The armed forces of the United States are pursuing acquisition of unconventional VTOL aircraft to perform niche missions, with Congress taking an increasing interest in these capabilities.
The primary advantage of vertical take-off and landing (VTOL) aircraft lies in their flexibility. They operate independent of runways and also from relatively confined areas including parking lots, roads, ship decks, and woodland clearings.
The US Air Force (USAF) is currently pursuing the Agility Prime programme which evaluates civilian electric-powered VTOL aircraft designs (commonly referred to as eVTOL or ‘air taxis’) for military applications. Agility Prime is conducted by the USAF’s technology incubator AFWERX (not an acronym but a play on ‘Air Force Works’). A subordinate unit of the Air Force Research Laboratory (AFRL), AFWERX brings together innovators from (mostly) small business and startup firms as well as academia to seek solutions to pressing military requirements. The organisation’s stated goals are to develop technology, to transition dual-use capabilities to operational capability, and to achieve this more quickly than through traditional development channels.
In 2021, AFWERX also launched the high-speed VTOL (HSVTOL) Concept Challenge to solicit industry proposals for high-performance designs optimised for special operations missions. Ultimately AFWERX did not pursue the programme past 2022, but the HSVTOL effort has been picked up by the Pentagon’s Defense Advanced Research Projects Agency (DARPA).
Credit: Bell-Textron
Agility Prime
The Agility Prime programme was initiated in 2020. Civilian market electric and hybrid propulsion aircraft are primarily conceived for urban environments. Electric propulsion significantly reduces decibel levels and heat signature, making eVTOL aircraft potentially suited for military applications which benefit from a stealth-approach capability. Alternately they could be used in scenarios where low emissions are desired for other reasons, such as minimising disruption and inconvenience to personnel. Early in the programme, Air Force representatives suggested potential missions might include special operations insertion, SWAT transport, search and rescue (SAR), battlefield casualty evacuation (CASEVAC) or assault carrier. More recently, the USAF cited disaster relief, humanitarian efforts, medical transport, and short-range movement of personnel and materiel as potential applications.
Many eVTOL aircraft also feature optional autonomous flight capability, another desired attribute. On the other hand, eVTOL aircraft still tend to have shorter range and endurance than conventionally fuelled helicopters and tiltrotors. Agility Prime only considers aircraft with a minimum 160 km range and 160 km/h flight speed. For the moment, eVTOL aircraft seem best suited for augmenting and supporting rather than replacing traditionally fuelled aircraft. Taking CASEVAC as an example, a stealthy (and potentially unmanned) eVTOL would enter the combat zone, rescue the wounded, then ferry them to a waiting long-range aircraft for evacuation.
The Agility Prime programme is structured around three phases:
- Discover: the programme identifies and evaluates eVTOL technologies, engaging with industry partners and conducting flight demonstrations;
- Assess: eVTOL prototypes undergo rigorous testing, including safety, performance, and certification assessments;
- Integrate: The final phase focuses on integrating eVTOLs into operational scenarios and exercises.
Since 2020, AFWERX has awarded hundreds of contracts to stimulate development in the eVTOL industry (Phase 1) and invited numerous firms to demonstrate their air taxi concepts (Phase 2). However, only a good dozen have advanced to Phase 3 evaluation. These include:
Heaviside (Kitty Hawk): The first operational exercise involving an eVTOL was conducted in May 2021 utilising a Heaviside aircraft developed by California-based Kitty Hawk Corporation. Capabilities tested included CASEVAC, personnel recovery and logistics transport.
Credit: Kitty Hawk Corporation
The aircraft accommodates one person. Flight mode options include on-board pilot, remote control, or autonomous completion of a pre-programmed mission. The aircraft utilises eight fully electric propellers which tilt to transition from vertical to horizontal flight. Each rotor is independently powered, eliminating the risk of a central system failure disabling the aircraft. The operational range on one battery charge is approximately 160 km. The aircraft reportedly consumes half as much energy as a conventional electric car. At an altitude of 500 m, the acoustic signature on the ground is 35 dB, or approximately 1% that of a helicopter, thereby minimising detection risk.
Alia-250 (Beta Technologies): Vermont-based Beta Technologies has designed both a conventional take-off and landing (CTOL) and a VTOL version of its all-electric Alia aircraft. While Agility Prime’s focus is on eVTOL technology, it is also evaluating the conventional Alia prototype. Both variants have a dual-seat cockpit, but require only one pilot. The aft compartment accommodates four additional personnel or three pallets. The Alia achieves 220 km/h airspeed, a 460 km range and a recharging time under one hour using a 350 kW Level 3 DC fast charger. Agility Prime experiments have shown the aircraft consumes circa USD 25 worth of energy per flight hour.
The USAF’s 413th Flight Test Squadron conducted an intensive three-month testing phase of the aircraft at Duke Field, Florida from October 2023 to January 2024. During this period the aircraft – and the Agility Prime programme – achieved several milestones, according to a military press release. This included conducting the first-ever direct radio communication between an eVTOL and a military aircraft, and validating mobile charging solutions for contingency and austere operating locations. The three-month stint at Duke Field included numerous critical resupply, cargo delivery and personnel transport scenarios, and ended with a simulated CASEVAC mission during which the Alia transported a stretcher patient 130 km (70 NM) to the airbase, freeing up a HH-60W helicopter for a different mission.
Credit: Beta Technologies/Brian Jenkins
In addition to operations, maintenance personnel acquired a fuller picture of requirements for keeping the eVTOL airworthy. The exercise confirmed the potential of eVTOL aircraft to “augment the existing fleet with additional low-cost assets to assist in mission execution so battlefield aircraft can stay in the fight,” said Maj Riley Livermore, 413th FTS Futures Flight commander. The aircraft also demonstrated the ability to self-deploy over the 2,400 km distance from the corporate facility in Vermont to Florida to begin the trials.
S4 (Joby Aviation): The first acquisition contract to be placed under the Agility Prime programme was awarded to California-based Joby Aviation on 25 April 2023. The firm’s optionally manned S4 eVTOL aircraft accommodates up to five people (including a pilot) or a 450 kg payload. Performance parameters include a 3,400 m service ceiling, 370 km/h maximum speed, 275 km range, and a 45 dB noise level in forward flight. The first of up to nine possible aircraft authorised under the contract arrived at Edwards AFB, California, in September 2023. At least one additional delivery to Edwards is scheduled for 2024. Beyond that, eVTOLs under the contract might be delivered to other air bases or to different military services (the US Marine Corps has expressed a special interest in the S4). Evaluation at Edwards AFB is being led by the USAF’s Emerging Technologies Integrated Test Force (ET-ITF). Rather than scripted exercises, the aircraft will be integrated into regular base operations. The initial focus is placed on studying the S4’s potential for short-to-mid range cargo deliveries as well as for security and maintenance personnel transport between the sprawling (1,200 km2) air base’s various test ranges. Another area of interest is verifying the promise of consistently lower operating costs when compared to conventionally powered aircraft or even ground vehicles.
Credit: USAF
In addition to aircraft, Agility Prime is investigating and investing in technology to support eVTOL operations. This includes mobile high-speed charging systems to permit fast turnaround of aircraft while minimising wear and tear on aircraft batteries. The latest step in this regard was the December 2023 demonstration of a mobile battery care station developed by Utah-based Arctech Charge. The demonstration took place at Hill AFB (Utah). According to the accompanying AFRL press release, the station was specifically designed to support eVTOL aircraft testing and operations, and can charge up to 36 batteries simultaneously. Depending on operational requirements, personnel can select from two battery care profiles: one for extending battery life cycles and one for charging as quickly and safely as possible.
Agility Prime moving forward
By acquiring the first Joby Aviation S4 in September 2023 and conducting several hover tests over the ensuing weeks, the USAF met its stated goal of fielding an eVTOL before the end of 2023. The recent and ongoing Alia and S4 evaluations at Duke Field and Edwards AFB represent the next phase of the larger developmental test and evaluation (DT&E) effort being conducted by the USAF to assess electric aviation’s applicability for all military branches. The Pentagon views this process as a clear and effective means to mature eVTOL transportation to the point where it can transition to acquisition programmes of record. “This partnership between AFWERX, Agility Prime and the ITF is a new, direct, agile path to progress technology through the Acquisition Valley of Death,” said Maj Phillip Woodhull, ET-ITF director, upon delivery of the first Joby S4 to Edwards AFB.
For its part, AFWERX has been shifting its emphasis from foundational research to a greater focus on delivering capabilities to meet USAF’s operational imperatives or OIs. “AFWERX 3.0 will have a deliberate focus on fielding capability,” said Andrew Hunter, Assistant Secretary of the Air Force for Acquisition, Technology and Logistics in December 2022. “Therefore, we’ll apply additional effort to not only creating research and development funding opportunities, but also linking them to the procurement funding necessary to turn these projects into delivered capability at scale.” Congress seems intent on holding the Air Force to its word. The House Armed Services Committee’s proposed Fiscal Year 2025 National Defense Authorization Act (NDAA) would require USAF to establish a high-level working group tasked with the transition of Agility Prime technologies into acquisition or R&D programmes of record, either for the Air Force or throughout the military. As conceived, the working group would operate through the end of Fiscal Year 2027, and present annual progress reports to Congress.
HSVTOL challenge
The standards for high-speed VTOL concepts significantly exceed those required of eVTOL candidates. This reflects the fact that HSVTOL development serves a priority requirement of the joint US Special Operations Command (SOCOM) and the Air Force Special Operations Command (AFSOC), which are seeking an enhanced replacement for AFSOC’s CV-22 Osprey. Performance characteristics call for high-end, runway independent aircraft capable of 740 km/h or higher flight speed. Required attributes formulated by AFWERX in 2021 include VTOL capability, “jet-like” speeds (although not necessarily jet propulsion), rapid aerial refuelling ability, collision avoidance and detection systems, and configurable mission-specific payload accommodation. Advanced navigation, situational awareness/threat detection and stealth systems were also included in the AFWERX graphic announcing early contract awards for concept development. The aircraft must be capable of conducting infiltration/exfiltration of SOF personnel and equipment, tactical mobility missions (runway independent tactical airlift), personnel recovery and aeromedical evacuation. Taken together, the performance parameters are geared toward enhanced mobility over greater distances (exemplified by the Indo-Pacific theatre) and greater survivability in contested environments; the “jet-like” speed requirement not only improves response times, but also permits SOF aircraft to be escorted by and jointly operate with tactical fighters.
Credit: Transcend Air Corporation
AFWERX awarded Phase 1 (Concept Design) contracts to 11 firms in early 2022. At that point, Dr Reid Melville, Chief Innovation Officer at AFRL’s Transformational Capabilities Office, stated that the solutions being offered would “help us understand how to build a new class of air vehicles.” The firms submitted their reports in June 2022. The Air Force subsequently awarded Phase II small business technology transfer R&D contracts to a few of the participating firms, including Transcend Air Corporation which had presented the Vy 400 tilt-wing concept. The five-passenger design promised 640 km/h airspeed and a 720 km range. The proposal was initially considered to have the best price-performance ratio among the competitors, said Transcend’s President and COO Peter H. Schmidt. Ultimately, however, AFWERX chose not to pursue the HSVTOL Challenge initiative further.
SPRINT
Instead, in December 2022 DARPA reached out to the previous programme’s participants to announce that it was now pursuing its own HSTOVL programme under the designation Speed and Runway INdependent Technologies (SPRINT); the agency subsequently went public with the programme in early 2023. While SPRINT is not the official successor to the AFWERX HSVTOL Challenge, it clearly builds upon the earlier programme’s foundation. SOCOM is actively partnering with DARPA and carrying a major portion of the programme cost. According to DARPA, SPRINT aims to design, build, and fly a “proof-of-concept technology demonstrator” X-plane to “validate enabling technologies and integrated concepts that can be scaled to different size military aircraft.” Performance parameters are similar to those of the AFWERX HSVTOL programme, including a speed requirement of 740–830 km/h. Other requirements include a 320 km range, a flight altitude of 4,600–9,200 m, a 2,270 kg payload capacity, and a roughly 8×2.5 m cargo bay capable of accommodating a small vehicle or 2.5 pallets. The design should be scalable to permit building aircraft of various sizes and endurance levels.
In December 2022, DARPA solicited proposals from the firms which had participated in the AFWERX HSVTOL Challenge. Basic 10-page proposals were presented in March 2023, with detailed papers following in May 2023. Ultimately only Bell Textron, Aurora Flight Sciences (which had not participated in the AFWERX programme), Northrop Grumman Aeronautic Systems and Piesecki Aircraft Corporation were invited to proceed with the SPRINT programme.
SPRINT is divided into three main phases. Phase 1 (Design) is divided into two sub-phases, each running for six months. Phase 1A (Concept Design) began in late November 2023 with awards to the four abovementioned firms; results were expected to be presented in late May or early June 2024. Phase 1B (Preliminary Design) will continue and refine the basic concept designs, adding details regarding the component parts and specifications for the aircraft. DARPA is expected to eliminate at least one competitor going into 1B. On 20 May 2024, the agency announced that Aurora’s blended-wing/fan-in-wing design had advanced to 1B. In late May 2024, the agency announced it had selected the Aurora and Bell Textron concepts to advance to Phase 1B, while eliminating Northrop Grumman and Piesecki from further consideration.
Moving forward with SPRINT
DARPA is expected to downselect to only one competitor for Phase 2 of the SPRINT programme, which is expected to begin in 2025 and run for 12–15 months. It will cover both detailed design and construction of the flyable prototypes prototype, which are is expected to be unmanned and not more than one-third the size of the objective aircraft. It is likely that only one – or at most two – firms will advance to this stage. Phase 3 will see the initial prototype test flights, currently slated to begin in 2027. Phases 2 and 3 together are scheduled to last 30 months. Following in-depth ground and air testing by the design firm, the military may assume control over the prototype to conduct additional flight tests before making a decision regarding construction and test of a full-scale prototype.
This will not lead directly to an acquisition programme of record. As DARPA has made clear from the beginning, the SPRINT X-plane is a proof-of-concept technology demonstrator intended to validate enabling technologies and integrated concepts which could lay the foundations for future aircraft designs. However, given the recent cancellation of the US Army’s Future Attack and Reconnaissance Aircraft (FARA) programme and SOCOM’s need for enhanced capabilities, the results of SPRINT are almost certain to flow into future design and development programmes.
Sidney E. Dean
