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RM8 vs TF30

Old 2nd Jun 2019, 00:25
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Join Date: Nov 2003
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RM8 vs TF30

Hi
I've been watching a bit about the F111 recently, (Jeff Guinn on Aircrew Interview), there's always a comment made about the TF30 being a bad example of an afterburning turbofan. However the Viggen with its RM8 was an afterburning turbofan. As I understand it, and this is very basic, the Swede's grafted an afterburner onto an existing civilian turbofan, (JT-8D). Does anyone know why the Swede's didn't run into the same sort of trouble as P&W did on their TF30, or did P&W driven the Swede's in a direction that was the result of what they had learned on the TF30?
Regards Mark
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Old 2nd Jun 2019, 09:16
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Well, remember that the JT8D was a turbofan conversion of the turbojet J52 (see A-4 Skyhawk & A-6 Intruder), so there was little left of the original subsonic engine... and converting the JT8D to the RM8 involved more than just "sticking an afterburner on" - the Swedes had plenty of experience by then in modifying jet engines.

Note the comment in the Wiki article:
RM8A - AJ 37 Viggen
Since the original engine was constructed for subsonic speeds, most parts of the engine had to be redimensioned for the higher Mach-speeds in a military aircraft. Fans and turbine were altered, a new burn-chamber designed and a totally new fuel-control system for both engine and afterburner.

The TF30 was a clean-sheet design (but strongly based on the JT8D, ironically) specifically for SUBSONIC aircraft (FD6-1 Missileer and the VAL A-4 replacement program, won by the A-7). So the entire design had been created without thought of supersonic flight - the first stage really needs to be designed differently, and it takes more than intake design to allow a sub-sonic engine design to deal with supersonic flight.

That the A-7A/B/Cs had no issues with compressor stalls etc, while both the F-111 AND the F-14 with its completely different intake design did experience them regularly, indicates that there was something about the basic design that was not compatible with supersonic flight - and which Pratt's engineers either could not, or were not authorized by USAF/USN to spend the money to, correct on the production engines.

The TF30 in the A-7 had had an issue with steam ingestion during catapult launches, but this was easily corrected by modifying the 12th compressor stage.

Last edited by GreenKnight121; 2nd Jun 2019 at 09:29.
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