Refining my practicality assessment for the Astro Elroy…
Based on a prop diameter of 1.5m, the Astro has a total swept rotor area of a little over 14m^2. With a 360kg MGTOW (that includes the 120kg “payload” of people & stuff, which is the number it must be certified to), disc loading is about 25kg/m^2 (actually pretty good).
Assuming a hover lift efficiency as good as a small helicopter (not really because of fixed-pitch, tip losses etc. but let’s be optimistic because of the contrarotation feature), we arrive at ~4.7kg/kW. That means at 360kg the total rotor shaft power input is about 75kW. Assuming a combined inverter and motor efficiency of 85%, the battery output needs to be 88kW to hover at MGTOW out of ground effect with 120kg of pilot/pax/baggage.
If you assume the “25-30 minute” range is variable based on load, and only 15-20 minutes of operation is possible with full load at rated speed of 70km/h, we have a rational and fair case. Let’s call 15-20 minutes at 88kW about 25kWh total energy requirement.
Pretty reasonable of me.
With 10% of capacity unused (charge to 95%, discharge to 5%) the total battery rating will be about 27.5kWh. It will be discharging at a rate of just over 3C. If we assume cells with the best available specific energy of 300Wh/kg could discharge continuously at 3C (which they can’t), Elroy needs a little over 90kg of CELLS ALONE. Improve this at a rate of 5%/year (based on history this is rational and commensurate with the best performance by the auto industry); by 2024 you could say perhaps 30% weight improvement or 63kg. With high-discharge rate Li batteries there needs to be significant management (cell discharge monitoring and control plus a cooling system, rated at about 5% of max discharge rate, or 5kW) as well as crash-land shock and lightning protection and fire suppression. 15kg is wildly optimistic. The total weight MIGHT come in as low as 75kg.
Then add 16 propulsion units at continuous rating of 4.7kW each at about 4200 rpm (keep tip speed below sonic).
Motor + inverter: best case using IEEE projections with exotic PM motors and SiC-based inverters is 0.75kW/kg, so 6.25kg each or 100kg. Note that this is based on 24krpm high-speed motors, and no compliance with lightning strike requirement but OK. Don’t forget that at these power levels even with SiC there needs to be about 4-5kW of cooling capacity as well, probably a pumped glycol loop, that I hven't included.
Props: 2kg each for 1.5m two-blade, fixed pitch, or 32kg. Nothing like this available right now, BTW, but let’s be positive.
So we’re up to (75+100+32)kg, or 207kg. That leaves only 33 kg for all the avionics, associated cooling, high-voltage shielded conductors, structure and seats AFTER we have made hugely charitable efficiency and weight concessions for the must-haves in a world 5 years hence.
I know, I know: but it’s already flown.
Yes, it has: empty for <5 minutes at a time, and when loaded with one person it never left ground effect. While their flight was stable and encouraging there is nothng here that hasn't been accomplished even by amateurs.
This is ~10years from reality and certificability in an OECD regulatory environment, if at all. Any NPV evaluation factoring in all the development cost, revenue timing and risk says 0.25/share might make sense.
BTW: can anybody name the technical talent on the team?
