IEEE Spectrum January, 2016 - 53

MattEr of fact
In 1985, scientists at MIT used electromagnetic propulsion
to fire a Ping-Pong ball through an evacuated tube.

"Compressors won't go beyond a certain speed"
unless you expand the cross-sectional area of the
turbine's intake in comparison with its payload,
says Jeffrey Chin, an aerospace engineer at NASA
Glenn Research Center, in Cleveland. "Since you
can't shrink people, you must expand the tube."
Indeed, the most recent specs for the Hyperloop
require a tube more than twice as wide as the pod.
E v E n at a M E r E 1 , 2 0 0 - p l u s kilometers per hour
(760 miles per hour) the Hyperloop would still be
far faster than existing airliners-and faster yet in
effective speed because, like trains, it would connect cities at their centers. But also like trains,
the Hyperloop couldn't go just anywhere, so its
technological sweet spot would be middle-range
trips-such as the 560-km stretch from L.A. to
San Francisco.
Musk, who holds a degree in physics from the University of Pennsylvania, got the science essentially
right. You can indeed maintain a nearly friction-free
surface, either with air cushions-as the hovercraft
people will testify-or with magnets, as the maglev
people will affirm. Civil engineers can lay a very
straight track. Linear accelerators work.
And you need pull only a partial vacuum. HTT
is talking of a tube pressure below 100 pascals,
which is a thousandth of an atmosphere. It's not
onerous at all, insists Carl Brockmeyer, head of
business development at Oerlikon Leybold Vacuum, an outfit based in Cologne, Germany, that
is collaborating with HTT.
"I'm tempted to say it's easy, but better to just
say it's very achievable-this will not be the big
technology difficulty," says Brockmeyer. "We are
most likely suggesting a displacement pump," he
adds. "I'm not entitled to tell you how much equipment we're going to use, but it depends not only on
what's necessary to keep vacuum-we also need
a redundant system," he goes on. "Maybe we'll
install an extra pump in case we need to exchange
pumps or to not have to run them all at full speed."
To answer these and other questions, HTT plans
to begin constructing its 5-mile test track later
this year in Quay Valley, Calif., halfway between
L.A. and San Francisco. By the way, that's also
the site of a proposed solar-powered city-which
is convenient, given that Musk is now also making battery storage for solar homes and because
his specs call for powering the Hyperloop with
tube-mounted solar panels.

Other questions to be answered at the test facility include these:
How would you-pardon the expression-evacuate passengers in
an emergency? What happens to followers if a pod up ahead has to
stop short? How do you avoid pileups during the deceleration phase?
Restart the system once you've repressurized all or part of it? And
how well will civilians bear the stress, with high-def screens substituting for portholes to ward off claustrophobia and with g-forces
reminiscent of roller coasters?
About that last one, at least, some observers are squeamish. "It's not
transportation; it's a barf ride," comments Alon Levy, a Stockholmbased mathematician who blogs on transportation.
HTT is facing this and other aerodynamic problems, says Craig
Hodgetts, the chief architect of the project and a professor at the University of California, Los Angeles. Its evolving design is now rather
different from the original Musk vision, which he says is one reason
why HTT won't participate in the SpaceX competition.
"The artist's sketches that accompanied
Musk's proposal consider the capsule with
a kind of sloping nose akin to that on highspeed-rail trains," Hodgetts says. "That's
an appropriate configuration if you're not
in a tube, but in a tube the ideal shape is
symmetrical all the way around. So we
made quite a difference in terms of locating the capsule in the tube."
Because HTT will suspend its pod magnetically rather than pneumatically, it
will need a smaller frontal compressor
turbine than originally envisioned, he
says. That means the pod can get along
with lighter batteries and cooling systems.
That weight reduction should minimize
buffeting (and barfing). But work is still
going on to figure out how passengers
will cope with acceleration.
"My sense is there will be fine-tuning at the Quay Valley installation,"
Hodgetts concludes. "Some of our research indicates that at full speed
it'll pull 1.5 g's in a 15-mile turning radius-quite a lot. That's a big determinant in laying out a route. And it doesn't look like it'll be practical
to slow capsules down to any degree in order to maneuver, so there
has to be a very sophisticated analysis of speed, route, and sensation."

"It's not
transportation; it's
a barf ride,"
comments
Alon Levy, a
Stockholmbased mathematician
who blogs
on transportation

l E t ' s s aY t h E E r g o n o M I c I s s u E s can be solved-and that, with
enough backup systems, the safety challenges can, too. Now the
really big problem is, of course, money.
Hodgetts asserts that building the tube itself is actually not a hard
problem: "It's several tiers down from building a skyscraper 1,000 feet
high, where you have unpredictable wind forces, possible seismic
forces, shrinking concrete."
Other experts are a tad less sanguine. "This is a huge project that no one has ever done, and he [Hodgetts] is promisSPECTRUM.IEEE.ORG

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jan 2016

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Table of Contents for the Digital Edition of IEEE Spectrum January, 2016