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In case anyone was confused by the article's description, here's the Wikipedia: "SABRE (Synergetic Air Breathing Rocket Engine) is a concept under development

In case anyone was confused by the article's description, here's the Wikipedia: "SABRE (Synergetic Air Breathing Rocket Engine) is a concept under development by Reaction Engines Limited for a hypersonic precooled hybrid air-breathing rocket engine. The engine is being designed to achieve single-stage-to-orbit capability, propelling the proposed Skylon spaceplane to low Earth orbit. SABRE is an evolution of Alan Bond's series of liquid air cycle engine (LACE) and LACE-like designs that started in the early/mid-1980s for the HOTOL project. The design comprises a single combined cycle rocket engine with two modes of operation. The air-breathing mode combines a turbo-compressor with a lightweight air precooler positioned just behind the inlet cone. At high speeds this precooler cools the hot, ram-compressed air, which would otherwise reach a temperature that the engine could not withstand, leading to a very high pressure ratio within the engine. The compressed air is subsequently fed into the rocket combustion chamber where it is ignited along with stored liquid hydrogen. The high pressure ratio allows the engine to provide high thrust at very high speeds and altitudes. The low temperature of the air permits light alloy construction to be employed and allow a very lightweight engine—essential for reaching orbit. In addition, unlike the LACE concept, SABRE's precooler does not liquefy the air, letting it run more efficiently. After shutting the inlet cone off at Mach 5.14, and at an altitude of 28.5 km, the system continues as a closed-cycle high-performance rocket engine burning liquid oxygen and liquid hydrogen from on-board fuel tanks, potentially allowing a hybrid spaceplane concept like Skylon to reach orbital velocity after leaving the atmosphere on a steep climb." https://en.wikipedia.org/wiki/SABRE_(rocket_engine) https://en.wikipedia.org/wiki/SABRE_(rocket_engine)

trhway 7y ago
>After shutting the inlet cone off at Mach 5.14, and at an altitude of 28.5 km, the system continues as a closed-cycle high-performance rocket engine burning liquid oxygen and liquid hydrogen from on-board fuel tanks the benchmark of today - SpaceX - separates the first stage at Mach6-8 at 65-80km height. Thus, the SpaceX second stage has less delta-v to add and don't have to carry all that weight - the SABRE would naturally be bigger than the 2nd stage for the same payload. The economical space related case of HOTOL/SABRE single-stage-to-orbit pretty much disappeared for a foreseeable future when SpaceX started to recover the 1st stage. That not to say about suborbital flights SF-Shanghai though. Or even "just" hypersonic as SABRE tech seems to successfully deal with one of the major/key obstacles for it (and that is huge, i'm in no way trying to dismiss their work). And one can see how SABRE can be used in some cases as a convenient 1st stage, i.e. by being able to launch from any big airport instead of only specially designed spaceports (such a 1st stage would probably be better than that https://en.wikipedia.org/wiki/Pegasus_(rocket)#Carrier_aircraft https://en.wikipedia.org/wiki/Pegasus_(rocket)#Carrier_aircr...).
- SECProto 7y ago
  Imagine putting SABRE on the first stage though - the engines would be heavier, but using air instead of carrying oxygen, for the full burn of the first stage. The payload mass fraction would be dramatically higher without carrying the oxidizer mass on board.
  - deleted 7y ago
    [deleted]
  - trhway 7y ago
    It is possible though cost&complexity seems high - we theoretically can even today bundle a bunch of SR-71 engines and probably stretch them beyond designed Mach 3.3 for the short time. Having low thrust-to-weight (SABRE promises up to 14, SR-71 had 6) we'd need either a lot of them for VTOL which leads to high cost, or we're limited by the size of maximum realistically possible hypersonic capable plane, plus the plane body being the additional weight. For example VTOL calculation - say engine weights 1 ton, we can lift off 13 ton of fuel and payload. It will use 40 ton of O2 from the air. For rocket engine - 1 ton engine can lift off 100+ ton, and if we just take the 13 ton fuel plus 40 ton O2 we'd have 45 ton budget for the rest (body plus payload). In reality you'd need higher fuel ratio, and this is why VTOL for atmospheric breathing engine pretty much isn't an option . Going beyond pure rocket engine for the 1st stage i think air-augmented rocket (http://astronautix.com/g/gnom.html http://astronautix.com/g/gnom.html) being simpler (thus cheaper) would be the next step that would get us closer (specific impulse wise) to what SABRE is targeting.
  - deleted 7y ago
    [deleted]
- 7952 7y ago
  I think the benefit of launching horizontaly is overstated. Any airport would still need to handle LOX and hydrogen which could be pretty dangerous. And the airport is not necessarily going to be in the right place for launch. Also traditional rockets have most of the forces acting vertically. A horizontal launch space plane must also deal with a completely different set of forces.

trhway 7y ago

>After shutting the inlet cone off at Mach 5.14, and at an altitude of 28.5 km, the system continues as a closed-cycle high-performance rocket engine burning liquid oxygen and liquid hydrogen from on-board fuel tanks the benchmark of today - SpaceX - separates the first stage at Mach6-8 at 65-80km height. Thus, the SpaceX second stage has less delta-v to add and don't have to carry all that weight - the SABRE would naturally be bigger than the 2nd stage for the same payload. The economical space related case of HOTOL/SABRE single-stage-to-orbit pretty much disappeared for a foreseeable future when SpaceX started to recover the 1st stage. That not to say about suborbital flights SF-Shanghai though. Or even "just" hypersonic as SABRE tech seems to successfully deal with one of the major/key obstacles for it (and that is huge, i'm in no way trying to dismiss their work). And one can see how SABRE can be used in some cases as a convenient 1st stage, i.e. by being able to launch from any big airport instead of only specially designed spaceports (such a 1st stage would probably be better than that https://en.wikipedia.org/wiki/Pegasus_(rocket)#Carrier_aircraft https://en.wikipedia.org/wiki/Pegasus_(rocket)#Carrier_aircr...).

SECProto 7y ago
Imagine putting SABRE on the first stage though - the engines would be heavier, but using air instead of carrying oxygen, for the full burn of the first stage. The payload mass fraction would be dramatically higher without carrying the oxidizer mass on board.
- deleted 7y ago
  [deleted]
- trhway 7y ago
  It is possible though cost&complexity seems high - we theoretically can even today bundle a bunch of SR-71 engines and probably stretch them beyond designed Mach 3.3 for the short time. Having low thrust-to-weight (SABRE promises up to 14, SR-71 had 6) we'd need either a lot of them for VTOL which leads to high cost, or we're limited by the size of maximum realistically possible hypersonic capable plane, plus the plane body being the additional weight. For example VTOL calculation - say engine weights 1 ton, we can lift off 13 ton of fuel and payload. It will use 40 ton of O2 from the air. For rocket engine - 1 ton engine can lift off 100+ ton, and if we just take the 13 ton fuel plus 40 ton O2 we'd have 45 ton budget for the rest (body plus payload). In reality you'd need higher fuel ratio, and this is why VTOL for atmospheric breathing engine pretty much isn't an option . Going beyond pure rocket engine for the 1st stage i think air-augmented rocket (http://astronautix.com/g/gnom.html http://astronautix.com/g/gnom.html) being simpler (thus cheaper) would be the next step that would get us closer (specific impulse wise) to what SABRE is targeting.
- deleted 7y ago
  [deleted]
7952 7y ago
I think the benefit of launching horizontaly is overstated. Any airport would still need to handle LOX and hydrogen which could be pretty dangerous. And the airport is not necessarily going to be in the right place for launch. Also traditional rockets have most of the forces acting vertically. A horizontal launch space plane must also deal with a completely different set of forces.