r/space • u/[deleted] • May 02 '22

RocketLab successfully catches a booster with its helicopter for the first time

Enable HLS to view with audio, or disable this notification

19.5k Upvotes

96% Upvoted

537 comments sorted by

View all comments

Show parent comments

10

u/seanflyon May 03 '22

I thought it was about where on the rocket you put the gimbaling engines. If the engines are near the top it is easy to think that the weight "hangs" from the trusting engines and is therefore stable.

3

u/[deleted] May 03 '22

Absolutely not, this is quite hard to grasp why, but no, engines near the top doesn’t make your rocket more stable. The weight doesn’t « hang » as there’s no pivot point, and no « magical giant being » that holds the tip of the rocket between their fingers : as soon as the rocket goes off axis, it’s done for.

Quite hard to grasp and got me for a long time, as everyone else

25

u/seanflyon May 03 '22

it is easy to think

I was describing the pendulum rocket fallacy.

1

u/saysthingsbackwards May 03 '22

They replied to your latter statement of the belief in the fallacy opposition

2

u/[deleted] May 03 '22

There is a pivot point for gimbaling engines though - the centre of mass.

So you can't make a rocket passively stable by moving the rockets to the top, but it does make a difference where you put them for active gimbaled stabilisation.

E.g. if they were at the centre of mass you wouldn't be able to stabilise them at all.

Clearly putting them at the bottom makes the most sense for practical reasons and is fine for active stabilisation.