Mustang Origins

[drgondog]

Was the North American Aviation Mustang (later P-51) based on RAF Operational Requirement OR.73 and RAF Specification 18/39?
Does anyone have a copy of either of those documents, or can they provide a link to where they can be read?

NAA (Kindelberger/Atwood) received RAF Spec 18/39 (typo '18/29') on pg 50 of Bastard Stepchild) on Jan 5 1939. Sent by Air Ministry which was also seeking NAA to produce Spitfire. Per correspondence files NAA.

While the MB 5 may have been built to that spec, it was also the framework for changing the P-509 proposal and spec to the jointly developed Model Specification for XP-51 Airplane NAA Model Number NA-73, dated 4-24-40. While this was the foundation for XP-51, NA-73, NA-83 Mustang I, it makes no reference to the RAF Spec 18/39, The Mustang features and performance was tied to that spec - and expanded.

NA-73 had nothing to do with O.R. 73 nomenclature, it was just the next active charge number after NA-72 a Brazil BC-1A. NA-73X was next, on 4-20-40 and then NA-73.

While there may be confusion on January 1939 delivery date, one can suppose it may have been a draft, but in any case the title is as presented on page 50. Page 51 cites verbatim the Requirements.

 

[drgondog]

Thank you.
That table looks as if it is from Wikipedia. It doesn't mention the Mustang, but neither does any other RAF Specification. There are quite a few planes missing from that list, so I am not sure that omission of 'Mustang' from th right hand column means that F.18/39 was not relevant to the formulation of the NAA specification. Curious.

Or, Wikipedia is incomplete. A suggestion regarding NAA reluctance to cite OR 73 would be clearly to not prod AAC with suggestion that the XP-51 was built in accordance to RAF guidleines. The fact (or omission) of NA-73 in the British tables may simply be because it was not manufactured in UK or Canada.

 

[Engineman]

A summary of F.18/39, which was issued over year before the discussions with North American
1 May 1939, Single seat fighter, File Number S.51584/RDT1 issued 4 May 1939 to Martin Baker for type MB.3 (later MBT.4 and 5).

To meet Operational Requirement OR.73, a high speed single seat fighter capable of operating in any part of the world is required as a replacement for the Spitfire and Hurricane. The outstanding requirement is to obtain the greatest possible supremacy in maximum speed over the contemporary bomber. There may be advantages to be twin engine design which would be acceptable, provided the performance was superior to that which would be obtained from a single engine aeroplane. The AUM of an aircraft built to this specification is 12,000 pounds.

Performance: The speed at 15,000 must not be less than 400 mph with the 37.V/12 engine and it is essential that the engine selected should be equipped with a two speed supercharger so that speeds below 15,000 feet may be as high as possible. The normal fuel load is to be sufficient for 15 minutes flying at maximum engine speed for take off conditions, plus 2 hours at the most economical cruising air speed at 15,000 feet plus 15 minutes at 15,000 feet at maximum engine speed for level flight at 15,000 feet, plus 30% reserve.

Engine: The aircraft shall be designed to accommodate a 37.V/12 engine but a Rolls-Royce RM.2SM engine is to be installed in the first instance.

Strength: The design requirements of AP.970, any corrigenda thereto and all current ADMs applicable to TV class acceptances are to be satisfied at 1.1 times the AUW in the fully loaded condition at the factors give for "other than experimental aeroplanes".

Armament: Two cannon guns in each wing set to fire along the line of flight, with at least 1 drum of 60 rounds of ammunition per gun. Provision shall be made for the installation of a reflector gun sight and a head sight and a Cine Camera gun, mounted internally. Protection for the pilot against armour piercing 0.303 inch ammunition is required to cover a forward cone with an angle of 20 degrees to the thrust line of the aircraft.

Wireless: TR.9D or TR.1133.

Note the Spitfire I and Hurricane, after deducting 15 minutes fuel for take off, had endurances at economic cruise of 3.1and 3.4 hours, for the Spitfire that means around 22 gallons to fulfill the combat allowance and reserves requirements of F.18/39, The MB.3 is quoted as having 120 gallons of fuel for its 2,00 HP engine compared with 84 gallons for the 1,00 HP engine in the Spitfire I.

Digital Archive – Aircraft Production Lists North American Aviation Charge Numbers (XLS format)

66 Harvard
67 XB-28
68 n/a
69 P-64
70 n/a
71 Venezuela order for AT-6 accepted in June 1940
72 Brazil order for BC-1A
73 Mustang
74 AT-6
75 Harvard

Very interesting detail from May 1939.
I particularly like the engine reference 37.V/12, which I guess was an early reference to the 37l Griffon development (that did not yet exist).
The RM.2SM engine for initial installation was possibly the Merlin X, although that is listed by Lumsden as the RM.1SM, with no RM.2 listed!

Eng

 

[ThomasP]

Merlin IV was basically the Merlin III modified specifically for use with 70/30 water/glycol coolant instead of pure glycol.

RM2S was a detail improved & strengthened Merlin IV with an improved supercharger, designed from the start to be run on 100 grade, No production.
Hi: 1265 BHP at 3000 rpm at +9.375 lbs at 9,500 ft
Hi: 1150 BHP at 3000 rpm at +9.375 lbs for TO

RM2SM was the RM2S with a 2-speed supercharger. Slightly re-geared RM2S S gear became the RM2SM M gear. No production.
Hi: 1140 BHP at 3000 rpm at +9.5 lbs at 16,750 ft
Lo: 1245/1365 BHP at 3000 rpm at +9.5/+12.125 lbs at 9,500/6,250 ft
Lo: 1120/1280 BHP at 3000 rpm at +9.5/+12.125 lbs for TO

These were the ratings achieved on bench test before development switched to the Merlin XX.

The RM2SM was effectively the intermediary development model between the Merlin X and the Merlin XX, but without the Hooker based improvements.

 

[ThomasP]

Addition to the above.

Just noticed, apparently the Merlin RM2SM was fitted to the first of the Spitfire Mk III airframes in early-1940. The 2nd prototype had the RM3SM (Merlin XX).

 

[Charlie P]

Very interesting detail from May 1939.
I particularly like the engine reference 37.V/12, which I guess was an early reference to the 37l Griffon development (that did not yet exist).
The RM.2SM engine for initial installation was possibly the Merlin X, although that is listed by Lumsden as the RM.1SM, with no RM.2 listed!

Eng

The first engine you mention was indeed the Griffon.
The second was the Merlin XX (single-stage, two-speed)

 

[Charlie P]

NAA (Kindelberger/Atwood) received RAF Spec 18/39 (typo '18/29') on pg 50 of Bastard Stepchild) on Jan 5 1939. Sent by Air Ministry which was also seeking NAA to produce Spitfire. Per correspondence files NAA.

While the MB 5 may have been built to that spec, it was also the framework for changing the P-509 proposal and spec to the jointly developed Model Specification for XP-51 Airplane NAA Model Number NA-73, dated 4-24-40. While this was the foundation for XP-51, NA-73, NA-83 Mustang I, it makes no reference to the RAF Spec 18/39, The Mustang features and performance was tied to that spec - and expanded.

NA-73 had nothing to do with O.R. 73 nomenclature, it was just the next active charge number after NA-72 a Brazil BC-1A. NA-73X was next, on 4-20-40 and then NA-73.

While there may be confusion on January 1939 delivery date, one can suppose it may have been a draft, but in any case the title is as presented on page 50. Page 51 cites verbatim the Requirements.

(I still can't work out how to quote only part of someone's comment!)

With regard to the date of receipt of OR.73, could it be a simple mix up of US and UK dating format?

1/5/39 is 5th Jan in the US and 1st May in the UK.

Apart from in this book, all other references seem to state that the OR was released on 1 May.

 

[Engineman]

Merlin IV was basically the Merlin III modified specifically for use with 70/30 water/glycol coolant instead of pure glycol.

RM2S was a detail improved & strengthened Merlin IV with an improved supercharger, designed from the start to be run on 100 grade, No production.
Hi: 1265 BHP at 3000 rpm at +9.375 lbs at 9,500 ft
Hi: 1150 BHP at 3000 rpm at +9.375 lbs for TO

RM2SM was the RM2S with a 2-speed supercharger. Slightly re-geared RM2S S gear became the RM2SM M gear. No production.
Hi: 1140 BHP at 3000 rpm at +9.5 lbs at 16,750 ft
Lo: 1245/1365 BHP at 3000 rpm at +9.5/+12.125 lbs at 9,500/6,250 ft
Lo: 1120/1280 BHP at 3000 rpm at +9.5/+12.125 lbs for TO

These were the ratings achieved on bench test before development switched to the Merlin XX.

The RM2SM was effectively the intermediary development model between the Merlin X and the Merlin XX, but without the Hooker based improvements.

"These were the ratings achieved on bench test before development switched to the Merlin XX.
The RM2SM was effectively the intermediary development model between the Merlin X and the Merlin XX, but without the Hooker based improvements."

Hi Thomas!

Yes, I can see that this is likely the development situation from the progression of the RR/Ministry designation RM.2SM and the other engine details, but where did you find the details you quote for the RM.2S and the RM.2SM please?
Another factor here is that as a Spitfire Merlin powerplant, I would expect it to have the .477 propeller reduction ratio.

Thanks for your info, it seems like you have a good source?

Eng

 

[Engineman]

The first engine you mention was indeed the Griffon.
The second was the Merlin XX (single-stage, two-speed)

Hi,

See my reply to ThomasP about his post #24. It does seem that the RM.2SM was a development engine that did not get a Mk number

Eng

 

[drgondog]

(I still can't work out how to quote only part of someone's comment!)

With regard to the date of receipt of OR.73, could it be a simple mix up of US and UK dating format?

1/5/39 is 5th Jan in the US and 1st May in the UK.

Apart from in this book, all other references seem to state that the OR was released on 1 May.

Possible

 

[Charlie P]

I've tried doing what you said and I still can't get it right!