Merlin v. DB 601A
For decades, arguments as to the relative merits of the main opposing fighters during the Battle of Britain have been tediously rehearsed. However, Guy Black instead considers the relative merits of the two main engine types used by the fighter aircraft o
It is certainly true that without the Rolls-royce Merlin, or an equivalent aero engine, then the RAF would have lost the Battle of Britain, and ultimately the Allies would likely have lost the war. It was certainly an impressive piece of machinery; a typically thorough but complicated British design that was available at the right time and in just the right quantities. Comparing the Merlin III with the Daimler-benz 601A, though, makes for far less comfortable reading.
We have all heard the now well-worn and rather silly cliché, that: ‘…the Rolls-royce Merlin was the engine that won the war’. But no engine ‘wins wars’; victory is a combination of a whole mass of not necessarily connected events and factors, and - in the case of the Second World War - not least that one side was run by an often chaotic totalitarian regime and the other by what was an ultimately overwhelming force, later dominated by American mass production methods.
The prosecution of a war, of course, is a complex pattern of attributes introduced by both sides, and it is generally a carefully calculated process. The side with more or better trained personnel, with the best tactics and strategy and the most efficient defence or attack machinery is already at a huge advantage. Thus, if the German war machine could produce an engine of roughly the same power as the other side for its front-line fighters, but could make it with half the manpower and at half the cost, then they were already on to a winner. Factor in the cost in terms of manpower and materials, then the main opposing fighter, the Messerschmitt 109-E, was also a winner. It was a small aeroplane, designed and manufactured for mass production from the very outset, while the Spitfire and Hurricane were larger aeroplanes, hand-built from a great many small parts and with little or no perception of the huge quantities that were eventually to be made. Whether one aircraft was better than another, or its pilots better trained and more of them available, is an argument which has been rehearsed ad-nauseum across the decades. But what is often overlooked is any comparison between the two main power plants.
First, though, it is essential to look at how the RollsRoyce Merlin came in to being to be able to fully understand any comparison with the DB601A.
RECIPE FOR MECHANICAL FAILURE
Rolls-royce loved introducing complication in the solving of problems, and in response to the need for a powerful aero engine at the beginning of the First World War, the first V-12 that Rolls-royce designed was the Eagle.
When it came to designing lightweight aero engines, Rolls-royce had the opportunity to examine a six-cylinder German Benz racing car trapped in a London dealer’s showroom at the outset of the Great War. This had been fitted with a straight-six aero engine (probably a D-III Daimler-mercedes). This had an overhead camshaft and was constructed with separate cylinders on a common crankcase. These cylinders were of a pressed welded sheet water jacket design but were expensive and complicated to make. However, for a racing car this hardly mattered – it was the weight to horsepower ratio that mattered. Indeed, this essential criterion is the same in aviation. Ironically, then, a German engine became the background to the establishment of Rolls-royce’s aero-engine design and manufacturing philosophy. The initial result was the overly complicated but otherwise superb V-12 Eagle.
This engine had twelve separate forged cylinders with pressed sheet metal and welded water jackets. Contrary to engineering wisdom, it also had a one-piece camshaft running above the cylinders in a spindly bronze multipiece tunnel which was a recipe for camshaft seizures unless carefully set up. The engine was a nightmarish Chinese puzzle, containing over 2,000 parts. No engineer could design such a layout and expect the average RFC/ RAF trained mechanic to rebuild it - let alone maintain it. Consequently, the design was a recipe for mechanical failure if the exacting standards of Rolls-royce were not followed.
The only way this could work was if each engine was hand assembled and maintained by highly skilled RollsRoyce trained engine builders. With this care, it became one of the most reliable and powerful engines of the First World War, with roughly 60% of all British manufactured aircraft using this engine This included its slightly smaller ‘brother’, the Rolls-royce Falcon, and what was effectively half of that engine, the six cylinder Hawk, an engine which was used mainly in the small Submarine Scout Zero (SSZ) class airships.
Production of the Eagle could not be rapidly expanded because of a shortage of skilled machinists and fitters,
and so the engine became by design and application a low volume engine constructed in large numbers which absorbed a disproportionate number of engineers and technicians as well as costs. All of these were critical strategic resources.
Rolls-royce, of course, had started their business building cars of such high quality (regardless of cost) to guarantee exceptional smoothness, silence and power. The only way this could be achieved without modern close-tolerance manufacturing machinery and interchangeability of parts was to hand-fit and adjust everything and to have available an almost limitless selection of parts to choose from - all of them minutely different in size to another. The result was the magnificent Silver Ghost. This was the best and most expensive motor car in the world. And there was no reason to suppose this attitude would change with their aero engine design. In fact, it did not. Thus, the company was inextricably betrothed to this methodology.
It was obvious that Rolls-royce had to make changes, though, or they would fail to be in the aviation engine business post-war. However, the company was shocked when Fairey Aviation went on to select an American Curtiss D-12 engine in favour of an uprated Rolls-royce Eagle. Consequently, Rolls-royce acquired a D-12 to see why. What they found was a well-made and neat looking engine, which they then loosely copied. In doing so, they naturally introduced much complication! Ultimately, this engine became the Kestrel (or “F” engine).
In its ultimate development, the Kestrel XXX was almost identical in design principles and construction to that which followed: the slightly larger Rolls-royce Merlin.
Following the armistice, Germany was effectively banned from producing military aircraft and engines. As part of a survival plan, Benz, Mercedes and Daimler combined names and entered the lightweight civilian aero engine market. With the arrival of huge civilian-carrying Zeppelins, the manufacture of large V-12 engines was seen to be a legitimate development.
When Hitler came to power in 1933, a surreptitious military aircraft development and manufacturing programme was quickly initiated, and this led DaimlerBenz into the development of military aircraft engines. Here, their range of liquid-cooled DB 600 Mercedes inverted V-12 engines led to the subject engine: the DB 601A. (The main difference between the 600 and 601A engines being the addition of fuel injection in the latter)
This engine had nothing like the layout of the Merlin. For a start, it was inverted and enjoyed an advanced Bosch fuel injection system. It was supercharged by a variable speed centrifugal single stage unit which had a system of automatic boost control to ensure suitable manifold pressure was available at all achievable altitudes. Typical of most large continental aero engines of the time, it was a slow-revving and large capacity engine. It had a bore of 150mm (5.91”) and stroke of 160mm (6.3”), giving a capacity of 33.9 litres – significantly greater than the higher-speed Merlin III at 27 litres. The DB 601A was also a clean-looking and compact engine, with all its accessories either driven from the rear auxiliary gearbox or sitting between the ‘V’ of the cylinders. By doing this, the cross-sectional profile it presented was small and significantly less than the Merlin at the top of the engine.
Yet another innovation with this engine were the simple engine bearers: a pair of forged magnesium alloy beams supported by steel tubular compression struts. These had just four attachments to undo (plus a few sundry pipes and controls) and this allowed the engine to be rapidly removed, complete with the coolant header tank and sundry other accessories which enabled it to be easily replaced for overhaul as a single and compact unit.
It is important to stress, however, that the Merlin III and the DB 601A had an almost identical performance and were also roughly the same size and weight. However, this discussion is about how the two manufacturers designed and constructed their respective engines and the relative merits of operating them. It is also important to examine the two engines as parts of integrated weapons systems.
CLEAN-LOOKING AND COMPACT ENGINE
A COMPLICATED ENGINE
Simplistically, when designing a military aeroplane from scratch (and usually following a specification from the eventual customer or end user) the first thing to do is to draw where one wants the guns or bombs – after all, the aim of the military aeroplane is to shoot down the enemy before he shoots you down, or to bomb him before he is able to stop you. The next task is deciding where best to place the pilot. Then, a suitable engine is put in the right position so that the pilot can see who he is shooting at and ideally take off and land the aircraft safely. Finally, the airframe is designed around those parameters. The challenge is to meet the specification in the most economical and expedient manner.
The customer will then instruct the designer how many aircraft are to be made. And this is very important. Building just a handful means that every part can be, or is, handmade. If the plan is to build thousands, then mass production methods need to be planned at the outset. In this case, press tools, jigs etc. (as opposed to hand forming or machining individual parts) is called for. This is initially a massive investment, but it wins handsdown when economy and speed of production become strategic necessities. And this is where the Spitfire, the Hurricane and their Merlin engines lost heavily against the Messerschmitt 109-E and the DB 601A; that is, because building the British fighters in such quantities was never envisaged. Both the German products, however, were designed to be mass produced from the very outset and put the British at a severe disadvantage. Of course, in slight mitigation, the British did not plan for a major war. Nevertheless, as just one example, the number of parts in a DB 601A engine is roughly half that of the Rolls-royce Merlin. And that is not very efficient engineering.
In fact, The Merlin engine suffered from several design and production problems: most of the general-purpose machine tools were left over from the First World War and were well worn, so we see the same bugbear of costly selective assembly practices and lack of interchangeability. It was also an immensely complicated engine with some 12,000 parts. All of this consumed labour and money in prodigious quantities and it was initially an unreliable engine in service use. There are also many design differences between the two engines, and it is important to review some of the more outstanding ones. This is also where the two engines start diverging in efficiency of
production.
The Merlin suffered from excessive camshaft and rocker wear, because when it was not running, the oil drained down into the sump. Meanwhile, on the inverted DB 601A engine, the oil drained down into the two cam covers, over the camshafts. This was important, because when an engine is not running, any internal loaded pair of metallic parts (such as a camshaft and follower) quickly create an almost dry metal-to-metal joint. In the micro-second following start-up, and the initiation of movement and consequent abrasion, there is little or no lubrication. That is when wear is at its worst. But this is not so with the DB 601A, where the camshaft was always drenched in oil. The DB 601A also did away with frictional rubbing between cam
and rocker, as occurred on the Merlin, by having a roller in contact with the camshaft. This resulted in almost no friction at start up and consequently little wear. Clearly, a far better solution.
Fuel delivery, too, is an important element to compare between both engines.
GENIUS OF DB601A ENGINE
Whilst the Merlin III had a carburettor, it stopped working when it was upside-down. The DB 601A, meanwhile, was fuel injected and conversely would work in any attitude the aircraft found itself in. In fact, diving or spinning rapidly downwards, with what might appear to have an inevitably apparent fatal ending, was a ploy often used to escape RAF fighters in combat, especially in 1940. By rolling inverted, opening the throttle and then pushing the nose down, Messerschmitt 109 pilots knew they could often escape a pursuer with carburettor-fed engines.
It is true, though, that this fuel injection system had more parts than the Merlin carburettor, but this was simply because most of this unit was multiplied x 12 in number (one for each cylinder) and was not due to any added engineering complication.
The Merlin was also a very unreliable engine when it was first put into service, and this should not have been the case. Surely, lessons ought to have been learned from the almost identically designed Kestrel XXX ? Most of these issues were related to the complicated cylinder block design which constantly cracked and leaked coolant in service. Let us examine, then, what is probably the major design difference between the two engines: the cylinder blocks and how they were constructed.
Both engines had a pair of one-piece liquid cooled blocks of six cylinders, cast in aluminium. Both had steel cylinder liners, and both had four valves per cylinder and a single overhead (or ‘underhead’, in the case of the DB 601A!) camshaft per cylinder bank. That is just about where any commonality ends. The main development issues the Merlin suffered from lay in the cylinder blocks.
Both Rolls-royce and Curtiss had endless problems with sealing the so-called wet liners. This is where steel cylinder liners are directly exposed to coolant, and this type of liner was easy to assemble but hard to seal. One of the difficulties that Rolls Royce faced was that each seperate liner was designed to be trapped between the one-piece head seal and the crankcase. This meant that each one of six in each bank had to be exactly the same size as all the others in that one-piece block. Of course, that meant machining the recesses in the head to exactly the same height. And as we have already learned, RollsRoyce were not that good at repeatably being able to produce engine parts that were exactly the same size. So, the old bugbear of selective assembly came back to the forefront. A problem which evoked shades of the problems with Eagle camshafts and cylinder heights.
Both Curtiss and Rolls-royce, then, had rejected the idea of a dry liner as this had historically caused seizures due to the steel cylinder distorting due to poor heat transfer. And herein lay a large part of the genius of the DB 601A engine.
A SUPERIOR SOLUTION
This engine did indeed have a dry liner screwed into the cast aluminium block, but being inverted it was constantly and evenly cooled by oil splash from oil draining down into the cylinders, and from crankshaft spillage above, before being thrown out until it drained down into the (inverted) cam covers where scavenge pumps scooped the oil up and sent it back to the oil cooler and tank before returning it to lubricate the engine.
The pistons themselves were heavily finned underneath the crown, again assisting in heat transfer to the oil. And there was another stroke of innovative design. Instead of a myriad of studs, rubber seals, transfer tubes for the studs, and nuts and bolts holding the blocks of the Merlin engine onto the crankcase while trying to trap the cylinder liners in place, the DB 601A had a superior solution. The engine had a large diameter geared steel ring screwed on to the protruding plain ‘tube’ of each cylinder liner, and over a steel and a rubber ‘nest’ of washers. This was all that held the blocks in place! So – the DB 601A engine had a total of 12 parts (plus washers) to hold the blocks onto the crankcase. On the Merlin, however, it needed 140 parts for the self-same purpose and function.
Additionally, and in order to make the cylinders a tight fit, the DB 601A designers had an ingenious arrangement whereby the geared ring was rotated, and thus tightened, by a pair of pinion gears on long shafts which were inserted in a well-placed hole and then screwed up tightly. These pinions were then removed and replaced in the compact tool kit.
The DB 601A engine was also not free of running problems either, although most of the difficulties seem to have revolved around lubrication issues. The engine had myriad oil passages and drillings, and oil pressure was also used to regulate the supercharger. In common with all mechanical devices, without lubrication, seizure becomes inevitable. However, the author has a sneaking suspicion this was not the fault of the engine per se, but rather the fact that oil was a relatively scarce resource because Germany had no significant oil fields, but did have huge reserves of coal and lignite - a poor relation of coal.
German scientists soon discovered a way of massproducing synthetic oil out of coal by liquefaction or Kohleverflüssigung. And anyone working on a wartime German engine, from an excavated crash site for example, will immediately notice the very black nature of this oil. It
is almost impossible to wash off your hands. Also, it seems to create a thick sludge and many of the engines that had failed, and which were examined by the Allies, seem to have seized due to blocked oil drillings. Indeed, on two DB 601A engines which the author has worked on at Retrotec Ltd, one had failed due to a gear shaft seizing and in the other a main bearing had seized. Both failures were due to blocked oil drillings.
While the author does not feel competent to draw such a conclusion absolutely, it is certainly tempting to speculate as to whether this was the major cause of failures which occurred in this engine. Indeed, an engine examined in 1941 by De Haviland Aircraft Ltd for Rolls-royce was also found to have a seized main bearing due to an oilway being blocked. Similarly, so did another examined by the SAE (The Society of Automobile Engineers) in America. Maybe it is a little unfair to blame poor oil on such failures, but it is arguable the engine should have been designed to take account of the type of lubricant to be used.
INVERTED ENGINE
The illustration below demonstrates how compact an engine the DB 601A was in the Messerschmitt 109-E compared to the Merlin installation in a Hurricane. Not only is it a smaller target, but it uses less material to manufacture and (as we have seen) consequently less labour and cost. The inverted engine also allowed for guns to be placed right in front of the pilot - without obstructing his view - making it easier to aim the aeroplane and shoot at a target any distance away.
Meanwhile, wing gun bullet streams (as on the Hurricane and Spitfire) must be set to converge at a fixed point in front of the aircraft for greatest effect. Incidentally, a central cannon was tried experimentally in the early Messerschmitt 109 and mounted in the ‘V’ of the engine block to fire out through an aperture in the propeller spinner. However, vibration and other failings did not allow this development to proceed until the later Messerschmit 109-F model. The apparently ‘empty areas’ in the two diagrams contained such things as oil and coolant tanks, engine mounting frames, hydraulic tanks and other ancillaries. The exhaust ejectors lower down on the DB 601A installation had another possible benefit – that is, that there was less glare in the dark in the event the aircraft had to be flown at night. On the Spitfire and Hurricane (and it was not entirely unusual for either of these RAF aircraft to be used on night operations) the exhaust glare was horrendously dazzling. It could only be partly mitigated by the addition of bolt-on anti-dazzle shields between the pilot and exhaust ejectors. Also, on the DB601A, there was probably less chance of exhaust fumes entering the cockpit. However, being hard to quantify, there is scant evidence to support this.
As the author compiled
this feature, so the list of advantages between the two engines coming down in favour of the DB 601A seemed to grow ever longer the more the matter was considered.
They could be summarised as follows: DB 601A - Advantages: 1) It has only half as many parts as the Merlin, thus reducing manufacturing costs and specialist engineers dramatically. This meant that more engines could be produced in any comparative budget set against the Merlin III. 2) The DB 601A was built as a mass-production engine from the outset, speeding output and reducing costs by efficient tooling. 3) Total interchangeability of parts in the DB 601A, freedom of shims and selective assembly again reduced costs and assembly time. 4) Far easier to assemble by unskilled factory labour and consequently easier to maintain in the field, as well as making engine component parts straightforward to exchange. 5) It was simple to remove and replace the entire engine unit for maintenance or replacement. 6) An incomparably simpler engine bearer assembly, thus being far cheaper and more efficient than the British design which was a veritable mass of tubes with dozens of intricate close tolerance steel fittings, nuts, bolts etc. 7) The DB 601A would continue running when flown inverted, unlike the Merlin III. 8) Clever oil cooling of cylinders and immunity from coolant leakage around the liners. 9) Camshaft and rocker design far more wear resistant and a greater efficiency in design and performance. 10) A large capacity engine resulting in greater development potential. 11) Far fewer specialist maintenance tools required. The Merlin has a huge parts book devoted to just the tools, whilst the DB 601A has a tool kit that one man can easily carry.
12) The DB 601A can be run at full power immediately on starting. The Merlin cannot. 13) It has a slower engine speed, which equates to lower wear rate and higher reliability from reduced stress. 14) A multiple speed supercharger. The DB 601A Disadvantages over the Rolls-royce Merlin III: 1) There were problems with oil quality affecting reliability. 2) Oil consumption likely to be higher, due to inverted cylinders 3) Risk of cylinder hydraulic lock if left long enough unrun without draining oil from cylinders. 4) The small airframe of the Bf 109-E meant less fuel capacity and consequently fighting time over enemy territory was reduced. This was a major disadvantage during the Battle of Britain, when most of the air fighting was over England. (However, this was an airframe design issue rather than an engine issue.)
A FAR BETTER SUITED ENGINE
It has in no way been the intention of the author to denigrate the prodigious bravery and ability of Allied fighter pilots, or the toils of designers, engineers, machinists and sub-contractors who contributed to the production of the Merlin engine, or the aircraft that were powered by it, in this feature. Instead, in pure isolation and with the benefit of historical hindsight, this has been an examination of the two engines principally used by both sides during the Battle of Britain and an objective look at the engineering merits (or otherwise) of both.
In conclusion, it might be worth looking at something which Henry Ford is reputed to have said when he remarked:
“An Engineer is a man who can make for a penny what a fool can make for a pound.”
There is no doubting that Rolls-royce were very far removed from being fools, but there is also no question in the author’s mind that, in 1940, the DB 601A was the better strategic power unit. Not only that, but writing as a former piston engine designer, I find it hard not to be impressed by the German thoroughness, precision and economy of design that is to be found in the DB 601A engine. And it is certainly the author’s assessment that it was a far better suited engine for the coming war than its nemesis, the Rolls-royce Merlin III.
On the other hand, if the author had to choose which engine he should have as a polished and sectioned exhibition piece in his drawing room, then the Rolls-royce Merlin would absolutely win hands down. Like an extremely complicated and handmade watch, it is a jewel of an engine which is a joy to behold and a pleasure to possess.
Despite everything, and notwithstanding the engine’s associated manufacturing difficulties, the Rolls-royce Merlin became a vital part of the Allied armoury in victory. Consequently, it has gained its own mystique and legend. Meanwhile, the DB 601A is barely remembered. Ad Victorem Spolias et Gloria.