Some FFO/APOD information on high level bombing with much background info coming from British Secret Projects and RAF Bomber Command and its Aircraft.
http://francefightson.yuku.com/topic/406/The-RAF-Super-Bombers
Under a special and secret specification in 1939 (B.23/39), the Air Ministry called on Vickers to develop a uniquely challenging bomber using technologies of the ragged edge of those available in 1939. B.23/39 called for a Wellington variant able to operate at 40,000, pressurised internally to 10,000 (a differences of 7.5psi).
The first design was the Vickers Type 407, with two Hercules VIII. The decision was taken to do the work at the Weybridge plant, and two Wellington Mk I were taken from the production line and fitted with a long cylindrical pressure vessel, the pilot having a Perspex dome. The machine was otherwise little changed. With the Hercules VIII unavailable, Hercules III were fitted for trials and the machine was referred to as the Type 421.
Designated the Wellington V, the first machine flew in September 1940. Flights up to 30,000 were made, and these revealed a multitude of unexpected problems.
- The main flying controls and trimmer circuits became almost immobile. The was traced to the control hinge bearing grease freezing it had to be thinned with paraffin
- The rear turret and bomb doors became unusable. A special hydraulic fluid had to be developed to remedy the problem.
- The pilots dome and windows iced on the inside. Drying the air solved the problem but created discomfort for the crew.
- Accumulations of ice on the skin often made escape from the aircraft impossible, earning the sobriquet The Flying Coffin, although no accidents occurred in flight testing.
The second prototype flew with Hercules VIII and the first was refitted with Hercules XI, themselves fitted with GEC exhaust-driven turbo-superchargers. It was then referred to as the Type 436.
On 1 March 1940 30 Mk Vs with Hercules VIII were ordered to specification B.17/40, as the Wellington V. This machine entered operational service in July 1940 in theory. In fact it was never an operational machine, it was used to explore the difficulties of building a high altitude bomber. Meanwhile, a Wellington VI fitted with Merlin RM6SM was ordered to be designed as a replacement for the Wellington V.
And a story based on the use of high level bombing using smaller versions of the Tallboy
http://francefightson.yuku.com/topic/383/109-Squadron
109 Squadron originally a WWI bomber squadron which disbanded in 1919. It was re-formed in December 1940 from the Wireless Intelligence Development Unit (WIDU) whose headquarters were at Boscombe Down, Wiltshire. The unit was immediately stationed at Stradishall for this work, and a flight remained there when the unit relocated to RAF Tempsford in early 1941. Tempsford was recently completed and was planned as the RAF SOE and classified trials base. The unit was tasked with developmental work at this stage at both Tempsford and Stradishall.
This was in two parts. The first was using Anson and conventional Wellington aircraft to develop radio counter-measures and new radar aids, notably the blind bombing system known as Oboe. The other part of the Squadrons activity was high altitude research and operational development in support of the 1937 Air Ministry decision to develop very high altitude bombers. The initial machine converted for this task was the ubiquitous Wellington.
This was the Wellington Mk. V, an experimental high altitude, pressurised cabin machine with turbo-charged Hercules VIII. The machine was built under B.23/39 with an operating altitude of 40,000 being called for. The pressurised crew cabin was set to 10,000, so the pressure differential was 7.5 psi, a very stringent demand in 1939. Two prototypes were ordered in November 1939 and two Mk I from the Weybridge plant were converted. Due to delays in building the Hercules VIII, these two machines were fitted with a standard Hercules III, and altitudes of 30,000 achieved in September 1940. These flights revealed problems with the extreme cold (-40 degrees C) as it froze flying controls and the tail turret. Modified greases and hydraulic fluids were developed to overcome these problems.
Thirty more Wellingtons were ordered in March 1940 with Hercules Mk VIII engines (Wellington Type 436) to specification B.17/40, and shortly after this 30 Wellington Mk VI with Rolls Royce Merlin RM6SM two stage engines were ordered. This was the Wellington Type 442.
Meanwhile, 109 SQN continued very high altitude (VHA) development flights. With Mk VI, 38,000 feet was attained and further problems developed. These revolved around bearing greases freezing at -65C, but the problem was quickly overcome. The compressor tended to put smoke into the cabin, and the constant speed propellers tended to run away. These problems were solved by early 1941, but 40,000 remained elusive until the Mk VI wing outer panels were extended. The aircraft were routinely reaching over 40,000 by March 1941.
This machine retained its tail turret and represented the first RAF attempt at a high altitude bomber. The SQN had, on receiving the first Mk V immediately christened the type The Flying Coffin on delivery to 109 SQN. Oddly enough, the VHA Wellington force proved to be safe (if very uncomfortable) machines and 109 SQN was not to suffer a single loss in training or from an operational accident. The Mk V provided essential experience of high altitude operations and allowed lubrication and hydraulic problems to be identified and overcome.
The RAF was concentrating on a crew centralisation concept to save weight, complexity and development time. The Wellington V fleet proved that arming these VHA bombers presented very serious technical problems. Basically, providing a remote controlled tail turret was possible, but was very difficult to do. This resulted in altitude being seen as the requirement for the first operational VHA bombers (Mk VI, Victoria and Lincoln) with the next generation (Vickers C) being developed for both altitude and defensive armament.
Intelligence reported that the Germans were working on VHA interceptors with 40,000 being about their useful maximum ceiling, implying a service ceiling somewhat below this figure. 109 SQN was operating at or slightly above 30,000 by April 1941, when their first Wellington VI with Rolls-Royce Merlin R6SM Type: 1600hp V-12 liquid cooled engines. The extended wing version got it somewhat above 40,000 routinely.
At this point the intractable problems with the tail turret manifested themselves and it was decided to delete it and lighten the machine. This took two months to do and a modification kit enabled existing machines to be modified at depot level. While the weight saving was not enormous, the lengthened tail cone improved air flow. Modified in this form, the now-unarmed Wellington Mk VI was able to reach 44,000 with a 4000lb bomb load. It could reach 1500 miles range with this load.
The operational Wellington VI was not armed, relying exclusively on its altitude for defence. One of the major surprises of the program proved to be the extraordinary manoeuvrability of these machines at altitude. A steady stream of problems caused by extreme cold and thin air was identified and overcome. The Merlin R6SM engines were also debugged to become the Merlin 60, and all of these fixes flowed through to the Lincoln and Victoria programs. By mid 1941, the RAF was building a formidable depth of expertise in VHA testing and evaluation of aircraft systems.
However, the 109 SQN bombing trials did not go well. Precision bombing proved to be impossible with the standard range of GP bombs. Discussions within Vickers with Mr Barnes-Wallis resulted in experiments with spin-stabilised bombs. Tizard agreed with these provided that the Air Ministrys wish that the aircraft remain able to drop all bombs in inventory was respected. This helped, but it became obvious that the existing bomb shapes were too crude to permit this avenue to work. By mid 1941, the experimental program had determined that the smallest bomb which could be used with any accuracy from such altitudes was a 2000lb streamlined, spin stabilised device. These weapons proved to be capable of very different affects compared to conventional bombs due to the very high speed on impact. The SAP versions could be fused to penetrate quite deeply into the ground before detonation. It was quickly realised that these weapons were useful to attack building and heavy machinery foundations and mountings. Trials against an old railway bridge and tunnel showed that they could actually destroy abutments and collapse tunnels. Against a city, they were considered to be weapons to disable firemains and underground transport systems. It was recognised that they offered a way to attack heavy infrastructure and industry if the required accuracy could be achieved. Special sights helped, but trials indicated that daylight operations were needed. The tactics developed were for two machines to bomb ahead of the formation to provide calibration. The formation then bombed. Ten machines in a diamond array were used to try and lay a partly overlapping pattern of bombs over a heavy industry plant. SAP bombs were used, bursting 15-25 underground. These were found in trials to be devastating against heavy industry as they buckled concrete slab mounted machinery and damaged the massive, rigid foundations of forges, blast furnaces and the like. However, these bombs were expensive.
, the faster and higher it goes the more difficult the intercept is (and if you have suitably modified Mosquitos as escort...). Remember in WW2 its a numbers game - losses/damage done. This way we are using more expensive bombers and escorts, but probably have a loss rate no worse than OTL. And if they are doing a lot more damage, you win. It also gives the option of hitting some high value prescision targets that simply werent easily damageable the OTL bomber force - U-boat pens, chemical works, marshaling yards. You may have higher percentage losses, but if you dont have to keep going back...