STOBAR aircraft Ski-jump performance (SU-33/J-15)

Discussion in 'World Armed Forces' started by Bltizo, Sep 1, 2016.

  1. Air Force Brat
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    Yep, somebody else is trying to get esoteric and wonky, lovin on those little hills on the front of those little ships,,, I think they got that idea from the Wright Brothers, launching those gliders off the dunes of Kill Devil Hills, yeah, them dam things can kill ya!

    Anyway the F-35B is far more efficient at carrying a load off the ramp than the Su-33 or J-15, those are big airplanes that fly off "under gross", very moderate load and about 1/2 fuel,, and even though it has much higher specific fuel consumption than the AL-31's, theres only one F-135 engine, and the SU-33's/J-15's, take a great deal more space on the boat, particularly the elevator... and the hangar deck....

    I think I saw two Bravo's on the QE elevator....
     
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  2. Tirdent
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    Tirdent Junior Member
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    Only by about 5% (Su-33UB), which might well mean wing loading didn't change at all compared to the basic Su-33 due to the weight increase driven by the new side-by-side cockpit section and the larger aerodynamic surfaces themselves. Bear in mind that the basic Su-33 is already better than the MiG-29K in this regard.

    On the Su-27KM (note to readers unfamiliar with Sukhoi's designation mess: this was NOT a true Flanker-variant but a navalized Su-47!), wing area was quite a bit less - making FSW and interesting weight saving measure, same performance at lower area/weight. Though the real-world feasibility of the FSW concept has yet to be validated of course, and it would likely be a cost-prohibitive path to follow for a post-Soviet Russia.

    That's just the point though - T/W ratio is a driver of aircraft empty weight (so is stealth)!

    Consider the following thought experiment: let's take the F/A-18C and look at the changes we might need to make in order to turn it into a STOBAR aircraft still capable of carrying the same payload weight over the same range.

    First of all, we'd need bigger engines to do away with catapult assistance -> higher empty weight (due to the engines themselves being heavier and structure beefed up to take the greater engine weight & thrust), even though the *ratio* of thrust to weight ends up improving.

    To maintain the same fraction of fuel weight to total take-off weight and account for the bigger engines (to meet the same range spec), we now need to increase fuel capacity however. That means bigger internal tank volume as well as structural strengthening to withstand the greater loads, further pushing up weight.

    That in turn means our wing loading is out of whack, we need more wing area -> yet more empty weight.

    But wait! Now weight has grown to the point where we need even bigger engines, so return to the first point - rinse and repeat.

    After a couple of iterations we'll be able to make ends meet again, but the resulting aircraft will be way heavier than (and look considerably different to) the F/A-18C we started out with, although it doesn't deliver more weapons or fly any further. All we've done is to get rid of the catapult, but weight is a slippery slope in this regard - we've only been able to succeed by, in essence, reducing the fraction of weapons weight to total take-off weight (and since weapons weight is constant, that meant increasing aircraft weight).

    Such as?

    Other than the considerable weight penalty to maintain the same payload/range (which is the root cause of the other drawbacks: more deck and hangar area required to handle the bulkier aircraft, greater fuel bunkerage needed to support the same number of sorties before the carrier needs replenishment), STOBAR actually looks pretty favourable. No catapults required (saving considerable development cost, ship mechanical complexity and ship operating cost), likely shorter launch cycle time (no catapult preparation), MUCH higher bring back weight than STOVL...

    Time on station at a given distance from the carrier with a given number of missiles is driven by the same considerations as range with a given payload. While orbiting, the aircraft is still moving, even though it doesn't cover any distance - the optimum speed is slightly lower, but more fuel per weight still means more endurance in the same way that it means more range. What avionics does the F/A-18 lack in your opinion? It has a good look-down-shoot-down radar with track-while-scan capability, a data-link (recently) and can carry a comparable number of missiles:

    [​IMG]

    This configuration has a very slightly better fuel fraction (0.322 to 0.318, accounting for a possible drag penalty of those dual launch rails) as a basic Su-33/J-15 with 10 MRAAM and 2 SRAAM.
     
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  3. Air Force Brat
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    I'm assuming you're talking about the Su-47 prototype with the "forward swept" wing,, (prolly never gonna be a real airplane?, not even sure its still airworthy?),

    and your assertion is that the F-18 will work as well off the ramp as the Su-33/J-15?? I absolutely agree that it would

    We have never seen a picture of an Su-33/J-15 operating off either boat, Admiral Kuznetsov/Liaoning, with a load out pictured above? the weapons load is always balanced with sufficient fuel for a mission?

    I rather doubt the Super Hornet would carry that load out off the ramp with significant fuel for a mission??
     
  4. Tirdent
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    Tirdent Junior Member
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    STOBAR can launch a 32000kg Su-33 (and potentially speaking a 34500kg improved Su-33), although it takes a 195m run :)

    The big difference: with a catapult, around 9% of that weight might be weapons while with STOBAR it's only about 6%, so with STOBAR you need ~1.5x the aircraft+fuel weight (and cost) to deliver the same weapons on the same target.

    Nope, the F-35 doesn't match the Flankers - if it launches with the same fraction of weapons weight to total take-off weight, fuel fraction (and hence range/endurance) will be worse. With the same fuel fraction, weapons load will be lower. This assumes a launch from the rear, 195m position for the Flanker, but IIRC the F-35B uses a ~200m run on QE as well (precluding simultaneous landing ops in both cases), so that's fair enough.

    As I said, I'm not sure if it is theoretically possible to engineer a STOVL aircraft which provides comparable performance by removing some of the limitations inherent in the F-35B (smaller size, stealth), but the point remains that it falls short.

    The F135 should be a bit more efficient than the AL-31F: same bypass ratio, comparable or higher overall pressure ratio and much higher turbine entry temperature. Specific fuel consumption is independent of the number of engines BTW, it's just fuel burned per thrust delivered. Given the same specific fuel consumption and total thrust, two engines with half the thrust require the same amount of fuel as one big engine.

    As for the size problem, yes, that's the big disadvantage of STOBAR, but the comparison with the F-35B is skewed (the Lightning is smaller than the Flanker, but it can't deliver the same performance either).
     
    #44 Tirdent, Oct 27, 2018
    Last edited: Oct 27, 2018
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  5. Tirdent
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    Tirdent Junior Member
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    Yes, that's the one I mean, and no, it will definitely never be a real aircraft. Too risky and costly, as I said.

    No, the Hornet would NOT work as well as a Su-33/J-15 off the ramp (too little power, too little weight margin for payload and fuel).

    It does work better off a catapult than the Flankers do off the ramp UNLESS you upgrade the latter (then they manage to match the F/A-18). In other words, the 19t (empty) Flanker gets no more load off the deck than the 11t Hornet, and then only if it receives the Su-35 treatment (more fuel, improved thrust) and some other fine-tuning measures.

    That's the advantage of CATOBAR - you get away with much smaller, cheaper aircraft to do the same job. OTOH the prevalent Western attitude that STOBAR imposes severe limitations of the same magnitude as STOVL is a misconception. If you go out and design a sufficiently large and powerful aircraft you can get a level of performance which is quite useful (comparable to F/A-18C or Rafale M) and beats any STOVL aircraft built to date.



    For kicks, here's a Su-33 successfully taking off from one of the forward (!) spots with either the parking brake mistakenly still set or a brake system failure which causes the wheels to lock up:



    No payload, but the friction from those skidding wheels is going to equate to a lot of (virtual) weight added!!! Not something I suspect some Western critics of STOBAR would believe possible.

    Certainly not, even the SH is too small.
     
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  6. Tirdent
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    Tirdent Junior Member
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    Since this thread is about the technical aspects of STOBAR, that launch with brakes set demonstrates quite graphically why the retractable chocks are necessary to hold the aircraft back before launch. The brakes will not be able to keep it stationary while the engines spool up to full afterburning thrust because the coefficient of friction between the tires and deck is significantly below 1.0, and maximum braking force is that coefficient times normal force (i.e. aircraft weight).

    With a T/W ratio sufficient for STOBAR take-off, thrust will exceed the available braking force before full power is reached, as even on a modern jet engine like the AL-31F the spool up time from idle is several seconds. If it wasn't for the chocks, the aircraft would start moving before maximum thrust is achieved and cover part of its take-off run at a lower acceleration, resulting in a less speed by the time it leaves the end of the ramp.

    So that the Su-33 in afterburner gets going even with wheels locked after the chocks are released should not come as a surprise, that it makes it to flying speed is impressive though.
     
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  7. Gloire_bb
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    Gloire_bb Junior Member
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    Su-47 was even further from the real thing than ATF prototypes were, and appeared long after Russian navy lost these ambitions.
    It used availible elements as much as possible.
    IMG_20181030_113224_574.jpg
    The closest we have to the real thing is this leaked model.(there was another dubious model later on, but it is unknown to be unintentional)

    Btw, note what su-47 was yet again much heavier than even the su-33.
     
  8. bd popeye
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    bd popeye The Last Jedi
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    I'd like to interject something here... I served with the US Navy for 20 years and six days. I made 7 major deployments aboard five different carriers i.e John F Kennedy, Midway, Hancock ,America & Nimitz. I worked on the flight deck of three of those ships. Never once in all those years of service did I ever hear the terms...CATOBAR, STOBAR & STOVL..Never not one time. I don't think anyone now serving with the USN uses these terms.
     
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  9. Tirdent
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    Tirdent Junior Member
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    Since the USN isn't really in the business of anything other than CATOBAR, I'm not surprised the alternatives aren't a hot topic - no reason to make a distinction.

    IIRC the acronyms CATOBAR and STOBAR go back to the deliberations in the UK defence establishment and industry on what mode of fixed wing take-off and landing the future carriers (which have in the meantime become the QE class) should use, so they are certainly 'official'. STOVL was used in this context too, but has before been associated with the Harrier in the trade press for as long as I can think. It gets even better though - the JSF/F-35 project coined a new term for conventional runway-based take-off and landing: CTOL!

    I suppose this is a long-winded way of saying that common USN jargon simply isn't the ultimate and only benchmark in this regard.
     
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