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Comment by VLM | Hacker News Reader

Historically this is the rocket equation, which led to multi-stage rockets. I'm not sure we'll see multi-stage electric cars anytime soon. Although with progress in autonomous vehicles, some kind of first stage or trailer could drop off and drive itself back home...

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At that point, you do not even need rocket style staging. Instead, when you are low, have a battery car meet you and dock while you are driving along the highway. Once you are recharged, the battery car can go back to the charging station, or to another customer.

Of course, I do not forsee this being a cost effective approach, even without the regularty hurdles of highway speed docking.

You could pull over just for docking. Then spend an hour towing the battery car, instead of waiting in one spot.

Or maybe something like the Bombardier Primove http://primove.bombardier.com/ inductive charging system could be viable for private cars on motorways? Apparently Bombardier has started to pitch it as something suitable for cars: http://primove.bombardier.com/media/news/news-detail-page/ar... . I can only imagine there would be some nasty transmission losses at various stages, though.

It seems this system requires the car to be stationary while charging but it would be like using cars as electric trollies. Hmmm I wonder if cars on hwys could have an automatically deployed arm for contacting a third rail or overhead wire?

s/towing/being towed by/

I think swapping batteries [0] or even whole cars would be viable before any kind of staged-power, especially for autonomous vehicles.

[0]: http://www.teslamotors.com/videos/battery-swap-event

I fail to see how the rocket equation applies for land-based vehicles.

It makes as much sense as calculating Delta-V of a car.

Adding more batteries increases the weight, thereby reducing fuel economy. So to get the intended effect, you have to add more batteries...

It's going to give you something like the equation for max velocity/altitude from a rocket, even if not precisely the same.

Fuel efficiency is dominated by other factors, so no it won't.

When every pound of fuel requires 5 pounds to lift it, things go one way. When every pound of fuel requires a tenth of a pound to push it, things go another way.

Not really, consider the extreme of a car shaped object with a volume of 99.9% battery, something of that shape can only go a certain distance X, where X is big, but moving a car shaped object X+1 is not possible with current (oh the pun) battery technology. One way to travel X+Y would be to put it on a flatbed electric tow truck with loaded range of Y, correct? In that case the tow truck is the first stage and the car is the second.

I am serious about the self driving 4 wheeled trailer full of batteries. Given the irrational claim that an electric car is useless to (all) Americans unless it can drive over 1000 miles per day, the best way to fight that irrational claim is to provide an equally irrational solution. So the proper way to fight an irrational thought experiment of needing to drive over existing electric car range, is provide the irrational solution of something like "mid-air refueling" but mid-highway instead of mid-air and electrical power instead of Jet-A. I've toyed with the idea of patenting it and publicizing it to make a political statement, but I generate good ideas a lot faster than I implement them, so ....

No need to own the self driving battery trailer, just pop up a smartphone app and your GPS talks to the closest charger's GPS and nature takes its course. With advanced enough communications it can drive in formation with a semi-autonomous car, tailgating is perfectly safe at 85 if your reflexes are faster than a couple nanoseconds and the existing wireless charging systems means the charger trailer doesn't need to actually touch your car.

Its amusing to think of a charge cart like this rolling around a parking lot as a public service to attract shoppers. No need to pull ultra high current charge cables to every lamp post or embed chargers in asphalt.

Running out of space to put more fuel is not related to the rocket equation.

With a battery-powered sedan, you could go much further without bothering to detach the trailer, because it wouldn't contribute much to the air resistance.

You need to be driving in near-vacuum if you want it to be about fuel pushing fuel.

You can make something very sleek, slow, ultra-high-distance and invoke the rocket equation... but it's not a sedan.

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