Yeah, 20000ft is roughly 6km. The air density is about 1/2 but from what I can determine that doesn't equal 1/2 air resistance, but something more like 90-95% air resistance of sea level.

Having said that, I haven't studied aerospace engineering, so I might be getting the details wrong.

There are definitely some minor advantages to building on a mountainside, but I don't think they outweigh logistical difficulties under normal circumstances.

The idea has a good bit of scifi (and probably scientific) history behind it though. I believe Heinlein used a railgun cargo launcher from the moon in Moon is a Harsh Mistress and a mountainside sled rocket in one of his earlier books.

Project Rho is a great resource for hard scifi and rocketry research for writers. http://www.projectrho.com/public_html/rocket/surfaceorbit.php is the link to a page that discusses maglev, railguns and rocketsleds.

Using a mountainside might help with structural integrity, but it's not likely to give much air resistance advantage if I'm reading the math correctly. The 5 highest peaks in the US are all in Alaska and and range from just under 5km to just over 6km. Commercial jets using air resistance/density for lift fly at about 10km and even at 38km aerodynamic lift still carries 98% of the weight of the plane (https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line)

Air density is halved at 5km compared to sea level, but air resistance doesn't diminish as quickly (due to it being multiplied by velocity squared and drag coefficient), and only becomes irrelevant (for short-term purposes) around 100km at the Karman Line.

If we had a 5km peak in Florida, the lack of logistical costs might make the benefits worth it, and if we could build on one of Equador's 5km peaks, then there's the further advantage of equatorial location for optimal rotational advantage (part of the reason we launch from South Florida)

Human terminal velocity is around 120mph when falling in breathable altitudes in a belly-down (flat) position. Pulling in your limbs will get you closer to 200mph, which is roughly what a peregrine falcon hits in its dive and in the ballpark of what a .30-06 round in freefall (ie, after being fired straight up) would reach. Professional speed skydivers fly head-down and reach 330mph. The higher you start, the lower air density and thus higher terminal velocity you get. Felix Baumgartner jumped from 128,000 feet and reached 840mph.

Source: https://en.m.wikipedia.org/wiki/Terminal_velocity

I sent this to my brother -- he's an ex-Air Force pilot, current Air Force pilot simulator trainer for C17s. He said this:

Big planes capable of carrying big weights but then not carrying big weights become a sports car. It looked like summer so the air density may have been 7,000 or 8,000 feet standard day. On rotation he kicks up a lot of dirt so he definitely used all the pavement.

C-17's do this for a living.

Why couldn't it be cheap to maintain a vacuum? It doesn't have to be anywhere near a total vacuum (which is mechanically impossible, BTW), just a relative vacuum. Any reduction in air density results in decreased air resistance, and increased top speed. I don't know the physics of it, but I'm guessing that even a 50% air density vacuum would result in a massive top speed increase. In a perfect vacuum, the theoretical top speed would be (acceleration force) x (track length). This 6,500 km/h number may be a limit imposed only by how good a vacuum they think can be reliably produced.

And about safety, obviously the track wouldn't be built in a bare tube that could dent or break. The tube could be whatever material, but then encased in something else and/or buried so nothing could fall on it/crash into it/etc. Before it went into operation, for consumer confidence (and even before production, for investor confidence) it would have to be demonstrated that the structure is strong enough to withstand any reasonably foreseeable event. I'd be mostly worried about earthquakes, personally, especially when crossing the Pacific.

Just like with aviation and normal trains, it will have its growing pains and regular disasters, but I bet it ends up being on par safety-wise with flight, and significantly cheaper, given the fuel savings.>> ^shole:

as said in the video, the cars don't move by sucking it through a straw, but by magnetic levitation
they draw the air out of the tubes and use magnets to speed it up, which is very efficient due to lack of air friction
there's a ton of problems with this though
it would need to be (relatively) airtight and stable throughout
that can't be too cheap, whatever the material
constant maintenance
what if there was an external accident that dents the tube, like a failing support structure, and the train-car later comes to that dent at this huge speed?
it would be worse than a plane coming apart midair
great for scifi, but i don't see it being reality any time soon

One thing I do wonder about, which is particularly pertinent to the result the mythbusters obtained, is compression.



They ultimately found that their two bullets struck the ground within a period shorter than the duration of a normal video frame. Assuming the bullet was successfully released level and at the same instant as their dropped bullet, would the slight increase in air density around the bullet, from compression when travelling at speed, explain the very slightly longer fall time? It appears that once you surpass 220mph, the compressive behaviors of gasses are no longer negligible.

>> ^djsunkid:
I like the screaming sound it makes just before! Any physicists in the crowd care to work out how fast it was going? Assume that the rocket's energy expenditure was totally linear and lasted exactly 6 seconds.
Hmm.. I wonder if you need to know how strong the propulsion was, and/or the mass of the rocket to figure that out, or if the time of ascent and descent is enough?


A rough formula for the fall of the rocket would be

v = g*t - 0,5*t*c_w*A*p*v^2

The stuff behind the g*t would be the air resistence. You should consider this at high speeds like 450 kmh.

So. If you solve this to get v you would get something like this

v = - m/(c_w*A*p*t) +- sqrt( (m/(c_w*A*p*t))^2 + (2*m*g)/(c_w*A*p) )

Please correct me if I'm wrong. I just wrote a test about dynamics and I didn't get the results back yet - so maybe I failed which means I suck.

c_w = 0.1 (air drag coefficient for a rocket)
A = 0.07 m^2 (estimated value for the area of the rocket if you would look directley into it)
p = 1.2 kg/m^3 (air density)
m = 50kg (I don't know how much it weighs... 50kg??? *shrug*)

t = 13s

If you calculate that you'll get 113 m/s or roughly 400 kmh. Hm thats very interesting, I would have thougt that the air resistence would be much higher. But 50 kmh less is still a bit.