megan,
A reading of "SR-71 Revealed" by Richard Graham, pilot and the 9th Strategic Reconnaissance Wing Commander, he says, "at cruise, the rotor of the engine actually has a small negative thrust load on the engine".
I'm trying to understand the import of his statement.
In normal operational circumstances of a turbojet I'd expect a fairly large axial drag component on the rotor (negative thrust if you like, since the turbine is driving the compressor). Without identifying the engine or thrust capability, a NASA report cites a axial load of 3,000lb on a medium size engine. The engine at SR-71 cruise provides 17% of the thrust, the rest being 54% from the inlet and 29% from the ejector..
Is this merely indicative that most of the compression is coming from the inlet rather than the compressor?
I've seen statement that the axial loading of a turbojet rotor can reverse direction depending on circumstance, such as RPM. Anyone with insight?
I assume the "at cruise" to be above Mach 3 in which case most of the compression is coming from the inlet. I assume that was what Graham was conveying.
Axial loading on rotors do reverse with thrust settings, acceleration (pouring on the coals) and deceleration (hitting the brakes). These changes have to be accounted for in the design of the rotor components. The rotors even move slightly in the axial direction forward and aft as a result of acceleration or deceleration, so spacing between the stationary vanes and rotors has to be calculated plus a safety margin to prevent clashing between rotors and stators in either direction.
Thrust bearings are of a ball bearing design of which there are several types. They are designed to be contact as uniformly as possible and never free floating. Roller bearings are not used in the axial direction, but are used in some situations in a radial direction.