The Hypersonic Turbojet Engine

[marcos]

http://www.reactionengines.co.uk/sabre_howworks.html

 

SABRE is at heart a rocket engine designed to power aircraft directly into space (single-stage to orbit) to allow reliable, responsive and cost effective space access, and in a different configuration to allow aircraft to cruise at high speeds (five times the speed of sound) within the atmosphere.

 

 

In the past, attempts to design single stage to orbit propulsion systems have been unsuccessful largely due to the weight of an on-board oxidiser such as liquid oxygen, needed by conventional rocket engines. One possible solution to reduce the quantity of on-board oxidizer required is by using oxygen already present in the atmosphere in the combustion process just like an ordinary jet engine. This weight saving would enable the transition from single-use multi-stage launch vehicles to multi-use single stage launch vehicles.

 

SABRE is the first engine to achieve this goal by operating in two rocket modes: initially in air-breathing mode and subsequently in conventional rocket mode:

Air breathing mode - the rocket engine sucks in atmospheric air as a source of oxygen (as in a typical jet engine) to burn with its liquid hydrogen fuel in the rocket combustion chamber

Conventional rocket mode - the engine is above the atmosphere and transitions to using conventional on-board liquid oxygen.

 

In both modes the thrust is generated using the rocket combustion chamber and nozzles. This is made possible through a synthesis of elements from rocket and gas turbine technology.

 

This approach enables SABRE-powered vehicles to save carrying over 250 tons of on-board oxidant on their way to orbit, and removes the necessity for massive throw-away first stages that are jettisoned once the oxidant they contain has been used up, allowing the development of the first fully re-usable space access vehicles such as SKYLON.

 

While this sounds simple, the problem is that in air-breathing mode, the air must be compressed to around 140 atmospheres before injection into the combustion chambers which raises its temperature so high that it would melt any known material. SABRE avoids this by first cooling the air using a Pre-cooler heat exchanger until it is almost a liquid. Then a relatively conventional turbo compressor using jet engine technology can be used to compress the air to the required pressure.

 

This means when SABRE is in the Earth's atmosphere the engine can use air to burn with the hydrogen fuel rather than the liquid oxygen used when in rocket mode, which gives an 8 fold improvement in propellant consumption. The air-breathing mode can be used until the engine has reached over 5 times the speed of sound and an altitude of 25 kilometres which is 20% of the speed and 20% of the altitude needed to reach orbit. The remaining 80% can be achieved using the SABRE engines in rocket mode.

 

For space access, the thrust during air-breathing ascent is variable but around 200 tonnes per engine. During rocket ascent this rises to 300 tonnes but is then throttled down towards the end of the ascent to limit the longitudinal acceleration to 3.0g.

 

[Speed]

I've been keeping my fingers crossed on this one ever since I first heard about the Skylon a couple years ago. Seems almost too good to be true. I do wonder how can they cool the air so fast. I'm not an ME or CE so I don't know my thermo very well though.

[marcos]

It uses a helium heat exchanger AFAIK. It can cool air from 1000degC to -150degC in something like 0.01s. The difficult part is doing it without causing ice formation.

 

http://en.wikipedia.org/wiki/SABRE_(rocket_engine)#Design

 

It's something that's only really practical on larger aircraft where extra weight is less of an issue.

Edited February 22, 2013 by marcos

[marcos]

Here's another effort using similar principles.

 

http://www.jaxa.jp/projects/aero/sst/data/htt.pdf