WWII Jet Engine Power

[mikewint]

I have been able to locate a very few MOTORJETS which says it all in my opinion. The Caproni Campini N.1, which has already been mentioned and the A.I. Mikoyan-M.I. Gurevich I-250(N). The MiG was a Soviet jet aircraft that was the only motorjet ever used by a military. It flew from 1946 to 1950 and was considered a hybrid, as it also had a nose propeller.
I have also been able to determine that late in World War II, the Japanese also experimented with the concept. They developed the Tsu-11 motorjet that was used in the Japanese Ohka kamikaze plane. It was reasoned that use of the Tsu-11 would overcome a major liability of the plane. It had to get very close to enemy aircraft in order to strike and was frequently shot down before it could carry out its mission, a problem the Japanese hoped the Tsu-11 would correct. An example of this is in the Smithsonian National Air and Space Museum in Washington, DC.
In 1942, Americans investigated motorjets in the Naca Jeep project, but never completed research because of interest in turbojets. The German company BMW experimented with motorjets and turbojets in the early 1940s. In the late 1940s, interest in motorjets had declined because turbojets tend to be more fuel-efficient than motorjets which would seem obvious to me. As shortround has already stated, you have to fuel the piston engine AND fuel the jet. So back to my first post REDUNDANT.
Now what we really need is an airplane that flies by flapping its wings an ORNDITHOPTER!

 

[JoeB]

The fuel economy is going to be atrocious. Not only are you running the piston engine but the combustion chamber "jet" reaction set up is far from efficient. Just a like a piston engine, the economy of a JET engine depends a lot on the compression ratio. Early jet engines ran on a compression ratio of about 3 or 4 to 1. The lower you go the less LESS thrust you get per pound of fuel burned.

The potential efficiency advantage of the motorjet is exactly in that respect though. Pressure ratio tends to be limited by turbine inlet temperature. The more you compress the intake air the hotter it is entering the combustion chamber, since you can't practically fit an intercooler into an airplane turbine. And given a particular turbine inlet temp (ie combustion chamber outlet temp, more or less) the less fuel energy you can add without exceeding that turbine limit. The motorjet compressor can be designed to a higher compression ratio because of the less restrictive limit on combustion chamber temperature, w/o high temp turbine metallurgy to worry about. Also, the efficiency of mid 1940's compressors and turbines was pretty low, so subsituting the thermally inefficient piston engine for the turbine in driving the compressor didn't cost as much as it would have later on.

As far as your thumbnail calculation, I think it might be too simple to really conclude much. But even though calculational thermo and fluid dynamics at that time were much less advanced than now, I don't think it's plausible to assume designers had to build motorjets in order to find out they were less efficient. I would assume that at some design speed around 500mph, at the edge or practical piston/prop capability, and a speed early jets took awhile to get to, the motorjet could be in theory compete with either on efficiency. As I mentioned above the 'you're dragging around a piston engine for nothing' inuition is not correct: the energy input of the piston engine comes out as thrust too, and even a cold ducted fan arrangement (what a motorjet would be with the combustion fuel turned off) will beat a (1940's state of art) propeller for efficency as the prop blades near or pass Mach 1.

But, the motorjet did have serious practical issues (any design of multiple mechanical connections to fast spinning stuff, like a prop up front and a motorjet in tail, like the Soviet concept, would be ripe for vibration problems at the then state of art in understanding those, just as one example). And the turbojet was a fast moving target (both literally and figuratively as a technology). A motorjet based around a 2000hp piston engine might have a total power equivalent of 3-4000hp including the energy added in the combustion chamber, but definitive postwar turbojets were already past that, and only 15 or so years further down the road an F-4 had seveal 10's thousand equivalent hp installed. There was no way to scale up power that way with a reciprocating or hybrid part-reciprocating propulsion device w/i the weight limits of an airplane. So the motorjet was a dead end, but so was the piston engine/prop combo by the mid 1940's, for high performance fighters.

Joe

 

[nuuumannn]

it's such a big and complex beastie - the guys working on it only got it fitted out, but not to a working state and it would have been so impressive seeing that thing roaring away on a test stand.

I was actually referring to the one we had at work, Wuzak. Yes, the engine flew in the nose of a Lincoln.

Such a "concept of a reciprocating engine driving a compressor" was put into practice.

I also know this, what I was referring to was that it wasn't put into widespread use (note to self - explain things more clearly)

One issue which Mike and Shorty mentioned is the type of fuel such an engine was to use; one area where the Napier was efficient was that it was a 2-stroke diesel, otherwise you'd need two different fuel systems - utter redundancy. As you probably know, diesels can also burn kerosine or jet fuel.

 

[tyrodtom]

The Mig I-250 or Mig 13 as it was called was in service with Soviet naval aviation till 1948 or 50, it depends on the source you check. Not a failure, not exactly a success either.
The compressor was on the same shaft that powered the supercharger, the same air duct fed the supercharger, coolant radiator, and motojet. From descriptions it sounds like the motojet used the same gas as the prop engine, and why not, with no output turbine to overheat.

The Sukhoi effort at a motojet wasn't as successful, some of it's engineers spent some time in the gulag for industrial sabotage.

 

[nuuumannn]

From descriptions it sounds like the motojet used the same gas as the prop engine, and why not, with no output turbine to overheat.

Because heat is what makes jet engines efficient.

Regarding the Russian aircraft, the MiG-I-250, N-1 and MiG-13 and the Sukhoi Su-5, both of these used the jet portion; the combustion chambers for high speed dashes only; both used piston power driving a propeller in the cruise. The jet engine was shut off in the cruise. They are described as VRDK Vozdushno-reaktivnyi Dvigatyel Kompressornyi - air reaction engine compressor and used the same fuel as the piston engine, which fuel type is not listed.

Sppeds for the MiG-1-250 listed as Max speed at 7 km; 513 mph, climb with VRDK; 3.9 min to 5 km, ceiling 10,5 km with VRDK, range 920 km with VRDK, 1380 without VRDK running. The VRDK produced 300 kg or 661 lb at low level.

Sukhoi figures soon...

 

[Shortround6]

The potential efficiency advantage of the motorjet is exactly in that respect though. Pressure ratio tends to be limited by turbine inlet temperature. The more you compress the intake air the hotter it is entering the combustion chamber, since you can't practically fit an intercooler into an airplane turbine. And given a particular turbine inlet temp (ie combustion chamber outlet temp, more or less) the less fuel energy you can add without exceeding that turbine limit. The motorjet compressor can be designed to a higher compression ratio because of the less restrictive limit on combustion chamber temperature, w/o high temp turbine metallurgy to worry about. Also, the efficiency of mid 1940's compressors and turbines was pretty low, so subsituting the thermally inefficient piston engine for the turbine in driving the compressor didn't cost as much as it would have later on.

The Jumo 004 had a compression ratio of 3 to 1 and needed 8 stages in the compressor to reach that. I would imagine that if they could have reached a 3 to 1 ratio with 6 stages they would have done it. The BMW 003 used 7 stages for 3.1 to 1. The early British jet engines had compression ratios of 4 to 1 or under from their single stage centrifugal compressors. The axial compressor on the Metropolitan Vickers was either 9 or 10 stages for a 4 to 1 ratio depending on exact model. The first J-35s in America used 11 stages to get a 4.0 to 1 compression ratio. Things got a whole lot better in just a few years. I would note that ALL of the above engines used a single stage turbine in jet engine versions ( turbo prop, turbojet fans both ducted and open used more stages in the turbine)

As far as your thumbnail calculation, I think it might be too simple to really conclude much. But even though calculational thermo and fluid dynamics at that time were much less advanced than now, I don't think it's plausible to assume designers had to build motorjets in order to find out they were less efficient. I would assume that at some design speed around 500mph, at the edge or practical piston/prop capability, and a speed early jets took awhile to get to, the motorjet could be in theory compete with either on efficiency. As I mentioned above the 'you're dragging around a piston engine for nothing' inuition is not correct: the energy input of the piston engine comes out as thrust too, and even a cold ducted fan arrangement (what a motorjet would be with the combustion fuel turned off) will beat a (1940's state of art) propeller for efficency as the prop blades near or pass Mach 1.

My thumbnail could be way off. I have no idea how much power it really takes to run the compressors on the JUMO 004 so I don't know how big an engine is really needed or what the fuel burn is. I also do not question that the "cold ducted fan arrangement" may be more efficient as the prop blades pass Mach 1. The problem is that the " efficiency" may not take into account the larger air frame and the friction loss inside the duct. The Friction loss is dependent on the size of the duct in relation to the air flow mass and in the speed of the airflow. The Russians used a much shorter duct than the Caproni. Even in cruising flight how fast does the "exhaust' have to be going to get any kind of "thrust/speed" ? Velocity X Mass equals thrust, a short duct may have little drag/friction loss (like in a ducted fan) a 20 ft duct may be a different story. Some jets ( Hawker Sea Hawk for one) used split intakes and exhaust to minimize duct losses, it may have been one of the reasons for the twin boom Vampire. As I have said, the efficiency of the propulsion unit may be higher but the efficiency of the complete airplane may not be. What would be the performance of a normal plane using a 500 kW Isotta Fraschini Asso L.121/R.C. 40 radial engine and having a 1250lb payload ( crew of 2 fuel, oil, etc) ?

 

[krieghund]

??? Which one ???

The Model 22 of which about 50 were produced were powered by a Campini type compressor engine. The rest of the production types (about 850+) used various turbojets or rocket engines.

 

[razor1uk]

That Hitachi looks slightly akin to a Argus or a DH Gipsy Minor (..or is a Major) engine... then again, an inverted aerial aircooled 4 cyl' engine could only sort look like that.

 

[FLYBOYJ]

The Model 22 of which about 50 were produced were powered by a Campini type compressor engine. The rest of the production types (about 850+) used various turbojets or rocket engines.

These were never used in combat.

From Wiki;

Operational variants

Kugisho/Yokosuka MXY-7 "Ohka" Model 11 Rocket Suicide Attacker. 755 built.

Non-operational variants

Kugisho/Yokosuka "Ohka" Model 21 Rocket Suicide Attacker. Steel wings; one built.
Kugisho/Yokosuka "Ohka" Model 22 Motorjet Suicide Attacker. 50 built.
Kugisho/Yokosuka "Ohka" Model 33 Turbojet Suicide Attacker. Renzan drop launch.
Kugisho/Yokosuka "Ohka" Model 43A Ko Turbojet Suicide Attacker. Submarine launched.
Kugisho/Yokosuka "Ohka" Model 43B Otsu Turbojet Suicide Attacker. Cave launched.
Kugisho/Yokosuka "Ohka" Model 53 Turbojet Suicide Attacker. Towed launch.

Trainer variants

Kugisho/Yokosuka "Ohka" K-1 Suicide Attack Trainer
Kugisho/Yokosuka "Ohka" Model 43 K-1 Kai Suicide Attack Trainer

 

[krieghund]

True only one Model 22 was test flown or I should say crashed. However the other models when they were successfully launched created some havoc.

From Rene Francillon's "Japanese Aircraft of the Pacific War";


Production: A total of 852 Ohkas were built by the following major contractors with the co-operation of a series of sub-contractors:
Dai-Ichi Kaigun Koku Gijitsusho, Yokosuka:

155 - Ohka Model 11
50 - Ohka Model 22
45 - Ohka Model K-l
2 - Ohka Model 43 K-l KAI

Dai-lchi Kaigun Kokusho, Kasumigaura:
600 - Ohka Model 11

 

[FLYBOYJ]

True only one Model 22 was test flown or I should say crashed. However the other models when they were successfully launched created some havoc.

From Rene Francillon's "Japanese Aircraft of the Pacific War";


Production: A total of 852 Ohkas were built by the following major contractors with the co-operation of a series of sub-contractors:
Dai-Ichi Kaigun Koku Gijitsusho, Yokosuka:

155 - Ohka Model 11
50 - Ohka Model 22
45 - Ohka Model K-l
2 - Ohka Model 43 K-l KAI

Dai-lchi Kaigun Kokusho, Kasumigaura:
600 - Ohka Model 11

ASAIK the only model used in combat was the model 11

 

[MiTasol]

I've always wondered why piston-powered jet engines did not see extensive use during ww2?

I know there was a little research and even a few aircraft built but nothing substantial.

Most of the problems of creating a jet engine special materials revolved around the turbine section of the engine. It seems to me that if they had eliminated the turbine section and powered the compressor with a piston engine instead, that a high-thrust engine could have been produced with relative ease. Say, using an R-2800 to power a compressor. Dumping 2000HP into a compressor should be suffice to produce enough thrust to power an aircraft like the ME-262.

I suppose I should put the pencil to paper and do some calculations.... But I wanted to see what everyone here had to say first. I'll post my findings later.
Thanks

The Japanese did this for some of the suicide aircraft.
From memory a 100hp piston engine gave an aircraft the weight of a Cessna 180 and a wing loading of 75lb/sq ft (very high) a 300mph top speed.

I will have to un-bury my Francillon Japanese aircraft to confirm

 

[GregP]

I see at least one claim that only 3 of the Tsu-11 thermojet engines were known to have been built, and there is no record of any having flown operationally.