From Academic Kids

For other uses, see Sabre (disambiguation).

SABRE (Synergic Air BReathing Engine) is a proposal for a hydrogen-fuelled airbreathing rocket engine/jet engine for propelling launch vehicles into low earth orbit.

SABRE is the ultimate design of Alan Bond's series of LACE and LACE-like designs that started in the early/mid-1980s for the HOTOL project. In 1989, after funding was killed, Bond and several others formed Reaction Engines Limited ( to continue research. However, his original LACE designs had been classified as top secret and could not be used; so Bond went on to develop SABRE in its place.



The precooler-based LACE idea was originally explored by Marquardt in the 1960s, which eventually led to medium-thrust engines that ran for several minutes at a time. Although the program was generally successful, changing priorities and USAF funding led to the idea being abandoned.

HOTOL's engine, the RB545, used a heat exchanger cooled by liquid hydrogen. Air was to be slowed and compressed in a supersonic air intake, then passed through a heat exchanger to be liquified. The liquified air would be separated, and the nitrogen and other components dumped, whilst the liquid oxygen would be fed to the engine. Since the warmed hydrogen is difficult to feed back into the engine, it was largely vented.

A new design

The RB545's liquid hydrogen precooler had issues with embrittlement, patents and The Official Secrets Act, and so the newer SABRE design instead uses a closed-cycle precooling system with gaseous helium coolant. Unlike the RB545, SABRE does not actually separate the oxygen from the nitrogen, nor does it liquify the air. Instead, after precooling, a turbocompressor is employed to raise the air's pressure before injecting it into a combustion chamber, in very similar fashion to the compressor on a conventional jet engine. The helium coolant is in turn cooled by a heat exchanger with the liquid hydrogen fuel. The liquid hydrogen fuel boils and is sent off to a ramjet to be burnt. The helium coolant loop is designed to be self starting, and in fact heat collected by the helium coolant loop is used to power the turbocompressor and the fuel and LOX turbopumps.

One potential problem with the precooler is to prevent freezing atmospheric water vapour and blocking the precooler. Research suggests that the precooler is possible, or atleast quite plausible and, specifically, rejection of condensed water before it freezes has been demonstrated.

Another change is that the SABRE system uses a number of engines of various sizes, each tuned to a different performance niche. Instead of one large "one size fits all" engine as in HOTOL, SABRE uses two pure-rocket engines centered behind a large shock cone in the center, surrounded by a ring of smaller combustors similar to an afterburner on a conventional jet engine. After being launched and brought to speed by a short burst of the rockets, the ramjets are started, fed by air bled from the shock cone. As the craft ascends the air is increasingly fed into the center through the precooler, first to provide denser air to the ramjets, and later to provide oxygen to the rocket engines. At Mach 5.5 the ramjets become inefficient and are powered down, and stored liquid oxygen is used for the rest of the ascent. Again, the losses due to carrying around a number of engines that will be turned off for some portion of the flight appear to be heavy, yet the gains in efficiency while running appear to more than make up for this.


The designed thrust/weight ratio of SABRE ends up several times higher—up to 14, compared to about 5 for conventional jet engines, and 2 for scramjets; whilst giving very good fuel efficiency—about 2800 seconds. It also gives much better performance than the RB545 engine.

The combination of high fuel efficiency and low mass engines means that a single stage to orbit approach for Skylon can be employed, with airbreathing to mach 5.5+, and with the vehicle reaching orbit with more payload mass per take-off mass than just about any non-nuclear launch vehicle ever proposed.

Like the RB545, the precooler idea adds mass and complexity to the system, normally the antithesis of rocket design. In addition, the precooler is also the most aggressive and difficult part of the whole SABRE design; the mass of this heat exchanger is an order of magnitude better than has been achieved previously; experimental work gives good reason to believe that this can be achieved, and small sections of a real precooler now exist.


  • A Comparison of Propulsions Concepts for SSTO Reusable launchers [1] (
  • An Experimental Precooler for Airbreathing Rocket Engines [2] (
  • The Skylon Spaceplane [3] (

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