Flight International – 6 August 2019

ightglobal.com 6-12 August 2019 | Flight International | 29

SMALL TURBINES

its Advanced Turbine Technologies for Af-

fordable Mission (ATTAM) programme.

Much of that investment is intended to bene-

fit 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 au-

tonomous, low-cost intelligence surveillance

and reconnaissance and strike UAVs. The

extra electricity would power directed ener-

gy weapons and electronic warfare hard-

ware. 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. Greater

fuel economy is a performance target that

could 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, GE Aviation, Honeywell, Kratos,

Lockheed Martin, Northrop Grumman, Pratt

& Whitney, Rolls-Royce LibertyWorks, and

Williams International.

MINIATURISATION CHALLENGES

Not surprisingly, one of the leading limita-

tions of small jet turbines is size. There is no

hard and fast rule for what defines a small jet

turbine, although generally engines that pro-

duce 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), al-

though 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 high-

er temperatures while using cooling systems

to keep turbine blades from melting. Such

coolant plumbing is difficult to replicate on

smaller jet turbines, although the AFRL’s Tur-

bine Engine Division believes there are prom-

ising 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 (diame-

ter), 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 tol-

erances, or casting tolerances or additive tol-

erances,” 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.”

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 addi-

tive part,” says Brostmeyer. “A heat exchang-

er is a good example of a part with a lot of in-

ternal plumbing, which would be considered

to be made using an additive process.”

USING HEAT

A heat exchanger could help improve the effi-

ciency of a small jet turbine by using otherwise

wasted exhaust heat to increase the tempera-

ture of the air heading into the combustor, thus

improving the efficiency of the fuel burn, says

Ken Suder, aerospace engineer at NASA’s

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 manufac-

turing, and it has 3D-printed parts of turbines

from Inconel or aluminium for new engines.

Beyond traditional measures of perfor-

mance, 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 says turbine engines without oil lubri-

cant are easier to store. “If the operations are

not long, then they can be lubricated by fuel

only,” says Katerina Fisova, manager of turbo-

jet 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

PBS marketing director Marek Fiala. “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 solu-

tions, although it declines to elaborate.

Ultimately, engine developers need to create

low-cost options. Suder says that may present

challenges to manufacturers because eliminat-

ing non-essential engineering while maintain-

ing 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 enough 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

US Air Forcehappen,” says Brostmeyer. ■

After subsonic Tomahawk, ATTAM project could enable

cruise missiles with Mach 3 operating performance

“The tools aren’t sophisticated

enough to predict only a few

hours of life, so we actually

overdesign everything”

Ken Suder

Aerospace engineer, NASA Glenn Research Center

FIN_060819_028-029.indd 29 31/07/2019 14:06