High efficiency fuel sleds for polar traverses
Highlights
► Mobility is key to efficient over-snow resupply in Antarctic and Greenland. ► We have developed lightweight, high efficiency fuel sleds for Polar traverses. ► These plastic sleds cost less and triple the fuel delivered per towing tractor. ► Sled sliding friction decreases with increasing sled–snow interface temperature. ► Solar gain using black bladders could double again per-tractor fuel delivery.
Introduction
The National Science Foundation’s Office of Polar Programs (NSF-OPP) operates year-round research stations at South Pole, Antarctica, and Summit, Greenland. Historically, all resupply operations for these stations were conducted using ski-equipped LC130 aircraft landing on prepared-snow runways. While effective, drawbacks of aircraft resupply include high fuel consumption and air emissions, payload weight and size limits, weather restrictions and high costs.
Beginning in 2003, NSF-OPP initiated an over-snow traverse to resupply South Pole Station from McMurdo Station, a one-way distance of 1660 km. The intent was to use large rubber-tracked agricultural tractors to tow fuel and cargo sleds directly over natural snow. However, conventional steel sleds developed very high towing resistance and unacceptably large motions when towed inline with the tractors. These mobility problems threatened the very feasibility of over-snow resupply. Lever et al. [1] proposed expedient solutions including spacing skis outside of tractor ruts, increasing ski area and improving nose shapes. These solutions worked remarkably well [2], [3] and established a key principle: over-snow traverses must achieve high transport efficiency to become an acceptable alternative to aircraft operations.
To this end, NSF-OPP initiated a cycle of innovation, laboratory tests, field validation, and refinement to improve the efficiency of its sleds. Because diesel fuel constitutes about half the cargo delivered to South Pole and Summit stations, initial development focused on fuel sleds. By 2008, this effort showed sufficient promise to initiate a traverse to resupply Summit Station from Thule, Greenland, 1170 km one-way [4], [5].
We describe here the evolution and performance of high-efficiency fuel sleds now used for Antarctic and Greenland traverses. These sleds consist of flexible fuel bladders strapped to sheets of flexible plastic (Fig. 1). By virtue of their low towing resistance, tare weight and cost, these sleds have transformed fuel delivery to US Polar research stations.
Section snippets
Design approach and performance monitoring
The primary South Pole Traverse (SPoT) towing tractors are Case STX530 Quadtracs and AGCO MT865s. The Greenland Inland Traverse (GrIT) uses Case STX485 Quadtracs. These tractors are commercial models fitted with cold-weather packages and optionally wider tracks. They perform quite well over natural Polar snow.
Table 1 summarizes the mobility characteristics of these tractors. Drawbar pull (DBP) values are average test and en-route values. Note the lower drawbar coefficients measured in Greenland
Steel versus flexible fuel sleds
Steel fuel sleds have design constraints that contribute to high towing resistance. The structural objective is to carry fuel in a rigid container and distribute that weight over large skis to minimize contact pressure. The skis must be strong enough to prevent buckling as they support the sled over rough snow. These structural requirements add tare weight and cost to steel sleds and limit the practical width and length of skis. Furthermore, over rough or varying-strength snow, only a portion
Sliding friction and sled temperature
During SPoT08-09, we measured towing forces during startup on cold days (<−20 °C) that were double the eventual steady state resistance for groups of 8–12 tan fuel bladders. These startup values could cause tractors to break traction during the 10–30 min needed for the sleds to “warm up.” Frictional heating amounted to ∼1–3 kW/m2 of contact area during this period. Stops for lunch or other breaks could recreate this transient behavior (with slightly shorter warm-up periods). In fact, high startup
Discussion and conclusions
Mobility performance lies at the heart of Polar traverses. The Antarctic and Greenland fleets are towing heavy cargo directly across natural snow. Frequent immobilizations limit daily progress, disrupt schedules, abuse equipment and stress crews. This puts pressure to reduce weight towed per tractor and thus reduce delivered payload. However, the fleet must maximize delivered payload to justify its capital and operating costs. With the support of NSF and traverse managers, we have advanced the
Acknowledgements
The authors gratefully acknowledge the commitment of NSF Program managers George Blaisdell and Pat Haggerty to advance Polar sled technology, and the support of traverse managers Paul Thur, Jay Burnside, Brad Johnson and Terry Billings to help us implement and test these new sleds across Antarctica and Greenland.
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