Performance
The designed thrust/weight ratio of SABRE ends is high—up to 14—compared to about 5 for conventional jet engines, and just 2 for scramjets. This high performance is a combination of the cooled air being denser and hence requiring less compression, but more importantly, of the low air temperatures permitting lighter alloy to be used in much of the engine. Overall performance is much better than the RB545 engine or scramjets.
The engine gives good fuel efficiency peaking at about 3500 seconds within the atmosphere. Typical all-rocket systems are around 450 at best, and even "typical" nuclear thermal rockets only about 900 seconds.
The combination of high fuel efficiency and low mass engines means that a single stage to orbit approach for Skylon can be employed, with air breathing to mach 5.14+ at 28.5 km altitude, 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. 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; however, experimental work has proved that this can be achieved. The experimental heat exchanger has achieved heat exchange of almost 1 GW/m³, believed to be a world record. Small sections of a real precooler, referred to as modules, now exist.
The losses from carrying the added weight of systems shut down during the closed cycle mode (namely the precooler and turbo-compressor) as well as the added weight of Skylon’s wings would appear to be heavy, yet the gains in overall efficiency more than make up for this. These losses are greatly offset by the different flight plan. Conventional launch vehicles such as the Space Shuttle usually start a launch by spending around a minute climbing almost vertically at relatively low speeds; this is inefficient, but optimal for pure-rocket vehicles. In contrast, the SABRE engine permits a much slower, shallower climb, air breathing and using wings to support the vehicle, giving far lower fuel usage before lighting the rockets to do the orbital insertion.
Read more about this topic: SABRE (rocket Engine)
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