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Comment by somatic | Hacker News Reader

> When the spacecraft is about 2,000 ft (610 m) from target, it switches to the landing phase. This is when the computer hands over manual control to the Commander, who guides it in for final touchdown. Slowed to a hover, the module can be steered by tilting it like a helicopter to make any necessary corrections.

A rocket landing on (top of) a column of thrust and a helicopter suspended below a disc of thrust are not even remotely the same thing. Classic failure to reason from first principles.

Here’s a suitable analogy: it’s like the difference between having a center of mass behind the center of pressure, and a center of mass in front of the center of pressure.

For the interested layman: https://en.wikipedia.org/wiki/Longitudinal_static_stability

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The idea that a rocket sitting on top of a column of thrust is significantly different than a rocket suspended underneath a column of thrust is actually a common fallacy, known as the pendulum rocket fallacy: https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy

You are correct that longitudinal stability is applicable and important for most aircraft, but that is due to angle of attack and lift forces. Neither of which apply in a vacuum.

> Here’s a suitable analogy: it’s like the difference between having a center of mass behind the center of pressure, and a center of mass in front of the center of pressure.

This "analogy" is more abstract than the original statement and is only going to make sense to someone who more-or-less also understands that one.

How about “it’s like pulling a floating balloon by a string vs. pushing the balloon from the side and trying to keep it moving in a straight line in both cases”

Or maybe “it’s like balancing a vertically-oriented baseball bat while holding it at the top vs. balancing a broom while holding it at the bottom.”

Erm, sorry, my editing was only half baked above; I tried to change my example from a broom to a baseball bat and accidentally left both in.

It's actually not that different, since the disc of thrust's direction in a helicopter is bound to the helicopter's orientation itself. Thus, there's no stabilizing momentum intrinsic to the system. The pilot needs to constantly balance it manually (if there's no computer help of course), which makes a helicopter so hard to operate.

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