Planes for SAI May Be Closer and Cheaper Than Previously Thought | SRM360

One of the primary technical barriers to a deployment of stratospheric aerosol injection (SAI) is right there in the name: the stratosphere. For the most commonly studied and cited SAI deployment scenarios, the material used to reflect a small percentage of sunlight back into space – most likely sulphate aerosols – would need to be hauled up to around 18 to 20 kilometres (about 59,000 to 65,000 feet) in altitude. Most of the world’s airplanes simply do not fly that high.

Though it has been widely accepted that this is a surmountable challenge, new work from a company called Iris Aero suggests the runway towards an SAI-capable aircraft might be substantially shorter and cheaper than previously assumed. According to Dr John Langford, an engineer and founder of several aeronautics companies including Iris Aero, two working prototypes of an aircraft that could feasibly be used for SAI deployment could be built within only three years for about $120 million.

“It can be done for significantly less money than some of the folks in the SRM field were coming to believe it was going to take”, he told SRM360. Previous estimates varied somewhat, but those numbers would chop at least a few years and a substantial number of dollars off the project.

Langford presented his company’s study on a plane design they call the IR-1 at the recent Frontiers in Climate Systems Engineering conference, hosted by the University of Chicago’s Climate Systems Engineering initiative (CSEi) in May. Over the course of about a year, funded by a grant from CSEi, his team sketched out the design of an unmanned aircraft, capable of bringing material up to the high altitudes required.

Some other experts seem persuaded that the timeline is realistic. “The IR-1 is another confirmation that this aircraft is reasonably buildable from technologies that already exist”, said Wake Smith, a lecturer at Yale School of the Environment who has studied aircraft and other technical details of SRM, during the CSEi panel discussion on SAI deployment technologies. “There aren’t big technological breakthroughs that would be required to do this.”

A rendering of the IR-1 design by Iris Aero

Details of the IR-1

A key reason the aircraft could be made so quickly is that it would use a commercially available engine – specifically, the Rolls Royce AE 3007H. This engine is capable of high-altitude flight, and is in use both for military applications and on some commercial and business jets.

“You’ve got to start with the engine, because if you’re trying to do something from scratch, it’s a totally different ballgame”, Langford said.

The IR-1 would be built of composite materials and feature two of those Rolls Royce engines, and would be capable of carrying 15,000 pounds (almost 7,000 kg) of material to the stratosphere. Importantly, that material and the mechanisms to disperse it would be contained in a swappable tank attached to the bottom of the fuselage. This would allow for more efficient and safer refilling between flights and means that changing the material or dispersal method would not require a redesign of the aircraft.

“It will be a little bit heavier for sure, but right now we think it’s probably a pretty big win, and is sort of a fundamental part of our concept at this stage”, Langford told SRM360. Because no crew would be on board, the plane would not need to be pressurised, reducing costs and weight. The team is preparing a paper detailing the IR-1’s design, likely to be published in early 2027.

Previous designs and other methods

There have been other aircraft designs for theoretical SAI deployments, such as the SAIL (Stratospheric Aerosol Injection Lofter). Described in a 2018 paper from Smith and Gernot Wagner, now at Columbia Business School, they imagined a 15-year development plan for the aircraft. This manned aircraft design would be substantially larger than the IR-1, weighing almost eight times as much, and would carry twice the payload. And critically, it would rely on an engine that is quite a bit harder to obtain.

The GE F118 is a military-only engine, and was designed to power the B-2 Spirit, commonly known as the stealth bomber. “The SAIL isn’t an aircraft that one could go out and build because the military likely won’t sell you that engine”, Smith said during the CSEi panel.

Notably, other recent research has shown that a form of SAI known as high-latitude low-altitude could also feasibly provide effective (though less efficient) global cooling. By dispersing the aerosols closer to the poles, the altitude requirements come down into ranges flown by existing aircraft, slashing the development timeline substantially.

Scaling up

How quickly could this design be scaled up to cool the planet?

The IR-1 planes would fly short, 90-minute missions up to around 18 km (60,000 feet) to disperse the material from their tanks before heading back to base to swap out...