(Continued from Part 4)

“That’s the problem,” Mr. Jackson said. “Earth atmosphere re-entry is a critical maneuver and the weight has to be known with some precision to fire the re-entry engines just right; to get the decent just right.”

“So what happens if they get too many rocks, or not enough to get the weight right?”

“The total amount of weight is not the problem. Measuring and knowing the actual weight is what’s required.”

Gregory looked confused.

“Let me explain. When the spacecraft is returning to Earth, well in advance we need to approach Earth at just the right angle to assure re-entry. You’ve heard from the Apollo missions that if the angle is too shallow, the atmosphere will produce more lift than drag and cause the spacecraft to lift upward, like skipping a rock on the lake. If the angle is too steep, the spacecraft will penetrate too quickly, generate too much heat, and burn up.”

“So what do the rocks have to do with the re-entry angle?”

“As I said, well before atmospheric penetration, we have to get the angle right. While the spacecraft is speeding through space toward Earth, in order to make changes in the direction, small rockets or thrust engines on the sides of the spacecraft ignite and push sideways on the vehicle, vectoring or turning the direction of motion by a small angle. The delta-V, we call it, or change in velocity, speed or direction, is dependent on the mass or weight of the whole spacecraft including it’s passengers and cargo.”

“Including the rocks.”

“That’s right. As you recall from Newtons second law, to change velocity in a particular direction, you need to accelerate or decelerate in that direction.”

“F=m*a?”

“That’s right. Acceleration, or a change in velocity delta-V is the Force of the rocket divided by the mass of the total vehicle. The maneuvering equations are a little more complicated than that, but in general, that’s the idea.”

“Wow. I didn’t realize how useful Newtons laws can be,” Gregory said.

“Here.” Mr. Jackson handed a wrench to Gregory and reached under the wooden box with a bolt. “Help me attach these bearings to the container.”

(To be continued)