electric turbofans

post by bhauth · 2024-11-02T22:50:59.807Z · LW · GW · 2 comments

This is a link post for https://bhauth.com/blog/flawed%20ideas/electric%20turbofans.html

Contents

  response from founder
  supersonic flight is less efficient
  supersonic aircraft engines
  supersonic dreams
  practical aircraft ideas
None
2 comments

It seems I've been typecast as a person to ask about "hard tech startups" with big claims that are getting attention. This time it's been Astro Mechanica, a Y Combinator startup making aircraft engines. Here's a video, possibly set up by a PR agency. Apparently Paul Graham compared the significance of their engine to James Watt’s steam engine.

Most large aircraft today are powered by turbofans, which of course involve a gas turbine that powers a fan. Some new turbofans have a gear stage between the turbine and the fan, so the (larger) fan can rotate slower. That seems to work pretty well; driveshafts and planetary gears usually do.

What Astro Mechanica does is replace that driveshaft and gear with electric motors. That lets you continuously vary the relative speed of the fan and gas turbine - but is that useful? If you change the speed ratio, power production and power usage become mismatched, and then where do you use/get the extra power?

Even modern axial-flux electric motors have similar power-weight to modern turbofans, so sending most of the power through 2 electric motors means you've tripled the engine weight, reduced efficiency, and increased production cost. Benefits would have to be substantial to justify that.

Supposedly, it's "efficient at every speed", including supersonic speeds. But why would that be the case? Variable fan speed doesn't get you that; what you want for that is variable bypass ratio. The faster an aircraft travels, the faster you want the engines to send the air backwards. Faster air ➔ more compression ➔ less air compressed per gas turbine flow ➔ lower bypass ratio. But electric motors don't let you vary the bypass ratio.

response from founder

Ian Brooke is the founder of Astro Mechanica. Since I didn't see how their approach was supposed to be better, I tried asking him.

He first wanted to clarify that at high speeds, most of the fuel usage happens outside the turbogenerator:

 

That was already my understanding, and it doesn't explain how electric motors help. It just describes an afterburning turbofan, which has low efficiency. At any speed from 0 to Mach 3, efficiency goes: low-bypass turbofan > turbojet > afterburning turbofan. That's worse efficiency, not better, so I asked about a specific fuel consumption graph, and got this:

That was a puzzling graph to me, because it shows better efficiency than modern high-bypass turbofans at Mach 0.25 and also better efficiency than turbojets at Mach 3. (A modern turbojet optimized for Mach 3 can give ~2450s specific impulse.) The former can't be achieved without a large intake area that makes Mach 3 impractical, and there's still no reason electric motors are helpful.

Ian replied that:

• That's with methane, which has 17% more energy per mass.
• They "also use a distributed propulsion design that increases effective bypass area with less ram drag".
• All that matters is that the system works.

Ah, reporting per-mass fuel efficiency with methane instead of kerosene. Very cheeky. But methane is harder to store and not a good tradeoff for aircraft...unless they're much bigger than current aircraft, I suppose, but Astro Mechanica says they want to enable smaller aircraft.

NASA has been researching turboelectric distributed propulsion designs for a while, but while there are some aerodynamic benefits, they're generally considered not worth the weight and efficiency penalty. Distributed propulsion matters with propellers, because of interaction between the wakes of propellers and the airframe, but with ducted fans...you can still try to do boundary layer ingestion, but it's definitely less worthwhile than with propellers. In any case, I consider describing benefits from that as "engine Isp" as incorrect, and it still doesn't make that graph plausible to me.

Yes, if the system works, that's fine, but if that's a chart specifically for an aircraft with integrated propulsion and aerodynamic interactions, well, that's obviously not something that's working yet. If that's what the benefit is from, it doesn't make sense to cheer and claim success from demonstrating a standalone engine. Neither is some distributed propulsion system a thing that was mentioned in the videos or articles about Astro Mechanica I looked at, or on their website. A google search for that also returns no relevant results.

Well, I do appreciate the founder taking the time to reply. Perhaps someone else will have a different interpretation of his comments.

supersonic flight is less efficient

Supersonic aircraft, compared to subsonic aircraft with the same level of competence, always have 0.5x the lift/drag ratio or worse. Why is that?

Wings produce lift largely because the pressure on their top side is lower than ambient pressure, which implies higher air speed there, but above the speed of sound that flow becomes choked.

We can reduce the effective air speed of wings by sweeping them. The problem is, lift is proportional to speed^2 across an airfoil chord, and by sweeping a wing, we reduce that but don't reduce drag. So, changing effective speed by X with sweep, we change lift/drag by X^2. Which clearly makes supersonic aircraft less efficient. That used to be partially compensated for by increased propulsive efficiency at high speeds, but modern turbofans have a high bypass ratio that better matches subsonic speeds.

You'll often see delta wings on supersonic aircraft. Those are effectively the same as highly swept wings; what matters for sweep is the sweep of the lines of equal air pressure (isobars). For low drag at high speeds you just need to make sure none of the isobars have sections with insufficient sweep. (Doing that reasonably well without CFD is one of my useless talents.)

supersonic aircraft engines

For supersonic aircraft, what you want is a turbofan with lower bypass ratio than subsonic aircraft use. The higher the speed, the lower the optimum bypass ratio, until at some point you just want an engine with no bypass, which is a turbojet. But generally, even for supersonic aircraft, you want a bypass ratio > 1, unless you're trying to minimize engine costs in which case you might still want turbojets.

The Concorde used turbojets; modern low-bypass turbofans can give you ~0.7x the fuel burn at its cruise speed. You can also burn the compressed air of a low-bypass turbofan in an afterburner, but afterburning turbofans are less efficient than turbojets at every speed.

supersonic dreams

There's a pattern of thought I've seen many times, and multiple times about supersonic aircraft in particular. It goes like this:

1. future = inevitable progress
2. progress = faster travel = supersonic travel
3. Therefore supersonic travel is practical and will be common in the future.
4. Here is the only route to practical supersonic travel we can think of, so it must work.

Of course, that logic is backwards. But how about that blogger? Why are they so enthusiastic about this startup?

It might surprise you to learn that I am not an aerospace engineer, so to figure out whether an aerospace engineer like Ian is legit, I need to look for clues. One clue might be a video of a working prototype, like the one we started the piece with. Another might be an endorsement from someone like Matt Parlmer. Another, more important one is the ability to recruit [former SpaceX] engineers. On that count, Astro Mechanica is legit.

So, clearly none of those are good indicators. Neither is endorsement by Paul Graham - or Bill Gates, or Elon Musk. Most American business leaders and popular bloggers are just completely unable to evaluate novel technology; you might as well roll dice. This is why China has been beating the US at some stuff lately: smarter leadership.

practical aircraft ideas

OK, so I'm not optimistic about replacing turbofan gears with electric motors, or cost-effective supersonic transport. (Unless you count delivering explosives as "transport".) But I wouldn't want people to think I just reject everything new. What novel aircraft things do I think are feasible?

BWB aircraft are a good idea, as I've said. They can have better efficiency and more space for passengers. They're common for new bomber designs. Passenger aircraft are harder because they need to be pressurized, but they still seem feasible. The real problem is development cost given certification requirements for passenger aircraft; it might simply be too expensive to actually get a BWB passenger aircraft into production. The 787 cost over $30 billion to develop, and a BWB is a bigger change than the 787 was.

Truss-braced transonic wings can significantly improve efficiency by allowing for a higher wing aspect ratio. The thing is, that means either:

• a longer wing, and passenger aircraft are already as wide as airports can handle
• or less wing surface area, which means increased takeoff distance, which is also a problem

Because of airport size limits, braced transonic wing designs are often proposed with folding wingtips, but of course that has its own problems.

That "Mentour Now" channel is actually decent; here's its video on propfans, which are also feasible. Yes, you can get better efficiency with those, at the cost of slightly lower cruise speeds, larger engine diameter, and increased noise.

Long-range low-cost autonomous military UAVs. They're coming at some point. Aren't you excited for that ✨ progress ✨?

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