TL;DR This is legislation Boom wants enacted to allow for more relaxed noise emission levels.

"Current engine noise rules for new supersonic aircraft are more stringent than those for the existing subsonic fleet. By setting our engine noise to the same levels as existing subsonic airplanes, we could make our engines produce so much less drag that we would save 20–40 percent on fuel, depending on whether you use this year’s or next year’s rules as a baseline."

Curious if they've hit a wall in their aerodynamic design and need this to move forward.

They want the regulations banning overland supersonic flight removed, and are trying to frame it as a fuel efficiency issue rather than the "we will make people's lives miserable with noise" issue that led to the ban.

It can be done sanely, but they don't have anything flying yet (was supposed to be May 2017 but that came and went).

Also, don't be fooled. The smaller test aircraft, if it ever flies, will have a much lower sonic boom than the larger airliner. Sonic boom is largely proportional to aircraft weight.

Reading between the lines it would appear that they are so far from having anything that flies that they're still willing to redesign their engine and presumably plane as a result.

(And yet they have the hubris to talk about how much a ticket will cost)

> They want the regulations banning overland supersonic flight removed

True, but

> and are trying to frame it as a fuel efficiency issue

No, they are trying to frame the changes they want to engine noise regs as a fuel efficiency issue; that's a separate rule change from (in the same legislation as) the one removing the overland supersonic flight ban.

> What I don't understand is the engine size rule.

It's not a size rule, it's an engine noise rule which ends up (per the article, I don't know enough about the relevant fields to confirm this) requiring wider engines which end up producing more drag.