can handle. Jet engines get
around it by being inherently high-temperature tolerant. |
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This is OK though. We don't need 2 huge engines. You
could easily have 20 smaller units, and take advantage of
propeller/prop-fan efficiency gains. Large numbers of
smaller units keeps diameter, and therefore tip speed
down. Supersonic tips are noisy and inefficient. Small
units could be built into the wing and used in creative
ways to increase lift through whole-wing blown flaps and
the like. Control surfaces could be eliminated and
functionally replaced by differential thrust. |
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Some thrust units could be closed off, eliminating drag
during cruise. Better yet, some energy harvesting could
be done during the descent phase when losing energy
quickly is a real advantage (see the Hawker Siddeley
Trident/C17). |
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// Jet engines get around it by being inherently high-
temperature tolerant.// That is a good point. On the other
hand, modern jet engines are the result of decades of
metallurgical tweaking. With a similar effort, we ought to be
able to make magnets that will take a bit of heat. Plus not all
electric motors rely on permanent magnets. |
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//magnets that will take a bit of heat. Plus not all
electric motors rely on permanent magnets.// |
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Depending on how hot you want to get, the Curie point
stops permanent magnets at ~1000K, ruling those. I had
to learn quite a lot about magnetism to
understand how it operates with temperature. I'm still
largely a moron on the subject, but it hit me pretty hard
that the magnetic field of the Earth isn't anything to do
with it being mostly Iron, it's above the Curie point by a
lot, so it's really about the movement of charge. Iron's got
bugger all to do with it, it could be copper or gold. I need
to have a word with whoever it was that taught me
physical geography. |
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Anyhow, you'd think that any charge would disipate in a
liquid metal, to the point where it moving material
wouldn't have any real bearing. As it happens, the
resistivity of stuff goes up a lot with temperature, usually
linearly, but it's all over the place. That has problems
with induction motors at high temperatures. You only
need a few hundred K to double the resistance in your
aircraft motor. |
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That's actually a serious issue, we know battery density
can't approach kerosene by a long shot, but I'm not sure
people realize that there's a power density limit, cooling
a rotor spinning at 50-200,000 rpm isn't trivial. You can
make the motor superconducting, perhaps, but then
there's a magnetic field density limit, I'm not sure how far
away that is from practical applications though. |
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