Hacker News Reader

Comment by Robotbeat | Hacker News Reader

It should be noted the California bullet train is planned to run at 220mph for its fastest leg, so this is within spitting distance already.

If you pump the (vast majority of the) air out, no reason you couldn't get supersonic. That's much faster than any non-rocket-propelled train has ever gone.

The fastest train in service is around 270mph, more experimental maglevs have reached 375mph. Supersonic is 767mph, so over twice the fastest maglev.

View on HN

> If you pump the (vast majority of the) air out, no reason you couldn't get supersonic.

Doesn't this sound like a maintenance nightmare? I'm not clear on a couple of things. What happens when the vacuum fails? What happens to passengers if the seal fails on the vehicle, and then all the air is sucked into the vacuum? Maintaining long tunnels is by itself hard, but it seems like maintaining long tunnels that also need to maintain a vacuum would be that much more complex.

I don't see much discussion on this aspect of the hyperloop.

The maintainance of a vacuum tube this long and the consequences of a failure are a main theme in most hyperloop "debunking" videos on YouTube.

I'm guessing that any leak is likely to be small, and thus slow. And depending on how many vehicles are 'in flight' at once, and how easy it is to detect, they could be slowed down before hitting atmospheric pressure to avoid shocks or vehicle damage.

We already run vehicles in 0.14 atm environments. The issue of running these vehicles in an environment hostile to human life has been mitigated pretty well.

In regards to there being more than expected air in the tunnel - the vehicle would be slower, but not much else right?

Less air than expected inside the vehicle could be life-threatening for breathers. For humans we could solve this with an emergency oxygen hose at every seat like in planes. Pets and other oxygen dependant lifeforms wouldn't be protected by this system though. An alternative could be to have an electronic hatch at regular distances in the tunnel. So when a vehicle detects a massive oxygen leak inside the cabin, it could signal the nearby tunnel hatch to open. The whole tunnel, and thus also the vehicle [1] would fill with air pretty fast.

[1] The vehicle would have a large hole in order for oxygen levels to suddenly go from life-sustaining to not. Thus that same hole would serve to refill the vehicle with oxygen coming from the tunnel hatch. However there could be an additional hatch for the vehicle to guarantee airflow. This could maybe even mount directly to a tunnel hatch, so that outside air would come to the vehicle and skip the tunnel.

> What happens to passengers if the seal fails on the vehicle, and then all the air is sucked into the vacuum?

I fly a lot for work. Every week, someone gives me a demonstration of what to do when this happens.

Unfortunately, that won't work. Oxygen masks only work up to a certain altitude, which is roughly the altitude where airliners cruise. At pressures lower than that, even pure oxygen isn't enough to sustain you.

You don't die instantaneously when exposed to vacuum, although it is pretty quick. I wonder if it would be possible to repressurize the tube (by blowing a big hole in it?) within 15 seconds or so.

The trouble with blowing a hole in the tube somewhere to re-pressurise is that whatever is inside the hole is hit by a wall of air travelling somewhere around the speed of sound. There are a LOT of problems with hyperloop not limited to someone walking up to the tube and shooting it with a .50 Desert Eagle, earthquakes, and the expansion and contraction of the tube as the sun rises and sets.

It's definitely the most unlikely of Elon Musk's plans.

The hyperlink won't be a vacuum. It's just a low air pressure chamber. The plan has never been to maintain a vacuum since that's too difficult at such large scales.

Hyperloop is planned to run at roughly 0.1% of sea level air pressure. For the purposes of a discussion about human survival in the event of pressurization failure, that's effectively a vacuum.

Have you considered, that, in light of the fact that Elon wants to go to Mars, that research in having vehicles perform in a near-vacuum is a possible side-benefit for that goal?

My understanding is that labeling it a vacuum tube is a bit of a misnomer. It should be called a low-pressure environment or near-vacuum. Achieving that much reduction in air pressure will greatly reduce drag and allow for increased speeds. It doesn't need to be in a vacuum.

> The fastest train in service is around 270mph [...]

French TGV has broken the 350mph limit in non-commercial run.

Could "normal" wheeled train could go this fast reliably in a vacuum chamber? I suspect heat in the wheels and bearings would cause problems, especially without air cooling, but it would take much less energy to drive the train.

China seems like the place to build the first hyperloop. It would cost a lot less and they’ve already embraced HSR.

They’re increasing their trains to 248 mph.

https://www.nbcnews.com/mach/science/china-reclaim-title-wor...

Long routes like Beijing/Shanghai would greatly benefit from a hyperloop. They transport almost 1.5 billion people yearly on 12,000 miles of HSR, and they’re building another 10,000 miles.

I agree -- in fact I'd wager that California is likely to be a laggard in this race. Highway expansion is very slow, causing clogged freeways, and local commutes in populated areas like LA and San Fran are among the worst in the nation. BART is embarrassingly old and dilapidated. The new Bay Bridge was way over schedule and budget (and with serious quality issues)...Given the track record with "traditional" infrastructure, I just don't see California being a leader in this area. Maybe some of the tech will be conceived here, but implementation would be different.

Except this is the whole reason Musk developed the Hyperloop concept. He was annoyed at sitting through California traffic.

Ironically, it is California's crappy traditional infrastructure that birthed Hyperloop.

The idea is to build the first one sooner rather than later. Then everyone who wants it can build it.

Sort of like Bill Gates working with China to get the next generation of nuclear energy developed in China.

https://qz.com/627113/bill-gates-says-china-is-the-best-plac...

Sadly it seems that (broadly speaking) bribery is the only thing holding human achievement back, across our planet.

That’s exactly why China is doing that, and why China in the past years bought from SIEMENS the entire IP for TransRapid (the only high speed maglev system that actually shipped in production, and is used daily by many travellers)

If you want to go very fast you can basically only go in a straight line and you need a long time for accelerating and braking. G-Forces on passengers are fairly limited, typically to around 1m/s^2.

Much more than that (~4m/s^2) is experienced nominally by jet liner passengers.

Roller coasters do 50-60m/s^2.

I don't think this is a major problem. Just accelerate slower.

Source? From what I know 6g is the realm of anti-g suit, it seems quite strange to me that it is deemed normal in an attraction used also by children.

Humans are pretty tough, it turns out. And people often are overly conservative in these kinds of discussions.

http://rollercoaster.wikia.com/wiki/Highest_G-Force_on_a_Rol...

Maybe you don't want to have a transportation system that is as uncomfortable to ride as a rollercoaster. You might want to be able to transport frail elderly people and small children as well.

This is a test track and a competition. Nobody is claiming to build a passenger hyperloop with these accelerations except for fun.

Google's search result blurb says:

> Tower of Terror holds the record for most G-force on any roller coaster in the world, an intense 6.3G. This is nearly twice the amount that the average astronaut experiences when making their way through the atmosphere.

So world record for a roller coaster was ~60 m/s2, and that was presumably for a very short duration.

On a transit system you don't want roller coaster style restraint systems so g-forces have to be much much lower.

Some people might be into "ride a roller coaster from NYC to Boston" but that sounds like a niche market to me.

To clarify the referenced ride is in Gold Reef City in South Africa and is not the same as the Twilight Zone Tower of Terror at Disneyland

> Twilight Zone Tower of Terror at Disneyland

Which doesn't exist anymore. It's now a Guardians of the Galaxy ride (much better IMO).

In California. It still exists in Florida.

Just to add, a competition Jetsprint Boat can pull 7G's in the corners... And that's pretty intense.

https://www.knfilters.com/news/news.aspx?id=1444

It highly depends on a direction, see https://en.wikipedia.org/wiki/G-force#/media/File:Human_line...

6 _sustained_ gs would require a g-suit, but for very brief durations it's not bad. It would be very uncomfortable for passengers though.

That's sustained 6g. Momentary 6g is easy to deal with. But it's not like most people will be experiencing such accelerations in a transportation setting normally.

270mph is 120 m/s. Two minutes of acceleration.

Passenger jets take off at more like 4m/s^2.

"supersonic" relative to the sound speed at normal density I assume. With no air, there would be no sound!

Edit: corrected pressure to density

https://mobile.twitter.com/elonmusk/status/90197511779785113... @elonmusk: "No such thing as true vacuum"

BTW, speed of sound doesn't change (to first order) with changing pressure (edit: nor does it change (to first order) with density, assuming temperature is the same). Mostly effected by temperature.

Hyperloop uses a partial vacuum, of course. No true vacuum exists.

"No true vacuum exist."

How high is density of atoms in space? Could there be a 1mm x 1mm space without atoms?

My understanding is that there would still be quantum fluctuatons causing particles to pop in and out of existence.

Would this be protons? Can vacuum contain photons and still be a vacuum? Not sure :-)

https://en.wikipedia.org/wiki/Quantum_fluctuation#cite_note-...

According to Wikipedia:

"Plasma with a number density of less than one hydrogen atom per cubic metre and a temperature of millions of kelvins in the space between galaxies accounts for most of the baryonic (ordinary) matter in outer space; local concentrations have condensed into stars and galaxies." - https://en.wikipedia.org/wiki/Outer_space

I knew that intergalactic space could see densities of one atom per some human-relevant volume of space, but I had no idea that actually accounted for most of what there is!

The fastest train in service by the way: https://en.wikipedia.org/wiki/Shanghai_Maglev_Train

(Which is a version of the SIEMENS Transrapid Maglev system)

Reply on news.ycombinator.com