# Why does Blue Origin "like landing on a moving ship"?

In the Blue Origin YouTube video Replay of New Shepard Mission 8 Livestream, at 32:53 when the narrator is discussing future plans for the landing of the New Glenn rocket, the narrator says:

The first stage when it comes back to land, you see that center engine will relight, and it actually comes in to land on a moving ship. Why do we like ships? We like the moving ship because it’s actually more stable than a barge out there, which means that we can actually launch and land in higher sea sates which means that we can have a more reliable schedule for our customers when they come fly with us.

What is it about landing on a moving ship that is better than a barge (particularly the moving part)? Is it nautical vessel dynamics, or patent infringement avoidance, or something else?

• It seems that you're right to focus on moving-vs-stationary, rather than ship-vs-barge. It seems that the stabilization systems could be applied to a towed barge, too. Apr 30, 2018 at 14:45
• @DavidRicherby I think they are needling SpaceX just a tiny bit. There's a YouTube link somewhere where E. Musk explains that their barges are ships, not barges, because they have propulsion. I'll look for it...
– uhoh
Apr 30, 2018 at 15:47
• @DavidRicherby found it! youtu.be/Nz60GcmKOvc?t=883
– uhoh
Apr 30, 2018 at 16:21

A moving ship can use stabilizers to reduce roll. Stabilizers are underwater wings that need to move relative to the water for them to work.

Ship stabilizers are fins or rotors mounted beneath the waterline and emerging laterally from the hull to reduce a ship's roll due to wind or waves. Active fins are controlled by a gyroscopic control system. When the gyroscope senses the ship roll, it changes the fins' angle of attack to exert force to counteract the roll.

• A moving ship could hold the position relative to the waves. It is better to hit the waves with the bow instead of a side of the ship. A ship may capsize if hit by a huge wave from the side.
– Uwe
Apr 30, 2018 at 10:48
• Waves are fairly slow, stabilizers become more effective with speed. Apr 30, 2018 at 11:29
• Stabilisation with moving fins is possible also for ships with slow or zero speed. See the Wikipedia link you wrote.
– Uwe
Apr 30, 2018 at 11:52
• @Uwe - that seems doubtful, or at least far less effective - you'd probably need either much larger fins and actuators, or thrusters. Stabilization with net water flow over the fin (and hull) is going to be a lot more effective for a given size of fin and actuator. In contrast, a modern control system likely finds little difficulty with a steady net velocity, so if the "wind across the deck" doesn't exceed capabilities, why not? Apr 30, 2018 at 18:41
• – uhoh
May 1, 2018 at 4:11

The entire quoted passage could be better worded for sure, as it goes around without really explaining anything.

A barge has a totally flat bottom; this makes it much more sensitive to waves and surface currents, and that's the reason barges are used in the calm waters of river, lakes, and canals. Nobody in a sane state of mind would use a barge in open water, as no matter its size just a little amount of wind and waves would easily sink it to the bottom of the sea.

A ship, on the other hand, has a...err...well, a shiphull. A shiphull is designed for high sea, for stability: every time a wave try to move laterally the ship, the mass it has to move is not only given by the ship, but by the mass of water displaced by the fin. Which is nicely multiplied by a loss lever -the force and the fulcrum are in the same place, the resistance (the mass) is on the far end of the leverage. Good luck with that.

SpaceX landing ships have been erroneously called barges for a while by the public (and from Elon Musk himself), due to their shape and their lack of engines, but at the beginning they were mostly "stationary ships". Now that they have engines they are called "moving ships", just to add a little more confusion.

Then, with engines comes an added lateral stability, too. Many modern ships have side thrusters: when stationary they use the engines to power up powerful water pumps, pumping in water and then pushing it at high pressure from specific points in the hull, generating small compensation forces to keep the ship stable and in position. Plus, or in alternative, they can have the stabilizers mentioned by Hobbes.

Finally, as previously said, a ship can be deployed to sea while a barge cannot. If you take this into account and the fact that it appears to be a lot of sea on the planet, it's easy to see how it gives them the huge advantage to be able to choose the most comfortable landing spot for the next launch, or for recovery missions to be as close as possible to the customer.

• Aren't the barges at the core of SpaceX ASDSs regular flat bottomed barges? And aren't those barges built with a great many water tight compartments inside, so no matter how many waves they take, they will always float? Apr 30, 2018 at 19:21
• I feel like a great deal is made of barges being completely inappropriate for ocean use, yet the only people landing rockets in the ocean on a vessel are using barges with added propulsion - no matter what Elon would like to call them. Example - mcdonoughmarine.com/assets/mcd-spec-sheets_v8-marmac_303.pdf Apr 30, 2018 at 19:25
• The vessels SpaceX uses are barges. Their propulsion is used for stationkeeping only, they are towed to/from the landing spot. From the dictionary: Barge n: A long, large, usually flatbottom boat for transporting freight that is generally unpowered and towed or pushed by other craft. May 1, 2018 at 9:35
• Voting down for the opinionated and (as far as I can tell) factually wrong answers. Respond to the critiques here with either explanations or corrections to change my vote. May 1, 2018 at 16:27
• There are also ocean going barges, see, towed by ocean tugboats. Both build to the requirements of open sea use. But successful landing of a rocket on a ship or barge during very heavy waves may be difficult.
– Uwe
May 2, 2018 at 8:05