Thursday, May 21, 2009

The King of Grunt

According to Aerotech Catalogs, the 9-grain J570W relaod for the 38/1080 casing is the 'King of Grunt'. And indeed it is. The single most powerful 38mm motor available, it packs 1060 Ns into 18" of metal tubing and releases it in just over 2 seconds. It peaks at 2000N of thrust and can lift a 20-pound rocket.

And that's just from the earth. Operating in the 1/6th gravity and no-drag conditions of the moon, in which APCP can still burn, it can throw a 4-ounce rocket up to 545,922m (that's 545.9km, or 1792807 ft, or 339.55 miles). It'll reach 1307m/s (4292fps or 2926 mph), which is 81% of lunar orbital speed.* In case you're wondering, the optimum delay is 832.7s, or just under 14 minutes. I haven't tried it with larger motors yet, but I suspect a K motor (or a J with an upper stage) could reach orbital speed, and that simple, fairly cheap 98mm M and N motors could easily loft 20 pounds into lunar orbit - at under 20 bucks a pound.

* I learned a nifty trick somewhere for approximating the mimimum low-orbital speed around any large body. Find the distance a body will drop in one second. For the moon, that's about 2'8", for earth it's about 16'. Then find the distance that you gotta travel in a straight line for the surface to fall away that much from under you. For the moon, it's just about a mile. For earth, it's 4.9 miles. Presto! In order to fall constantly and not hit the moon, you merely have to travel about 1 mile per second, or about 1600mps. For the earth, it's 4.9 miles per second, or about 17000 mph. Confusing, I know. I gotta reword it.

Also, since the moon has no atmosphere, you can have a very low lunar orbit - 6.5km is all you need as that'll take you above the highest point on the moon, which is on the far side and about a kilometer taller than Mons Huygens, which is usually listed as the highest point. On earth, you need to be up at least 100 miles or so to avoid atmospheric drag.

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