After successfully launching a resupply capsule, the SpaceX Falcon 9 rocket attempted to land on a barge in the ocean. As you can see, the landing attempt was not successful. Really, should we be surprised? I’m surprised the rocket was so close to actually landing at all. Landing a rocket like this is quite difficult.

So, why is this rocket difficult to land? Before I give an explanation, let me just give a reminder. I’m a physicist and not a rocket scientist. I am going to talk about the general physics principles and not technical details of the rocket landing.

Lunar Lander Is Easy

Yes, we landed several spacecraft on the moon with the lunar lander in the Apollo missions.

Actually, this there is also the famous arcade game called Lunar Lander. Here is an online version if you want to play it. The goal is to change the angle and thrust for a lander to safely land on the moon.

Ok, the real Lunar Lander game isn’t always so easy – but it’s easier than landing the SpaceX Falcon. What’s the difference? The lunar lander has a rocket at the bottom, but it rotates with other thrusters on the side. The Falcon 9 has a rocket engine on the bottom and it uses this rocket for both thrust AND rotation. This makes the Falcon 9 a bit harder to maneuver (also the lunar lander was on, you know, the moon – where the gravitational field is smaller).

Three Motions for a Rocket

The Falcon 9 rocket can do three different things with the main thruster:

Vertical acceleration: this is useful for things such as slowing down the rocket’s decent so it doesn’t, you know…crash.

Horizontal acceleration: used to change the rocket’s horizontal velocity. This is very useful for changing the horizontal position of the rocket so that it can land on a barge in the ocean.

Angular acceleration: this changes the rotational motion of the spacecraft about its center of mass. This would be useful if you wanted to make sure the rocket landed in a vertical position.

Maybe this will make more sense with a quick example. Suppose the Falcon rocket is coming in for a landing and it has some horizontal velocity. In order to slow down for a safe landing, the rocket must thrust in the opposite direction. Here’s what happens.

In order to accelerate to the right, the rocket angles a little bit to point to the right. However, since this thrust force doesn’t act in a line that goes through the center of mass, there is a torque on the space craft that changes its rotational motion. Add on top of this the fact that you have to also change the thrust value in order to accelerate the rocket up and down also.

It’s a pretty tough problem to land a rocket like this. Actually, you can try something like this yourself. Get a broom or long stick and head outside where you won’t hit anything. Now try to walk while balancing the broom on your hand just by placing the end of the broom on your hand. How do you stop walking? Here is an example.

Yes, in this example I did indeed stop the broom and it didn’t fall over. However, with the rocket you need to both stop at AND keep it vertical at the end.

Why Not Use a Different Rocket Design?

This is pure speculation, but let me consider two rocket designs. First, there is the Falcon 9. Second, there is a flatter design that would be easier to land. It would look something like the the lunar lander.

This “Easy Lander” would be much easier to control. First, it isn’t tall and skinny like the Falcon 9. The center of mass is much closer to the main thrusters so that they wouldn’t exert as much torque to change the rotational motion. On top of that, there are multiple thrusters so that you could vary the thrust to create zero torque if you wanted. Finally, this design also has side thrusters. You could change the horizontal motion of the Easy Lander without even rotating the spacecraft. Seems like a better rocket, right?

Although the Easy Lander would be easier to land, it wouldn’t be as good as the Falcon 9. The Falcon 9 is not designed to land on a barge in the ocean. No, it is designed to launch a payload into orbit. That is it’s primary function, a function that the Easy Lander would do a very poor job at. Rockets are tall and skinny like they are so that it will have a lower air drag on it as it accelerates through the atmosphere. The smaller the cross sectional area of the front of the rocket, the lower the air resistance. If the Easy Lander were to launch a payload into space, it would need MUCH more fuel to compensate for the larger air resistance. With more fuel, you would need bigger rockets (for the increased fuel mass) which would need even more fuel. When launching a rocket, every little bit of mass matters.

Of course, that’s just speculation about the shape of a rocket. Either way, I think we can all agree that making a rocket launch a payload into orbit and then safely land is a pretty difficult thing to do.