Elon Musk has been dropping hints about his Hyperloop idea. (We cannot call it a proposal because he has not actually proposed it yet.) There is a lot of curiosity about what it might be. Given Elon's history, the idea will sound audacious and yet will actually be workable.
Jacques Mattheij recently speculated on the topic. His proposal has the serious problem in that the friction of the air on the sides of the tunnel would lose way too much energy. But it got me thinking, and I have what may be a more realistic proposal.
First, imagine a tube that goes in a loop from Los Angeles to San Francisco. Let's put flaps on the walls. When they are open, air pressure can equalize. When they are closed, they don't leak much. Now let's put large, heavy objects going around and around the loop. For lack of a better name, let's call them plungers. The plungers can be floated and moved very efficiently with maglev technology. As each plunger approaches, the flaps open so that air can get pushed out, then closes so that it doesn't come back in. This is not an evacuated tube (Elon explicitly says that his technology isn't an evacuated tube), but results in a decent vacuum away from the shockwaves in front of each of plunger. That eliminates most of your friction losses. I don't know how low, but Elon claims low enough that solar panels on top of the device provide more than enough energy to keep it permanently going. I see no reason to disbelieve that solar panels could do that.
Now where do the people fit in? People go into vehicles that I'll call cars, even though they aren't really cars. These cars can be fired by a railgun to match speeds with the tube, and injected in front of a plunger. We can build the plunger with a space in its front that the car fits in. This space has air trapped in it by the shock wave and so the people can breathe. On its own that space would heat up due to the friction on the gas, but you can put a heat sink (eg a block of ice) in the car and keep it comfortable inside. Near the end of your journey the plunger ejects the car from this space on a course that launches it out of the tube while the plunger continues on its way. The car is then stopped with regenerative braking that recovers most of the launch energy, resulting in surprisingly little energy loss for taking the trip.
Now what would some of the specs be? Well, Elon claims 30 minutes from downtown Los Angeles to downtown San Francisco. According to google maps that's 382 miles, which is about 600 km. So the loop should be going around 1200 km/hour. If we put 12 plungers on the loop, and have plenty of vehicles, then you get in your vehicle and have a launch opportunity every 5 minutes. Increase the number of plungers, and the time to launch can be decreased while the capacity of the system can be increased. If we space the plungers a third of a km apart, we would have 3600 of them and could be launching every second into the system. It probably is more efficient if you instead make plungers larger so that cars carry more people. So instead of car think "bus". But after the initial system is built, you can later add new entrance/exit ramps and ramp up capacity. As Elon has promised, you would not need to reserve tickets - you'd pretty much arrive and then go.
Elon also claims that his system could store a lot of energy, enough to collect energy during the day and run off of it at night. The obvious place to store energy is in the kinetic energy of the plungers. How much energy are we talking? Well 1200 km/hour is a third of a km per second. That's 55.5 kJ of energy per kg of plunger. So 64,800 kg (about 143,000 pounds) of plunger is a megawatt-hour of power. Suppose that is one plunger. If you've got a thousand plungers, and each is storing a full megawatt-hour, you could permanently consume 50 megawatts of energy. You'd use up over half of it at night, then regain it during the day. Trips taken in the early morning commute might take 50% longer than during the evening, but it is doable. This gets better if we make plungers bigger, have a more efficient system, or have more plungers. I'm sure that Elon has thought about the ideal parameters. But if heavy plungers are good, well, put in enough metal for maglev to work and then add rock. You'd store a lot of energy.
Heck, the solar panel angle is fun but not really necessary. From the point of view of the electric power grid, it would be very, very good to have a large energy sink that can even out power fluctuations. Renewable energy sources often arrive at different times than we'd like to get power out. Sometimes, like with wind, we get very sharp spikes that we need to even out. If designed properly, the Hyperloop can absorb pretty much any power spike, and can bleed enough power out to be interesting. Therefore if the power utilities are smart then they should be willing to pay to add more plungers. Not because they care about the fact that they are improving peak capacity and reducing waiting time, but because they want to be able to store more power in it.
I'm sure that there are many improvements on this design that I have not thought of but which Elon has. I'm also sure that Elon has detailed blueprints that take this from a half-baked concept to something you can start to put cost estimates on. But this idea looks doable to me, and looks like it could - at least in principle - justify all of the claims that Elon has been making for the Hyperloop.
Finally I'd love to see this built. I'd love to see it built in California. But, unless someone like Elon pushes it, I'd be willing to bet that the Chinese get it first.
BTW for further discussion, see Hacker News