DARPA’s SPRINT X-Plane Contenders Mix Unknowns with One Known

DARPA has selected four firms to design a 500-mph VTOL X-plane. Only one has a relatively proven design.

The Defense Advanced Research Projects Agency (DARPA) announced on Monday that it has selected Aurora Flight Sciences, Bell Textron, Inc., Northrop Grumman Aeronautic Systems and Piasecki Aircraft Corporation to submit conceptual designs for its Speed and Runway Independent Technologies (SPRINT) project.

SPRINT is a joint DARPA/U.S. Special Operations Command project which seeks tactical transport aircraft which combine high subsonic speed, substantial range and significant internal payloads with “runway independence” – a euphemism for vertical takeoff and landing (VTOL) capability.

Such high-speed VTOL aircraft are envisioned as relevant to the tactical realities of the Indo-Pacific where long over-water distances with few long runways in between present a challenge for U.S. force mobility.

Aircraft which can bridge those distances, set down and alight from short, unprepared runways (or merely landing zones) on remote islands and elsewhere could offer in-demand capability. The same characteristics make them suitable for a range of special operations missions as well.

While SPRINT ultimately intends to design, build, and fly an X-Plane, DARPA will use Phase 1A to evaluate designs from the firms above. It will do so with an eye firmly on the goal of moving to an aircraft “with the ability to cruise at speeds from 400 to 450 knots at relevant altitudes and hover in austere environments from unprepared surfaces,” according to its release on the project.

Aurora Flight Sciences will submit a turbojet-powered blended wing body design. Aurora describes it as a “a high lift, low drag fan-in-wing (FIW) demonstrator aircraft…with embedded engines and moderate sweep, with a vertical flight design comprised of embedded lift fans linked to the engines via mechanical drives.”

The design potentially offers low-drag, yielding speed and range but it also may confer stealth to the concept. Despite the presence of twin vertical tails in the concept drawing Aurora provided, it shares the familiar basic stealth shape of the B-2 and B-21 bombers and other experimental aircraft.

While blended wing body designs are generally thought to offer significant cargo volume, none has been physically executed at scale yet to prove that assumption. The embedded lift fan feature that Aurora mentions has already long been flying in Lockheed Martin’s F-35B Lighting II though the concept here places the fans in the wings rather than the fuselage a la F-35.

The arrangement looks like it could be effective in high-speed cruise flight but there may be questions about the weight and complexity of a blended wing body aircraft with multiple lift fans. One will surely center on the design’s suitability to VTOL operation in austere environments where dirt, dust, debris, heat and other impediments challenge integrated engine/lift-fan configurations.

Aurora has some experience with lift fans via its Excalibur UAS and its partner, Boeing has previous experience with its own X-48 blended wing body aircraft which featured external engines. Neither firm has the operational lift fan experience that Lockheed has gained.

Neither Northrop Grumman nor Piasecki has so far revealed information on the designs they will submit for SPRINT. It’s possible that Northrop Grumman may leverage design work it has done for DARPA for an unmanned AdvaNced airCraft Infrastructure-Less Launch And RecoverY (ANCILLARY) demonstrator. Little information is available on the UAV but it looks to be a sort of tilt-rotor design.

Piasecki is also participating in ANCILLARY and has experimented with unmanned ducted-fan VTOL prototypes but it’s not clear if such a configuration figures in their SPRINT design.

That leaves Bell, the only firm among the four awardees with an operational medium speed tactical transport (the V-22) and a relatively high-speed future operational tactical transport, the V-280 Valor.

The company has been at work on what it calls an HSVTOL (High Speed Vertical Takeoff and Landing) concept for several years. It envisions a possible family of HSVTOL aircraft of different sizes with potentially different propulsion configurations. All would share the same tilt rotor-esque basic shape and the folding rotor concept that Bell is presently experimenting with in New Mexico.

It’s reasonable to say that concept though HSVTOL is, it is the only real “known” among the likely design submissions for SPRINT. Bell has already been at work on the concept as one of 11 companies participating in the AFWERX HSVTOL Concept Challenge, a crowdsourcing effort to advance such concepts.

In essence, Bell’s V-22, V-280 and previous HSVTOL work represent what may be a more technologically-ready concept than the other SPRINT awardees are bringing to the table. Bell could be said to have done its homework in advance.

In a set of email responses, a Bell spokesperson told me that, “Bell’s DARPA SPRINT X-plane concept leverages this work and allows for rapid development.” Its projected scalability (touted as ranging from 6,000 to 100,000 pounds takeoff weight) raises the question of what size aircraft Bell is offering. That, Bell says, is contingent upon DARPA’s requirements.

DARPA has previously specified that for SPRINT it seeks a potentially optionally-piloted aircraft capable of carrying a payload of 5,000 pounds, with a substantial 30-foot-long, eight-foot-wide cargo bay capable of carrying a small vehicle or two and a half pallets. The ask includes endurance of 1.5 hours and a 200 nautical mile range.

That puts the SPRINT requirements, aside from speed, slightly below the size, capacity, range and endurance of the V-280. As such it’s reasonable to expect Bell’s design to be near-V-280-sized.

Given what HSVTOL work has already been done, how much more I wondered will be required to meet DARPA’s submission requirements?

“Bell’s investment to mature HSVTOL concepts has reduced risk to the DARPA SPRINT program,” the company said. “We will look to move rapidly into subsequent phases if the Government is interested.”

With its seemingly ready design and a foreseeable potential market for high-speed VTOL, I asked if Bell views the DARPA project merely as an adjunct to its development arc. In other words, will Bell proceed with HSVTOL development irrespective of SPRINT?

“As we look at rapid development of the X-plane we will continue to evaluate transition opportunities as well as what other technology development efforts are needed to begin to shorten overall timeline to a fielded capability,” the company’s spokesperson replied.

DARPA SPRINT participation and subsequent feedback surely furthers Bell’s design work (from a funding perspective alone) but regardless of whether it advances past Phase 1a, it appears to be on course to building HSVTOL prototypes and eventually bringing them to market.

Whether that could be said of the other SPRINT participants and their designs is tough to call. It’s another reason why there’s arguably one-known in this X-plane project.