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SR380
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Cryogenic Fuel On Aircrafts

Thu Jan 05, 2017 7:43 pm

Hey guys,

I am checking on Wikipedia the back story of the Tupolev 244. A huge SST developed after the withdrawn of the Tu-144. It would have been twice as big, carry 300+ pax, over 9000kms, using cryogenic fuel!

Some other projects were conducted on Tupolev 154, dubbed 156.

All of those projects were canceled in the early 1990s, following the dismantlement of the USSR.

How fare did they go studying such fuel? Would it be suitable in a short term future? Why A, B, or any other aircraft manufacturer aren't taking any leap towards this technology?

Cheers,
 
kalvado
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Re: Cryogenic Fuel On Aircrafts

Thu Jan 05, 2017 8:24 pm

Cryogenic fuel can be either some flavor of liquid natural gas, preferably methane - or liquid hydrogen.
In both cases, you get higher energy density per pound/kilogram, but lower density per liter/gallon. Besides,, you need a bulky and heavy thermally insulated tank, and incur losses at idle and at liquefying stage.
If weight is critical, H2 may be advantageous - look at space applications. Short lifetime of loaded tank and tricks of dealing with hydrogen make it expensive - don't touch until you ABSOLUTELY need that. Volume may be a big issue for the airplane. Overall similar to using gold wires - sometimes you just cannot use copper, but for house wiring.. Maybe Al is coming as a trend, but gold - no way.
Methane/LNG is a more reasonable idea. Still flat wing tanks are too lossy heat-wise, you may find out that you have dry and warm tanks after plane was parked for several days.
Also considered for space applications, but mostly due to advantages not applicable to planes: similar temperature to LOx oxidizer, less sooth in the engine for reusable runs.
I doubt anything in terms of cryogenic fuel would make it to mass produced airplanes. If anything, I can see planes being last mechanisms burning hydrocarbons as source of energy.
 
wingscrubber
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Re: Cryogenic Fuel On Aircrafts

Thu Jan 05, 2017 9:42 pm

Hi SR380,

I actually did my dissertation on this topic at Uni - Tupolev got further along with this technology than any other manufacturer, but Airbus also had a Cryoplane concept on the drawing board too. A Cryoplane is simply a plane fueled from a cryogenic fuel, i.e. liquid natural gas or liquid hydrogen, it's difficult because it must be stored in a pressure vessel, refrigerated.

I can't find the website I remember finding, but I did discover some old Tupolev archives which detailed the multiple Cryoplane concepts they had developed, including a Turboprop variant, plus the 244 you mention. They also laid out the potential infrastructure they would need to accomplish a liquid natural gas distribution network at airports.

The goal with Cryoplanes, at least for Russia, was to transition initially to liquid natural gas fuel for aircraft as a first step (as LNG is very cheap and plentiful there, in comparison to other markets), but the concept would work for Liquid Hydrogen too.

The attractiveness of liquid hydrogen, and the reason I studied it for my diss 11 years ago, is that you could potentially implement a carbon-neutral propulsion system for an airliner, which is environmentally attractive. LNG was meant to be a stepping stone to LH fueled aircraft.

The challenges though, are that the massive tank size either takes up a large internal cabin volume that could otherwise be used for passengers, or incurs drag if you house it externally Beluga-style. Also, there are crashworthiness and flammability problems, loss of bending relief provided by fuel weight, no suitably mature turbine technology, and principally: no infrastructure.

Airports are not set up to deliver LNG, let alone Liquid Hydrogen (and Hydrogen is not a fuel, it is just energy currency, you have to 'buy' it from somewhere, be that gassification of methane, or electrolysis of water, which then needs its own fuel...)

Because you cannot produce or supply LNG or Hydrogen to commercial airports at a cheaper rate than Kerosene, the business model is a non-starter, even if the environmental model is attractive.

What's actually happening in industry, is that Biofuel is becoming more attractive, as it can now be produced in sufficient quantity and quality to supply to aircraft, and is in principle carbon-neutral, so it's solving both the supply chain problem and the environmental problem without any technical impact on the aircraft.

I really believed Cryoplanes were the logical path in my Uni days, however in the time since I've changed tact on the evils of CO2, I'm less convinced now that it is the woe of the world, there is quite the Pascal's wager that CO2 is a very bad thing, the facts that H2O has a higher radiative forcing, higher specific heat capacity and is a billion times more plentiful fall by the wayside.

Anyhoo - back to Tupolev, I found some of the pics of the Tu-155 and Tu-156 I used in my dissertation, below. The Tu-155 actually flew with one engine powered by LNG from the large central tank, and the other two with regular Kerosene from the wing tanks.
It's mothballed somewhere in Russia now I think. The Tu-155 was just the flight test version though, the passenger version was intended to be the Tu-166 never built, but this also shares a designation with an EWACS plane which is completely different.

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SR380
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Re: Cryogenic Fuel On Aircrafts

Fri Jan 06, 2017 11:17 am

Thanks for this very complete response!
 
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DocLightning
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Re: Cryogenic Fuel On Aircrafts

Fri Jan 06, 2017 8:40 pm

wingscrubber wrote:
Anyhoo - back to Tupolev, I found some of the pics of the Tu-155 and Tu-156 I used in my dissertation, below. The Tu-155 actually flew with one engine powered by LNG from the large central tank, and the other two with regular Kerosene from the wing tanks.
It's mothballed somewhere in Russia now I think. The Tu-155 was just the flight test version though, the passenger version was intended to be the Tu-166 never built, but this also shares a designation with an EWACS plane which is completely different.


I think this really shows why cryofuels are impractical. If that one tank only powered one engine, imagine if you needed three times that tankage. Why, you'd have no room for any passengers at all!
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan
 
kalvado
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Re: Cryogenic Fuel On Aircrafts

Fri Jan 06, 2017 9:29 pm

DocLightning wrote:
I think this really shows why cryofuels are impractical. If that one tank only powered one engine, imagine if you needed three times that tankage. Why, you'd have no room for any passengers at all!

I believe tank was already sized for 3 engines, and probably fitting that into existing structure was less than optimal.
Given energy density of LNG per volume is 2/3 of than for Jet-A, I don't see sheer volume as a catastrophic problem, especially for short haul - and if the price is right. Difficulties with squeezing thermally insulated tanks into wings, however, is more interesting. BWB may make it more practical..
 
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DocLightning
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Re: Cryogenic Fuel On Aircrafts

Sun Jan 08, 2017 8:24 pm

I believe tank was already sized for 3 engines, and probably fitting that into existing structure was less than optimal.


OK, fair enough. Still not a good arrangement.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
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Okie
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Re: Cryogenic Fuel On Aircrafts

Mon Jan 09, 2017 1:48 am

DocLightning wrote:
I think this really shows why cryofuels are impractical. If that one tank only powered one engine, imagine if you needed three times that tankage. Why, you'd have no room for any passengers at all!

Absolutely.
The main issue with cryofuels is the tank insulation.
If using solid insulation or micro spheres you end up with enough thickness on all sides of the tankage that the wing does not end up with much tankage space because of the thickness of the insulation much less dealing with irregular shaped pressure vessel.
That only leaves vacuum insulation in which now you have increased weight because you have to have a double wall tank and makes irregular shapes impractical.
The larger the diameter the more effective the tank becomes for the loss of volume for the insulation which brings you to installing the tank large diameter in the fuselage or structurally part of the fuselage.
The next issue is the weight which will indicate the tank will have to be near the center of lift essentially dividing the fuselage into a front and rear section.


Okie
 
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Matt6461
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Re: Cryogenic Fuel On Aircrafts

Mon Jan 09, 2017 6:56 pm

Wingscrubber wrote:
I actually did my dissertation on this topic at Uni


Thanks for all the info! I have a few follow-up questions, related to Okie's points:

Okie wrote:
The main issue with cryofuels is the tank insulation.
If using solid insulation or micro spheres you end up with enough thickness on all sides of the tankage that the wing does not end up with much tankage space because of the thickness of the insulation much less dealing with irregular shaped pressure vessel.
That only leaves vacuum insulation in which now you have increased weight because you have to have a double wall tank and makes irregular shapes impractical.
The larger the diameter the more effective the tank becomes for the loss of volume for the insulation which brings you to installing the tank large diameter in the fuselage or structurally part of the fuselage.


Thanks and very interesting. I have never seen a cryo proposal with in-wing tanks, only fuselage body tanks. There are a few published studies on this idea, going back as far the '60's. Maybe I've even read Wingscrubber's study at some point (browsed, skipping over the heavy-math parts I don't understand).

Do either of you (or others) have a rough weight estimate for a state-of-the-art cryo tank for LNG? Or for hydrogen?

Re the wing-bending relief, here's an idea that I'd love to see studied and/or explained to me why it wouldn't work out (aside from impracticality of the cryoplane generally):

  • Assume we're building a VLA-size plane with a big fuselage
  • Instead of building in-wing or in-fuselage cryo tanks, put tanks in wing-mounted side fuselages. Advantages are:
    • The fuel tanks provide bending relief, such that for a big plane running into square-cube law, the tank weight approaches being "free"
    • As you're already adding wing fuselages, it seems a good idea to move baggage/cargo load into those fuselages, rather than in the main pax fuselage. Escalation of volume to accomodate cargo would escalate wing-fuselage wetted area and weight less than volume. The now-empty cargo space in the main fuselage would become space for more seats and/or galleys/lavs. This also provides further wing bending relief for a given payload design parameter.
    • Wing-fuselage drag could be greatly ameliorated by mounting the engines at the rear of them and using them as Goldschmied propulsors. The biggest obstacle to building a Goldschmied propulsor currently is the need to build the empennage around it. That wouldn't be an issue for wing-fuselages, obviously.
    • Outboard of the wing-fuselages, and assuming they are mounted under the wing, it would be easy to use a strut-braced wing: A big obstacle to SBW now is the need to use a high-wing as that poses water-landing difficulties. Obviously evacuation concerns wouldn't apply to wing-fuselages containing only tanks and cargo/bags. Depending on design, we might end up with very little net MZFW bending moment at the wing roots (due to bending relief), plus very efficient handling of bending moments outboard of the wing-fuselages (due to struts). Could be a good combination that, on net, outweighs the additional wing-fuselage weight and drag.
  • Drawback would include wing-body join for wing-fuselages at the acute angle with a sloping wing-lost of friction there potentially. Also the issue of water landing - we'd have to assume both wings will tear off or else design a way for the wing-fuselages to detach the way engines are supposed to. Perhaps some of the space freed up by moving cargo/baggage into wing fuselages could hold a few inflatable "hulls" or cushions to keep a waterborne and wingless fuselage stable enough for evacuation.

That's just thinking out loud...
 
DfwRevolution
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Re: Cryogenic Fuel On Aircrafts

Mon Jan 09, 2017 8:13 pm

wingscrubber wrote:
The attractiveness of liquid hydrogen, and the reason I studied it for my diss 11 years ago, is that you could potentially implement a carbon-neutral propulsion system for an airliner, which is environmentally attractive. LNG was meant to be a stepping stone to LH fueled aircraft.

The challenges though, are that the massive tank size either takes up a large internal cabin volume that could otherwise be used for passengers, or incurs drag if you house it externally Beluga-style. Also, there are crashworthiness and flammability problems, loss of bending relief provided by fuel weight, no suitably mature turbine technology, and principally: no infrastructure.


The LNG industry has changed dramatically in the last 11 years.

Infrastructure: The discovery of abundant and cheap U.S. natural gas has motivated efforts to produce, distribute, and export LNG. Engineering contractors are offering cookie-cutter LNG plants in a variety of sizes. It's a buyers market. China already has a big fleet of mid-scale LNG plants for truck distribution. You can have a mid-scale LNG plant online in 12-18 months.

Turbine technology: The aircraft OEMs have been selling industrial versions of their aero engines (LM2500, LM6000, iRB211, iTrent) that burn natural gas for the last 40 years. The technology is super mature with millions of operating hours. In fact, there is a reliability and emissions benefit from running natural gas over liquid fuel. Maintenance intervals are roughly double when run on gas fuel versus liquid fuel.

Flammability: LNG is more stable than liquid fuels

The principle challenge is unquestionably the aircraft integration. I would propose under-wing tanks like you see on combat aircraft. The drag penalty is minor compared to the huge cost spread between LNG and kerosene. You maintain the wing-bending relief of storing fuel in the wings and no cabin space is lost. It's a minimum change solution. However, I have no expectation of anyone taking a leap on this. The industry is justifiably conservative.
I have a three post per topic limit. You're welcome to have the last word.
 
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DocLightning
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Re: Cryogenic Fuel On Aircrafts

Mon Jan 09, 2017 8:17 pm

What about solid-state metal hydride storage for hydrogen? What are the issues with that? Yes, I know that hydrogen is more of an energy storage solution than a proper "fuel," but let's assume we can electrolyze water using our unlimited clean energy generated by our extensive fusion powerplant network on the ground, so hydrogen is cheap to make. What are the issues with using metal hydride solid-state systems for storage in aircraft?
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AirlineCritic
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Re: Cryogenic Fuel On Aircrafts

Mon Jan 09, 2017 8:26 pm

Interesting topic. Wingscrubber, would your dissertation be online by any chance? Care to give us a pointer, would be a fun read I'm sure!
 
DfwRevolution
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Re: Cryogenic Fuel On Aircrafts

Mon Jan 09, 2017 9:30 pm

DocLightning wrote:
What about solid-state metal hydride storage for hydrogen? What are the issues with that? Yes, I know that hydrogen is more of an energy storage solution than a proper "fuel," but let's assume we can electrolyze water using our unlimited clean energy generated by our extensive fusion powerplant network on the ground, so hydrogen is cheap to make. What are the issues with using metal hydride solid-state systems for storage in aircraft?


I've traditionally seen metal hydrides proposed in the context of micro storage for things like laptops, cell phones, and electric vehicles that are fuel cell powered. I've never thought about it for aviation, but it could be interesting. Distributing pebbles of metal hydride in wing cavities might allow you to store hydrogen in a modern long-thin airfoil without a ton of insulation.

Without running any numbers, my gut reaction is that the cost, energy/weight density, and storage capacity might be uncompetitive with simple tanks. An aluminum tank can hold 10x its weight in LH2. The feasibility would also be impacted by whether you plan to use the hydrogen to power fuel cells + electric motors or combustion gas turbines. It's a very complex multi-variable scenario.
I have a three post per topic limit. You're welcome to have the last word.
 
kalvado
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Re: Cryogenic Fuel On Aircrafts

Tue Jan 10, 2017 2:04 am

DocLightning wrote:
What about solid-state metal hydride storage for hydrogen? What are the issues with that? Yes, I know that hydrogen is more of an energy storage solution than a proper "fuel," but let's assume we can electrolyze water using our unlimited clean energy generated by our extensive fusion powerplant network on the ground, so hydrogen is cheap to make. What are the issues with using metal hydride solid-state systems for storage in aircraft?

I suspect things will boil down again to energy density.
Best result achieved so far is 10% weight H2 - in lab conditions. H2 has weight energy density about 3x of Jet-A, so best result is about 30% of Jet-A power per kg/lb of hydride. LiH can be around 13%, as far as I understand; borane - BH3 - 23% but borane is not really something you can put into tanks..
Then you need to take into account that only 10% of fuel is actually burning off in flight, so you carry more weight around and need to compensate for that. Last, but not the least, I am not sure if removable tanks for hydrides will be practical, and if tanks are factory-preloaded then effectively plane would have to carry almost max fuel at all times.
I doubt changes in energy extraction strategy (burn/fuel cell) would be nearly enough to compensate for that.
 
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LAX772LR
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Re: Cryogenic Fuel On Aircrafts

Thu Jan 12, 2017 6:58 am

DfwRevolution wrote:
Flammability: LNG is more stable than liquid fuels.

Interesting. I would've intuitively thought the opposite, though admittedly not based on any tangible fact.

What makes it more stable?
I myself, suspect a more prosaic motive... ~Thranduil
 
WIederling
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Re: Cryogenic Fuel On Aircrafts

Thu Jan 12, 2017 11:46 am

DfwRevolution wrote:
wingscrubber wrote:
The attractiveness of liquid hydrogen, and the reason I studied it for my diss 11 years ago, is that you could potentially implement a carbon-neutral propulsion system for an airliner, which is environmentally attractive. LNG was meant to be a stepping stone to LH fueled aircraft.


Just the liquefaction takes an added 20% of the LNG created. irrecoverable ( in contrast to compression energy in gaseous pipeline transfers.

LNG is pushed for political reasons.
Murphy is an optimist
 
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rjsampson
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Re: Cryogenic Fuel On Aircrafts

Sat Jan 14, 2017 12:04 am

LAX772LR wrote:
Interesting. I would've intuitively thought the opposite, though admittedly not based on any tangible fact.

What makes it more stable?


It's the same concept as the tank of propane on your grill being more stable in liquid state. You could shoot it with a bullet (contrary to the movies) and it wouldn't ignite. Many combustible fuels need to be aerosolized, or in a gaseous state to detonate.

Heck, even the coffee creamer or flour in your pantry is highly combustible when dispersed in air:

http://youtu.be/yRw4ZRqmxOc?t=37
"..your eyes will be forever turned skyward, for there.." yeah we know the DaVinci quote. Unfortunately, we're grounded :(

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