RAF Mustang mods for V-1 interception

[BarnOwlLover]

I'm trying to figure out what mods/changes the RAF used on the Mustang IIIs (and maybe IVs?) that were used to intercept V1 flying bombs. Other than running 25 lbs of supercharger boost, what other changes were made to boost performance? I do believe that 25 lbs of boost was equal to 80 in/Hg, which is what the V-1650-9 in the P-51H used (along with 90 in/Hg, which I thing equated to 30 lbs of boost).

 

[EwenS]

Mark III only. Mk.IV didn't begin to arrive in the UK in quantity until Sept 1944 with the first issues to UK squadrons in Jan 1945.

 

[Mike Williams]

I'm trying to figure out what mods/changes the RAF used on the Mustang IIIs (and maybe IVs?) that were used to intercept V1 flying bombs. Other than running 25 lbs of supercharger boost, what other changes were made to boost performance? I do believe that 25 lbs of boost was equal to 80 in/Hg, which is what the V-1650-9 in the P-51H used (along with 90 in/Hg, which I thing equated to 30 lbs of boost).

The RAE's Technical Note No.Aero.1501(Flight) on Improvement of Performance of Fighter Aircraft Operating Against the German Flying Bomb dated August 1944 describes improvements and results for the Mustang III:

I don't know to what extent these improvements were adopted by operational squadrons.

Also, ADGB decided to retard the ignition timing on the Mustang III's V-1650-7 engine. (ADGB/S.36777/Eng.4.).

 

[BarnOwlLover]

One thing that I read did say that the V-1 Mustang IIIs were capable of 415-420 mph at 2000 ft. So I'm wondering, one, how valid is that claim, and two, what was done to get there. I'm also interested in how that compared to a Mustang III that was used by Rolls-Royce as a Merlin 100 test bed.

 

[BarnOwlLover]

I do wonder how the V-1 mods compare to what Rolls Royce did with a Mustang III that used Merlin 100 and 113 engines (quoting an entry from Wikipedia on the subject, but citing official reports):

"While North American was concentrating on improving the performance of the P-51 through the development of the lightweight Mustangs, in Britain, other avenues of development were being pursued. To this end, two Mustang Mk IIIs (P-51Bs and P-51Cs), FX858 and FX901, were fitted with different Merlin engine variants. The first of these, FX858, was fitted with a Merlin 100 by Rolls-Royce at Hucknall; this engine was similar to the RM 14 SM fitted to the XP-51G and was capable of generating 2,080 hp (1,550 kW) at 22,800 ft (7,000 m) using a boost pressure of +25 lbf/in2 (170 kPa; 80 inHg) in war emergency setting. With this engine, FX858 reached a maximum speed of 455 mph (732 km/h) at 17,800 ft (5,425 m), and 451 mph could be maintained to 25,000 ft (7,600 m). The climb rate was 4,500 ft/min (22.9 m/s) at 1,600 ft (486 m) and 4,000 ft/min (20.3 m/s) at 13,000 ft (3,962 m).[60]

FX901 was fitted with a Merlin 113 (also used in the de Havilland Mosquito B.35). This engine was similar to the Merlin 100, fitted with a supercharger rated for higher altitudes. FX901 was capable of 454 mph (730 km/h) at 30,000 ft (9,100 m) and 414 mph (666 km/h) at 40,000 ft (12,200 m).[61]"

 

[Mike Williams]

One thing that I read did say that the V-1 Mustang IIIs were capable of 415-420 mph at 2000 ft. So I'm wondering, one, how valid is that claim, and two, what was done to get there. I'm also interested in how that compared to a Mustang III that was used by Rolls-Royce as a Merlin 100 test bed.

 

[Deleted member 68059]

This might help a little.

(below is from same file, also on V1 modified fighters)

 

[Sisu]

Why did they not work up an ADI system before the P-51H, given that round engines had apparently been fitted with water injection well before this time?

Also, how did they deal with engine cooling in these operations? At the Reno races, run at about 10,000 feet density altitude, the racers found early on that a stock Mustang cooling system simply cannot deal with the heat loads of sustained operation at above 60 inches -- after just a few minutes the radiator exit door has to be fully open with a big loss of speed. In Reno operations nearly everyone uses "spray bars" to dump water on the radiator face -- this was not available during WWII. Did they have cooling issues back then?

 

[Deleted member 68059]

Why did they not work up an ADI system before the P-51H, given that round engines had apparently been fitted with wate injection well before this time?

Also, how did they deal with engine cooling in these operations? At the Reno races, run at about 10,000 feet density altitude, the racers found early on that a stock Mustang cooling system simply cannot deal with the heat loads of sustained operation at above 60 inches -- after just a few minutes the radiator exit door has to be fully open with a big loss of speed. In Reno operations nearly everyone uses "spray bars" to dump water on the radiator face -- this was not available during WWII. Did they have cooling issues back then?

water/methanol gets you almost no increase in high altitude performance, and weighs a LOT. The whole point of the 51H was to be light.

I admit to not having NAA documents on the matter, but this is my guess pending anyone coming along with an NAA memo.

 

[Sisu]

I never thought about weight being a big issue with a water-alcohol system. There's a pump, regulators, piping, and of course the water itself. How much heavier was a dash 9 than an earlier engine? Seems like ADI would be useful at V-1 chasing altitudes.

Could cooling be an additional issue? If the cooling system struggled with a stock engine at high power, more power would simply make the problem worse...

 

[Deleted member 68059]

I never thought about weight being a big issue with a water-alcohol system. There's a pump, regulators, piping, and of course the water itself. How much heavier was a dash 9 than an earlier engine? Seems like ADI would be useful at V-1 chasing altitudes.

Could cooling be an additional issue? If the cooling system struggled with a stock engine at high power, more power would simply make the problem worse...

Well you cant have P-51H for chasing flying bombs anyway, it was not in service until after the V1`s had ceased to be launched.

 

[Mike Williams]

I do wonder how the V-1 mods compare to what Rolls Royce did with a Mustang III that used Merlin 100 and 113 engines (quoting an entry from Wikipedia on the subject, but citing official reports):

"While North American was concentrating on improving the performance of the P-51 through the development of the lightweight Mustangs, in Britain, other avenues of development were being pursued. To this end, two Mustang Mk IIIs (P-51Bs and P-51Cs), FX858 and FX901, were fitted with different Merlin engine variants. The first of these, FX858, was fitted with a Merlin 100 by Rolls-Royce at Hucknall; this engine was similar to the RM 14 SM fitted to the XP-51G and was capable of generating 2,080 hp (1,550 kW) at 22,800 ft (7,000 m) using a boost pressure of +25 lbf/in2 (170 kPa; 80 inHg) in war emergency setting. With this engine, FX858 reached a maximum speed of 455 mph (732 km/h) at 17,800 ft (5,425 m), and 451 mph could be maintained to 25,000 ft (7,600 m). The climb rate was 4,500 ft/min (22.9 m/s) at 1,600 ft (486 m) and 4,000 ft/min (20.3 m/s) at 13,000 ft (3,962 m).[60]

There was little to choose between a Mustang III with V-1650-7 (+25 boost) and a Mustang III with Merlin 100 (+25 boost), at the low altitudes where the V-1 flew. The engine powers of the two engines were similar when operating at +25 lbs. with 150 grade fuel.

Mustang III FX.858 (Merlin 100) +25 lbs. boost (A.& A.E.E. trials report here)
2,000' - 404 mph / 4,000' - 413 mph

Mustang III FB.377 (Merlin V-1650-7) +25 lbs. boost (see post #3 and #6 above)
2,000' - 410 mph / 4,000' - 413 mph

 

[MIflyer]

One interesting problem that had for intercepting V-1's at night was the flame from the pulsejet. While the flame was easy to see at night it also made it very difficult to judge the range to the flying bomb in a tail chase - and you did not want to be too close if you hit one and it exploded, nor too far away so that the gunfire was ineffective. So they took a Typhoon and added a APS-13 or Monica tail warning radar focused forward. When the warning light came on the pilot knew how far he was from the V-1 and thus judge much better when to open fire. This apparently was fitted only to one aircraft for test purposes.

 

[Mike Williams]

One interesting problem that had for intercepting V-1's at night was the flame from the pulsejet. While the flame was easy to see at night it also made it very difficult to judge the range to the flying bomb in a tail chase - and you did not want to be too close if you hit one and it exploded, nor too far away so that the gunfire was ineffective. So they took a Typhoon and added a APQ-13 or Monica tail warning radar focused forward. When the warning light came on the pilot knew how far he was from the V-1 and thus judge much better when to open fire. This apparently was fitted only to one aircraft for test purposes.

Interesting. There must be some stories around about how the Mosquitoes managed. I've a very fuzzy recollection of having read something somewhere about that, although memory fails me at the moment.

 

[MIflyer]

A mod was done to some P-51's on B-29 escort missions out of Iwo Jima. They replaced the momentary contact spring loaded switch that was intended to enable a test of the 2nd speed of the supercharger with a regular toggle switch so the pilot could engage the high speed setting and still have enough hands left to fly the airplane. The daylight raids of the B-29's occurred at 20,000 ft and above. Given the fact that Japanese fighter mostly started running out of performance above 15,000 ft, the Japanese pilots preferred to lure the Mustangs down to where they had peak performance but the P-51 low speed supercharger was topping out. Of course the P-51 pilots could have just ignored the Japanese fighters and protected the bombers, but fighter pilots do not tend to think that way. So one P-51 squadron commander had his Mustang modified with the toggle switch that enabled him to engage the higher speed at below the nominal 18,000 ft aneroid device switching point. The Packard rep said something like, 'You canna do that Captain, you'll blow up my engines!" But it worked.

 

[Mike Williams]

Why did they not work up an ADI system before the P-51H, given that round engines had apparently been fitted with water injection well before this time?

Fwiw, see attached:

 

[EwenS]

Interesting. There must be some stories around about how the Mosquitoes managed. I've a very fuzzy recollection of having read something somewhere about that, although memory fails me at the moment.

Generally the Mosquitos patrolled along the French coast at around 8,000 ft in clear weather in an attempt to spot the flashes from the launchings and then the flames from the pulsejet. They then dived to the roughly 2,500ft (but heights were variable, with one attack occurring at 12,000ft) at which the bombs flew at 320-350mph. In other cases they were vectored out to meet them by land based radar. Other than the problems of getting on a V1's tail and closing for the attack, the third problem in completing the kill was to do it in the space of about 30 miles before it reached the gun belt on the English coast or over land between the gun belt and the balloon belt immediately south of London.

And aircraft were lost to engine and radiator damage after flying through the debris of a V1 kill. The other danger was having your Mossie set on fire and losing its paint by passing through the explosion!

By July some 9 squadrons were on anti-Diver duty either full or part time, including 418 & 605 flying non-radar equipped Mk.VI. The top scorer was the Canadian Russ Bannock of 418 with 18.5 kills to his credit (After 5 July a V1 downed over land only counted as half a kill).

Mosquitos claimed 623 V1.

 

[Mike Williams]

One of those fuzzy memories- Aid for the daylight location of flying bombs by fighter aircraft:

 

[Daggerr]

One of those fuzzy memories- Aid for the daylight location of flying bombs by fighter aircraft:

".......... Rolls-Royce have done some careful test and assessment work wherein they found that the slight extra cooling effect of the injection is more than off-set by the slightly lowered thermal content of the total charge."

Not clear what is meant by that but it sounds silly.

I can imagine that Merlin engines with an intercooler just upstream the piston engine would not benefit much from water-methanol injection upstream the carburetor which is upstream the supercharger.
Although a water cooled intercooler would be smaller than an aircooled one and therefor easier to incorporate, the argument itself has nothing to do with whether the engine is cooled by water or air.

The P-47 has an aircooled intercooler between the turbocharger and carburetor so any liquid injection is downstream supercharger and intercooler and therefor has a larger impact on charge manifold temperature, but that has nothing to do with using air instead of water for engine cooling.

Rolls-Royce should simply have said that the design of their Merlin engines is not really suitable for ADI, but for obvious reasons they did not.
It's not whether the engine is water or air cooled but the sequence of steps from ambient air to piston engine that makes the difference between a Spitfire/Mustang and a P-47.

 

[BarnOwlLover]

Strangely, later Merlins (Packard V-1650-9/-11, Merlin 100 series) could use ADI to make a notable performance difference. The -9 Packard Merlin was able to make 220bhp on WEP/sprint with ADI, with the -11 capable of nearly 2300 bhp, as well as the not mass produced Merlin RM 17SM that could make 2600 bhp with ADI, 36 lbs supercharger boost and 150 octane fuel. I do wonder what changed between the Merlin 60 series/ pre -9 V-1650s and the later engines.