This talk was given to the Royal Aeronautical Society Weybridge Branch on 3 February 2016 by Charlie Muir, an aerospace engineering undergraduate at the University of the West of England.

Starting with definitions, the speaker said that it was generally agreed that the hypersonic regime started at M5.5 where air dissociation begins to happen.

Ariane 5 is an example of a conventional rocket. ESA.

For space flight rocket boosters had been the only propulsion method; is there a better way? The flaws in rocket propulsion are cost and propellant storage. The cost to launch a vehicle into geostationary transfer orbit is £5k to £10k per lb and all the propellant has to be carried on board. The Ariane 5 launch weight is made up of 88% propellant and just 2% payload compared with a Boeing 787 take-off weight made up of 33% propellant and 18% payload. Rockets have a low specific impulse when compared with jets and ram jets. Subsonic turbofans deliver a specific impulse of 8,000-6,000, ramjets at M3-6 deliver 2,000-1,000, whereas rockets at all speeds deliver 500.

There are potential improvements available to rocket performance by matching the nozzle exit plane pressure to the ambient atmospheric pressure for maximum efficiency. A conventional nozzle exhaust is over expanded at sea level and under expanded at high altitude. Altitude compensation could be achieved by utilising a movable centrebody or by employing a two-contour dual bell nozzle.

Costs could be reduced by introducing space industry standards, for example by having common docking mechanisms and standardised off-the-shelf modules for satellites - like ‘Cubesat’. Also, reusability could be increased from today’s partially reusable vehicles.

X-51A WAVERIDER

X-51A Waverider on the B-52 mothership. USAF.

Alternative propulsion methods could be developed in the long term such as the air breathing supersonic combustion ramjet or Scramjet. The problems include establishing stable combustion in supersonic flow over a range of Mach numbers. The technology has been demonstrated by the Boeing X-51 unmanned vehicle, launched at 50,000ft from a B-52 then boosted to M4.5 by a solid rocket which is jettisoned as the Pratt & Whitney-Rocketdyne Scramjet is lit to accelerate the vehicle to M6 for 6 minutes.

SABRE

A cutaway drawing of the Reaction Engines SABRE. Reaction Engines.

A UK project is the SABRE Synergetic Air Breathing Rocket Engine which is designed to operate much like a conventional jet engine to around Mach 5.5 beyond which the air inlet closes and the engine operates as a highly efficient rocket accelerating the vehicle to orbital speed. The SABRE engine is not a scramjet, but a jet engine running combined cycles of a pre-cooled jet engine, rocket engine and ramjet. The key technology for this type of pre-cooled jet engine is a very low weight heat exchanger, about one tenth that of a conventional unit. This has been demonstrated. Reaction Engines plans to run a complete engine at the end of this decade.

Such engines could power single stage to orbit vehicles not needing to carry large quantities of liquid oxygen and taking off conventionally from runways. This would allow affordable space access for placing satellites, launching spacecraft and deliveries to space stations. Sub-orbital very long-range hypersonic passenger carrying vehicles would also be feasible. Possible military applications include reconnaissance vehicles and hypersonic cruise missiles.

After the lecture the speaker was joined by Neil Taylor, head of performance at Reaction Engines, developers of the SABRE power plant, for a lively questions and answers session. Neil also gave the vote of thanks.

Chris Farara


10 June 2016