Discussion about GM-1 system

[tomo pauk]

An invitation for a discussion about the GM-1 system, used (and planned to be used) on some LW fighters. I'm interested in how well it's use was spread, experiences of the pilots that used it in combat, pros cons etc. Any constructive info is appreciated

 

[Balljoint]

It's nitrous oxide as is now used with performance cars.

More specifics,
Bomber Aircraft - Luftwaffe Resource Center - A Warbirds Resource Group Site

 

[tomo pauk]

Thanks for the link. About the GM-1 it says same as Wikipedia, and maybe someone could put more light on this:

Initially intended as standard equipment for the Luftwaffe, in operational service it was found that GM-1 had some drawbacks. The additional weight of the equipment reduced performance on all missions, while the system was only used in the cases where the aircraft went to very high altitudes. GM-1 also became less attractive than originally imagined when in 1943, the previous trend towards ever increasing combat altitudes ended.

The LW fighters were between the rock and a hard place once the Allies started to throw 400+ mph fighters at altitudes between 25-30000 ft, from mid 1943 on. And LW become 'disillusioned' at GM-1 then?? The Fw-190A-8 was at 9 km almost 100 km/h faster with GM-1 than without it; it was faster some 20 km/h than the A-9 (the A-9 without GM-1) up there, and almost as fast as D-9 above 8 km (but with almost twice the firepower).

added: sure enough, German language Wikipedia is more up to the point:

Mit einer Bf 109 G-1/R2 ließen sich mit GM-1-Anlage 100 km/h Geschwindigkeitsgewinn in 12.000 m Höhe und eine Gipfelhöhe von 13.800 m erzielen.

or: With GM-1 system, the speed of BF-109G-1/R2 was increased by 100 km/h at 12000m, achieving the service ceiling of 13800 m.

Beim Einsatz in Höhenaufklärern Ju 88T mit BMW-801-G-2-Motoren konnte die Leistung, die bei diesen Maschinen in 10.000 m Höhe von 1.730 auf 880 PS abfiel, durch GM-1 auf 1.430 PS gesteigert werden.

saying basically: when used on hi-alt recce Ju-88T, with BMW-801G2 engines [equivalent of the better known 801D-2] capable for 1730 PS [at low altitudes], the GM-1 increased engine power from 880PS to 1430 PS at 10000 m.

in square brackets are my comments

 

[fastmongrel]

Wasnt there some use of Nitrous Oxide by RAF PR Spits and Mossies. Was it a similar system to the GM-1.

 

[OldSkeptic]

Yes, a bunch of Mossie nightfighters were converted. Rawnsley's book, Night Fighter (about him and John Cunningham) mentions it. Added 50mph.

 

[Shortround6]

Some of the installations were very heavy. The Installation used in the JU-88S-1 was supposed to provide enough nitrous oxide for 45 minutes at 7.95lbs per engine per minute "normal" rate and 27 minutes at 13.2lbs per engine per minute at "emergency" rate.
Installation weighed 400lbs empty and held 900lb of nitrous ( and compressed air?). Tanks filled rear bomb bay. Please note that the range of the JU-88 can vary widely depending on if wing tanks alone are used, wing and rear bomb bay tanks or wing, rear and forward bomb bay. If the rear bomb bay is already filled with nitrous tanks it rather limits range/endurance unless bombs are hung outside.

The nitrous would evaporate out of the tank/s if the plane was not flown. Kurfurst's site has a document giving evaporation rates at different temperatures, winter wasn't so bad. but in summer two days could see most of the nitrous gone after filling.

Kurfürst - Kurz-Betriebsanleitung für Flugzeugführer und Bodenpersonal für GM 1-Anlagen in Bf 109 G.

 

[tomo pauk]

The added weight was then 1300 lbs, ie. 650 lbs per engine for a twin engined plane. The GM-1 tank + mixture for the Ta-152 (85L) weighted 104 kg (~230 lbs); 10 - 15 min worth?

 

[Aozora]

Yes, a bunch of Mossie nightfighters were converted. Rawnsley's book, Night Fighter (about him and John Cunningham) mentions it. Added 50mph.

http://www.wwiiaircraftperformance.org/mosquito/N2O_Mosquito_XIX.jpg

I came across an old forum posting by "butch2k" real name Olivier Lefebvre who apparently has heaps of original 109 documents in which he stated: Time limit for MW50 usage > 109

Using GM-1 was an hassle, the product being highly volatile you could not fill the tank and just wait a few days before using it.
Moreover it lost most of it's interest when the engine being produced reached FTH in the 7-8km range, indeed GM-1 could not really be used below FTH+1500/2000m meaning that you had to fly at 9000-10000m to use it.
At such an altitude you most likely need a pressurised cockpit and it's very unlikely you'll encounter anyone except for some recce a/c.
G-5/U2 and G-5/U2/AS were kept with GM-1 just for such business, intercepting high flying mossies and the likes.

Most usefull was the MW-50 boost which permitted higher outputs for a long time at usual combat altitude.

In another post: Bf109 vs Fw190 speed at low level

Beginning in April 1944 the G-6/U2 were transformed on the production line for MW-50 use rather than GM-1 moreover 200 kits were issued to transform the existing GM-1 using G-6/U2.
THe aircraft transformed on the production line were issued the light tank similar to the later G-14, but the modified G-6/U2 kept their the heavy GM-1 tank.

So it looks as though the GM-1 had some limitations. Pity there weren't any documents posted. (Don't know what happened to Lefebvre, who apparently was writing a "definitive" book on the 109 at the time?)

 

[tomo pauk]

Thanks for feedback.
Buch2k has his own forum for some time now, the invitation was (is?) needed to register there. Now, on his comments:

Using GM-1 was an hassle, the product being highly volatile you could not fill the tank and just wait a few days before using it.

This need to be in connection with time and place. From 1943, the LW fighters were scrambled on daily bases (wetter permitting) vs. Allied daylight incursions in German-held Europe - fill in the morning, and you can be sure you gonna need it within couple of hours.

Moreover it lost most of it's interest when the engine being produced reached FTH in the 7-8km range, indeed GM-1 could not really be used below FTH+1500/2000m meaning that you had to fly at 9000-10000m to use it.

The 9-10000m means 29500-32800 ft - right where the P-51 have had the edge vs. Bf-109. The normal operating altitude for the Fw-190A-8 equipped with GM-1 was from 8-11 km (26300-36100 ft), and I really look forward to read any info about the 190s use of the GM-1.

At such an altitude [9-10 km] you most likely need a pressurised cockpit and it's very unlikely you'll encounter anyone except for some recce a/c.

?? Just the right altitude for the P-51 and P-47 to bounce on you. Neither had much of a real pressurized cockpit. Suffice to say that Ta-152 was to have the GM-1 installed, it certainly would've not if the GM-1 was considered such a hassle.

Most usefull was the MW-50 boost which permitted higher outputs for a long time at usual combat altitude.

?? again. The MW-50 was of no use above ~7km for the Bf-109 at the ETO, unless the Bf-109 was equipped with the DB-605ASM (or later) engine. The 'usual combat altitude' started, for many combats, above 7.5 km (~25000 ft).

 

[Balljoint]

To speculate as to what the GM-1 hassles may have been, direct injection could deal with water/methanol injection as a function of manifold absolute pressure since the methanol/water injection would cool (increase oxygen density) while diluting the oxygen present in the fuel mixture, i.e. a rough offset. Gm-1 would cool while further leaning the mixture with increased oxygen thus requiring new mixture controls. Also, GM-1 could be provided in three strengths, further complicating matters. It's not that the injection couldn't be rejigged to handle the additional demands, but the complications were perhaps not worth the tradeoffs in 1943 and 44.

Also with fuel, munitions etc. being made from a coal feedstock, production of nitrous from ammonium nitrate may have been very high on the priority list. As with pilots and alloys, resources were in short supply.

 

[OldSkeptic]

Thanks for that Aozora, what's the book (though I'd got the lot abut Mossies...)?

In this case they were using NO2 mostly as an oxygen booster, a simple (if heavy) alternative to a 2nd stage engine. Given how quickly they threw it together they did a good job.

There were a lot of experiment on Spits) using liquid oxygen as a high altitude booster, right from the Spit1, then Spit V and also the VII. Abandoned, mainly due to lack of need, but they did gain a lot of experience.
LOX is even better than NO2 as a high altitude oxygen booster, as you need less and is correspondingly lighter (and it lowers inlet temps therefore improving efficiency).
The downside is the ground requirements, the fact that it boils off so quickly meant if a plane was filled up it had to be topped up often. Then again NO2 is difficult too.

But, if it had been needed they would have used it and/or NO2. Just that Messers Rolls and Royce kept coming to the party with more ever powerful engines and of course an effective (and light) 2nd stage.
Plus using 150 octane to kick up performance even more meant a much simpler logistics chain, post D-Day.

 

[stona]

From a practical standpoint GM-1 remained in use for aircraft operating at high altitudes and expected to remain there

Whatever altitudes many combats may have started at most continued at a lower altitude. Some finished at close to sea level. Without digging through dozens of references I would conjecture that the added performance at very high altitudes obtained by the use of GM-1 injection was considered not worth the trade off in weight and other problems for day fighters.

There are literally hundreds of combat/encounter reports available. How many of these include combat above 8000-9000 metres (26,000-30,000 feet)? I have been sifting the encounter reports of the P-47s 0f the 56th FG for someone recently. Generally combats (on escort missions) commenced between 18,000 and 22,000 feet (roughly 5,000 to 7,000 metres) and descended from there. At this altitude GM-1 was not going to be useful to a Luftwaffe fighter. MW 50 on the other hand would have been. I don't believe that it was an accident that many Luftwaffe fighters were MW 50 equipped.

On other mission types the combats took place much lower.

Cheers

Steve

 

[tomo pauk]

From a practical standpoint GM-1 remained in use for aircraft operating at high altitudes and expected to remain there

The aircraft with single-stage engine and GM-1 can be fast at 25000 ft and above, and then either fight at those high altitudes, or dive at prey several thousands of feet under (disengaging the GM-1). Such an airplane, but without GM-1, cannot choose the option 1 vs an enemy with a 2-stage engine aboard, and it might not be in position to take full advantage of option 2.

Whatever altitudes many combats may have started at most continued at a lower altitude. Some finished at close to sea level. Without digging through dozens of references I would conjecture that the added performance at very high altitudes obtained by the use of GM-1 injection was considered not worth the trade off in weight and other problems for day fighters.

Fair point. Seems we lack the 1st hand reports and opinions of people that used it and/or were making decisions about it, those things needed for forming a credible picture about the system.
At any rate, the escort fighter that is driven far to much down does not contribute it's task, unless the defending fighter must go down too (since it's hi-alt performance is low, like the 1944 Fw-190s?).

There are literally hundreds of combat/encounter reports available. How many of these include combat above 8000-9000 metres (26,000-30,000 feet)? I have been sifting the encounter reports of the P-47s 0f the 56th FG for someone recently. Generally combats (on escort missions) commenced between 18,000 and 22,000 feet (roughly 5,000 to 7,000 metres) and descended from there. At this altitude GM-1 was not going to be useful to a Luftwaffe fighter. MW 50 on the other hand would have been. I don't believe that it was an accident that many Luftwaffe fighters were MW 50 equipped.

I've gone through some reports right now (thanks, Mike), and many of them are like this: we (P-47s) went down against Fw-190s. I guess that any fighter that P-47 is going down against it is in shambles, and generally giving away the altitude advantage was seldom a wise move. It confirms that (1) Fw-190As were worse that P-47s above 20-25000 ft, and (2) that Fw-190As needed more HP above 25000 ft. GM-1 is an easy solution?
MW-50 was a no-go for Fw-190, instead they upped the boost, much like the Allies were doing for their engines, thus taking advantage of better octane rating (vs. B4) of the C3 fuel.

On other mission types the combats took place much lower.

Agreed. It were the hi-alt missions that sealed fate of LW in 1944, however.

 

[stona]

Just to clarify, those P-47s did not necessarily engage in combat at 18,000 to 22,000 feet. This was the altitude at which they were flying, the word cruising is often seen, before initiating the engagemant.
Cheers
Steve

 

[tomo pauk]

I've read maybe 15 of the reports within few minutes (top ones listed here), many of them (most?) state their altitude as being 25-26000 ft, diving at LW fighters under them.
At any rate, the increased boost used on Fw-190s was not doing anything to improve Fw-190A-8's performance above 6.3 km for speed, and above 6 km for RoC (20700 and 19700 ft respectively; here, shaded areas are of interest). It was of better use for the lighter and more streamlined A-5/A-6, up to 7 km of altitude (for high speed).
Another thing to consider is the cruise altitude of B-17s in the ETO - that was around 25000 ft IIRC. The LW fighters need to go up and play, and better play good when up there. In case they decide they can attack from under those 25000, they easily end up mentioned at the USAF's combat report as victim.

 

[stona]

I thought you must be reading different reports to me so I checked the first ten on your link.
Only one mentions starting at above 25,000 feet stating 28,000 feet.
On three the initial altitude is unclear.
Five start at 25,000 feet or less. One states that they climbed to engage the enemy at 26,000 feet, which means six out of ten started from 25,000 feet or less.
Now, those ten are not representative of the whole body of data but they tend to confirm what I have seen in other reports.
25,000 feet is lower than 8,000 metres.

Looking from the other side I have seen many claims from Luftwaffe pilots for B-17s and B-24s. It is remarkable how often the figures 6,000 and 7,000 metres appear considering popular wisdom is that US bombers flew higher than this. 8,000 metres is less common, though not rare . The highest height given for a bomber "pulk" that I have seen with a quick look through my notes is 8,500 metres.

This would be a representative account, from Lt. Ekkehard Tichy of 9./JG 3.

"At 10.15 hour I took off in a Bf 109 G-6 with three MG 151/20s and two MG 131 as Staffelfuhrer in the Gruppe of Major Dahl against an enemy formation reported in the area of Mannheim. It contained 60-70 aircraft staggered from 6,000 to 7,000 metres."

I have one individual report where a Fw 190 pilot (Uffz. Gerd Wiegand, 8./JG 26) claimed to have dived on the bombers, through the escorting P-47s from 11,000 metres, but this is a rare exception. More often the Luftwaffe fighters attack from an altitude similar to that of the bombers.

Cheers
Steve

 

[tomo pauk]

I thought you must be reading different reports to me so I checked the first ten on your link.
Only one mentions starting at above 25,000 feet stating 28,000 feet.
On three the initial altitude is unclear.
Five start at 25,000 feet or less. One states that they climbed to engage the enemy at 26,000 feet, which means six out of ten started from 25,000 feet or less.
Now, those ten are not representative of the whole body of data but they tend to confirm what I have seen in other reports.
25,000 feet is lower than 8,000 metres.

Indeed, 25000 ft is 7620 meters.
The point I'm trying to make is this: one that has altitude and/or speed advantage has better chances in air combat. Eg. if the bombers are cruising at 25000 ft (+ and - 1000 ft), the escorts will better be a tad above them (25-28000 ft), so they can punish the interceptor trying to get the bombers. The escorts will have both speed and height advantage when doing that, and interceptors know that (or don't, ending up as a claim in MACR).
The interceptors can try to reverse things - cruise at 27-30000 ft, and once the enemy is at sight, either open up the throttle and engage it, or dive and engage (depending whether the enemy is at same altitude or lower). Curiously enough, we don't see many MACRs where the P-47s engaged an interceptor that came from higher altitude and at higher speed - having altitude and/or speed advantage really worked for the LW?
The Fw-190A (no GM-1) was, however, ill able to do that to the P-47s, since above 20-25000 ft it performed worse than it (and than Merlin Mustang).

Looking from the other side I have seen many claims from Luftwaffe pilots for B-17s and B-24s. It is remarkable how often the figures 6,000 and 7,000 metres appear considering popular wisdom is that US bombers flew higher than this. 8,000 metres is less common, though not rare . The highest height given for a bomber "pulk" that I have seen with a quick look through my notes is 8,500 metres.

Killing a 4-engined bomber was not a same thing as killing the P-47/51. The bombers were likely to be 1st damaged, and then destroyed. The 1st hits will be likely received at normal cruise altitude (guns, rockets, Flak), in case the engine or two were lost the B-17 was likely to be slowly descending. After that the LW pilot (not always the same one that damaged that bomber) might deliver the blow that would've finally destroyed the B-17. The 'stragglers' were in similar position, and LW fighters were eager to gang up upon those, most often well damaged birds.

This would be a representative account, from Lt. Ekkehard Tichy of 9./JG 3.

"At 10.15 hour I took off in a Bf 109 G-6 with three MG 151/20s and two MG 131 as Staffelfuhrer in the Gruppe of Major Dahl against an enemy formation reported in the area of Mannheim. It contained 60-70 aircraft staggered from 6,000 to 7,000 metres."

I have one individual report where a Fw 190 pilot (Uffz. Gerd Wiegand, 8./JG 26) claimed to have dived on the bombers, through the escorting P-47s from 11,000 metres, but this is a rare exception. More often the Luftwaffe fighters attack from an altitude similar to that of the bombers.

Cheers
Steve

The fighter going vs. bomber at same altitude is just fine when one does not to worry about the escorts. The interceptors were also not flying fast, in order not to collide with a bomber when making a head-on pass, and not overtake the bomber once in tail-chase. Once the escorts that 'thrived on thin air' were introduced, flying at 20000 ft at 250-300 mph was an recipe for disaster. And there was no two-stage BMW-801s or two-stage DB-605s to even the odds.

BTW, it goes without saying that Bill/drgondog is invited to share his stuff here

 

[stona]

There were tactics involved.
It was often Fw 190s that were attacking the bombers, particularly after the formation of the "Sturmgruppen". These very often had an escort which would position itself higher and attempt to intercept the American escort. One element of the Luftwaffe escort would position itself below (and to the left or right) the bombers in an attempt to provide protection for the sturmgruppe as it re-assembled.
The re-assembly direction was given as part of the original order to engage as in "Pauke,Pauke, Abgang Links (or Rechts)". A variation was "Pauke, Pauke, Versammeln Links (or Rechts)"

This all looks good on paper but once the US escort fighters actually got involved I imagine it all went to hell in a handcart anyway.

Cheers
Steve

 

[davebender]

RR Merlin had a nice compact two stage supercharger system but I suspect GM-1 was no heavier then some American supercharger installations. How much did that monster size P-47 supercharger system weigh, including additional fuselage volume necessary to contain it?

 

[Shortround6]

You know, just maybe, the P-47 had a big supercharger system because it's system would supply 2000-2500hp worth of air at 25,000ft.

Roughly double the amount of air that a Merlin needed. And way more air than even a DB 603E or 603AA. An easy 33% more air even for 2000hp let alone the later versions.

You want 2000-2500hp at 25,000ft you need a lot of air.