Fireworks over the Mississippi River in downtown Minneapolis. This one is of the July 4th display. 2005:07:04 21:59:29
Tentative as all such complex endeavours must be, the plan is to put the NASA Space Transportation System -- the Space Shuttle -- back in service tomorrow. Given the amount of engineering (both mechanical and procedural) that has gone into the return to flight, this may well be one of the safest manned space launches ever. True, I'd probably have been in less peril taking a walking tour of Gaza City than riding the Shuttle, but by the standards of spaceflight, that's good.
Pop quiz: to you, was that a revelation about spaceflight or about Gaza City?
The Space Shuttle came to mind this weekend during the Aquatennial fireworks display, as a barrage of blue sparks launched off the Central Avenue bridge. For many years I've tried to make a mental note whenever blue shows up in a fireworks show, because of a chemical connection to space travel. And possibly an economic one, although I've never been able to confirm that aspect.
The Space Shuttle uses two kinds of rocket engine for launch. The main engines (SSMEs) are your classic liquid hydrogen-liquid oxygen system, a horrendously complex set of beasts. Then there are the Solid Rocket Boosters, glorified and meticulously constructed bottle-rockets, each filled with around 500 metric tonnes of fuel. The stuff comes in gigantic cylindrical blocks supposedly having roughly the consistency of a pencil eraser. The texture would be quite different, though, as the SRB fuel is essentially a gravel of ammonium perchlorate (an explosive in its own right, used here as a source of oxygen) and aluminum embedded in a rubber matrix. Literally rubber -- that's technically what the SRB is burning, although there is considerably more perchlorate by mass.
Some ages back, I read (probably on Usenet, although Google Groups has thus far failed to unearth the post I'm thinking of) that the Space Shuttle program poses something of a difficulty for the fireworks industry. Supposedly, ammonium perchlorate happens to be one of a very few substances able to produce a good, bright blue flame, and when launching in quick succession the Space Shuttle SRBs consume most of the nation's ammonium perchlorate production capacity, driving up the price. Thus, the writer claimed, blue firework explosions had become quite rare in the late 80s.
I don't find this story especially plausible, as it happens. Ammonium perchlorate has various industrial applications, and is consumed in large quantities in the production and maintenance of solid rocket motors for the military, as well. Then again, the Henderson, Nevada explosion in 1988 suggests that the SRBs consumed at least some respectable portion of perchlorate production at that time. The past two years, with the Shuttle fleet grounded again, would be a good test. Has anyone noticed a significant increase in the amount of blue used in fireworks displays recently?
Regardless of the accuracy of some old Usenet post, the story has stuck with me. Whenever I see a shell burst with blue sparks or trails now, I think of the Space Shuttle. It's a risky business, strapping astronauts onto a pair of several-hundred-tonne fireworks, but thus far the engineers of NASA have done a remarkable job of pulling it off, again and again.