More KC-46A woes....

[KenV]

…or the 32 tonnes in the centre tank.

We considered the tanks in the center wing to be part of the wing tankage. We view that as wing structure, not fuselage structure.

[Just This Once...]

Nice try...

[tdracer]

Just This Once, are trying to say the center wing box isn't part of the wing?

[Just This Once...]

My remarks were aimed at the missing fuel volume from our resident A330 tanker expert and his suggestion that the USAF option carried less than standard A330s.

Comical Ali comes to mind.

[Davef68]

Originally Posted by KenV

And about those TriStars: fuel capacity of the -200 is 180Klb and OEW is 248Klb. So fully fueled an empty TriStar weighs 428Klb. MTOGW is 466Klb. So fully fueled a TriStar can still carry a 38Klb payload. If RAF TriStars can reach MTOGW with fuel alone, then they also must have additional tanks installed.

They did (and the RAF ones were -500s) - if my memory is correct the additional tanks added approx 100Klb of fuel capacity

[RAFEngO74to09]

Tristar KC1 MTOW & Fuel

According to this: Tristar bows out | Wings of History


MTOW 539,000 lb with 212,410lb fuel in wings and 98,285lb fuel in 2 cargo bays.

[D-IFF_ident]

Regards the 12,000 / 13,000 / 10,000 runway argument. I'm pretty sure the A330 MRTT can manage the 12,000 or 13,000 runways too.

[KenV]

My remarks were aimed at the missing fuel volume from our resident A330 tanker expert and his suggestion that the USAF option carried less than standard A330s.

Comical Ali comes to mind.

1. There is no missing fuel volume. The centerwing tank has always been part of the wing and its volume was considered. The tail tank volume was also considered, although I admitted I forgot to specifically mention it.

2. Neither you nor anyone else have provided any data indicating that the MRTT offered to USAF has less fuel volume than "standard A330s". The only data provided for the Voyager was highly suspect, showing that even at zero payload, the tanks could not be filled, a highly unlikely design. However there MAY be a simple explanation for that: OEW (Operating Empty Weight). If the Voyager has a high OEW because of modifications or onboard equipment, then it will mass out before reaching its max fuel capacity. But that seems highly unlikely. Why? Because it would mean the Voyager is flying around with over 20,000 lbs of extra weight. More llikely is that the numbers provided were erroneous.

Here's some hard data for a "standard" commercial A330s:
(Since some folks take offense at wiki data, all the data below came from Jane's, which coincidentally is exactly the same as the wiki data)

A330-300
OEW = 273.5Klbs
Fuel capacity = 175.2Klbs
MTOGW = 534.0Klbs

A330-200
OEW = 263.7Klbs
Fuel capacity = 249.8Klbs
MTOGW = 534.0Klbs

Now, lets "do the math"

"standard" A330-300
273.5 + 175.2 = 448.7 = ramp weight w/max fuel w/zero payload
534.0 - 448.7 = 85.3 = payload capacity (in Klbs) with full fuel load

"standard" A330-200 (Which is what the MRTT is based on).
263.7 + 249.8 = 513.5 = ramp weight w/max fuel w/zero payload
534.0 - 513.5 = 20.5 = payload capacity (in Klbs) with full fuel load

Imagine that!! BOTH versions can carry substantial payloads with a full fuel load, just like every ariliner ever designed. Who'd have thought?

And my goodness, when a "standard A330-200" is full of fuel, it can still carry 20.5 THOUSAND pounds of payload. I don't remember the OEW of the MRTT offered to USAF, but I'm very confident it was not 20.5Klbs heavier than a "standard A330". Indeed if memory serves it was a bit lighter.

Comical indeed.

And about that "A330 tanker expert" remark. I never claimed nor even remotely suggested that I was an "A330 tanker expert." I am however somewhat familiar with the two tankers offered to USAF and some of the criteria used to choose between the two offers. And I chose to share some of that information here. As for the various "true believers" who got their knickers in a twist.......yes, comical does indeed come to mind.

[vascodegama]

Ken

Sorry to have to tell you that my figures are correct. It may well be that the base ac theoretical weights give the possible "substantial" payload that you refer to but the figures that I have used are the practical ones. It's a bit like the theoretical max fuel of the Tristar KC1- not possible to achieve on a day to day
basis.

[KenV]

Ken, Sorry to have to tell you that my figures are correct. It may well be that the base ac theoretical weights give the possible "substantial" payload that you refer to but the figures that I have used are the practical ones. It's a bit like the theoretical max fuel of the Tristar KC1- not possible to achieve on a day to day basis.

OK, your fuel and weight figures for the Voyager are correct. So? It's still an odd design that makes it impossible to put on a full fuel load (on the other hand, if the Voyager's max ramp weight is 3t higher than MTOGW then the crew can burn off 3t of fuel before taking off. Still kinda odd though.) However a number of folks have stated that the RAF's other air tankers have belly tanks with the TriStar's belly tanks having near 100Klb capacity. A "standard" A330 cannot reach MTOGW with just fuel, and you never answered if the RAF did or did not put belly tanks in the Voyager to enable it to reach MTOGW with just fuel. Just This Once in post #93 implies that the Voyager, unlike the other RAF air tankers, has no belly tanks. If that is true, then either the Voyager's empty weight is 20Klb higher than a "standard" A330, or either the fuel capacity figures provided by both Janes and wiki are off by 20Klb, or the figures provided by you are off by 20Klb.

This much I'm very confident of: the MRTT offered to USAF did not have belly tanks and it could not reach MTOGW with just fuel. I have no idea why the Voyager is different in that regard. I also have no idea why that difference is so important to some folks that they've gotten their knickers in a twist over them. But here we are.

[D-IFF_ident]

It's a fairly common tanker design to not reach MAX FOB without exceeding MAX TOW. You can't get anywhere near tanks full in a KC-10 for example. Fuel tank capacity is a function of volume - or the space available in the airframe. For the engineers it's probably more important that they avoid the situation where MAX FOB gives less than MAX TOW.

KC45 OEW was around 130,000KG. MAX FOB was 111,000KG.

MAX TOW still 233,000KG

[BEagle]

Voyager does not have any additional fuel tanks; unlike the KC-46A it doesn't need any.

Voyager's under floor cargo area isn't compromised by additional centre tank 'plugs', whereas KC-46A's is. Hence it needs an upper deck cargo door whereas Voyager does not.

From what has been posted, typical ZFW for Voyager means that it's about 2.1T short of being able to operate with full fuel. But it can still take about 98% of max fuel. Why is it heavier than an A330-200? Consider the weight of AAR pods, FRU, camera equipment, additional avionics and DASS, the MSO's equipment etc. and there you have the answer.

I doubt whether the KC-45A would have been able to operate with 111T of fuel at start either - particularly given the weight of the ARBS.

Incidentally, the A330 centre tank is indeed considered to be a 'centre wing' tank rather than a 'fuselage' tank.

D-IFF_ident makes a good point regarding fuel volume. Airliner brochures tend to use low SG values in order to delude customers into thinking that a higher payload will be available than will actually turn out to be the case in service. Whereas a tanker manufacturer is more interested in ensuring that a high fuel mass, rather than volume, is available - so will quote a higher SG value. The A330MRTT brochure uses a realistic SG value of 0.799, whereas if the value of 0.785 as used in other brochures was quoted, 139000 litre would have a mass of 109T....and MTOW with max fuel (apart from that used during taxying) would be possible.

[KenV]

Voyager does not have any additional fuel tanks; unlike the KC-46A it doesn't need any.

Voyager's under floor cargo area isn't compromised by additional centre tank 'plugs', whereas KC-46A's is. Hence it needs an upper deck cargo door whereas Voyager does not.

Once again, "need" is in the eye of the beholder. Your eye might not see those as needs. But the USAF eye decided that a cargo door and cargo floor were priorities. The KC-10 for example (which is even larger than the A330) has a cargo door, cargo floor, and belly tanks. Why? Besides the ability to handle fully loaded military 463L pallets, the cargo door/floor facilitates medevac. Airbus not even offering them hurt our proposal. We dearly wanted Airbus to offer an A330-200F based MRTT, which had the cargo door, cargo floor, and the revised nose gear. But they did not want to move up the freighter development schedule to meet the first competition's schedule. They simply refused to offer it for the later competitions and still refuse to do so. Don't know why.

And oh yes, if Voyager has no belly tanks, then I don't know how it could possibly mass out before it volumed out. It's hard to imagine Voyager's empty weight is 20klbs higher than an A330's empty weight. I do know that the MRTT offered to USAF in the first round was very slightly lighter than the A330-200 even with the addition of all the refueling gear because it had the lightweight passenger floor, and no seats, galleys, lavs, etc. I can't remember for sure, but it may not even have had the cargo handling gear in the belly. It was highly optimized as a pure air tanker.

[BEagle]

KenV wrote:

And oh yes, if Voyager has no belly tanks, then I don't know how it could possibly mass out before it volumed out.

Do you actually understand the concepts of specific gravity and the effect of temperature on SG? Such factors may be insignificant in some little mini-jet such as an A-4, but they are highly significant in large aircraft such as the A330.

Total tank volume in the A330-MRTT is 139000 litre. Assuming you can do sums, you might like to calculate the total mass at different specific gravity values, then adjust that for temperature deviation.

Again, the USAF needed cargo floor and door in the Frankentanker because the B767's normal, somewhat limited underfloor cargo space is further compromised by the center tank plugs needed to meet the AAR requirements of the KC-X competition.

[Heathrow Harry]

" USAF eye decided that a cargo door and cargo floor were priorities"

then why not buy more freighters? tha damn thing is supposed to be a TANKER, not a glorified DC-8F

The USAF wanted a Boeing aircraft - and they're going to get one.........

and they will pay the price

[D-IFF_ident]

The KC-10 is not larger than the A330. The KC-10 measures 50 x 55 m - the A330-200 is 58 x 60m.

[tdracer]

then why not buy more freighters? tha damn thing is supposed to be a TANKER, not a glorified DC-8F

Really HH? You're really going to argue that the A330 is a better deal because you also need to buy another cargo aircraft to provide similar capability to the KC-46?
Listen, some of the USAF requirements during the tanker completion were quite frankly silly - but when we went back and tried to point out how some of the requirements made no sense in the real world, the basic response was along the line of "what part of mandatory don't you understand?" After contract award, we tried again - same response.


D-IFF_ident, I don't think aircraft footprint relates much to it's cargo carrying capability - and in that regard the KC-10 fuselage is ~1 ft. larger in diameter than the A330.

[stilton]

Lots of xenophobic attitudes here and a clear bias to the far less flexible Airbus platform from our European friends.


Fact is the 'frankentanker' is just a better aircraft for the USAF.

[Heathrow Harry]

The US will discover, as the British have, that once you have a single supplier then your negotiating position is zero - you HAVE to buy from them

And so you finish up with even bigger cost overruns and kit that just doesn't work

[D-IFF_ident]

Not sure where the xenophobia is - I can only see people putting their opinions about different airframes forward. Personally, my opinion on which is better - the KC-45 or the KC-46 is "none of the above". If I was in a position to make decisions in strategic multi-role tanker procurement I'd want at least 3 engines, cargo door, flexible cabin arrangement able to take a minimum of between 0 pallets:250 passengers to 20 pallets: 0 passengers, with a minimum combi-load of 10 pallets:100 passengers and no height compromise for strengthening the floor or having overhead lockers. I'd want a comprehensive C2 avionics suite with modular intel/sensor/MAWS/LIRCM systems, an integrated mission planning system, inbuilt W&B sensors connected to the onboard flight planning systems. I'd want a variety of options for internal configurations for Aeromed/VIP etc and fuel tanks that can be isolated so I could do the all-important fuel deliveries to keep the support staff oil heaters working. In general I'd want a self-supporting command and control, passenger, cargo, tanker, battle-damage resistant jet aircraft with the shortest take-off/landing roll possible.

In short I'd want a purpose-built aircraft, not a modified civilian airliner.

Since that option doesn't exist I'd settle for the Airbus airframe, with a Boeing boom and Cobham pods. Parker can supply the UARRSI and Eaton can sort out the plumbing.