Unleashing the Potential of Hypersonic Flight
 
Today, the United states has clear air superiority over its great-power adversaries, Russia and China.  While the Russian Sukhoi SU-57 and the Chinese J-20 fighters are comparable to our F-22, they are just beginning to be delivered.  And while Russia has comparable ICBM and submarine fleets, neither country has the aircraft carriers, airtankers, and stealth bombers needed to project military power around the world.

To compensate, they’ve announced progress in the embryonic field of “hypersonic weapons.”  By definition, hypersonic weapons can travel at least five times the speed of sound, without being locked into the predictable trajectories of ballistic missiles.

There are two types of hypersonic weapons emerging: hypersonic glide vehicles and hypersonic cruise missiles. The announced Russian and Chinese weapons are all hypersonic glide vehicles.  Hypersonic glide vehicles are placed on top of rockets, launched, and then glide on top of the atmosphere. They are like airplanes with no engines on them, which can fly above 100,000 feet.  China’s DF-ZF boost-glide weapon, Russias Avangard hypersonic glide vehicle, and the American TBG and “Hacksaw” projects, discussed in our July 2018 issue, are all hypersonic glide vehicles. They use aerodynamic forces to maintain stability, to fly along, and to maneuver.  And, unlike a ballistic missile, a hypersonic glide vehicle is somewhat maneuverable. That means it can keep its target a secret up until the last few seconds of its flight.

“Hypersonic cruise missiles” such as America’s Arrow and HAWC projects are intended to be powered all the way to their targets using so-called SCRAMJETs.  SCRAMJETs use the vehicle’s forward motion to shovel air at supersonic speeds into the engine, causing thrust. One big problem with scramjets, is their inability to operate at slower speeds. That means, a booster stage typically carries them to a velocity and altitude where the scramjet engine can take over.  Hypersonic cruise missiles are very fast and maneuverable.

And they can fly at altitudes up to 100,000 feet.  Defenders may have just a few minutes from the time a trans-Atlantic attack is launched until the time it strikes.  And the hypersonic cruise missile can maneuver just like a conventional cruise missile, keeping defenders guessing about its intentions.

Until now, no one has been able to build a reliable hypersonic engine for cruise missiles or other applications, but that’s quickly changing.  After passing a preliminary design review by the European Space Agency (or ESA) a new air-breathing hypersonic engine is finally ready for a major round of testing in the next 18 months.
 
The Synergistic Air-Breathing Rocket Engine (or SABRE) is being developed by a U.K.-based company called Reaction Engines.  This engine can switch between two modes.  In aircraft-engine mode, it uses oxygen from the atmosphere.   In rocket-engine mode, it burns an oxidizer carried onboard together with its liquid hydrogen fuel. The real breakthrough involves the aircraft-engine mode, which has already been tested at airspeeds up to 2500 miles an hour.  This test indicates that large-scale hypersonic engines could be fitted to passenger jets to ferry passengers around the world in hours and other aircraft to deliver goods into orbit inexpensively.  During the next stage of tests, the SABRE technology will be tested at 4,200mph.
 
Until now, hypersonic engine designs have failed because of the enormous temperatures and physical stresses involved.  But the researchers have solved this problem with a so-called pre cooler.

The pre-cooler, allows the aircraft to travel at hypersonic speed without hot air causing the engine to melt.  To do this, a heat-exchanger manages very high temperature airflows and could ultimately form the basis for the SABRE engine to provide low-cost repeatable access to space.

To date, Reaction Engines has managed to make SABRE’s pre cooler work at a simulated speed of Mach 3.3.  For that test, they built a testing facility on the ground in Colorado and used a General Electric J79 turbojet engine to replicate the conditions that an engine will experience at hypersonic speeds.
 
The pre-cooling technology lowers the temperature of the air coming into the engine from more than 1,000°C (or 1,832°F) to room temperature in one twentieth of a second. From take-off to 5.5 times the speed of sound, it would take oxygen from the atmosphere, which would be fed into a rocket combustion chamber.  This will let the SABRE engine act as an air breathing rocket engine.  As such, it would carry significantly less oxidizer than a conventional rocket, making it much lighter.

That means that at low altitude and speeds up to 4200 miles per hour, it would behave like a jet, burning its fuel in a stream of air scooped from the atmosphere.  Then, at higher speeds and higher altitudes, it would transition to full rocket mode, combining the fuel with the liquid oxygen carried inside. 
 
What’s really special is that this new class of aerospace engine will enable aircraft to operate from standstill on a runway to speeds of over five times the speed of sound in the atmosphere. It can then transition to a rocket flight mode, allowing spaceflight at speeds up to orbital velocity, equivalent to 25 times the speed of sound.

Reaction Engines was founded 30 years ago by three scientists who had worked on these challenges at Rolls Royce.  At the time, the necessary technology was far way, but these scientists weren’t daunted.  With support from the European Space Agency and others, they continued their work, leading to solid preliminary results in 2012.

Then, in November 2015, BAE Systems bought a twenty percent stake in the company for ￡20.6 million. That was followed by a ￡10 million contract with the European Space Agency in July 2016, along with ￡50 million in grants from the UK Space Agency. And in April 2018, Reaction Engines received ￡26.5 million in investments from Rolls-Royce and Boeing HorizonX Ventures.

In 2018 thanks to a contract from the U.S. Defense Advanced Research Projects Agency (or DARPA) it conducted the breakthrough testing in Colorado.  And now, Reaction Engines aims to test the first integrated SABRE engine core in 2020 at a new facility under construction in the U.K.

Today, SABRE is decades away from use in passenger jets, where cost and safety trump performance.  But it may be a game-changer for hypersonic cruise missiles and manned military aircraft.  

As such, SABRE promises to unleash a wave of rapid progress in hypersonic aviation research.

Given this Trend, we offer the following forecasts for your consideration.

First, by 2025 the U.S. will introduce the first Hypersonic cruise missile and initiate work on a hypersonic fighter plane.

Just as the F-117 stealth fighter was developed as a “black project” in the Lockheed Skunk Works, these innovations are unlikely to be unveiled until they are ready to deploy.  DARPA’s support of Reaction Engines’ testing program indicates that the U.S. military is keenly aware of the potential.  As usual, expect the Chinese and Russians to emulate these achievements.

Second, just as post-War military research on jet engines paved the way for the Boeing 707 and Douglas DC-8, military research on hypersonic engines will pave the way for Hypersonic commercial aircraft by 2035.

The big challenges will be flight safety and sonic booms.  Only time will tell whether hypersonic commercial aircraft are limited to a high-end niche like the Concorde or become the default for intercontinental fights.  And,

Third, SABRE will open-up the possibility of spaceplanes traveling from runways to low-earth-orbit (or LEO) by the 2040s.

The sort of trips from runways to space stations envisioned in the film 2001: A Space Odyssey will become technically feasible.  But as with commercial hypersonic air travel, it’s unclear whether it makes economic sense.

References
Resource List:
1. Space.com.  April 18, 2019.  Tereza Pultarova.  Air-Breathing Rocket Engine Gets Green Light for Major Tests.

https://www.space.com/sabre-rocket-engine-cleared-major-tests.html

2. The New York Post.  April 8, 2019.  Charlotte Edwards.  ‘Spaceplane’ that could fly from NYC to London in 1 hour makes breakthrough.

https://nypost.com/2019/04/08/spaceplane-that-could-fly-from-nyc-to-london-in-1-hour-makes-breakthrough/

3. New Atlas.  April 8, 2019.  David Szondy.  SABRE hypersonic jet/rocket hybrid engine passes key precooler test.

https://newatlas.com/sabre-hypersonic-engine-precooler-test/59204/

4. IEEE GlobalSpec.  April 25, 2018. Eric Olson.  SABRE: A Hypersonic Precooled Hybrid Air-breathing Rocket Engine.

https://insights.globalspec.com/article/8642/sabre-a-hypersonic-precooled-hybrid-air-breathing-rocket-engine