# Question About Electric Aircraft Propulsion

#### Klystron

Gold Member
I'd really like to see a calculation on the volume of lifting gas required to carry 100,000 tons, to replace a cargo ship.

Then I'd like to see the kinetic energy of impact if one breaks apart at 10,000 ft.
My post stipulated an upper limit of one cargo container built from light weight materials similar to cargo space for a typical delivery truck that carry a few tons at most or equivalent 10 passengers with luggage. I fail to see the humor in citing a ridiculous 100,000 tons cargo criteria for a single aircraft. Unless you meant kilograms?

The largest civilian aircraft I have entered a C-5A Galaxy could possibly lift 10E5 Kg under stringent take-off and landing conditions, minimum fuel, minimum crew, perfect weather conditions but so what?
...but the maximum allowable payload was reduced from 220,000 to 190,000 lb (100,000 to 86,000 kg). At the time, a 90% probability was predicted that no more than 10% of the fleet of 79 airframes would reach their fatigue life of 19,000 hours without cracking of the wing.[15]

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#### cjl

The point of the 100k tons is because that would make it similar to modern cargo ships in capacity, since the suggestion was to replace earthbound transportation with airships (not to replace existing cargo aircraft). You're right that that would be a pretty crazy amount to move by air though, since the largest existing cargo plane can only carry around 250,000 kg of payload.

#### Klystron

Gold Member
The point of the 100k tons is because that would make it similar to modern cargo ships in capacity, since the suggestion was to replace earthbound transportation with airships (not to replace existing cargo aircraft). You're right that that would be a pretty crazy amount to move by air though, since the largest existing cargo plane can only carry around 250,000 kg of payload.
Negative. I made no such suggestion. Read the post. While unclear, perhaps, the reference to marine freight was only to compare the requirement of time in transit. Ships are slow relative to jet aircraft. The counter-argument devoid of sarcasm would be that ships carry large tonnage as the weight is supported by water. This argument could lead to comparison of transit times versus payload. Can we state that generally the faster the transportation, the lighter the payload?

Earthbound transportation is unclear as the atmosphere is earthbound.

My reference to LTA and airships was a carry-over from a parallel thread on NASA airships for exploring solar system objects such as Titan and Mars. No 'giant gasbags' mentioned. Amazon, among others, is diverting cargo from trucks to aerial delivery systems, all electric powered AFAIK.

#### russ_watters

Mentor
My post stipulated an upper limit of one cargo container built from light weight materials similar to cargo space for a typical delivery truck that carry a few tons at most or equivalent 10 passengers with luggage. I fail to see the humor in citing a ridiculous 100,000 tons cargo criteria for a single aircraft. Unless you meant kilograms?

The largest civilian aircraft I have entered a C-5A Galaxy could possibly lift 10E5 Kg under stringent take-off and landing conditions, minimum fuel, minimum crew, perfect weather conditions but so what?
It was 6 weeks ago, so I don't recall if I read the post correctly (whether you intended an exact container ship replacement), but I do know my post was way less than 50% sarcastic. Whether what you suggest would take a large number of obscenely enormous airships or an obscenely enormous number of large airships is just two sides of the same coin. So, some related numbers I'd like to see for the other side:

1. How many airplanes are in flight over the US at any given time?
2. How many single container trucks are on the road?
3. What is the global annual helium production volume?
4. What volume of helium would be required to lift that many containers simultaneously?

A quick google tells me that replacing one large container ship with single container airships would roughly double the number of aircraft in the air at one time, globally.

If you're going to propose an outside-the-box idea, you really should do a little work to check on feasibility.

Can we state that generally the faster the transportation, the lighter the payload?
No, I wouldn't say that. There are several constraints affecting payload and transit speed. The constraints tend to be more fundamental to the mode of transport than related to speed/capacity.

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#### Klystron

Gold Member
OK. Leave it at 6 weeks, sleepless night, mixed threads on my part. The LTA concept was asked on a speculative forum. I must have merged airship with electric propulsion. An interesting concept but I never mentioned helium or gas bags. You are using a form of argument of going to extremes in order to ridicule. ALL ground vehicles. ALL marine tonnage. Largest bulk cargo aircraft ever built.

Other engineers on this thread got it right that current e-propelled aircraft frames are designed for minimum weight. You are stuck in Leviathan. Current electric aerial delivery systems mimic hummingbird.

#### sophiecentaur

Gold Member
Current electric aerial delivery systems mimic hummingbird.
Local deliveries by drone could work - except that there WILL be accidents. Get indoors if you ever hear one!

#### russ_watters

Mentor
OK. Leave it at 6 weeks, sleepless night, mixed threads on my part....
Ok....yeah, looking back at the original discussion, you already acknowledged the original description was "inapt". So I'm not even sure why you are circling back to it (I was letting it drop at that).
[first post]
....canny aero engineers should reconsider LTA ships for practical transport including cargo. Time is not a prohibitive factor for tons of cargo and passengers on the water. LTA ships could replace much ground transportation dependent on fossil fuels, even using similar modular containers.

[second post]
The reference to water-borne cargo was meant as analogy, perhaps inapt. I have seen an articulated truck loading three standard containers. Most ground vehicles carry only one or two containers.
I didn't reply to the second, but my reply would have been the same as the one I gave today; a lot of one-container airships or fewer 10,000 container airships are two sides of the same coin/problem.
The LTA concept was asked on a speculative forum. I must have merged airship with electric propulsion. An interesting concept but I never mentioned helium or gas bags.
Now I don't follow: if you didn't mean helium or gas bags, what does "LTA" mean? As far as I know, that's the only way to achieve it.
You are using a form of argument of going to extremes in order to ridicule. ALL ground vehicles. ALL marine tonnage. Largest bulk cargo aircraft ever built.
I am not intending that, nor did I say most of those -- indeed, I think my examples substantially understate the scale problem in what you are suggesting. Most of my examples were for replacement of a tiny fraction of our shipping (one container ship). Asking how many trucks are on the road was more for scale -- I have no idea how many shipping containers are in transit at any one time. And on the other end, saying how many planes were in the air wasn't to say I think you're talking about replacing all planes, but using that number to show how unworkable what you propose is, to replace even one container ship. But if you have a more specific idea in mind as to what "much ground transportation" means, please say so. 1%? 10%? 40%? If you don't say what you mean, what choice do I have but to guess what you mean?

#### Klystron

Gold Member
Thanks. I finally remember my point in returning to this thread. Not as important or profound as I thought but here is my reasoning.

Around the time this type of electric aircraft was reported in the media, my family noticed a grocery delivery truck with an enormous fairing stretching from the tractor cab over part of the top of the refrigerated cargo section high balling through the Mojave desert on a lower section of highway.

My artist granddaughter commented on the "decorative panels" and thick coils running under the fairing into the refrigeration unit. My engineer granddaughter recognized solar panels on the fairing and uncovered area of the roof of the cargo container from school projects and working with her dad. Were the panels providing electricity from solar to help power the refrigerators?

Discussion led to comparing the super-light solar powered aircraft with broad wing geometry carrying two passengers to a speculative electric delivery truck with batteries recharged by the sun between roadside charging stations.

What was the optimum "solar area" for an e-truck? An e-train? E-car?
Should the truck "follow the Sun" as the solar aircraft was reported to have done?
How efficient were the solar panels versus weight and size?
Instead of frozen groceries, should our hypothetical E-truck just carry solar panels to market...?

#### sophiecentaur

Gold Member
"much ground transportation"
much ground transport can be achieved on the ground on rails. Very efficient and nothing to drop out of the sky. Sea transport costs more to run and is slow but, as in the days of canal transport, a delay is not necessarily a problem if the arrival rate is high enough. The safety aspect makes sea transport attractive, too. Except when approaching land, engine failure just means you stop. Near land, you may need rescuing but that probably has a timescale measurable in hours.
Passenger transport is a different matter but, as I have remarked several times in the past, there are alternatives to many journeys by plane. Modifying lifestyles is not an unthinkable concept if the result is to reduce 'carbon' footprint.

#### merriam

In the overall picture, this is one of the most relevant facts. It's only when all electric transport energy produces almost no climate effect that the quoted massive ratio can be ignored. An intermediate solution would be to store energy in the form of Hydrogen, which sits somewhere in between. Hydrogen is something that seems to vary in popularity over the years.
Actually, Hydrogen has an energy density (per kg) of approximately 3 times that of jet fuel (varying slightly with temperature) and at least 100 times that of Lithium polymer batteries. However, it is tough to use. Among its drawbacks: Takes a lot of volume store as a gas, though hydrogen fuel cell cars (like Toyota Mirai) have a range comparable to that of gasoline vehicles 2) hydrogen tends to degrade any metal it comes in contact with (see hydrogen embrittlement) though this may be overcome with coatings and liners. 3) it is explosive at all concentrations between 5% and 95% (safety issue). The most serious however is that hydrogen gas contains some molecules traveling at escape velocity, so it might depart the earth's atmosphere (as helium does). Normally it is attached to oxygen (or carbon) so this isn't a problem, but in a hydrogen economy it could be a problem.

#### cjl

The most serious however is that hydrogen gas contains some molecules traveling at escape velocity, so it might depart the earth's atmosphere (as helium does). Normally it is attached to oxygen (or carbon) so this isn't a problem, but in a hydrogen economy it could be a problem.
Nah, that isn't an issue at all. We have so much hydrogen available that the small amount lost to atmospheric escape is completely irrelevant.

#### 256bits

Gold Member
What was the optimum "solar area" for an e-truck? An e-train? E-car?
Should the truck "follow the Sun" as the solar aircraft was reported to have done?
How efficient were the solar panels versus weight and size?
Instead of frozen groceries, should our hypothetical E-truck just carry solar panels to market...?
If the top roof area of an 18 wheeler was covered with solar panels, expect about 5 kW production bright sun overhead, and then decreasing to 0 at night. Considering that a reefer ( the refrigeration container ) will produce about 10kW of refrigeration, and that depends on ambient / interior unit temperature difference, one comes up a bit short even at peak sunlight. One could thicken the insulation to reduce the cooling load, if one live and make a profit with reduced cargo carrying capacity compared to the competitors running on current technology. People are probably working on it. Perhaps you saw a test model.

Are you sure the fairing was not just there for reduced aerodynamic drag - very common these days.
I don't know what the hoses would be except the hookup from the tractor to trailer for lighting and braking for an 18 wheeler if that is what it was.

#### Klystron

Gold Member
If the top roof area of an 18 wheeler was covered with solar panels, expect about 5 kW production bright sun overhead, and then decreasing to 0 at night. Considering that a reefer ( the refrigeration container ) will produce about 10kW of refrigeration, and that depends on ambient / interior unit temperature difference, one comes up a bit short even at peak sunlight. One could thicken the insulation to reduce the cooling load, if one live and make a profit with reduced cargo carrying capacity compared to the competitors running on current technology. People are probably working on it. Perhaps you saw a test model.

Are you sure the fairing was not just there for reduced aerodynamic drag - very common these days.
I don't know what the hoses would be except the hookup from the tractor to trailer for lighting and braking for an 18 wheeler if that is what it was.
Right you are about each detail. +10.
As driver I only glanced at the grocery truck. The side I saw was encased in removable film announcing experimental delivery technology for regional stores (now owned by Kroger). Most likely a test model, as you say.

The front fairing looked enormous, certainly to reduce aerodynamic drag and protect the film. In retrospect the cable bundles and coils were probably part of the reefer-truck mechanism but also for radio communication. If memory serves, I saw a tall whip antenna for citizens band (CB), an AM/FM antenna and possibly (?) some kind of compact microwave rig -- like a Lewis antenna but smaller.

My (artist) granddaughter corrected my recollection. She liked the irredescent colors of the truck. My daughter and her daughter, who had some experience with home solar panels, noticed what they thought were solar cells on the trailer but not on the cab. Certainly that section of the Mojave high desert would be ideal for testing solar-augmented reefer trucks. The road alert signs, some lights and all emergency phones have solar panels that charge during the day. Thanks.

#### zekise

Pretty much all modern turbofans run with a blade tip speed at full throttle of mach 1.5 or so.
4MW isn't anywhere close to enough. Propulsive power at cruise for a modern jetliner the size of a 787 is more on the order of 40MW. At takeoff, each engine has to be making more like 50-60MW of shaft power just to run the front fan. Rerun the electric numbers knowing that and you'll see why running jetliners on electric power is a pipedream without a massive breakthrough in technology.
I think you need about 10 tons of thrust per engine during cruise for a 737. If the thrust to power ratio of an electric fan is 6 kg/kW, then you need 10,000 / 6 = 1,660 kW or 1.7 MW per engine. This assumes 95% motor efficiency and 90% propulser efficiency. Developers are building 0.75 MW motors. So four of these will do.

#### cjl

I think you need about 10 tons of thrust per engine during cruise for a 737. If the thrust to power ratio of an electric fan is 6 kg/kW, then you need 10,000 / 6 = 1,660 kW or 1.7 MW per engine. This assumes 95% motor efficiency and 90% propulser efficiency. Developers are building 0.75 MW motors. So four of these will do.
You actually don't need anywhere close to 40klb of thrust at cruise for a 737 - that's about a factor of 4 or 5 high. In reality, a 737 might have between 7000 and 10,000 pounds of drag (and thus thrust) during cruise, depending on gross weight, which 737 variant it is, etc.

Unfortunately, though, your thrust to power ratio is also way, way too high. At cruise, a 737 engine might be ingesting around 300kg/s of air at 250m/s. To generate 20kN (4500 lb) of thrust (since it has 2 engines), this air will need to be accelerated to 317m/s by the engine. This involves adding about 11MW of power to the air jet, so using your propulsor efficiency value of 90%, we need 12.5MW of shaft power per engine to generate 4500lb of thrust, for a thrust to power ratio of less than 2N/w.

The reason the power level needs to be so high is because of the high speed and the low density of air at altitude, resulting in a fairly low massflow. With a higher massflow (larger prop disk size, lower altitude) and lower airspeed, you probably could get a 737 to fly on only 3 or 4MW, but you'll never get anywhere close to the altitude or airspeed without 20-30MW.

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#### Z0dCHiY8

Summary: Question About Electric Aircraft Propulsion

Now the question:
Could we reach supersonic speed with battery powered aircraft considering (4) and (2). Or, could we power gas turbine with electricity?
the very heart of any transportation & machinery is fuel == if you have one w/ sufficient specific energy, you could do anything. All-electric propulsion has many solutions. For instance, magnetic-plasma ramjet == such engine has no moving parts == it inlets air, heats it w/ microwaves, focuses plasma w/ magnetic fields & pushes it w/ electric field to outlet. needless to say, mp-ramjet can variate thrust & Isp. For vacuum, we can have powerful ion thrusters. But we need such a minor trifle == BATTERY to meet those requirements on specific energy.. such a trifle :)

#### phinds

Gold Member
But we need such a minor trifle == BATTERY to meet those requirements on specific energy.. such a trifle :)
HUH ? Do you have any idea what is the energy density of a battery vs the energy density of gasoline?

#### russ_watters

Mentor
HUH ? Do you have any idea what is the energy density of a battery vs the energy density of gasoline?
I was detecting sarcasm in that statement.

#### sophiecentaur

Gold Member
I was detecting sarcasm in that statement.
Perhaps it was more Irony (a more sophisticated weapon). I can't imagine the word "trifle" used seriously in that way.

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