Researchers reviewed all lofting technologies that seem plausible as methods to put 100,000 tons per year of sulphur to an altitude of up to ~20 km in 2033. The program then scales to 5 million tons per year. Their main research involved engaging directly with commercial aerospace vendors to elicit what current and near-term technology platforms can achieve at what cost. We have met or corresponded directly with: Airbus, Boeing, Bombardier, Gulfstream, Lockheed Martin, Northrup Grumman; GE Engines, Rolls Royce Engines; Atlas Air, Near Space Corporation, Scaled Composites, The Spaceship Company, Virgin Orbit, and NASA, the latter in respect of its high-altitude research aircraft fleet.
They eliminated aerostats and hoses because the technology is not ready or untested. Modified business jets, noted prominently in McClellan et al (2010, 2012) study, are incapable of reaching altitudes above ~16 km. High payload, high altitude aerostats have been hypothesized but not yet successfully tested, and in all events, are operationally fragile, unable to operate in adverse weather conditions. Tethered hoses are even less technologically mature and to-date untested. Military fighters such as the F-15 have reached altitudes of ~18 km in the context of record-setting ballistic climbs in ideal conditions, but they are incapable of either sustained flight or regular operations at such altitudes.
They estimate total development costs of $~2 billion for a special airframe, and a further $350 million for modifying existing low-bypass engines. These numbers are toward the lower end of McClellan et al (2010, 2012) range of $2.1 to $5.6 billion and significantly below the TU Delft students’ estimates of $14 billion for its purpose-built Stratospheric Aerosol Geoengineering Aircraft, or SAG.
The required SAIL plane is equivalent in weight to a large narrow-body passenger aircraft such as the A321, or in Boeing terms, sized between the 737–800 and the 757–200. In order to sustain level flight in the thin air encountered at altitudes approaching ~20 kms, SAIL requires roughly double the wing area of an equivalently sized airliner, and double the thrust, with four engines instead of two. (While maximum thrust requirements of most aircraft are defined by takeoff, SAIL’s engines are configured to perform at high altitudes.) At the same time, its fuselage would seem stubby and narrow, sized to accommodate a heavy but dense mass of molten sulfur rather than the large volume of space and air required for passenger comfort. SAIL would therefore have considerably wider wingspan than length. Its compact fuselage, however, would sit behind a conventional manned cockpit. While it is easy to imagine SAIL migrating to unmanned cockpits over time, under current certification rules, it would be substantially faster and therefore cheaper to certify the aircraft with onboard pilots.
The preliminary design for SAIL calls for a length of ~46 m, a wingspan of ~55 m, and a wing area of ~250 m2, with an aspect ratio of ~12:1. The maximum structural payload would be ~25 t, with maximum takeoff weight (MTOW) of ~100 t, operating empty weight (OEW) of ~50 t, and maximum fuel load of ~32 t. The aircraft would have 4 wing-mounted low-bypass engines, modified for high-altitude operations with an aggregate take-off thrust of ~25–30 t and a thrust-to-weight ratio of ~30%. (GE Engines considers its F118 engine adequate, noting that it powers the NASA Global Hawk aircraft to similar altitudes; its Passport 20 engine may similarly be capable. Rolls Royce suggests its BR710 or BR725 engines.) The design will require a smaller fifth centerline auxiliary power unit for bleed air and onboard combustion of the molten sulfur payload.
Researchers reviewed the capabilities and costs of various lofting methods intended to deliver sulfates into the lower stratosphere. They lay out a future solar geoengineering deployment scenario of halving the increase in anthropogenic radiative forcing beginning 15 years hence, by deploying material to altitudes as high as ~20 km. After surveying an exhaustive list of potential deployment techniques, they settle upon an aircraft-based delivery system. They conclude that no existing aircraft design—even with extensive modifications—can reasonably fulfill this mission. However, they also conclude that developing a new, purpose-built high-altitude tanker with substantial payload capabilities would neither be technologically difficult nor prohibitively expensive. They calculate early-year costs of ~$1500 per ton of material deployed, resulting in average costs of ~$2.25 billion per year over the first 15 years of deployment. They further calculate the number of flights at ~4000 in year one, linearly increasing by ~4000 per year. They conclude by arguing that, while cheap, such an aircraft-based program would unlikely be a secret, given the need for thousands of flights annually by airliner-sized aircraft operating from an international array of bases.