Science-based what-if questions

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andykhang
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Re: Science-based what-if questions

Postby andykhang » Sat Feb 10, 2018 3:14 am UTC

With that much power though, it's probably more cost-effective to just used the laser to blast the air let inside the jet pack and push up that way lol. Probably only need a fraction of the cost.

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Re: Science-based what-if questions

Postby gmalivuk » Sat Feb 10, 2018 5:16 am UTC

Yeah, the "advantage" of photon drives is that you need very little propellant to get a lot of velocity change, since you're sending your "exhaust" at the speed of light so you don't need a lot of it, mass-wise.

The problem is, "not a lot" mass-wise is still a huge amount, energy-wise. One gram of photon exhaust is almost 10^14 J of energy.

If you put that same amount of energy into something much more massive to speed it up to a tiny fraction of the speed of light, you'd get far more thrust out of the deal.
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Re: Science-based what-if questions

Postby gmalivuk » Sun Feb 11, 2018 3:12 pm UTC

Some additional numbers for context:

10^14 J of photon energy is about 334,000 kg m/s of momentum change. The payload the Space Shuttle could deliver to LEO was 27,500kg, and this amount of thrust could give that payload a measly 12 m/s of velocity change.

If instead that much energy were put all at once into a 200kg mass, it would accelerate it to 500,000m/s, for 100,000,000 kg m/s of momentum change. This means the 27,300kg (minus the projectile) remaining mass would get 3663 m/s of velocity change. That's enough to escape Earth's orbit entirely.

(That's obviously not quite the way rockets work, since they spray out exhaust as fine particles and not 200kg chunks, but the principle is the same: your energy use is much more efficient if you're willing to use up more propellant.)
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Re: Science-based what-if questions

Postby Sableagle » Sun Feb 11, 2018 9:58 pm UTC

That's 23.9 kT TNT.

The distance over which that projectile, fired straight down, is going to release that energy as heat is very short.

Maybe to a molecular biologist it's a vast distance, but in terms of 23.9 kT TNT, it's Not Far Enough.

That's very slightly more than the Fat Man plutonium bomb, which made one heck of a mess.

Anything you're trying to launch with that needs a very tough back end and a very remote launch-site.
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Re: Science-based what-if questions

Postby gmalivuk » Mon Feb 12, 2018 3:38 am UTC

Yeah obviously that's a lot of energy, that was my point.

But also I was talking about LEO, where presumably wouldn't shoot it straight down.
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Re: Science-based what-if questions

Postby Eebster the Great » Mon Feb 12, 2018 11:07 am UTC

I feel like even in LEO, pumping that much energy into the atmosphere would make something happen that would provide more impulse to the shuttle than 12 m/s.

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Re: Science-based what-if questions

Postby gmalivuk » Mon Feb 12, 2018 1:52 pm UTC

Why are you pumping any of it into the atmosphere?
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Re: Science-based what-if questions

Postby p1t1o » Mon Feb 12, 2018 2:17 pm UTC

Eebster the Great wrote:I feel like even in LEO, pumping that much energy into the atmosphere would make something happen that would provide more impulse to the shuttle than 12 m/s.


Hmmm...I doubt it.

If we are talking the equivalent of a Hiroshima detonation every second: depending on the frequency of the beam, the energy will be released partly in a column of atmosphere and partly at ground level. Energy absorption will be proportional to density of the medium and considering that 99% of the atmosphere lies below 20miles altitude, not much will be happening very close to the shuttle orbiting at ~200 miles. Combine that with the empirical knowledge that ground-level bursts of up to 50megatons do not knock spacecraft about in LEO.

When a large amount of energy is dumped into a small volume of atmosphere, not much (relatively speaking) air actually gets moved very far, a large bubble of hot gas will expand and rise, and there will be energy dissipation via shockwaves, but in order to have an effect on a shuttle at 200miles altitude, an ungodly amount of mass would have to be lofted very, very high and fast, and I just dont see a mechanism at this level of energy expenditure.

gmalivuk wrote:Why are you pumping any of it into the atmosphere?


Because he really, really, really hates whoever is living on that planet?
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Re: Science-based what-if questions

Postby gmalivuk » Mon Feb 12, 2018 5:45 pm UTC

It also of course depends on how fast you're releasing that energy. Sure, 100TW is a lot, but I never specified that it was a gram per second. Do it slow enough and it won't be slagging anything.
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Re: Science-based what-if questions

Postby p1t1o » Mon Feb 12, 2018 7:58 pm UTC

gmalivuk wrote:It also of course depends on how fast you're releasing that energy. Sure, 100TW is a lot, but I never specified that it was a gram per second. Do it slow enough and it won't be slagging anything.


Naturally.
I did combine some assumptions for the sake of argument ;)
It certainly wouldnt be much of a photon drive if it only had a 100TJ capacity and as a weapon "only" about as capable as a common-or-garden nuke, and vastly more complex and massive.

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Re: Science-based what-if questions

Postby gmalivuk » Mon Feb 12, 2018 11:47 pm UTC

Yeah, 100TW in a photon drive produces less thrust than a typical large airliner. Not really what you'd want to use for significant space travel.
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