Intro

On Feb 10th 2025 the Boom XB-1 completed her 13th   and final flight. Baby Boom got to 36,514 feet in altitude, went supersonic all the way to Mach 1.18, flew for 41 minutes and was captured in vivid schlieren images going supersonic. While all these are stunning achievements, there are several standouts. The first is boomless cruise, the XB-1 went supersonic with no audible sonic boom and the second was this aircraft was almost directly responsible for having the 52 year old supersonic over land ban in the United States overturned and finally the Boom XB-1 is the very first privately funded aircraft to go supersonic. This is the story of ‘The Little Plane That Could’.

The Idea

The idea of the ‘Baby Boom’ as a technology demonstrator finds its underpinnings in 2013. Their CEO Blake Scholl was always interested in supersonic flight post a visit to the British Airways Concorde G-BOAG at the Museum of Flight in Seattle and had set up a Google alert if anyone ever came up with the idea of making a supersonic airliner, nothing ever came about. 

In 2013 after yet another transatlantic flight delay, he pondered if this was something do-able. Like most ‘cowboy entrepreneurs’ he just decided to do it and Boom was born in 2014 with seed funding of approx $1-2 million and 10 driven engineers.

By 2016 the team at Boom had its first iteration of what the Boom Overture airliner would look like. It was a trijet design with two engines underwing and the third buried inside the tail. The aircraft looked very similar to Concorde being a tailless delta and was designed to carry 65-88 passengers and cruise at Mach 2.2. The aircraft renderings were unveiled at the Paris Air Show and the Boom revealed a letter of intent from Virgin Atlantic (that is a story by itself and speaks volumes of Richard Branson & Blake himself). This triggered a Series A funding of $50 million . The media highlighted the idea of ‘democratization of supersonic air travel’. 

Developing a full scale airworthy Overture is a $8bn venture. There are designs to be proven, manufacturing processes to be validated and of course the need to prove that a small start up with almost no background in aerospace can build a supersonic aircraft.

It’s around this time a pivotal realization emerges within the Boom Team. They needed a ‘Falcon 1 moment’, they needed a demonstrator.

The Demonstrator

Small aerospace companies such as Boom have no track record but are extremely ambitious about their disruptive technologies and need a proving platform. Successful subscale (a percentage of full size) demonstrators provide the foundation their technologies and capabilities need at a fraction of the cost (Boom spent $13 million on the XB-1 & $156 million on the project, including early Overture development costs).

The Falcon-1 moment that Blake was referring to was the fourth launch of Space X’s first little rocket. After three failures between 2006-08 and on the verge of bankruptcy Space X had enough money for one more launch. The fourth of course proved private rocket launches were viable and the rest is history.

The XB-1 was a 1:3 scale prototype of the full scale Overture (first iteration of the Overture was a trijet which changed to four engines in 2022. The XB-1 was ready to start testing by 2020 itself, and hence stayed a trijet) and the project goals were to prove privately funded supersonic flight was possible by validating aerodynamic design & highlighting Boom’s manufacturing capabilities.

The Design

The XB-1 had two designs. The first iteration had a length of 68 feet and a wingspan of 17 feet. The final iteration had a length of 62.6 feet and a wingspan of 21 feet. The increased wing span of the slender delta on the latter gives the aircraft an improved aspect ratio (even though the wing had a shorter chord at the root, the original had the wing chines right up to the cockpit, while in the latter the wings chines end behind the cockpit and the aircraft was shorter by over 5 feet) which in turn improves aerodynamic handling especially at lower speeds. The wing itself is an ogival delta, where the leading edge has a S shaped curve on the leading edge to manage vortices as lifting devices.

The XB-1 uses area ruling right through its entire length. Area Ruling a.k.a Whitcomb Rule is an aerodynamic principle that states the wave drag on an aircraft flying at transonic speeds (Mach 0.8-1.2) is minimized if the total cross sectional area of the aircraft changes smoothly and gradually along its length. An example is the manner in which the cockpit canopy tapers off where the wings begin, and a top view of the XB-1 shows off the horizontal stabilizer roots beginning where the wing trailing edge root ends. 

At the very tip of the nose is a ram air pressure sensor (works as a pitot tube). The ram air pressure tube reads the speed of the aircraft. The tube has two vanes attached to it , the alpha vane for the AoA of the aircraft and the beta vane to measure yaw (lateral directional movement relative to oncoming wind). Such a set up is used on experimental aircraft where data is still being gathered in real time as against an aircraft which already has a data history and is in clean air (undisturbed by the aircraft’s turbulence). Such a system is critical to validating the performance of the aircraft across the entire speed regime.

The nose of the XB-1 looks like a cone that has been flattened on the bottom. The contoured underside of the nose is critical to aircraft stability during take off and landing when the aircraft has a high Angle of Attack (AoA).The original iteration of the XB-1 nose was more conical but computational fluid dynamics (CFD) directed the nose to be flattened on the bottom. The nose design itself took about two years! The shape of the nose had a direct impact on the shape of the wing,the leading edges of the  wings of the final XB-1 are much further back on the fuselage than the original iteration as this combination gave the maximum stability at low speeds (more on this later). To sum up, having a perfectly conical nose means it sheds vortices in unpredictable directions to small changes in aircraft speed and attitude which interfere with the lifting surfaces and the vertical stabilizer. Behind the cockpit on the port side is the AC inlet that creates cold air via a turbocharger inside and bleed vanes behind.

The nose slopes up towards a canopy that is contoured into the curve of the upper nose cone. This gives the canopy a very merged look relative to the nose curve.  The nature of the curve of the canopy in tandem with the overall design of the aircraft means the pilot has very low visibility at landing and takeoff (slender delta wings have high AoA at low speeds). The Concorde had a dropping nose which is a relatively heavy mechanism, The XB-1 uses augmented reality, more on this later.The entire underbelly of the aircraft appears to be relatively flat and heavily contoured, all the way to the rear trijet configuration. The contours merge with the redesigned wings seamlessly. Right through the length of the underbelly of the aircraft  are a series of access panels that merge seamlessly with the surrounding fuselage contour. The panels are held in place by quarter turn fasteners such as those made by Howmet Aerospace. These panels give access to various avionics bays and fuel tanks (refueling happens off the top of the aircraft).

The landing gear on the aircraft have been optimized at the lowest possible cost. The first thing that strikes you about the nose gear is the heavily machined trunnion and pivot arm, they are made of titanium, these parts are mated to mechanisms salvaged for an F-4 & a T-38 to complete the gear. On the gear are two HD cameras and these are part of the augmented reality system that XB-1 uses to overcome the poor cockpit visibility mentioned earlier. They have a 12.5° down angle to compensate for aircraft attitude at low speeds. There are two cameras for redundancy and are spaced more than a standard bird wingspan apart, this ensures a birdstrike cannot take out both cameras at once. As the gear retracts, the strut first compresses to have a smaller profile as it sweeps up & forward into the fuselage.

The main landing gear continues the heavily machined theme and is capable of taking a hard landing load of 200,000 pounds and bouncing right back! The gear is a combination of three arms. The main strut is made of Aermet100, an ultra high strength steel alloy. The alloy has iron, cobalt and nickel as additives. The drag arm is grade 5 titanium as is the main shock absorber. The arms converge just above the wheels and radiate out into the bay. The twin main gear of the aircraft converge into the bay with the wheels laterally radiating out. The bay has two hydraulic reservoirs for redundancy, one on each side and an emergency DC pump as a backup. The XB-1 has no RAT, the DC pump is the back up. The landing gear bays are absolutely packed with different systems and include a generator control unit in there as well. All extremely well laid out. This gear was the second iteration.

Under each wing is an engine inlet (the third is on top of the fuselage with an S shaped duct to the third engine). The inlets look relatively small for a supersonic aircraft, what strikes you is the careful contouring of the inlet to slow supersonic air down to subsonic speeds for the engine compressors to work optimally. The front profile of the engine inlet is rectangular in 2D. The upper lip of the inlet starts about two feet in front of the lower lip. The sideview of the engine inlet shows a line running from the upper lip and converging with the vertical line running up from the lower lip at about 80% of the height of the inlet (there are no specs in the public domain).The inlet has no moving parts and the air is slowed by the inlet shockwave off the upper lip. The third engine of the trijet has a similar looking inlet, only thing is the geometry appears to run in reverse to the under wing inlets.The engine inlet geometry took about a year and half to finalize and hugely enhanced the engineering team’s capabilities.

A subtle design feature on the XB-1 is the gap between the underwing engine inlets and the wing. This gap is a boundary layer diverter to ensure the air the engines operate in clear air free of any turbulent vortices off the aircraft. While the diverters under the wings are thinner when compared to the center engine diverter on top of the fuselage as the boundary layer there is much thicker. The boundary layer itself is the layer of air that clings to the aircraft and is pulled with it. It is generally much thinner at the nose of the aircraft than the rear. In fact the boundary layer to the rear of the aircraft is so thick, the fasteners on the titanium engine bay covers do not need to be recessed and they actually protrude a bit with no additional drag.

The rudder on the vertical tail which is around six feet tall looks small but actually has about 20% higher authority than predicted by CFD. The all moving horizontal stabilizer (necessary for supersonic flight) is pivoted on a titanium torque tube that runs through the fuselage and connects to the stabilizer on the other side.This ensures the two horizontal stabilizers always move in unison. The horizontal stabilizer itself is positioned below the wings on the horizontal plane to avoid being blanketed by wing vortices.

The XB-1 has five fuel tanks running longitudinally down the length of the aircraft. The onboard flight control computer controls the centre of gravity by pumping fuel forwards or backwards depending on the acceleration or deceleration regime.

The Assembly

Industry 4.0 has revolutionized manufacturing by integrating digital technologies for efficient design, simulation and production. An example of this is computational fluid dynamics (CFD) where engineers ran thousands of simulations to optimize the aerodynamics of the XB-1 at a fraction of the time and cost. Traditionally the designs were validated in physical wind tunnel testing and reiterated upon, but now computers and artificial intelligence run the process end to end. It needs to be acknowledged that modern computing power and software have enabled Industry 4.0. 

Virtual prototyping has ensured a digital replica of the actual physical XB-1 is available at the touch of a keyboard. Generative design has ensured that algorithms can create complex component designs beyond human capability. End to end digitization is a digital information thread maintained from initial design to current status.

Boom has excelled by compartmentalizing the entire end-to-end creation of XB-1 by optimizing each component through the most suitable design and manufacturing process, and then bringing them together in perfect synergy across the whole aircraft.

A quick glance at the XB-1 immediately tells you the aircraft essentially has two colors. Most of the aircraft is white and towards the rear of the aircraft the engine bays are metal. All the white on the XB-1 is carbon composite and most of the metal on the aircraft is titanium.

Boom has designed and constructed the composite parts of the XB-1 inhouse at a facility close to Denver, Colorado. The carbon fibre composites are used for most of the fuselage, wings and tail empennage. The material is lightweight, has very high tensile & flexural strength in addition to high impact resistance & a high strength to weight ratio. The heat resistance is very good. Additionally composites are easier to work with when creating complex aerodynamic shapes as compared to traditional materials such as aluminium.

Boom partnered with TenCate Advanced Composites (now Toray Group) to supply high temperature epoxy resins & prepregs for hot sections such as leading edges (temps as high as 307°F/153°C) and nose. Incidentally these are the same materials used on the Space X Falcon 9. Wing spar load testing was done in 2017.

For the Carbon composite Boom used the hand layup process where engineers manually layered carbon fibre prepreg sheets (pre-impregnated with TenCate’s epoxy resin) into molds to form the fuselage halves in addition to other components such as the wings. The sheet fibers are layered at 0°,45° & 90°, such layering gives the final product the longitudinal, shear / torsional & transverse strength we spoke of earlier. The process began in 2019, the fuselage halves were joined together in 2020. 

Post the layering the layered sheets with their molds, Boom used both autoclave & out of autoclave (OoA) curing using an inhouse oven for the curing of parts. The difference between an autoclave and an oven is the autoclave cures the composite sheets under heat & pressure while the oven heats the composites & their molds in a vacuum. 

During the post processing, the cured parts are checked for fit, surface quality & performance. Excess material is trimmed out and subjected to non destructive testing such as ultrasonic scans or tap tests to detect voids or delamination. Finishing involves surface preparation with primers and cleaners specific to the composite materials, followed by painting. The XB-1 was painted in high gloss white PPG Aerospace CA 9800 paint and added roughly 125 pounds to the aircraft while staying within the centre of gravity limits. The choice of white paint was prioritized by thermal management. The titanium sections were unpainted.

The titanium parts of the XB-1 are at critical high-temperature or high-strength areas of the aircraft . The XB-1 had a total of 21 titanium parts including engine bay covers, exhaust components and structural elements such as parts of the landing gear. Titanium has a high melting point and thermal stability which makes it ideal for such sections.

Boom partnered with VELO3D to produce the titanium parts using additive manufacturing (3D printing). VELO3D used their Sapphire 3D printing system which uses laser powder bed fusion (LPBF). In this process a laser fuses titanium powder (probably Ti-6Al-4V a.k.a titanium-aluminium-vanadium) layer upon layer on a digital CAD model. This method allows for complex geometries & reduced material waste for rapid prototyping which is ideal for XB-1s small batch, high precision needs. The precision & complexity enabled by the process is far superior to traditional processes such as using CNC machines. VELO3D’s expertise helped accelerate XB-1’s production timeline.

The post processing of the 3D printed titanium parts involved heat treating to enhance mechanical properties, surface finishing to remove any surface irregularities & non destructive testing using X-rays and ultrasonic testing. The finished titanium parts were integrated with the carbon composite fuselage.

The use of 3D titanium parts validated additive manufacturing processes for the Overture. In fact the XB-1 has a total of 193 parts on it that are 3D printed!

Most of the avionics used on the aircraft are standard off the shelf and a few that have been created specifically for the XB-1. The XB-1 sits midway between a fly by wire & a manual system. The reason for this is Boom needed the reliability and simplicity of a manual system for the experimental XB-1 at the same time they wanted to develop engineering expertise for a digital flight control system. The Overture will be a fly by wire aircraft. The engines on the XB-1 are General Electric J-85s with afterburners that produce approximately 4100 pounds of thrust each. The engines themselves date back to the 1950s. The XB-1 was rolled out in 2020, it was only much later (2022) that Boom decided to make its own Symphony engines which were unveiled at the Farnborough International Airshow.

The Overture

As of Jun’24 the Boom Superfactory was completed in North Carolina and currently the Superfactory is being tooled up. The factory will eventually produce 66 aircraft per year. 

Boom is currently prototyping its Symphony engine sprint core using surprise surprise 3D printing! Boom Symphony expects to produce thrust early 2026.

The Overture prototype should be ready by 2029.

The XB-1 ‘ The Little Plane that Could’….. 

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