Airbus refuelling boom - hey, it works!
"The INTRODUCER"
Thread Starter
Ah, the old "getting a liquid to run downhill in a pipe" routine.
Well tricky is that.
Well tricky is that.
The USAF's boom-mafia are some of the keenest empire-builders around. Theirs is an essentially simple task fluffed up to appear mind-blowingly complicated.
If you have a spare evening, locate one of the 1950s era Flight Refuelling old hands and sit back for an entertaining monologue.
Making fuel run downhill on demand (and stop when you want it to) is non-trivial.
Making fuel run downhill on demand (and stop when you want it to) is non-trivial.
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Why the fuss?
Good question LJR. Airbus and others have achieved a lot to demonstrate AAR tankers. I think you have to appreciate the two different aspects of what is being done in relation to all the new generation of tanker aircraft. Firstly, can receiver aircraft make or gain contact with the tanker and receive fuel. Well over the years we know the answer is yes. It might be the lazy way where some highly paid extra crew member steers a computer controlled boom into a contact receptacle on some aircraft that is positioned somewhere near the tanker, or it may be the result of an incredibly skilled pilot flying to Red Arrows tolerances to gently insert his probe at the right speed where it is meant to be... 
Not much news in that.
The second aspect is the tanker, and it is here that the demonstrations are so important. This is a very complex area, but let me scratch the surface.
Refuelling equipment. This has to demonstrated and tested to show it can, for a boom, extend and "fly" in the range of receiver movement relative to the tanker, and absorb changes in relative positions. For hose systems, the hose must fly in a stable manner, and the trailing system must be able to "fly" the hose in such a manner to absorb the contact relative speeds (3 - 5 knots) and then continually balance the drag from the drogue and receiver against the "pull" from the trailing mechanism. Moreover, it must be able to do this over a range of IASs dependant on the receiver type, and over a range of heights. Many designs have tried to do this, but only FRL in the UK, and Sargent Fletcher in the US, later bought out by FRL, produced equipment that could do this reliably. The VC 10 and Tristars took existing equipment designs that were well proven and went into service. The new generation tankers have to have completely redesigned hose/boom equipment, and therefore their demonstration is key to entering service.
Aircraft systems. Fuel. A commercial fuel system design has a simple requirement - get the fuel at the right pressure to the engines whilst maintaining the COG. In a tanker, large quantities of fuel are moved around, COG is critical, additional fuel system piping is required, the system needs higher capacity tank pumps, and the whole tested for its capability to absorb surge pressures in the air higher than the basic aircraft fuel system. In addition, there may be a requirement for additional fuel tanks requiring not only ground rig testing, but also air testing.
Aircraft systems. Power. Not much to say here, but AAR equipment needs a lot of power to operate the hose winches or boom "flying" controls. Thus any civil design has to be shown to have sufficient power capacity, and power reserves, to operate the equipment and additional fuel pumps.
Aircraft systems. Avionics. Tanker aircraft require additional avionics capable of operating without affecting the aircrafts normal navigation and communications systems. These have to be demonstrated.
Airframe structure. A lot of standard airframe structure has to be cut, strengthened, or new built in, when an airliner is turned into a tanker, and new stress paths and airframe lives re-calculated. This can be straight forward as with the VC 10, or DC 10 and KC 10, or impossible. The Tristar was never modified for the carriage of pods - although it was purchased to be a three point tanker, because a total rebuild of the outer wings and splitting the ailerons would have been necessary. The cost of this and the re-certification of the aircraft would have cost more per airframe than the original cost of the aircraft by several orders of magnitude.
Aerodynamics. Critical to successful AAR is the "smooth" airflow around and in the receiver volume of movement. Critical areas of "no go" have to be determined. Despite careful CFD, the prediction of actual airflow direction and speed behind the various parts of a tanker is not a defined art. In other words, the only way to be sure is to suck it and see under very careful test conditions, not only for the centre-line positions, but also the wing pods. The Victor had strong wing tip vortices that had to be avoided at all costs. (cv the Buc/Victor mid air collision) The VC 10 had predictable airflow round the pods, but tended to "hold in" receivers on the centre-line hose. As noted above, despite limited air trials, the Tristar never had pods fitted or demonstrated, but there were grave reservations about the suitability of an aircraft with active ailerons being suitable for wing pod refuelling.
So ... long post, but despite the plethora of civil aircraft designs being converted for tanker purposes, I have no doubt that each one deserves very careful testing, and the actual demonstration of fuel transfer in the air. Boeing and Airbus have now done this.
Not much news in that.The second aspect is the tanker, and it is here that the demonstrations are so important. This is a very complex area, but let me scratch the surface.
Refuelling equipment. This has to demonstrated and tested to show it can, for a boom, extend and "fly" in the range of receiver movement relative to the tanker, and absorb changes in relative positions. For hose systems, the hose must fly in a stable manner, and the trailing system must be able to "fly" the hose in such a manner to absorb the contact relative speeds (3 - 5 knots) and then continually balance the drag from the drogue and receiver against the "pull" from the trailing mechanism. Moreover, it must be able to do this over a range of IASs dependant on the receiver type, and over a range of heights. Many designs have tried to do this, but only FRL in the UK, and Sargent Fletcher in the US, later bought out by FRL, produced equipment that could do this reliably. The VC 10 and Tristars took existing equipment designs that were well proven and went into service. The new generation tankers have to have completely redesigned hose/boom equipment, and therefore their demonstration is key to entering service.
Aircraft systems. Fuel. A commercial fuel system design has a simple requirement - get the fuel at the right pressure to the engines whilst maintaining the COG. In a tanker, large quantities of fuel are moved around, COG is critical, additional fuel system piping is required, the system needs higher capacity tank pumps, and the whole tested for its capability to absorb surge pressures in the air higher than the basic aircraft fuel system. In addition, there may be a requirement for additional fuel tanks requiring not only ground rig testing, but also air testing.
Aircraft systems. Power. Not much to say here, but AAR equipment needs a lot of power to operate the hose winches or boom "flying" controls. Thus any civil design has to be shown to have sufficient power capacity, and power reserves, to operate the equipment and additional fuel pumps.
Aircraft systems. Avionics. Tanker aircraft require additional avionics capable of operating without affecting the aircrafts normal navigation and communications systems. These have to be demonstrated.
Airframe structure. A lot of standard airframe structure has to be cut, strengthened, or new built in, when an airliner is turned into a tanker, and new stress paths and airframe lives re-calculated. This can be straight forward as with the VC 10, or DC 10 and KC 10, or impossible. The Tristar was never modified for the carriage of pods - although it was purchased to be a three point tanker, because a total rebuild of the outer wings and splitting the ailerons would have been necessary. The cost of this and the re-certification of the aircraft would have cost more per airframe than the original cost of the aircraft by several orders of magnitude.
Aerodynamics. Critical to successful AAR is the "smooth" airflow around and in the receiver volume of movement. Critical areas of "no go" have to be determined. Despite careful CFD, the prediction of actual airflow direction and speed behind the various parts of a tanker is not a defined art. In other words, the only way to be sure is to suck it and see under very careful test conditions, not only for the centre-line positions, but also the wing pods. The Victor had strong wing tip vortices that had to be avoided at all costs. (cv the Buc/Victor mid air collision) The VC 10 had predictable airflow round the pods, but tended to "hold in" receivers on the centre-line hose. As noted above, despite limited air trials, the Tristar never had pods fitted or demonstrated, but there were grave reservations about the suitability of an aircraft with active ailerons being suitable for wing pod refuelling.
So ... long post, but despite the plethora of civil aircraft designs being converted for tanker purposes, I have no doubt that each one deserves very careful testing, and the actual demonstration of fuel transfer in the air. Boeing and Airbus have now done this.
LJR,
The Airbus boom was cleared some time ago on the A310 testbed.
The significance of this is that EXACTLY the advanced boom proposed for the USAF has now been shown to work on almost exactly the version of the A330 that is proposed for the USAF.
That is not true of its competitor.
The Airbus boom was cleared some time ago on the A310 testbed.
The significance of this is that EXACTLY the advanced boom proposed for the USAF has now been shown to work on almost exactly the version of the A330 that is proposed for the USAF.
That is not true of its competitor.
All Airbus tankers use Cobham Mission Equipment (FRL as was!) AAR pods; the A330 FSTA will also use a Cobham centreline hose as will the KC-45A.
Boeing's 767 doesn't. Mistake....big mistake!
Boeing's 767 doesn't. Mistake....big mistake!
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Why did we not go for a dual boom and drogue tanker with FSTA? Surely that would have given flexibility in allowing tanking of all NATO airctaft. Also it would give huge flexibility and potential savings in future purchasing of fighters, UCAVs etc
Cost, for one thing.
It is simpler to fit a fighter such as an F-16 with a system such as the Sargent Fletcher 370-Gallon Aerial Refueling Tank/System rather than add the weight, drag and complexity of a boom to an existing tanker.
The US has plenty of boom equipped aircraft for its large receivers; the RAAF's KC-30A has a boom system to meet the requirements of its national large aircraft receivers.
Whilst large offload rates and quantities might be more suitable for large aircraft, the demands of modern fighter aircraft formations are better met with the multi-point hose and drogue system. Not that you'll get the USAF admitting that.....
It is simpler to fit a fighter such as an F-16 with a system such as the Sargent Fletcher 370-Gallon Aerial Refueling Tank/System rather than add the weight, drag and complexity of a boom to an existing tanker.
The US has plenty of boom equipped aircraft for its large receivers; the RAAF's KC-30A has a boom system to meet the requirements of its national large aircraft receivers.
Whilst large offload rates and quantities might be more suitable for large aircraft, the demands of modern fighter aircraft formations are better met with the multi-point hose and drogue system. Not that you'll get the USAF admitting that.....
