mobile offshore bases





Wing lift is proportional to velocity^2.

Ignoring drag, speed at the end of a runway is proportional to sqrt(length).

As such, until drag becomes comparable to thrust and/or velocity significantly reduces thrust, max takeoff weight is approximately proportional to runway length. Drag force is proportional to velocity^2 so a point of small returns to length is rarely reached in practice for jet aircraft; for example, a Boeing 787 at liftoff has only ~1/3 its cruise drag.

Increasing max takeoff weight means more fuel and payload can be carried. If the aircraft is also redesigned for a longer runway, range and payload can be more than linear with runway length.



The mobile offshore base (MOB) is a concept for something like an aircraft carrier but with a much longer runway, long enough for aircraft not specifically designed for carriers. Because dry docks and ports have limited size, this typically involves modular designs where several independent ships join together to form a large runway. Most such designs are slower than an aircraft carrier and don't have an aircraft catapult.

The military did this study on the MOB concept. But the authors didn't understand the above Fun Aircraft Fact; I guess they were all ship guys. The report treats takeoff distance as a threshold that each aircraft has which is either met or unmet.

They concluded that a MOB could operate C-130s and potentially C-17s. They considered it otherwise equivalent to a slow-moving aircraft carrier with a high sortie rate and lots of deck space. They considered the main advantage over a conventional carrier to be the ability of a MOB to act as a sort of logistics hub that could support land-based operations with transport aircraft. But that's not right.

Even if you just operate existing carrier aircraft, a longer runway means higher max takeoff weight.

A 2000m runway on a moving ship means it's possible to operate 737s and 787s. The 787 has a range of 14000 km. That is about the range of a B-52. Therefore, it's possible to operate long-range bombers from a 2000m runway. And this doesn't include the option of takeoff with low fuel loads and immediate in-flight refueling; "buddy refueling" is commonly used with aircraft carriers now.

They also compared a MOB to heavy bombers operating from US airbases, without appreciating that a MOB could potentially support bombers of comparable performance.

Increased aircraft range doesn't just increase combat ranges from a MOB: it also means more ability to transfer aircraft between airbases and a MOB as needed.

Their cost estimates for a MOB were based on the estimated cost of 5x large (1200 ft) slow (12 knot) ships which would be built in a standard way and duct taped together.

They concluded based on this methodology that a 5000 ft runway MOB that moves at 12 knots would cost $8 billion to $13 billion in 2004 dollars. This was considered approximately 2x the cost of an aircraft carrier, and they concluded that 2 carriers would be preferable to 1 MOB.

As I discussed above, that study underestimated the overall utility of a MOB relative to a carrier. But it also, I believe, significantly overestimated the required cost, because it didn't consider more novel approaches to construction. Also, there's been some potentially relevant progress in composites, concrete, and steel alloys since it was done. If I was making a MOB, I'm estimating that a 20 knot MOB with a 2000m runway could be realistically built for $3B to $10B in 2020 dollars.

Meanwhile, in 2018 dollars, a Nimitz class carrier is $9B and a Ford class carrier is $13B.

As far as historical precedent goes, the closest thing to a MOB built so far is perhaps the Mega-Float project in Japan. A 1000m moored floating runway was successfully built as a demonstrator for a proposed 5000m runway in Tokyo Bay, but that larger project never went forwards.

In conclusion, America should build a mobile offshore base. Not so much because it's useful militarily - although it would be - as because it's a thing to do that could develop some engineering capability by being new and is kind of inspiring by being really big. If the enervation of America is something comparable to major depression, then perhaps the individual-level advice of trying to do stuff one used to enjoy (in this case, construction of a new ship class) is applicable.


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