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[Red13]

Thunda - thanks for that, it's much more than I expected anyone to come up with.
This was obviously an official mod produced by Rolls Royce for Merlin 21s and 23s, but I can find no reference to it in otherwise authoritative titles such as Putnams' 'DeHavilland Aircraft since 1909' nor in 'British Piston Aero Engines' by Lumsden. As I said above, I believe the wartime use of N20 on a Merlin installation was unique to the Mosquito - a British equivalent of the German GM-1 equipment.

As I'm sure you know, the British generally preferred mechanical means for increasing charge density and cooling (via superchargers and intercoolers), rather than chemical injection methods, so I'm surprised to find N20 in use here.
I can only assume that the Merlin 21 series, being single stage, two speed engines, simply ran out of puff above 20,000ft and some expedient means of quickly increasing their rated altitude was required. From my reading it seems that the two-stage Merlin 61 series could not be fitted to the Mosquito because the latter's radiators were higher than the engines (not beneath them, as on Beaufighters, Lancs etc.), which required that the engine be redesigned to allow for 'reverse coolant flow' (the new engine design being labelled Merlin 70). I suspect there was a shortage of two-stage, Merlin 70s (most being fitted to photo recon Mosquito variants), so N20 equipment was retro-fitted to home-defence Mosquitos as a stop gap.
According to the Lumsden book, the 1390 h.p Merlin 21 was cleared for 14lbs boost in MS gear and 16 lb boost in FS gear, so the 12lb applied in the Cunningham/Bennett test was not the limit, but I suppose they didn't want to overspeed the engines in a dive and they reached compressibility anyway.

[I don't understand the reference to 'resins' in those reports...(!?)]

[Thunda]

Red13, when they talk about 'showing resins', I take it to mean the position indicator lamps on the wings and tail, being made out of resin, before plastic was available.

There is more mention of the NO2 fitment in 'Night Fighter'- will take a look and see what it says. I think that it was used exactly as is stated in the first combat report- when they were chasing a fast target, like a Me410, and needed a bit of extra 'grunt', they would dial in the nitrous for a short period to close the range- obviously prolonged use would blow the engine up, as the use of NO2 increases the pressure in the cylinder, and I think thats what the reference to +12lb boost is all about- higher boost + nitrous = large bang!!! I imagine the system would be fairly simple "direct port" or "wet induction"- nitrous being introduced directly into the inlet port, so the chances of blowing the engine through 'detonation' would be very great. However, even with a simple system like this, the extra power delivery would be dramatic- especially if you are sitting between two howling V12's!!!

[Red13]

Hi Thunda - yes, I'm sure you're right about the simplicity of the system. In Morgan and Shacklady's book about the Spitfire they describe the Liquid Qxygen installation used on the Spitfire VI, which was surprisingly unsophisticated - just a tube squirting liquid oxygen directly into the carburettor air intake. I believe LOX was ultimately rejected because, although it gives a higher calorific value than N2O in combustion with a hydrocarbon, it is more dangerous to store and use in service and it caused carburettor icing problems.

Still, the fascination for me lies in the fact that Rolls must have persevered with these tests and when the RAF put out an urgent request for more Mosquito power in 1944, the company was able to quickly respond with N2O equipment. It leaves open the question as to whether the installations were fitted in the field or done as factory conversions. I'd imagine De Havilland must have done the work, as the copper tubing had to be fed through the wings from the bomb bay tank and into the cockpit, then everything pressure tested. A bit involved for most fitters, I'd say.

There must be an official pilots manual or DH factory conversion reference to the mod somewhere, but it seems to have eluded all the authors I've consulted, so far. 

[Thunda]

It wouldn't suprise me if it was fitters at squadron level-say at the FIU or somewhere who came up with a working solution- as you have pointed out, it wasn't the type of power boosting tech that the RAF went in for.

[Red13]

Well, it seems I should have looked in the less likely places first , as I've found a reference to nitrous oxide injection in 'The De Havilland Mosquito SAM Modellers Datafile 1' - and it includes an indication of the performance boost provided:

"Operational use of the NF series Mosquito had shown that there were some areas which needed improvement. The first was for better speed at around 20,000ft, but also a steeper climb was required to get the aircraft up to the interception point sooner. A quick method of clearing ice from the flat armoured windscreen was also needed, as icing problems had been encountered on many operations. To give the Merlin 21/23/25 engines more power for short periods, a nitrous oxide (N2O) injection system was developed. The system was fitted to two machines, one from the Fighter Interception Unit and the other (HK374) of 85 Sqn, by the RAE and this resulted in an increase in speed of up to 47mph at 27,000ft. The system was to be retro-fitted to fifty NF Mk XIIIs and this work was carried out by Heston Aircraft Ltd. Most of these machines were operated by No.96 and No.410 (Cougar) Squadrons. With the strengthened wing of the fighter/bomber series, NF XIIIs were also able to carry bombs under the outer wing panels...Wg. Cdr. John Cunningham flew an NF MkXIII (HK374) for the first time on the night of 2/3rd January 1944 and successfully shot down an Me410."

[Thunda]

LOL- yes, no sooner had I written that I thought it was a squadron level conversion, I came across this:

"British World War II usage of nitrous oxide injector systems were modifications of Merlin engines carried out by the Heston Aircraft Company for use in certain night fighter variants of the de Havilland Mosquito and PR versions of the Supermarine Spitfire."

Also:

"To tackle Me410s, two Mosquitoes were modified to provide nitrous oxide injection to boost engine power and provide an extra 40 knots plus at 28,000 ft for about six minutes. Fifty more planes were then modified. ‘Engines which had run around 200 hours with the use of nitrous oxide [presumably 200 hrs total running!] were found to be in excellent condition’.
Source of all the above: ‘The Merlin in Perspective - the Combat Years‘, page 27."

[Thunda]

Again, Im guessing, but the nitrous was probably introduced straight into the supercharger, in a similar way to the Luftwaffe's GM-1, but without the 3 stage injection- think it was just on or off. Obviously, the GM-1 was being used on engines with fuel injection so accurate adjustment of fuel to nitrous was easier to accomplish:

"GM-1 was typically sprayed in liquid form directly into the supercharger intake from two jets of different bore while at the same time, the fuel flow was increased to take advantage of the additional oxygen from the nitrous oxide. The jets could be operated individually or in combination, yielding three steps of power increase, for example 120/240/360 HP at different GM-1 flow rates (60, 100 and 150 grams/sec). The development of a continuously variable injection system was considered, but apparently it never saw operational use.

[crazyflak]

This thread is gold indeed    I completely missed it.

[sputnikshock]

@Crazyflak:    That#s what I meant. Even if the mod is outdated now this stuff here by the Mossiemaniacs is just too good to be lost.
Thanks very much for cleaning and moving!

[melissa]

Animation the radars, indication in display
AI MkIV

AI MkVIII

[sputnikshock]

Thanks melissa.

Thought I'd add the C-/B-scope display of the AI Mk.X



The three together are a nice visualization of the evolution from longwave to cm wave spiral scan and finally helical scan radar scopes.

[Thunda]

Thats really interesting- wonder what the MkXV ASV radar that the Intruder FB MkVI's used was like on its mighty 3" tube!!!