By Bjorn Fehrm
December 20, 2017, ©. Leeham Co: The Super Sonic Transport (SST) has a new spring. Aerion announced its new partner Lockheed Martin Friday and Boom got a new investor in Japan Airlines (JAL) the week before.
How to design a supersonic aircraft was mastered in the 1950s. I’ve flown one of these designs, at the speeds the new SST’s aim for, the Mach 2 SAAB Draken.
The SAAB Draken was designed in the mid-1950s, like the Mirage III and Lockheed Starfighter. All capable of Mach 2.0 at high altitude.
The challenge the new SST projects wrestle with is not how to design the SST airframe. How was learned then. And it hasn’t changed.
Yes, Aerion introduces interesting supersonic laminar flow technology to make the SST fly farther on less fuel. This wasn’t available then and it’s new and exciting stuff. But it’s not the sauce that cracks the real SST nut.
The real problem all these projects wrestle with is the engine.
But the Mach 2.0 Draken had an engine, didn’t it? Yes, but not one that would be accepted in a modern SST. Too thirsty, too noisy.
Since the days of the straight jet-driven Draken/Mirage/Starfighter, the bypass turbofan has solved the “too thirsty, too noisy” problem. The higher the bypass, the quieter and more economic the engine. Yet this is the problem. High bypass turbofans don’t work on an SST.
Why? We have learned the higher By-Pass-Ratio (BPR), the better?
We didn’t learn the full story. The mantra, “higher BPR” is better, only applies for subsonic airliners. It’s all in the fundamentals of how jet thrust is created. And it isn’t complicated.
We have several times presented the thrust of a turbofan as:
Thrust = Air mass flow * Air over-speed
The engine thrust increases when the air mass flow through the engine increases or the air’s speed out the back increases. We also learned; the propulsive efficiency of the engine increases when the over-speed (“specific thrust” in expert speak) is close to the free stream’s speed.
Consequently, today’s high BPR engines get a lot of air accelerated to a modest over-speed.
Now to a version of the formula that tells us a more detailed story (still not complicated):
The Air mass flow is the air entering the engine in the front, through the inlet. The Air over-speed is the difference between the air entering the inlet (freestream air) and the air going out the engine’s back. We can also write the formula as (read here to drill deeper):
Thrust = Air mass flow * Exit airspeed – Air mass flow * Inlet airspeed
The first term is called Gross Thrust and the second term Ram drag. The Thrust we now call Net Thrust, to distinguish from Gross thrust. So Net Thrust is Gross thrust minus the Ram drag.
The increasing Ram drag is our supersonic problem.
To understand it, we compare two engines: the engine powering the Boeing 737 MAX and the Aerion engine GE is working on.
The 737 MAX engine is the LEAP-1B, built by GE (core) and Safran (low-pressure part). Static thrust is between 22klbf and 29klbf, with a bypass ratio of 9. Overall Pressure Ratio is around 40, with the high-pressure compressor at 20. It’s a state of the art engine, with low noise and low fuel consumption.
The Aerion engine is projected as an 18klbf static thrust engine with a “medium BPR,” according to Friday’s press conference. It’s based on a GE commercial core which “has billions of hours of operation,” surrounded by a new, lower bypass, low-pressure system.
We will assume a BPR of 4 and an overall PR of 30.
Based on the information, we assume the core comes from the CFM56. It has a static thrust between 22klbf and 34klbf dependent on the version. Static Pressure Ratio for the most common version is around 27, with the high-pressure compressor at 10.
Its core design is from the 1970’s Boeing B1 bomber project, a 40-year old base design. We will come to why this core would be more suitable than the 40 years younger LEAP core.
We will simulate “flying” these engines at the M0.78 of the 737 MAX and then the M1.4 of the Aerion SST. We will use our engine simulation software, GasTurb, for the simulation.
As these are different size engines, we will focus on key ratios between the engines rather than absolute values. Our aim is to get a qualitative understanding of the SST problem.
Here the results:
One could wonder if the Aerion engine would work for BOOM’s M2.2 aircraft. It wouldn’t. Ram drag-to-net thrust at M2.2 would be at 20 times and the TSFC goes through the roof. The faster you cruise, the less air you shall let pass your inlet. And the faster you should kick it out the back.
The dilemma is engine noise also scales with Specific thrust, but in the wrong direction. The lower the over-speed, the less noise generated by the air leaving the engine. Noise regulations coming 2020 requires an engine with a compatible Specific Thrust.
The SST engine design problem then is:
If you want to go faster, you need a variable cycle engine (one with high BPR at low speed and low BPR at high-speed).
The BPR (or really Specific Thrust) dilemma is the principle problem. There is one more key problem (and then many smaller), needing a solution. An SST engine gets hot.
The air entering our LEAP at 35,000ft is at -54°C. When braking from M0.78, the air is heated to -28°C before it enters the compressor. At the end of our 40:1 PR compression, the air is around 520°C, below what the best compressor materials can handle. The subsequent higher temperatures in the combustor and turbines can be handled with cooling air tapped from the 520°C compressor.
If we fly the LEAP at the Aerion data, M1.4 and 45kft, the air entering the compressor is at +28°C, a 56°C rise. Now the end of the compressor is around 650°C. This is uncomfortably high for a three-hour cruise and the cooling air for the turbines and combustor is a bit hot.
To fix the high engine temperatures we need an engine with a lower compression ratio. This is where the lower compression core of the CFM56 fits.
For a Mach 2 engine, the entry temperature of the compressor air is 150°C. Now we will fry the engine with a compressor which has a PR higher than 15.
The cruise speed of the Aerion SST has gradually gone down over the years from M1.6 to M1.4. And for good reasons.
The Ram drag problem becomes more manageable and the engine runs cooler. The SST engine can be built on a modest compression ratio civil core, paired with a suitable low-pressure system.
If one wants to go faster, a suitable core is harder to find. The engine needs a lower bypass ratio at cruise, to not suffer under the created Ram drag, and a lower overall pressure ratio, to not get too hot from the high inlet temperature.