Hybrid Plane/Airship

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Hybrid Plane/Airship

Postby gumOnShoe » Tue Sep 10, 2013 3:05 pm UTC

1) I know enough physics to sound stupid to people who know physics.
2) I will now look stupid.

x = helieum tank
# = fusilage

The basic idea of construction for what I'm talking about, of course more arrowdynamic and less borg cube like:

wing with jet engine
xxxxxxxxxxxxxxxxx
#################
xxxxxxxxxxxxxxxxx
wing with jet engine

The idea is to reduce the weight of the structure so that more energy can go towards moving forward, getting a more arrodynamic vehicle and worrying less about lift. Complete waste of time?

The helium tanks could perhaps surround the top and bottom of the structure. You could use balast tanks to decrease or increase your boyancy, but the hope here is to have a more rigid structure than a regular airship so that high speeds can still be achieved.

My guess is that the real problem is air resistance and not lift with a jet, but I'm a writer and I'm looking for a scientifically plausible new type of ship to populate a world that's had an environmental catastrophe that has slightly thickened the air. If if this does turn out to be stupid, I still may use it. Soft-science fiction is easier to write than hard.

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Re: Hybrid Plane/Airship

Postby Izawwlgood » Tue Sep 10, 2013 3:57 pm UTC

My understanding of airships is that they require big honking balloons of buoyant gas to stay afloat. My understanding of jets is that they push air out really really fast and require fairly high speeds to function, as well as aerodynamic bodies for lift.

It sounds like you're basically nerfing the best features of both systems.
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Re: Hybrid Plane/Airship

Postby Adam H » Tue Sep 10, 2013 4:19 pm UTC

Yes, at high speeds, air resistance is more significant than lift. And with the thickened air in your story, air resistance and lift forces would both be increased anyways. That is, planes could just have stubbier wings to generate lift, which would make the plane more aerodynamic and decrease air resistance. I'm not 100% sure that the decreased air resistance from the improved plane design would outweigh the increased air resistance from the atmosphere, but I suspect it would. Which would mean that THINNER air makes your hybrid idea more plausible.

Lighter than air, rigid, and aerodynamic helium tanks would need to have such thin walls that it's just impossible.
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Re: Hybrid Plane/Airship

Postby davidstarlingm » Tue Sep 10, 2013 4:48 pm UTC

Izawwlgood wrote:My understanding of airships is that they require big honking balloons of buoyant gas to stay afloat. My understanding of jets is that they push air out really really fast and require fairly high speeds to function, as well as aerodynamic bodies for lift.

It sounds like you're basically nerfing the best features of both systems.

What he said.

The problem with airships is embodied in two equations:

A = 4πr2
V = (4/3)πr3

Those equations embody the surface area and volume of a sphere. The critical bit is the exponents. Double the radius of a sphere, and its surface area (along with its weight) quadruple, but its volume (along with the amount of gas it can hold) octuples. In other words, bigger is better. The larger a balloon is, the better lift-to-weight ratio it can get.

Of course, the smaller a balloon is, the worse its lift-to-weight ratio becomes. For the lifting requirements of an airship, it's just not going to be enough.

However, there's a difference -- the whole "thickening of the atmosphere" thing. Increasing the density of the atmosphere will increase the lifting force that the helium can produce. It will also increase the drag on your object, as well as the lift generated by the wings. Of course, if the catastrophe that increased the density of the air involved gravity, then that's going to counteract any advantage you might have received from lift, making a buoyant airship more feasible in general.

Still might not be enough, though. Aerodynamics is about reducing drag over the body of the craft; adding bulky helium chambers isn't going to help with that. How bulky? Well, a 747 is around 400 tonnes. To reduce its effective weight by half, you'll need a 4.4 million cubic meters of helium -- a sphere substantially larger than the wingspan of a 747.

But the real question is, why? If you want to increase the lift, just increase the size of your wings.

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Re: Hybrid Plane/Airship

Postby JBJ » Tue Sep 10, 2013 8:51 pm UTC

Are you thinking of something like this?
Spoiler:
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Re: Hybrid Plane/Airship

Postby Izawwlgood » Tue Sep 10, 2013 11:30 pm UTC

Curious; I wonder if you could compress the gas, and inflate some sort of balloon structure after a big altitude gain. You could then deflate the balloon, and send the plane into a dive while you ramp the jets up.

Silly idea?
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Re: Hybrid Plane/Airship

Postby Copper Bezel » Wed Sep 11, 2013 1:44 am UTC

Can't be as silly as the gravity plane (thanks for sharing, JBJ; I'm a little awed by the step where compressed air is used as a power source to compress air, increasing ballast.) But Izawwlgood, you mean something like a collapsible blimp with a functional plane as the gondola? What's the benefit?
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Re: Hybrid Plane/Airship

Postby Izawwlgood » Wed Sep 11, 2013 2:16 am UTC

Yeah, basically; the benefit is that it could do what I wager the OP is after; fly on jet propulsion, 'park' on blimp mode.
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Re: Hybrid Plane/Airship

Postby scarecrovv » Wed Sep 11, 2013 3:48 am UTC

Lockheed Martin seems to think it's a good idea. My understanding is that they're working on a blimp-like thing that makes use of both aerodynamic lift and buoyancy. It should be faster than blimps, cheaper than planes, have a large cargo capacity, and be able to land in any sufficiently large open space.

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Re: Hybrid Plane/Airship

Postby speising » Wed Sep 11, 2013 8:23 am UTC

anything that promotes airships is a good thing.

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Re: Hybrid Plane/Airship

Postby Zamfir » Wed Sep 11, 2013 9:39 am UTC

If the planet has higher air density on the ground level, jets can still climb to a cruising altitude with the same density as their current cruiding altitude.

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Re: Hybrid Plane/Airship

Postby davidstarlingm » Wed Sep 11, 2013 6:32 pm UTC

Copper Bezel wrote:Can't be as silly as the gravity plane (thanks for sharing, JBJ; I'm a little awed by the step where compressed air is used as a power source to compress air, increasing ballast.)

I don't think it's particularly silly. Build a lighter-than-air craft with an overall aerodynamic glider-like shape. Release it and let it rise to altitude. Once you're at altitude, compress surrounding air and pump it into collapsible bags inside your hydrogen chamber, which will decrease your buoyancy; then, fly it down to your destination like a glider. Continue pumping air into your chamber to match atmospheric pressure until you reach a low enough altitude to land.

I'm guessing you'd want a solar-panel skin to power the air compressor and a few ducted fans for fine maneuvering and a controlled landing (for the landing, you'll probably want to reduce your buoyancy-to-weight ratio to around 0.9 and then land vertically), with some batteries for backup.

Why wouldn't that work?

scarecrovv wrote:Lockheed Martin seems to think it's a good idea. My understanding is that they're working on a blimp-like thing that makes use of both aerodynamic lift and buoyancy. It should be faster than blimps, cheaper than planes, have a large cargo capacity, and be able to land in any sufficiently large open space.

Not quite what the OP was thinking of, but it's a good example of how much lifting gas you actually need.

I'm wondering how this sucker takes off. Is it just VTOL with vectored fans that slowly switch to forward thrust as you build up aerodynamic lift from forward velocity?

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Re: Hybrid Plane/Airship

Postby thoughtfully » Wed Sep 11, 2013 6:58 pm UTC

Yeah, there's not much "jet" going on there. Pretty neat though, especially the ability to land without any infrastructure/support. I'd prefer the limited supply of Helium went to MRIs and research, though :)

There's a link to a youtube video at the LM page; it's got some details and action shots.
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Re: Hybrid Plane/Airship

Postby Copper Bezel » Wed Sep 11, 2013 7:49 pm UTC

Bah. Build it, use hydrogen. It's totally worth it. It's today's goddamn zeppelin of the future of the past, and I want one.

davidstarlingm wrote:
Copper Bezel wrote:Can't be as silly as the gravity plane (thanks for sharing, JBJ; I'm a little awed by the step where compressed air is used as a power source to compress air, increasing ballast.)

I don't think it's particularly silly. Build a lighter-than-air craft with an overall aerodynamic glider-like shape. Release it and let it rise to altitude. Once you're at altitude, compress surrounding air and pump it into collapsible bags inside your hydrogen chamber, which will decrease your buoyancy; then, fly it down to your destination like a glider. Continue pumping air into your chamber to match atmospheric pressure until you reach a low enough altitude to land.

I'm guessing you'd want a solar-panel skin to power the air compressor and a few ducted fans for fine maneuvering and a controlled landing (for the landing, you'll probably want to reduce your buoyancy-to-weight ratio to around 0.9 and then land vertically), with some batteries for backup.

Why wouldn't that work?

It would, fair and well, but you're describing a solar blimp, not a "gravity plane." "Solar blimps" are a thing that could make sense. Even solar hybrid blimps. The Lockheed Martin design is fairly sexy, and it'd be even better with a layer of solar cells up top.

The vessel described on the page JBJ linked is a bit different. Check out the video.

Spoiler:
It's a perpetual motion machine, to start with. It's also heavier than air at a glance, since it's half a 747 wrapped around a pair of small zeppelins, but depends on acting as a lighter than air aircraft for the "up" part of its cycle.

Best bit, though, it stores energy exclusively as compressed air, while air compression is also used at altitude to increase ballast. This means that compressed air is being released to compress more air, enough to change the buoyancy [cannot spell that word] of the plane itself from net positive to net negative, at altitude. This breaks my brain.

It's of course not entirely impossible that the plane could gain a small amount of mass that way, though - the perpetual motion bit only comes in afterward, when the plane falls, and it uses turbines to extract energy from its fall, which compress some external air....
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Re: Hybrid Plane/Airship

Postby speising » Wed Sep 11, 2013 8:03 pm UTC

that gravity thing is weird. i mean, at least they got a professionally designed website, but the science is unconvincing.
he starts of by describig the weather cycle, with a nod to the "small input" of solar power to heat the water, but then claims the whole thing gains its energy mainly by gravity? i believe some people would call this "woo".

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Re: Hybrid Plane/Airship

Postby JBJ » Wed Sep 11, 2013 8:22 pm UTC

Yeah, I linked to the Gizmodo article on it, rather than the project's site, because when is cyan text on a star-field background ever a good idea? Also, since it was soft science the OP was writing towards I figured the general flight profile might be plausible (without the perpetual motion aspect).
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Re: Hybrid Plane/Airship

Postby davidstarlingm » Wed Sep 11, 2013 8:25 pm UTC

Copper Bezel wrote:Bah. Build it, use hydrogen. It's totally worth it. It's today's goddamn zeppelin of the future of the past, and I want one.

Yeah, hydrogen FTW.

Query: how much would a helium-hydrogen mixture decrease the probability of an accidental ignition? Would it even work?

It would, fair and well, but you're describing a solar blimp, not a "gravity plane." "Solar blimps" are a thing that could make sense. Even solar hybrid blimps. The Lockheed Martin design is fairly sexy, and it'd be even better with a layer of solar cells up top.

The vessel described on the page JBJ linked is a bit different. Check out the video.

Hmm, yes, it's not quite what I had in mind.

Even so, it's not a perpetual motion machine. At least, I don't think it is. Granted, his lovely little website has all the painful earmarks of a conspiracy theorist den (seriously, where do these people get the idea that black backgrounds make them look credible?), but it's not a perpetual motion machine per se.

As best as I can make out, it isn't really supposed to be gravity-powered; its real power source is a rudimentary Carnot engine. The cycle is a heat exchange between the warmer low atmosphere and the cooler high atmosphere. Now, the website has some gobbledygook about a "proprietary low-boiling-point liquid that acts as a lighter-than-air lifting gas" but that doesn't seem possible.

Even so, the heat exchanger aspect seems sound (in theory). Start with a rigid lighter-than-air craft at the surface (with an insulated lifting gas) and float skyward. At altitude, use a highly efficient heat exchanger to cool the lifting gas, which both reduces buoyancy and provides mechanical energy (which can then be used to take in and compress atmospheric air for ballast to further reduce buoyancy). Glide down to the ground, and run the heat exchanger in reverse to heat the lifting gas while again gaining energy. You're very slightly altering the ground-to-altitude temperature gradient, but you should be gaining energy both going up and coming down.

As far as the other idea was concerned: though the blimp would indeed be prettier with solar panels on top, I was thinking of something with a profile more like the fuelless thing. Faster, you know.

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Re: Hybrid Plane/Airship

Postby Copper Bezel » Thu Sep 12, 2013 5:05 am UTC

Yeah, that writeup makes surprisingly more sense than the video did, with the emphasis on heat transfer as the power source. I really think the effects would still be too subtle to get a high descent speed out of the thing, and that's where it's doing most of its transit, so I'm not really convinced, still. Again, throwing on some solar cells and fans would make it a good bit more convincing.

The idea of making it more streamlined for greater speed takes us back to the problem Izawwlgood started off with - if you want a more aerodynamic body with an airplane-like profile, it just means building bigger for the same amount of buoyant lift, and increasing drag that way. The gravity-plane-that's-really-a-temperature-gradient-plane requires a phase of lighter-than-air activity. (Just like the LM design doesn't.)

For that matter, if a high-volume fuselage is needed with as little drag as possible, but only a limited amount of aerodynamic lift, why wouldn't you use a lifting body design? Why waste structural weight on surface area that isn't, you know, wrapped around a volume containing gas, particularly on such large wings that seem to assume a much denser craft? I just don't believe that the parts that are zeppelin could lift the parts that are glider, or that the parts that are glider make aerodynamic sense attached to a zeppelin. The LM design just seems like a more completely thought out version of the same idea (power source aside.)
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Re: Hybrid Plane/Airship

Postby davidstarlingm » Thu Sep 12, 2013 1:59 pm UTC

Copper Bezel wrote:Yeah, that writeup makes surprisingly more sense than the video did, with the emphasis on heat transfer as the power source. I really think the effects would still be too subtle to get a high descent speed out of the thing, and that's where it's doing most of its transit, so I'm not really convinced, still. Again, throwing on some solar cells and fans would make it a good bit more convincing.

The idea of making it more streamlined for greater speed takes us back to the problem Izawwlgood started off with - if you want a more aerodynamic body with an airplane-like profile, it just means building bigger for the same amount of buoyant lift, and increasing drag that way. The gravity-plane-that's-really-a-temperature-gradient-plane requires a phase of lighter-than-air activity. (Just like the LM design doesn't.)

I'm pretty sure that the mere contraction/expansion of the gas wouldn't be enough to do the trick; you'd have to derive mechanical energy from the heat exchange and use that to compress air and really substantially decrease your buoyancy that way.

For that matter, if a high-volume fuselage is needed with as little drag as possible, but only a limited amount of aerodynamic lift, why wouldn't you use a lifting body design? Why waste structural weight on surface area that isn't, you know, wrapped around a volume containing gas, particularly on such large wings that seem to assume a much denser craft? I just don't believe that the parts that are zeppelin could lift the parts that are glider, or that the parts that are glider make aerodynamic sense attached to a zeppelin. The LM design just seems like a more completely thought out version of the same idea (power source aside.)

Maybe the LM design isn't set up for ever having such a low-buoyancy stage?

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Re: Hybrid Plane/Airship

Postby Copper Bezel » Thu Sep 12, 2013 3:03 pm UTC

Well, no, it looks to be permanently just a bit heavier than air, and make up for the difference via thrust vectoring until at speed , when it's using aerodynamic lift.
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Re: Hybrid Plane/Airship

Postby davidstarlingm » Thu Sep 12, 2013 5:02 pm UTC

Copper Bezel wrote:Well, no, it looks to be permanently just a bit heavier than air, and make up for the difference via thrust vectoring until at speed , when it's using aerodynamic lift.

Which is a friggin' awesome design, by the way. But the giant-puffy-flying-wing design is pretty much the opposite of what you'd want from an unpowered glider, in terms of aerodynamicity. We're looking for a long, rapid gliding descent. The only reason the LM can get any lift is from the thrusters; it has WAY too much drag to glide on its own.

Another option is to dispense with the whole heat exchanger design and use a collapsible balloon and a few decent compressors. Lift a fairly standard glider to altitude using regular balloons, then use an onboard compressor (perhaps supplemented by solar panels) to compress the lifting gas into tanks and pull the empty balloon in to stow, then glide to your destination.

Heck, for that matter, you could use hydrogen balloons and burn a portion of the H2 as your energy source for the compressors. Instead of refueling your entire plane, you'd merely need to top off the hydrogen in your tanks each time you land. Surely that would be more efficient than current cargo freighting.

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Re: Hybrid Plane/Airship

Postby nitePhyyre » Thu Sep 12, 2013 6:27 pm UTC

davidstarlingm wrote:Heck, for that matter, you could use hydrogen balloons and burn a portion of the H2 as your energy source for the compressors. Instead of refueling your entire plane, you'd merely need to top off the hydrogen in your tanks each time you land. Surely that would be more efficient than current cargo freighting.
More energy efficient? Maybe. More time efficient? Nah.
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Re: Hybrid Plane/Airship

Postby davidstarlingm » Thu Sep 12, 2013 8:17 pm UTC

nitePhyyre wrote:
davidstarlingm wrote:Heck, for that matter, you could use hydrogen balloons and burn a portion of the H2 as your energy source for the compressors. Instead of refueling your entire plane, you'd merely need to top off the hydrogen in your tanks each time you land. Surely that would be more efficient than current cargo freighting.
More energy efficient? Maybe. More time efficient? Nah.

Probably faster than shipping by truck. Maybe even cheaper, too.

The "proprietary liquid" mentioned by the gravity-plane quacks is probably some sort of ammonia solution. The problem there is that evaporation cools everything, and increases pressure (along with boiling point), so I doubt it would work as easily as they claim.

Compression of hydrogen is difficult, and containment even moreso. We could always use methane instead; it has about half the lifting strength of hydrogen, but it is twice as energy-dense by volume in its compressed state (and doesn't need to be cooled, unlike hydrogen) and nearly four times as energy-dense by volume in its gaseous state.

I'm not quite sure how to calculate the energy requirements of compressing a cubic meter of gaseous methane to 250 bar. Any ideas? I'm trying to figure out how much of the gas would be consumed in order to compress the gas that remained. Incidentally, the expansion of the stored methane could be used to power a generator while the craft was on the ground refueling.

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Re: Hybrid Plane/Airship

Postby Copper Bezel » Fri Sep 13, 2013 3:04 am UTC

Well, the ideal gas law and isothermal compression at least give a lower bound at 600 kj (~12 grams of methane, 1/60 what you started with.) I'm guessing I'm stating the obvious, although the compressibility factor for methane is only like 1.2 here, so long as the excess heat is magically tapped off somehow, the real gas properties can't be too far off, right? I'm a little surprised at how small it is.

Looking this stuff up has made the gravity plane's chosen energy storage method all the more odd. At 100 bar, 3/4 of the energy stored is lost to heat.
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Re: Hybrid Plane/Airship

Postby davidstarlingm » Fri Sep 13, 2013 3:47 pm UTC

Copper Bezel wrote:Well, the ideal gas law and isothermal compression at least give a lower bound at 600 kj (~12 grams of methane, 1/60 what you started with.) I'm guessing I'm stating the obvious, although the compressibility factor for methane is only like 1.2 here, so long as the excess heat is magically tapped off somehow, the real gas properties can't be too far off, right? I'm a little surprised at how small it is.

Yeah, CNG works well for car engines and the like (at least for inner-city buses and taxis and whatnot) because it doesn't require any thermally-sensitive storage.

Looking this stuff up has made the gravity plane's chosen energy storage method all the more odd. At 100 bar, 3/4 of the energy stored is lost to heat.

Magic insulation.

Looking back over my idea, I'm honestly a little puzzled, because it feels a little bit perpetual-motion-ey. Let me recap just to make sure I'm getting it right.

  1. Methane is pumped into the lifting balloon at sea level. The methane contains chemical potential energy in its bonds and positive potential gravitational energy due to its buoyancy.
  2. The craft rises to altitude, converting the positive potential gravitational energy of the methane's buoyancy into the negative potential gravitational energy of the craft.
  3. A small portion of the methane (call this "fuel methane") is burned with atmospheric oxygen, compressing the remainder of the methane by over 100x. This converts its chemical potential energy into the potential energy of the pressurized methane. This step also dramatically increases the negative potential gravitational energy of the craft....which is odd.
  4. The craft glides to its destination, converting the negative potential gravitational energy of the craft into kinetic energy, which is used to move a long distance and so forth (one could even place turbines on the outside of the craft to extract mechanical energy at the expense of a little more drag).
  5. On the ground, the fuel methane will need to be replaced. Paradoxically, the potential energy of the pressurized methane can be extracted as it refills the balloon; this potential energy happens to be the same amount of energy as one would have acquired from burning the amount of methane you're replacing, adjusting for the various inefficiencies. That's awkward.

It really feels like we've got a perpetual motion thing going on here. What's the catch? Am I not factoring in the potential energy of atmospheric oxygen that we use to burn? Or is there a trick involving the positive/negative gravitational potentials that I'm not seeing?

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Re: Hybrid Plane/Airship

Postby speising » Fri Sep 13, 2013 3:54 pm UTC

Very few perpetual motion machines can afford to burn fuel.

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Re: Hybrid Plane/Airship

Postby davidstarlingm » Fri Sep 13, 2013 4:07 pm UTC

speising wrote:Very few perpetual motion machines can afford to burn fuel.

If the efficiency of the machine is greater than 100% after taking into account the potential energy of the fuel being burned, it's still perpetual motion.

For a more closed-cycle design, you could dispense with the glider altogether and just envision everything in a large shaft. Empty tanks and a compressor are lifted up the shaft by the methane balloon, doing work in the process. A small portion of the methane is burned at the top of the shaft to power the compressor, which fills the tanks with methane. The entire apparatus descends under the force of gravity, doing work in the process. When you get to the bottom, you top off the methane....but the potential energy stored in the compression of the methane is identical to the energy you would have gotten if you had burned the same amount of methane to compress it all at the ground level, meaning that the work done to raise and lower the apparatus came from nowhere at all.

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Re: Hybrid Plane/Airship

Postby Izawwlgood » Fri Sep 13, 2013 4:14 pm UTC

Maybe I'm missing something, but isn't the 'pump methane into the plane at sea level' how energy is added? It's not a perpetual motion machine; it's got an input of energy.
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Re: Hybrid Plane/Airship

Postby davidstarlingm » Fri Sep 13, 2013 4:24 pm UTC

Izawwlgood wrote:Maybe I'm missing something, but isn't the 'pump methane into the plane at sea level' how energy is added? It's not a perpetual motion machine; it's got an input of energy.

Right, but that input of energy is not actually consumed.

Consider two stations.

In one station, gaseous methane is burned to power a compressor, which compresses more methane, thus storing the fuel methane's potential energy in a tank of CNG.

At the other station, the same amount of methane is pumped into an aircraft, which leaves and returns with a tank of CNG.

Same amount of methane going in; same amount of potential energy in the CNG coming back. Only, in the latter case, you also had a flight. For free.

Wait, is it going to take more energy to compress the methane at altitude than it would at sea level because the atmospheric pressure is lower? That might be the difference.

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Re: Hybrid Plane/Airship

Postby Copper Bezel » Fri Sep 13, 2013 6:07 pm UTC

Yeah, you're right. That same pressure gradient is the thing that provides all lift, so it checks out. I thought at first that it was only a perpetual motion machine in the way that an empty elevator car on a counterweight is until I realized that the payload didn't seem to automatically introduce any new terms (if the "counterweight system" proper is the whole system of the gases, internal and external, involved, and the"payload" is the craft and anything it carries.) But there's a proportional difference in the amount of gas you'll need to compress at altitude.
So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.

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Re: Hybrid Plane/Airship

Postby davidstarlingm » Fri Sep 13, 2013 6:46 pm UTC

Copper Bezel wrote:Yeah, you're right. That same pressure gradient is the thing that provides all lift, so it checks out. I thought at first that it was only a perpetual motion machine in the way that an empty elevator car on a counterweight is until I realized that the payload didn't seem to automatically introduce any new terms (if the "counterweight system" proper is the whole system of the gases, internal and external, involved, and the"payload" is the craft and anything it carries.) But there's a proportional difference in the amount of gas you'll need to compress at altitude.

I work in gas and oil pipeline regulation, so I was asking one of the engineers about burning gas to compress gas. Apparently, pipeline companies do burn small amounts of methane to run compressors to pressurize the rest of the methane. Unfortunately, that's typically happening at a pressure station where the step-up in pressure is very small and what's coming in is already under pressure. So that's not too helpful.

I guess what we need to find out is the chemical potential energy of methane at STP, then figure out how much energy it takes to compress one kg of methane from STP to CNG (which should be the same as the work potential of one kg of CNG expanding to STP). That will tell us what percentage of your lifting gas would need to be burned in order to compress the remainder. We'd need to repeat the process for our "target altitude" temp and pressure. The difference between these two (after adjusting for compressor inefficiencies) represents the actual energy cost per kg of methane.

Then, we'd need to multiply this by the mass of methane needed to lift a small glider, a compressor, a motor, and a collapsible balloon. If the final "energy bill" is lower than the energy cost of towing a glider up with a prop plane, it's more economical.

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Re: Hybrid Plane/Airship

Postby Copper Bezel » Sat Sep 14, 2013 10:40 pm UTC

I think it's both simpler in concept and more complex in application than that. Methane's caloric value by mass is known, so there's no mystery there. If having it in a less compressed form (expanded into the balloon) makes it more difficult to engineer a turbine and pump, then you might end up needing an auxiliary power source to start the process, and that might be enough weight cost to make the whole idea fall apart.

The compressor efficiency is also an unknown, more on the generating turbine end than the compression fan end. I'm not sure how to estimate that for such an unusual case. I think that's much more an unknown than the work required.

Once you get the system running, the balloon reinflates on the stored energy from the compression at altitude, and thanks to ambient heat transfer, it'll expand to the volume and pressure it would have regardless of how it was compressed. I don't see how you could reclaim any meaningful energy at that stage in a useful form, particularly at the launch site - at best you pool your CNG across flights (since it's at slightly lower compression than it started at, but this doesn't really matter for inflating the balloon.) I think the only energy cost you need to calculate is the cost of compression at altitude.

And a plane-towed glider does have the extra energy input of its lateral momentum. To be equivalent, the balloon has to hit a higher altitude, so the glider can turn potential energy into forward motion. Whether that will make a significant difference to the energy needed to compress the gas, I don't know (most likely not, but worth noting on the balance sheet.)
So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.

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Re: Hybrid Plane/Airship

Postby davidstarlingm » Mon Sep 16, 2013 3:29 pm UTC

Copper Bezel wrote:I think it's both simpler in concept and more complex in application than that. Methane's caloric value by mass is known, so there's no mystery there. If having it in a less compressed form (expanded into the balloon) makes it more difficult to engineer a turbine and pump, then you might end up needing an auxiliary power source to start the process, and that might be enough weight cost to make the whole idea fall apart.

It's just that there aren't many engines/compressors designed to run on STP methane. Sure, methane has a certain caloric value by mass, but its value as a combustible fuel will include the stored energy represented by its compression. One kg of CNG contains more energy than one kg of STP methane.

Of course, we can get around this by simply running the turbine off of CNG, "priming" the CNG storage tanks with enough fuel to start the process and then looping a fraction of the CNG produced back into the turbine.

The compressor efficiency is also an unknown, more on the generating turbine end than the compression fan end. I'm not sure how to estimate that for such an unusual case. I think that's much more an unknown than the work required.

Compressor efficiency can range from 50-85% when we're dealing with a step-up in pressure across a gas pipeline compressor station. Unfortunately, that's not what we're dealing with. The biggest unknown I see is the difference in energy between one kg of STP methane and one kg of CNG. It's been too long since I took thermodynamics; not exactly sure how to estimate that. Any ideas?

Once you get the system running, the balloon reinflates on the stored energy from the compression at altitude, and thanks to ambient heat transfer, it'll expand to the volume and pressure it would have regardless of how it was compressed. I don't see how you could reclaim any meaningful energy at that stage in a useful form, particularly at the launch site.

Can't you use it to run a turbine as it expands?

at best you pool your CNG across flights (since it's at slightly lower compression than it started at, but this doesn't really matter for inflating the balloon.) I think the only energy cost you need to calculate is the cost of compression at altitude.

Yeah, that's the whole point: to have an essentially reusable balloon. Balloon-launched gliders have been around for a while, but the amount of gas needed to lift a glider is rather expensive considering that it's wasted. The ability to recover this gas and stow it within the craft is novel, I think. Heck, you could even use the CNG to run a prop or small jet turbine engine.

And a plane-towed glider does have the extra energy input of its lateral momentum. To be equivalent, the balloon has to hit a higher altitude, so the glider can turn potential energy into forward motion. Whether that will make a significant difference to the energy needed to compress the gas, I don't know (most likely not, but worth noting on the balance sheet.)

Definitely a good point.

Could it conceivably make sense to use a balloon to lift an orbital insertion rocket with the balloon lifting gas as a major component of the fuel?


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