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Comment by jessriedel | Hacker News Reader

> 1. Extremely high duty cycle 100 gigawatt laser. (edit: now apparently it's an "array" because someone with a brain mentioned that a 100 gigawatt laser is ludicrous) That's right 7 peak summer output Grand Coulee Dams and 12 Palo Verde nuclear power plants with all reactors operating simultaneously COMBINED would be needed to power this laser.

That's not how this stuff works. You would use some sort of large capacitor/battery bank which is charged by a much smaller power draw over a long time, and then dump the energy over a short time.

The laser array currently operating at the National Ignition Facility has a peak output of 500 TW, more than three orders of magnitude higher than the number you called ludicrous. Obviously Starshot would be applying that power for a much longer time interval, and indeed would be a global undertaking dwarfing NIF. But it doesn't need 100GW of dedicated power plants.

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Do they not need to run it/them continuously for years for a solar sail approach to make sense? NIF is not a good comparison for exactly that reason.

No, the acceleration phase is only for a few minutes. (I agree with you that if it requires 100 GW of power for years, his criticism would be accurate.) After a few minutes, the probe is already so far away from Earth that you can't focus the lasers effectively on it.

From wikipedia:

> ground-based lasers would then focus a light beam on the crafts' sails to accelerate them one by one to the target speed within 10 minutes, with an average acceleration on the order of 100 km/s2 (10,000 ɡ)

Well damn, then. I missed that when I was reading about it before. https://en.wikipedia.org/wiki/Breakthrough_Starshot

NIF's laser works for only 1microsecond, took over a decade to build and the price was over 3.5 bln. "Peak current for the system exceeds 100 million amperes and the peak power exceeds 1 terawatt (1 trillion watts)" https://lasers.llnl.gov/about/how-nif-works/power-conditioni...

Peak NIF power is actually only ~a nanosecond. And I acknowledged that Starshot would be a global undertaking that dwarfs NIF. I'm just saying that it's wrong to say this is going to require 100 GW of industrial power plants.

> > high duty cycle

This means that the laser is on most of the time.

If the laser has 70% efficiency, and is on 35% of the time, you need 100 / .7 * .35 = 50GW average (and 100 / .7 = 142GW when lasing). Any storage you have would have to be long term, because you need to lase when the spacecraft is overhead and not at night.

He's using it to mean "high duty cycle over a time-period of minutes". That would be relevant if his criticism was "we don't know how to build lasers that use that much power for that much time" (which is true, currently), but in fact his criticism was that it would require industrial power plants capable of outputting 100 GW (which is wrong). The big power draw of 100 GW over ~minute timescales is for the purposes of launching one probe. After that your power draw stops and you can recharge for as long as you want before the next probe launch. So it's not high duty cycle over timescales longer than hours or days or years.

The thing is, it'd fundamentally different from lasers that have timescale of microseconds or less. He mentioned "capacitor banks."

Launching 20 per day over a 10 minute span each is 200 minutes; you still need 12GW average over the day.

And you need an insane amount of storage-- if 12% of the energy comes directly from the power plant, you need to store 53 terajoules, which is about 60,000 tonnes of lithium batteries per daily shot. (It's more than that, because you're power/discharge rate limited, and it's going to wear out that minimum amount of batteries quickly).

> He mentioned "capacitor banks."

No he didn't. I did.

> Launching 20 per day over a 10 minute span each is 200 minutes; you still need 12GW average over the day.

Where are you getting 20/day from? I didn't see it on this OP link or the Wikipedia page. I'd have guessed more like once a week, which is 100 MW. That's a reasonable size for a gas turbine power plant, which seems like an appropriate expenditure for a project like this.

As a point of reference, the entire globe launches something to space every couple days on average.

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