# Foxbat's Speed



## silence (Mar 12, 2014)

I recall reading somewhere that the speed of the Foxbat was the result of the type of engines it had rather than the engine's sheer thrust. According to the Wiki articles, each engine in a Foxhound is rated at approx. 12,000 more pounds of thrust, yet its max speed is "only" Mach 2.83 while the Foxbat could hit Mach 3.2, albeit by ruining the engines.

Can anyone explain this to me?


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## FLYBOYJ (Mar 12, 2014)

During full power operations you're probably looking at exceeding a specified Turbine Inlet Temperature (TIT). In most turbine engines, if you exceed that temperature over a certain period of time, the turbine section is ruined. This is what Wiki says about the engine...

http://en.wikipedia.org/wiki/Tumansky_R-15


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## razor1uk (Mar 12, 2014)

The augmentors or afterburners would generate most of the power and speed. The Limit was the engines maximum rev/speed since the airframe shape is good upto Mach 4 or 5 AFAIK.

The M3.2 is true, but the engines were over sped severely reducing their life spans to the point the engines when the plane landed had to be replaced and junked - after the Turmanuski Bureau studied them or didn't.
The intake compression ramps probably never had a setting for such high speeds, so the ram air and shock compression forces pressures would have also added to the variables leading to the engine cores overs peed resonance damaging effects.


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## silence (Mar 15, 2014)

So the ramjet nature of the engine at high speeds keeps generating thrust, not the engine itself. The non-mechanical compression of the inlet air feeds directly into the afterburner, and as this compression keeps increasing so does the thrust. The engine itself develops up to 22-odd kips; the ramjet effect keeps producing more. Is that correct? I think the (better-designed and built!) J58 works the same way.

If so, I wonder what kind of thrust is being put out at mach 3.2?


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## razor1uk (Mar 15, 2014)

silence said:


> So the ramjet nature of the engine at high speeds keeps generating thrust, not the engine itself. The non-mechanical compression of the inlet air feeds directly into the afterburner, and as this compression keeps increasing so does the thrust. The engine itself develops up to 22-odd kips; the ramjet effect keeps producing more. Is that correct? I think the (better-designed and built!) J58 works the same way.



Yes, basically, though (as you know,) the Mach number is also heavily influenced by the altitude too as, is the fuel efficiency and the resulting thrust - I'd imagine that a related engine/A/C performance diagram could be used to guesstimate an very rough figure - say 10 - 33+ % more max thrust at best operating height.
Soviet era terbojet engines were never made for longevity, only to operate to within their A/C's role service limitations for as cheap new unit cost AFAIK with shorter lives than allied equivelents.


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## FLYBOYJ (Mar 15, 2014)

And also consider with this extra thrust, how much N1 or T1 over speed could this engine take? I know some engines that could go as high as 10% over. Wiki talks about some pumps over speeding, I'm wondering about that as I've seen pumps with shear shafts that would break if the pump locked up or was over sped. It would be interesting to find a MiG-25 maintainer somewhere.


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## silence (Mar 17, 2014)

I wonder if you could install a J58 in place of a R-15? The raw sizes seem to work.


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## razor1uk (Mar 18, 2014)

While the unit size might be similar, there's a lot of ancillary bits and bobs and other gubbins that are specific to each engine, even the fuels aren't compatable (and thats ignoring the expensively exotic afterburner lighting fuel for the J57) due to different heat, material and fuel tank constructions.

Also while the Foxbats shape is theoretically ok for high transonic machs, its structure isn't able to handle them like the SR-71 is/was able to - the bird used its fuel as a heat exhanging coolant for the electronics, hydraulics and the airframe cooling heatsinks. I've heard some rumours that the bird could often go above M4 -since generally misinformations, its cruise speed is listed the same as its max speed.

Yep FBJ, a rocking horse poo rare (for this website; perhaps too western) foxbat maintainer could provide some good insights so long as they didn't risk foxhound.


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