How would you by definition separate the aircraft with mixed propulsion (example: Ryan Fireball) from the airplanes with auxiliary engines (example: C-123J), or does both mean the same thing?
good question. Let me throw my two cents.
There are more or less four reasons / four broad cases why engine pods were added, or why jet engines were mixed with piston-engine.
- the B-36
- the C-123
- the KC-97
...
- the Ryan Fireball
I think a good case could be make that the only difference is that the Fireball was designed, FROM THE DRAWING BOARD, with a jet engine in the tail. When all three others got jet engines added in wing pods, during their lives. As a cheap-and-dirty expedient to help their performance. But what performance ?
For the B-36 (as said above) it was a cheap way to get faster over the target.
For the C-123 and Noratlas and many other transports, it was to help at liftoff, not to get faster by any way. Transports don't really care about speed. Piston-powered transport aircrafts were underpowered, and when flying in very hot climates as in Algeria (Noratlas) or Vietnam (C-123) then their performances took a major hit. Jets pods helped getting safer liftoff. The pilots then shut the jets, otherwise they would have cut into the range, being so thirsty.
...and then there is the strange case of the KC-97: that piston engine tanker was too slow to refuel fast jets ! It had to get into a shallow dive to accelerate, and this is no good for in-flight refueling, which is already risky enough. so they put jet pods on KC-97 to get them faster.
and now the Fireball. Piston + jet. In this case, it was because early jets were awfully unreliable, and guzzled huge amount of fuel. Which was particularly catastrophic for naval fighters. Range was bad, reliability was bad, and veteran WWII pilots had spent their lives flying piston engine fighters.
Also when landing aboard a carrier, if you miss your approach, then you hit full throttle, and climb again to try again. While piston engines, as in the F-4U Corsair, were pretty good at that, early jets had very, very long reaction times. So if you missed a carrier approach with an early jet, then, well, you died. That's the reason why there was no
operational naval Vampire or P-80 variants (I said operational).
It would have been a slaughter of naval pilots, except of course if you were Eric Winkle Brown, a pilot gifted by God.
So when Ryan got a contract for the Fireball in 1943 the raison d'etre was
a) reassure the pilots with a proven and reliable piston engine that don't explodes or take fire just before landing on the carrier
b) preserve range by flying on the piston engine, light the jet only to accelerate in combat
While the P-80 completely trashed the P-51 with superior speed, and climb and height, its range was abysmal when compared to a Mustang.
For the record, at the end of WWII, Mustangs could fly 10 hours /1500 miles - long missions with small drop tanks: Great Britain to near Polish border and back: Okinawa to Tokyo and back. It took many years for any jet fighter to achieve such range, as late as 1950 or even 1955, the Mustang escort range was still unmatched by any jet. See the Air force many atempts at create a non-piston powered escort fighter: P-81, F-84, F-88...
More generally, mixed piston-jet were a maintenance PITA for a simple reason. The piston engine burned avgas when the jet needed kerosene. So they needed separate tanks. Took me a while to realize this, I thought you could burn avgas into a jet.