A proposal from rotorcraft manufacturer Bell has advanced in a Pentagon X-plane competition to explore novel technologies for vertical take-off and landing (VTOL) aircraft.

Known as Speed and Runway Independent Technologies (SPRINT), the project aims to produce flight-capable prototypes that can achieve fixed-wing jet speeds in horizontal flight, without relying on traditional airfields.

The programme is being overseen by the secretive Defense Advanced Research Projects Agency (DARPA), which selected Bell, Aurora Flight Sciences, Piasecki Aircraft and Northrop Grumman as participants in November 2023.

Aurora’s design proposal – a blended-wing body type incorporating fan-in-wing rotors for vertical lift – was advanced by DARPA into the next phase of SPRINT on 30 April.

Now Bell is joining Aurora in Phase 1B, with an aircraft that looks almost nothing like its competitor’s.


Source: Bell

Bell’s proposal involves a tiltrotor-like aircraft, with the addition of a third, jet-powered flight mode in addition to the rotor-driven vertical and horizontal flight modes found in existing designs

Building on its expertise developing the world’s first, and thus far only, operational tiltrotor, Bell’s SPRINT design appears to build upon the company’s experience pioneering the Bell-Boeing V-22 Osprey.

Called High-Speed Vertical Takeoff and Landing (HSVTOL), the concept could represent the next stage of tiltrotor technology. Earlier descriptions from Bell, alongside a new concept rendering, depict a tiltrotor-like aircraft, with the addition of a third, jet-powered flight mode.

In addition to the rotor-driven vertical and horizontal flight modes found in the V-22, the HSVTOL concept would be able to convert into this third mode for long distance and high-altitude flights.

Diagram from 'Tiltrotor aircraft having rotary and nonrotary flight modes' patent published in 2017 c Bell and US Patent & Trademark Office

Source: US Patent and Trademark Office

A 2017 patent request from Bell describes a “tiltrotor aircraft having rotary and non-rotary flight modes” that incorporates folding rotorblades to add a third, jet-powered flight mode

“Bell’s HSVTOL technology blends the hover capability of a helicopter with the speed, range, and survivability of jet aircraft,” the company says.

Drawings submitted to the US Patent and Trademark Office in 2017 depict a tiltrotor-style aircraft with rotorblades capable off folding down into the engine nacelles for a streamlined profile during high-speed flight. The patent filing describes the HSVTOL concept as a “tiltrotor aircraft having rotary and non-rotary flight modes”.

During a January 2023 visit to Bell headquarters in Fort Worth, Texas, the company’s vice-president of engineering Jason Hurst described the concept as something “brand new” in aviation.

At the time, Bell was internally funding its HSVTOL research and still seeking a government partner for the effort. That development work included sled testing an HSVTOL propulsion system at Holloman AFB in September 2023. 

DARPA launched the SPRINT effort later that year in November.

“We completed our initial risk-reduction efforts with our sled test demonstration at Holloman Air Force Base, and we look forward to building on this success with our continued work with DARPA,” Hurst said on 28 May following Bell’s advancement to the next phase of SPRINT.

DARPA has set ambitious goals for the programme, including an airspeed target of 400-450kt (740-830km/h) for the prototype aircraft.

Aurora claims its blended-wing design will be able to maintain a cruising speed of 450kt, while Bell says the HSVTOL technology can surpass 400kt.

HSVTOL Risk Reduction Testing

Source: Bell

Bell began sled testing technology for the High-Speed Vertical Take-off and Landing concept in September 2023

Currently the only aircraft capable of achieving such airspeeds, while also boasting VTOL capability, are a handful of single-seat fighter jets such as the Lockheed Martin F-35B and BAE Systems/Boeing AV-8B Harrier II.

However, in the case of SPRINT, the goal appears to be a more versatile platform that would be capable of delivering troops or supplies into remote and austere locations.

DARPA is partnering on the project with US Special Operations Command (SOCOM), which procures equipment for the Pentagon’s coterie of elite commando units that are often called to missions far from friendly infrastructure.

Notably, SOCOM in 2023 told FlightGlobal the goal of SPRINT is to deliver “drastically” better speeds than even the US Army’s new Future Vertical Lift rotorcraft – another Bell design.

The Bell V-280 Valor tiltrotor, which the army selected in 2023 to succeed its existing fleet of Sikorsky UH-60 Black Hawks, can reach cruising speeds over 280kt.

DARPA says Phase 1B of SPRINT will last approximately 12 months, culminating with a preliminary design review that could lead to subsequent fabrication and flight testing.