The problem with bringing two aerodynamic bodies together under high flow rates is far more challenging than just controlling two bodies that are quasi-rigidly attached. The problem is three-fold.
The first, as mentioned by the o.p., is that the flow conditions of one body will change the lift and control conditions of the other. This has to be considered in the aerodynamic response of the vehicle, and is not at all the same as vehicle flying in close formation where (aside from end tip vortexing) there is really no aerodynamic coupling between vehicles. For instance, a vehicle coming in underneath another will create a pseudo-jet at the top and bottom surfaces of the respective vehicles become close enough that they accelerate air streams into each other, which in turn will create a vacuum effect which will draw the vehicles toward each other in an unstable manner.
The second is that any dynamic rendezvous operation (e.g. two bodies joining together with a significant relative velocity) requires some kind of energy moderating/rejection system, e.g. some kind of spring and damper system to both spread out the impulse and convert the kinetic energy into some other form. However, this system also introduces novel physical dynamics (e.g. vibrational modes) of the system which themselves have to be accounted for. It is suprising how quickly this becomes complicated to simulate and control, especially for any system which has multiple connections, especially if those connections are in any way coupled, and especially if the modes are in low frequencies which may couple to the aerodynamic control modes.
The third issue is what is called a radical state change; that is, the system goes from two independently controlled bodies to one integrated body. From an abstract controls problem this is trivial; the two “boxes” (vehicles) just switch to one box, and the connections between them disappear. The reality is that this handoff is never that clean. The details of such a radical state change are always challenging, even when it the easier case of going from one body to two (as in weapon release or separation of a downstage from a rocket); in the case of joining two bodies together, the sequencing of how the control is maintained and concomitant events (see the discussion of dynamics above) are dealt with is crucial.
Even in orbital mechanics where aerodynamics are not of concern, bringing together two bodies into a smooth rendezvous is highly challenging, and is the reason that the ISS uses the Mobile Servicing System (the Canadarm and associated systems) for docking unmanned supply capsules and assembling modules. At high flow rates and inherently unstable aerodynamic conditions of atmospheric flight, this becomes an inherently hazardous operation. The necessity of precision that no human pilot can achieve (steady control within a few centimeters of relative displacement), designing a system to operate in radically different structural and aerodynamic configurations, and dealing with the dynamics of the rendezvous operation are all beyond the current state of the art at typical aircraft speeds.
Someone will doubtless come along and say that we can just solve it in software models. In fact, even among people who work in aerospace, the default approach to any physical problem is to push it down to the presumably technomagical mages of “software” to fix. (Norman Augustine’s eponymous Laws are largely a rant about the tendency to attempt to fix all problems in software and the nightmares that creates, both for the actual software engineers and the program managers who don’t understand why all the promises they made about fixing instabilities by just generating some code can’t just magically be compiled like so many cantrips.) The reality is that software, and the computers that run it, are like idiots savant; modern computers can calculate very, very fast–almost certainly faster than you really require–but can only calculate based upon the models you provide them in the forms of algorithms. Unfortuantely, those models are all approximations–often, very coarse approximations–of reality.
Sure, we can control a vehicle like the F-22 or F-35 which is inherently aerodynamically unstable, but in order to do so feasibly we generally have to ‘linearize’ (i.e. further simplify) the model into certain highly predictible regimes. This linearization often causes problems when you find that it doesn’t, in fact, cover all of your flight regimes, requiring expensive regression testing and refactoring of code, which then has to be simulated, vetted, peer reviewed, documented, configuation controlled, et cetera. It is no wonder that in complex, bleeding edge systems, the massive overruns all tend to happen in the software development; it’s not the fault of the poor software engineers and programmers that a mass of physically impossible and often contradictory requirements were shoved down at them to compensate for all of the problems glossed over in the physical design and development of vehicle propulsion, control, lift, structure, and other systems. “We’ll fix it in software” is generally a euphamism for “I don’t know how to deal with this, let’s make it someone else’s problem.”
In short, while mid-air rigid docking between aircraft may not be physically impossible, it is far more challenging that most people suspect or understand, and even with modern avionics systems over a century of flying stuff from ground level all the way to orbit, we still don’t have a good general approach to doing this reliably short of designing one vehicle to be able to essentially crash into another.
Stranger
I appreciate your response Stranger. It is very intelligent as always. I know this a difficult problem to solve but I am also confident that it could be done on a limited scale if there was an actual need for it plus lots of money.
The OP didn’t specify the size of the planes required. We already have plenty of planes of all sizes that can fly in very tight formations so I don’t believe the aerodynamics are a show-stopper. I say you could build some sort of attachment platform that could dock the two at least temporarily if you really wanted to. It may be a stunt that is roughly the same as extended formation flying combined with a large and semi-rigid tether but I truly believe it could be done. The real question is why you would want to do such a thing in the first place?
Sure, if you wanted to build to planes that are specifically designed around the objective of being able to dock with each other, with a connection that provided enough standoff and controlled flexibility to allieviate the issues discussed above, it’s feasible; that is pretty much what the “trapeze systems” used on experimental carrier aircraft did. But if you tried to do so with two aircraft of comparable size, you’d end up with something that wasn’t particularly functional for any propose. Conversely, trying to take two existing useful aircraft, like, say, a 747 and a B-1 bomber and design a system that would allow you to connect them together in flight would be nigh on impossible, which is more in line with the scenario implied by the o.p.
You can teach a bear to dance if you work at it and don’t mind too much about being clubbed over the head by a really annoyed bear, but he won’t win on “Dancing with the Stars”. On the other hand, you won’t ever teach a bear to reinact the umbrella scene from “Singing in the Rain” no matter how hard you work at it.
Stranger
Stranger :
The scenario I had in mind is a specific one, from a television show. I did mention it in the OP and even showed a screenshot from the show.
In the show, the main characters have an aircraft that is about the size of a 747 but has 2 extra engines. As you can see from the screenshot, the aircraft docking with them is considerably smaller, and carries only 2 passengers. Both aircraft do not exist in the present day and this docking capability must have been designed in.
For some reason I am reminded of the technique used to land a helicopter on a ship in bad seas. Helicopter drops a cable, which is coupled to hook in the middle of the helipad, and the helicopter winches itself onto the helipad, all the time keeping the cable taught with lots of power. When there is a large disparity in mass between the two craft, this idea would seem to help. The OP’s linked picture even has a hint of some sort of docking boom.
The technique doesn’t solve all the problems Stranger lists. It might ameliorate some. Having the small craft fly up to a docking boom that is sticking up well outside any zone of aerodynamic interaction and then hook on, then adopt a flight attitude and power that is trying to fly away from the mother craft. The boom would need to have lots of damping and compliance, but since the small craft is much much smaller than the big one, the oscillatory modes might be easier to keep out of the way. Then you crank the small craft into a hard dock using the boom. You still have significant issues when the craft get close to one another, and a single boom may not be enough to prevent the small craft being slammed into the big one when it gets close. Maybe you would need three docking booms. This then becomes a matter of docking outside of the interaction area, and brute forcing the docking, relying upon the very large size disparity to avoid all the other problems. Why am I reminded of the old Thunderbirds TV program from when I was a kid?
Had a high school teacher who used to fly B-52s back in the day (from the Vietnam War through Desert Storm). He mentioned that mid-air refueling from a tanker is either the most difficult or easiest thing to do in an airplane, depending on how much you overthink it. Watch the tanker’s position and movements relative to yours like a hawk, carefully tune your engine outputs, elevator trim, flaps, ailerons, etc. ever mindful of the distance between the two large jets cruising at five hundred knots 20,000 feet over the Earth, and it will be the single most exhausting and difficult accomplishment of your career.
Or, slowly approach the tanker from below and let your plane get sucked into the vacuum envelope behind and below the tanker, while the Boomer maneuvers the refueling boom to make the actual connection, as the Boeing engineers intended both planes to do by design, and it becomes the easiest thing ever. You still have to be careful, but other people have already figured out the worst parts about how the two aircraft will aerodynamically interact.
Again though, that’s two jets that were designed to connect like that (the KC-135 and B-52 Stratofortress are both Boeing jets, and since the 135s have been in service since the 50’s, most newer tanker and jet designs in Air Force service are designed with compatibility in mind)
Just saw today, a beautiful and unusual – planar, heh – photo of mid-air fueling.
That’s an F-35, right? (Whatever happened to the F-22?).
F-22 is in service now, there’s just not a whole lot of them. You can find lots of pictures online of Alaska-based F-22s escorting Russian Bear bombers out of NORAD airspace.
Speaking of mid-air refueling photos, there was this catch someone found on Google Earth a few years back, of a KC-135 refueling a C-5 Galaxy, as caught by a satellite photo.