What’s the Most Stable Shape for an Interstellar Lightsail?
In 2015, Russian billionaire Yuri Milner founded Breakthrough Initiatives with the intention of bolstering the search for extra-terrestrial life.
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CrazyNeutrino said:
What’s the Most Stable Shape for an Interstellar Lightsail?In 2015, Russian billionaire Yuri Milner founded Breakthrough Initiatives with the intention of bolstering the search for extra-terrestrial life.
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Why is it that no billionaire ever sponsors an experiment on the origin of life? The most expensive experiment on the origin of life ever done probably cost about $200. Which I find infuriatingly short-sighted.
An interstellar lightsail with optimal geometry could be designed fairly easily by Rev Dodgson using standard engineering principles. Or by me.
CrazyNeutrino said:
What’s the Most Stable Shape for an Interstellar Lightsail?In 2015, Russian billionaire Yuri Milner founded Breakthrough Initiatives with the intention of bolstering the search for extra-terrestrial life.
More…
“The most ambitious of these is arguably Project Starshot, an interstellar mission that would make the journey to the nearest star in just 20 years. This concept involves an ultra-light nanocraft that would rely on a laser-driven sail to achieve speeds of up to 20% the speed of light.”
We’ve discussed this before. The energy from the laser beam heats the nanocraft up to vapourisation point in no time flat, unless the mirror is a low temperature superconductor, in which case keeping it cold is the dominant engineering challenge.
“What they found was that the simplest, stable configuration would involve a sail that was spherical in shape. It would also require that the StarChip be tethered at a sufficient distance from the sail, one which would be longer than the curvature radius of the sail itself.”
Huh? Are they placing the nanoship between the sail and the laser source? That is the worst possible configuration for thermal design. It would fail
Aren’t there any engineers working on visionary projects any more? This is the third recent thread on this forum that describes a proposal that can be easily demonstrated to be impossible after less than a minute of engineering thought.
The following relevant article from Wikipedia is interesting, though.
https://en.wikipedia.org/wiki/Optical_coating
“Gold has 99% reflectivity. A dielectric coating can be constructed from thin layers of materials such as magnesium fluoride, calcium fluoride, and various metal oxides, which are deposited onto the optical substrate. By careful choice of the exact composition, thickness, and number of these layers, it is possible to tailor the reflectivity and transmitivity of the coating to produce reflection coefficients of surfaces greater than 99.99%, producing a high-reflector (HR) coating.”
In other words, with an optical coating tuned to the monochromatic light source, the vapourisation of the proposed nanocraft could perhaps be delayed in time by a factor of 100. I hadn’t thought of that when I looked into this earlier.
Here’s the previous thread on project StarShot.
http://tokyo3.org/forums/holiday/?main=http%3A//tokyo3.org/forums/holiday/topics/7242/
Here’s what I said before:
“I find the idea of interstellar nanocraft extremely intriguing. I calculated many moons ago that no spacecraft powered even by hydrogen fusion could travel at even 1% of the speed of light. But my “small” spacecraft for the calculation weighed several kilograms. I also tried a spacecraft powered by a black hole, and one powered by antimatter. Even those weren’t good enough to get to high relativistic speeds. The only sensible way to get beyond that sort of speed, for an interstellar nanocraft, would be some sort of linear accelerator. Sort of a cross between a railgun and a SLAC. This would have to be built off-Earth because otherwise the atmospheric drag would both slow down the craft and heat it up to unacceptable levels. The biggest problem I can see is the trade-off between image resolution and speed. Let’s do a calculation of size vs speed. CERN uses 180 megawatts on occasions. Let’s assume that that is all the power available (remember that this is off-Earth) where power is more difficult to obtain. Assume that passive cooling in space means that superconducting magnets use a negligible fraction of that power. Calculate a G-force. Calculate one that limits uniaxial compression stresses to 250 MPa (The nanocraft is solid state, Some solids can take much higher stresses than that). Give it a density of 3100 kg/m^3. Then acceleration times length = stress divided by density = 80,000 m^2/s^2. For a length of 0.1 metres that’s 80,000 G. Let’s choose 5 cm long and 1,600,000 m/s^2. The time to reach relativistic speeds is therefore about 120 seconds. So let’s see what mass we could accelerate using 180 megawatts.”
“Power = mass times velocity times acceleration = m*v*a
First calculate this using peak velocity, say 20% of the speed of light.
m = 180*10^6 (watts) / 1.6*10^6 (m/s^2) / 0.2*300*10^6 (m/s)
m = 1.87 milligrams.
That’s a heck of a small mass for a spacecraft.
“With reduced spacecraft length (L<<5 cm) the stress on the craft components drops off, reduced acceleration (a<<1.6*10^6 m/s^2) would drop both the stress level and the power requirements, and variable acceleration (eg. acceleration proportional to 1/velocity at large velocities) would even out the power requirements. By how much could we reduce the acceleration, if at all? Let’s suppose the linear accelerator is s = 27 km long, the same as the circumference at CERN. Remember that this is off-Earth. IIRC, acceleration is calculated from a = v^2/2s = (0.2*300*10^6)^2 / 27,000 = 133*10^9 m/s^2 >> 1.6*10^6 m/s^2 calculated earlier. That’s an error by a factor of 83,000. In conclusion, it can’t be done. No nanocraft with even a mass as small as 1.87 milligrams could be made to fly at a relativistic speed with conceivable future power availability and space limitations.”
It was someone else who pointed out that applying that much power to a nanocraft would instantly vapourise it.
“I just calculated the theoretical minimum mass for a solar sail that could reflect without melting the specified 100 GW for 20 minutes. I checked reflection from six metals (aluminium, lithium, gold, osmium, palladium, silver) at the wavelength of maximum reflectivity (which happens to be in the microwave), heating up from a temperature of zero Kelvin. The result. The mass of the solar sail would have to be at least 148 tons, and made of lithium. So the specification of “a few grams” for the solar sail is totally impossible.”
Now, applying the new information from optical coatings in Wikipedia, the heating could theoretically be made 100 times less. Perhaps I should add here that curving the solar sail makes control of the thickness of the optical coating trickier, but not impossible. This reduces the minimum mass for the solar sail from 148 tons to a mere 1.48 tons. Not possible for a nanocraft.
but …
… let’s suppose that that 20 minutes of power was spread out over a very large number of small pulses, with enough time between the pulses for the craft to cool off over and over again. Then that 1.48 tons for the solar sail could be reduced considerably. By how much would require some more engineering calculations.
There’s also the small engineering problem that the cooling off will slow the spacecraft as the radiation will be dominated by that in the forward direction, applying acceleration in the opposite direction, but a back-of-envelope calculation says that we can ignore the slowing due to cooling.
mollwollfumble said:
Here’s the previous thread on project StarShot.http://tokyo3.org/forums/holiday/?main=http%3A//tokyo3.org/forums/holiday/topics/7242/
Here’s what I said before:
“I find the idea of interstellar nanocraft extremely intriguing. I calculated many moons ago that no spacecraft powered even by hydrogen fusion could travel at even 1% of the speed of light. But my “small” spacecraft for the calculation weighed several kilograms. I also tried a spacecraft powered by a black hole, and one powered by antimatter. Even those weren’t good enough to get to high relativistic speeds. The only sensible way to get beyond that sort of speed, for an interstellar nanocraft, would be some sort of linear accelerator. Sort of a cross between a railgun and a SLAC. This would have to be built off-Earth because otherwise the atmospheric drag would both slow down the craft and heat it up to unacceptable levels. The biggest problem I can see is the trade-off between image resolution and speed. Let’s do a calculation of size vs speed. CERN uses 180 megawatts on occasions. Let’s assume that that is all the power available (remember that this is off-Earth) where power is more difficult to obtain. Assume that passive cooling in space means that superconducting magnets use a negligible fraction of that power. Calculate a G-force. Calculate one that limits uniaxial compression stresses to 250 MPa (The nanocraft is solid state, Some solids can take much higher stresses than that). Give it a density of 3100 kg/m^3. Then acceleration times length = stress divided by density = 80,000 m^2/s^2. For a length of 0.1 metres that’s 80,000 G. Let’s choose 5 cm long and 1,600,000 m/s^2. The time to reach relativistic speeds is therefore about 120 seconds. So let’s see what mass we could accelerate using 180 megawatts.”
“Power = mass times velocity times acceleration = m*v*a
First calculate this using peak velocity, say 20% of the speed of light.
m = 180*10^6 (watts) / 1.6*10^6 (m/s^2) / 0.2*300*10^6 (m/s)
m = 1.87 milligrams.
That’s a heck of a small mass for a spacecraft.“With reduced spacecraft length (L<<5 cm) the stress on the craft components drops off, reduced acceleration (a<<1.6*10^6 m/s^2) would drop both the stress level and the power requirements, and variable acceleration (eg. acceleration proportional to 1/velocity at large velocities) would even out the power requirements. By how much could we reduce the acceleration, if at all? Let’s suppose the linear accelerator is s = 27 km long, the same as the circumference at CERN. Remember that this is off-Earth. IIRC, acceleration is calculated from a = v^2/2s = (0.2*300*10^6)^2 / 27,000 = 133*10^9 m/s^2 >> 1.6*10^6 m/s^2 calculated earlier. That’s an error by a factor of 83,000. In conclusion, it can’t be done. No nanocraft with even a mass as small as 1.87 milligrams could be made to fly at a relativistic speed with conceivable future power availability and space limitations.”
It was someone else who pointed out that applying that much power to a nanocraft would instantly vapourise it.
“I just calculated the theoretical minimum mass for a solar sail that could reflect without melting the specified 100 GW for 20 minutes. I checked reflection from six metals (aluminium, lithium, gold, osmium, palladium, silver) at the wavelength of maximum reflectivity (which happens to be in the microwave), heating up from a temperature of zero Kelvin. The result. The mass of the solar sail would have to be at least 148 tons, and made of lithium. So the specification of “a few grams” for the solar sail is totally impossible.”
Now, applying the new information from optical coatings in Wikipedia, the heating could theoretically be made 100 times less. Perhaps I should add here that curving the solar sail makes control of the thickness of the optical coating trickier, but not impossible. This reduces the minimum mass for the solar sail from 148 tons to a mere 1.48 tons. Not possible for a nanocraft.
but …
… let’s suppose that that 20 minutes of power was spread out over a very large number of small pulses, with enough time between the pulses for the craft to cool off over and over again. Then that 1.48 tons for the solar sail could be reduced considerably. By how much would require some more engineering calculations.
There’s also the small engineering problem that the cooling off will slow the spacecraft as the radiation will be dominated by that in the forward direction, applying acceleration in the opposite direction, but a back-of-envelope calculation says that we can ignore the slowing due to cooling.
What about the speed of “thought or imagination”? you can imagine a space ship here on Earth and a destination, say Alpha Centauri, you can then imagine it taking off doing a loop around Alpha Centauri and returning in say 10 seconds thus exceeding “C”?
bob(from black rock) said:
What about the speed of “thought or imagination”? you can imagine a space ship here on Earth and a destination, say Alpha Centauri, you can then imagine it taking off doing a loop around Alpha Centauri and returning in say 10 seconds thus exceeding “C”?
The speed of thought is up to a maximum of 120 metres per second. It only appears fast because it doesn’t have very far to travel. By contrast, the speed of New Horizons is 16,200 metres per second.
Not what you meant?
At the speed C it takes no time to travel everywhere. That’s because of the Fitzgerald contraction. Travel fast enough, and Alpha Centauri becomes closer than your next door neighbour.
mollwollfumble said:
bob(from black rock) said:What about the speed of “thought or imagination”? you can imagine a space ship here on Earth and a destination, say Alpha Centauri, you can then imagine it taking off doing a loop around Alpha Centauri and returning in say 10 seconds thus exceeding “C”?
The speed of thought is up to a maximum of 120 metres per second. It only appears fast because it doesn’t have very far to travel. By contrast, the speed of New Horizons is 16,200 metres per second.
Not what you meant?
So do I get a “tic an elephant stamp and 10/10”?
At the speed C it takes no time to travel everywhere. That’s because of the Fitzgerald contraction. Travel fast enough, and Alpha Centauri becomes closer than your next door neighbour.
bob(from black rock) said:
mollwollfumble said:
bob(from black rock) said:What about the speed of “thought or imagination”? you can imagine a space ship here on Earth and a destination, say Alpha Centauri, you can then imagine it taking off doing a loop around Alpha Centauri and returning in say 10 seconds thus exceeding “C”?
The speed of thought is up to a maximum of 120 metres per second. It only appears fast because it doesn’t have very far to travel. By contrast, the speed of New Horizons is 16,200 metres per second.
Not what you meant?
So do I get a “tic an elephant stamp and 10/10”?
At the speed C it takes no time to travel everywhere. That’s because of the Fitzgerald contraction. Travel fast enough, and Alpha Centauri becomes closer than your next door neighbour.
So, no “tic elephant stamp and 10/10”? is that what you are saying? so go back to being a 76 year old retired engineer, bugger.
bob(from black rock) said:
bob(from black rock) said:
mollwollfumble said:The speed of thought is up to a maximum of 120 metres per second. It only appears fast because it doesn’t have very far to travel. By contrast, the speed of New Horizons is 16,200 metres per second.
Not what you meant?
So do I get a “tic an elephant stamp and 10/10”?
At the speed C it takes no time to travel everywhere. That’s because of the Fitzgerald contraction. Travel fast enough, and Alpha Centauri becomes closer than your next door neighbour.
So, no “tic elephant stamp and 10/10”? is that what you are saying? so go back to being a 76 year old retired engineer, bugger.
Ah, now I understand. Here we use gold stars rather than elephant stamps. Elephant stamps hurt more.
Would like to visit you soon bob, (especially if you have a bathroom I can use onece or twice during our renovations).
mollwollfumble said:
bob(from black rock) said:
bob(from black rock) said:So, no “tic elephant stamp and 10/10”? is that what you are saying? so go back to being a 76 year old retired engineer, bugger.
Ah, now I understand. Here we use gold stars rather than elephant stamps. Elephant stamps hurt more.
Would like to visit you soon bob, (especially if you have a bathroom I can use onece or twice during our renovations).
Molly you are always welcome, suggest you ‘phone first to make sure I’m at home
bob(from black rock) said:
mollwollfumble said:
bob(from black rock) said:So, no “tic elephant stamp and 10/10”? is that what you are saying? so go back to being a 76 year old retired engineer, bugger.
Ah, now I understand. Here we use gold stars rather than elephant stamps. Elephant stamps hurt more.
Would like to visit you soon bob, (especially if you have a bathroom I can use onece or twice during our renovations).
Molly you are always welcome, suggest you ‘phone first to make sure I’m at home
Molly, don’t rely on the Holiday Forum as a means of contact, as I don’t visit it every day.
bob(from black rock) said:
bob(from black rock) said:
mollwollfumble said:Ah, now I understand. Here we use gold stars rather than elephant stamps. Elephant stamps hurt more.
Would like to visit you soon bob, (especially if you have a bathroom I can use onece or twice during our renovations).
Molly you are always welcome, suggest you ‘phone first to make sure I’m at home
Molly, don’t rely on the Holiday Forum as a means of contact, as I don’t visit it every day.
Newly Doted.
It’s a pity we can’t create smart light or perhaps a beam that contains some nanoprobes that arrive at their destination and then self assemble a probe from available resources to send back information
Cymek said:
It’s a pity we can’t create smart light or perhaps a beam that contains some nanoprobes that arrive at their destination and then self assemble a probe from available resources to send back information
Well, I suppose we could, for certain definitions of “nano”.
A “nanosatellite” is allowed to weigh up to 10 kg. That’s big enough to self-assemble a probe from available resources at the destination.
I wouldn’t like to try it with a spacecraft weighing less than one gram.
I’ve just sent an email to the lead author of the paper that reads:
Dear Elena Popov,
How do you stop this spacecraft from being instantly vaporised by the heat of the laser?
The time to melt (t) can be easily calculated from the received power of the laser (P), the reflectivity of the surface ®, the specific heat of the solar sail bulk material (Cp), the mass of the solar sail (m), and the melting point temperature (dT).
If the reflectivity and specific heat are that of the best possible metal, lithium, then the solar sail has to weigh at least 150 tonnes.
If the reflectivity is that of super-reflective optical coating, then the light source has to be a monochromatic laser tunable over a very wide wavelength range because of the Doppler shift, and the solar sail still has to weigh at least 1.5 tonnes.
If the material reflectivity is increased beyond that of a normal metal by being a low temperature superconductor, then keeping the temperature that low becomes a problem.
If the laser is fired in a series of much shorter pulses, with a cooling off period in between, then laser focus over such a vast distance becomes impossible.
So how do you overcome this vaporisation problem with the Breakthrough Starshot program, which requires a solar sail whose weight is less than 1 gram?
Yours sincerely,