|
Post by Kerr on Jun 23, 2017 17:41:43 GMT
Guys, let's work out an example, using equations and data from this discussion on laser sails: toughsf.blogspot.com/2017/04/interstellar-trade-is-possible-part-ii.htmlLet's use a 10MW laser firing far-infrared light (5 um). We will use Starwisp figures of 100kg/km^2 and 99.9% reflectivity, so a 2m wide sail will mass 0.31 grams. Let's make this military grade and all LCD reflector/absorbant strips that can steer the lightsail, so a 1 gram lightsail 2m wide can be made. Normally, a single bounce laser sail reflective 99.9% of 10MW produces 0.13N of thrust and would accelerate 1 gram at 133m/s^2. After 10 seconds, the laser sail would reach 1.33km/s, after a minute, about 8km/s. It accelerates over a distance of 239.4km and delivers 32kJ to the target. This laser heats up the laser sail at a rate of 10kW. This leads to an equilibrium temperature of 486K over 3.14m^2 (one side reflective, one side black). We can push this temperature as high as 800K for aluminium, 2000K for carbon-based meshes. At 2000K, we can dissipate 2.84MW of waste heat. So, we can reflect 284MW and accelerate a 2m wide laser sail to 266km/s within a minute, to deliver 25MJ upon impact. Bouncing a beam ten times allows us to generate ten times more thrust per watt. So, a 284MW/99.9%/2m/1gram laser sail will reach 2266km/s in one minute and deliver 2.56GJ impacts. The LCD shutters would have to be replaced by actuators that expose to hide extra segments of mesh for manoeuvring due to the high temperatures, but you only need to dedicate 0.0026% of the incoming light to match the movements of a target accelerating at 10m/s^2. I do not, however, understand how bouncing light violates the conservation of energy. Because mommentum is conserved, every time it's reflected equal amount of force goes into the photon and the sail. livestream.com/viewnow/niac2015seattle/videos/105034354 Here is a video from NIAC explaining DEEP IN/DE-STAR.
|
|
|
Post by shiolle on Jun 23, 2017 18:14:35 GMT
I do not, however, understand how bouncing light violates the conservation of energy. Thanks for the nice analysis. There is no violation of conservation of energy. Let's consider a single pulse of infntely small length dt. Nu=0.999 of its energy is reflected, 0.1% is absorbed by the sail. It is bounced against a similar mirror, where 0.1% f the energy is absorbed and the rest is reflectd. So when the pulse arrives back at the sal again, it only contains the fraction of ts enery, whch s equal to Nu^2. You might notice this is a geometric progression with multiplier of Nu 2. So total thrust power for n bounces is equal to Weff = dE/dt = W*(1-Nu 2n)/(1-Nu 2). This ignores any losses caused by imperfect fcusing of the beam (there is a theoretical threshold for that). Then the amount of absorbed power is W*n - Weff Wabs = W*(1-Nu)*(1-Nu 2n)/(1-Nu 2).
|
|
|
Post by Kerr on Jun 23, 2017 18:20:37 GMT
I do not, however, understand how bouncing light violates the conservation of energy. Thanks for the nice analysis. There is no violation of conservation of energy. Let's consider a single pulse of infntely small length dt. Nu=0.999 of its energy is reflected, 0.1% is absorbed by the sail. It is bounced against a similar mirror, where 0.1% f the energy is absorbed and the rest is reflectd. So when the pulse arrives back at the sal again, it only contains the fraction of ts enery, whch s equal to Nu^2. You might notice this is a geometric progression with multiplier of Nu 2. So total thrust power for n bounces is equal to Weff = dE/dt = W*(1-Nu 2n)/(1-Nu 2). This ignores any losses caused by imperfect fcusing of the beam (there is a theoretical threshold for that). Then the amount of absorbed power is W*n - Weff. Well, he always had a grudge against momentum equations, prefered kinetic energy ones. The x input y output, higher output than input really is a bit wacky.
|
|
|
Post by n2maniac on Jun 24, 2017 4:15:38 GMT
The mirror needs to be both 100% reflective and 100% smooth. Any imperfection in the mirror will vaporize the sail. Any wrinkle will tear it apart. All these qualities need to be maintained under 2 million g acceleration. Yes, totally legit. 1) Photon pressure = 20kPA, the yield strength of the sail is over 100 GPa, an difference of over 6 magnitudes. 2) "100% reflective and 100% smooth." In that case the thrust will be amplified by infinity. But as soon they start moving apart the doppler-effect will limit the amount of bounces. 99.9% Reflectivity is more than sufficient for 1000x bounces. Imperfections are avoidable with sufficient effort. We also can increase the size from 1m² to 16m². Which would (If we use an meta-graphene mirror that's few hundred atoms thick) still weigh roughly 1g. Put decreasing the power intensity from 300MW/m² to 18,75MW/m². The 99.9% Mirror used in the NIAC experiment had a threshold of 50MW/cm². Doppler effect will also sap energy (and momentum) from the photons. If the mirrors lose no photons between them (100% reflection, 0% diffraction/etc losses) then the setup becomes effectively a gas piston with photons as the working gas that happen to have a high speed of sound.
|
|
|
Post by tukuro on Jun 25, 2017 0:21:04 GMT
How does the sail survive that?
|
|
|
Post by Kerr on Jun 25, 2017 7:23:55 GMT
How does the sail survive that? Inside Voitenko Compressors payloads already survived acceleration of 2Gm/s or 200 million G, 100 times higher, plus the sail is based made out of graphene. The real problem right now is the temperature of the sail. I am gonna edit the values.
|
|
|
Post by leerooooooy on Jun 25, 2017 10:38:49 GMT
It was already demonstrated to work. By NASA Innovative Advanced Concepts Sure, and I am a wizard. Extraordinary claims require extraordinary evidence, and "bounces create more momentum change" is extraordinary
|
|
|
Post by Kerr on Jun 25, 2017 12:14:13 GMT
|
|
|
Post by leerooooooy on Jun 25, 2017 16:05:21 GMT
"conservation of momentum" is the reason the setup in the first link uses two spacecrafts bouncing a laser between each other. If one spacecraft accelerates in one direction, the other accelerates in the opposite direction, and total momentum is zero. You cannot get a laser sail up to megameters of speed without the other spacecraft reaching the same momentum. Also energy conservation, you cannot get infinite energy just by bouncing a photon between two mirrors and harvesting the bounce energy.
|
|
|
Post by Kerr on Jun 25, 2017 16:46:04 GMT
"conservation of momentum" is the reason the setup in the first link uses two spacecrafts bouncing a laser between each other. If one spacecraft accelerates in one direction, the other accelerates in the opposite direction, and total momentum is zero. You cannot get a laser sail up to megameters of speed without the other spacecraft reaching the same momentum. Also energy conservation, you cannot get infinite energy just by bouncing a photon between two mirrors and harvesting the bounce energy. And...? I never said it wasn't the case. The Laser fires a stream of photons to the sail, the sail reflects them to a mirror on-board the ship and so on. The Beam creates 667N every second. If the ship weights 100t the total velocity gained from shooting such a sail is 2.8 centimeter per second. I even mentioned photon recycling and photonic thruster. Both imply such a setup as described in the first link. Conservation of energy is only relevant for heating in this case. Because the energy of a photon can't accelerate an object directly, photons lack rest mass which means they don't have any kinetic energy, but they can have momentum. Does doubling momentum gain from photon by using material compositions sounds extraordinary? Because that what's happens if you reflect a photon, there is no reason why you can't repeat this with the same photons instead of constantly creating new ones.
|
|
|
Post by leerooooooy on Jun 25, 2017 18:26:38 GMT
|
|
|
Post by Kerr on Jun 25, 2017 18:57:00 GMT
I thought the equation expresses Electromagnetic Wave energy. Or does EMR Energy count as kinetic energy?
|
|
|
Post by thorneel on Jun 25, 2017 19:39:20 GMT
Wouldn't photons rebound each time with decreased frequency (thus with less energy)?
|
|
|
Post by Kerr on Jun 25, 2017 19:46:37 GMT
Wouldn't photons rebound each time with decreased frequency (thus with less energy)? Yes, in a none perfect mirror photons redshift (decrease in wavelenght)
|
|