Second string substitute Â relief blogger john hare.
Some years ago I did a few posts about an air turborocket with the bladeing based on the squirrel cage fan. October of 2008 if you’re interested. Â Some varieties of the squirrel cage fan have blade geometries that are simultaneously useful as compressors and radial inflow turbines. By using the blades as compressors on 75% of the cycle and as turbines on 25% of the cycle, 100% of the incoming air regeneratively cooled the blades so they could run a considerably hotter turbine inlet temperature than normal. The higher the allowable temperature, the higher the available performance. The other benefit of this blade geometry is that all moving components were on a single wheel, which allows for massive weight reduction compared to conventional turbine based engines.
The downside of the concept is that the cycle doesn’t close. Using the same blade for outflow compressor and inflow turbine means that the turbine inlet pressure must be considerably higher than the compressor will deliver. It was only as an air turborocket that the concept works as originally conceived. As an air turborocket though, thrust/weight ratios of 25 are quite attainable with specific impulses of nearly a thousand. By adding multiple wheels the specific impulses were somewhat closer to that of turbojets, though not turbofans. Adding extra wheels was still like adding epicycles to make the concept attractive.
The cagejet turborocket will always be a niche engine if it ever gets built. Thrust /weight will be far less than rockets, while fuel economy will be worse than turbojets and far worse than modern turbofans. Attractive for Launch Assist Platforms that want high acceleration for limited time in the atmosphere, but not for the long cruises some of these platforms want. Also attractive for some limited military applications.
It was called to my attention a few years back that perhaps I was too focused on reaction turbines when impulse turbines were useful in some applications. An impulse turbine can be a bit less fussy in the bladeing in exchange for considerably more critical nozzles to drive them. If I could use the impulse turbine concept, it might be possible to work a radial outflow turbine with the same blade that is a compressor on the rest of the cycle. Â If this can be done, the cycle might close an allow an engine that doesn’t require a rocket to drive the engine. The elimination of the oxidizer turns it into a very light turbojet with high thrust/weight.
A second thing pointed out to me was that the squirrel cage blades were speed limited by the mach number at the leading edge of the blade. This is the same problem of centrifugal compressors. The speed limit forces the inlet area down in relation to the wheel diameter. It also puts a limit on available compression ratio. Â A compression ratio of 2 is respectable for an air turborocket, and insufficient for a turbojet. By putting a fan in the inlet plane of the cage, it is possible to power prewhirl the incoming air so that the cage blades can run faster. Double the possible compression ratio faster. So I added a fan that prewhirls the incoming air, but also creates some compression in its’ own right. Compression ratio of 4 is now possible which is barely in turbojet country.
The third modification to the concept is the fuel handling. By using blades with fuel passages and film cooling holes that are also fuel injection holes, it is possible to get very fast mixing, while also using the fuel to regeneratively cool the blades as well as supply film cooling to them. With the blades being cooled by 100% of the air during 75% of the cycle, and simultaneously cooled by 100% of the fuel during 100% of the cycle, it is possible to run this turbojet at stochiometric mixtures without damaging the blades. This allows a high thrust/weight ratio from the turbojet even with a compression ratio of 4, and eliminates fuel hungry afterburners.
This side view shows the incoming air in light blue Â that goes through the compressor/turbine blades into the volute for pressure recovery. From the volute into the burner that is mostly not shown except for the section close to the turbine nozzles. After the burn the hot gas is through the turbine nozzles to the turbine blades. Into the thrust nozzle after driving the turbines to produce the thrust.
This is a side view of the engine. The air enters through the prewhirl fan on the left. It enters the compressor blades on 75% of the cage interior perimeter shown here on the bottom. Leaves the compressor blades into the volute shown on the very bottom. Hits the flameholders in the hot section. Burns and enters the turbine nozzles and turbine blades. Leaves the turbine blades into the thrust nozzle. Provides thrust.
By running the liquid fuel through blades as shown here, the blades are both regeneratively Â and film cooled by the entire fuel flow. The compressing air strips the fuel film from the blades and mixes with it in the volute even as it is doing pressure recovery. The air and fuel will be well mixed before the mixture hits the flame holders.
What results from all this is a true turbojet capable of a thrust/weight of 25 at sea level with a specific impulse of over 2,000. In cruise considerably more. The double regenerative blade cooling along with the fuel film cooling means that this engine could run stochiometric up to around Mach 5 without throttling down. It also means that the use of liquid oxygen for mass injection precooling would allow even higher thrust at any altitude or airspeed. Enough fuel could be used to burn all the oxygen for high thrust even at high altitudes.
An auxiliary rocket could be used in the burn chamber in air turborocket mode for extreme altitudes and to guard against flame outs if there is an inlet unstart.
This type engine could be used as an add on for a conventional air launch aircraft. Use the normal engines for cruise with the cagejets at take off and the gamma maneuver. Mach 0.9 in a near vertical climb at 50,000 feet would seem a good place to be compared to the drop and light of most air launch concepts. Â If the cagejets are wanted to cruise also, then they could be used as jets during cruise and turborockets when a lot of extra thrust is needed.
If it was desirable to modify this engine further for turbofan class fuel economy, a second smaller cage could be used to bring the total compression ratio to 16 for the burn in the second spool. Specific impulse to several thousand with the possibility of getting extreme thrust levels with the flick of the switch.