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.
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.
Unlike traditional rocket engines, and like other types of air breathing jet engine, a hybrid jet engine can utilise air to create combustion saving on propellant weight and therefore increasing payload fraction.
Ramjets and Scramjets must spend a significant amount of time within the lower atmosphere to build speed to reach orbital velocity creating issues with extremely high drag leading to intense heating and the subsequent weight and complexity of required thermal protection. A hybrid jet like SABRE needs only reach low hypersonic speeds inside the lower atmosphere before engaging its closed cycle mode, whilst climbing, to build speed.
Unlike ramjet or scramjet engines the design is able to provide high thrust from zero speed up to Mach 5.5, with excellent thrust over the entire flight, from the ground to very high altitude, with high efficiency throughout.