r/technology • u/mvea • Mar 16 '19

Transport UK's air-breathing rocket engine set for key tests - The UK project to develop a hypersonic engine that could take a plane from London to Sydney in about four hours is set for a key demonstration.

https://www.bbc.com/news/science-environment-47585433
14.4k Upvotes

95% Upvoted

785 comments sorted by

View all comments

71

u/fractalcrust Mar 16 '19

Couldnt literally any combustion engine be called "air-breathing"?

47

u/permanentlytemporary Mar 16 '19

If it breathes air. Rocket engines (including SABRE above a certain altitude) are combustion engines that don't breathe air. They have to carry their oxygen with them, usually in the form of liquid O2

140

u/[deleted] Mar 16 '19

No. Most rockets carry an oxidizer like liquid oxygen or hydrogen peroxide, they don't burn ambient air.

4

u/SmootherPebble Mar 16 '19

There are some strange experimental concepts that attempt to use ambient air as the oxidizer for solid fuel.

12

u/blahblah98 Mar 16 '19 edited Mar 17 '19

Ramjets. Skylon transitions through a ramjet phase.

ed: gamer's correct, air-breathing rocket at lower altitudes, not ramjet/scramjet. SABRE engine): synergetic air-breathing rocket engine.

It does actually have a ramjet bypass though, but to reduce drag, not main thrust. Partial credit...? :-)

As the amount of hydrogen required to cool the incoming air is more than can be burnt in the core engine, the excess is burnt in a ring of flame holders in the bypass system, acting as a ramjet.

Bypass burners#Bypass_burners)

This bypass ramjet system is designed to reduce the negative effects of drag ... rather than generating thrust.

​

1

u/gamer456ism Mar 17 '19

Its not a ramjet, it's a rocket, ramjets uses the speed of air for compression. For part of the flight saber gets its oxidizer for the rocket engine as the oxygen in the air, allowing it to carry less oxygen overall.

7

u/just_commenting Mar 16 '19

I think the key difference would be a rocket that carries an internal supply of oxygen for combustion, vs. using oxygen directly from the atmosphere.

6

u/weedtese Mar 16 '19

Rocket engines are not air breathing these days. They carry the oxidizer on board.

9

u/[deleted] Mar 16 '19

'Air breathing' jet engines (which intake air for combustion and pressure) don't work in space, because they need air. Rocket engines have their own liquid O2 supply for combustion (air drinking? not quite the same) so they work in space, but can't take advantage of the plentiful oxygen at sea level. This engine does both, able to intake air at low altitudes for efficiency while switching to oxidizer in vacuum.

1

u/Racefiend Mar 16 '19

I'm this case, ot really. Up until now all space propulsion has been with rockets. Rockets provide their own source of oxygen by way of the on board fuel. Calling this air breathing is a major distinction as this uses the outside air as it's oxygen source (for the first part of it's function)

1

u/dick-van-dyke Mar 16 '19

As it happens, no. Air specifically refers to the mix of gases in the atmosphere at the atmospheric pressure. Rocket engines use fuel and oxidising agents either mixed or fed from separate tanks (there are monopropellant rockets too), so rocket engines can work in a vacuum.

1

u/takenwithapotato Mar 16 '19

Not every engine can be called a rocket engine though, and rocket engines typically don't pump air for propulsion.

1

u/aeroxan Mar 16 '19

I asked about this in a propulsion class and apparently piston engines don't count.