Hacker News Reader

Comment by anovikov | Hacker News Reader

Trick of SpaceX success wasn't in vertically landing rockets - with mass being concentrated in the bottom in form of engines, thrust structure, and legs, it's not all that hard.

What was extremely hard, is havigation and control to enable return back to launch site optimally, going through narrow corridors of acceptable parameters (go too shallow and you risk being unable to correct your position in the end as precision drops, go too steep and rocket breaks up due to aerodynamic forces, and so on), while burning minimum fuel. That was an insanely difficult GNC task which took so many iterations to perfect out...

Plus yes, on landing they didn't have a second chance as rocket couldn't hover - minimum engine thrust was bigger than it's mass so if it missed, it either crashed, or flew back up until running out of fuel and crashing, too.

View on HN

If I understood correctly, the way to land when even minimum thrust is too much is both simple and clever. The goal is to reach zero velocity at zero altitude. But there's always some uncertainty in how rapidly an engine will fire up and come up to power, so you can't just precisely time the engine startup. So what you do is to time the startup so that the predicted zero velocity point is underground. Then once the engine has started and spooled up, you continuously measure altitude and velocity, update the predicted zero velocity altitude, and progressively increase thrust until the zero velocity altitude is at zero altitude.

I mean, seems pretty hard to be honest. They got the trajectory down pretty close for the vast majority of these (otherwise would be too far away to see with cameras). But so much extremely expensive space hardware destroyed in the pursuit of this goal. They earned it: https://youtu.be/bvim4rsNHkQ

Mass distribution isn’t part of it as long as things are symmetrical (https://en.m.wikipedia.org/wiki/Pendulum_rocket_fallacy) but yeah, threading the needle like SpaceX does is not easy.

Due to fuel slosh, things aren't symmetrical really.

Got to say, I don't understand how the argument in that article works. If Goddard's rocket that's on the photo had a canvas stretched between the wires/pipes that form its body (I assume it's a part of the rocket, and not a launch platform), then I'd expect aerodynamic forces to act on them, countering the tilt. Conversely, if such surfaces were above of the engine, I'd expect them to make the construction unstable.

You're right that any canvas stretched around the frame could act as aerodynamic stabilizer fins. The pendulum rocket fallacy is the assumption that a higher mounted engine will stabilize the rocket's direction due to it 'hanging' from the engine, by analogy with a pendulum (where the higher a point you 'hang it from' the more stable it is). The fallacy lies in the fact that the pendulum is hung from a fixed point which can apply forces to generate a stabilizing torque, whereas there are no such outside forces generated by the rocket engine (other than the thrust it produces).

Fins totally work and that's why almost all rockets use them to some degree for stabilisation while in the atmosphere.

But for landings, it's totally a part of it. If there is a mentionable lateral velocity component, rocket will tip over on landing unless it's center of gravity is very low. It happened just once with SpaceX rocket though.

The pendulum rocket fallacy refers to the placement of the engine itself, not center of mass. Mass distribution absolutely matters, because with vectored thrust, your engine can exert a higher torque on the rocket if the center of mass is farther away. You can try it yourself with a long pole and weight (e.g. broom with big brush?). Balancing the pole with the weight on top is easier than with the weight on the bottom.

Reply on news.ycombinator.com