Alternative fuel aircraft

Calliban · 2021-03-23 09:29:19

I have raised this topic so that Kbd512, Louis and others, have an appropriate thread in which to discuss Earth based (or potentially Mars based) aircraft topics with the goal of eliminating or greatly reducing, the consumption of light oil (kerosene) or other petroleum based products in aircraft.

Calliban · 2021-03-23 09:36:00

I will kick off with what I believe to be an existing technology option that is applicable to short to medium range flights. My proposal is that we use turboprop aircraft instead of standard small passenger jets, for flights less than about 3000km in length. Using entirely existing technology, this should cut fuel consumption per passenger-km by about one third. The price to pay for this innovation is lower cruising speed, lower ceiling height and more problems with noise. On the plus side, planes can take off from shorter runways. Some technical stretch options, roughly in order of difficulty.

1. Substitute biofuel oils for jet fuel. This reduces greenhouse gas emissions and improves sustainability, but slightly reduces energy density. There are problems with waxing of biofuels in fuel lines at temperatures beneath freezing point. This may require insulation of fuel lines and the use of heaters in fuel tanks, which would warm the fuel prior to take off.

2. The use of LPG instead of jet fuel. This can be stored at room temperature under pressure of ~9bar(a). Its mass energy density is 17% greater than Jet A, though volumetric energy density is lower. It can be stored as a liquid at 1atm if chilled to -44C. At room temperature, its vapour pressure is 8.5bar(a), so pressurised tanks would be needed. Greenhouse gas emissions could be cut by around one third by using LPG instead of Jet A and LPG is a more abundant product in tight oil and natural gas liquids, which are a growing proportion of total oil production, so sustainability is improved compared to Jet A. For a turboprop burning LPG fuel, total greenhouse gas emissions per passenger-km would be about half those of a jet burning Jet A fuel. LPG is cheaper and its greater mass energy density should allow greater range.

3. Use liquid methane (LNG or biogas) as fuel. Methane mass energy density is 29% greater than Jet A, though volumetric energy density of liquid methane is lower. Methane is a cryogen with a boiling point of 111.6K. For planes to use this fuel airports would need to be equipped with LNG storage tanks, fitted with coolers that minimise boiloff. Refuelling will be more hazardous, as spilled LNG would rapidly evaporate giving rise to flammable vapours. The LNG tanks would need insulation to prevent thermal shocking of the airframe and other exposed components. A significant advantage of LNG is its ability to provide interstage cooling of air entering the compressor. This would substantially reduce compressor work, increasing the propulsive power of the engine. Combining these advantages and accounting for the lower mass of fuel, an LNG fuelled turboprop should be twice as energy efficient as a standard jet per passenger-km and reduce greenhouse gas emissions by 70%.

4. Use liquid air or liquid oxygen to fuel the engine. In a gas turbine engine, about one half to two-thirds of all mechanical work harvested by the turbine is consumed by the compressor, which compresses the air into the combustion chamber. The greater the compression ratio, the higher the performance and efficiency of the turbine. However, greater compression ratio increases the mass and power consumption of the compressor. There is a compromise between efficiency and weight, in terms of overall fuel consumption. One way of improving the compression ratio would be to spray liquid air into the air stream between the compressor stages. As the liquid air has a boiling temperature -160C, small droplets will rapidly phase change, cooling the air between the stages. This would allow a substantial improvement in pressure ratio, whilst the compressor work and mass would be reduced compared to baseline. The logical conclusion would be to dispose of the compressor all together and run the turboprop on stored liquid air or liquid oxygen and LNG. This would result in the problem of having to carry a much greater mass of propellant, as the aeroplane is effectively carrying all the air or oxygen its engines burn. So this option only makes sense over relatively short ranges. However, by eliminating the compressor, the efficiency of the engines increases substantially, because the pressure ratio of an engine burning liquid fuel and liquid oxidiser is potentially huge. The power-weight of the engines is also greatly increased and the aircraft can generally be a lot more streamlined, as the engines no longer require air intake. For short to medium range flights, this option could cut fuel consumption dramatically, but would become less and less efficient as journey length increased.

5. Dimethylether, methanol and ammonia fuel. If we get to the point of producing synthetic fuels from biomass or using hydrogen and captured CO2, or hydrogen and nitrogen, these three fuels are the most portable options for an aircraft. Methanol is not a good option, as it is highly corrosive to aluminium alloys. Ammonia causes stree corrosion cracking in aluminium alloys, so should be avoided. Dimethylether does not have any materials compatability issues and can be stored as a chilled liquid at -24°C. Heat of combustion is 31.68MJ/kg, which is about the same as Jet A. This is a fuel that could probably be burned in existing technology engines. An advantage to this fuel is low freezing point, avoiding the need to insulate tanks and lines.

6. Hydrogen. I will look at this in more detail later. It is probably the most challenging fuel to use in any application. It is a deep cryogen with a boiling point of just 20K, a very high mass energy density of 142MJ/kg - about 3.3 times greater than Jet A and 2.6 times greater than LNG. However, volumetric energy density is only about 1/4 of Jet A. This leads to a requirement for bulky fuel tanks, implying more drag. It is for this reason, that hydrogen fuelled aeroplane concepts tend to incorporate a lifting body design, to keep lift-drag ratio from deteriorating too far. The very low temperature of liquid hydrogen complicates the design of fuel tanks and fuel lines, as materials experience embrittlement and air may liquefy on contact with unlagged surfaces. The poor energy density of hydrogen also presents a problem for engine power density.

Last edited by Calliban (2021-03-23 11:12:40)

tahanson43206 · 2021-03-23 10:02:15

For Calliban ... best wishes for success with this interesting new topic.

I'd be interested in seeing development of a part of this new topic with Hydrogen as the fuel. I'm ** pretty ** sure kbd512 has already posted along these lines in other topics.

There ought to be a business opportunity at hand, for investors with a long enough tolerance for payback, and deep enough pockets to fund development and rollout of new systems and procedures.

(th)

Calliban · 2021-03-23 11:54:21

Solid fuels are a possible fuel option. I am thinking of pyrolytic carbon residue from biomass pyrolysis. This would work for just about any biomaterial that is heated in an inert or reducing atmosphere. The material breaks down into volatile products (many of which are combustible), water vapour, CO2, methane, char and ash. The char is close to being pure carbon and can be compressed at high pressure with a binder to produce high density fuel slugs, up to 3 times denser than water, with a heat of combustion of 32MJ/kg. These would be loaded into a pyrolysis chamber in the fuselage. The chamber would insulate the slug and keep it at a temperature of 1000°C and pressure of 30-100 bar. Pure oxygen and steam would be injected and the material would partially burn into a mixture of CO, CO2 and H2 gas, which would then be directed into the engine combustion chambers.

Advantages are high fuel density and the wide availability of biochar. The fuel has a mass energy density similar to Jet A, and volumetric energy density is better than any other fuel.

Disadvantages are (1) design complication; (2) the added weight of the pyrolysis chamber; (3) the need to deal with a solid fuel that cannot be pumped through pipes; (4) the need to carry enough oxygen for pyrolysis; (5) the need to channel low energy density fuel gases into the engines; (6) the need for heat exchangers. None the less, this would appear to be a viable power source for many vehicles, including aircraft.

Last edited by Calliban (2021-03-23 12:02:23)

louis · 2021-03-23 13:39:27

Just a fairly obvious observation...you could manufacture the methane from air and water using green energy - that would make it essentially zero carbon emissions

What about using the free fuel from the sun when above cloud cover.

What if you had a delta shape craft (seriously considered the way forward in the 1950s) to maximise solar power film area on the external area. This might include under-carriage as well given 30% of solar radiation is reflected back - especially off white clouds (might be more like 40-50% I expect).

With a delta wing configuration, for a 100 metre long plane, you might get 5000 x 0.3 Kws =1500 Kws of power from the top side and let's say 400 Kws from below, so a very respectable 1.9MWs.

Now, if the jet were also to unfurl and tow a "solar sail" this might be augmented substantially.

For daylight flights this might be a significant fuel substitute.

Calliban · 2021-03-25 10:00:44

louis wrote:

Just a fairly obvious observation...you could manufacture the methane from air and water using green energy - that would make it essentially zero carbon emissions
What about using the free fuel from the sun when above cloud cover.
What if you had a delta shape craft (seriously considered the way forward in the 1950s) to maximise solar power film area on the external area. This might include under-carriage as well given 30% of solar radiation is reflected back - especially off white clouds (might be more like 40-50% I expect).
With a delta wing configuration, for a 100 metre long plane, you might get 5000 x 0.3 Kws =1500 Kws of power from the top side and let's say 400 Kws from below, so a very respectable 1.9MWs.
Now, if the jet were also to unfurl and tow a "solar sail" this might be augmented substantially.
For daylight flights this might be a significant fuel substitute.

A few scoping calculations on solar power for aircraft.

Let's examine the scenario for the 747-400, which is a moderately well optimised air frame with a lift-drag ratio of 18.
http://www.dutchops.com/AC_Data/Boeing/ … nsions.htm

The fuselage is 68.63m long and 6.5m wide. Wing span is 68.92m and wings have an average width of about 10m. So the upper surface area of a 747-400 is about 1100m2. Insolation will vary between zero and 1kW.m-2 at noon at the equator. At European latitude, insolation has a maximum of 400W.m-2 at peak summer noon, to less than 100W.m-2 at noon in winter, and obviously zero after sundown. So 300W.m-2 is probably a good time average insolation for the world as a whole, so mounting 20% efficient thin-film solar cells of 747-400 wings would generate an average power of 66kW.

The fully loaded take-off weight of a 747-400 is 870,000lb or 395 metric tonnes. With a lift-drag ratio of 18, total drag would 21.94 tonne-force, or 215.275KN. At a cruising speed of 550mph (246m/s), total work performed by the engines would be 53MW. Take-off power is much greater. It's contribution to total fuel consumption depends on the length of the flight. So the average sunlight arriving on a 747-400 upper surface is about three order of magnitude too small to play a useful role in propulsion.

That being said, thin film solar on the upper surfaces could play a useful role in powering some on-board services, provided that the cells are extremely light and add nothing to aerodynamic drag. How light? For each kg of weight added to the plane, engine power must increase by 134.2 watts. Assuming a 40% engine efficiency, that is an extra 9.7MJ of fuel (225 grams) in the course of an 8-hour transatlantic flight. So the panels must generate at least 60W/kg (1kg/m2 @20% efficiency) at beginning of life, for it to be worthwhile including them. This appears to be achievable, so long as including the panels does nothing to increase drag.

I can see applications for small solar-electric aeroplanes in aiding long range communications, monitoring of weather systems and military reconnaissance. These are applications where the plane is unmanned, payload weight may be small, speed is not important and the ability of the plane to remain airborne for long durations without refuelling is very valuable. But solar-electric aeroplanes don't make sense for economical delivery of people or freight between locations. The low propulsive power means that weight margins are very constrained and meaningful payload fraction is small.

Likewise, battery electric aeroplanes may have niche applications for short distance flights. We already have battery powered drones that are useful for some applications, provided that required range is relatively short. Even over distances of a few hundred kilometres, the low energy density of even the best lithium ion batteries would seriously eat into payload capacity. But if you wanted a simple and reliable aircraft that can island hop say, and were concerned about pollution or didn't want the bother of having to ship diesel to the locations, then a battery electric aircraft may be more flexible within it's limitations. This isn't something you could fly across the Atlantic. But it may be something that could fly you from London to Manchester, or from Glasgow to some of those remote Scottish Islands. There is a market that this type of aeroplane could fulfill. One possible stumbling block, in addition to reduced payload, may be charging time. It typically takes several hours to recharge lithium ion batteries. Trying to accelerate that time increases the required generating capability of the grid and may severely limit battery life. Short haul flights generally require rapid turn around to be competitive. So this may be something that is problematic.

A plane powered by solid oxide fuel cell may be a more appropriate solution, in terms of refuellingtime and power-weight. This could burn diesel, biofuels, LPG, liquefied natural gas or even compressed hydrogen.

Last edited by Calliban (2021-03-25 10:21:14)

louis · 2021-03-25 15:55:35

Well I did specify a delta shaped aircraft which would clearly have a larger surface area. In addition above the cloud cover, you could probably justify putting solar power film on the under-parts of the craft. Lastly, as I suggested, there is the possibility of the plane towing behind long solar "sails". We know small aircraft can tow behind advertising banners many times longer than the planes themselves. I don't know whether this would be practical but it might be.

Clearly I wasn't suggesting that this approach would suffice to get the plane off the ground. With VTOL craft, this might be something where that microwave/laser power could be used. Imagine a microwave or laser beam delivering energy to an onboard system till the plane is 5 miles high, at which point onboard batteries take over. This technology might become a reality, though I accept it isn't currently available.

Calliban wrote:

louis wrote:
Just a fairly obvious observation...you could manufacture the methane from air and water using green energy - that would make it essentially zero carbon emissions
What about using the free fuel from the sun when above cloud cover.
What if you had a delta shape craft (seriously considered the way forward in the 1950s) to maximise solar power film area on the external area. This might include under-carriage as well given 30% of solar radiation is reflected back - especially off white clouds (might be more like 40-50% I expect).
With a delta wing configuration, for a 100 metre long plane, you might get 5000 x 0.3 Kws =1500 Kws of power from the top side and let's say 400 Kws from below, so a very respectable 1.9MWs.
Now, if the jet were also to unfurl and tow a "solar sail" this might be augmented substantially.
For daylight flights this might be a significant fuel substitute.
A few scoping calculations on solar power for aircraft.
Let's examine the scenario for the 747-400, which is a moderately well optimised air frame with a lift-drag ratio of 18.
http://www.dutchops.com/AC_Data/Boeing/ … nsions.htm
The fuselage is 68.63m long and 6.5m wide. Wing span is 68.92m and wings have an average width of about 10m. So the upper surface area of a 747-400 is about 1100m2. Insolation will vary between zero and 1kW.m-2 at noon at the equator. At European latitude, insolation has a maximum of 400W.m-2 at peak summer noon, to less than 100W.m-2 at noon in winter, and obviously zero after sundown. So 300W.m-2 is probably a good time average insolation for the world as a whole, so mounting 20% efficient thin-film solar cells of 747-400 wings would generate an average power of 66kW.
The fully loaded take-off weight of a 747-400 is 870,000lb or 395 metric tonnes. With a lift-drag ratio of 18, total drag would 21.94 tonne-force, or 215.275KN. At a cruising speed of 550mph (246m/s), total work performed by the engines would be 53MW. Take-off power is much greater. It's contribution to total fuel consumption depends on the length of the flight. So the average sunlight arriving on a 747-400 upper surface is about three order of magnitude too small to play a useful role in propulsion.
That being said, thin film solar on the upper surfaces could play a useful role in powering some on-board services, provided that the cells are extremely light and add nothing to aerodynamic drag. How light? For each kg of weight added to the plane, engine power must increase by 134.2 watts. Assuming a 40% engine efficiency, that is an extra 9.7MJ of fuel (225 grams) in the course of an 8-hour transatlantic flight. So the panels must generate at least 60W/kg (1kg/m2 @20% efficiency) at beginning of life, for it to be worthwhile including them. This appears to be achievable, so long as including the panels does nothing to increase drag.
I can see applications for small solar-electric aeroplanes in aiding long range communications, monitoring of weather systems and military reconnaissance. These are applications where the plane is unmanned, payload weight may be small, speed is not important and the ability of the plane to remain airborne for long durations without refuelling is very valuable. But solar-electric aeroplanes don't make sense for economical delivery of people or freight between locations. The low propulsive power means that weight margins are very constrained and meaningful payload fraction is small.
Likewise, battery electric aeroplanes may have niche applications for short distance flights. We already have battery powered drones that are useful for some applications, provided that required range is relatively short. Even over distances of a few hundred kilometres, the low energy density of even the best lithium ion batteries would seriously eat into payload capacity. But if you wanted a simple and reliable aircraft that can island hop say, and were concerned about pollution or didn't want the bother of having to ship diesel to the locations, then a battery electric aircraft may be more flexible within it's limitations. This isn't something you could fly across the Atlantic. But it may be something that could fly you from London to Manchester, or from Glasgow to some of those remote Scottish Islands. There is a market that this type of aeroplane could fulfill. One possible stumbling block, in addition to reduced payload, may be charging time. It typically takes several hours to recharge lithium ion batteries. Trying to accelerate that time increases the required generating capability of the grid and may severely limit battery life. Short haul flights generally require rapid turn around to be competitive. So this may be something that is problematic.
A plane powered by solid oxide fuel cell may be a more appropriate solution, in terms of refuellingtime and power-weight. This could burn diesel, biofuels, LPG, liquefied natural gas or even compressed hydrogen.

tahanson43206 · 2021-03-25 17:47:45

For electric aircraft supporters ...

Today's news feed included an item about a design for an electric powered aircraft system using a hybrid approach ...

A passenger vehicle is designed to fly 300 miles at 200 miles an hour, with (I ** think **) six passengers.

The system would feature a drone to lift the passenger plane from a small inner-city port, carry it to altitude, and release it to fly to the destination.

At the destination, a similar drone would capture the aircraft and land it gently in vertical flight mode to a similar small landing field.

At this point (I got the impression) the system is purely conceptual, but I liked the concept.

The capture of link and text got lost along the way, but (hopefully) other forum members will be able to find it.

(th)

louis · 2021-03-25 18:13:05

Interesting concept.

There may be other possibilities - including ground electric rail systems to accelerate the craft to very high take off speeds, doing most of the energy work from the surface of Earth.

tahanson43206 wrote:

For electric aircraft supporters ...
Today's news feed included an item about a design for an electric powered aircraft system using a hybrid approach ...
A passenger vehicle is designed to fly 300 miles at 200 miles an hour, with (I ** think **) six passengers.
The system would feature a drone to lift the passenger plane from a small inner-city port, carry it to altitude, and release it to fly to the destination.
At the destination, a similar drone would capture the aircraft and land it gently in vertical flight mode to a similar small landing field.
At this point (I got the impression) the system is purely conceptual, but I liked the concept.
The capture of link and text got lost along the way, but (hopefully) other forum members will be able to find it.
(th)

Calliban · 2021-03-25 19:52:41

louis wrote:

Well I did specify a delta shaped aircraft which would clearly have a larger surface area. In addition above the cloud cover, you could probably justify putting solar power film on the under-parts of the craft. Lastly, as I suggested, there is the possibility of the plane towing behind long solar "sails". We know small aircraft can tow behind advertising banners many times longer than the planes themselves. I don't know whether this would be practical but it might be.
Clearly I wasn't suggesting that this approach would suffice to get the plane off the ground. With VTOL craft, this might be something where that microwave/laser power could be used. Imagine a microwave or laser beam delivering energy to an onboard system till the plane is 5 miles high, at which point onboard batteries take over. This technology might become a reality, though I accept it isn't currently available.

A towed array is not a viable option, as it would increase drag dramatically and engine power is very limited. A delta wing does increase total wing area, but at low speeds, a high angle of attack is needed to generate adequate lift. This makes lift-drag ratio relatively poor at the very low speeds of <100mph at which the solar powered plane would be constrained to operate. Unfortunately, even if you could double or triple the solar arrays specific power output, the picture doesn't change much.
https://en.m.wikipedia.org/wiki/Ultralight_aviation

The best option is to minimise both drag and weight as much as possible, by using a flying wing type concept. At low speed, the dynamic stress on the airframe would be low and an ultralight airframe with stretched carbon-fibre polymer composite could be used. Weight should be distributed throughout the wing as much as possible, as the frame would likely be too flimsy to withstand heavy point loads.

Last edited by Calliban (2021-03-25 19:56:24)

louis · 2021-03-25 20:36:32

OK, looks like I got confused between delta and flying wing there as I definitely meant the latter - so basically your plane is a triangle with no fuselage. That will maximise surface area.

Think you are a bit dismissive of the towed solar sail idea...are we sure the drag is more negative than the gained power?

Calliban wrote:

louis wrote:
Well I did specify a delta shaped aircraft which would clearly have a larger surface area. In addition above the cloud cover, you could probably justify putting solar power film on the under-parts of the craft. Lastly, as I suggested, there is the possibility of the plane towing behind long solar "sails". We know small aircraft can tow behind advertising banners many times longer than the planes themselves. I don't know whether this would be practical but it might be.
Clearly I wasn't suggesting that this approach would suffice to get the plane off the ground. With VTOL craft, this might be something where that microwave/laser power could be used. Imagine a microwave or laser beam delivering energy to an onboard system till the plane is 5 miles high, at which point onboard batteries take over. This technology might become a reality, though I accept it isn't currently available.
A towed array is not a viable option, as it would increase drag dramatically and engine power is very limited. A delta wing does increase total wing area, but at low speeds, a high angle of attack is needed to generate adequate lift. This makes lift-drag ratio relatively poor at the very low speeds of <100mph at which the solar powered plane would be constrained to operate. Unfortunately, even if you could double or triple the solar arrays specific power output, the picture doesn't change much.
https://en.m.wikipedia.org/wiki/Ultralight_aviation
The best option is to minimise both drag and weight as much as possible, by using a flying wing type concept. At low speed, the dynamic stress on the airframe would be low and an ultralight airframe with stretched carbon-fibre polymer composite could be used. Weight should be distributed throughout the wing as much as possible, as the frame would likely be too flimsy to withstand heavy point loads.

tahanson43206 · 2021-03-26 05:40:59

For Louis ... thanks for picking up on the hybrid electric aircraft concept in your post #9

Here is the original quote:

https://www.yahoo.com/lifestyle/evtol-f … 00823.html

Daniel Bachmann
Thu, March 25, 2021 10:00 AM
About two years ago, two space-exploration aeronauts got together to find a way for people to skip airports and fly straight to their destinations. Both had just worked on the Falcon 9—the first orbital class rocket capable of re-flight—and the SpaceX Crew Dragon, a class of reusable spacecraft that carries up to seven passengers and cargo to and from Earth’s orbit.
Inspired by their space rocket pioneering, Jamie Gull and Evan Mucasey, co-founders of LA-based Talyn Air, designed an aircraft that can fly like a plane, but launches and lands with the help of a large drone, much like a tugboat that maneuvers a ship to and from the dock.
More from Robb Report
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The unmanned drone will consume its own battery power to lift the Talyn eVTOL to its flight level, where it will release the eVTOL once its rotors reach flight sustaining speed. The drone will then return to its base to be recharged for another lift or landing mission.
- ADVERTISEMENT -
“The system takes off together vertically; transitions to forward flight; the two vehicles separate; the VTOL portion returns to the take-off pad,” Gull told FlightGlobal.com. “It’s an electric aircraft at that point: very efficient, very aerodynamic, lower mass, so it can go much farther. It will then do a mid-air docking with another VTOL [drone] vehicle at the destination, transition to vertical flight and do a landing.”

(th)

Quaoar · 2021-04-01 16:34:12

In a future dominated by green economy we can imagine hybrid electro-turboprop airplanes: with superconductive electric motors driven by hydrogen fuel cells. LH2 cools the superconductive coils of the stator, then is burned in the combustion cells generating electricity and the exhaust is recuperated by a turbine mechanically connected to the propeller, like a conventional turboprop (or by a turbo-alternator) giving extra power, with an overall efficiency near to 80%.

GW Johnson · 2021-04-01 19:48:55

Alternate aircraft fuels? I used to do that for a living. About a quarter century ago.

Why not look at ethanol made from cellulose (not corn-the-foodstuff, but corn cobs, say). It has about 2/3 the energy per unit volume of gasoline and jet fuel. It's all biomass carbon, not fossil carbon. And we already know how to use it in existing hardware. For spark ignition recip engines, it has octane near that of the old purple 115/145 avgas (way to hell and gone higher than 100LL). It will burn in a turbine, too. It is NOT a diesel fuel. OK in a neoprene fuel bladder, not OK in a urethane fuel bladder. OK with neoprene seals and teflon-lined fuel hoses. Vapor exposure destroys the two clear structural plastics very quickly; liquid exposure takes almost as long as with gasoline.

For that matter, there is biodiesel you can blend with jet fuel. I've tested it at B-20 to B-30 blends, and I found an ether that could be used in trace quantities to get the freezepoint of a B-30 blend with Jet-A back down under -60F. Worthless as a spark ignition recip fuel, but works great in turbine (or diesel) engines. Biodiesel exposure was found (in a contract overseen by the FAA) to rejuvenate old, stiff fuel bladder materials. Not sure you want to use it neat, it freezes too easily.

GW

kbd512 · 2021-04-01 21:13:57

louis wrote:

Well I did specify a delta shaped aircraft which would clearly have a larger surface area. In addition above the cloud cover, you could probably justify putting solar power film on the under-parts of the craft. Lastly, as I suggested, there is the possibility of the plane towing behind long solar "sails". We know small aircraft can tow behind advertising banners many times longer than the planes themselves. I don't know whether this would be practical but it might be.
Clearly I wasn't suggesting that this approach would suffice to get the plane off the ground. With VTOL craft, this might be something where that microwave/laser power could be used. Imagine a microwave or laser beam delivering energy to an onboard system till the plane is 5 miles high, at which point onboard batteries take over. This technology might become a reality, though I accept it isn't currently available.

Louis,

The first thing you need to accept is that there is no "winning" here, merely solutions that are more or less efficient than what we currently use, but all will have similar power requirements, because it takes power to move weight through the air at a given speed, period and end of story. No matter what type of design you specify, solar panels can't come close to providing enough energy to keep all but the flimsiest, slowest, and lowest payload capacity aircraft airborne. It's a basic physics problem that no amount of religious belief in "green energy" will ever overcome.

From the Wikipedia Article about Solar Impulse II:

The aircraft's major design constraint is the capacity of the lithium polymer batteries. Over an optimum 24-hour cycle, the motors can deliver a combined average of about 8hp (6kW), roughly the power used by the Wright brothers' Flyer, the first successful powered aircraft, in 1903. In addition to the charge stored in its batteries, the aircraft uses the potential energy of height gained during the day to power its night flights.

The Wright Flyer was about the size of a small 2 seat GA aircraft. The time-averaged power output that could be generated by Solar Impulse II, over a 24 hour time period, was equivalent to what the Wright Flyer was able to generate. In other words, something grossly insufficient to ever become an airliner. 8hp is sufficient to keep 1 pilot in the air inside a rather flimsy 2,000kg machine that was incapable of making subsequent flights due to flight loads / stresses placed upon an airframe design that was at the absolute limits of technological feasibility, without an unacceptable risk of structural failure in flight, using existing CFRP and thin film plastic technology. Current CNT fabric could feasibly cut the airframe weight in half, which means an aircraft with a wingspan equivalent to an Airbus A380 that could carry a single pilot and a single passenger, but never fly more than a handful of times before flight stresses rendered the airframe too near to an impending in-flight structural failure for re-flight.

Solar Impulse II was equipped with 4 lightweight 10hp motors, for a total of 40hp. That means it has a maximum power-to-weight ratio of 15kW per metric ton. There is no such thing as an airliner with that kind of power-to-weight ratio, nor will there ever be. The Boeing 747 has an average power requirement of 140MW during cruise, and that equates to generating an average of 314.6kW per metric ton during the cruise phase of the flight. You can cut the airframe's empty weight in half with extensive use of CFRP, but that still leaves you more than an order of magnitude away from what you can supply using solar panels, and the batteries only exacerbate the weight problem, so now we're in a technological "corner", if you will, and we can't get out of it. Flying a little slower always helps, but not enough to change the fact that current photovoltaics couldn't come close to supplying enough power if they were 100% efficient.

The Boeing 747 has 525m^2 of wing area, which is quite a bit. At high noon, at the equator, with perfectly clear skies, it could generate 525kW from 100% efficient wing mounted thin film solar. There's just one problem. It still needs 70MW if it's made from CFRP and flying at airliner speeds. If it flies a little bit slower, like the AGA-33 concept from France, it could get away with 45MW. At that power level, the 747 needs 85.7 TIMES as much surface area to generate equivalent power output, but to cut the weight in half, we need to make the wing smaller so it generates less drag and requires less power to move it through the air.

Now let's consider your 100m "flying Dorito" / "flying wing" proposal. The surface area of that vehicle is 4,330m^2. If it had 100% efficient thin film array (1,360W/m^2), it would generate 5,888,800W of power under the most ideal of conditions. That sounds great, right? That's quite a bit of power to be sure, but it's nowhere near 45MW. Since our flying wing has an absolutely crazy amount of volume, it's a safe bet that it's going to weigh more to remain stiff enough to deal with flight loads, even at 450mph vs 600mph cruise speeds. Even if we invoke CNT, we're still not light enough, and we can't decrease our surface area to reduce our power / thrust requirement to overcome viscous (pushing an object through a fluid) or induced (generating lift) drag, despite being lighter.

In short, making the aircraft larger will only make the power-to-weight problem worse, not better, because more structural weight is required to provide sufficient airframe stiffness as volume increases and you get more drag that you have to overcome to remain airborne. If you increase the volume of the aircraft by using a "flying wing" or "flying Dorito" design, then it will have both a greater inert mass fraction and more skin drag (drag generated by pushing the airframe through a viscous fluid, aka "air") and more induced drag from generating more aerodynamic lift (which it must have to remain airborne- a blessing and a curse), and consequently require even more power to propel it through the air at a speed above its stall speed.

There are no banner towing planes that use less fuel (energy) when towing the banner than not towing the banner, so that eliminates the feasibility of towing a solar sail. If the aircraft requires more surface area to generate more power from a skin-mounted solar array, then the most feasible engineering solution is to incorporate more surface area into the airframe design of the aircraft. That's what the designers of Solar Impulse / Solar Impulse II actually did to create a solar-powered aircraft that could fly around the world while carrying a single pilot. In fact, one could validly claim that they pretty much knew what they were doing and how to do it, and their bird made it around the world one time, just like Voyager, so mission accomplished.

There is one and only one practical way in which solar panels could ever generate enough power to propel an airliner-sized aircraft through the air, and that is if these "plasma engine" devices can be made sufficiently light / durable / powerful enough to use on an airliner-sized vehicle. That's because they can generate 55 TIMES more thrust per kilowatt of input power than a turbofan engine. The fly in the ointment is the special materials, making them lighter and more durable (not BBQ'ing the electrodes in operation), and having a vehicle with enough surface area to mount the thruster array (a "flying Dorito" would actually be a pretty good airframe design for something like this). They also become less efficient as flight speeds increase. If all those problems are solved and we come up with thin film that's dramatically more efficient than what we currently have, then we can reevaluate solar powered airliners. We should see how this technology progresses over the next ten years or so, as it's still in its infancy.

A solar powered airship is perfectly feasible with current technology. It's so feasible that Lockheed-Martin built and operated one for more than a year.

A solar powered train is perfectly feasible with current technology. It's so feasible that it's providing routine passenger transport services in multiple cities around the world.

A solar powered ship might be feasible, although it's dubious as to whether or not we could use it in a practical way, and nobody has ever built one.

A solar powered car / truck / aircraft will never be feasible, let alone practical, even if we invoke 100% efficient solar panels and CNT composites. Unfortunately, short of some heretofore unseen or unproven technology, it really doesn't matter what emerging technologies we invoke, because the power generated from the surface area available is grossly insufficient to move these vehicles at any appreciable speed.

Now that we've done enough simple math to know that we're never flying on a solar powered airliner, even if the solar panels are 100% efficient and CNTRP becomes as cheap and widely available as CFRP, unless those air breathing ion engines / plasma engines become practical, can we proceed to discuss some practical alternatives that could feasibly work using existing technologies?

There's plenty of practical applications for solar panels, but airliners aren't one of them. I know it's a real drag, but all engineering has its limitations. It's not a lack of imagination or a money problem, it's a physics problem. The Sun only provides so many photons / Watts per square meter to play with.

kbd512 · 2021-04-01 21:45:03

Given everything I just stated about a solar powered aircraft, there is still a very practical way to use solar power and electric motors on existing airliners. Every day, airliners burn through copious quantities of fuel on the ground while taxiing. For large airliners, an electric taxi system could save $250K to $500K per aircraft, per year. Taxiing can be as much as 5% of the fuel budget. Furthermore, when flying above the clouds, the VFGs that presently supply electrical power to all the electronic gadgets that the airliner itself and all its passengers use, could save another very healthy chunk of the fuel budget, because VFGs are fantastic at power-to-weight, but kinda suck relative to EV motors, as far as overall efficiency is concerned. EV type motors are 95%+ efficiency, whereas VFGs are 75% to 85% efficient, at most. So... Thin film on existing airliners. You bet. The benefits are well worth the cost. There's a very minor weight penalty associated with doing this, and having a backup electricity source is a good thing. If the engines quit and the APU won't start, then you'll want a solar array to pressurize the hydraulically-actuated flight controls.

Edit:

Perfect Flight by George Rhodes - Assistant Director Safety and Flight Ops ATM - Africa And Middle East

Last edited by kbd512 (2021-04-01 22:35:50)

louis · 2021-04-02 12:59:27

Wave power ships are probably the way to go with powered marine vehicles.

https://www.bbc.com/future/article/2020 … -the-ocean

This technology looks very promising.

Re solar powered aircraft I don't think at this stage we would be looking at night flying, so averaging out Solar Impulse's power over a 24 hour cycle is irrelevant. I was thinking more about short haul flights of up to maybe 1000 Kms (covers probably 350 million people in Europe. It would all be about flying during the day probably between 10 am and 3pm.

Not sure solar powered trains are quite as advanced as you suggest. The first one only began operation in 2018.

https://byronbaytrain.com.au/sustainability/

But maybe you are referring to electric rail tracks powered by solar?

But if so, then you should also allow for electric roads with induction electric power which could feed EVs with power, meaning the EVs could have much smaller batters.

kbd512 wrote:

louis wrote:
Well I did specify a delta shaped aircraft which would clearly have a larger surface area. In addition above the cloud cover, you could probably justify putting solar power film on the under-parts of the craft. Lastly, as I suggested, there is the possibility of the plane towing behind long solar "sails". We know small aircraft can tow behind advertising banners many times longer than the planes themselves. I don't know whether this would be practical but it might be.
Clearly I wasn't suggesting that this approach would suffice to get the plane off the ground. With VTOL craft, this might be something where that microwave/laser power could be used. Imagine a microwave or laser beam delivering energy to an onboard system till the plane is 5 miles high, at which point onboard batteries take over. This technology might become a reality, though I accept it isn't currently available.
Louis,
The first thing you need to accept is that there is no "winning" here, merely solutions that are more or less efficient than what we currently use, but all will have similar power requirements, because it takes power to move weight through the air at a given speed, period and end of story. No matter what type of design you specify, solar panels can't come close to providing enough energy to keep all but the flimsiest, slowest, and lowest payload capacity aircraft airborne. It's a basic physics problem that no amount of religious belief in "green energy" will ever overcome.
From the Wikipedia Article about Solar Impulse II:
The aircraft's major design constraint is the capacity of the lithium polymer batteries. Over an optimum 24-hour cycle, the motors can deliver a combined average of about 8hp (6kW), roughly the power used by the Wright brothers' Flyer, the first successful powered aircraft, in 1903. In addition to the charge stored in its batteries, the aircraft uses the potential energy of height gained during the day to power its night flights.
The Wright Flyer was about the size of a small 2 seat GA aircraft. The time-averaged power output that could be generated by Solar Impulse II, over a 24 hour time period, was equivalent to what the Wright Flyer was able to generate. In other words, something grossly insufficient to ever become an airliner. 8hp is sufficient to keep 1 pilot in the air inside a rather flimsy 2,000kg machine that was incapable of making subsequent flights due to flight loads / stresses placed upon an airframe design that was at the absolute limits of technological feasibility, without an unacceptable risk of structural failure in flight, using existing CFRP and thin film plastic technology. Current CNT fabric could feasibly cut the airframe weight in half, which means an aircraft with a wingspan equivalent to an Airbus A380 that could carry a single pilot and a single passenger, but never fly more than a handful of times before flight stresses rendered the airframe too near to an impending in-flight structural failure for re-flight.
Solar Impulse II was equipped with 4 lightweight 10hp motors, for a total of 40hp. That means it has a maximum power-to-weight ratio of 15kW per metric ton. There is no such thing as an airliner with that kind of power-to-weight ratio, nor will there ever be. The Boeing 747 has an average power requirement of 140MW during cruise, and that equates to generating an average of 314.6kW per metric ton during the cruise phase of the flight. You can cut the airframe's empty weight in half with extensive use of CFRP, but that still leaves you more than an order of magnitude away from what you can supply using solar panels, and the batteries only exacerbate the weight problem, so now we're in a technological "corner", if you will, and we can't get out of it. Flying a little slower always helps, but not enough to change the fact that current photovoltaics couldn't come close to supplying enough power if they were 100% efficient.
The Boeing 747 has 525m^2 of wing area, which is quite a bit. At high noon, at the equator, with perfectly clear skies, it could generate 525kW from 100% efficient wing mounted thin film solar. There's just one problem. It still needs 70MW if it's made from CFRP and flying at airliner speeds. If it flies a little bit slower, like the AGA-33 concept from France, it could get away with 45MW. At that power level, the 747 needs 85.7 TIMES as much surface area to generate equivalent power output, but to cut the weight in half, we need to make the wing smaller so it generates less drag and requires less power to move it through the air.
Now let's consider your 100m "flying Dorito" / "flying wing" proposal. The surface area of that vehicle is 4,330m^2. If it had 100% efficient thin film array (1,360W/m^2), it would generate 5,888,800W of power under the most ideal of conditions. That sounds great, right? That's quite a bit of power to be sure, but it's nowhere near 45MW. Since our flying wing has an absolutely crazy amount of volume, it's a safe bet that it's going to weigh more to remain stiff enough to deal with flight loads, even at 450mph vs 600mph cruise speeds. Even if we invoke CNT, we're still not light enough, and we can't decrease our surface area to reduce our power / thrust requirement to overcome viscous (pushing an object through a fluid) or induced (generating lift) drag, despite being lighter.
In short, making the aircraft larger will only make the power-to-weight problem worse, not better, because more structural weight is required to provide sufficient airframe stiffness as volume increases and you get more drag that you have to overcome to remain airborne. If you increase the volume of the aircraft by using a "flying wing" or "flying Dorito" design, then it will have both a greater inert mass fraction and more skin drag (drag generated by pushing the airframe through a viscous fluid, aka "air") and more induced drag from generating more aerodynamic lift (which it must have to remain airborne- a blessing and a curse), and consequently require even more power to propel it through the air at a speed above its stall speed.
There are no banner towing planes that use less fuel (energy) when towing the banner than not towing the banner, so that eliminates the feasibility of towing a solar sail. If the aircraft requires more surface area to generate more power from a skin-mounted solar array, then the most feasible engineering solution is to incorporate more surface area into the airframe design of the aircraft. That's what the designers of Solar Impulse / Solar Impulse II actually did to create a solar-powered aircraft that could fly around the world while carrying a single pilot. In fact, one could validly claim that they pretty much knew what they were doing and how to do it, and their bird made it around the world one time, just like Voyager, so mission accomplished.
There is one and only one practical way in which solar panels could ever generate enough power to propel an airliner-sized aircraft through the air, and that is if these "plasma engine" devices can be made sufficiently light / durable / powerful enough to use on an airliner-sized vehicle. That's because they can generate 55 TIMES more thrust per kilowatt of input power than a turbofan engine. The fly in the ointment is the special materials, making them lighter and more durable (not BBQ'ing the electrodes in operation), and having a vehicle with enough surface area to mount the thruster array (a "flying Dorito" would actually be a pretty good airframe design for something like this). They also become less efficient as flight speeds increase. If all those problems are solved and we come up with thin film that's dramatically more efficient than what we currently have, then we can reevaluate solar powered airliners. We should see how this technology progresses over the next ten years or so, as it's still in its infancy.
A solar powered airship is perfectly feasible with current technology. It's so feasible that Lockheed-Martin built and operated one for more than a year.
A solar powered train is perfectly feasible with current technology. It's so feasible that it's providing routine passenger transport services in multiple cities around the world.
A solar powered ship might be feasible, although it's dubious as to whether or not we could use it in a practical way, and nobody has ever built one.
A solar powered car / truck / aircraft will never be feasible, let alone practical, even if we invoke 100% efficient solar panels and CNT composites. Unfortunately, short of some heretofore unseen or unproven technology, it really doesn't matter what emerging technologies we invoke, because the power generated from the surface area available is grossly insufficient to move these vehicles at any appreciable speed.
Now that we've done enough simple math to know that we're never flying on a solar powered airliner, even if the solar panels are 100% efficient and CNTRP becomes as cheap and widely available as CFRP, unless those air breathing ion engines / plasma engines become practical, can we proceed to discuss some practical alternatives that could feasibly work using existing technologies?
There's plenty of practical applications for solar panels, but airliners aren't one of them. I know it's a real drag, but all engineering has its limitations. It's not a lack of imagination or a money problem, it's a physics problem. The Sun only provides so many photons / Watts per square meter to play with.

GW Johnson · 2021-04-02 13:18:24

I would rather investigate using the solar electricity on the ground to make some sort of fuel with which to fly the airplane. That divorces the solar panel area problem from the wing area problem, and it also divorces the weights of the electrics from the flying weight problem. Plus it more-or-less lets you continue flying existing airframe and engine hardware here on Earth.

That sort of approach is more suited to Earth than Mars. Here, we need only carry the fuel, we are flying in freely-available oxidant, at significant density. That is NOT true on Mars.

Here the atmosphere is dense, and there are feasible technological solutions to building practical airplanes with materials that we already have, and also building and operating practical combustion engines. Neither is true on Mars, the new helicopter there notwithstanding.

That Mars helicopter thing only works in small sizes, at infeasibly low weights for the size, and with supersonic rotor blades. Precisely because the Martian atmosphere is so thin (comparable to Earth at 105-110 thousand feet = 33 km).

Plus, Mars's atmosphere is NOT an oxidant, it is pretty much an inert. Between that and its low density, combustion engines as we know them here are pretty much impossible. There, you supply (and carry) both the fuel and the oxidizer. And you have to make them both. And you must come up with a suitable engine design in which to use them, especially if your oxidant is straight oxygen, no dilution gas to lower flame temperatures.

GW

Calliban · 2021-04-02 16:27:22

As Kbd512 has explained, a solar powered aeroplane cannot generate sufficient lift to carry passengers or cargo. This is a physics problem stemming from the limited energy flux of sunlight. The aerodynamic lift and drag acting on the airframe both scale in proportion to the square of velocity. The engine power needed to overcome drag is drag force x velocity, so basically proportional to v-cubed. Flying more slowly reduces drag more quickly than lift, but ultimately there are limits to how far lift can decline, as the achievable strength of materials set lower limits on airframe mass. The designers of the Solar Impulse II chose the flying wing concept to maximise lift-drag ratio and used ultra-lightweight materials with minimal design factor to keep total structural weight within the limits of the meagre lift that can be generated by the motors. Even so, fatigue limited the life of the plane to a single flight with one passenger. The sensible conclusion to draw is that this demonstration proved that solar powered flight is technically possible at the extreme limits of material specific strength, and solar cell and electric motor power-weight. But given the extreme structural weight limits and limited fatigue life, no sensible person would conclude that this technology was destined to replace jet fuel in hauling freight and passengers.

There may be a few specialised niche applications for a solar powered plane, where its ability to remain airborne for long periods without refuelling is more valuable than its ability to haul weight. As I said previously, it may be useful as a reconnaissance drone or maybe an aerial booster for mobile phone signals. Here, the ability to remain airborne for long periods without landing could be valuable and payload mass need not be large. The surveillance and transceiver equipment could be miniaturised and would probably weigh no more than a few pounds. The plane does not need a pilot or any hotel facilities, just electronic control systems, which eliminates a significant amount of weight. So for these limited applications, solar powered flight might fit the bill. A small surveillance drone that can remain airborne for long periods would be militarily useful in keeping constant watch over the mountain passes of Afghanistan. It could not realistically carry any munitions, but it could provide a constant eye in the sky.

Last edited by Calliban (2021-04-02 16:33:08)

louis · 2021-04-02 16:47:00

On Mars, the hyperloop, if it proves feasible, would be perfect for long distance travel. Transplanet Starships might also operate, together with shorter rocket hoppers. But hyperloops might just be more convenient. Starships and rocket hoppers won't be able to operate from the middle of an urban settlement, for instance. It would be very convenient if hyperloop stations could be located bang in the middle of a settlement - none of the equivalent of travelling 1-2 hours to get to a major airport hub.

Hyperloop travel on Mars could be one of its many joys - no airport style security, shuffling along in huge queues. No need to tow your suitcase - a wheeled robot will accompany you all the way on to the hyperloop. External cameras would feed images of the landscape you are passing through to onboard "windows".

GW Johnson wrote:

I would rather investigate using the solar electricity on the ground to make some sort of fuel with which to fly the airplane. That divorces the solar panel area problem from the wing area problem, and it also divorces the weights of the electrics from the flying weight problem. Plus it more-or-less lets you continue flying existing airframe and engine hardware here on Earth.
That sort of approach is more suited to Earth than Mars. Here, we need only carry the fuel, we are flying in freely-available oxidant, at significant density. That is NOT true on Mars.
Here the atmosphere is dense, and there are feasible technological solutions to building practical airplanes with materials that we already have, and also building and operating practical combustion engines. Neither is true on Mars, the new helicopter there notwithstanding.
That Mars helicopter thing only works in small sizes, at infeasibly low weights for the size, and with supersonic rotor blades. Precisely because the Martian atmosphere is so thin (comparable to Earth at 105-110 thousand feet = 33 km).
Plus, Mars's atmosphere is NOT an oxidant, it is pretty much an inert. Between that and its low density, combustion engines as we know them here are pretty much impossible. There, you supply (and carry) both the fuel and the oxidizer. And you have to make them both. And you must come up with a suitable engine design in which to use them, especially if your oxidant is straight oxygen, no dilution gas to lower flame temperatures.
GW

kbd512 · 2021-04-19 03:36:24

Speaking of alternative fuels...

I propose we use HAN / AF-M315E monopropellant for powered flights on the surface of Mars. We need a dense, storable liquid fuel for powered flight that doesn't require a separate oxidizer, cryogenics, or extreme pressurization. Hydrazine has been experimented with here on Earth for high altitude drones (NASA's "Sniffer" drones that were intended to study potentially damaging atmospheric effects from flying hypersonic airliners at high altitudes), but HAN provides a healthy 50% fuel economy boost for our application. This somewhat novel propellant is of particular interest to us for powering a crewed aircraft that uses aerodynamic lift to actually fly on Mars.

GW, I have a question regarding transonic flow effects on Cl/Cd at very low pressures.

This relates to the Perlan II glider's operation at pressure altitudes closely resembling that of Mars sea level. As I understand it, atmospheric differences state that Mach 0.5 at MSL is 120m/s vs 172m/s for Earth, and above that you start getting into some complex transonic effects. Anyway, the question is how does operation above Mach 0.5 affect Cl/Cd, because in order to use an airframe design similar to Perlan II, with an engine, on Mars, we're very likely to have transonic flow over the wings. A swept wing and some quality time with a super computer appears to be a hard requirement. By the time we're at 10km above MSL, we're well past the "coffin corner" for Perlan II's airfoil design. Given how much lift force the wing generates, relative to the airframe's weight on Mars, I presume we could fly more slowly. Do you have any data / books / links regarding how Cl/Cd would be affected by operating at Perlan II's Va speed? Is it even feasible to execute a 2g bank at, say, 5km above MSL?

Substitute the Cl of 0.397 / Cd of 0.047 for the APEX-16 airfoil design, for whatever it is for the Perlan II airfoil design (I guess that's proprietary since I haven't found it), presume that our wing area is 24.4m^2 to roughly match that of Perlan II, and then use GW magic to tell me how nuts I am for even proposing this. Can we perform that maneuver while parts of the wing are going supersonic, or do we rip the wings off? Dependent upon atmospheric density at that specific moment and place on Mars, just how close are we to killing everyone aboard by either stalling or going supersonic using an airfoil that most definitely wasn't designed for that? Please note that this is just a crazy idea. I haven't done enough math to know how silly the idea is and I have zero knowledge of what Cl/Cd does at transonic flow speeds. Would the drag rise simply bleed airspeed like mad before we destroyed the wing? I feel like this should be doable with reduced airspeeds, although perhaps it requires a highly specialized airfoil design, swept wings to delay the onset of transonic flow, and careful attention to indicated airspeeds to avoid going supersonic while maneuvering.

NASA's wind tunnel simulations seem to indicate that gliding at 10,000m above Mars is almost identical to gliding at 30,840m / 101,181ft on Earth. I should note that the "Vs = Vne" for Perlan II's airfoil is around 96,000ft and that the designer optimized the airfoil to operate most efficiently at 60,000ft, whereas the APEX-16 airfoil was purposefully optimized to operate between 70,000ft and 100,000ft by some fellow from MIT named Mark Drela. There's a NASA airfoil, PSU94-97, that they did a bunch of testing on that will supposedly contend with the expected operating conditions on Mars, but I've no clue how well developed that idea is. My guess is that it was more than a passing curiosity since they built and tested some prototype hardware.

The lift force calculation seems to indicate that Perlan II at max gross weight would get airborne at around the same speed as a Cessna 182.

For everyone else...

The Perlan II Glider by Dr. Daniel Johnson

The measured 0.02kg/m^3 density altitude on Mars at the Viking landing site is just above the 30,000m / 98,425ft Earth-equivalent, which is measured at 0.01841kg/m^3 here on Earth, according to the Engineer's Toolbox website. This seems to indicate that some form of powered flight using aerodynamic lift falls within the capabilities of at least one existing Earth-bound all-CFRP glider named "Perlan II".

Perlan II's empty weight is 1,265 pounds, gross weight is 1,800lbs. Vs is 38kts IAS or 256kts TAS / Va is 52kts IAS or 350kts TAS, Vne is 56kts or 377kts TAS at 90,000 ft, according to their graph. Mars gravity means Perlan II only weighs 682.2 pounds, so it's objectively generating nearly 3 times as much lift force there as its Earth Vs. Actual stall speed appears to fall somewhere in the range of a heavy lift aircraft, so take-off would be a rather sporty proposition for a glider of its size. Perlan II is also pressurized to 8.5psi, so the fuselage contains a rather large pressure vessel for two crew members. That means it could be made lighter for an astronaut wearing a space suit. The wings of Perlan II are also optimized for best performance at 60,000ft. Since we'd operate at a density altitude of 90,000ft+ at all times, the airfoil design could also be further optimized for generating lift and avoiding transonic flow effects at the density altitudes associated with Mars.

Advanced materials like CNT fabric would be required to reduce the weight and therefore stall speed to something comparable with a light STOL aircraft, but that appears to be doable. I want to consider the use of a "biplane" or "X-wing" airfoils to reduce the span, decrease the structural weight associated with very long spans, to provide improve maneuverability.

I also want to eliminate the drag penalty associated with the large mono wheel and exposed tail wheel by using a hand-crank retractable 4-poster (similar to the B-52) landing gear arrangement with lighter tires and CNT torsion bar trailing arm suspension (OV-10) to permit modest rough field landings.

In case it's not clear, this is a modification of the Perlan II design to make it a propeller-powered / AF-M315E fueled, single seat light STOL biplane airframe capable of operating at modest speeds and altitudes. After we prove that a single seat aircraft design works reliably, then we can worry about carrying passengers and cargo. For now, this is strictly a type of one-off experimental design intended to prove the feasibility of a practical crewed aircraft on Mars. We can design supersonic vertical-takeoff business jets or regional airliners after we demonstrate that the "KBD Flyer" is airworthy and reliable on Mars.

Calliban · 2021-04-19 05:56:31

Kbd512, great idea regarding use of monopropellant. The energy density advantage of using bipropellant appears to disappear when it becomes necessary to carry oxidiser as well as fuel. Liquid hydrogen - oxygen propellant would have energy density of 15.7MJ/kg and it would be difficult to store, even on Mars.

As the CO2 dominated Martian atmosphere is at temperatures far beneath its critical point, compressor work should be small and a turbofan engine burning HAN fuel could be extremely efficient. This partially mitigates the burden of having to carry both fuel and oxidiser.

Louis's idea of an electric aeroplane got me thinking. There are propellants like silane or magnesium that can be made on Mars that will burn in a CO2 atmosphere. The problem is that the exhaust would contain solids which would shot blast a turbine to smithereens. But there is no reason why the energy driving a turbojet compressor has to be provided by a mechanically coupled turbine. If the compressor is powered separately using batteries or a gas turbine burning a small quantity of monopropellant, then the main engine can work more like an air-fed rocket, without any moving parts exposed to the sooty and sandy exhaust gases. Hence, an airbreathing Martian jet engine can be fashioned that burns silane. I think this is only possible because the Martian atmosphere is composed almost entirely of CO2 far beneath its critical temperature. Hence, relatively little compressor power is needed. The achievable expansion ratio should be excellent for the same reason. So Martian airbreathing engines should be very efficient.

The easy compressibility of Martian air may also result in unconventional aerodynamic effects. I'm not sure how this would effect the lift or the drag acting on the air frame. But one most definitely cannot make a like for like density corrected comparison between Martian conditions and Earth conditions, as air will exhibit ideal gas properties, whilst CO2 at 220K will not. At those temperatures, CO2 turns to liquid at about 6bar. I would be interested to know how that would effect the practical generation of lift. Does it improve lift relative to ideal gas conditions, or does it degrade lift? How would it effect drag at high subsonic speeds?

Last edited by Calliban (2021-04-19 06:39:46)

tahanson43206 · 2021-04-19 06:39:29

For kbd512 re #21
For Calliban re #22

SearchTerm:monopropellant for flight at Mars
SearchTerm:glider powered flight at Mars
SearchTerm:solid fuel option possible for flight at Mars (fuel expresses as solid) (will contribute to dust on Mars)

(th)

kbd512 · 2021-04-20 02:03:49

Let's further explore this idea of a monopropellant gas turbine powered light aircraft, using the Space Shuttle Hydrazine-powered Improved APU / I-APU as our power plant. The I-APU is rated to produce up to 110kW of output power (more than 100% of the rated power of the original design), with nominal maximum output of around 100.7kW. We're powering an aircraft that weighs 682lbs on Mars, and 100.7kW / 135hp would be a LOT of power for an aircraft that light here on Earth. It's easily more than double the amount of power you typically see for aircraft in this weight class, so take-off performance should be nothing short of spectacular with a propeller design suitable for the thin atmosphere on Mars.

APU/Hydraulic/Water Spray Boiler Systems Training Manual

The total weight of the Space Shuttle I-APU device, with all of the hydraulic equipment included, is only 88lbs / 40kg and produces 135hp / 100.6kW. It's a pretty safe bet that all of that attached hydraulic equipment is at least 20kg of the weight of that device, if not more. For sake of argument, since we don't know the exact weight, unless RobertDyck can tell us, let's stipulate that a complete power plant with gearbox weighs 30kg. Power-to-weight is moderately high for a small gas turbine, but nothing too spectacular, especially since there is no compressor stage or burners. It's directly blowing hot expanding gas expelled from the catalyst bed across the turbine wheel to produce power, and is a dual pass system that passes through / over the turbine wheel twice before exiting overboard. In the I-APU, Hydrazine decomposes at 1,700F / 927C in the gas generator catalyst bed to produce expanding steam that exits the turbine wheel at a temperature of 1,000F / 538C. Those temps correspond incredibly closely with the typical hot section and exhaust temperatures of a modern garden variety Pratt & Whitney Canada PT-6A gas turbine, despite the very different fuels used to generate the power. The hot section of the PT-6A is slightly hotter and the exhaust temp is an exact match. In short, service temperatures are well within the continuous service temperature limits of Nickel-based super alloys such as Inconel 625.

The turbine wheel speed of the I-APU at 100% of maximum continuous rated output is 72,000rpm, at 115% or peak output is 83,000rpm, 129% or 92,280rpm is considered to be over-speeding that should trigger an automatic shutdown to prevent damage, and 80% is 56,700rpm which is also considered to be under-speeding that should likewise trigger an automatic shutdown. The fuel pump operates at 3,918rpm and the oil pump operates at 12,515rpm to produce a nominal 60psi of oil pressure and minimum of 15psi. The lube oil lubes the fuel pump, turbine wheel bearings, and gearbox. The oil used tolerates temps of up to 270F / 132C and the water-spray cooled oil is 250F / 121C. I think we need an air cooler to cool the oil to negate the requirement for the water spray coolant used in the I-APU. There are some other features of the I-APU, such as lube oil accumulators / expansion tanks, that may not be strictly required in the presence of gravity, but I think we should keep these features so that accidental inverted flight or unusual flight attitudes don't starve the turbine or accessories of lube oil.

We need to make some changes to accommodate the catalyst bed to break down HAN / AF-M315E monopropellant. The catalyst bed requires pre-heating to 545F / 285C, but 599F / 315C is considered to be the nominal start temperature.

Before I forget again, I need to correct my previous 50% "fuel economy" performance increase remark, which should have stated that you get roughly 50% more output per unit of propellant tank volume. The density of AF-M315E / HAN is 47% greater than Hydrazine (1.47g/cm^3 for AF-M315E vs 1.00g/cm^3 for Hydrazine vs 0.81g/cm^3 for RP-1), but the specific impulse increase is only 12% greater (257s vs 235s). That said, fuel weight / volume matters greatly in this application, so having a pressurized propellant tank that contains 47% more propellant per unit volume is very important. Defense Systems Information Analysis Center claims AF-M315E provides a 64% density impulse increase over Hydrazine, although all the AFRL and NASA documents claim 50%, and perhaps DSIAC is referring to the cumulative effects of greater Isp and Id.

Review of State-of-the-Art Green Monopropellants: For Propulsion Systems Analysts and Designers

Testing of a 1N AF-M315E Thruster

THERMAL AND ELECTROLYTIC DECOMPOSITION AND IGNITION OF HAN–WATER SOLUTIONS

The Decomposition of Hydroxylammonium Nitrate under Vacuum Conditions

Catalytic Decomposition of Hydroxylammonium Nitrate Ionic Liquid: Enhancement of NO Formation.

Ultra High Temperature Ceramics: Densication, Properties and Thermal Stability.

From the documentation, it should be obvious that HAN presents some real material challenges due to the heat and species of exhaust products generated, as compared to Hydrazine. Particular emphasis on decomposition products must be paid to assure the longevity of the major components of the engine through adequate oxidation resistance at extreme temperatures.

Adiabatic Flame Temperatures:
Hydrazine - 926.85C
AF-M315E - 2,126.85C
LOX/RP-1 - 3,396.85C

Although I'm concerned about the oxidation resistance of ZrC at the temperatures involved, I'm specifying the use of Zirconium Carbide for the gas generator catalyst bed housing, gas turbine housing, and turbine wheel until I understand all of the species of combustion products produced during decomposition. It seems as if there are a number of tweaks that alter this. Perhaps we need a good thermal barrier coating that won't oxidize at the temperatures involved. This is clearly a higher level of technology than the simple Inconel that the I-APU uses, but ZrC and the thermal barrier coatings are widely used in current generation gas turbine hot section fan blades / burner cans / exhaust components, and the benefits justify the added expense. One significant benefit of ZrC (6.73g/cm^3) that it's significantly lighter than Inconel (8.44g/cm^3). I could be wrong, but I think we're still going to require some CO2 coolant gas passed through some coolant channels from an onboard LCO2 tank to avoid oxidizing the hell out of the gas generator and turbine blades.

louis · 2021-04-20 05:26:12

Having seen a helicopter fly on Mars, I'm now coming round to the idea a plane might fly as well!

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