Yeah, I had written a section about this but must have messed up my tags and Slashdot ate it.. Delta clipper highest achieved altitude: 1 kilometer. Falcon 9 first stage alone highest achieved altitude: 130km. Delta clipper furthest flown from the landing pad before landing: 300 meters. Falcon 9 first stage alone, furthest flown from the landing pad before landing: 300km. Delta clipper mass ratio, 2,5. Falcon 9 first stage alone, mass ratio 20 (and the boosters on the Falcon Heavy have a mass ratio of 30). And on and on and on. Not to mention that they're built utterly differently.

Aww, Slashdot ate my whole paragraph comparing Falcon 9 to the Delta Clipper, that was the best one.... must have messed up my tagging :(

Internet satellite thingy - almost identical to Teledesic

Teledesic: Launched on Pegasus rockets which cost your firstborn child. SpaceX: Launched on Falcon rockets which are cheaper than the Russians and Chinese even without reuse. Teledescic: 90s computer and communications tech (this was the era where playing the original Doom game took a high end computer and nerds envied those with ISDN connections). SpaceX: 10 iterations of Moore's Law later. Teledescic: Communcation sats have to be large objects with heavy hydrazine thrusters for stationkeeping. SpaceX: Much smaller satellites available (all the way down to cubesats), with a wide variety of ion thrusters for stationkeeping available.

Yeah, totally the same situation.

Hyperloop - first theorised by Robert Goddard nearly a century ago and a staple of SF for decades

Goddard and sci-fi: vaccuum tube. Hyperloop: tube full of thin air. Goddard and sci-fi: maglev. Hyperloop: ground-effect aerofoils. Compressor on each craft. Goddard and sci-fi: massive trains holding huge numbers of passengers. Hyperloop: small computer-timed trains to spread out the load on the track and thus reduce construction costs. Goddard and sci-fi: Trains implausibly deep underground. Hyperloop: built like a monorail. Goddard and sci-fi: tubes take the shortest route to their destination. Hyperloop: Trains go primarily over already-built and permitted infrastructure to reduce right of way and environmental costs / challenges.

Yeah, totally the same situation.

Falcon 9 - It can land vertically, like errr, the lunar module or the Delta Clipper

Tesla - Okay, they're quite nice but electric cars aren't exactly a new idea

Aww, you didn't give me an example to compare it to! Let's just go with the EV-1, since that was probably the most modern commercially-produced EV before Tesla EV-1, range 60 miles (older version) to 100 miles (newer version). Tesla Roadster, range 230 miles, and Model S, up to 300. EV-1, 0-60=8 seconds. Tesla Roadster and Model S Performance, 4 seconds. EV-1 production: about 1100. Tesla: produces that many cars in *1 1/2 weeks*. EV-1: Loved by owners but panned by critics. Tesla Model S: not only loved by owners but has been getting some of the highest ratings for any kind of car period.

Your "analogies" are akin to saying "So what if he won the Indy 500 - I raced my go-cart down the street the other day and beat a soap-box racer!"

"Cruising speed" is otherwise known as "terminal velocity" and is hundreds of meters per second. And I'll reiterate: the *point* is to go slow. Drag is a *good thing* on the way back down.

Other corrections: It's false that there's no part of the rocket that reliably faces a given angle - it doesn't tumble, it maintains an orientation generally between 0 and 15 degrees relative to the direction of travel. And the concept that bloody air is going to kill a pneumatic piston in a matter of minutes is the height of absurdity. .

Apart from actually launching a rocket to space and then having it descend and attempt to land, what's your proposed method to determine how much the fins have to move during a real-world descent and thus how much hydraulic fluid they'll consume? (beyond the simulations, which SpaceX uses extensively; they're invaluable but don't correspond 100% to real-world flight scenarios)

Yeah, it's kind of funny, but with this "failure" SpaceX will still end up recovering more of their first stage than is recovered for 99% of first stages launched in the history of man. Not like any of it will likely be of any use except for analysis and then scrap, but still...

I believe what they're saying is instead of any hydraulic system at all, which would be a simplification, no? And I have no idea what you mean by "efficiency leeching ramscoop", the whole point is to slow down.

I'm sure that SpaceX had reasons for not going with such a design. One that comes to mind for example would be during hover/low speeds - no ram air. But you don't need to be mean to the GP for suggesting that.

Yep. From 1300 meters per second to maybe 3 meters per second, with a CEP that would make military ballistics engineers salivate. So, so close.

It's funny, but there's really three analogies I use to explain the "whats" and "whys" of the hyperloop concept, and one of them is a roller coaster (the other two being the "super-high altitude airplane" analogy and the "building a pipeline" analogy).

Compare a roller coaster ride with going on a train. Are roller coasters built suchly that you have to wait half an hour or more between rides because they haul many hundreds of people at once? Do you have to spend 5 minutes boarding and later 5 minutes disembarking because of the scale? Does a pilot have to take the controls to maintain spacing and occasionally handle the risks of merging traffic and the like? And the tracks massively heavy and expensive to support these giant roller coaster cars?

No, of course not. Roller coasters are well optimized. Roller coaster cars are small, maybe two dozen or so riders at once. Because of this, they load and unload quickly. They're predominantly computer controlled with only a bit of human "central control" to send craft on their way and the like. They're all "expressways", no intersections, so all the computer has to do is make sure that it's not too close to the cars ahead of or behind it. Because the cars are small, the track can be made light, which makes it a lot cheaper.

Hyperloop implements the roller coaster paradigm to a tee.

That said, the current stage they're at, I wouldn't put people on it. They need to make sure that things are going to go as expected. Most of what they're doing is mature tech, but a few of the things, like the air-bearing skis, are going to need a lot of testing to prove their reliability. Right now they need a proof of concept and to iron out the basics. The next step up, where they have to prove the predicted reliability, repeatability, throughput, economics, maintenance etc, that would be more of the stage where an amusement part ride would be a possibility. Though I'd personally prefer that their next testing stage be built as something that, if it goes well, one could just expand into an actual hyperloop route. Maybe several dozen kilometers here - that should be enough room to accelerate up to top speed, coast a bit and deal with some curves and the like, then decelerate back down. And if it works out well, I have trouble picturing that some Vegas casino magnates wouldn't pay to link it up between them and LA. 6-ish billion dollars to enable millions of people in the LA area to pop over to Vegas in half an hour for $20 and unload a couple hundred dollars in the casinos? The amount of additional traffic they'd get would pay that off in a heartbeat.

Although... hmm, you know, they designed Hyperloop to limit passenger vertical acceleration to 1G and lateral acceleration to 0,5Gs, for reasons of passenger comfort - but not reasons of structural integrity or acceleration capability. So you know, even on actual routes, they actually could potentially let people purchase tickets to a... ahem... less G-force limited experience. ;) It'd require more car spacing, so the tickets would cost more, but when your base price is only $20... Plus, you'd get there a little faster. ;)

but why would it be lighter than a conventional train?

The main reason is that hyperloop isn't designed to achieve throughput by bundling everyone together into (proportionally) rarely launched trains, but by frequently launching smaller trains fully under computer control - spacing on the order of a few minutes instead of a half hour or so. There's only 28 passengers per pod. That launch rate is easier than many other computer controlled transportation systems, mind you, because there's no intersections - the only thing you could possibly hit is the car in front of you or the car behind you, and only by doing something really ridiculous (it's also more than enough time to stop if something goes wrong, as per the calculations, and the numbers look quite realistic).

but at the least it's going to have to support the tube plus the train cars itself

The tube isn't actually as heavy as you might think. Unfortunately I dont have my numbers on me right now, but it works out to not materially change the picture.

Is it simply that new materials and not having to share tracks with existing trains allow for different, lighter construction?

The real enabling technology for this is high launch rate, and the enabling technology for that is computer control with a simplified control problem (one way, don't hit the car ahead of or behind you). Yes, they'll use modern materials to try to keep things light, but that's not the key factor; the key factor is spreading out the load. It also adds a great deal of convenience for passengers - near constant departures and quick to get in and out of. The proposal really has more in common with a roller coaster ride than with a train: frequent small cars, quick loading and unloading, computer control maintaining spacing, etc.

But I'm concerned that it might be too optimistic

In the beginning I did too. But I've read the proposal and cross-referenced the numbers, and I'm sold. The cost figure is totally different from rail because it's really nothing like rail. For example, the track construction is far more like pipeline construction (really, it *is* a pipeline construction), so you need to compare to pipeline construction costs, not track construction costs. And it actually favors comparably with most types of pipeline, like oil pipeline, which are bogged down in environmental regulations and almost always lots of lawsuits, plus face high construction costs from having to generally go through wilderness areas, and a ton of other things. In most aspects oil pipelines have a far tougher time of it; the only thing that Hyperloop has harder is establishing and maintaining tolerances (but they have some very good proposed solutions for achieving the them quickly and affordably).

But since Musk intends to use the I-5 median on the long straight San Joaquin stretch, why will he use columns that entire distance?

I'm not sure what you're not understanding here. You have two seven-foot diameter pipes here, where are you picturing they should go if not "up"? I doubt anyone would approve of you eating up the entire median the whole way, if they'd even fit to begin with. If you're thinking about expanding the road, that takes land acquisition and all sorts of added hassle. Also, as straight and flat as the road seems to you, it's not so straight and flat to someone moving at 740 miles per hour. The columns vary in height and how they hold the pipe to compensate for the irregularities in the landscape.

And given that he will be using columns to save cost in urban areas, why not come right imto the city? Bringing it to the vicinity of Union Station would be a powerful selling point.

I do find that a valid criticism, something that one may well pass off to thinking in terms of airports - ignoring that airports are positioned on the outskirts of cities because they *have to be*, not because that's a good thing. That said, I'm sure the people permitting any actual implementation would pretty much force them to go into town - especially if they're also begging for funding. The highways get a bit twisty in town but there's some nice straight railways they could go over on both ends.

True. Although to be fair oil pipelines often have to be built in places where the ground is terrible - for example, permafrost, bogs, etc, vs. the median of an already-built highway.

Hyperloop does have deflection calculations worked into the proposal. The uniformity of the interior surface proposed to be accomplished by mounting a rotating buffing disk to a pipe-crawling robot (like those already in use for sewer and water pipe maintenance), having it grind and polish out the welds and any irregularities in the steel as it goes. Since pipe-crawling robots are on the market today, I don't see this as a serious hurdle. Certainly way cheaper to accomplish then fighting protracted legal battles over environmental regulations ;)

Either way, the people saying that Hyperloop should cost many dozens or hundreds of billions of dollars are clearly out of the ballpark. That's just not what long elevated steel pipes and associated hardware cost.

Hmm, another thought: if instead of air you maintained a sparse methane atmosphere, you could get a 140% the speed you could in air. More challenging to maintain such an environment, of course, since leaks into the pipeline would be air (unless the pipeline was surrounded by a thin methane sheath). At least it wouldn't be flammable - at such low partial pressures, there's no amount of air that could leak in that would lead to a flammable mixture.

Ammonia has similar performance to methane, but it's corrosive, so methane would probably be a better choice than ammonia. Neon also has similar performance to methane, but is way more expensive.

For the excellent performers, helium has a speed of sound 3x higher than air, and hydrogen 4x higher. But helium is rare and increasingly expensive, while hydrogen embrittles steel, leaks through almost anything, and leaks into the atmosphere have adverse consequences to the ozone layer. So I imagine both of those options are out.

If one scrubbed oxygen from the pipeline, with any sort of easily-oxidized material placed regularly in the pipeline, you should be able to get a couple percent boost in max speed, nitrogen has a slightly higher speed of sound than oxygen. But whether that would be worth it, probably not, unless the oxygen is problematic in other ways.

All of that said, I think the best option would be water vapor; at such low pressures, any water in the tube will automatically vaporize. Such a low partial pressure should pose no rust risk (that's actually very dry!), it's cheap, and most importantly, your vacuum pumps can simply discharge it and you can just feed more into the pipeline as needed, there's no need to filter it out or neutralize it first or anything. The more you approach a 100% water vapor atmosphere, the more you approach having 150% the max speed that air would give you. Instead of the 1190 kph/740 mph that the current Hyperloop design tops out at, you could potentially go upwards of 1790 kph/1110 mph. The downside is of course the increased pumping effort to try to keep the atmosphere as close to 100% water vapor.

If one could achieve a practical average 1000mph then that's 2 1/2 to 3 hours New York to Los Angeles, depending on how straight the line would be. For an express that stayed in the countryside, that is; each stop along the way would cost time. Hopefully the system would be smart enough to let passengers bundle together into "express" pods and let them bypass stations they don't want to stop at (although the lower in-town speeds would still be a hindrance)

Said who? It costs the same as a BMW 535i, yet only goes 265 miles. That's *not* revolutionary.

Given that the previous longest range before Tesla came around was in the ballpark of 40% that far and was produced by the hundreds, not the tens of thousands, and that the model S outperforms the BMW 535i, and has higher customer satisfaction ratings, and the whole teensy detail that no new US manufacturer that has anywhere near that order of sales for any type of car (let alone a radical new one) has been established since 1925... yes, that is damned impressive.

Using an engine designed by someone else

Where'd you get the impression that the Merlin was designed by someone else? Merlin is the most from-scratch engine design for an orbital launch vehicle in the US since the 1950s. It shares a few parts with older engines, such as the pintle injectors, but the vast majority of the engine is of brand-new design. The engine shares some similarities with work done at TRW, but it's not a TRW engine (doesn't even burn the same fuels). The reason that it's sometimes referred to as a descendent of work done at TRW is because TRW's former chief engineer is SpaceX's head of propulsion. He was tinkering on rocket engines in his garage that he felt he couldn't get support for at TRW when Musk picked him up; he proceeded to use his new position to create what became the Merlin series.

It's great engineering and no-nonsense construction from a company that hasn't (yet) become bloated by sucking on the DoD & NASA teats, , but that is *no* revolutionary.

Nice dodge: let me repeat: #Beating Ares 1 to the ISS for 2% of the development cost, on a rocket cheaper than the Russians and the Chinese, *without* the reuse that it was designed for": how the heck is that not bloody amazing and something to be celebrated? If it's so easy, then why hasn't everyone been doing it? And yes, people like you were all over the place here a few years ago saying they'll never get off the ground.

For it to be revolutionary, they'd have to come up with something *really* game changing, like... a fuel better than LH2/LOX which doesn't corrode everything it gets near

No, something that's "really game changing" is dramatic reductions in the price of getting to orbit, with serious potential for even more significant drops if reuse works out. That is bloody game changing if the term "game changing" has any meaning. The propellent mix is irrelevant. You can have the highest ISP fuel mix on earth and still cost a bloody fortune to get to orbit if it's not economical. The Russians beat the US for the longest time with much lower performance engines for that reason.

However, and sadly, getting a booster to land on a floating platform is "mere" engineering

Any more difficult challenge than that and you might as well just call it "magic". You don't get much harder in the rocketry world than something like that. Rocketry *is* engineering, and adding the word "mere" is just an insult.

The air on the outside is still going to *aggressively* want to rush in through any little crack.

Air is not magical. You can't put a pinprick in a partially evacuated tube and have it just suddenly equalize. Viscosity on the order of the size of small cracks highly limits the rate at which air can migrate in. A little crack or a leaky seal is simply not enough to overcome an air compressor.

To put it another way: the pressure differential here is approximately one atmosphere. Large trunk natural gas pipelines have a pressure differential of about 13 atmospheres. By your logic, a natural gas distribution infrastructure is utterly impossible because "the natural gas on the inside is still going to *aggressively* want to rush out through any little crack".

Let me see if I've got this straight: we can't build regular maglev trains because they're super-expensive (the engineering, construction and maintenance would be incredibly difficult), so... we'll just make it that much harder by wrapping a (partial) vacuum tube around it???

First off, let's make this clear. Hyperloop is not Maglev. In fact, the design document notes that they could use Maglev, but dismisses it as too expensive: "A viable technical solution is magnetic levitation; however the cost associated with material and construction is prohibitive." Hyperloop uses air bearings - skis operating in ground effect with the pipe.

Maglev trains are expensive for many reasons. The cost of having the track be able to provide forward propulsion however usually represents only the tiniest fraction thereof. First off, you have the reasons that rail is expensive, period (right of way costs, environmental reviews, and all of the other overhead). Then you have to have the entire route be able to lift up a multi-dozen to multi-hundred-tonne train. Not just propel, but actually hold it stably in the air, which is a far more difficult challenge for many reasons than propulsion - you either have to have an extremely precise computer-controlled fluctuating magnetic field in a train with hanging magnets, or you have to have the entire track be magnetized or be able to magnetize, in a manner that resists dynamic instability.

Hyperloop only involves propulsion, and the accelerators represent just a few percent of the length of the track. It's a tried and tested technology, use around the world, and their budget for it is in-line with industry norms. There are all sorts of trains today that use linear accelerators, almost all of which represent way more length of accelerator than Hyperloop needs. Examples include

                Airport Express in Beijing (opened 2008)
                AirTrain JFK in New York (opened 2003)
                Detroit People Mover in Detroit (using ICTS) opened 1987
                EverLine Rapid Transit System in Yongin (opened 2013)
                Kelana Jaya Line in Kuala Lumpur (opened 1998)
                Scarborough RT in Toronto (using UTDC's (predecessor) ICTS technology - opened 1985)
                UTDC ICTS test track in Millhaven, Ontario
                SkyTrain in Vancouver (Expo Line (using ITCS) opened 1985 and Millennium Line opened in 2002)
                Limtrain in Saitama (short-lived demonstration track, 1988)
                Nagahori Tsurumi-ryokuchi Line in Osaka (opened 1990)
                Toei edo Line in Tokyo (opened 2000)
                Kaigan Line in Kobe (opened 2001)
                Nanakuma Line in Fukuoka (opened 2005)
                Imazatosuji Line in Osaka (opened 2006)
                Green Line in Yokohama (opened 2008)
                Sendai Subway Tzai Line in Sendai, Japan (under construction, opening 2015)
                Line 4 of Guangzhou Metro in Guangzhou, China (opened 2005).[5]
                Line 5 of Guangzhou Metro in Guangzhou, China (opened 2009).
                Line 6 of Guangzhou Metro in Guangzhou, China (opened 2013).

If maglev was a good idea, it would have already been built.

1. This isn't maglev. It's a ground effect air vehicle inside a tube that simulates high altitude. Levitation works via air pressure.
2. Maglev has been built: Shanghai's Transrapid, Japan's Linimo, and there are two under construction, one in Beijing and one in Seoul.

That's not to say that there's no engineering challenges - there are. I think the biggest thing for the test track to prove will be the air bearings, to show that there's no high speed instability modes with them in a realistic tube. Most of the other stuff is pretty mature tech, however.