Do We Need Aerial Tankers?

Do we really need carrier based, aerial tankers?  Your immediate reaction is, of course we do!  However, let’s hold off before we make that our final answer and take a moment to look just a bit closer at the tanker question.
 
Let’s start by answering the most basic question:  why do we have tankers, currently?  This is not a trick question.  There are two general answers:
 

• Overhead Tanking provides tanking for overhead aircraft who need a just a bit of extra fuel to get back aboard the carrier.  Perhaps the pilot had to execute one too many wave offs and go-arounds and has run just a bit low on fuel or maybe the aircraft came back from the mission low on fuel due to any number of possible reasons.  Those aircraft need fuel.
• Mission Tanking extends the reach of a mission.  WWII aircraft were limited to a range of whatever their onboard fuel tank allowed them.  Tanking is a means of extending the range of an aircraft by refueling during the mission.

 
Understanding those two basic requirements, let’s look a bit deeper and bit further into the future of tanking.
 
 
Mission Tanking
 
Let’s start with the mission extension requirement.
 
Here’s a question you may not have previously considered:  what determines the maximum range of a mission?
 
Simplistically, the range is determined by the aircraft’s unrefueled range plus any aerial refueling provided as part of the mission … but is that the whole story?  Since we’re doing a post on this, you can assume it’s not!
 
If fuel were the only determinant of range, we could, in theory, have carrier aircraft fly global missions.  The carriers could stay in port, launch aircraft, and those aircraft could strike/fight on the other side of the world;  after all, it’s just a matter of sufficient refueling events, right?  However, a moment’s thought suggests that the pilot of a single seat aircraft can’t remain awake, alert, and combat effective beyond a certain number of hours in the cockpit.  Anyone who’s driven long hours in a car understands the debilitating effect of cramped quarters even with occasional pit stops for relief, food, rest, and just to stretch one’s legs.  How much worse must it be for a pilot who, literally, is strapped in and can’t move or stretch, and struggles even to relieve himself.  At some point, the pilot becomes combat ineffective.  It’s analogous to the infantryman who quickly becomes ineffective in a landing craft due to seasickness after a brief period.
 
What is the time period beyond which a pilot becomes combat ineffective?  I don’t know – and it will vary somewhat from person to person – but a reasonable estimate is around three hours.  Beyond that point, the pilot begins to lose effectiveness.  Sure, there’s nothing like the adrenalin surge of combat to wake one up but there’s no escaping the underlying decrease in alertness, reflexes, and mental agility (which declines precipitously with fatigue).  A less than completely optimal pilot is another way to describe a dead pilot.  This is not to say that a mission longer than three hours can’t be accomplished but you’re dipping into diminishing returns at that point.  Diminishing returns is another way to describe a dead pilot and failed mission.  Modern combat requires 100% efficiency in order to have a hope of survival and success.  This, by the way, is the main reason why modular ships are inherent failures – they’re not 100% optimized.  But, I digress …
 
Let’s set aside range limitations and consider enemy threats.  Submarines, cruise and ballistic missiles, supersonic aircraft, very long range SAMs, and the like have resulted in being forced to doctrinally move our carriers further and further back from the target.  We’re now talking about having to operate many hundreds of miles away or even out to a thousand miles or more.  What does that do to the mission time frame?  Using subsonic aircraft with, say, a cruise speed of 550 mph, it would take 3.6 hours to fly a thousand mile, straight, out and back mission.  Now, throw in realistic time delays for departure assembly at the carrier, tanking, non-linear routes, in-flight refueling, actual mission execution time (air to air combat or loitering), landing pattern time, etc. and that bare minimum of 3.6 hours becomes something on the order of five hours.  Wait … what did we say about cockpit time beyond which a pilot’s performance begins to degrade?  Yeah, something on the order of three hours.  Uh, oh …
 
Returning now to the tanker issue, we can see that simply adding tankers to provide longer and longer ranges is not a correct or viable approach.  Tanking is beneficial only until it extends the mission time beyond the magic three hour limit.  After that, it becomes counterproductive.  Thus, even if we had a tanker that could deliver infinite fuel at infinite range, it would be useful only within fairly narrow constraints. 
 
The pilot’s combat effectiveness is the limiting factor, not fuel !
 
Thus, bigger, better, longer ranged tankers are not the answer beyond a certain point.
 
Note:  An almost semantic variation of the range extending, mission tanking is station time extension where we want to keep an aircraft on station for an extended time at a shorter range.  For example, an aircraft flying cap at, say, 300 miles, might need refueling to enable it to loiter on station for a couple hours even though it has sufficient onboard fuel for the 600 mile round trip.
 
 
Overhead Tanking
 
Not much to say about this.  Overhead/recovery tanking is a mandatory aspect of carrier operations.  There’s no getting around the need.
 
 
Conclusion
 
Single seat aircraft are constrained by the physical and mental fatigue limits of the pilot.  As we noted, a thousand mile mission is about the limit of a pilot’s combat effectiveness.  Thus, our attempts to design and build aircraft with combat radii greater than a thousand miles and/or to provide tankers that can extend missions beyond a thousand miles are pointless.
 
Of course, if our aircraft have only an inherent combat radius of, say, 200 miles then, yes, we need to provide tanking to accomplish a thousand mile mission.  However, we have, in the past, built aircraft with unrefueled, thousand mile radii, or nearly so, so that should be our design goal.  An aircraft with a thousand mile unrefueled radius pretty much eliminates the need for mission tanking except in the extreme of, say, maximum range, air-to-air combat which requires full power/afterburner once arriving on station.
 
The conclusion is that, yes, we most definitely need tanker aircraft but we need to be careful to recognize that we’re bumping up against pilot limitations, not fuel limitations.  This recognition should impact our tanker needs (number, size, capacity, etc.) and design.
 
 
 
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Note:  I selected a value of three hours as the point beyond which a pilot becomes ineffective.  It could be two hours, or four, or 3.187.  The exact value doesn’t change the premise and there is no exact value, anyway, since it would vary from pilot to pilot and would depend, in part, on the circumstances of the mission.  Therefore, I’m not going to entertain debates about the exact value.  Fair warning!

Combat Radius

So many discussions about aircraft include mention and claims about range.  Unfortunately, range is a very imprecise concept and range numbers are highly variable.  There’s an old saying in statistics that you can make the numbers say anything you want.  The same is true in discussions of range - you can get any range number you want by playing with the conditions. 

Let’s look a bit closer at the concept of range by looking at the specific example of an F-18E/F Super Hornet (SH).  In a recent post, a SH range figure was quoted and I expressed doubt and stated that it was a very optimistic number.  Let’s see why.

As a general prelude to this discussion, we need to agree on some terms. 

Range is, technically, the distance from one point to another – in other words, a straight line, one-way travel.  Carrier aircraft don’t generally do that.  Instead, they fly out, execute a mission task, and return to the carrier.  So, their “range” is actually a radius.  Thus, their maximum radius is half their maximum range. 

Unfortunately, many people use “range” and “radius” interchangeably.  For the rest of this discussion, we’ll attempt to use “range” as meaning a one-way, straight line travel and radius to mean an out and back trip. 

Now, let’s plunge right in and check the range and radius for the F-18E/F.  From the F-18E/F NATOPS Performance Data Manual, Specific Range Chart Fig. 5-37, Alt = 15,000 ft, Wt = 50,000 lbs, we get the following data.

Using the Optimum Cruise line at the median Drag Index gives

Speed = 456 mph

Fuel Flow = 6500 pph

Specific Range = 0.058 nm per pound of fuel

So, for a SH with a full internal fuel load of 14,400 lbs, that gives a range of (14,400 lbs x 0.058 nm per lb = 835 nm), radius = 417 nm

Calculated alternatively,

Flight time = (14,400 lbs / 6500 lbs/hr = 2.2 hrs)

Flight range = (2.2 hrs x 456 mph = 1003 miles or 872 nm), radius = 501 miles or 436 nm

We see, then, that the combat radius of the SH is about 420 nm at a cruise altitude of 15,000 ft and a weight of 50,000 lbs and using optimum cruise speed.  As a reminder, the recent post quoted the SH range figure as 450 nm.  Well, that’s almost exactly the figure we just calculated so that seems like a reasonable and valid range number, right?  Why did ComNavOps express doubt and claim it was extremely optimistic?

Well, here’s where the discussion breaks down.  The calculated range is for straight line cruising at the specified altitude, speed, and weight but how did the aircraft get to that altitude?  It didn’t just appear there.  It launched from a carrier using maximum thrust which consumes huge amounts of fuel.  It had to climb to altitude which consumes additional fuel over and above that needed to simply cruise.  So, in the real (operational) world, the fuel required to launch and climb to altitude has to be subtracted from the available fuel in our calculations.  How much fuel is that?  I have no idea – 20%, maybe?  A pilot would have to tell us or I would have to dig even deeper into the NATOPS manual.  The point is that the calculation assumes the aircraft starts at altitude and with a full load of fuel which is impossible.  So, the calculated range/radius must be significantly reduced.  That 420 nm radius now becomes 350 nm, maybe?

But wait, there’s more confusion.

The 420 nm radius assumes a straight line, unwavering, constant speed flight.  On a combat mission, aircraft don’t generally fly straight, level, and constant speed.  Typically, a combat mission will involve changes in direction to weave around known dangers (radar sites, enemy bases, etc.) and/or to approach from a direction other than straight on.  So, even if you could fly 420 nm in a straight line, if you throw in several course changes that take you off that straight line, you’ll consume additional fuel which has the effect of shortening the apparent radius. 

Further, on a combat mission the aircraft don’t fly at a constant speed or altitude.  They increase speed, especially near the target when they use fuel gulping maximum power.  They fly at different altitudes.  For example, a mission might consist of a high altitude cruise to the target, a low altitude approach, and a high altitude return cruise.  The altitude changes not only consume additional fuel but change the flight efficiency – lower altitudes are generally less fuel efficient.

So, the actual combat mission flight profile is going to consume additional fuel.  That 350 nm radius now becomes 260 nm, maybe?

Of course, if the aircraft expends ordnance at the target, the aircraft’s weight decreases which improves the fuel efficiency on the return leg.  Also, as fuel is burned, the aircraft becomes lighter, further improving fuel efficiency and that 260 nm radius becomes 290 nm, maybe?

So, we see that combat radius is totally dependent on the specific conditions of the combat mission.  The point is that the published range/radius is generally significantly unrealistic.  Nowhere is this more evident than in discussing the much-hyped and much-lied about F-35.  You know those ranges are works of fiction!

So, is that the end of the discussion?  No, not by a long shot!

The problem is further compounded by actual carrier aircraft operating procedures.  Aircraft don’t actually launch with a full load of fuel, go out, execute their mission, and return, all on one load of fuel.  Instead, the aircraft usually launch with a partial fuel load and top off over the carrier, after launch.  Additionally, they are often partially refueled during the return leg and/or at the carrier for recovery.  Making the issue more complicated is that fact that aircraft don’t usually take off and immediately begin flying straight to the target – they take off and marshal at some point waiting for the rest of the strike package to launch and assemble.  This waiting burns more fuel.  The assembly point may also include a partial refueling. 

So, what does combat radius refer to?  Is it the radius after having launched and refueled and factoring in a return refueling or two?  If so, then the combat radius can be any distance you want just by adding in more refuelings!  When the Navy cites a combat radius of xxx nm, do they mean the radius with the benefit of multiple refuelings because that’s what a typical mission consists of?

Carrier aircraft rarely fly “unrefueled” missions so citing an unrefueled radius is a combination of unrealistic, pointless, meaningless, and confusing.  Yikes!

So how do we usefully compare the combat radius of one aircraft to another?  We intuitively know that aircraft differ in their inherent “range” but how do we compare the differences?  Well, for practical purposes, we can’t.  About the best we can do is get our hands on flight manuals and extract specific range numbers (nm miles per pound of fuel) under a specific set of conditions.  This will give us directly comparable fuel efficiency numbers which will reveal which aircraft is more fuel efficient and from that we can infer, and calculate, actual combat radii.  But, even that isn’t the end of the story.

Different aircraft fly different types of missions.  An air superiority fighter flies a different mission and flight profile than a strike aircraft so trying to compare combat radius at a single, arbitrary set of conditions is pointless and unrealistic. 

Further, different aircraft have different fuel efficiencies under different conditions.  Some aircraft are optimized for lower level flight while others are optimized for higher level flight.  How do we compare those?  Again, we really can’t.  The best we can do is try to compare apples and apples.  We can’t meaningfully compare the combat radius of an A-10 low level close air support plane to the combat radius of a high altitude F-22 air superiority fighter.  We can, however, compare the F-35C to the F-18E if we specify the mission, weapons load, and flight profile – assuming we can get our hands on NATOPS flight data.

What’s the overall conclusion from this discussion aside from the realization that published range/radius values are virtually meaningless?  The conclusion is to assume the published range/radius numbers are maximum and unrealistic values that would only be obtained under unrealistic, non-operational conditions.  If you want an “actual” operational combat radius, take 60% of the published figure and you’ll be somewhere in the realm of an actual combat mission radius.

So, did that clear things up?  Yeah, I didn’t think so but it’s the best we can do.  Hopefully, it will at least allow you to more intelligently discuss combat “range” in the future and more realistically assess published values.

On a related note, this discussion makes the published range/radius values for the SH and F-35 all the more disappointing when compared to the actual Pacific theater requirements.  It also emphasizes the need for an operationally effective tanker.  Above all, though, it clearly points out the need to build aircraft that are inherently longer ranged.  When we're talking about thousand mile A2/AD zones, it quickly becomes obvious that the F-18 and F-35 are both woefully short on their useful combat radii.