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Skylon
Saber Engine


Schematic of engine

Reactions Engines, the British company working on a partially air breathing cryogenic engine which would power a single stage to orbit spacecraft, Skylon, as well as a hypersonic transport, the A2.

This project has taken a major step forward with both the European Space Agency and the Air Force Research Laboratory (AFRL) have found the basic concept sound, including a heat exchanger that cools the incoming air by hundreds of degrees in a fraction of a second without choking up without being choked with frost:

It is a well-established truism in aerospace that leaps in propulsion technology almost always precede major advances in spacecraft or aircraft design.

As the clamor for affordable access to space continues to grow, there is mounting interest in the Synergetic Air-Breathing Rocket Engine (Sabre) concept under development by U.K.-based Reaction Engines. This hybrid powerplant is designed to bridge the infamous power gap between air breathers and rockets, potentially enabling a vehicle to accelerate from a standing start on the runway all the way to low Earth orbit.

Such an engine could power high-speed aircraft, suborbital craft or even multi- and single-stage-to-orbit vehicles. Even more encouraging to Sabre proponents is that, while earlier attempts to harvest oxygen from the atmosphere succumbed to thermodynamic reality, the Reaction design continues to pass muster with experts in Europe and the U.S. The company’s most recent—and possibly most valuable—vote of confidence comes from the U.S. Air Force Research Laboratory (AFRL), which analyzed Sabre under a cooperative research and development agreement.

AFRL’s validation followed a detailed study of the entire concept, particularly the precooler heat exchanger technology, which allows for the practical extraction of oxygen from the air without clogging up the mechanism with frost and ice. Reaction unveiled initial details of the methanol-based frost-control system at the American Institute of Aeronautics and Astronautics Hypersonics and Spaceplanes conference in Glasgow in early July.

AFRL program manager Barry Hellman says analysis “confirmed the feasibility and potential performance of the Sabre engine cycle. While development of the Sabre represents a substantial engineering challenge, the engine cycle is a very innovative approach and warrants further investigation.” As a result, Reaction Engines and AFRL plan to continue collaborating on Sabre, with potential follow-on work focusing on evaluation of various air-breathing-powered vehicle concepts and testing of specific engine components.

The AFRL study will also evaluate other potential uses for the Sabre’s heat exchanger technologies, including looking at broader defense applications. “The question to answer next is what benefit the Sabre could bring to high-speed aerospace vehicles compared to other propulsion systems,” says Hellman. “AFRL is analyzing vehicle designs based on the Sabre engine concept. We are also considering testing their heat-exchanger technology at Mach 5 flight conditions in a high-temperature wind tunnel.”

While AFRL acknowledges that Sabre’s original target—a single-stage-to-orbit space access vehicle dubbed Skylon—remains technically “very risky as a first application,” Hellman says: “Sabre may provide some unique advantages in more manageable two-stage-to-orbit configurations.”

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The precooler chills the incoming air from more than 1,000C (1832F) to -150C in less than 1/100th of a second, before passing it through a turbo-compressor and into the rocket combustion chamber, where it is burned with subcooled liquid hydrogen fuel. For higher altitude operation and the jump to orbit, the engine switches to an onboard liquid oxygen supply and runs as a conventional closed-cycle rocket engine (AW&ST Nov. 26, 2012, p. 47).

What this means in the short term is not space travel, but it does mean that they are far more likely to get government and private sector funding.

Good folks at Av Week have a description of how Reaction Engines made this work, but I cannot make heads nor tails of it:

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But after endorsement of the basic technology from the European Space Agency and, more recently, the U.S. Air Force’s Research Laboratory, the company’s synergetic air-breathing rocket engine (Sabre) concept is being taken far more seriously. Designed to power a vehicle from a standing start to Mach 5.5 in air-breathing mode, and from the edge of the atmosphere to low Earth orbit in pure rocket mode, the Sabre engine with a heat exchanger at the heart of the design is attracting widespread interest for potential application on a range of atmospheric and space vehicles.

With patents pending and negotiations with new industrial partners apparently at an advanced stage, Reaction Engines has made the surprise decision to unveil the first details of the critical technology at the core of its hybrid hypersonic propulsion system.

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“It is pretty mind-bending stuff,” says Reaction Engines technical director and chief designer, Richard Varvill. Speaking at the American Institute of Aeronautics and Astronautics International Space Planes and Hypersonics conference here, he says the system counters the frost that precipitates out of the air as it becomes saturated with increasing relative humidity during the rapid cooling process. The precipitation “looks like the white feathery frost you’d see on a cold winter’s day. Unfortunately, that frost is sufficiently mechanically strong that it can bridge the gaps between the tubes and will block the matrix solid in about 3 sec. flat if you don’t do anything about it.

“So—surprise, surprise—we use an anti-freeze, and in this case it is methanol. But we use the methanol in a rather sophisticated way, with the objective of minimizing the amount you need. Also we don’t want to spray the methanol in and leave it in the air flow because we are actually cooling down the air to the point at which the methanol would freeze itself,” he says.

To do this, Reaction Engines has “borrowed a trick from the chemical process industry,” says Varvill. “We inject the methanol at one of the coldest points, and we effectively get the mix of water and methanol to flow forward in the matrix—against the direction of the airflow.” He concedes this seems counterintuitive, but explains the system generates an effective reverse flow by catching the water-methane mix and reinjecting it further upstream. “We have multiple injection and extraction points in the matrix, but the overall effect is the mix of methanol and water is actually flowing forward in the matrix against the airflow direction.”

The reasoning, he says, is that the condensate composition at the cold end of the matrix is nearly all methanol, and as it flows forward the methanol picks up the water. “At the inlet [of the matrix] it is nearly all water, so the composition is more methanol-concentrated at the cold end than it is at the warm end," Varvill says. "That then reduces because you have extracted most of the water at the warm end, and that reduces the absolute amount of methanol you need to throw into the pre-cooler to stop it freezing.” And because the amount of liquid water reduces so does the relative humidity. “Eventually you end up with a situation where you have extracted all the water vapor as liquid from the airflow, and that leaves you essentially with dry air below 215K. The partial pressure of the water vapor at this point is so low that you can allow it to pass through the heat exchanger and it does not freeze.”

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Reaction Engines decided to go public on the frost-control technology because of pending patent applications. “The trigger for patenting was the awareness that to execute this program we are going to have to involve other companies,” says Mark Thomas, the former chief engineer for technology and future programs at Rolls-Royce and now managing director at Reaction Engines. “You can’t keep trade secrets very long in that situation, so it is better to be protected formally and legally on the clever stuff.”

This is all going on while the engine is moving faster than mach 5, though it is slowed to subsonic speeds (the cooling allows the system to avoid the complexities of a scramjet, the shock cone in the inlet is the tell here).

I would really like to see an animation of this, because for the life of me I cannot see how they get coolant to flow forward against that sort of air flow.

It's weird, but it is a good kind of weird.