RM8 vs TF30
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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
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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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:
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.
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.
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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Hi GreenKnight121
Wow, thanks for such insightful comment. 3 questions has arisen from your comments, (and bear in mind I am just a simple engineer).
1. When you say "... it takes more than intake design to allow a sub-sonic engine design to deal with supersonic flight," does this mean the velocity triangles are changed for the first stage, i.e. your angles are lessened and you accept a lower pressure rise coz you are getting a stage that can accept wider flow regimes?
2. Was the steam ingestion problem the result in the mass flow increasing or did the steam alter the total temperature of the flow?
3. Do you know why the 12th stage compressor was targeted for the fix?
Regards
Mark
Wow, thanks for such insightful comment. 3 questions has arisen from your comments, (and bear in mind I am just a simple engineer).
1. When you say "... it takes more than intake design to allow a sub-sonic engine design to deal with supersonic flight," does this mean the velocity triangles are changed for the first stage, i.e. your angles are lessened and you accept a lower pressure rise coz you are getting a stage that can accept wider flow regimes?
2. Was the steam ingestion problem the result in the mass flow increasing or did the steam alter the total temperature of the flow?
3. Do you know why the 12th stage compressor was targeted for the fix?
Regards
Mark
