History of small gas turbines: The Commercial challenge.

The history of small gas turbines (SGT), like many, has been one full of ups and downs. There have been periods of active development large scale production and   periods where engineers forget that such a category of gas turbines exist totally. On the technical front, these SGTs pose a  set of challenges different from that of the larger engines. The main source of most the issues, as expected, is their diminutive size. Overall, these SGTs are no less complicated than their larger counter parts, if not more.

The definition of small gas turbines, or SGTs, has been evolving with time. What is a medium gas turbine in our generation is a small gas turbine in the next generation. This is mainly because we engineers have been squeezing ever more mass flow into these small machines and extracting more and more thrust Out of them. In the 1950's engines giving up to 900 shaft horse power were considered small. By the late 1990's the definition of SGT's included engines producing up to 6000 shp. A point to note here is that the large gas turbines of 1950's have become small gas turbines by 1990. The predominant application of these SGTs has primarily been target practice drones and a few military trainer aircraft. Target practice drones, for those who don't know, are basically small unmanned aircraft that are flow via radio so that the anti-aircraft gun crews can shoot them down for practice. The Ryan "Firebee" is one of the most successful target practice drones that used a SGT. 

The popular Firebee drone, with a Continental-J69 engine, image from wikipedia.

When introduced, these gas turbines faced an interesting set of commercial challenges. One main challenge they faced was that, they had a huge competition from piston engines of the time. The large gas turbines however, did not face any threatening competition from piston engines. The Merlin 66 engine, which was one of the largest piston engines of the time, topped at 1750 shp. If you had an application that needed more power than this, you went running to the large gas turbines. However in the power range of SGTs, there were numerous piston engines that had been perfected through decades of experience and tuning. The customer especially did not have an incentive to choose a SGT over a proven piston engine, especially when the SGT had lower time between overhaul than the piston engine. 

One of the largest piston engines of the time, the Merlin 66, image from http://nhi.a.la9.jp

The Kuznetsov NK-12 engine, a large turboprop with no equivalent piston engine, image from savine 

In spite of this major commercial challenge and  motivated by the competition, engineers from Fairchild, Williams International and the like have worked hard to solve the many unique technical challenges of the SGT and have made it a niche sector by itself within the huge gas turbine industry. 

Find few of  the interesting technical challenges of the SGT and the innovative solutions of the engineers in the next post.