Flight International – 6 August 2019

22 | Flight International | 6-12 August 2019 flightglobal.com

MILITARY ENGINES

For air-breathing hypersonic weapons, scramjets are the

crucial ingredient – and developers believe advanced 3D

printing can beat technical hurdles to practical motors

Materially

improved

GARRETT REIM LOS ANGELES

A

fter decades of false starts, the

USA’s leading scramjet engine de-

velopers say hypersonic flight

using this type of powerplant is

now within reach. That is partly thanks to the

coming of new 3D printing technology.

Two competitors in the US Defense Ad-

vanced Research Projects Agency (DARPA)

Hypersonic Air-breathing Weapon Concept

programme – a Raytheon-Northrop Grumman

team and a Lockheed Martin-Aerojet Rocket-

dyne pairing – say they are leaning on 3D

printing to bring their scramjet-powered hyper-

sonic cruise missiles into test flights in 2020.

The companies say that their programmes

are deeply indebted to the X-51A Waverider,

which last flew in 2013. Built by Boeing and

Pratt & Whitney Rocketdyne (now Aerojet

Rocketdyne) for the US Air Force Research

Laboratory and DARPA, the X-51A was the

first hypersonic vehicle to be propelled over a

substantial period of time by a scramjet en-

gine that used JP-7 jet fuel. That hydrocarbon

propellant was once used by the Lockheed

SR-71 Blackbird. The military-grade jet fuel is

more readily available than the exotic hydro-

gen gas that had been used to fuel previous

hypersonic vehicles, such as the scramjet-

powered NASA X-43A in 2004.

“X-51 was really a stake in the ground. It

was a propulsion demonstrator. It was intend-

ed to demonstrate the capability, to solve the

physics of supersonic combustion,” says

Tyler Evans, vice-president of Aerojet Rocket-

dyne’s Rocket Shop. “Ever since then, we’ve

been evolving the state of the art around mak-

ing it more affordable and producible.”

Aerojet Rocketdyne and Northrop are the

engine developers on their respective teams,

and believe 3D printing will help save on pro-

duction costs and lead times. Those improve-

ments will be needed to manufacture scramjet

engines in large numbers, should the US De-

partment of Defense ever decide to move be-

yond funding prototypes and towards order-

ing production models.

“I think we are in a good position now

where we are actually starting to drive afford-

ability into hypersonics,” says John Wilcox,

Northrop’s vice-president of advanced pro-

grammes and technology.

MATCH IN A HURRICANE

Like a conventional turbojet engine, a scram-

jet inhales air through its inlet, compresses

and mixes it with fuel, and ignites the mixture

to produce thrust out its nozzle. Unlike a tur-

bojet engine, it does not have any turbine

blades to compress air, but instead relies on

air being forced through its inlet as it is

pushed through the atmosphere at high

speeds, often propelled by a rocket booster.

The result is a jet engine with no moving

parts, which cannot produce thrust on its

own from a standstill.

Scramjets – supersonic combustion ramjets

• are similar to ramjets, which ram air into
  their inlet to create compression. However,
  combustion of air and fuel in a ramjet takes
  place at velocities below Mach 1, while com-
  bustion of air and fuel in a scramjet takes
  place at supersonic speeds. That means each
  engine has limits.
  “M5 is considered the upper operating
  limit of the ramjet,” says Dora Musielak, an
  adjunct professor of mechanical and aero-
  space engineering at the University of Texas at
  Arlington. “A scramjet will not operate below
  speeds of about M5.”
  So difficult is igniting fuel and air at super-
  sonic speeds inside a scramjet that producing
  thrust with the technology has been com-
  pared to lighting a match in a hurricane and
  trying to keep it burning.
  “If you take the length of a typical combus-
  tor, say 20cm [8in] or 50cm and divide it by
  the velocity of 3,000mph [4,800km/h] to
  6,000mph, you will find that the typical time
  that an air molecule spends within the com-
  bustor is of the order of a fraction of a millisec-
  ond,” says Javier Urzay, a senior research en-
  gineer in aerospace at the Center for
  Turbulence Research at Stanford University.
  “The main evolution has been in trying to in-
  crease that flow-through time, making flow-
  through times that are longer so you have
  longer times to burn fuel and oxidiser.”
  In addition, the flight corridor of a scramjet-
  powered hypersonic vehicle is in the strato-
  sphere at 66,000-98,000ft. Go too high and the

X-51A experimental vehicle provided vital data about

overcoming challenges inherent in supersonic combustion

US Air Force

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