Why not a jet driven P-38?

[delcyros]

I had an interesting discussion on this. For the search for the most reasonable USAAF jet fighter in ww2 I stumbled over the possibility to refit a jet engine to a P-38 Lighting.

Since we also tend to like what ifs, it´s not going to totally sidetrack this board.

Bell Aircraft had designed a single-seat airplane powered by two turbojets built by General Electric (called I-16 or GEJ31) under British license. The first XP-59 was shipped to Muroc Army Air Base in California, where it flew on Oct. 1, 1942. Called the Airacomet, it offered little advantage over conventional fighters, and the few that were produced served mainly as test beds or trainers.

I wonder why the Lockheed company never considered mounting an early jet engine centrally into the P-38 fuselage. The jet-prop mix A/C would have been a reasonable start to deal with the new propulsion, offering a high degree of safety (never test a new engine in a new airframe) and fullfilling some tactical needs (hunt down V-1 bombs, high speed interception and recon).

Assuming a single GE J-31 turbojet engine is used for reference (weight: 386 Kg, thrust: 748 Kg, avaiability late 1943 for serial production):

engine installation:
The J31 unit has to be installed into the central fuselage section. CoG issues require to move the cockpit section about 2 ft. forward, closer to the nose. In order to avoid a major redesign of the wole section it is advised to use side air intakes under the wingroots for the engine (the nose part remains identical to the historic one), which are not expected to interfere with the cartridge propellant gases of the upper nose mounted guns.

airframe:
The airframe does not need a structural reinforcement. Maximum take off gross weight increases to ~23.000 lbs. It does not appear that the limiting capacity of the landing gear is reached with this loading.

fuel system:

The fuel system requires some major modifications, esspeccially in the midwing section for a larger seperated tankage. The jet engine arguably can use low grade fuel as it´s performance is indifferent to octane grades.
The increased internal fuel buncerage consists of 110 gallons jet fuel and 350 gallons high grade fuel (460 total)
Either two standart 165 gallons or 300 gallons drop tanks can be used for a total buncerage of 790 to 1060 galloons. Mixed low grade - high grade tanks are not in production but can be put into if required, as long as these are not avaiable, the pilot has to choose whether he uses high grade or jet fuel drop tanks depending on mission parameters.

Serial production:

Could be implemented rapidly without shutting down Lockheeds P-38 production lines. 85% of the tooling can be overtaken.

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My estimated charackteristics of the hypothetical P-38 with jet engine:

A. GROUND HANDLING:
No special caution has to be made except for a larger clearence sector behind the tail during the warm up of the jet engine.

B. TAKE OFF:
The avaiable power for take off with jet engine is greatly increased. A fully-loaded P-38(mod) without use of the jet engine could take off in estimated 1200 ft under zero wind conditions and concrete runways, while with jet assistance, the take off distance reduces to estimated to under 1000 ft. at full load, giving this P-38 modification the shortest take off run of any USAAF Fighter in ww2 service. The aircraft would lift off the runway between 120 to 125 mph, and required very little effort to pull it up in the air.

C. CLIMB:
In clean condition, with normal weight, an altiutde of 20.000ft. can be established in less than 6 minutes estimated at full power rating, one of the finest time to altitude figures for ww2 fighters.

D. ACCELERATION:
The P-38 arguably had the fastest linear acceleration of all US propeller planes during WW2 (This was true to for all variants for their respective times). For example, starting at sea level at 250 mph with 14.100lbs gross weight and applying COMBAT power the P-38L's linear acceleration was 4.13 ft/s^2 (1.26 m/s2), whereas the P-51D's linear acceleration was 3.85 ft/s2 (1.17 m/s^2) = (recalculated thrust) to weight ratio of 0.3184. The jet modification will help to improve this outstanding rate by close to 20%, giving a linear acceleration of 4.92 ft/s^2 (1.5 m/s^2) at 16.200 lbs gross weight = thrust to weight ratio of 0.3791. It´s unpredecented acceleration is estimated to be second only to the Me-163 (substantially outaccelerating Me-262, Ar-234, He-162, P-80 and Meteor III!).

E. TOP SPEED:
At sea level, the 36.7% power increase would translate into ~9% more speed or about 382 mp/h (615 Km/h) at sea level applying full power (jet engine) and pushing 70" HG using 44-1 fuel (piston engine setting). The best speed is expected to be 452 mp/h /728 Km/h) at around 18.000 - 20.000ft (this figure is already corrected for the increased parasite drag). This figure is surprisingly 39 mp/h better than the contemporary Bell P-59 Airacobra and 37mp/h better than the contemporary Meteor-I using an almost identic jet engine type, even the improved Meteor-III under investigation in the UK is not expected to exceed 455-475 mp/h at best altitudes (short nacelles version). This would make the P-38 modification one of the fastest allied Fighter of ww2. It should be noted that the comparably low limiting Mach number (0.69) is not affected by the modification and caution must be made not to exceed this speed at altitudes.

F. MANEUVERABILITY:
The increased wing loading results in trade offs with maneuverability. Roll rate nevertheless is excellent at high speeds due to hydraulic boost. All other figures are slightly under the serial P-38J maneuverability.

G. CRUISE AND RANGE:
Endurance and Range are widely dependant on altitude. The specific fuel consumption of the J31 jet engine is worst at sea level and excellent at high altitudes. Full power endurance is estimated to be 45 minutes on internal fuel only at medium altitude. The most economic range is expected to be about 1850 mls. at 25.000ft. and 230 mp/h cruise speed with two 300 gallons drop tanks.
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Altough not as perfect as possible, such a modification would give the USAAF a reliable jet Fighter with excellent performances and outstanding range (for a jet driven plane) for rather minor modifications. The resulting P-38 could be produced and deployed in larger numbers than either the P-80 or Meteor and pilots were already familar with the behavior of the airframe.

Instead of redrawing the P-38 from europe the USAAF could reequip some squadrons with this plane in late 44 (a realistic timeframe) and use the fighter in capacities like V-1 hunting, long range (high altitude) recon and medium altitude air superiority fighter.

 

[FLYBOYJ]

The wing, nose section and stabilizer tips of the P-38 were used on the P-80. Close enough?

 

[ccheese]

Didn't the P-38 have compressability problems in a steep dive ? How would
a jet driven P-38 manage this ?

Charles

 

[Graeme]

I wonder why the Lockheed company never considered mounting an early jet engine centrally into the P-38 fuselage.

I suspect that men like Hall Hibbard and Kelly Johnson were always thinking ahead, and a composite powered aircraft wouldn't be the 'Lockheed way'.

Pictured is Hall Hibbard with a model of the Lockheed Model 133, the precursor to the P-80 that was designed around Lockheed's L-1000 jet engine.

 

[FLYBOYJ]

The P-80 used the model 133s wings...

 

[Graeme]

This is what Bell was working on before the P-80 was eventually selected. A single engine version of the Bell P-59, the XP-59B. A jet powered Airacobra of sorts, with air intakes in the wing roots and jetpipe emerging under the tail. Looks sad compared to Lockheed's design/s.

 

[evangilder]

The P-59 was woefully underpowered in any of it's configurations, unfortunately. I don't think just slapping jet engines on a P-38 would be a wise decision. As Charles brought up, the compressibility problem would be made worse at higher speeds. It would have required more engineering than building a new airplane, which is what they did.

While the P-59 was a disappointment in speed and general performance, it did pave the way for other jets that would soon start to appear.

 

[drgondog]

The P-59 was woefully underpowered in any of it's configurations, unfortunately. I don't think just slapping jet engines on a P-38 would be a wise decision. As Charles brought up, the compressibility problem would be made worse at higher speeds. It would have required more engineering than building a new airplane, which is what they did.

While the P-59 was a disappointment in speed and general performance, it did pave the way for other jets that would soon start to appear.

I believe you are absolutely correct.. without a drastic redesign of the P-38, the issues of flow separation on the fuselage creating the wake over the horizontal stabilizer, added to provisioning the 38 for jet engines, doesn't suggest dramatic improvement in high altitude performance (except maybe ceiling and climb near service ceiling) but does imply less range

- if true why screw with jet engines to create an aircraft with marginally better (but distinctly better) low to medium altitude performance with much less range when the P-80 (and Me262) were already better?

The P-82 was probably as good as the 38 for long range escort but thankfully we never had to test that thesis as neither one could handle a MiG (or a 262).

 

[delcyros]

Cheese wrote: Didn't the P-38 have compressability problems in a steep dive ? How would
a jet driven P-38 manage this ?

A jet driven P-38 would still suffer from the same problem, which also affected all other ww2 planes and USAAF planes up to including early F-84 models. The serial P-38 had dive breaks which adressed parts of the problem, so that the dive speed remained under the critical speed. Neither fast ww2 plane could dive without encountering serious compressibility buffeting. The problem with the P-38 is that it happens sooner than on other designs and convincibly, the Me-262 could outdive the P-38 as it experienced the same issues at a significantly higher speed, altough that was unknown in the US by then.

Graeme wrote: I suspect that men like Hall Hibbard and Kelly Johnson were always thinking ahead, and a composite powered aircraft wouldn't be the 'Lockheed way'.

I share this estimation. It is an explenation how it didn´t came off. The proposed P-38 mix, while strictly hypothetic, uses 1942-43 US technology and therefore is contemporary with the L1000 and M133, but while the P-38 is a reasonable (say realistic) suggestion, the sophisticated 133 airframe and the advanced L1000 engine were dead end developments, which never had a chance to materialize, altough the project contributed to the succeeeding P-80 design in several aspects, notably the wings.

Evanglider wrote: The P-59 was woefully underpowered in any of it's configurations, unfortunately. I don't think just slapping jet engines on a P-38 would be a wise decision. As Charles brought up, the compressibility problem would be made worse at higher speeds. It would have required more engineering than building a new airplane, which is what they did.

While the P-59 was a disappointment in speed and general performance, it did pave the way for other jets that would soon start to appear.

I am confident that this never have been shown to be the case. The P-59 was slow, true, but it definetely was not underpowered. There was more power (in thrust) relative to each single ft^2 area or lbs weight than either the Me-262 or the P-80 had. Acceleration and powerload was excellent. The slow speed is a direct result of unfortunate aerodynamics, most notably the very thick wing and drag sensitive large area of fuselage and wings.

As I showed above, the P-38 mix is based on very early jet engine technology , avaiable to the US. If the P-38 mix would have flown against the P-59, it´s superior performance may have saved the day. While the P-59 was not selected, the P-38mix gives performances mostly equal to or better than the UK Gloster Meteor III, which uses more recent stages of jet technology.
Applying thinner propellors to the P-38 (say P-38L like) and more potent jet engines (say GE J33) is not impossible, there at least is some window for limited further development (~500 mp/h top speed at lower altitudes are not impossible at all using late ww2 technologies). Unlike complete new designs, such as the Meteor, Vampire, P-80 and the P-59, the required new tooling processes were limited and a substantial number of planes could be build in the timeframe in question. The pilots did not need a major training on new types and the testing program could have been shortened due to the known properties of the P-38.

drgondog wrote: - if true why screw with jet engines to create an aircraft with marginally better (but distinctly better) low to medium altitude performance with much less range when the P-80 (and Me262) were already better?

Neither the Me-262 nor the P-80 are avaiable in the timeframe in question. Both will eventually turn out to be better all around jet planes, enjoing a higher top speed and critical Mach speed altough both having some points of tactical inferiority against the P-38: service ceiling, acceleration, range, climb rate and possibly reliability and avaiability in numbers, too. A good range particularely was no keysign for first generation jets (P-80A-late: 1440 mls., P-59B: 950 mls., Meteor-III: 1340mls max. with ventral tank, Vampire MK I: 330 mls., Vampire MK-III: 550 mls., XP-83: 1730 mls.) and the P-38 with a best range of 1850 mls. actually fullfills the 1943 US specification requirements for a long range escort fighter, despite using the early jet engine.
I would put forward time as the reason why such a modification should be considered. Not only would this give the US a more reasonable jet fighter than was the P-59 but it would also give the USAAF total technical superiority over the Luftwaffe in 1944.

One of the more serious shortcomings of early jet engines was the design of the air intake and exhoust pipes. The loss of static thrust caused by long air pipes (P-80, P-84) was up to 8% but could be lowered to 5-6% if properly adressed. Hence the move to gondolas (B-47), even if they add parasite drag. I find the layout of the P-38 allows this to be largely bypassed as the jet engine can be installed into the central fuselage section without exhoust pipes and extremely short intake pipes, indeed. I have been unable to find this detail in other prop-jet mix planes so easily.

best regards thanks for Your comments,
delc

 

[Graeme]

 

[evangilder]

Where on earth did you get the idea that the acceleration on the Aircomet was excellent?

You may want to take a read at this:
Bell P-59 Airacomet

And again, you are talking about drastic changes to the P-38's airframe. You will add weight and drag by adding the engine and will likely have an effect on the overall performance. Lockheed did the right thing by developing a new airframe to go with the powerplant.

 

[delcyros]

Compare the thrust to weight ratio of ww2 jets. The XP-59 is superior to the P-80, Meteor I and III, Me-262 and He-162 and almost as good as the Me-163 (depending on load). This is more than enough justification to disproove that the P-59 was underpowered. It´s only when both are nearing the P-59´s top speed when the remaining acceleration of the P-80 is better than the P-59s.
Physics remains validate for everything unless one is the pope and orders Newton to be burned !
Neither does Your link contradict this. There is nothing to be read regarding a bad acceleration but spool up time (something pretty different). It´s the top speed and take off run was to be critisized, not the acceleration once full power was developed by the engines.

The P-59B had a thrust of 4.000 lbs and a weight of around 11.000lbs. The P-80a had a thrust of 3.850 lbs and a weight of 11.700 lbs.

I don´t think that Lockheed did something wrong with the P-80. I proposed the P-38 mix as a replacement for the 1941-1944 P-59 not the subsequent P-80a 1943-1946 design. A reasonable estimation for prototype stage is late 42 and for serial production start is nov. 1943. From a strategic point of view, the P-38 mix could be deployed in numbers about contemporary to or instead of the P-51B in europe (at around may 44) as a long range escort or interceptor.

A tradeoff exists in the delayed development of the P-80. However, this does not seem bad to me as A) the P-80 still was in testing stage at wars end and B) I could imagine that the P-80 benefits from additional developmental time.

[edit] thanks Graeme for the P-59 successor picture. It´s the first time I see a pic from the single engined P-59 derivate.

 

[delcyros]

I have spent some minutes retoushing a P-38 bmp into how I think the P-38(i) could look like.

While the appearence is quite different, You see that the modifications are limited. The booms with engines and the complete landing gear as well as the nose and cockpit section, the outer wing section and the tail section can remain identic. The midwing section and the rear and mid fuselage needs to be of a new design. Since the cockpit is moved more forwardly, the mid wing buncerage can be improved in the way I suggested, without drawing an entirely new wing.

I suspect that the weapons arrangement get´s a bit cramped in this appearence, so perhaps it´s better to adopt a normal, long nose version instead?

As pointed out above, this is purely hypothetical. -

 

[evangilder]

Read it again. The RAF test pilots found the aircraft to be badly underpowered, with an unacceptably-long takeoff run. Like all other early jet-powered fighters, the Airacomet suffered from very poor engine acceleration.

You can true to puff up the P-59, but the fact of the matter was that the aircraft was a turd, plain and simple.

You need to look at a cutaway drawing of the P-38 before you start moving parts of the airframe around. You do realize that their are very few open spaces in that airplane, right? Moving the cockpit forward moved it right into the cannon ammunition drum. The air intakes for the jet engine happen to sit right where the nose wheel retracts, plus it is dangerously close to the spent ammo chutes. And now that you have moved the cockpit forward, all of the control cables will need to be rerouted and extended.

Again, more engineering than it's worth. Better to spend the energy on a new airframe around the new engine than to re-engineer an existing design around an engine.

 

[delcyros]

Sorry but engine acceleration is spool up time and this has nothing to do with the airframe of the Bell P-59 but all to do with the jet engine (put the same engine in another plane and You get the same result). Underpowered is unjustified as the two jet engines deliver more than enough thrust compared to the weight of the plane. You have not showed that the opposite is the case, I had hoped You would reflect test pilot accounts of the very time with high performance piston planes and new jet planes with more caution.

You may blame the 1942 jet engine for this, but later mods of the same engine had an improved spool up time, negating Your argument.


Engine acceleration is not acceleration of the plane. I have showed this with powerload figures and thrust to weight figures. While the engine acceleration (spool up time) of new jet engines (not only this one) was indeed worser than -what the RAF test pilots experienced- f.e. high performance piston fighters, the acceleration of the plane itselfe, once full power was delivered by the turbojet was simply superior.
Galland wrote that the Me-262 due to the poor engine spool up time needed more "acceleration ellbows" than piston fighters - exactly what the RAF testpilots describe. One side is better described as rapidly applied power (in piston A/C of this time) while the jets had slowly applied power. This is not to misinterprete as acceleration because max. acceleration is measured when full power is applied, already. How badly underpowered would be the vaunted Me-262 or even worser: the Ar-234B in Your eyes! It´s two Jumo-004 B3 jet engines delivered only 3.923 lbs of thrust (with B4 engines up to 1960 lbs) while the plane itselfe weighted ~18.000 lbs compared to the "featherweighted" ~11.000lbs P-59B (which had 4000 lbs thrust avaiable). If You insist to say the P-59 was underpowered than I have to remind You that all ww2 jet planes were underpowered and all even to a larger degree than the P-59!

Your repeated equaling engine acceleration with linear acceleration and use of unreflected texts, while neglecting physics here shows that You have a particularely poor understanding of the matter. Test pilots which tested 1st generation jets never were impressed by stand off acceleration performance after having experienced advanced piston A/C´s - no wonder! What they were impressed of is acceleration at high speed (preferably well above the performance envelope of piston A/C), something the P-59 never was able to show as it had only comparable fight envelopes to piston fighters. Under full power, the P-59 had the potential to outaccelerate all contemporary US/UK and german piston or jet A/C, at least at 250 mp/h. It was not until the Meteor-IV that a plane experienced better acceleration.

The latter part of Your post can be confirmed with my estimations. I have already expressed my doubts that the nose config get´s to cramped. However, it is entirely feasable (altough this forces me to redo the bmp). The normal nose cockpit section is moved forward around 2 ft. This leaves the normal space for nose gear, ammo boxes, cockpit and weaponry. The midwing section with the modifed tankage assembly, a new rear fuselage containing the J31 + side air intakes (external, they do not interfere with the landing gear) are the only new parts next to minor changes in the electric systems. Altough the nose section is moved forward, the cg changes are countered with the jet engine itselfe.

It just has been an idea, nothing to upset us. I could imagine that this layout combines the advantages of piston A/C (rapid applied power) with some of the jet advantages (low weight for power). Another aspect comes to mind: While Jets were expected to have a generally higher performance envelope than piston A/C (and equal performance was considered unstatifying = P59 and He-178 ), all nations regarded the rather shortliving mix A/C as beeing intermediary. Used in such a capacity, the P-38mix is very attractive, proposing a plane with the performance envelope beeing better than the F7 Tigercat but avaiable 1 year earlier...

 

[evangilder]

You are assuming that RAF test pilots only flew the P-59 after flying only piston fighters. Since the Brits were quite ahead of the US in jet aircraft development, I am sure that the comparisons were against the Meteor. I understand how acceleration and spool up time work. It doesn't change the fact that the P-59 was a big disappointment, period.

 

[SoD Stitch]

Good idea, but you used the wrong airframe . . . the XP-58 Chain Lightning would've been a better candidate for a jet engine installation. Basically an enlarged P-38 with more powerful engines (Allison V-3420's), more airframe room to work with, and a lot more fuel onboard (2,000 gals.+). Initial tests with the aircraft weren't exactly successful, mainly due to the tempermental engines (another Allison trademark), but the airframe had a lot of room for development, including room for installing a jet engine in the rear of the centrally-mounted fuselage.

 

[FLYBOYJ]

Agree.....

 

[johnbr]

Or how about a pair of 3000shp turbine.

 

[SoD Stitch]

Or how about a pair of 3000shp turbine.

Yeah, if they'd had turboprops in '45, which they didn't.

Jet turbine technology was in it's infancy as that time; as it was, it was difficult enough to get a jet turbine to operate reliably without trying to bolt on an output shaft to the compressor section, and add a bunch of other stuff (accessory drives, etc.) to an otherwise complicated (for 1945) piece of technology. They had enough trouble with just the engine by itself, let alone an output shaft, harmonic balancers, etc.