Environmental Research Letters LETTER • OPEN ACCESS Stratospheric aerosol injection tactics and costs in the first 15 years of deployment To cite this article: Wake Smith and Gernot Wagner 2018 Environ. Res. Lett. 13 124001 View the article online for updates and enhancements. This content was downloaded from IP address 54.235.163.141 on 22/12/2018 at 22:53
Environ. Res. Lett.13(2018)124001https://doi.org/10.1088/1748-9326/aae98d LETTER Stratospheric aerosol injection tactics and costs in thefirst 15 years of deployment Wake Smith1and Gernot Wagner2 1Associate Fellow, Trumbull College, Yale University, and Lecturer in Yale College, New Haven CT, United States of America2Harvard University Center for the Environment, 26 Oxford Street, MA 02138, United States of America E-mail:gwagner@fas.harvard.edu Keywords:solar geoengineering, albedo modification, solar radiation management, high-altitude aircraft Abstract We review the capabilities and costs of various lofting methods intended to deliver sulfates into the lower stratosphere. We 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, we settle upon an aircraft-based delivery system. Unlike the one prior comprehensive study on the topic (McClellanet al2012Environ. Res. Lett.7034019), we conclude that no existing aircraft design—even with extensive modifications—can reasonably fulfill this mission. However, we also conclude that developing a new, purpose-built high-altitude tanker with substantial payload capabilities would neither be technologically difficult nor prohibitively expensive. We calculate early-year costs of ∼$1500 ton 1of material deployed, resulting in average costs of∼$2.25 billion yr1over thefirst 15 years of deployment. We further calculate the number offlights at∼4000 in year one, linearly increasing by∼4000 yr 1. We conclude by arguing that, while cheap, such an aircraft-based program would unlikely be a secret, given the need for thousands offlights annually by airliner-sized aircraft operating from an international array of bases. 1. Introduction Solar geoengineering is commonly seen to be subject to what some call its‘incredible economics’(Barrett 2008)and, more specifically, its‘free driver’effect: its direct costs are so cheap compared to its potential climate impacts so as to reverse many of the properties of the so-called‘free rider’problem governing carbon mitigation decisions and climate policy more broadly (Wagner and Weitzman2012,2015, Weitzman2015). The governance problem becomes one of cooperation to restrain rather than increase action. Here we probe these economic assertions and review the capabilities and costs of various lofting methods intended to deploy sulfates into the lower stratosphere, the leading proposed method of solar geoengineering(Keith 2000, Crutzen2006, National Research Council2015). Stratospheric Aerosol Injection(SAI)would require lofting hundreds of thousands to millions of tons of material each year to altitudes up to∼20 km. Here we seek answers to three questions: if SAI deploymentwere to commence within the foreseeable future with the tools and technologies at our disposal, how would such deployment be physically achieved, how much would it cost, and could it be done in secret? National Academies of Sciences(NAS), Engineer- ing and Medicine(1992)provides an early review of SAI deployment options, deriving detailed pricing for naval rifles and two different balloon systems (appendix Q.11). McClellanet al(2012)attempt to provide thefirst comprehensive answer to this ques- tion, publishing results from an earlier Aurora Flight Science Corporation analysis(McClellanet al2010). Like McClellanet al(2010,2012), and later reviewed by Moriyamaet al(2017 ), we explore an array of dif- ferent SAI lofting technologies and given our more specific mission criteria, we conclude that aircraft are the only reasonable option. Unlike them, we conclude that modified existing business jets are incapable of flying above∼16 km, a conclusion confirmed directly by the manufacturers of the jets in question. This directly contradicts both McClellanet al(2010,2012) OPEN ACCESS RECEIVED23 July 2018 REVISED14 October 2018 ACCEPTED FOR PUBLICATION19 October 2018 PUBLISHED23 November 2018 Original content from this work may be used under the terms of theCreative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s)and the title of the work, journal citation and DOI. © 2018 The Author(s). Published by IOP Publishing Ltd
and IPCC(2018). The latter demonstrates the large influence McClellanet al’s analysis has had on the broader conversation. IPCC(2018)states that‘there is high agreementthat aircrafts after some modifications could inject millions of tons of SO 2in the lower strato- sphere(∼20 km)’(chapter 4). IPCC cites three studies in support of that statement, including McClellanet al (2012). However, both of the other two studies, in turn, base their conclusions, in large part, on McClellanet al’s earlier analysis. Irvineet al(2016) also cites the other(Davidsonet al2012), which, in turn, cites McClellanet al(2010). Robocket al(2009) provides one further independent analysis, reviewing capabilities of militaryfighters and tankers. We agree with Robocket al(2009)that militaryfighters are capable of reaching∼20 km, but they are incapable of sustainedflight at that altitude(see table2below). We further conclude that no other existing aircraft have the combination of altitude and payload cap- abilities required for the mission, leading us instead to the design of a new plane. We propose such a plane and call it SAI Lofter (SAIL), describing its basic specifications and provid- ing detailed cost estimates for its design, manufacture, and operation under a hypothesized solar geoengi- neering scenario of halving the increase in radiative forcing from a date 15 years hence. We do not seek to foretell future technological breakthroughs, nor do we guess at costs in 50 or 100 years when next-generation deployment technologies would likely become avail- able. Further, we do not consider solar geoengineering methodologies other than SAI or materials other than sulfate aerosols(Keith2000, Keithet al2016).We instead hope to illuminate discussions of direct SAI deployment costs based on existing technologies, thereby facilitating further benefit-cost comparisons and grounding‘ free driver’discussions in concrete numbers supported by science-based SAI deployment scenarios and sound aerospace engineering. 2. Stratospheric aerosol deployment scenario Following a research hypothesis proposed by Keith and Irvine(2016), we consider a limited SAI deploy- ment scenario(Sugiyamaet al2018)intended to cut in half the rate of temperature change from thefirst year of the program onward. While such a scenario is less ambitious(and less environmentally risky)than those aimed at keeping temperatures constant from a certain date forward, it is more ambitious than SAI merely holding the rate of temperature change constant (MacMartinet al2014). We further assume anthropogenically driven radiative forcing of∼2.70 W m 2by 2030, with an assumed decadal increase of∼0.5 W m2that is roughly consistent with the Representative Con- centration Pathway(RCP)6.0 scenario(Mosset al2010, IPCC2013). Assuming the desire to cut this rate of increase in half implies the need for SAI to reduce radiative forcing by∼0.25 W m 2by the end of the first decade of deployment. The implied change in glo- bal average surface temperatures from SAI deploy- ment is0.2 K per decade, with an assumed global average temperature sensitivity of 0.8 K per W m 2. We focus on SAI using sulfates, not because they are optimal—they may not be(Keithet al2016)—but because the long record of prior analyses on both effi- cacy and risks of sulfate deployment(National Research Council2015)renders them the best under- stood and therefore least uncertain material with which to commence in this hypothetical scenario of partial deployment. In the base case, we assume a top- of-atmosphere(TOA)sulfate forcing sensitivity of 0.25 W m 2per Tg S yr1, a value toward the lower end of recent estimates. Pierceet al(2010)estimates 0.34 W m 2and Daiet al(2018)derives a range from below0.50 to over2Wm2for injections between 30°N and 30°S. Other estimates for different injection scenarios, roughly converted to TOA values, range from0.15 W m 2(Kuebbeleret al2012)to 0.33 W m2(Niemeier and Timmreck2015), while Pitariet al(2014)shows results from the Geoengineer- ing Model Intercomparison Project(GeoMIP), here roughly converted to TOA, for one point of injection at the equator ranging from0.47 to0.98 W m 2. Table1summarizes the base-case SAI deployment scenario for thefirst 15 years of a program commen- cing in 15 years. The year 2033 is entirely hypothetical. It is not the most likely start date, nor are we suggesting it is an optimal one, but any deployment much sooner seems highly unlikely based on scientific and political Table 1.Hypothesized base-case SAI scenario in thefirst 15 years of deployment commencing in 15 years. Tons of S carried are half of tons SO 2dispersed. YearUnabated forcing (Wm 2)Target forcing (Wm2)SO 2dis- persed (Mt) aTemperature reduced(K)b 2033 2.850 2.825 0.20.02 2034 2.900 2.850 0.40.04 2035 2.950 2.875 0.60.06 2036 3.000 2.900 0.80.08 2037 3.050 2.925 1.00.10 2038 3.100 2.950 1.20.12 2039 3.150 2.975 1.40.14 2040 3.200 3.000 1.60.16 2041 3.250 3.025 1.80.18 2042 3.300 3.050 2.00.20 2043 3.350 3.075 2.20.22 2044 3.400 3.100 2.40.24 2045 3.450 3.125 2.60.26 2046 3.500 3.150 2.80.28 2047 3.550 3.175 3.00.30 aAssumes0.25 W m2per Tg S.bAssumes 0.8 K per W m2average temperature sensitivity(see text). 2 Environ. Res. Lett.13(2018)124001
considerations. Later deployment may mean the approaches explored here can be revised in light of new scientific and technological developments. The assumed linear ramp-up, and assumed lofting of sulfate for the purpose of dispersing SO 2(Smithet al 2018), implies the need to loft∼0.1 Mt of S in year one, increasing at a rate of∼0.1 Mt yr 1linearly there- after. Note this is significantly less material than McClellanet al(2012)assumed massfluxes of either 1 or 5 Mt yr 1of S, presenting a more limited and phased deployment scenario(Sugiyamaet al2018). Another important consideration is the location for SAI. Following Tilmeset al(2018a), we assume base-case injection sites at latitudes of 15°and 30° North and South of the equator. This is no verdict as to these four latitudes being optimal or definitive. It is a statement that, if forced to choose today, these four latitudes appear like a good starting point for discus- sions(Kravitzet al2017, MacMartinet al2017, Richter et al2017, Tilmeset al2017, Daiet al2018). Note that while SAI latitudes matter, longitudes appear not to, as injections at any one longitude mix rapidly to all oth- ers. Latitudes, meanwhile, influence the height of injections. At 15°N and S, injections may be required as high as∼20 km(Pierceet al2010). Some argue that even higher injection altitudes would provide greater radiative benefit(Tilmeset al2018b). For the purpose of defining the deployment scenario, we define the service ceiling necessary for the lofting platform at∼20 km. 3. Review of possible lofting technologies We have undertaken a review of all lofting technolo- gies that seem plausible as methods to hoist 0.1 Mt S to an altitude of up to∼20 km in 2033. Our 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 aircraftfleet. Table2summaries ourfindings across lofting technologies. We eliminate technologies we deem insufficiently mature to be used for deployment 15 years hence and those incapable of reaching the required altitude. Existing commercial and military transport aircraft cannot achieve the required alti- tudes, even with extensive modifications. Modified business jets, noted prominently in McClellanet 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 tooperate in adverse weather conditions. Tethered hoses are even less technologically mature and to-date untested. Militaryfighters 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 sustainedflight or regular opera- tions at such altitudes. Among technologies capable of achieving the mis- sion, costs are often prohibitive. NASA’s existing high- altitude aircraft that can reach appropriate altitudes do so with∼1 t payloads, making them very costly. Rock- ets are intended to reach altitudes 15–25×higher than are required to reach the lower stratosphere, rendering them both ill-suited and extremely costly. Even if the unit-costs of the massive SpaceX Falcon Heavy were reduced by 95% to account for the ratio of its normal target altitude to the∼20 km assumed here, it is still roughly 50×costlier than SAIL. Balloons and large naval-style guns are capable and plausible alternatives, but their per-ton costs are at least 10×as high as those we estimate for SAIL. Table2also shows McClellanet al(2010,2012) new high-altitude aircraft, which posits a cost-per-ton similar to that of SAIL. While we derive a similar unit- cost, SAIL’s numbers apply to the initial years of deployment, while McClellanet alconsider annual masses of both 1 and 5 Mt, the latter of which implies a larger and more mature program that may have achieved substantial economies of scale. For reference, our estimate is that a second-generation platform loft- ing the same 5 Mt yr 1might have unit costs at least 20% lower than the $1400 calculated here for afirst- generation SAIL technology. 4. SAI lofter(SAIL) Given the apparent inadequacy of existing technolo- gies, especially of previously assumed-to-be-adequate modifications to existing aircraft(McClellanet al 2010,2012), we propose a novel aircraft with dispro- portionally large wings relative to its narrow fuselage. We also describe the aircraftfleet requirements, and we calculate development and deployment costs from conception through year 15 of the hypothetical program. 4.1. Design The aircraft is designed to meet the assumed require- ments outlined in section3above. In particular, it is capable of levelflight at an altitude of∼20 km while carrying a 25 ton payload—large enough to lower operational costs significantly relative to existing high- altitude aircraft, yet small enough to make the mission possible. We have developed the design with direct input from several of the aerospace and engine companies consulted. It assumes a novel aircraft design but utilizes modified pre-existing low-bypass engines, which, though disfavored in commercial 3 Environ. Res. Lett.13(2018)124001
service due to their reduced fuel efficiency, will perform better at high altitudes. Broadly, SAIL 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 levelflight in the thin air encountered at altitudes approaching∼20 kms, SAIL requires roughly double the wing area of an equiva- lently 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 alti- tudes.)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 com- fort. 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 unman- ned cockpits over time, under current certification rules, it would be substantially faster and therefore cheaper to certify the aircraft with onboard pilots. More specifically, the preliminary design for SAIL calls for a length of∼46 m, a wingspan of∼55 m, and a wing area of∼250 m 2, 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 max- imum 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 ade- quate, 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 smallerfifth centerline auxiliary power unit for bleed air and onboard combustion of the molten sulfur payload. This highlights another advantage of aircraft as a lofting platform, since they can take advantage of the onboard combustion system from S to SO 2explored by Smithet al(2018). Lofting S would cut in half the payload required compared with lofting SO 2. More- over, S is a less dangerous substance than SO 2to han- dle on the ground or contend with in the event of an accident. Other possible lofting methods such a bal- loons and guns could not accommodate thisin situ conversion with existing technologies and would, therefore, need to loft SO 2with twice the mass of SAIL’s payload. SAIL is designed for a service ceiling of∼20 km, with a maximum altitude of up to∼19.8 km in a Table 2.Cost and capabilities comparison of lofting technologies. Platform Cost(‘000 $/t)SAIL multiple Source Mission capable SAIL a1.4 1× McClellan New High Altitude Aircraft 1.5b∼1×McClellanet al(2010,2012) Delft SAGAc4.0∼3×Delft Reportc McClellan Modernized Gun 19∼14×McClellanet al(2010,2012) Balloons∼40∼28×Near Spaced NASA WB57 43∼30×NASAd NASA ER2 50∼35×NASAd NASA Global Hawk 70∼50×NASAd SpaceX Falcon Heavy Rocket 71e∼50×Chang(2018) Gun Mark 7 16’137∼100×McClellanet al(2010,2012) Vector Rocket 1180 e∼850×Chang(2018) Virgin Orbit Rocket 2000e∼1400×Virgin Orbitd Mission incapable Existing Commercial Aircraft Not capable of reaching∼20 kmf Modified Commercial Aircraft Not capable of reaching∼20 kmg Existing Military TransportershNot capable of reaching∼20 kmg Military Fighters Not capable of sustainedflight at∼20 kmg Tethered Hose Not sufficiently mature technologyg Aerostats/Airships Not sufficiently mature technologyg aSee section4for cost derivations.bAssumes a program deploying∼1Mt yr1.cTU Delft student report developing SAGA, the Stratospheric Aerosol Geoengineering Aircraft(Design Synthesis Exercise Group 22016).dPersonal communications with individuals at respective entities.eReduced by 95% to account for 20 km target altitude relative to 200 km for Earth orbit; Chang(2018)’s estimates for Vector Rocket confirmed by Vector Launch. fMcClellanet al(2010,2012)and authors’analysis(see text).gAuthors’analysis(see text), including, for militaryfighters, personal communication with Boeing, Lockheed Martin, and Northrup Grumman. hIncluding existing military tankers. 4 Environ. Res. Lett.13(2018)124001
typical mission. Each mission would last∼5 h, with ∼2 h of ascent and descent time each, plus∼1 h on sta- tion. The∼2 h for ascent and descent time situates SAIL reasonably between the performance rates of the Global Hawk and U2/ER2. That assumes a∼25 t pay- load and a conversion of S to SO 2at∼0.5 t S per min- ute. Operationalflights areflown out and back to the same base, with a range of∼4500 km for each plane at maximum payload. While Tilmeset al(2018b)have noted that injections at altitudes 5 km higher would add perhaps 50% to the radiative benefit derived from deployed aerosols, SAIL and similar aircraft deploying conventional engine technology to haul large payloads are unable to substantially exceed∼20 km. The design assumes 2 pilots plus 1 payload opera- tor, and accommodates 1 supernumerary, possibly a scientific observer. Crucially, there are no passengers, which simplifies regulatory certification for the newly designed plane. SAIL would only have one mission and at most a handful of operators. Ferry and position- ingflights aside, SAIL can be expected tofly only in a few remote air corridors, likely enabling it to operate as an experimental aircraft in a restricted category without full commercial certification. This in turn would substantially reduce developmental costs. 4.2. Fleet We calculate that in year 1 of the deployment program (assumed to be 2033), the SAILfleet would require 8 new aircraft including oneflight-ready spare plane at each of the two initial bases. This assumes that one spare does not substantially influence our cost esti- mates. Table3summarizes SAILfleet and activity in thefirst 15 years of deployment. Such a scenario also assumes that by year 16, the ‘first-generation’SAIL technology is supplanted by a second-generation lofting solution for which muchhigher development sums would be expended to achieve substantially lower subsequent operating costs. No new SAILs would be manufactured there- after, though the existing SAILfleet would serve out its remaining economically useful lifespan. We therefore consider development costs of thisfirst-generation SAIL technology, commencing 7 years before year 1 of the program, but do not include any additional devel- opment costs to further refine or supplant the technology. 4.3. Development Costs We estimate total development costs of $∼2 billion for the airframe, and a further $350 million for modifying existing low-bypass engines. These numbers are toward the lower end of McClellanet al(2010,2012) range of $2.1 to $5.6 billion and signifi cantly below the TU Delft students’estimates of $14 billion for its purpose-built Stratospheric Aerosol Geoengineering Aircraft, or SAGA(Design Synthesis Exercise Group 22016). The former base their estimates largely on RAND Corporation’s Development and Procurement Costs of Aircraft(DAPCA)modelfirst developed in the 1960s and 1970s(Boren1976, Raymer1999). The latter use McClellanet al(2010,2012), and thus DAPCA indirectly, as one data point, but also consider a more granular build-up of development costs by category, andfinally compare those numbers to the developmental budget for the A380. We arrive at our numbers by developing the preliminary aircraft design described in section4.1and then budgeting the elements of that design in a series of personal conversations with relevant commercial vendors. Among the importantfindings derived from that approach was that while both McClellanet aland TU Delft devoted roughly half their developmental budget to the development of new engines, we found several Table 3.Totalfleet andflight activity by hypothesized deployment year. Year New aircraft aTotal aircraftaTotal payload (Mt S)bFlights/year BasesMonthlyflight hours/ aircraftcFlights/ base/day 2033 8 8 0.1 4007 2 278 5 2034 6 14 0.2 8015 2 278 11 2035 8 22 0.3 12 022 4 278 8 2036 6 28 0.4 16 029 4 278 11 2037 6 34 0.5 20 036 4 278 14 2038 6 40 0.6 24 044 4 278 16 2039 7 47 0.7 28 051 4 272 19 2040 6 53 0.8 32 058 4 273 22 2041 6 59 0.9 36 065 4 273 25 2042 6 65 1.0 40 073 4 274 27 2043 6 71 1.1 44 080 4 274 30 2044 6 77 1.2 48 087 4 274 33 2045 6 83 1.3 52 095 4 275 36 2046 6 89 1.4 56 102 4 275 38 2047 6 95 1.5 60 109 4 275 41 aIncludes one spare aircraft per base.bS burnedin situto disperse 2×SO2(see table1).cExcludes spare aircraft. 5 Environ. Res. Lett.13(2018)124001
pre-existing engines that can power SAIL, though with substantial modifications to account for the high- altitude operations. We de-emphasize commercial aircraft develop- ment programs as relevant data points, since it is very different and significantly costlier to design aflexible aircraft for a range of commercial operations than to design a small batch of specialty aircraft like SAIL that is intended for a novel but very specific mission. SAIL must demonstrate that it can fulfill its mission, but its testing and certification process does not need to explore the entireflight envelope to determine the range of operations for which a variety of operators might purpose the aircraft. Moreover, SAIL does not need to compete against other aircraft based on oper- ating costs. In these senses it is more like a military design exercise—what matters is that the aircraft can achieve the specified mission, but the optimization of operating costs is a substantially lesser consideration. Much of the design, certification, and testing costs for commercial manufacturers like Boeing, Airbus, and Bombardier lies in optimizing the aircraft for opera- tional cost by reducing drag, fuel consumption, and maintenance cost, while increasing operational relia- bility. These same considerations would be applicable to a second-generation SAI lofting solution, when (and if)the desirability of this intervention has been proven and the lofted masses need to be substantially greater. This may be a more advanced and potentially unmanned aircraft, or a non-aircraft lofting technol- ogy. Thefirst-generation solution on the other hand would favor‘quick and cheap’experimental aircraft for an experimental mission. Moreover, the small production run of SAIL is unlikely to attract the world’s biggest airframe devel- opers and is more likely the province of experimental aircraft designers. Two such companies have reviewed detailed SAIL specifications and contributed to the conclusion that development costs for SAIL would be less than the reported $300 million budget for Strato- launch(Foust2011), the massive catamaran aircraft currently being built with funding from the late Paul Allen. Given that the 650t MTOW of Stratolaunch is more than 6 times that of SAIL, a $250 million budget for the demonstrator aircraft seems generous. Testing costs for a restricted category certification, meanwhile, would run two to three times that, placing the total air- frame budget at $0.75 to $1 billion. We arrive at our $2 billion airframe developmentfigure by taking the high end of the range, and arbitrarily doubling it to account for the well-established history of cost overruns in air- craft developmental programs. To this airframe budget we have added $350 mil- lion for engine modifications and testing, which per- sonal communications with Rolls Royce indicates would be sufficient to purpose one of its existing engines to this program. Both Scaled Composites and The Spaceship Com- pany estimatefive years to be the best-casedevelopment timeframe, and would suggest allocating 7 years from the commencement of a fully funded pro- gram through certi fication in the relevant jurisdictions and entry into service of thefirst production aircraft. All this assumes a deliberate but standard develop- ment program rather than a crash effort intended to deploy SAI as soon as possible in response to a(per- ceived)crisis. Such a military-style deployment effort could cut several years off the assume 7-year develop- ment phase and bypass the required civil certification process, while substantially increasing costs. 4.4. Operating costs We build a SAIL operating budget using modeling conventions and cost factors common to the air freight industry, including aircraftfinancing assumptions. Table4details SAIL operating cost assumptions based upon the relevant cost drivers. We assume $2.00 per gallon for fuel, which comprises one of the largest elements of operating cost, while the cost of sulfur comprises a mere 3% of the budget. We assume a cost of $80/t for molten S(US Geological Survey2018,pp 160–161)with an assumed additional $20/t for transport. We assume the average marginal cost(i.e. exclud- ing amortization of development costs)for each addi- tional aircraft to be $100 million, roughly equal to the actual purchase price(as opposed to list price)of B767-300 and A330 freighters, both of which have about twice the OEW of SAIL. This assumes that SAIL aircraft will be priced at a substantial premium relative to OEW peers such as the A321 because of the much lower projected production volume. Given that the low annual aircraft production rates will not facilitate optimization of the production line, we assume conservatively that the build time for one SAIL aircraft is two years. That implies that prior to the commencement of operation, the program will have funded not only the $800 million required for the initial complement of 8 aircraft, but an additional $300 million in progress payments towards the addi- tional six aircraft required in year 2. In addition to pre-start capital costs, we assume the need for an aggregate of $∼40 million to fund an administrative entity that will manage the develop- ment program for the aircraft over its seven-year gestation cycle as well as to plan for the commence- ment of operations. During the two years immediately preceding deployment, a yet larger sum will be required for start-up costs such as hiring and training staff, setting up bases, procuring inventory, and certi- fying the airline(s)that will actually operate theflights. We estimate the capital required for this purpose to be 50% of thefirst year operating budget, excluding aircraft capital costs—a sum equal to roughly $100 million. Table5summarizes total SAIL capital require- ments during the assumed seven-year development 6 Environ. Res. Lett.13(2018)124001
phase and thefirst 15 years of operation. Total pre- deployment capital requirements are∼$3.6 billion. All costs(pre-start and operational)through Year 5 are ∼$10 billion. Total costs through Year 15 are∼$36 billion. Total ops costs in table5presents the resulting annual operating costs, including capital costs forfleet procurement as well as the amortization of total devel- opment costs. All told, year 1 operating costs are ∼$310 m, increasing annually in rough proportion to the growing deployment masses. Unit costs per deployed t SO 2decrease slightly in each year due to accumulating but limited economies of scale. Both simple and weighted average operating costs are ∼$1400/tSO 2deployed, in 2018 US $. That places total costs well below any other alternative currently available technology and roughly equivalent toMcClellanet al(2010,2012)$1500/t unit cost estimate for 1 Mt deployed via its proposed new aircraft pro- gram. For the reasons outlined above, we have sig- nificantly more confidence in our estimate. While $1400/t may convey a false sense of precision, we are confident to conclude that average operating costs are