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  • ANALYSIS: USAF eyes revolution in small jet turbines

ANALYSIS: USAF eyes revolution in small jet turbines

Kratos Defense & Security Solutions believes there is a big future in small things – in particular, small jet turbine engines.

The target drone manufacturer, which is also developing a fleet of tactical unmanned air vehicles (UAVs), put $60 million behind that belief when in February 2019 it bought a controlling stake in Florida Turbine Technologies, a manufacturer of small turbofan and turbojet engines that has since been renamed Kratos Turbine Technologies

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XQ-58A Valkyrie pictured during test flight, June 2019

USAF

Explaining the acquisition, Kratos chief executive Eric DeMarco said at the time: "The projected market for advanced turbojet and turbofan engines in our class alone is easily in the many thousands over the next five years, given the projected number of extended-range and low-cost cruise missile systems, and next-generation unmanned weapons systems to be acquired."

Kratos is getting its cues from a US Air Force (USAF) vision of developing a fleet of low-cost missiles and attritable UAVs that can overwhelm, evade or outdistance sophisticated and far-reaching Chinese and Russian air defence systems. For instance, UAVs such as the XQ-58A Valkyrie loyal wingman demonstrator, developed by the US Air Force Research Laboratory (AFRL) and Kratos.

"We want to complicate their surveillance and targeting problem so we can survive, and so that we can penetrate through their air defences," says Thomas Karako, director of the Missile Defense Project at the Center for Strategic and International Studies. "It's presenting them such a complex problem that their command and control, their sensors and their effectors are going to be overwhelmed."

Indeed, to complicate adversaries' air defence problems, the USAF wants a wide variety of tactical UAVs and cruise missiles. That means various engines with combinations of greater thrust, better fuel economy, more reliability, reduced maintenance needs and lower overall cost.

To that end, USAF funding has come around at the right time, says Joe Brostmeyer, senior vice president of Kratos Turbine Technologies. "What limits innovation is funding, and often your manufacturing and materials technology," he says. "So, if you have the funding you have a chance to take advantage of the latest manufacturing and materials technologies. That enables new designs which can multiply the effects of those other things."

Funds from the USAF, combined with emerging manufacturing technologies such as 3D printing, are enabling improved performance from small jet turbines – in some cases for the first time in decades, he says.

Between fiscal year 2018 and FY2026, the AFRL plans to invest up to $725 million in jet turbine research and development, through its Advanced Turbine Technologies for Affordable Mission (ATTAM) programme. Much of that investment is intended to benefit performance of small jet turbines, says the Turbine Engine Division Systems Branch at the AFRL’s Aerospace Systems Directorate.

The ATTAM goals are ambitious. The aim is to develop small jet engines that provide a 20-fold increase in electrical power for autonomous, low-cost intelligence surveillance and reconnaissance and strike UAVs. The extra electricity would power directed energy weapons and electronic warfare hardware. And the Turbine Engine Division wants to see a 30% reduction in mission fuel use and reduced maintenance requirements for UAVs.

Moreover, the division also wants engines with enough thrust to fly small "expendable strike" weapons, including cruise missiles, at speeds of more than Mach 3 against time-critical targets. Other low-cost engines would power swarms of subsonic munitions. Still more should have a greater fuel economy that would facilitate an increase of 60-70% in stand-off range.

Nine companies have been awarded ATTAM Phase 1 contracts relevant to small turbine engine development. Those include Boeing, Kratos, GE Aviation, Honeywell, Lockheed Martin, Northrop Grumman, Pratt & Whitney, Rolls-Royce LibertyWorks, and Williams International.

SIZE MATTERS

Not surprisingly, one of the leading limitations of small jet turbines is size. There is no hard and fast rule for what defines a small jet turbine, although generally engines that produce less than 3,000lb-thrust (13.3kN) are considered small. These engines can be around 300mm (12in) in diameter, 1m (3ft) long and weigh less than 90kg (200lb), although characteristics vary.

Small jet turbines can be pure turbojets or even turbofans. They typically do not have all the bells and whistles of their larger cousins, which power manned military aircraft or commercial airliners – because it is difficult to miniaturise many turbine innovations.

For instance, over the past several decades larger turbines have been able to generate more and more power by burning fuel at higher temperatures and using cooling systems to keep turbine blades from melting. Such coolant plumbing is difficult to replicate on smaller jet turbines, although the AFRL’s Turbine Engine Division believes there are promising technologies that could enable running small jet turbines at higher combustor and turbine temperatures.

"Turbojet performance is directly related to mass flow rate and exhaust exit velocity. Mass flow is fixed for a given engine size (diameter), hence we go after efforts to increase the exhaust exit velocity or expansion," says the Turbine Engine Division. "The temperature limits are material-based; hence we look to thermal barrier coatings, advanced ceramics and cooling to achieve engine life at higher temperatures."

In addition to temperature issues, small jet turbines also suffer from relatively larger turbine blade tip clearances. That gap between the end of the turbine blade and the inside wall of the engine is a leakage point for pressure, meaning losses in power and efficiency.

"A lot of this is driven from what you can do from the manufacturing standpoint as you get smaller. It is harder to hold machining tolerances, or casting tolerances or additive tolerances," says Brostmeyer. "And, when you can't hold tolerances, you can't hold your tip clearances on your turbine machinery; your leakages go up and these are losses."

ADDITIVE TECHNOLOGY

Despite these challenges, Kratos sees opportunities to use new technologies, such as 3D printing, to improve small jet turbine performance and cost. "Additive parts buy their way into a design the fastest when replacing a function that historically required an assembly of many parts with a single additive part," says Brostmeyer. "A heat exchanger is a good example part with a lot of internal plumbing, thus would be considered to be made using an additive process."

A heat exchanger could help improve the efficiency of a small jet turbine by using otherwise wasted exhaust heat to increase the temperature of the air heading into the combustor, thus improving the efficiency of the fuel burn, says Ken Suder, aerospace engineer at NASA Glenn Research Center in Cleveland, Ohio.

PBS Aerospace, a Czech manufacturer of small jet turbines for target drones and UAVs, says it also sees promise in additive manufacturing, and it has 3D printed parts of turbines from Inconel or aluminium for new engines.

Beyond traditional measures of performance, other areas of possible improvement include better storability. Cruise missiles and tactical UAVs may have to sit idle inside launch canisters for months, or even years, with little maintenance.

PBS Aerospace says turbine engines without oil lubricant are easier to store. "If the operations are not long, then they can be lubricated by fuel only," says Katerina Fisova, manager of turbojet engine sales. "There is no restriction on the position of the engine. They can even use it on vertical take-off of the UAV."

There are also performance benefits, adds Marek Fiala, PBS Aerospace’s director of marketing. "If you can produce an engine without oil systems, you can save some weight," he says. "You can carry more fuel."

Kratos says it also is looking at storage solutions, although it declines to elaborate.

Ultimately, engine developers need to create low-cost options. Suder says that may present challenges to manufacturers because eliminating non-essential engineering while maintaining high reliability for expendable engines is not something the jet turbine industry is well set up to do. "I only need this thing to last for 20h, I don't need 10,000h of life in it," he says. "The tools aren't sophisticated to really accurately predict only a few hours of life, so we actually overdesign everything."

Solving the low-cost problem is critical to fulfilling the USAF's visions of swarming weapons, agrees Kratos. "You know, without low-cost, small engines this is not going to happen," says Brostmeyer.

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AGM-86B air-launched cruise missile

USAF

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