SBSP for electric aircraft, cont.

Chris speaking.

Along with Jon, I’ve given a lot of thought to this specific application of space-based solar power (SBSP).

Yes, there are thousands of aircraft in the air at any one time, but I find this to be one of the strongpoints of this concept: The market is huge, and it’s not competing with grid power. Over 5 million barrels of jet fuel are burned each day, translating into an average of about 250 Gigawatts of power, about half of the US’s average electrical power. (Only a fraction of this is turned into useful thrust, of course, because jet engines are only so efficient, just like your typical piston engine… though they’re larger and are used constantly, so probably do better than your SUV… From my research, the total efficiency of a jet engine is about 20-40%. For an electric propeller with a high-efficiency motor, about 85%.)

Many of these flights are likely short-haul, and could be handled by batteries, such as advanced lithium ion and lithium sulfur batteries. (Lithium-Air batteries are also a real possibility… Some research on them is being done at NASA Langley Research Center.)

Also, there are still improvements to be made to efficiency using blended wing, larger engines, advanced alloys, higher carbon fiber usage, topological optimization of structure, etc. And many of the aircraft are older (especially in the developing world), not taking advantage of the latest improvements. Ultimately, we could probably reduce consumption to about a half, to 125GW, with almost half again being taken care of by ground-charged batteries (the ascent uses a lot of fuel, but low enough that it can be handled by advanced batteries), so let’s say 75GW average.

The market will grow substantially through the rest of this decade (a factor of 5?) as the rest of the world comes up to US standards of living. But we’ll set that aside for now.

50MW is about right, I think, for the solar power production of a SBSP satellite, each made with 1-10 launches, depending on how powerful your launcher is. A challenging figure, but possible. That gives about 1500 SBSP satellites to produce average 75GW of power, not counting eclipsing. But of course, that’s only equivalent to our average power if we can transfer that power to the aircraft and into thrust at the same efficiency that jet fuel can be transferred into thrust. Let’s say around 25% round-trip efficiency, so at least 50% efficiency transmitting and at least 50% efficiency receiving (including beam losses).

There are 2 main options for transmitting power. Microwave or laser.

But first, let’s pick an orbit. I’m going to pick a low Medium Earth Orbit altitude of about 3000km, with a total distance from transmitter to receiver of about 5000km (the receiver won’t be directly underneath very often). Geosynchronous is so far away that a mobile receiver is almost hopeless. LEO is so close that you have big slew-rate issues as well as more shadowing and a more difficult thermal environment plus atomic oxygen. (Shadowing is less of a problem than you might initially think, since there are much fewer flights at night… although for long-haul flights, this is less true.)
(BTW, if launch costs are low enough, it may actually make sense to put batteries on the satellites… but it’ll require very low launch costs… below $50/kg IMLEO.)
The drawback is higher radiation. The solar arrays will need to be self-healing (which thin-film cells are capable of, when heated).

To be economical to launch, we’d likely require 500-1000W/kg solar arrays (which is doable in a few different ways, but would require cleverness). They’ll also need to be regularly healed of radiation damage. So 50-100 tons of solar arrays. But I think the radiators will end up quite heavy as well, perhaps rivaling the solar arrays. 50 tons, because that’s a complicated problem.

The optics/antenna will be significant, too, but it’s harder just to estimate parametrically. Let’s look at the size required.

Microwaves sound great at first. High efficiency, easy conversion, just chicken wire receiver. Except none of that applies at high frequency.

With a 100m dish, 1mm wavelength, 5000km distance, you have:
5E6m/(1E2m/(1E-3m))= 50m receiver. We can just about fit that on a large aircraft.
That is 300GHz, by the way. And with 1mm wavelength, you can’t just use a mesh, you basically need a solid dish (which would need to be unfolded or pieced together on orbit).

But 300GHz sources are very hard to come by (and are often called Terahertz sources). Gyrotrons are a good option since they’re compact (200kg?) and high-power (1-2Megawatts apiece are fairly easy to come by) and fairly inexpensive (~$1/Watt), but they usually max out at around 100-200GHz for high power. In the 100GHz range, you can get around 50-55% efficiency with 1-2 Megawatt output (but usually are lower efficiency). To try to operate at higher frequency causes huge reductions in power and efficiency, and especially as you push to 300-500GHz, you may need to operate just in pulsed mode. The reason why is because they need a high magnetic field strength. About 28GHz per Tesla. So a 100GHz gyrotron needs just a relatively modest 3 Tesla. A 500GHz gyrotron needs 15Tesla, which is super difficult to come by except pulsed or using a superconductor. So realistically, we’re limited to around 100-200GHz until superconductor technology significantly improves and becomes cheaper. And I don’t think that receiver rectennas in this frequency range are much better than 50% efficiency.

So we need more like a 150m diameter antenna… even at just 3000km altitude and 25% round-trip efficiency. That’s pretty horrible.

What about lasers? Next post. Lasers are also a good option, and direct diode lasers are available at 100kW and fairly inexpensive ($1-10/Watt?) and efficient (wall-plug of about 50% or more), but they really need to wait until diffraction-limited weapon-class (>100kW) lasers are available and become more efficient since direct-diode lasers have high divergence and horrible beam quality, necessitating optics nearly as big as our microwave sources.

A good rule of thumb, by the way: The laser satellite will need to be close to the same price as the aircraft it powers, so around $150-300 million, including launch costs. That is really, really challenging, but not quite impossible.

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7 Responses to SBSP for electric aircraft, cont.

  1. Andrew_W says:

    In the previous thread I mentioned Jerry Pournelle’s description of a laser powered aircraft, he was referring to a study entitled ‘Laser Aircraft Propulsion.’ by Dr. Abraham Hertzberg & Kenneth Sun presented at the Third NASA Conference on Radiation Energy Conversion in 1978.

    In summary, the paper focused on a Boeing 747 powered solely by a jet engine with a heat exchanger in place of the combustion chamber. A laser from an SPS strikes a target attached to the heat exchanger, and the energy is used to heat up & expand the air entering the turbofan, producing thrust. The aircraft has to use conventional jet engines during take-offs & landings. Pournelle talks about using light frequencies not capable of reaching the ground (doesn’t sound realistic to me) and argues that even if the laser misses the aircraft the damage would be minor compared to say an airline crash (maybe), but I suppose if the laser was to automatically cut off if the laser if light from a reflector on the target aircraft stopped giving a handshake there wouldn’t be much risk of damage to people and property on the ground.

  2. The military price of aircraft fuel can be very different from the civilian price. In mainland USA and Europe the costs are similar. However fuel prices in Afghanistan and Iraq were very much higher because the supply lines were under attack. So the military may be willing to pay significantly more for energy supplies that terrorists can not stop by mining the roads. (And not just for aircraft.)

  3. Chris Stelter says:

    Correct both of you. Military applications are the obvious first customers. And Andrew_W, there are definitely wavelengths of light and microwaves that are strongly absorbed by the atmosphere, such that they’re reduced in intensity by 100x, 1000x, 10,000x or more. You’d want aircraft that fly VERY high… Which kind of goes hand in hand with flying very fast. It may make sense to start out SBSP aircraft as long-haul supersonic, since that makes the best use of the capability to do something that is both unusually energy-intensive and also makes the most use of not having an exponential term in your fuel-vs-range equation (yes, aircraft also obey the rocket equation!).

  4. Andrew_W says:

    There are problems with using frequencies that are strongly absorbed by the atmosphere, obviously loses are one,thermal blooming the other.

  5. N/A says:

    Laser thermal heat exchanger turbo electric turbofan might be an interesting setup, especially if the turbine shaft can be decoupled from the fan shaft. For example, for a single shaft

    fan+electric motor/generator+compressor+external heat transfer HX+conventional combustor+power exhaust turbine

    conventional can combustors are optional, and if you can bypass some of the heated air past the power turbine, you get high altitude jet thrust (since high altitude thrust is more from the jet than the fan). In some ways this would resemble an externally heated turboramjet (sorta SERJ-esqe).

    Now, if the fan is not direct shaft coupled, then the capability to run the fan at power levels different from the turbine core is possible (say core power+battery). Ring drive motor fan rather than hub motor fan is possible. Decoupling should also allow running the core turbine for not just jet thrust but additional power generation when external power is plentiful, to recharge the battery stack. Decoupling also in theory allows running the fan like an auxilary RAT power generator when descending.

    Wonder about the air heating though, is it better to route compressor air through the laser receiver HX surface directly, or use a helium, nitrogen, or a supercritical CO2 intermediate loop.

    Conventinal business case is an aircraft with jet fuel capability (when no laser available), that can operate all electric when the laser is available. Engine maker goes beyond conventional engine lease, to a “fuel”/”power” purchase agreement, locking in revenue from operators. Operators like this because “fuel” cost becomes more static and is no longer dependent on local infrastructure (tank farm/fuel supplier). Miltary usage is the forever UAV HALE/AWACS/JSTARS that can act as a low flying satellite for theater C3ISR, to provide additional power to payloads. Actually, a radar dish would make a convenient spot for a receiver…

    For added crazy, air tractors cable towing conventional aircraft (requiring mid-air cable capture…), and superheated hot air diridgibles.

  6. Pingback: Electric Aircraft Followup

  7. Axel says:

    Jon posted:
    “Aircraft typically cruise at altitudes above 30kft, where you’re above clouds, and also above enough atmospheric moisture […] The fact that the lower atmosphere absorbs them efficiently means that any misses just heat up moisture in the air on the way down.”

    Does that mean you must not fly over areas with dry and clear weather? I just imagine a cabin anouncement like “… due to clear weather we have to land at Albuquerque Airport for the night. We will continue our flight to Los Angeles as soon as the weather becomes cloudy over the desert.”

    Andrew_W says:
    “if the laser was to automatically cut off […] there wouldn’t be much risk of damage to people”

    Please do not take this for granted. With the given example (3000km high, ca. 10 Satellites, latidude of +/-40°) the aircraft can be up to 6000 km away. Then a round-trip visual control loop at light speed has a delay of at least 40 ms. That’s just the blink of an eye, but what if you look into the sky and do *not* blink? Think about eye damage. We are talking about energy densities in excess of 1000 times higher than sunlight. Could it cause damage to the cornea? How much of it will be focused on the retina? Visible laser light would “obviously” be dangerous, but some invisible frequencies (e.g. in the near IR) are as dangerous. Also check out anmimal eyes. You wouldn’t want to cause an epidemic of blind foxes or such, would you?

    After some BOTE guestimates I would say 40 ms could well be long enough to seriously burn skin. Much depends on details.

    By the way: laser pointers are not allowed to be operated around airports. So be careful how to safely operate a high powered laser around your aircraft. And other aircrafts.

    The “automatic cut off” feature is an add on, which needs electronics and computers to work and adds complexity to the system. Think about all possible failure modes. Electronics can break down with age, radiation, etc., software has bugs, can be hacked, reprogrammed with a software update, could have backdoors installed. Think about terrorists hacking your laser satellites. Think about military interests in having a secret laser weapon in orbit. Call me paranoid, but if a foreign nation claims to have learned your laser satellite can be turned into a space based laser weapon (still banned, are they?) and shoots down your network in self defense I would give them the benefit of the doubt of doing the right thing.

    Things might be better with microwaves. But I’ve recently read the term “microwave weapon”, so add some public relations budget to your project to advertise your project as friendly. Say it reduces CO2 footprint, saves the environment, creates jobs, is good for health, … Prove it is completely harmless. Phrase it in a way everyone can understand and believe.

    For added crazy: consider to do microwave launching the Kevin Parkin way with your satellites. Needs more power, but once you managed supersonic, there you have a next challenge.

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