I picture the Griffon Windsor sooner, but I have a tendency to want to push things as early as possible, so take that with a few grains of salt.
Dropping the Wimpy after contract completion makes sense, too, but it makes me wonder if the Air Ministry doesn't just re-engine it & continue, rather than switch.
It will take longer to redesign the Wellington to take the Griffon than it will to put the Griffon-Warwick into production.
IOTL:
- Development of the Merlin-Wellington began in January 1938. The prototype flew in March 1939 and the Mk II went into production in October 1940 at Vickers, Weybridge.
- Detail design of the Hercules-Wellington began in January 1938 too. The prototype flew in May 1939 and the Mk III went into production in October 1940 at Vickers, Blackpool.
- The contract to develop the Twin Wasp Wellington was signed on 9th September 1939 and the prototype flew at Vickers, Chester in December 1940. This factory built a total of 220 Mk IVs (including the prototype) in 2 batches. The first 25 were delivered between December 1940 and May 1941. The other 195 were delivered between June 1941 and March 1942.
If the Air Ministry authorised development of a Griffon-Wellington in January 1938 ITTL my guess (based on the above) is that the prototype wouldn't fly until May 1939 and the first production aircraft wouldn't be delivered until October 1940.
The Warwick had been in development since October 1935 and because we're expecting development of the Griffon to be less troublesome than the Vulture's the first flight will be in the summer of 1938 instead of August 1939. If production contracts are placed at the same time as the second Weybridge and first Chester Wellington contracts it will be delivered to the RAF in August 1939 (from Chester) and November 1939 (from Weybridge).
Therefore, I think the Air Ministry will order the Griffon-Warwick into production because it will be available one year before the Griffon-Wellington.
War hasn't (quite) started, but are they getting nervous enough not to risk it? (As I'm reading the timing, end of contract is cutting it pretty fine, though they can't know it yet TTL, can they?)
This is a non-issue.
The decision to put the Griffon-Warwick into production will be taken nearly 18 months before the declaration of war. Furthermore, my understanding is that when the decision would be made, war wasn't expected until the early 1940s.
- The first Vickers, Weybridge contract was placed in August 1936. The 180 aircraft in this batch were delivered between October 1938 and August 1939.
- The second Vickers, Weybridge contract was placed in May 1938. The 120 aircraft in this batch were delivered between November and December 1939.
- 200 Wellingtons were ordered from Gloster in October 1937. This contract was transferred to Vickers, Chester which delivered the aircraft between August 1939 and August 1940.
- The order to start Wellington production at Vickers, Blackpool wasn't given until December 1939 and the first aircraft was delivered in August 1940.
ITTL the decision to order the Griffon-Warwick into production at Vickers, Weybridge would be made in May 1938 when 120 would be ordered instead of the factory's second Wellington contract. This is nearly 6 months before the Munich Crisis, nearly a year before the German occupation of Bohemia-Moravia and nearly a year-and-a-half before war broke out. AFAIK at this time the expectation in the spring of 1938 was that war would not come until the early 1940s.
The Air Ministry will order Griffon-Warwicks from from Gloster in October 1937 and transfer the contract to Vickers, Chester at a later date. October 1937 was nearly two years before war broke out so I think time will not be an issue at the time the decision was made. Furthermore, it would have been just as hard for these factories to tool-up to build Warwicks as it was to tool-up to build Wellingtons ITTL. Ditto for Vickers, Blackpool.
Or is the performance improvement enough, & the production changes small enough, to warrant it?
Yes, the improvement in performance is big enough. These are the performance figures for the main marks of Wellington bomber from the copies of the "Fight" series of official histories of the RAF in World War II from the Hyperwar website.
| Aircraft | Maximum Speed | Service Ceiling | Range and Associated Bomb Load |
| m.p.h. at feet | feet | miles—lb. |
|
| Wellington IC | 235 at 15,500 | 18,000 | 2,550—1,000 or 1,200—4,500 |
| Wellington II | 247 at 17,000 | 20,000 | 2,450—1,250 or 1,400—4,500 |
| Wellington III | 255 at 12,500 | 19,500 | 2,200―1,500 or 1,540―4,500 |
| Wellington IV | 229 at 13,000 | 20,000 | 2,180—500 or 980—4,000 |
| Wellington X | 255 at 14,500 | 19,600 | 2,085―1,500 or 1,470―4,500 |
According to the Puthams on Vickers aircraft the Warwick Mk II (with two 2,000hp Centaurus IV engines) had a maximum speed of 300 mph at 20,000ft. Its range was 2,075 miles at 185 mph at 15,000ft. It could carry 8,000lb of bombs. Unfortunately, it doesn't say whether the range of 2,075 miles was when it was carrying 8,000lbs of bombs. My guess is that the early marks of Griffon-Warwick would have had similar performance.
Yes, the production changes are small enough to warrant re-tooling of the Weybridge factory.
Re-tooling is a non-issue for Blackpool and Chester because they weren't switching from Wellingtons to Warwicks. They were building Warwicks from the start.