Junkers 390 with jet engines?

Discussion in 'Aviation' started by DerGiLLster, Jun 28, 2015.

  1. DerGiLLster

    DerGiLLster Member

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    Would this modification have been possible? What if the Junkers had not used 6 BMW-801G engines but in fact had used eight Junkers Jumo 004 engines? Now this would defeat it's purpose of the Amerika Bomber program the RLM had in mind, but imagine what it could have been used for on the fronts at home. Range would be limited but if it could have a combat range of around 1500 miles then it would be perhaps, in my opinion, more effective than German medium bombers.

    I have believed that German medium bombers were not as effective as the American or British heavy bombers, since the heavy planes could pack more boom in the area increasing causalities. Germany would have created Schwerschnellbomber, a plane that would have been too fast to catch up for the allies, and having a higher bomb-load increased to a total payload of 20-25,000 lbs.

    Would it have been possible to reconfigure the Junkers 390 with eight junkers jet engines? Would this effect have increased payload and speed as I have mentioned?
     
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  2. GregP

    GregP Well-Known Member

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    #2 GregP, Jun 28, 2015
    Last edited: Jun 29, 2015
    Could have been done, but the airframe wasn't an especially high-speed airframe and early jet engines were anemic and thirsty. I don't really see the point in doing it.
     
  3. Koopernic

    Koopernic Active Member

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    I doubt it would work, perhaps as an hybrid aircraft.

    I have the Creek/Forsyth book on the Me 264 as well as Griehl’s “Luftwaffe over Amerika” (which shows how non committed many in the Luftwaffe were, especially Milch)

    What is obvious from the Creek/Forsyth book is how the powers that be in the Luftwaffe and RLM thought a hybrid aircraft was a great advance. They produced conceptual designs of hybrid Me 264 and Ta 400 with jet engines buried in the wing roots, some with swept wings.

    The advantages of this system were confirmed with several post war American hybrid aircraft such as the B-36, Neptune, Mercator.

    Basically the jet engine is relatively easy to integrate due to its low vibration, light weight, absence of a propeller weight and arc. The additional power of the jet allows a greater load of fuel to be lifted of a runway which increases range. Speed is also increased and further range increases are possible by shutting down the jets in flight. Safety is enhanced due to the redundancy.

    However the Ju 290 /390 is not likely to be adaptable as a jet. It’s thick fuselage and wings for instance will create drag at high speed.

    A wing has two sources of drag:
    1 Induced drag, which is the drag that comes from creating lift, it is usually about 100 units of lift per 1 unit of drag.
    2 Parasitic drag also called form drag which is the drag that comes out of the wing but does not produce lift. It also comes from the fuselage of course.
    3 The above figures are usually offered as a single coefficient of drag of combined parasitic and induced drag. It is fairly linear but as the stall at CLmax is approached drag increases disporportionatly, which is why a good wing area can help a fighter.

    A thick curvy wing can usually offer higher coefficients of lift. However as speed increases the parasitic drag of the wing goes up with the square of speed. The drag induced from lift does not go up because the angle of attack is reduced to produce the same lift.

    Thus at a certain point a thick wing has so much drag its overall lift/drag ratio looks very poor and also the effects of compressibillity. This is the reason the P-51 did so well, its wing didn’t have the highest lift/drag ratio and CLmax but at high speed its Lift to Frag ratio was superior to that of conventional aircraft and so it could manoeuvre better.

    The ultra-long range jet proposals that show up in Luft46 offered ranges similar to the piston engine aircraft even with the thirsty jets. Why is that?

    The reason is that jet engine does not loose efficiency at high speed nor at high altitude. The prop doesn’t work near sonic speeds while the engines exhaust pressure or crankshaft is being used to power a supercharger rather than to propel the aircraft.

    A jet despite a small or thin or low aspect ratio wing can simply fly faster to gain the lift it needs to fly at high altitude because its engine doesn’t lose efficiency, only power. Once at high altitude the thin air means there is far less parasitic drag on either the wing or the fuselage and long ranges are possible.

    You’ve created a potential take-off problem but technology comes to the rescue. There were advanced Fowler, double slotted, travelling flaps possible. And apart from leading edge slats German researchers did much work on what are called Krueger Flaps (developed by Rudiger Kosin who designed the Ar 234) and most importantly the nose flap or leading edge flap (also called droop snoot in the USA). This type of flap doesn’t work well with already thick wings but it has a tremendous effect on thin wings and swept wings effectively making them equal to thick wings when deployed.

    The Ju 290/390m with its thick transport fuselage and wings really wasn’t so well adapted to take advantage of jet engines, however with a new wing it might be possible I feel as a sort of ‘wide body. It might work better as a hybrid, duel prop jet.

    Big wings benefit jets as well, they can with such a wing fly at 50,000ft or more as was seen in British post war bombers like the Vulcan and Canberra. At this altitude the drag is even less. The Germans had this type of design as well for their Luft46 proposals, hence the Luft46 flying delta wings.

    Speed is of the essence. The days of bombers being able to survive a few hits from machine guns were over. The effect of 4 x 20mm guns on an bomber is truly horrific, I’ve read of the accounts of what a Beaufighter or Mosquito could do to a He 177 or Do 217.

    Armour piercing rounds that smash through most armour, incendiaries that smash into fuel tanks and set them into an immediate explosion and explosive rounds that rip of high chinks of aircraft skin and structure. Best not get intercepted in the first place.

    Ju 290 used in the maritime reconnaissance roll could avoid this. They had a Hohtenweil radar that could easily range to 80km and beyond, no only would they see individual ships in the convoy they could see aircraft taking off from the escort carriers. Hence the could just avoid these aircraft. Ju 290 mad regular zig zag missions over the Bay of Biscay and avoided interception this way. In any theoretical mission such aircraft could help avoid picket ships and aircraft carriers.

    The equation that relates power to thrust is as follows
    P=F.v Where:
    P is in Watts
    F in Newtons (about 0.1kg)
    v in meters per second.

    Thus if our Me 262 is producing 2 x 8000N thurst at sea level and has a speed of 250m/s (500mph) then it has a power of 4,000,000W ie 4000kW or about 5400hp ie 2700hp at the prop per engine, possibly equal to 3300hp per engine.

    The faster you fly the more apparent power you have.

    The equation can be reordered for a piston aircraft F = P/v thus the faster you go in a piston engine aircraft the less thrust you have, whereas a jet stays constant.
     
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  4. fastmongrel

    fastmongrel Well-Known Member

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    It would have been an awesome sight taking off but it would have had an epic fuel consumption. Still going to be a big lumbering not quite so slow target.
     
  5. Shortround6

    Shortround6 Well-Known Member

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    For a simplistic graphic:

    TotalDragVsJet&Piston.gif

    You might get a Ju 390 to "fly" with 8 early jet engines, getting it off the ground was the problem. It's normal take-off weight was about 8 times that of a Me 262's normal take-off weight. and the 262 was not noted for either a short take-off run or low speed acceleration.
     
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  6. DerGiLLster

    DerGiLLster Member

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    Thank you Koopernic! You answered my thread and another question that has been bothering me. How to convert thrust into horsepower!

    Also, what if the Ju 390 had six Walter rockets to assist in takeoff? It had a durable thick wing-frame, I would assume it would be able to take the stress of rocket takeoff.
     
  7. DerGiLLster

    DerGiLLster Member

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    Thank you for the visual Shortround6, it will be a great reference for whatever I would like to work on in the future.
     
  8. Koopernic

    Koopernic Active Member

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    #8 Koopernic, Jun 29, 2015
    Last edited: Jun 29, 2015
    I'm sure you are right, the world of bomber versus fighter was changing

    The Ju 290A5 wasn't particularly fast, about the same 280 mph performance league as a Lancaster but its larger brother the Ju 390 was fairly sprightly at 314 mph with a range of 6000 miles. S

    Both versions if pressurised and equipped with the turbo charged BMW801TJ were expected to be have an impressive fast cruise at 11000m. Thin air forgives a big fuselage. This is the Ju 290B or Ju 390C

    As you say, it would have been a target. It would have been satisfactory only for maritime patrol, by 1945 or 46 there might have been some standoff missiles that would give it some assault capability. Passive radar homing glide bombs such as the BV-246 which one might expect to get active radar homing like the USN's BAT missile or TV guidance. The range of these missiles, well in excess of 100km might work on some targets.

    I think its main use would have been to support the Type XXI U-boats.
     
  9. Koopernic

    Koopernic Active Member

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    Unfortunately I'm working overseas and don't have my library, I believe Ju 290's were tested with RATO which helps get a big load of fuel of the ground and Ju 390V1 perhaps as well.

    I don't know the thickness/chord ratio, it may be quite low given how large a wing chord the Ju 290/390 had. In general fast jets need wings in which the thickest portion is at about 45% to 50% not 20% or so that was common on say the spitfire. However there you have and aircraft with the thickest point at 20% which nevertheless had good high speed performance due to overall thinness.

    The Ju 390 had 6 piston engines so I suspect that 6 pair (12 engines) podded as in the Arado 234 would work. A BMW003 is only 600kg yet an BMW801 with prop might have been twice as much.

    Thrusts of a Jumo 004B at 10000m was about 380kg, the BMW003 about 11% less so about 340kg. 12 engines would provide 9.6 tons take-off thrust and 4.08 tons at 10000m. specific fuel consumption is about 1.3 so fuel consumption at takeoff power would be 13 tons an hour and about 5.2 tons at full power at 11000m.

    At 440mph (200m/sec) this is the equal of 8000kW/11000hp. So this is overpowered compared to even BMW turbocharged version.

    No one cruises at full power but assuming 16 tons fuel it would have a range of 1600 miles or 2600km at 440mph or so, maybe 10 less to allow for climb.

    This is a very crude guess.
     
  10. SpicyJuan11

    SpicyJuan11 Member

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    Koopernic, how much of a speed increase would the 290/390 with Jumo 222'z have?
     
  11. Koopernic

    Koopernic Active Member

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    #11 Koopernic, Jun 30, 2015
    Last edited: Jul 1, 2015
    Most web pages give the speed of the Ju 290V1 as 314mph with 1700hp BMW801G2/D2 engines. The aircraft flew in 1943 and that was certainly the maximum power available at that time. It's possible these ratings are for the more powerful BMW801E but I doubt it.

    Assuming the Jumo 222A2/B2 had the same full throttle height and drag and that the extra weight has not effect (a fair assumption) the speed increase would be
    314 x (2500/1700)^0.33 = 357mph.
    Assuming 1900hp had been available 344mph.

    The Jumo 222E/F at least 10% faster again due to the operation in thinner air, about 392mph.

    The aircraft had much lower wing loading than the B-29. With the Jumo 222E/F or BMW801TJ it would have good altitude and speed.

    The Ju 290 has its origins in the Ju 89 bomber which was like the B-17 a low wing monoplane. The bomb bay would have to go between the wing spars as on the B17, He 111. I do not see that as a problem for a long range aircraft. A B17 bomb load out might be 6 x 1000lbs and surely the bigger Ju 290 could do better in terms of size and number of bombs. Nevertheless the Ju 290/390 was I believe to receive a pannier for carrying bombs. I suspect since the pannier could be used to carry a ventral gun it would allow the elimination of the ventral gun gondola leading to a minimal increase in overall drag.

    A typical B17 load out was load 6 x 1000lb, the Ju 290 surely could have carrier more and bigger bombs if it had been given an internal bomb bay. However if you can make a pannier act as a ventral gun station then it makes sense to go with the pannier.
     
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  12. SpicyJuan11

    SpicyJuan11 Member

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    Thank you very much, Koopernic. You say that the bomb bay would not have been a problem at long ranges, but what about shorter ones? Also, could bomb bays have been added in the engine nacelles as the Fw 200 used?
     
  13. Koopernic

    Koopernic Active Member

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    #13 Koopernic, Jul 1, 2015
    Last edited: Jul 1, 2015
    I suspect for ultra long range missions the 3-5 tons of 500kg to 1000kg bombs that might fit between the wing spars of a Ju 290/390 is as much as can be carried in weight anyway.

    In terms of short range missions, I suspect that there would be no opportunity to do that. On the Ju 89 they were planning vertical suspension of the bombs as was used in the Ju 52 and He 111. The B-17 had them in stacked vertical rows.

    The bomb pannier on the Ju 388K over the Reconnaissance version Ju 388L had maybe a 3.5 % reduction in speed at most. In the case of the Ju 390 the pannier would allow the elimination of the gun gondola as it could incorporate the guns, so this is not a bad solution as the net gain in drag is offset by the elimination of the gondola.

    The Ju 290E was the version with the bomb panier: was to carry 40000lbs about 18 tons.

    The Ju 390 actually had fantastic performance in all aspects: range, speed, operational ceiling bomb load and armament.

    In its roll as a reconnaissance aircraft, equipped with radar, it could have executed its job well easily using its radar and speed to keep out of harms way while carrying a heavy punch. The Ju 290A7 had twin 20mm tail guns and the Ju 390C's would have received British style tail turrets only with 20mm cannon and heavy armour to protect the tail gunner.
     
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  14. SpicyJuan11

    SpicyJuan11 Member

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    I'm not sure if I quite catch you there. Perhaps as a maritime patrol aircraft, but as a bomber it was outclassed by its competition whether it be on paper (Ta 400, He 277), or actually flying (Me 264, B-29), not just in bomb-load but also being a full 35-40 mph slower than all of them, and having a measly service cieling of 20,000 ft, compared to the average of 30,000 of it competitors.
     
  15. Koopernic

    Koopernic Active Member

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    The Ju 290 or 390 when fitted with the turbo charged BMW801TJ or TQ would have had superb speed and altitude performance. This engine did see service in some Ju 88S3 and a few evaluation Ju 388L1 however the speed of the Ju 390 of of 314mph with plane jane mechanically supercharged BMW801 is quite good. The Ju 290 or 390 (non turbocharged non intercooled) can't compete with a B-29 (turbo charged) however an turbo charged BMW801 was available and could no doubt be fitted with relative ease. Similar performance could be expected from the Jumo 213E1 that was used on the Ta 152H and Ju 88G7.

    Non of the above aircraft would in my view be survivable. One aircraft which might have been survivable was the Ju 488. Black Cross Ju 288/388/488 gives speed estimates for the Ju 488, they seem initially unimpressive (low 400+ mph range) until you realise these are cruise not maximum speeds, its range was however less.

    The Ju 390 had a big advantage over the He 277 and Ta 400. In that it could be actually gotten into production very quickly.
     
  16. SpicyJuan11

    SpicyJuan11 Member

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    How much better? Because I'm sure if you give the other bombers Jumo 222's or BMW 801TJ's they'll also have a speed and altitude boost while not having the primary disadvantage of the 390: a low max internal bomb load.

    Without escorts I agree, but they do have some pretty nasty defensive armament.

    The Me P. 1075/1085 and Ta 400 would have had the same or high cruising speeds although not only does this require jets and would take longer to get in service. Why was the Ju 488's cruising speed that high? Was it the Jumo's or the a/c itself, because I'm wondering how fast the other projects would have been with it.

    For the Ta 400 that is absolutely correct, for the He 277 less so as it was based largely on the He 177/177B. What about the Me 264, would it have been quicker to get the 390 in production as well?
     
  17. kool kitty89

    kool kitty89 Well-Known Member

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    Would there be any possible use of auxiliary jet engines applied for take-off boost? From my understanding the advantages of non-jettisonable JATO units (be it rocket or turbine) was relatively high thrust relative to weight and static drag added and high fuel consumption being a non-issue given they weren't intended for sustained use in-flight.

    But perhaps the relatively poor thrust to weight ratio and fuel consumption of the Jumo 004 or even BMW 003 would preclude it from such applications.
     
  18. SpicyJuan11

    SpicyJuan11 Member

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    #18 SpicyJuan11, Jul 2, 2015
    Last edited: Jul 2, 2015
    Yes. The jets in the Ta 400 and various Me 264 projects would also be used for take-off, as for the designs that did not incorporate jets, they'd use the small and efficient Walter RATO's.
     
  19. kool kitty89

    kool kitty89 Well-Known Member

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    I still wonder if pulse jets would make decent JATO units. You'd need to address vibration issues, but that might be somewhat less problematic for short-term only use. Static thrust should be improved if the jets were within range of the prop wash as well. (expendable jettisonable pulse jet units should also be cheap enough to just throw away rather than worrying about recovery chutes as walter rockets used)
     
  20. SpicyJuan11

    SpicyJuan11 Member

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    I'm not sure about that, I know that there were plans to equip Arado 240's and Me 410's with Jumo 004's and there were trials with a Ju 88 with a Jumo 004 pod underneath.
     
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