October 26, 2018, ©. Leeham News: In the previous Corner we discussed the noise challenge an SST engine has. To be effective at Supersonic speed it needs a high Specific Thrust (a fast jet out the back) but this creates Takeoff and Landing noise.
We now look at some key data for SST engines.
The first SST engine, the Concorde Olympus was designed in the 1960s, almost 60 years ago. One would think it should be easy to design an engine for SuperSonic Transport (SST) aircraft which would be more efficient than the Rolls-Royce/Snecma Olympus 593.
By today’s standards, its data is antiquated. It’s a straight jet engine with an Overall Pressure Ratio (OPR) of 15.5 and with a cruise specific fuel consumption at Mach 2 of 1.2 lb/lbf/hr. Modern Turbofans are below 0.55 lb/lbf/hr, less than half the Olympus.
As we shall see it’s hard to beat the Olympus for a Mach 2 engine. The reason is the Olympus combines a low mass flow at cruise of less than 90kg/s or 200lbm/s with a high specific thrust of more than 500m/s or 1,600ft/s (the 900m/s value we talked about in the previous Corner was for Takeoff with Afterburner). The low mass flow gives a low Ram drag, which compensates for other inefficiencies.
But a straight jet is impossible to use with today’s noise regulations. We must use bypass turbofans to reduce takeoff and landing noise.
The Mach 1.4 engine
Before we look at Mach 2 engines, let’s start with comparing some key data for the GE Affinity, an engine optimized for a modest SST speed of Mach 1.4. We compare it to its donor engine, the CFM56.
The CFM56-5 sitting on an A320 has a cruise thrust of around 5,000lbf at 37,000ft, a bypass ratio of 5 and a fuel consumption of 0.6 lb/lbf/hr.
To produce this thrust the engine produces a Gross thrust of 12,500lbf. Gross thrust minus Net thrust gives us our Ram drag. It means we have a Ram drag at Mach 0.78 of 7,500lbf and we have a Gross thrust to Net thrust ratio of 2.5.
We now compare these values with the values for Affinity at Mach 1.4. To keep Ram drag within bounds, GE set the ByPass Ratio (BPR) of Affinity to around 3.
If the engine shall produce 3,500lbf thrust per engine at Mach 1.4 and 50,000ft we need to inject fuel in the combustor to produce 11,000lbf of Gross thrust. Our Gross thrust to Net thrust ratio is now 3.15.
The Ram drag increase for the Affinity is reasonable. This has been achieved by keeping the Bypass ratio down at ~3 instead of the original engines 5. The cost is a loss in propulsive efficiency.
The increased Ram drag and the lower propulsive efficiency increases the cruise fuel consumption of the Affinity at Mach 1.4 with about 50% over the CFM56-5 at Mach 0.78.
Mach 2 engines
In the next Corner, we will look at engines for SSTs flying at Mach 2 or faster.