Sky Whale II... feasible?

RecoveringYuppy said:
So recover a small amount.
Click to expand...

No, it's not worth it.

Airplanes are always losing energy. It's called drag. That's why you need to keep your engines on just to cruise.

The most energy efficient thing to do for descent is to power down your engines and let the loss of altitude cover most of your drag (you don't shut the engines off for obvious safety reasons). Yes, you're losing energy, but you have to lose it anyways, because again, drag.

If you want to descend faster, so that your altitude drop can cover more than just your minimum drag, thereby powering your turbine, well, that means that you will have had to cruise, with engines at normal power, for longer. So you're not just recovering energy, you're spending MORE energy, and then recovering some of it.

But that's not a net gain. You're better off not spending it in the first place.

Regenerative braking works on cars for two reasons. The first is that the relative energy cost of extra weight (the battery capacity to capture recovered energy) is much less than for a plane. The second is that, unlike planes, you generally need to stop cars MUCH faster than drag will allow.
 
Ziggurat said:
The second is that, unlike planes, you generally need to stop cars MUCH faster than drag will allow.
Click to expand...

IIRC, airliners typically do come in relatively steeply. For both for noise abatement and safety, they like to spend as little time as possible at low altitude, so they cruise a little longer and then use flaps and spoilers to bleed off energy in a relatively steep approach.

I'd assume that the fraction of total energy that could theoretically be recovered would be vastly lower for aircraft than for cars, and of course for aircraft, you'd have to actually lift the ******* battery and generator up to cruise altitude first, so it's hard to imagine how you'd ever come out ahead.
 
Ziggurat said:
Regenerative braking works on cars for two reasons. The first is that the relative energy cost of extra weight (the battery capacity to capture recovered energy) is much less than for a plane. The second is that, unlike planes, you generally need to stop cars MUCH faster than drag will allow.
Click to expand...

On average, a car never has too much kinetic energy. 40 mph driving = 20 m/s = 200 joules of kinetic energy per kg of car mass. If you have 200 J/kg stored in batteries, you can accelerate a car out of a green light. If you have 200 J/kg of empty storage capacity, you can fill it up while regenerative-braking when approaching a red light. A typical car trip involves doing that over and over and over, charging and discharging that little battery (or capacitor, or flywheel, etc.) every few minutes. It's worthwhile to haul around a battery of that size if you're going to use it every few miles.

An airplane needs a lot of kinetic energy. 250 m/s = 31,000 J of kinetic energy per kg of airplane mass. That's a lot. An airplane would need 5-10% of its weight to be made of lithium batteries in order to accelerate once. And a typical airplane trip involves going hundreds or thousands of miles without hitting a red light, during which time those batteries are useless. Terrible.
 
Nihilianth said:
I don't think craig4 is planning on a fiery ball of flames as his destination. Though, I could be wrong. I don't know craig4.
Click to expand...

No, I'm hoping to die suddenly while doing something I enjoy like my grandfather did, not writhing and screaming in terror like the prostitute he was playing trust games with when he had the massive heart attack.
 
dasmiller said:
IIRC, airliners typically do come in relatively steeply. For both for noise abatement and safety, they like to spend as little time as possible at low altitude, so they cruise a little longer and then use flaps and spoilers to bleed off energy in a relatively steep approach.
Click to expand...

The safety issue isn't about the plane, it's about air traffic control. The planes don't descend continuously, they descend in steps, so that they hang out at fixed altitudes for a significant fraction of the descent phase. But more advanced air traffic control systems (read: computers) make it possible to manage continuous descents. In fact, there are significant safety advantages to continuous descent approaches (also called optimized profile descents), because the pilot has fewer changes they need to make. And it also reduces noise: the step-wise descents require that the planes actually periodically accelerate to level off, which requires more engine use, and because they level off at a fairly low level on the last step, they actually spend more time close to the ground than continuous descent approaches.
 
ben m said:
I don't think the creator is "prone to woo". I think he's an artist who enjoys thinking about and drawing futuristic-sounding airplanes, and who doesn't know thing one about physics, energy, or engineering.
Click to expand...
It reminds me of an idea from the '80s that I vaguely remember for a nuclear powered airship, using the reactor to heat lifting gas as well as propel the craft.
 
catsmate said:
It reminds me of an idea from the '80s that I vaguely remember for a nuclear powered airship, using the reactor to heat lifting gas as well as propel the craft.
Click to expand...

That sounds distressingly close to sensible.

Typical hot air balloon:
Power needed: 20 gallons propane per hour = 20 kilowatts heating power.
Lift obtained: 1 tonne

Now double the radius.
Power needed = 80 kW (r^2)
lift obtained = 8 tonnes (r^3)
power requirement per weight = 10 kW per tonne

That's about the power density of one-year-old spent nuclear fuel! Stick some of them fuel rods in a big hot-air balloon and it'll stay aloft for a year at a time. (Not with you *in* it, mind you, unless you're fond of gamma rays.)

Scale up another factor of 20 and you can probably hoist a small not-actually-existing-but-not-implausible miniature reactor.
 
ben m said:
That sounds distressingly close to sensible.

Typical hot air balloon:
Power needed: 20 gallons propane per hour = 20 kilowatts heating power.
Lift obtained: 1 tonne

Now double the radius.
Power needed = 80 kW (r^2)
lift obtained = 8 tonnes (r^3)
power requirement per weight = 10 kW per tonne

That's about the power density of one-year-old spent nuclear fuel! Stick some of them fuel rods in a big hot-air balloon and it'll stay aloft for a year at a time. (Not with you *in* it, mind you, unless you're fond of gamma rays.)

Scale up another factor of 20 and you can probably hoist a small not-actually-existing-but-not-implausible miniature reactor.
Click to expand...
Thanks!
I must crunch the numbers and come up with a vaguely plausible design, it'd be fun in a RPG scenario. Giant nuclear powered airship raining destruction on the ground below.
 
catsmate said:
Thanks!
I must crunch the numbers and come up with a vaguely plausible design, it'd be fun in a RPG scenario. Giant nuclear powered airship raining destruction on the ground below.
Click to expand...

If you want a nightmare RPG scenario, why go with a nuclear powered hot air balloon when you could go with a giant nuclear-powered ram jet cruise missile that would drop multiple nuclear bombs? And here's the best part: we actually had preliminary designs for a real one!
 
Ziggurat said:
If you want a nightmare RPG scenario, why go with a nuclear powered hot air balloon when you could go with a giant nuclear-powered ram jet cruise missile that would drop multiple nuclear bombs? And here's the best part: we actually had preliminary designs for a real one!
Click to expand...
Which just happens to be what the nuclear powered airship is armed with
 
ben m said:
That sounds distressingly close to sensible.

Typical hot air balloon:
Power needed: 20 gallons propane per hour = 20 kilowatts heating power.
Lift obtained: 1 tonne

Now double the radius.
Power needed = 80 kW (r^2)
lift obtained = 8 tonnes (r^3)
power requirement per weight = 10 kW per tonne

That's about the power density of one-year-old spent nuclear fuel! Stick some of them fuel rods in a big hot-air balloon and it'll stay aloft for a year at a time. (Not with you *in* it, mind you, unless you're fond of gamma rays.)

Scale up another factor of 20 and you can probably hoist a small not-actually-existing-but-not-implausible miniature reactor.
Click to expand...

I am going to have to run those numbers by my not-actually-existing-but-not-implausible girlfriend.
 
Ziggurat said:
If you want a nightmare RPG scenario, why go with a nuclear powered hot air balloon when you could go with a giant nuclear-powered ram jet cruise missile that would drop multiple nuclear bombs? And here's the best part: we actually had preliminary designs for a real one!
Click to expand...
Bah, airships are far coooler than mere missiles. Plus there's room inside for suff to happen.
Maybe add in a nuclear powered aircraft carrying ekranoplan and a fleet of NB-36s.

Grashtel said:
Which just happens to be what the nuclear powered airship is armed with
Click to expand...
Indeed. Build it big enough and it could carry it's own strike aircraft.
 
ben m said:
I don't think the creator is "prone to woo". I think he's an artist who enjoys thinking about and drawing futuristic-sounding airplanes, and who doesn't know thing one about physics, energy, or engineering.
Click to expand...

In all honesty, that fits most criteria one would have for being prone to woo.

@Catsmate, I assume you're voting yes in the age of the Presidency referendum?
 
Last edited:
You must log in or register to reply here.

Back
Top Bottom