Spitfire XIVs equipped with nitrogen drop tanks

[Mike Williams]

This passage from 430 Squadron's Operations Record Book regarding Spitfire XIVs equipped with nitrogen drop tanks has me puzzled:

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The operational use of nitrous oxide by Mosquito aircraft comes to mind, see for example this combat report. Does anyone have any knowledge or ideas as to what these Spitfire XIVs were doing carrying nitrogen around? Thanks.

 

[Crimea_River]

It's possible they were ferrying empty used tanks with a nitrogen purge on them to render any fumes inside inert.

 

[Koopernic]

NOX can be used in two ways: either compressed or cryogenic. In Luftwaffe service it was initially compressed but this system was unsafe and heavy. Cryogenic systems could, for some reason, be made much safer.

The problem with cryogenic systems is the boil off hence one needs a fast distribution system. Perhaps ferrying by drop tank was one way to do this swiftly and safely. P-51's tasked against Me 262 used NOX, presumably in the tail tank area. If one assumes a 1/3rd tank of about 20 gallons or 84 Litres there is about 10 minutes of NOX for a Merlin at full power.

Im assuming Nitrogen is a an incorrect abbreviation. For Nitrous. I assume nitrogen has its uses as a safe tank purging system.

 

[Crimea_River]

...... I assume nitrogen has its uses as a safe tank purging system.

It does even today. Commonly used in the oil and chemical industries. That's why I suggested this might be the answer.

 

[kool kitty89]

Might it also be nitrogen pressurized fuel tanks, not just empty purged tanks, but ones pressurized with nitrogen on the ground for altitude use without need for a pressurization system onboard the aircraft. (I might be mistaken, but I believe CO2 was mainly used for onboard purging and fire extinguishing of fuel tanks, partially due to relative ease of liquification under pressure -hence CO2 fire extinguishers -the compressed liquid quickly vaporizes in the air creating dry ice snow -solid CO2- and CO2 gas)


I'm a bit surprised that high pressure nitrous oxide tanks were so problematic. I'd thought the liquification properties weren't so different from CO2 (or perhaps even propane) let alone potential for compromise with high-pressure near-cryogenic containment to minimize boil-off but still employ a check valve or other regulator of some sort to vent excess pressure.

Granted, you also have the issues with needing to properly vaporize that liquid and avoid solid super-cooled particles from accumulating. (some sort of pre-heating/diffusion chamber before feeding into the cylinders -same should apply to a pressurized CO2 system for fuel tank purging, you really wouldn't want solid CO2 clogging the system or getting into the fuel tanks and causing potential solid/waxy supercooled fuel granules)

 

[fastmongrel]

Werent some aircraft suspension systems charged with Nitrogen. Possibly they were used to ferry Nitrogen to forward airbases, though why anyone would use a XIV when there must have been plenty of other aircraft that could do the same.

 

[pbehn]

I'm a bit surprised that high pressure nitrous oxide tanks were so problematic. I'd thought the liquification properties weren't so different from CO2 (or perhaps even propane) let alone potential for compromise with high-pressure near-cryogenic containment to minimize boil-off but still employ a check valve or other regulator of some sort to vent excess pressure.

The problem is that Nitrous oxide is a violent oxidant. In certain conditions materials such as stainless steel can be a fuel for nitrous oxide. Nitrogen and CO2 may be similar as liquids in their performance but they are inert, using Nitrous as a fire extinguisher isnt a great move.

 

[kool kitty89]

The problem is that Nitrous oxide is a violent oxidant. In certain conditions materials such as stainless steel can be a fuel for nitrous oxide. Nitrogen and CO2 may be similar as liquids in their performance but they are inert, using Nitrous as a fire extinguisher isnt a great move.

Nitrogen isn't much like CO2 at all ... as far as physical properties go. Impossible to liquify under pressure at standard temperature (must be cryogenic) with much lower boiling points than either CO2 or N2O.

N2O really isn't that reactive. Yes, it would be a bad idea to try and extinguish a fire with it (not nearly as bad as pure oxygen, but a worse than atmospheric air) as would spraying propane or butane at a fire ... but it's rather stable and non-corrosive under normal conditions. It's not like nitric oxides (NOx, representative of both NO and NO2) those are highly reactive and corrosive and form nitric acid on contact with moisture though Nitrogen dioxide has the advantage of being liquid just below room temperature. (useful for rocket propellant ... far too corrosive to use as a more conventional engine boosting agent)

CO2 and N2O are also both non-toxic (and both used as propellants in food products -and of course carbonated drinks in the case of CO2), nitric oxides on the other hand are poisonous as well as dangerously corrosive.



In any case, my point still stands regarding GM-1 systems and the potential to use pressurized COLD (vented) N2O containers as a practical compromise between cryogenic and warm pressurized arrangements. At 1 atmosphere, nitrous oxide also has a very narrow liquid range (melting point at -90.86C, and boils at just -88.48C) so you'd be much closer to just having bricks of dry-ice-like consistency with a tiny amount of liquid as well, so it'd really need to be under significant pressure to be useful in any case. Perhaps that's already what they did do for 'cryogenic' GM-1 systems. (cold, liquified N2O under less extreme pressure)



The other advantage of 'low pressure' cryogenic systems is if ruptured, they don't explode with the force of high pressure systems (operating at nominal atmospheric temperatures) but instead just spew out supercooled liquid in a relatively mild explosion, with said liquid then vaporizing along with forming some super-cooled frost.

Did the 'low pressure' oxygen systems introduced pre-war in US aircraft work similarly? (low pressure, low temperature liquified oxygen storage rather than high pressure gas cylinders?)

 

[GregP]

I've seen more than 10 engines blow up and fail with Nitrous Oxide systems designed and engineered for that particular engine ... all in just testing, nevermind driving or racing. Up in Portland, Oregon I watched a new Corvette with less than 2,000 miles on it do a dyno run and use his Nitrous system for about the 25th time. It was a wet system (Nitrous injected after the mass airflow sensor) and the extra solenoid to provide more fuel with the Nitrous shot due to it having more Oxygen than air failed.

In less than 5 seconds the Corvette owner lunched a really nice, heathy-sounding LS-1 engine. No point crying over spilled milk, but spilled money is different.

I don't think I'd try one in a plane with the glide ratio less than that of a poorly-designed paper airplane after the engine fails. My name isn't "Rocky the Flying Squirrel" and I don't have a partner named Bullwinkle.

In a car on a dragstrip? Sure, if I want to take the chance. In an F4U Corsair that glides like a piece of space debris? I'll refrain unless I'm going to die anyway from an attacker I didn't see until the last second.

 

[Mike Williams]

Thanks for the feedback so far. My interpretation of the ORB excerpt from post 1 above is that the pilots of 430 Squadron flew two Spitfire XIV aircraft equipped with auxiliary tanks containing (or capable of containing) nitrogen or nitrous oxide to 35 Wing, leaving them there - and then flew two different Spitfire XIVs as replacements from 35 Wing back to base. In the interest of providing context, 430 Squadron was the Spitfire XIV squadron with 39 Wing, 83 Group, 2nd TAF while 2 Squadron was the Spitfire XIV unit with 35 Wing, 84 Group, 2nd TAF (268 Squadron, also with 35 Wing, not yet having converted to Spitfire XIVs). Both 2 and 430 Squadrons were principally employed in tactical reconnaissance, with occasional photo, arty and weather reconnaissance also carried out. Tactical Recce as conducted by these units often meant shooting up targets of opportunity, including enemy aircraft – see for example 2 Squadron Combat Report and 430 Squadron Combat Report.

My working theory is simply that 430 Squadron was delivering Spitfires XIVs modified to carry Nitrous tanks to 2 Squadron. An initial long shot theory was that perhaps the Spitfires were delivering Nitrous tanks to 35 Wing for Mosquitoes, however, there were no Mosquito units assigned to either 35 or 39 Wings.

If the Spitfire XIVs were equipped to utilize Nitrous, the question arises – to what end? They were already running on 150 grade fuel at the time of this ORB record. Perhaps the Nitrous installation predated the switch over to 150 grade, as 2 and 430 squadrons converted to Spitfire XIVs in 1944, prior to adoption of 150 grade fuel by the 2nd TAF?

Nitrous Oxide was used operationally by Mosquitoes as noted earlier. Perhaps of interest:

Research on this continues…

 

[Koopernic]

In Luftwaffe service nitrous oxide was added at a mass flow rate approximately equal to the fuel flow rate. It was generally only used above the full throttle height of the engine, hence it didn't boost power above the rated power, it merely maintained it. The TA 152 had three settings, I think 80, 100, 130 grams per 100 grams of fuel. Germans like using centigrams.

Even with 150PN fuel nitrous oxide would increase power at below the full throttle height so long as the engine could take it. It acts as an anti detonation agent so that is the predetonation problem gone. I note they weren't planning on increasing boost merely to add nitrous oxide.

I suspect the RAF was taking interim measures to cope with the new generation of Luftwaffe jets or TA 152H.

 

[kool kitty89]

Nitrous oxide would indeed be for high altitude use, not for boosting max power. You'd only gain power when employing it above critical altitude (otherwise you'd be better off just increasing the boost pressure and employing water injection of detonation was a problem -and/or excessive losses due to charge heating). Nitrous oxide injection would tend to increase the likelihood of detonation to some extent though with it likely being rather cold, there would still be trade-offs there. (you'd better have the system maintaining a very rich mixture, though, enough to result in a rich burn with the combined fuel+air+N2O mixture, or the added oxygen will lead to running lean and hot)


Somewhat farther afield, but I wonder if nitromethane injection would be at all useful for high altitude performance boosting. (nowhere near the oxygen content -still a fuel and not an oxidizer- but also still reducing the oxygen requirements in the mixture and being a fairly stable liquid at standard pressure/temperature) The main problem with it I can see is just generally poor energy density and very high consumption rate. (the advantage still being, in theory, allowing engineering set-up rather similar to water/methanol injection) I think nitromethane is also a good deal more expensive to manufacture than nitrous oxide.

It's also worth noting that these high-alt boosting systems lack most of the corrosion concerns associated with water or methanol (introducing either into the cylinders can have problems, mostly related to residue left if water-injection was run very soon before landing) and also more of a problem if carbon steel cylinder liner alloys are used rather than chrome/stainless steel. Lower tolerances for carbon steel is one of the more notable problems that cropped up on the R-2600. I believe in that case I believe it was excessive rust forming in humid or marine/coastal climates, specifically on the A-20. (oddly I don't recall notes about it being a problem on any navy aircraft, but perhaps the models employed there used different alloys specific to naval/coastal operations)

 

[GregP]

They had better NOT try N2O of internediate throttle settings. Great way to blow and engine. Even in automovie use, it is injected at full throlle, or at least above 90% throttle.

I supoose there IS a use for 7.5 minutes of speed increase, but there would have to be some serious threat before anyone would likely use it. I'd say the crew chiefs would likely argue with the pilots for a long time if the N2O system was used. It sure would make for a LOT of work, but could also save your life when you needed it.

I seriously doubt it would ever be used, even in a Mosquito fighter type variant when attacking. It was probably reserved for escaping.

 

[Mike Williams]

<...>if the N2O system was used. <...> I seriously doubt it would ever be used, even in a Mosquito fighter type variant when attacking. It was probably reserved for escaping.

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[Koopernic]

The ratings for GM-1 use on a Ta 152 were 20 minutes at a time presumably a full flow rate. There was 40 minutes total of the stuff.

 

[Mike Williams]

Revisiting for a moment the initial excerpt for 430 Squadron's Operations Record Book:

An examination of 2 and 430 Squadrons' Form 541 - "Detail of Work Carried Out" for February 1945 shows that Spitfire XIVs RM.783 and RM.871 were transferred from 430 Squadron to 2 Squadron (apparently in exchange for Spitfire XIVs RN.115 and RM.925) and would be the aircraft delivered by the pilots of 430 Squadron noted in the Operations Record Book. Form 78 for these aircraft don't show anything out of the ordinary except perhaps that RM.871 underwent "mods" on 11 October 1944 prior to delivery.

I forgot all about the following:- Tests were carried out on Spitfire V P.8781 equipped with a nitrous oxide installation in May of 1943. The installation for this test weighed 92 lbs and increased power significantly for 1 minute at high altitudes. So, utilization of nitrous by Spitfire XIVs would seem to be a subject worthy of further research.

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[GregP]

Is one minute of extra power even worth the effort to investigate? If it did any good, we'd hear all about it in rave reports. Late model Merlins were aleady at or slightly over the power level that stock Merlin rods could handle. Adding more was looking for an engine failure.

They solve that Reno by using Allison G-series rods in Merlin blocks to handle the power gain in a racing engine. When they blow, it isn't the rods. Before they tried that it was usually the Merlin rods. In fact, they were above the nominal power level the rods could handle when WWII ended.

The only reason it wasn't rwally addressed is that they were working heavily on jets and simply soldiered on with existing Merlins until jets were developed enough to replace the pistons.

 

[Mike Williams]

Mods, anyone - Does this board have an ignore member capability? I can't seem to find it anywhere here. Thanks.

 

[GregP]

Great question there, Mike.

By the way, you have a very nice website. Thanks for posting all the stuff you do.

 

[wuzak]

Is one minute of extra power even worth the effort to investigate? If it did any good, we'd hear all about it in rave reports. Late model Merlins were aleady at or slightly over the power level that stock Merlin rods could handle. Adding more was looking for an engine failure.

They solve that Reno by using Allison G-series rods in Merlin blocks to handle the power gain in a racing engine. When they blow, it isn't the rods. Before they tried that it was usually the Merlin rods. In fact, they were above the nominal power level the rods could handle when WWII ended.

The only reason it wasn't rwally addressed is that they were working heavily on jets and simply soldiered on with existing Merlins until jets were developed enough to replace the pistons.

Merlin rods were fine for the power level obtained in WW2 (up to 2200hp in series production, 2600hp in testing). But they weren't designed for 3500hp+ that the Reno engines reputedly put out.