Question about Future Ramjet aircraft

**cr9527** · 20 Jan 12,, 01:27

From the articles and concept arts I've seen, almost all future Ramjet aircraft utilize seperate engines of Turbojet and Ramjet.

Why wouldn't they utilize what the Sr-71 did with the J58, where it is a combination of the two(does both well)?

**USSWisconsin** · 20 Jan 12,, 02:25

Originally Posted by cr9527

From the articles and concept arts I've seen, almost all future Ramjet aircraft utilize seperate engines of Turbojet and Ramjet.

Why wouldn't they utilize what the Sr-71 did with the J58, where it is a combination of the two(does both well)?

Could you show us an example of a separate engine concept that you're looking at? IMO, the J58 concept makes sense - perhaps the still secret recon aircraft have pulse ramjets - but that is speculation. The ramjet is a fuel hog, but speed is expensive.

**Jimmy** · 20 Jan 12,, 03:49

I'm pulling these completely out of my arse, but the SR-71 had a very stringent tolerances for airflow. It's probably simpler to keep the engines separate. My other guess is also simplicity: for maintenance.

**Stitch** · 20 Jan 12,, 06:01

Jimmy's pretty close to explaining (simply) why the J-58 wasn't perfect; it took Lockheed about two years and dozens of test flights to fine-tune the supersonic parameters for the J-58; several A-12 airframes (and pilots) were lost exploring the performance envelope of the J-58. It was probably the biggest hurdle the A-12/SR-71/YF-12A had to overcome to make it a capable M3+ aircraft. It wasn't really until the SAS (Stability Augmentation System) was installed in the early A-12's that the Blackbird became realiable; up until then, engine "unstarts" were a common occurance.

The J-58's optimum speed was about M3.2; it could go faster, but efficiency (ST) dropped off significantly above this speed. The theoretical maximum speed of the J-58 is close to M4, but it's not very efficient at this speed; the inlet shock pattern also becomes progressively more unstable above this speed, so pilots tended to avoid going above M3.2 unless absolutely necessary (like if there was an SA-2 coming after you).

The "latest" engines also tend to use separate fuels; a conventional turbofan engine will use JP-8, whereas a pulse-detonation engine (PDE) or scramjet will probably use liquid hydrogen, or something similar (like the X-43).

**cr9527** · 20 Jan 12,, 08:18

Originally Posted by Stitch

Jimmy's pretty close to explaining (simply) why the J-58 wasn't perfect; it took Lockheed about two years and dozens of test flights to fine-tune the supersonic parameters for the J-58; several A-12 airframes (and pilots) were lost exploring the performance envelope of the J-58. It was probably the biggest hurdle the A-12/SR-71/YF-12A had to overcome to make it a capable M3+ aircraft. It wasn't really until the SAS (Stability Augmentation System) was installed in the early A-12's that the Blackbird became realiable; up until then, engine "unstarts" were a common occurance.

The J-58's optimum speed was about M3.2; it could go faster, but efficiency (ST) dropped off significantly above this speed. The theoretical maximum speed of the J-58 is close to M4, but it's not very efficient at this speed; the inlet shock pattern also becomes progressively more unstable above this speed, so pilots tended to avoid going above M3.2 unless absolutely necessary (like if there was an SA-2 coming after you).

The "latest" engines also tend to use separate fuels; a conventional turbofan engine will use JP-8, whereas a pulse-detonation engine (PDE) or scramjet will probably use liquid hydrogen, or something similar (like the X-43).

Got it, thanks