A power-to-weight ratio of 10 screams possibility

Monday, November 14th, 2022

Electric aircraft have certain advantages and disadvantages, Casey Handmer notes:

Advantages include mechanical simplicity and reliability, reduced noise, reduced cost, increased efficiency, and reduced engine weight. The major disadvantage is that battery energy density is still, at best, about 50 times less than gasoline. Even factoring in other efficiency gains, electric aircraft have greatly reduced flying time and range.

The underlying reason that I believe electric aircraft can break the sound barrier is that electric motors can deliver far higher power-to-weight ratios than piston engines, jets, or turbines. The F-4 Phantom is a textbook example of high thrust, being able to (just) achieve a vertical climb. In contrast, for $100 I can buy a racing drone that can accelerate vertically at 10 gs. There are other factors at play but a power-to-weight ratio of 10 screams possibility.

In terms of fundamental physical limits, let’s consider the Concorde. While most fighter jets can fly supersonic for at most a few minutes, the Concorde couldn’t do in-flight refueling and had to cross the Atlantic in a single hop. It could cruise at Mach 2 for 201 minutes! Let’s say that when battery energy density and electric motor efficiency are factored in, electric systems with present technology would have 10x less range. Still, an electric Concorde could fly for 20 minutes, covering almost 450 miles. That’s more range than a Tesla!


Of course it should be possible to develop a better configuration than a Concorde clone, but it’s an interesting starting point. In particular, many supersonic aircraft use delta wings because of relatively consistent lift characteristics over a range of speeds. It’s not that Concorde needs that enormous wing to fly at Mach 2 at 60,000 feet. Concorde needs the huge, draggy wing to fly slowly enough to land on a runway. But electric aircraft can deliver the necessary power and control to take off and land vertically (VTOL) like a helicopter, obviating the need for much wing at all.

Before diving into the specifics of different subsystems, I will motivate an example point design by appealing to the obvious. A supersonic electric aircraft must have a lot of thrust and minimal drag. When we think about what it might look like, the F-104 Starfighter comes to mind. Long, pointy, and with the barest minimum of a wing.


Ordinarily, fast planes use jet engines for propulsion. Their compressor stages operate at subsonic speed so all supersonic jets have complex intake systems designed to decelerate inrushing air with a series of shocks prior to impacting the turbine. Building a turbine to ingest a supersonic stream ordinarily seems like a recipe for disaster. Jets need subsonic flow because combustion typically occurs subsonically. Electric propellers have no such constraint, and nor do they care that 80% of the atmosphere isn’t oxygen.


  1. Bomag says:

    Can we do practical supersonic with propellers?

    (Too lazy to Google.)

  2. Isegoria says:

    NASA’s predecessor, the National Advisory Committee for Aeronautics (NACA), believed that it was possible for propeller-driven aircraft to break the sound barrier:

    In the 1940s, NACA invested heavily in new designs for propellers through a high-speed propeller research program. Over time, it was able to design propeller blades capable of reaching Mach 1.0. This was accomplished by shortening and thinning the blades, sharpening the leading edges, minimizing camber, and increasing the blades’ angles.

    While the change was an overall success, parts of the blade would reach supersonic speeds before others. This was problematic for two reasons — first, sonic waves are created when an object nears the speed of sound. Because the blades were reaching Mach 1.0 unevenly, they were creating pockets of sonic waves powerful enough to destroy the propeller. Second, another problem was noise. Propeller-driven aircraft are loud enough, but when those spinning blades reach supersonic speeds, the noise level generated becomes a threat to the structural integrity of the aircraft — and its pilot.

  3. Slumlord says:

    Propeller efficiency drops rapidly as one approaches the speed of sound. i.e much of the power becomes devoted to generating shock waves and not propelling the aircraft forward.

  4. Gavin Longmuir says:

    “a power-to-weight ratio of 10 screams possibility.”

    One of the major possibilities is that this power-to-weight ratio ignores realities — such as the weight of the batteries the power source has to lug around.

    Hey! I have a great idea for bringing down the cost of air travel. All we have to do is ignore the cost of the plane and the cost of the fuel.

  5. Wilbur Hassenfus says:

    A quick Google search suggests that solid-fuel model rockets accelerate vertically at about 9 g.

    Scale matters a lot. This is reddit-tier reasoning.

  6. Lu An Li says:

    “The major disadvantage is that battery energy density is still, at best, about 50 times less than gasoline.”

    And takes about 20X longer to charge those batteries to full capacity than fuel a conventional fueled aircraft.

    Batteries unless tremendously improvement made and soon a no go. Why even try?

  7. Jim says:

    There were nuclear-powered aircraft in the fifties.

    Are we to believe that there are briefcase nukes but not briefcase nuclear reactors?

    Are we to believe that the technology for nuclear-electric vehicles is not musting away on a TS/SCI basement shelf somewhere?

  8. Michael van der Riet says:

    @Wilbur Hassenfus, absolutely! If a humming-bird were scaled up to twice its size, it would be unable to fly. Humans scaled up two times would collapse under their own weight. Even the balsa-wood gliders that I played with as a child would as full-scale aircraft be far too heavy to take off.

  9. Michael van der Riet says:

    Another problem with the propellor is that the rotational airspeed of the blade increases exponentially from root to tip. Those unfortunate enough to live near military flying schools that operated the T-6 Texan will remember the terrific roar that the blade tips made as they broke the sound barrier, even at modest airspeeds for the aircraft as a whole. Nor was it a pleasant experience for the pilots.

  10. Altitude Zero says:

    “There were nuclear-powered aircraft in the fifties.

    Are we to believe that there are briefcase nukes but not briefcase nuclear reactors?”

    As I recall, back in the 1960′s NASA developed a desk-sized nuclear reactor for Project NERVA. I wonder what ever happened to it?

  11. Jim says:

    Let us wonder indeed!

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