So we have an aircraft sitting on the flightline sometime in 2040 with the following stenciled under the pilot's window: YRBS-3G-15-BO.

What does it mean?

According to the Post-1962 Unified Aerospace Designation System used by all the US Armed Forces; the designation breaks down as:

Prototype, Spaceplane, Bomber, modified for the Reconnaissance Mission, third design, seventh major modification of it, production block 15; and manufactured by Boeing's Seattle plant.

Confused? Consult the handy guide below.

Status Letter

Location: First letter in the sequence.

Notes: Signifies the status of that particular aerospace vehicle.

Modifier Letter

Location: Second letter in the sequence.

Notes: Signifies any modifications made to the basic mission of the aerospace vehicle.

Mission Letter

Location: Third letter in the sequence.

Notes: Signifies the primary mission of that aerospace vehicle.

Vehicle Type Letter

Location: Fourth letter in the sequence.

Notes: Signifies the type of vehicle. Is only used if the vehicle itself is uncommon.

Design Number

Location: Numerical Sequence following Mission Type Letter.

Notes: According to the regulations, design numbers should be allocated serially, e.g. the F-35 Lightning II should have been designated the F-24 Lightning II. This is now routinely violated. In one notable instance, Boeing asked, and received the designation KC-767 for it's 767 tanker conversion, instead of the in-line KC-42A designation.

Series Letter

Location: First letter after design number.

Notes: The first craft in any design number series is always designated A (e.g. F-22A). Successive modifications follow alphabetically. (e.g. F-22B, -22C, etc). The exception are the letters O and I; which are skipped to avoid confusion with the numbers zero and one; hence if we were to produce a successor to the B-52H, it would be the B-52J. The same follows for N-series aircraft. For example, with the B-95N, it's successor would be the B-95P.

Block Number

Location: Follows Series Letter.

Notes: Introduced during World War II, and used to represent changes during production of an aircraft which did not warrant a new series letter. Assigned in Blocks of five, e.g. (Block 1, Block 5, Block 10) to allow for modifications to production aircraft after they have left the factory. Originally block numbers were written as “B-2A-30”, but the modern application of them is “B-2A Block 30”.

Manufacturer's Code

Location: Follows Block Number.

Notes: Two-digit letter code introduced in WWII to differentiate between the same aircraft produced by two different manufacturers. At first, the system was relatively easy, with designations such as NA for North American Aviation, and GR for Grumman. However, with the huge expansion in factories caused by World War II; a more rational system was needed; and so it was modified so that the first digit designates the manufacturer, while the second digit designates a specific factory owned by that manufacturer.

For example, the Enola Gay's designation is B-29-45-MO, indicating it was a Block 45 B-29 built by Martin's Omaha, Nebraska Plant.

There were special cases to the designation system, for example Boeing's Seattle plant should have received a designator of BS (Boeing, Seattle); but instead was given BO to avoid the connotations of “BS”.

The code was officially dropped from regulations in 1976, and many of the companies have merged since then into the three behemoths of the American aerospace industry, Boeing, Lockheed Martin, and Northrop Grumman.