Giving Mossies a bit more oomph…

If you fired the NOx at sea level then you would get a power boost over and above the rated output, but it was obviously used, much in the same vein as a Super/turbo charger to give back some of the tail-off of power at altitude.

There is a view that using NOx isn’t too detrimental as the cooling effect offsets the additional heat created, but the strain on the reciprocating parts must be taken into account.

I am mystified why the ‘Compound’ engine didn’t really get developed, where the exhaust gases are passed through an impellor directly attached to the Crankshaft (a la Constellation)

Compound engines are excellent for continuous power for example an airliner or bomber, but less useful when numerous throttle movements would be necessary, as in a fighter. This is demonstrated with the US army air force bombers Vs US navy fighters. The Army wanted to use the Turbo supercharger on all its aircraft, and all the heavy bombers were thus equipped and performed very well. The compound engine was an extension of this technology, but was not sufficiently developed and therefore not available during the war and was overtaken by the turbine engine, again a further development of similar technology.

US fighters with the turbo supercharger were far less successful, until employed in roles other that that for which they were originally intended. The P38 is the best example of this, performing better at low altitude in the fighter bomber role than as a high altitude interceptor. The P47 was more successful at altitude but was again better employed as a FB. The US navy developed two stage superchargers in a parallel to the developments at RR and the Hellcat and Corsair were very successful in their intended roles, despite using the same basic engine as the P47. Horses for courses as they say.

Another point to remember is the contribution of the exhausts to top speed in a fighter. In a turbo supercharged engine some of the exhaust energy is recovered via a turbine. In a full mechanically supercharged engine in a fighter the exhaust energy was used as jet thrust. On a Merlin at full chat and at 400mph this could be in the region of 300hp. This is a lot of power to dispense with for a recovery turbine that out of design necessity had to be optimised for either cruise economy or full power

If you fired the NOx at sea level then you would get a power boost over and above the rated output, but it was obviously used, much in the same vein as a Super/turbo charger to give back some of the tail-off of power at altitude.

There is a view that using NOx isn’t too detrimental as the cooling effect offsets the additional heat created, but the strain on the reciprocating parts must be taken into account.

I am mystified why the ‘Compound’ engine didn’t really get developed, where the exhaust gases are passed through an impellor directly attached to the Crankshaft (a la Constellation)

Ah – I think I´m with you. So its not a case of not being able to increase power output, more that the engine probably wouldn´t hold together if you tried it for too long. Makes sense!

cheers

Nitrous is an oxidising agent, but it also cools the mixture and it enables you to inject an extra shot of fuel. I don´t see why you would say that it wouldn´t increase power?

James, I’m not saying it wouldn’t under certain conditions, but if the Merlin 23 for example had a maximum rating of 1400hp @ 3000rpm at say 18,000 feet in hi blower, it would be limited to this power by the mechanical integrity of the engine components. This same engine would only produce maybe 650 hp @ 30,000 ft at the same max full throttle settings. Using NOx under these conditions would allow the engine to produce say 1000hp, by supplying additional oxygen. (1000hp @ 30,000ft was the target hp for the Merlin 60, the first of the two stage engines.) At no point would NOx be used below rated altitude to increase the max hp above that to which the engine is mechanically limited.

The Merlin 25 had a higher std rating of about 1650hp, at a lower rated altitude of 14 to 15,000ft. What’s more it had components that allowed short bursts of power at much higher boost pressures. However this extra power was most often used, and far more effective at low altitude, well below the rated altitude, and usually limited to low blower to reduce parasitic loses. ADI (water/methanol) would have been manner from heaven in these situations, but NOx was completely unnecessary as there was always enough Oxygen present below 10,000ft.

I hope this clarifies matters. By the way my figures are approximate and not exact, but close enough to illustrate the point. ADI was developed for the Merlin, but as the 2 stage Merlin 66 could run at @25lb boost and 2000+ hp, it was seen as unnecessary at the time. The Merlin had been cleared to run at 30lbs and 2200hp prior to the end of the war, and ADI had been developed to take it further still. Now it’s the racers of Reno that take advantage of all this good work.

I think you will find that the Nitrous Oxide was to basically add extra oxygen to the engine above the rated altitude of the single stage Merlin’s then fitted to the fighter Mosquito’s, pending the arrival of the Mk30 with two stage engines.

Nitrous is an oxidising agent, but it also cools the mixture and it enables you to inject an extra shot of fuel. I don´t see why you would say that it wouldn´t increase power?

I think you will find that the Nitrous Oxide was to basically add extra oxygen to the engine above the rated altitude of the single stage Merlin’s then fitted to the fighter Mosquito’s, pending the arrival of the Mk30 with two stage engines. At no time would the engines have produced additional power over and above the rated power for the particular engine model. MkXIII, were produced with both Merlin 23’s and 25’s, the later capable of running a higher boost. I haven’t got my references with me but I think that the 23s maxed out at 16lbs (or 14lbs) boost and the 25 at 18lbs. Some 25s were modified to run at 20 plus lbs for chasing V1s, but I’m not sure what the exact maximum was.

Cheers guys thats great stuff. The Me 410 incident is mentioned in the autobiography of Cunningham’s navigator, C. F. Rawnsley. Could anyone confirm that the 50 conversions were actually undertaken?

Best wishes
Steve P

Cunningham trialed the Nitrous installation in a 85 Sqn. Mossie and claimed a Me 410 on the night of 2-3 January 1944. It boosted the performance by 47mph at 28,000ft in tests, and could be used for 6 minutes. It was decided to equip 50 Mk.XIIIs of 96 and 410 squadrons.

Nitrous Oxide in Mossies

Many years ago I read a book about John “Catseyes” Cunningham’s nightfighter exploits during WW2 . I remember in one chapter he mentioned how his Mosquito’s engines had been fitted with nitrous oxide bottles to improve combat performance.

Colin