XB-1 Goes Supersonic, Retires from Service

Boom Supersonic’s XB-1 test aircraft has completed its mission. The plane operated at supersonic speeds on two separate test flights and now will be retired.

This was always the plan. XB-1’s purpose was simply to break the sound barrier and prove that a private company could do so. In theory that would help Boom prove its acumen and convince investors to seed the billions it will need to develop Overture, the proposed commercial supersonic variant. Whether that works remains to be seen.

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CEO Blake Scholl continues to press forward with an incredibly aggressive timeline for Overture. As he shared in the live stream for the final XB-1 flight:

We’re literally about a week away from calling design freeze on Overture. We’ll be calling design freeze on our Symphony engines in March, and our goal is to be making thrust on those engines around the end of this year. [We’ll] start building the first overture in about 18 months, roll it off the line in about three years, put it in the air in less than four and our goal is to be ready for passengers by the end of 2029.

Visually, that timeline looks something like this:

Putting aside the fundraising challenges, the timeline is spectacularly optimistic about the certification work. Boom will be certifying a brand new engine and a brand new airframe. And it will be doing so as a company that has never gone through such a program before. Assuming it will build the first Overture using the assembly line that also requires it to develop and build all the tooling for the assembly line. Supply chain resources and staffing could also pose challenges.

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Scholl also suggested that the company should be able to develop Overture in half the time it took to get XB-1 into the air, while omitting that XB-1 did not have a certification program. It used existing engines and flew under an Experimental authorization from the FAA. Scholl also seems to suggest that Overture will require only a quarter of the money needed to develop XB-1. Given his prior statements on the billions the program is expected to cost it is unclear where this change of budget comes from.

Supersonic, without the Boom

One limitation Boom faces is a prohibition of generating a sonic boom over land. But that is not stopping the company from teasing the possibility of M1.3 flights connecting domestic destinations.

Scholl suggested on the webcast that the company can operate at M1.3 – faster than the speed of sound but slower than Overture’s planned top speed of M1.7 – without causing a boom to reach the surface of the earth. This “boomless cruise” configuration, as Scholl describes it, was teased as available now, without any additional certification requirements.

That position appears to be based on a very generous reading of § 91.817. The law does allow aircraft flying to or from the US to operate at speeds greater than M1 provided “Information available to the flight crew includes flight limitations that ensure that flights entering or leaving the United States will not cause a sonic boom to reach the surface within the United States.” That comes from paragraph (b)(1) of the law, which is secondary to paragraph (a) which clearly states “No person may operate a civil aircraft in the United States at a true flight Mach number greater than 1 except in compliance with conditions and limitations in an authorization to exceed Mach 1 issued to the operator in accordance with § 91.818.”

Combining these two, it appears to permit supersonic flights only outside the USA, but flights to or from the US can go that fast, provided the boom never hits the ground in the US. That would be how Concorde operated legally.It does not, however, allow supersonic operations over the US, regardless of whether the boom reaches the surface.

The reference to § 91.818 suggests the flights could be permissible with special approval from the FAA Administrator. Alas, paragraph (a)(8) makes clear that the permission is only available for certain limited test scenarios. It does not allow for typical commercial operation.

Scholl has made clear his intention to lobby for changes to the rules, particularly based on expectation that the boom will not reach the surface. But the claim it is permissible today without any additional certification requirements does not appear to align with the laws as written.

More of Boom's winding path to supersonic flight

• An investment Boom from Japan Airlines
• Engine selection “core” to Boom Supersonic’s success
• Boom’s supersonic timing slips
• PaxEx Update: FTE Asia edition
• Behind the scenes with Boom Supersonic
• Supersonic sound standards set; Boom schedule slips
• Boom Supersonic picks an engine manufacturer (for further discussions)
• Virgin Galactic makes a move in supersonic travel
• United Airlines plans supersonic Boom with new order
• Boom fires up the engines on XB-1 supersonic demonstrator
• Boom building at Greensboro, new environmental concerns revealed
• Boom Overture adds engines in design revamp
• American Airlines buys in to Boom, despite significant uncertainties
• Boom plans new Symphony supersonic engine design
• XB-1 takes flight, beginning path to supersonic
• XB-1 Goes Supersonic, Retires from Service