Speaking of doing hard things. Last week I went to a lecture on the Sabre engine.

The Sabre is a  cross between a traditional engine and a rocket. Reaction engines, the company behind it hopes the sabre will allow them to develop an affordable spaceplane. Unlike conventional rockets, Space planes would afford reusable and cost-effective access to space.

One of the surprising statistics Dr Robert Bond mentioned in the talk was the sheer inefficiency of rockets as a means for getting into space. Take a typical Ariane five rocket pictured below


This rocket carries about 160 tons of fuel: 132 tons of liquid oxygen and 26 tons of liquid hydrogen. The fuel burns to produce water vapour. The huge inefficiency comes in the fact that most of this fuel is merely present to lift the rest of the fuel.

Reaction engines have decided to approach this problem by building an engine that operates in two stages:

  1. An air breathing first stage in which the engine operates as a jet. In this stage the engine burns liquid hydrogen and oxygen from the atmosphere.
  2. A conventional rocket engine stage in which the engine burns liquid hydrogen and liquid oxygen from its on-board tanks.

The fact that a sizeable fraction of the rocket’s ascent takes place in stage 1 means that it can carry far less liquid oxygen allowing for a substantial weight saving.

The talk touched on many fascinating details about the Sabre engine but I’ll just share one here. Take a look at the engine:



Why is it shaped like a banana?

Well the space plane’s wings are very small so the plane must have a very high angle of attack in order to generate enough lift. But a jet engine works best when air is entering it head on so the front of the engine needs to be tilted downward. Amazing diagram below:

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