​The Solar Aircraft Company and the HT-340 | Octane Press

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​The Solar Aircraft Company and the HT-340

The World's First Hydrostatic Drive Tractor

A replica of an International HT340 Turbine Tractor sold at the Mecum Auction in November 2019. This is the story of the real HT340s, including the location of the lone survivor as well as the mysterious examples that disappeared. 

In late 1959, Solar Aircraft Company lost several key military contracts for their turbine engines and so decided to court an investor. The one that best suited their fancy was IHC—and the relationship was mutual. The IHC leadership team was smitten with the idea of buying Solar Aircraft Company and developing turbine engines for their trucks, tractors, and construction equipment. It didn’t hurt that Solar’s engine product line included the name Titan, which had also been used in IHC’s past.

In 1960 IH purchased the Solar Aircraft Company, which built gas-turbine engines. Here, Solar’s craftsmen work with metals beryllium, titanium, refractory metals, dispersion-strengthened metals, stainless steels, and super-alloys in an air-conditioned hospital-clean room with glare-proof lights and a built-in vacuum system. Wisconsin Historical Society / 95230

The deal was made official on March 6, 1960. IHC was in the turbine engine business. One of their first experiments was to install a gas turbine engine into a tractor. As it turned out, they had an ideal platform for the test waiting in the wings.

International Harvester immediately began to work on incorporating a turbine engine into an experimental tractor, the HT-340. The engine ran at 57,000 rpm and produced 85 horsepower, and the engine and gear reduction weighed only 90 pounds. Wisconsin Historical Society / Imperial Studio / 11323

International Harvester engineers experimented with hydrostatic tractor drivelines in the 1950s by building both static test models and the world’s first hydrostatic-drive tractor: the model HD-340. This tractor started as a stock I-340 gasoline-powered utility tractor that had its entire drivetrain (clutch, transmission, and final drives) replaced by a variable-displacement hydrostatic pump and two radial hydrostatic final-drive motors (one for each real wheel). The HD-340 performed wonderfully, running circles around a gear-drive I-340. The HD-340’s single control lever allowed the operator to control travel speed and direction independent of the tractor’s engine speed. But simple operation was just one advantage of the hydrostatic drive. The drive also provides continuously variable speed control—the operator simply slides the lever to increase or decrease speed. This allows an unparalleled ability to match speed to load.

The HT-340 takes shape at Harvester Engineering Research Lab in 1961. Either two or three of these prototypes were built. The unpainted skin is the color of fiberglass. Slots in side of hood for turbine engine air intake will be covered with perforated sheet metal. The nose casting is aluminum. Wisconsin Historical Society / 75473

When IHC purchased Solar, the engineering team decided to blend their hydrostatic experiment with a Titan T62T gas turbine built by Solar.. The result was the new model HT-340.

The HT-340 coupled a Solar-built Titan T62T gas-turbine engine to a hydrostatic transmission. Many heavy-equipment manufacturers experimented with gas-turbine engines in the 1960s. The engines have a higher power-to-weight ratio than reciprocating engines, and offer terrific reliability. They can run a wide variety of fuels and have low emissions. By the 1970s, the diesel engine proved much more fuel-efficient, spelling the downfall of the gas-turbine engine for heavy equipment. Gregg Montgomery collection

The space race was in full swing in the early 1960s and influencing design trends in popular culture, and IHC created a tractor to match. The HT-340’s futuristic styling featured a sloped nose hood, wraparound rear gas tank, and futuristic dash panel. IH unveiled the HT-340 at the University of Nebraska’s Tenth Annual Tractor Day on July 20, 1961. Shortly after the show, the HT-340 was damaged when the truck hauling it overturned on the highway in Missouri.

The experimental HT-340 is tested. The tractor was first shown to the public in July 1961 at the University of Nebraska’s Tenth Annual Tractor Day. The machine was damaged in a traffic accident on the way home. Wisconsin Historical Society / 75472
It’s possible that when the tractor was returned to the IH engineering center for repairs its moniker was changed to HT-341. Early photos of the tractor clearly do not show the HT-341 emblems that are on the tractor today, nor its current white, red, and silver paint scheme. The original paint scheme was sky blue and white.

In July 1962, the original HT-340 prototype was retrofitted with new features and a new red and white paint scheme. The red model was called the HT-341, and it was used mainly for demonstrations around the country. Gregg Montgomery collection

IHC used the HT-341 at several farm shows and dealer open houses before finally storing it in 1962. On May 25, 1967, IHC announced that it was sending the HT-341 to the Smithsonian Institute to be part of a display marking the seventy-fifth anniversary of the gas-powered farm tractor. The HT-341 was later displayed at large IHC tractor shows in the Midwest and at the 2003 Farm Progress Show.

The experimental tractor was donated to the Smithsonian in 1967 and still occasionally travels to shows around the country. This image is dated June 1973. Wisconsin Historical Society / 86601

While the gas turbine tractor never made it to production (International also built a gas turbine–powered semi-truck, the Turbostar), Solar continued to be a productive and largely profitable division. Parts made by Solar were used on the Saturn V rocket and on the surface of the moon. The Solar division was eventually sold to help generate cash to keep IHC afloat in their final days.

To find out the rest of the incredible story of International Harvester, get your copy of Red Tractors 1958-2018 here!


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