Twin rotor helicopters with a pusher : why now and not before?

So, Sikorsky/Boeing are shopping around a design for an improved helicopter. It’s here. There’s nothing earth-shattering in the design. Twin rotors have several advantages in stability and have been used in helicopters for decades. A pusher prop is an obvious improvement to boost flight speed.

So, why wasn’t this tried 30 or 50 years ago with far older helicopter designs? What has changed in technology that makes this design possible when it wasn’t then? I don’t see anything radical about a pusher prop or using 2 rotors so you don’t need a tail rotor. Could the engineers of 1970 not work out the complex mechanism that allows for counter-rotating rotors?

Also, how does this helicopter design solve the problem of retreating blade stall? As I understand it, that in practice limits the top speed to a function of the size of the rotors, which is why the Chinook is the fastest. I would have assumed that to solve this problem you would need an aircraft that provides it’s own lift via some kind of wing - hence all the sci fi aircraft use ducted fans to take off vertically, have a shape like a lifting body, and turn the fans backwards so the aircraft can fly like a conventional aircraft. Though, for some reason, there’s a distinct lack of actual flying prototypes of such designs…

A relevant article:
http://spectrum.ieee.org/aerospace/aviation/the-fastest-helicopter-on-earth

Russian Kamov helicopters have indeed used coaxial counterrotating main rotors for decades.
https://en.wikipedia.org/wiki/Coaxial_rotors

The main drawback of the design is the obvious mechanical complexity.

In the design in the video, I don’t know if the pusher prop at the back is just a more efficient way of providing forward thrust, or whether significantly higher forward speeds are attained. If the latter, the resolution of retreating blade stall must presumably some combination of
(a) faster rotor RPM
(b) improved rotor airfoil design
(c) greater tolerance for retreating blade stall with the counterrotating design

A faster ship without the problem of retreating blade stall was going to be my guess.

FWIW, the single-rotor Lockheed AH-56 Cheyenne (first flight, 1967) had a pusher prop.

The rest of that paragraph is poorly-written. For example, ‘…and could be adjusted by collective pitch control changes,’ and ‘Rotor tilt was controlled through gyroscopic precession.’ I mean, no kidding. The collective pitch can be changed with the collective pitch control! Who knew? And while gyroscopic precession is something that happens when the rotor is tilted, the actual tilt is controlled with the cyclic pitch control. I believe the quoted bit is correct, but based on what follows, I wouldn’t take it as gospel without more research.

I’m not quite sure what you mean by this. Retreating blade stall is (ceteris paribus) just a function of forward airspeed - you don’t resolve the problem just by having additional forward thrust come from a separate source.

With a counter-rotating rotor system, the retreating blade still stalls. But lift is provided by the advancing blade on that side. At least, that’s how I understand it. I’ve only flown traditional single-rotor helicopters.

Ok, yes indeed. I had listed that as (c). I thought you were implying something else.

In the counterrotating system, the retreating blade still stalls at the same forward airspeed, but there’s somewhat greater tolerance for it because it doesn’t result in asymmetry of lift.

Not a helicopter but a sub.

In its last incarnation*, the USS Albacore had counter-rotating propellors driving it (go to the fifth image):

http://ussalbacore.org/html/virtual_tour.html

*The Albacore was a research vessel for testing designs. It was rebuilt numerous times.