February 07, 2008

Hypersonic vehicles designs and progress

From the designers of Skylon, a fairly practical spaceplane designs, comes the Mach 5 A2 commercial Concorde replacement. The A2 and the Scimitar engine are more affordable and longer lasting versions of the Skylon spaceplane and Sabre engine.

Analysis of the Development, Production and Operating costs suggests that the average ticket price would be comparable to an existing Business class ticket. The A2 vehicle could capture all of the current business and first class traffic due to the greatly reduced journey time of 4.6 hours compared to the current 22 hours.

Unlike Concorde the A2 vehicle has exceptional range (approx 20,000 km both subsonic and supersonic) and is therefore able to service a large number of routes whilst simultaneously avoiding supersonic overflight of populated areas. Its good subsonic performance enables it to service conventional subsonic overland routes thereby increasing its sales potential to airlines.

To achieve the range requirement liquid hydrogen fuel is mandatory since the specific calorific energy of hydrocarbon fuels is too low. Reaction Engines have conceived the Scimitar precooled engine concept which exploits the unique thermodynamic properties of liquid hydrogen.

The Scimitar Engine is a derivative of the Sabre spaceplane engine intended for SSTO launcher application. Consequently most of the Scimitar engine technology is similar to Sabre but designed for much longer life. Both engines are designed around existing gas turbine, rocket and subsonic ramjet technology. However the incorporation of lightweight heat exchangers in the main thermodynamic cycles of these engines is a new feature to aerospace propulsion.

The A2 is larger than an Airbus A380 super jumbo jet.

More pictures of the A2

The Reaction Engines site who have designed the Skylon space plane and the A2 commercial transport (The A2 is also referred to as the Lapcat project, Long-Term Advanced Propulsion Concepts and Technologies project)

Skylon Images

The key to the Skylon and A2 planes are precooled engines as explained in this 9 page pdf.

The issues relevant to propulsion design for Single Stage To Orbit (SSTO) vehicles are considered. In particular two airbreathing engine concepts involving precooling are compared; SABRE (Synergetic Air-Breathing and Rocket Engine) as designed for the Skylon SSTO launch vehicle, and a LACE (Liquid Air Cycle Engine) considered in the 1960’s by the Americans for an early generation spaceplane. It is shown that through entropy minimisation the SABRE has made substantial gains in performance over the traditional LACE precooled engine concept, and has shown itself as the basis of a viable means of realising a SSTO vehicle. Further, it is demonstrated that the precooler is a major source of thermodynamic irreversibility within the engine cycle and that further reduction in entropy can be realised by increasing the heat transfer coefficient on the air side of the precooler. If this were to be achieved, it would improve the payload mass delivered to orbit by the Skylon launch vehicle by between 5 and 10%.

The US hypersonic program is also looking to use fuel to cool the engine of its hypersonic planes.

Successful recent ground tests of jet-fueled, ramjet/scramjet demonstrator engines by Pratt & Whitney Rocketdyne and Aerojet represent important progress toward flight-testing of three separate hypersonic-vehicle programs.

Using JP-7 jet fuel, PWR ran the combustor successfully at a variety of Mach numbers from Mach 2.5 to Mach 6.0, demonstrating "desired operability and performance" at each speed, the company said.

FaCET aims to develop a hypersonic test vehicle -- which could fly in 2012 -- that would take off and land by itself, use an advanced turbojet to get up to a speed of at least Mach 4 and then use a liquid hydrogen-powered scramjet to get to Mach 10 and beyond. Jet fuel can't be used as a scramjet fuel at speeds as high as Mach 10.

The US should be flight testing an unmanned mach 10 aircraft in 2008

NASA hypersonics expert Dr Isaiah Blankson believes that MHD energy-conversion in the intakes can take 30-40% of the energy, letting a turbine engine run at up to Mach 7. One engine able to take the plane all the way to Mach 7 and have a lot of extra power for things like military lasers or railgun launchers.

The advantage of this proposal is that it seems like a simpler design than some other proposals for scramjets Scramjets promise to be better than rockets by not needing the 75% of the weight which is oxidizer, but designs need to simpler and not replace the oxidizer with a heavier and more expensive aircraft.

Reportedly, Blankson says extracting 30 to 40 per cent of the inflow energy would cut its speed by 50 to 75 per cent. That sounds counterintuitive, as kinetic energy is proportional to the square of velocity, but presumably a man with his background knows what he's on about. Potentially, a Mach 7 flow would slow to Mach 3 downstream of the MHD, and then a Blackbird type setup could handle it.