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Elon Musk's pea-shooter is full of air


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Elon Musk's pea-shooter is full of air, not ;-)

Elon Musk made a pile of cash out of Paypal. Now he runs battery car manufacturer Tesla Motors and space-trucking firm SpaceX, which delivers freight to the International Space Station. Later, he plans to set up a colony on Mars. Which might explain why he likes battery-powered cars. The atmosphere on Mars is very thin and only about one eighth of one percent of it is oxygen. Internal combustion engines would have to carry their own oxygen supply, just like they would on the moon. That’s also true for hydrogen fuel cells, which is why NASA’s six-wheeled moon-truck carries both oxygen and hydrogen for its fuel cell.

Now he's promoting an electric pea-shooter. At first glance, it looks like a space catapult called StarTram, which is a blow-pipe that would launch capsules into space. Only StarTram's tube has no air in it, but Musk's pea-shooter does. Just enough to make a big difference.

Air is useful. But it can get in the way. We can boost horsepower and thrust by pumping more air into an engine. But the top speed of pretty-much everything that moves on earth is limited by aerodynamic drag. Cycling down a hill, you can use it for speed control. Sit up straight to slow down. Hunch down to speed up.

Air resistance is so debilitating that some rockets have to throttle back their engines to get safely through the earth's atmosphere. A rocket must achieve at least twenty-nine thousand kilometres per hour to reach orbit. But it can’t go that fast until it leaves the atmosphere, otherwise friction would make it overheat. This wouldn't be such a problem on Mars. The Martian air is so thin rockets can go pedal to the metal all the way.

But we have to get there first. Right now, that’s not cheap, even if you fly with private space-truckers like Elon Musk.

Catapult launching could help. NASA’s Saturn V rocket could put a spacecraft the size of a truck (45 tonnes) on course for the moon. The Saturn V weighed 2,800 tonnes. Almost all of that weight was fuel and oxygen, most of which was burned up in the first hundred kilometres of the journey. A catapult would dramatically increase a spacecraft's range and payload.

A space catapult could be a scaled-up version of the steam catapults used on aircraft carriers. On the moon it could be built around a rocket sled. The moon has practically no air to create friction to heat up the spacecraft or the rocket-sled. Small rocket-sleds on earth have already gone faster than 8,600 km/h, fast enough for a spacecraft to escape the moon’s gravity. That’s too fast for wheels, so the rocket-sleds would probably use magnetic levitation, like super-high-speed trains.

Spacecraft launched from an earth-bound catapult would get hot enough to need specialised heat shielding. Unless we could get rid of the air. One possible solution is to fire the spacecraft through an empty tube. By “empty”, I mean the tube contains nothing. Except a vacuum. If there’s no air, there’s no friction, and if there’s no friction, the spacecraft won’t get hot. That’s one of the ideas behind the StarTram: A long empty tube that fires spacecraft off the top of a mountain. Inside the tube, the spacecraft is supported by magnetic levitation. Thrust comes from a linear electric motor, which is a series of electromagnets mounted along the tube. At the end of the tube the spacecraft punches through a “plasma window” and smacks into the atmosphere. It’s travelling so fast, at this point, that it gets into space before it gets too hot. Still, the higher the mountain, the thinner the air. And the thinner the air, the less the capsule heats up.

Which brings me to Elon Musk’s latest press release. Musk reckons California’s proposed high-speed train is a lemon. The second page of his 57-page press release says: “The train in question would be both slower, more expensive to operate (if unsubsidized) and less safe by two orders of magnitude than flying, so why would anyone use it?”

The rest of the tome outlines Musk’s “hyperloop”. This is an electric catapult that looks a bit like StarTram. Only, not as fast. And with brakes.

You pay money to sit in a 28-seat capsule with no steering wheel and no windows, just like a StarTram capsule. A linear motor shoots you along the tube, like a grain of rice zooming through a pea-shooter. The hyperloop won’t fire you into space. It will take you from Los Angeles to San Francisco, which is about 350 miles (564km). Right alongside it there’s another tube, so you can escape as quickly as you got there. Top speed is 760 mph (1,220 km/h).

There’s almost no air inside the tube. The internal air pressure is only one thousandth of the air pressure in Wellington. Or, if you’re planning on retiring to Mars, you could say: “The pressure of air in Hyperloop is about 1/6 the pressure of the atmosphere on Mars.” With almost no air, there’s almost no friction. The linear motors extend along only one percent of the tube’s length. Mostly the capsule just coasts along. Until it comes to the braking section, which is a linear motor working backwards.

Still, there’s enough air in the tube to support the capsule without maglev. The capsule has “air-skis”, which squirt air against the tube. This creates an air-cushion that supports the capsule, with a little help from aerodynamic lift.

Would Musk’s hyperloop work on Mars? We’ll have to await until the place has two cities with enough public transport enthusiasts to stump up the cash to build it. Musk reckons his earth-based system will cost about six billion US dollars.

Which raises an interesting question: What if we tilted the tube upward and increased the speed to, oh, let’s say 15,000 km/h? What would it cost? Could we use it to send Elon Musk and his battery-powered SUV to Mars?

If passengers were happy to tolerate about 2.5 g’s, which is less acceleration than a top-fuel dragster, the tube would be 330 kilometres long, a little more than half the length of the hyperloop. A rocket stage would boost the spacecraft up to 30,000 km/h and then drop away, like the second stage of a conventional three-stage rocket. The catapult eliminates the very expensive first stage.

StarTram Gen 2This is very similar to StarTram’s “Gen 1.5” system. StarTram haven’t estimated the cost of that. They reckon a “freight-only” system with a 100 kilometre tube would cost about $20 billion. If they're in the right ball-park, space-trucking would get a whole lot cheaper.

But this freight-only system accelerates capsules at 30g. That would be very hard on freight. Worse. To drive the forty-tonne capsule, the linear motors need to produce well over a hundred Gigawatts. The 330 km tube is easier on freight and it needs less power: about 23 GW (30 million hp) would drive the capsule plus a 120-tonne booster. That’s challenging. But it could be worth a try.

Musk’s hyperloop raises some interesting questions. He pointed out that it’s difficult to maintain a hard vacuum in such a large system. In contrast, a fleet of industrial vacuum pumps along the tube could overcome minor leaks. But then you have to deal aerodynamic issues. I have no idea if experience from an operational hyperloop would help the development of space catapults like the StarTram. The only way to find out is to go ahead and build the hyperloop and see what happens.

Humans have been firing capsules through tubes since whoever first invented the blow-pipe. Could an electric blowpipe be a better way to get into space?


Kevin Cudby is a Wellington-based Freelance Writer and Technologist who loves writing about cool new technology. Email him to discuss your communication requirements: hello {a}

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