Mosquito LB.IX

[wuzak]

What if in early 1943, during the time which the Mosquito B.IX (and PR.IX) was being developed, the low level daylight bombing raids by 105 and 139 squadrons impressed the RAF BC hierarchy enough that they decided to maintain one or two dedicated daylight Mosquito bomber squadrons.

Then as the IX came on stream in mid 1943 a decision was made to do to the Mosquito what was done with the Spitfire VIII IX - split into high altitude and low altitude versions.

Thus the LB.IX could be born, using a pair of Merlin 66s for low altitude performance. This would give a marginal performance boost, if at all, at low levels, but still improved performance at higher altitudes.

In early-mid 1944 when 150 grade fuel is approved for use the 66 could be turned up to give around 2000hp at low level with +25psi boost. That should give the Mosquito pretty good performance at 5000ft and below.

Then if we can get +28psi into the 66 power will nearly peak at 2200hp.

http://www.spitfireperformance.com/merlin66hpchart.jpg

Having looked over at Spitfire performance I now wonder if the single stage engine would be just as good for a low altitude LB.IV. I see some of the NF squadrons got to use 150 grade fuel when combating V-1s. Certainly the 200-250lb weight savings per engine would be of benefit.

Given how difficult the Luftwaffe could find the Mosquito to intercept at low level, how effective could a squadron using a LB.IX (or LB.IV), armed with 4 x 500lb bombs (+2 x 500lb on the wings as an option), 2 x 1000lb bombs (+2 x 500lb on the wings as an option) or 1 x 4000lb MC bomb (HC bomb didn't liek being dropped at high speed at low level, apparently), be?

 

[wuzak]

Just found http://www.wwiiaircraftperformance.org/Aircraft_Engines_of_the_World_Rolls-Royce_Merlin.pdf

This gives:
M66
Max (+25psi boost MS) 2000hp @ sea level
Max (+25psi boost FS) 1860hp @ 10,500ft.
Max (+18psi boost MS) 1705hp @ 5,750ft
Max (+18psi boost FS) 1580hp @ 16,000ft.
Normal (MS) 1400hp @ 9,250ft
Normal (FS) 1310hp @ 19,000ft
Cruising (MS) 1100hp @ 11,250ft
Cruising (FS) 1040hp @ 20,500ft


M24
Max (+25psi boost MS) 1730hp @ sea level
Max (+25psi boost FS) 1780hp @ 4,000ft.
Max (+18psi boost MS) 1640hp @ 2,000ft
Max (+18psi boost FS) 1500hp @ 9,500ft.
Normal (MS) 1240hp @ 10,000ft
Normal (FS) 1175hp @ 17,500ft
Cruising (MS) 1000hp @ 9,250ft
Cruising (FS) 970hp @ 16,000ft

The Merlin 25, as used in the Mosquito, is the same as the Merlin 24 but with reversed coolant flow.

 

[davebender]

Two light bomber squadrons are a drop in the bucket of total RAF strength. They aren't going to attract special attention.

 

[wuzak]

Two light bomber squadrons are a drop in the bucket of total RAF strength. They aren't going to attract special attention.

Not the point.

2 squadrons of low level Mosquitoes could cause quite a bit of damage, seeing as they would have greater accuracy than high level bombers.

Also, in mid 1943 it is unlikely that BC would commit more than 2 squadrons to low level daylight bombing. Remember that in early 1943 there were only a few Mosquito bomber squadrons (or was it two - 105 139?), and that the head of BC was very much a proponent of night time bombing raids by heavy bombers. Low level raids after mid-1943 (when 105 139 squadrons were transferred to night raids/pathfinding) would be largely be experimental.

 

[mhuxt]

Remember that in early 1943 there were only a few Mosquito bomber squadrons (or was it two - 105 139?), and that the head of BC was very much a proponent of night time bombing raids by heavy bombers. Low level raids after mid-1943 (when 105 139 squadrons were transferred to night raids/pathfinding) would be largely be experimental.

It was three squadrons in early 1943 - the two you've named plus 109 Squadron, the original Oboe markers in 8 Group.

105 and 139 were brought into 8 Group (under AVM Don Bennett - also very much committed to high-level night-time raids) at the start of June when the other squadrons of 2 Group were hived off to become the proto-2nd TAF.

The best bet to get a low-level force of Mosquitos into action in mid-1943 would probably have been to put them in 5 Group, where Bennett's great rival, Cochrane, was forging a force to rival 8 Group. He was in the ascendancy a year later, in April '44, when three of Bennett's squadrons, including the Mosquitos of 627 Squadron, were transferred to Cochrane's group. As I've noted previously, 627 was used by Cochrane for low-level marking, so a low-level Mosquito attack force in 5 Group is not unrealistic.

I don't know enough about the performance of the Merlin 66 to comment, but I think the 25 would have been a more likely choice for low-level ops. The first FB.VIs to go to 418 Squadron for intruder work in June or July 1943 had the 25.

 

[Kryten]

The Mossie may have been hard to intercept due to it's speed but low level makes it vulnerable to light flak, which was the Mossies biggest enemy!

 

[vinnye]

You are correct Kryten, flak was a big problem for any low flying aircraft.
The best defence against it was to keep low and fast and try to be in and out before the enemy knows you are around!
The Mossie was quite good at this - but even more speed may have helped.
The reverse is also a problem, a small error in judgement when low and fast is usually fatal!

 

[wuzak]

It was three squadrons in early 1943 - the two you've named plus 109 Squadron, the original Oboe markers in 8 Group.

105 and 139 were brought into 8 Group (under AVM Don Bennett - also very much committed to high-level night-time raids) at the start of June when the other squadrons of 2 Group were hived off to become the proto-2nd TAF.

The best bet to get a low-level force of Mosquitos into action in mid-1943 would probably have been to put them in 5 Group, where Bennett's great rival, Cochrane, was forging a force to rival 8 Group. He was in the ascendancy a year later, in April '44, when three of Bennett's squadrons, including the Mosquitos of 627 Squadron, were transferred to Cochrane's group. As I've noted previously, 627 was used by Cochrane for low-level marking, so a low-level Mosquito attack force in 5 Group is not unrealistic.

I don't know enough about the performance of the Merlin 66 to comment, but I think the 25 would have been a more likely choice for low-level ops. The first FB.VIs to go to 418 Squadron for intruder work in June or July 1943 had the 25.

Thanks mhuxt.

B.IVs could have easily been updated with Merlin 25s. I think, though, the Merlin 66 would have given more performance, though they were heavier, than the 25s, particularly if they got to use the 150 grade fuel. And the 66s would have given better altitude performance - but possibly worse fuel economy.

I read somewhere that 627 were not fond of the IXs they had, preferring the IV/XX for low level marking. Mainly because the IXs they did have had the bulged bomb bay and high altitude rated 70-series Merlins.

 

[mhuxt]

The Mossie may have been hard to intercept due to it's speed but low level makes it vulnerable to light flak, which was the Mossies biggest enemy!

Flak was indeed the Mossie's deadliest (only?) nemesis, however the 2 Group day bombers reduced their casualty rates significantly when they moved to predominantly low-level ops. Bear in mind the FB.VIs of 2 Group also undertook daylight bombing ops from late '43 and ran at a lower loss rate than the earlier B.IVs, also of 2 Group. Have the numbers upstairs, will check in the morning.

 

[Jabberwocky]

Just found http://www.wwiiaircraftperformance.org/Aircraft_Engines_of_the_World_Rolls-Royce_Merlin.pdf

This gives:
M66
Max (+25psi boost MS) 2000hp @ sea level
Max (+25psi boost FS) 1860hp @ 10,500ft.
Max (+18psi boost MS) 1705hp @ 5,750ft
Max (+18psi boost FS) 1580hp @ 16,000ft.
Normal (MS) 1400hp @ 9,250ft
Normal (FS) 1310hp @ 19,000ft
Cruising (MS) 1100hp @ 11,250ft
Cruising (FS) 1040hp @ 20,500ft

M24
Max (+25psi boost MS) 1730hp @ sea level
Max (+25psi boost FS) 1780hp @ 4,000ft.
Max (+18psi boost MS) 1640hp @ 2,000ft
Max (+18psi boost FS) 1500hp @ 9,500ft.
Normal (MS) 1240hp @ 10,000ft
Normal (FS) 1175hp @ 17,500ft
Cruising (MS) 1000hp @ 9,250ft
Cruising (FS) 970hp @ 16,000ft

The Merlin 25, as used in the Mosquito, is the same as the Merlin 24 but with reversed coolant flow.

I'm not sure the Merlin 24/25 figures are correct.

At +18 lbs, Rolls-Royce documentation (including ram effect) has 1600 hp at 6400 ft and 1480 hp at 14000 ft.
At +25 lbs, figures are 1950 hp at sea level and 1800 hp at 8000 ft.

The difference between a Merlin 25 equipped Mosquito and a Merlin 72 equipped Mosquito only favours the Merlin 72 powered aircraft by about 5 mph, at most. Adding 25 hp for a Merlin 66 really isnt going to cange that very much.

 

[wuzak]

The difference between a Merlin 25 equipped Mosquito and a Merlin 72 equipped Mosquito only favours the Merlin 72 powered aircraft by about 5 mph, at most. Adding 25 hp for a Merlin 66 really isnt going to cange that very much.

At what altitude?

 

[Jabberwocky]

At what altitude?

Sea-level to about 15,000 ft. Above that the Merlin 72 is noticably superior

 

[wuzak]

Sea-level to about 15,000 ft. Above that the Merlin 72 is noticably superior

Hence my suggestion to use Merlin 66s - they have more power at the low altitudes than the 72.

 

[mhuxt]

Had a squizz for the numbers. Sharp Bowyer say the FB.VIs of 2 Group, operating *almost* exclusively at low level, suffered a casualty rate of 2.25% in the period between October '43 and the end of May '44. As ever, the rates are higher at the start of operations - for all of 1944 the daylight loss rate for 2 Group Mossies is 1.84%. Again, most of their attacks will have been carried out at low level. Interestingly, Mossies operating in daylight which returned to base suffered battle damage at a rate in excess of 8%.

So, clearly the flak is a danger, not so much however to be prohibitive nor so deadly that the Mossies couldn't handle much damage.

 

[Jabberwocky]

Hence my suggestion to use Merlin 66s - they have more power at the low altitudes than the 72.

At MS supercharger levels, they have 40 hp at most over the Merlin 72. Maybe enough for an extra 2-3 mph.

I'd still prefer the Merlin 25s for low level work. Simpler, lighter (by around 220 lbs) and slightly more reliable.

I'll do an overaly of the Roll-Royce Merlin 66, Merlin 72/73 and Merlin 25 charts and see what the differences are.