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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    Exactly,, those who continue to proclaim that a fully loaded Su-33 or J-15 will fly away off the ramp really don't understand the process or the physics.
     
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  2. Intrepid
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    Intrepid Senior Member

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    There is a small flaw in your drawing: For the same speed, mass and flight vector, both planes should have the same pitch angle (or better, the same angle of attack relative to their flight vector).

    Unbenannt.png
     
  3. Obi Wan Russell
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    Obi Wan Russell Jedi Master
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    True, but I was probably trying to depict the point where the aircraft becomes fully wing-borne and can level out; apologies for not making that clear. I made that over ten years ago and I was probably in a hurry to get so much of the process on the page. It was during one of the periodic outbursts of "STOBAR AIRCRAFT LEAVE THE RAMP AT FLIGHT SPEED!" and/or "WHY DON'T WE FIT AN EMALS TO A SKI RAMP" nonsense that do the rounds of these forums every few years.
     
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  4. Tirdent
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    While that is all correct in principle, there will nevertheless be a significant difference in speed between a STOBAR and STOVL aircraft as they leave the ramp.

    Has to be, because the STOBAR aircraft must be closer to flying speed at that point for it not to violate the same minimum altitude limits during the "semi-ballistic" part of its trajectory. Unlike the STOVL counterpart it cannot rely on a lift contribution from the engine, but that's fine since using all available thrust for acceleration (especially aided by retractable chocks) provides for correspondingly greater aerodynamic lift.

    Decreased compared to a STOVL aircraft, at least the F-35B in particular? Nope.

    I don't know whether it is, potentially speaking, possible to design a STOVL fighter which beats the payload range of the Su-33/J-15. For example by removing the weight penalties imposed by stealth or increasing size to Flanker-levels. However, the F-35B definitely is NOT that aircraft - as mentioned in the carrier thread, not even when taking into account a F-14-like fuel reserve in the Su-33 for bolters. Do the math using the weights given in Blitzo's original posts and pay attention to fuel fraction at comparable payloads. And that's before we get to scenarios where bring-back is the long pole in the tent - the Su-33 has no problems at all in this regard.

    At the same time the point about MTOW compared to CATOBAR which the original source makes is admittedly misleading as well. Yes, STOBAR can get an aircraft comfortably in excess of 30t off the deck, but that neglects to consider what fraction of this total weight is payload and fuel. Launching an aircraft off the deck is not an exercise in breaking MTOW records, it's a means to an end: delivering explosives on a target!

    Using that criterion and assuming a "Su-35-ized Su-33" (slightly higher empty weight, more fuel & big thrust hike) as an indication of the full STOBAR potential, you find it approximately equivalent to a F/A-18C or Rafale M. That's a creditable showing (although the only reason it manages to just about match Rafale is the MTOW restriction due to the 75m catapults on CdG) but falls short of the Super Hornet.

    What's more, it means you need a ~19t OEW STOBAR aircraft (and correspondingly more fuel) to do the job of a ~11t CATOBAR aircraft! Effectively this at least partially negates the ship impact and cost benefits of losing the catapults too. For housing and handling an equivalent number of the bigger aircraft, a STOBAR carrier will require a larger hangar, flight deck and aviation fuel bunkerage. Essentially, you need a somewhat bigger ship!

    So no, STOBAR is not a viable replacement for CATOBAR, but all indications are that it is superior in terms of mission performance to STOVL.

    As impressive as it is in terms of an engineering achievement, the F-35B does strike me rather as a solution looking for a problem. Like I said, stealth with all its weight penalties due to internal weapons bays, coatings and large fuel capacity is pretty much the antithesis of STOVL requirements, so the very concept is insanely ambitious. To have pulled that off is a resounding success, how does it pay back though?

    The short legs mean the carrier has to approach pretty close to the enemy coast, which seems like a big risk if the enemy's defences are indeed sophisticated enough to require a stealthy aircraft. What's the use of a survivable fighter when you have to put the ship in jeopardy to use it? Yes, yes - sortie rate ("it's not a bug, it's a feature"). But what stops you from parking a CATOBAR carrier just as close to the opponent if and when conditions allow? On a 65000 ton, 280m by 73m platform I doubt CATOBAR deck ops will come with an efficiency penalty worth mentioning, anyway.

    Cost? Ok, the ship definitely gets cheaper without catapult and arresting gear, but isn't that penny-wise, pound-foolish when the airgroup becomes more expensive instead, with all the added complexity of a powered lift system? Not to mention the impossibility of providing equivalent AEW capability, no matter the money.

    The only real advantages I can see are training (a short take-off or landing is much the same on a ship as it is on land) and perhaps airframe life (catapult launches and arrested recoveries are hard on the structure). If these factors are really that critical though, maybe it's time to ask yourself if you really can afford a carrier capability in the first place rather than accepting a contrived half-way house.
     
    #34 Tirdent, Oct 22, 2018
    Last edited: Oct 22, 2018
  5. gelgoog
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    The main advantage of VTOL or VSTOL aircraft like the F-35B is that you can use them on the helicopter carriers that are widely used among US allies or in the USMC carriers which AFAIK do not have either catapults or ramps. But I personally think for a country with large resources which is building its navy from scratch it is a pointless waste of time. Like I have said here before more than once. These aircraft are IMHO an historical artifact. The UK used to use decent aircraft in its carriers like the Blackburn Buccaneer. But the UK spent so much money on the Harrier's development that they basically had to cancel a lot of aircraft designs and purchase others from abroad. Other countries, like France, had VTOL programs around the same time but decided to cancel them because they were wasting a lot of resources on these with poor results to show for it. They are supposed to have all sorts of advantages in operating from unprepared airfields, but, you seldom see them used in that capacity because they throw up a lot of dirt unless you use a prepared surface. The more power the engine has the worse the problem is. At that point, you should consider, is it worth it? You'll always have to carry that lift fan with you, while something like the Su-33 won't have to drag that weight if it gets access to an actual runway later in the campaign. I can see a point for rough field performance, or requirements to launch from highways like the Swedes do. But I think VTOL is pointless. I say this, but as a kid I thought the Harrier was really a great airplane. Then I grew up and read more about it, it was more about great pilots in the Falklands War than the airplane being great. TBH it was quite dire. The original versions did not even have a radar and could take off with minimal payload. Later on they developed VIFF and the ramps to increase takeoff payload, but it was never all that great. The USMC basically rebuilt their aircraft with all composite construction to further increase payload. It still isn't that great and you have to remember the airplane is subsonic. Even with all the improvements in the F-35B I would rather use something like a Rafale-M in a STOBAR. I think eventually this debate will become inconsequential. As IEPS and EMALS become more commonplace the traditional expense of having a CATOBAR will be diminished and STOBAR will vanish.
     
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  6. Tirdent
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    Tirdent Junior Member
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    Yeah, STOVL isn't really ever a contender if you are looking for more than a token strike and air defence capability - STOBAR can at least compete with CATOBAR up to a certain point before the aircraft size penalty becomes untenable.

    Nonetheless, the Harrier was a sensible tool for some tasks (maybe excepting the Sea Harrier and radar-equipped Harrier II+). It didn't pretend to be a supersonic multi-role fighter with added STOVL capability, so could do away with much heavy, expensive equipment. This made it an efficient CAS platform for semi-permissive airspace (once you commit to a landing operation, enemy defences should already have been rolled back sufficiently that stealth is redundant) which was/is exactly what the USMC needed/needs. With higher payload and speed than a helo it significantly raised the volume of ordnance which could be delivered on-shore.

    IMHO the trouble with the F-35B is actually that it represents a departure from this minimalist philosophy, with expensive and heavy gold-plated features which would require longer range to truly take advantage of (and which are to the detriment of its CAS payload). Perhaps this was driven by a realization that modern attack helicopters and long-endurance UAVs were increasingly encroaching on the Harrier's niche*, but the smart consequence would have been for the USMC to ditch its organic fixed-wing capability in that case.

    As for the RN? Two 65000t carriers with stealthy, supersonic aircraft that are however rendered unsuitable for missions where those capabilities might matter by STOVL - I don't get it. If CAS in relatively uncontested airspace was truly all the UK was looking for, what would seem more useful to me is four 30000t Cavour-style ships (ideally operating not F-35s but a non-stealthy, subsonic Harrier-successor). They can be in more places at once, and if the full might of a QE was needed, two could be deployed to the same theatre to join forces.

    * Nowadays helos carry Hellfire-type PGMs in large numbers and no matter how fast a jet is, it cannot react as quickly as an UAV which is already orbiting overhead. Throw-weight continues to favour jets, but maybe the time of forward-basing them on the landing ships has simply passed now, in light of these changed circumstances.

    As for EMALS/AAG, it is not at all clear to me that they will result in lower entry barriers to the CATOBAR game. France just entered into talks with the US on EMALS for its next-generation carrier, so it apparently continues to be the case that developing your own catapult solution from scratch is cost-prohibitive for nations without a huge carrier fleet. STOBAR should therefore remain an alternative for anyone not having ready access to catapult technology and balking at the cost of developing it independently. As before, STOVL has its niche for close air support of amphibious operations, but cannot adequately provide a genuine strike or air defence capability.
     
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  7. Gloire_bb
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    If you would really design STOBAR aircraft from ground up, it wouldn't look like Su-33. Su-33 always was meant to be an interim solution.
    Oh the irony of "interims".

    I actually posted both "true" contenders for this spot in other topic before.
    They were, su-27km and su-33ub. Both were very big improvements on take-off.
    P.s.su-33ub could be viewed as su-35 of the -33, though. But the scale of upgrade is obvious even to a naked eye.

    With all due respect, Mig-29k does the same just fine.
    Su-33 never was meant to be a deliverer of explosives to begin with.
    And what it was meant to do(CAP qualities), neither can match.
     
  8. Tirdent
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    Those improvements were mostly due to the superior high lift devices on the wing (particularly the mission-adaptive LE flap), which I assumed the "Su-35-ized Su-33" would get as well. More thrust to accelerate it to the same or greater speed in the same distance at higher weight (=more fuel, with some allowance for empty weight growth), better high lift devices to make sure this speed still suffices to generate the increased lift required by the greater weight.

    The MiG does not achieve the same payload/range as a F/A-18C (because internal fuel capacity is slightly lower and, notably, empty weight somewhat higher than 11t, quite apart from any MTOW limits imposed by STOBAR). It does outperform the basic Su-33, chiefly thanks to improved T/W ratio and provision for drop tanks, but these are features I credited the upgraded Su-33 with as well (2000l tanks, two wet wing hard points in addition to the centre line station, 14500kgf 117S engines).

    A strike range (well, fuel fraction, really) comparison at equivalent payload is a useful proxy for CAP endurance between aircraft which are this similar (all of them lack VG wings). The Hornet has slightly better wing aspect ratio, the MiG and Flanker have slightly more efficient engines*. Swings and roundabouts.

    * Yeah, you read that right - a consequence of bypass ratio (0.2 for the F404, 0.5 - 0.6 for the RD-33/AL-31F/117S). Some critics knock the Hornet for its short legs and attribute that to a supposedly low fuel capacity, but actually it is entirely in line with other supersonic multi-role fighters of the same weight class in this regard. The only way it looks inferior is if you compare to the A-7, which carries almost the same amount at >20% less empty weight, but that's a bit unfair (the SLUF was a subsonic, dedicated strike aircraft which makes no allowances for any other role).
     
  9. Gloire_bb
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    Larger wing, too.
    In case of su-27km - the whole point of FSW was intentionally launching aircraft well below the speed where it could fly on its own.
    With 5th gen T:W ratio it is sufficient to have even a very heavy thing flying. But Soviet Union collapsed, and for now no one really has initiative to repeat this cycle of developing specialized aircraft yet again.
    It's true.
    The point here is what mission is fullfilable at a comparable(if somewhat worse) level.
    Incurable downsides of stobar lie in other areas.
    But actual comparacement here goes in time on station at range, with all necessary avionics and armament. Toppled with the ability to dash to the position of interception ASAP(and launch weapons with the best initial energy state possible).
    Hornet is "fine" only in weapons department. And even this "fine" is a result of the end of the CW. Nothing else survived.
     
  10. bd popeye
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    bd popeye The Last Jedi
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    Huh??? to technical for me.... all I know is the best was to get ordnance laden aircraft off the ship is this! ...FF this video to 1:22;





    kick the tires..and light the fires!..up to 74,350 lb (33,720 kg) shot into the sky in 280ft(85m)
     
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