The difficulty of landing a rocket

Space X has taken it upon themselves to land a rocket vertically after takeoff. This may seem strange, but has many technological benefits. The first and foremost is learning how to control a very heavy and unstable object and bring it to a safe landing. So a large portion of the problem is controls. Since there’s no one on board the expended rocket body, everything has to be done by the computer with sensors either on board or on the ground. Now this isn’t to say computer control of flying vehicles isn’t common place. The launch of the rocket is entirely computer controlled. The timing required is too precise for humans to handle. Commercial airplanes are also largely computer controlled. However in both those cases, the dynamics are well understood and the vehicles have plenty of sensors on board and o the ground to obtain data from. However we’ve never really cared about bringing a rocket back to Earth intact so the sensors and knowledge necessary aren’t in place, so they have to make do with what they can and develop the knowledge as they go.

The second benefit of landing a rocket is it maybe allow the industry to reduce the cost of rockets. Instead of throwing the spent rocket away in the ocean, if we can recover it and refurbish it, it can be reused at a fraction of the cost.

So why is it so hard to land a rocket body? There are three main reasons, and lots of smaller but still important reasons.
1) The landing platform is floating on the ocean. Even if the ocean is smooth and calm, the platform is still drifting, so its position changes. That means the rocket is trying to hit a moving target.
2) The rocket body is not aerodynamically and dynamically stable. It is a large pole that weights thousands or tens of thousands of pounds. A breeze will created a force that can cause the rocket to topple over. It’s easier to land a hockey puck right side up than a pencil.
3) The rocket is really heavy. You’ve probabaly tried to balance a pencil on your finger before, and it’s not too hard. It’s easy to provide enough force to lift the pencil. However we’re talking about thousands of pounds of weight, and it’s being help up by thrusters that output thousands of pounds of thrust. So fine control of the thrust is very important. Unfortunately rocket engines usually don’t have fine throttles. So maybe you can either do 0, 10, 50, or 100 lbs of thrust, but not 1 lb. I’m exaggerating, but there’s no 2.1253 lb and then 2.1254 lb. With less fine thrust, you have less fine control.

Their first attempt to land a rocket body on a floating platform didn’t end so well. They release a short video of the rocket coming in to land from a camera located at the landing pad. It’s quite spectacular. The first thing to address is why are they landing on a floating platform instead of on land. You would think it’d be easier to land on solid Earth, which it is. However there are concerns of safety involved. If the rocket were to try and land on ground, even far away from cities, there is the chance it could go arwy and the crash could cause damage to the area, wildlife, or even people. So we land it in the ocean, and only the fish get hurt at worse.

Space X has tried a second landing at this point, and it didn’t do so well either, but they’re learning and will get better. Every launch will be an opportunity. It is a very difficult task, but worth doing and learning. Since only by doing and learning do we get advance technology and civilization.


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