Why does this refueling pod on Navy fighter jets have a propeller?

[berkeman]

I was watching some videos of aircraft carrier cat shots of fighter jets today (don't ask), and was surprised to see in a slow-motion closeup shot that one of the underwing pods on an FA-18 had a propeller spinning on its nose.

That seemed pretty strange to me, so I did some Google searching, and it looks like the pod is used for in-flight refuelling of other aircraft (?). But why would you use a propeller for powering such a pod instead of just drawing the power from the host aircraft?

http://tailspintopics.blogspot.com/2015/06/things-under-wings-inflight-refueling.html

 

[PeterDonis]

I don't know for sure but my guess would be that, since the F/A-18 is not primarily designed as a fuel tanker, putting the necessary connections to the fuel pod into the airplane would be more expensive or less reliable or both than just having the pod power itself. The hardpoint the pod is hanging from is primarily intended for weapons.

 

[DrJohn]

I suspect they can guide the pod into the refuelling basket and in older systems couldn't supply power to it. On some refuelling systems they can guide the basket onto the aircraft. Probably depends on the type and age of the system. In some older systems the pilot tries to fly into the basket thingy that hangs out the back while the basket controller just tries to hold the basket steady with control surfaces on the basket and their own control column in the tanker (they sit right at the back with a clear view of the approaching aircraft. When they connect, there is a lock of some sort to hold them there and activate the fuel delivery valves. When they are full, the inability to pump in more fuel disconnects the basket. On old systems they can break free accidentally and the fuel pump switches off automatically, and waits for them to reconnect.

All from talking to US airperson who did this, back in the mid 80s, during a tour of their military airbase. She was apparently the first female to qualify to control the basket thingy.

During said tour, in the air traffic control room, being small and at the back of a large group I couldn't see what they were talking about very clearly in a talk-through of what they did. So when the group moved on, I moved in closer for a quick refresher. And then an aircraft taking off disappeared at low altitude! So I watched them handle a real emergency while the others were a few meters away looking at another rehearsed explanation of something else, with their backs to the real action. Wasn't I a lucky person! Luckily no-one had crashed from the engine problem that caused them to drop to a very low level, literally under the radar at about ten feet AGL over lower ground and unable to climb for a couple of minutes.

 

[FactChecker]

The diagram in your link seems to indicate that the Ram Air Turbine (RAT) provides hydraulic pressure to power the fuel pump.

 

[berkeman]

I don't know for sure but my guess would be that, since the F/A-18 is not primarily designed as a fuel tanker, putting the necessary connections to the fuel pod into the airplane would be more expensive or less reliable or both than just having the pod power itself. The hardpoint the pod is hanging from is primarily intended for weapons.

Hmm, that might the the key. Maybe the hard points on a fighter are only designed for interfacing with weapons, so no power or very little power available. So to accommodate the refueling pod pump's electrical power demand, they need to add the external propeller.

What's weird is it looks like these types of pods have been in service for many years, and should have predated the design of newer fighter jets carrying them. So you'd think that the designers of the newer jets would have added the capability for the aircraft to power these refuelling pods, but apparently not.

 

[PeterDonis]

you'd think that the designers of the newer jets would have added the capability for the aircraft to power these refuelling pods

That might not have been a viable option. There isn't a lot of leeway in designing a fighter jet, particularly a carrier based one.

 

[FactChecker]

My knowledge is second-hand, but I believe that carrier catapult takeoffs and landings are hard on electrical connections. It wouldn't surprise me if this design is more robust, reliable, and versatile. And you wouldn't have to design the airplane's electrical system to have enough excess power (weight, cooling, etc.) for an occasional task.

 

[Tom.G]

And you wouldn't have to design the airplane's electrical system to have enough excess power (weight, cooling, etc.) for an occasional task.

Especially if you want to use it as an actual Fighter when needed!

 

[Vanadium 50]

Propeller adds extra burst of speed if needed.

Seriously, having the pod supply its own power makes things more modular. There are good and bad reasons to make things modular, but one is if you have half a dozen different variants of aircraft, you don't also need half a dozen variants of fuel tanks.

When fighting a war, logistics are a big, big thing. The Russians recently relearned this lesson.

 

[berkeman]

Propeller adds extra burst of speed if needed.

https://www.google.com/url?sa=i&url...ved=0CBIQjhxqFwoTCIixqO7qhP8CFQAAAAAdAAAAABAJ

 

[PeterDonis]

Propeller adds extra burst of speed if needed.

As I understand it, the propeller is to supply power to the pod, not to draw power from the pod to move the plane.

 

[Vanadium 50]

Was joke.

The clue was the next word, "seriously".

 

[berkeman]

Beep beep.

 

[hutchphd]

I've always heard it as meep-meep.

 

[Frabjous]

I've always heard it as meep-meep.

from https://en.wikipedia.org/wiki/Beep,_beep_(sound)#:~:text=Although commonly quoted as "meep,hmeep hmeep" or "mweep,

Although commonly quoted as "meep meep", Warner Bros., the current owner of all trademarks relating to the duo, lists "beep, beep" as the Road Runner's sound, along with "meep, meep." According to animation historian Michael Barrier, Julian's preferred spelling of the sound effect was either "hmeep hmeep"[2] or "mweep, mweep".

 

[Lnewqban]

The diagram in your link seems to indicate that the Ram Air Turbine (RAT) provides hydraulic pressure to power the fuel pump.

 

[FactChecker]

View attachment 326836

There might be a significant safety benefit to having the internals of the fuel tank run by hydraulic rather than electric power.

 

[Lnewqban]

Copied from
https://en.wikipedia.org/wiki/Ram_air_turbine

"RATs are common in military aircraft which must be capable of surviving sudden and complete loss of power.

In other military uses, pod-fitted systems such as the M61A1 Vulcan cannon or high-powered electronics such as the AN/ALQ-99 jamming system can be powered by a RAT in standard operation. This allows their installation on a standard hardpoint, without requiring a pod-specific power supply."

 

[Flyboy]

I was watching some videos of aircraft carrier cat shots of fighter jets today (don't ask), and was surprised to see in a slow-motion closeup shot that one of the underwing pods on an FA-18 had a propeller spinning on its nose.

That seemed pretty strange to me, so I did some Google searching, and it looks like the pod is used for in-flight refuelling of other aircraft (?). But why would you use a propeller for powering such a pod instead of just drawing the power from the host aircraft?

http://tailspintopics.blogspot.com/2015/06/things-under-wings-inflight-refueling.html

View attachment 326791

That's what's called a "Buddy Store" and its meant to be a completely self-contained unit to provide refueling capacity to an existing aircraft with only minor wiring changes to accommodate the needed controls. As others have mentioned, the propeller is a ram air turbine for powering the pod's hydraulics and electronics. IIRC, they were originally developed for use with the A-4 Skyhawk, which could carry a frankly ridiculous amount of external fuel for such a tiny plane, but the concept lives on today for use with the FA-18E/F Super Hornets for use as tankers on carriers.

Similar RATs are found on the front of the ALQ-99 electronic warfare pods used by the EA-18G Growler (and the EA-6B Prowler before it). They provide the required electrical power to drive the systems on the pods without requiring additional power generation on the host platform.

In both cases, it actually reduces the complexity of integrating such a system onto an existing aircraft, and today it's literally just "plug and play". There's no need to tap into the hydraulic system of the host aircraft, no need to run high power electrical wiring to the pylon. Just some extra light-duty wiring for communication between store and aircraft. The aircraft's stores management system on modern aircraft already knows how to interface with the Buddy Store and can display the correct menus and send/receive the correct information via the databus, and no additional wiring or plumbing is required.