Passenger Drones (Concept of Mine)

[kiwi1960]

I disagree, its not just technology, a flying car was made in the 1950's....

The real question is safety...

anyhow... this is from popularmechanics.com

[Red Iron Crown]

12 hours ago, Snark said:

Hydrogen's a pretty crappy answer for automobiles, too, which is why nobody's building commercial hydrogen-powered cars.

I don't often disagree with you Snark, but I have to on this one:

https://en.wikipedia.org/wiki/Hyundai_Tucson_FCEV
https://en.wikipedia.org/wiki/Toyota_Mirai
https://en.wikipedia.org/wiki/Honda_Clarity

Hydrogen fuel cell cars are a thing, mostly because the greater energy density of hydrogen compared to even the best batteries gives them greater range, and refuelling is a quicker process than charging a battery. They still suffer all the other drawbacks of hydrogen (no existing distribution infrastructure, storage issues, etc) and I suspect for most users a battery powered EV is a better choice.

[Snark]

1 hour ago, Red Iron Crown said:

I don't often disagree with you Snark, but I have to on this one:

https://en.wikipedia.org/wiki/Hyundai_Tucson_FCEV
https://en.wikipedia.org/wiki/Toyota_Mirai
https://en.wikipedia.org/wiki/Honda_Clarity

Hydrogen fuel cell cars are a thing, mostly because the greater energy density of hydrogen compared to even the best batteries gives them greater range, and refuelling is a quicker process than charging a battery. They still suffer all the other drawbacks of hydrogen (no existing distribution infrastructure, storage issues, etc) and I suspect for most users a battery powered EV is a better choice.

Cool!  Excellent, thanks for the links; hadn't been following for a while.  You're right, I went a bridge too far on that one.

Fuel cells are, indeed, highly cool.  You're right, they're usable for cars-- nice to see that they've gotten the range up to practical values.  Another area where I've heard they have good potential is for small electronic devices (i.e. run for much longer on a single battery charge), or the potential for large ones that could run a house's electrical needs.

My understanding, though, is that the limitations of energy density and mass requirements make this not a realistic option for aircraft.

[Red Iron Crown]

Just now, Snark said:

My understanding, though, is that the limitations of energy density and mass requirements make this not a realistic option for aircraft.

Indeed, at least for sustained flight (I think Skylon is supposed to be hydrogen fueled in both airbreathing and rocket modes). 

[shynung]

@Red Iron Crown Cool links on the cars there. Makes me drool already...^{I only have an itty bitty scooter, mind you.}

On Skylon using hydrogen, I think it has to do with the fact that hydrogen carries more energy by mass compared to hydrocarbon fuels. That, and the hydrogen gets used as the coolant in the precooler system.

[Snark]

36 minutes ago, shynung said:

@Red Iron Crown Cool links on the cars there. Makes me drool already...^{I only have an itty bitty scooter, mind you.}

On Skylon using hydrogen, I think it has to do with the fact that hydrogen carries more energy by mass compared to hydrocarbon fuels. That, and the hydrogen gets used as the coolant in the precooler system.

Also, don't forget the elephant in the room, I_sp. Anything that needs to get a lot of dV from pure reaction mass has to care about I_sp, a lot.  Hydrogen has the advantage of a high exhaust velocity, owing to low molecular mass.

[shynung]

1 minute ago, Snark said:

Also, don't forget the elephant in the room, I_sp. Anything that needs to get a lot of dV from pure reaction mass has to care about I_sp, a lot.  Hydrogen has the advantage of a high exhaust velocity, owing to low molecular mass.

Yep, that.

Also, in space vehicles, mass more expensive than volume, hence why space-going vehicles are OK with having more than 75% of their internal volume being occupied by propellant tanks. Should the space vehicles used enough hydrocarbon-based propellants to get the same dV, what they'll lose in tank size (and mass) they'll start to gain in propellant mass, due to lower I_sp. Tsiolkovsky's equation would then push the propellant requirements beyond the impractical range.