Nuts & Volts - September 2006

■ FIGURE 7

300

Boom Strength

250

200

Load (pounds)

150

100

Bare
Taped
Thin Ply/All Sides
Thick Ply/All Sides
Thin Ply/Top & Bottom
Thin Ply/Sides Only

calculate the average slope of each curve between the 0.0 and 0.1 inch deflection points. The chart in Figure 8 shows the stiffness of each of the booms.

Stiffness is measured in units of pounds per inch (or newtons per meter in the metric system). The chart above shows that a fully laminated boom is at least 500 times stiffer than a bare Styrofoam boom. It’s interesting to note that a Styrofoam boom laminated in 1/16” thick plywood has a stiffness close to one ton per inch.

50

CONCLUSION

0

0.1

0.2

0.3 Displacement (inches)

0.4

0.5

The first column is the amount of force measured by the load cell and the second column is the displacement of the ram in the press. Around 2,000 measurements were generated during each test.

I imported the results into an Excel spreadsheet and generated a chart shown in Figure 7. If you’re interested in this data, you can find a copy on the Nuts & Volts website ( www.nutsvolts.com).

ANALYSIS

A force of only 10 pounds will flex a bare 3/4-inch thick Styrofoam boom by its own thickness. A taped boom

Boom Stiffness

2000

0.6 So, what have I learned? If the amount of force that a boom must support is small (less than 10 pounds), then a bare Styrofoam boom can handle it and bounce back after deflecting. However, if the force acting on a boom is on the order of 100 pounds or more, then a fully laminated Styrofoam boom is required if the boom is not to break. If I can live with a boom 15% heavier than a 1/32” covered boom, then I can make a boom capable of supporting up to 250 pounds with 1/16-inch thick plywood.

The stiffness of a fully laminated boom is significantly greater (it flexes less) than a bare or even partially laminated boom. A fully laminated Styrofoam boom under a load of 100 pounds experiences less than 0.1 inches of deflection. That’s not bad for something you can make in your garage.

If you have a near spacecraft or

does so with a force of 11 pounds. Look at what plywood lamination does to Styrofoam, however. Booms laminated on two sides can withstand 40 pounds of force without failing and their deflection is minimal until they fail. Booms laminated on all four sides can withstand forces exceeding 100 pounds with minimal deflection. A 3/4-inch thick Styrofoam boom eight inches long and covered in 1/16” thick plywood can support my weight and hardly bend. And that’s a boom that only weighs 27 grams, or one ounce.