The Frosty Launch: Questions About a Nose Cone Latch

Launch Notes: The Nose Cone That Let Go Too Soon

1The Cedar Gap Rocketry Club launches model rockets from a pasture at the edge of town. This year eighth graders Annika Thorsby and Jomo Wanjiru built the Kittiwake, a rocket whose nose cone is held on by an electromagnet latch. The latch is a coil of insulated copper wire wrapped around an iron core and powered by a small battery pack. While current flows, the coil pulls on a steel plate glued inside the nose cone. At the top of the flight, a tiny flight computer switches the current off, a spring pushes the nose cone away, and the parachute comes out.

2The first launch day in March started with frost on the grass. While the Kittiwake sat on the pad for twenty minutes, Jomo tapped the launch rail to check that it was steady, and the nose cone popped off. In the warm workshop the week before, the same latch had held firmly through much harder shaking. Club adviser Mr. Percival Osei asked the pair to turn the problem into questions they could test. A testable question, he reminded them, names one thing to change on purpose and one thing to measure.

3Back in the workshop, the pair clamped the latch upside down with one layer of tape on the core, so the gap to the steel plate stayed the same in every test. They pulled down on the plate with a spring scale and recorded the holding force: the pull at the moment the plate let go. In Test 1 they powered the coil from the school's adjustable power supply and changed the current. In Test 2 they used the club's own battery pack after leaving it for one hour outdoors at −5 °C, in the workshop at 20 °C, or in an inside jacket pocket at about 30 °C, and they also read the current on a meter.

4Mr. Osei explained that reversing the current swaps the north and south ends of the coil. In Test 3 the pair swapped the battery wires. They tested the usual steel plate and then a plate with a small magnet glued to it.

5The club's rule is that the latch must hold with a force of 4.0 N or more, because the spring and the shaking of a launch can pull the nose cone outward by nearly that much.