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Tue April 30, 2013
The Boomerang Rocket Ship: Shoot It Up, Back It Comes
What in heaven's name is happening here?
We're in McGregor, Texas, surrounded by farms (and the ghost of Johnny Cash). There, on a launch pad, is a 10-story rocket ready to take off. Its engines ignite. Up it goes, higher, then higher, then higher still, until at 820 feet, something weird happens. It stops moving, hovers in the sky for about 15 seconds as if wondering what to do next. The wind is blowing, (you can see the smoke blowing off in one direction, so there's definitely a breeze up there) but it doesn't wobble, doesn't tilt, it just hangs there like a candle in the sky, and then, suddenly, it switches direction, and looking down, you can see it heading unerringly for a pinpoint spot on the ground below. It's going back to the exact spot where it began, hits its mark, "legs extended" and turns itself off, like a shy student going back to his seat in a classroom.
Why Build A Rocket That Goes Nowhere?
It's such an odd idea, a rocket ship that behaves like an elevator, an elevator, by the way, that doesn't go as high as the one they've got in the Empire State Building. What's being accomplished here?
Well, first off, this is a "student" rocket. It's practicing. On its first launch last year, it went up just eight feet; more recently it reached 262 feet, now 820, so it's doing better and better.
Second, it's a bottom stage booster. It's not the part of the rocket ship that goes into orbit; it's the part that gets things off the ground. In a normal launch, it would detach from the payload (and upper parts that might be carrying people) and either fall into the ocean and have to be retrieved, or drop down softened by a parachute to the ground, where it would be fetched and repaired. This one, as you've just seen, doesn't hurt itself. It just goes back home.
That's huge. Because rockets are exceedingly expensive. SpaceX, the private company that is building this booster (which they call "The Grasshopper") isn't sharing numbers, but while scanning the YouTube comment section, I learned that rockets like these can cost "about 60 million dollars." That's a pricey gizmo to keep dropping and repairing. To build one that doesn't need fetching and that's ready to go again as soon as you need it, must be an enormous money-saver.
What About The Extra Fuel?
It takes more fuel to power the booster to the ground. Up untill now, boosters just fall for free. What does it cost for the extra rocket fuel? SpaceX isn't saying, but common sense says when the rocket comes down, most of that fuel has been burned, so it is much, much lighter. I couldn't find any numbers, but I did find this bit of accounting from a self-described rocket engineer in the comments:
To put it in perspective, the cost of one of these rockets is about $60,000,000, and the total fuel cost for one trip is about $200,000. That means the cost of fuel is 0.3% of the vehicle.
Let's say you spread the vehicle cost over 100 launches in its working life (all LEO launches with 29,000lbs payloads) . That averages out to $20.76 per lb cargo (Space Shuttle was $8200 per lb).
If that's true, SpaceX is on its way to making routine space voyaging much cheaper. As for environmental costs, I worried a little about rocket exhaust. Burned rocket fuel throws off CO2 and water vapor, I don't know how much, but on this second video, it looks like a lot. The underbelly view of the rocket rising and landing is pretty wild. Especially the elegant finish.
Then there's that bird, the one you see in the first video about 11 seconds in, that swoops close for a look-see and then swooshes off. Maybe they should have loudspeakers at the launch site broadcast a couple of bangs to clear the flight zone.
Local farmers, I figure, have to live with these launches; they might even love them. (I think I would. I imagine myself sitting in the living room and out the window there's a ten story object hanging in the air, slowly sinking down behind the cottonwoods. Most people have sunsets. I'd have rocket-sets, a rich roar followed by a deep quiet. I think I could get used to that.)