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Comment by otras | Hacker News Reader

This is probably a silly question, but for the test run, did they launch the rocket straight up? From my limited understanding of gravity, I’d expect it to come straight down if anything went wrong, and it looked like there were quite a few things for it to come down on.

Edit: looking at the video closer, it looks like there is a slight angle to the launch, tilting away from the building next to it, which makes sense.

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Earth is a rotating reference frame. So, no, anything going high enough up won't come "straight down"

I think that requirement of “high enough” is why I was surprised, given that a) it was a 1/3 scale model, b) it was a trial run, and c) things frequently go wrong. I guess sitting below it shows their confidence in its ability to get things up there!

Edit: that being said, it does look like there is a slight angle.

Will come straight down even though the earth is spinning... unless atmosphere drags it aside.

No, even if there were no atmosphere it would not.

There's a point at which you can no longer ignore the rotation of the earth.

Maybe the effect still isn't huge at the scales we're talking about but it's not straight up and straight down.

Doesn't the angular velocity of the projectile match the angular velocity of the launcher at launch?

That's incorrect. Anything released from earth carries the same inertial momentum. It's like when you throw a ball in a running car.

You're not throwing the ball high enough in a car that the difference in radius from the center of the Earth makes a difference that you can detect.

For objects above the earth's surface, the radius of the 'orbit' around the center of earth increases, and thus the 'forward' component of the velocity has to increase in order to stay over the same point on the earth. If you shoot something high enough straight up, it will land to the West of the point it was launched from. (This does ignore effects of wind and such, but I believe so did your argument).

It's nothing like throwing a ball up from a running car. Look up the physics of rotating reference frames. I'm sure others can explain it better than I can.

If it were at even a slight angle the projectile wouldn’t come straight down, wind notwithstanding.

It most likely has a self-destruct sequence. It's basically the same as firing a missile so I wouldn't be surprised if the same type of persuasions were taken as when military drills take place.

One mock up images shows it at an angle.

I did see that, and that’s what I would expect for the full version (much like launching rockets over the Atlantic from Cape Canaveral). Just a little surprised to see it looking a little more vertical for the trial run.

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