SAILING TO THE STARS WITH NUCLEAR

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Since at least WW2, scientists and science fans have been speculating about how to accelerate a spaceship fast enough to reach other star systems. The most sought-after method seems to be nuclear fusion (I think,) currently out of reach.

But what’s wrong with nuclear fission? If the power of the Hiroshima bomb could be channelled, I would imagine that would get us to alpha centauri pretty damn swiftly.

I did hear once that governments will not allow the use of nuclear for space exploration. Anyone know if this is true?

Cheerz
GIAN🙂xx

PS As you've probably guessed I'm not a scientist but I hope to be one day

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Nuclear fission converts ~ 0.1% of the mass into energy.  If all of that energy was convert into KE for the remaining mass (acting as the reaction mass), then you might get a exhaust velocity of ~.045c.

So let's say that you want to reach 10% of c.(43 yrs to Alpha Centauri).  Using the rocket equation gives us an answer of needing over 8kg of fissile fuel per kg of payload you want to get to Alpha C.  If you want the trip to end with you being at rest with respect to your destination, this jumps to 75 kg of fuel per kg of payload.

This is impractical.

Fusion is the better option since it converts a larger percentage of the mass into energy, thus giving you a higher exhaust velocity, which decreases the fuel to payload ratio needed to reach any given velocity.

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It’s not only an issue of energy, but having a reaction mass to expel*. The longer the period of acceleration, the more mass you need, and this is inefficient because in the beginning, you’re accelerating all that mass.

*unless you use photons, which is really inefficient

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I believe you can get "free" acceleration by using the mass of planets or moons in a "slingshot" effect. I don't know if there's a limit on what you can take from it. Any atmosphere will limit how low you can pass a body, and of course the craft has to be robust enough to withstand the forces involved. But it's substantial enough for long-distance probes to use it on a regular basis.

Fusion does allow for a greater percentage of the fuel to be used, but the hardware involved will be extremely heavy for centuries to come, so the weight advantage of a light fuel will be nullified. Controlled fission needs a lot of heavy hardware too. Maybe uncontrolled fission (as in a bomb) could give a craft a hefty kick??

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2 hours ago, mistermack said:

I believe you can get "free" acceleration by using the mass of planets or moons in a "slingshot" effect. I don't know if there's a limit on what you can take from it. Any atmosphere will limit how low you can pass a body, and of course the craft has to be robust enough to withstand the forces involved. But it's substantial enough for long-distance probes to use it on a regular basis.

Fusion does allow for a greater percentage of the fuel to be used, but the hardware involved will be extremely heavy for centuries to come, so the weight advantage of a light fuel will be nullified. Controlled fission needs a lot of heavy hardware too. Maybe uncontrolled fission (as in a bomb) could give a craft a hefty kick??

The gravity "slingshot" uses the planet's gravity to alter the trajectory in such a matter that part of the planet's momentum/orbital velocity is transferred to the craft.

The theoretical maximum gain from such a maneuver  is twice the orbital velocity of the planet.  However, this would require placing the craft ahead of the planet in it's orbit, at just the right spot and at rest with respect to the Sun.  In practice, this is not practical ( and you'd likely end up wasting more fuel trying to do so than you'd save with the slingshot).  In practice, you will always end up in a scenario where you get a smaller boost.  This is further complicated by the fact that if you have a final destination in mind, you are limited as to which types of trajectory/boost you can use.

My calculations using fission was assuming 100% efficiency as far as the released energy being converted into propulsion. Using a bomb would waste a good percentage of the energy and be less efficient.  Also, it is important not to confuse thrust with engine efficiency.   For example, Chemical rockets tend to be high thrust and lower efficiency, while ION engines are low thrust and high efficiency.

Higher exhaust velocity equals greater delta v for the fuel used, but lower exhaust velocities give you better thrust for the energy used.

Edited by Janus
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On 9/16/2023 at 4:25 PM, Janus said:

Nuclear fission converts ~ 0.1% of the mass into energy.  If all of that energy was convert into KE for the remaining mass (acting as the reaction mass), then you might get a exhaust velocity of ~.045c...

Thanks! So would nuclear be a practical possibility for a tiny space probe? Eg a camera and the computing ability of my cellphone?

I would imagine a single cellphone could gather quite alot of data and then send it home? Or if you got it upto say 50%c turn around and bring it home?

Would it be possible to use the Hiroshima bomb or bombs to send a projectile upto speed say 50%c from earth or from the moon without it having to power itself?

Yes I know I'm ignorant but it's so important

GIAN🙂×

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1 hour ago, Gian said:

Thanks! So would nuclear be a practical possibility for a tiny space probe? Eg a camera and the computing ability of my cellphone?

I would imagine a single cellphone could gather quite alot of data and then send it home? Or if you got it upto say 50%c turn around and bring it home?

Would it be possible to use the Hiroshima bomb or bombs to send a projectile upto speed say 50%c from earth or from the moon without it having to power itself?

Yes I know I'm ignorant but it's so important

GIAN🙂×

The issue would be how would you make that bomb focus all it's released energy towards accelerating the probe?

The closest example we have in this respect is an underground nuclear bomb test from 1957.  The bomb was placed at the bottom of a shaft with a iron cap. When the bomb was detonated, it blew the cap off.  Estimates have put the speed of the cap at 5 times the escape velocity from the Earth.

Now, given the size of the cap and the density of Iron, you can get an estimate of how much KE it had.  If you then take that KE and apply it to something with the mass of a cellphone, you can get its equivalent speed. It works out to ~ 2% of light speed.  And this was using a nuclear devise many times more powerful than the Hiroshima bomb.

To reach 50% of c, it would have had to had more than 625 times more energy than that (At this velocity you'd need to use the relativistic KE formula to get an accurate value)

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41 minutes ago, Janus said:

The issue would be how would you make that bomb focus all it's released energy towards accelerating the probe?

The closest example we have in this respect is an underground nuclear bomb test from 1957.  The bomb was placed at the bottom of a shaft with a iron cap. When the bomb was detonated, it blew the cap off.  Estimates have put the speed of the cap at 5 times the escape velocity from the Earth.

Now, given the size of the cap and the density of Iron, you can get an estimate of how much KE it had.  If you then take that KE and apply it to something with the mass of a cellphone, you can get its equivalent speed. It works out to ~ 2% of light speed.  And this was using a nuclear devise many times more powerful than the Hiroshima bomb.

To reach 50% of c, it would have had to had more than 625 times more energy than that (At this velocity you'd need to use the relativistic KE formula to get an accurate value)

Thanks Janus!

Well that sounds a bit more hopeful, especially if launched from the moon with its gravity of 16%G.

If a nuke 100x Hiroshima were exploded at the bottom of a shaft on the moon, I reckon that could get my cellphone past 50%c?

Cheerz

GIAN🙂x

Edited by Gian
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21 hours ago, Gian said:

Thanks Janus!

Well that sounds a bit more hopeful, especially if launched from the moon with its gravity of 16%G.

If a nuke 100x Hiroshima were exploded at the bottom of a shaft on the moon, I reckon that could get my cellphone past 50%c?

Cheerz

The real problem with any quick form of acceleration, whether nuclear or chemical, is how to avoid destroying what you are accelerating. The forces need to be so high that virtually anything would be obliterated. Your cell phone would be dust. If you find a way to accelerate something over a much longer period, the forces can be more survivable. But then you have the problem of accelerating your fuels and reaction mass before you use them.

Other less obvious problems would also provide a barrier to interstellar travel. At a speed of 50%c, the tiniest particle of dust would cause catastrophic damage, so you would need shielding, and that would have considerable mass, which is the enemy of interstellar travel. The same applies to cosmic ray shielding, it's all extra mass that needs to be accelerated and decelerated at the destination. Everything seems to work against interstellar travel, even to the nearest stars, and that's why it's not surprising that we haven't been visited by aliens.

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47 minutes ago, mistermack said:

Everything seems to work against interstellar travel, even to the nearest stars, and that's why it's not surprising that we haven't been visited by aliens.

Agreed.

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• 2 weeks later...

I just came across this youtube video, on this very subject, by a physicist who really knows what he's talking about, and works on this very subject for a living for NASA. Les Johnson.

It's worth viewing, if you're interested, after the introductions. He knows his stuff, examines all of the possible, and some of the more speculative methods, of travel to planets and stars.

Some of the bottom lines are that with current rocket power, getting to the nearest star with a lightweight probe would take in the region of 100,000 years. And he takes as his base number a period of 1,000 years, as a realistic target, if more advanced methods become available. He does actually address the nuclear fission theoretical boom boom method, although he doesn't address how you stop it destroying the craft, other than saying you would need a massive shock absorber.

But it's a very good video, worth watching, and bottom line, to get men to Proxima Centauri, the nearest star system, with very speculative technology that we don't have yet, would take an optimistic tens of thousands of years travel time. And that's the nearest star.

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Propulsion systems using reaction mass seem just wrong to me, for reasons touched on already.  And exterior acceleration (laser sail, say) confines exploration of Proxima to a very fast fly-by, given we can't zip on ahead and set up a deceleration laser there.  Alcubierre metric drive looks good albeit maybe impossible.

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On 9/16/2023 at 10:25 AM, Janus said:

Nuclear fission converts ~ 0.1% of the mass into energy.  If all of that energy was convert into KE for the remaining mass (acting as the reaction mass), then you might get a exhaust velocity of ~.045c.

So let's say that you want to reach 10% of c.(43 yrs to Alpha Centauri).  Using the rocket equation gives us an answer of needing over 8kg of fissile fuel per kg of payload you want to get to Alpha C.  If you want the trip to end with you being at rest with respect to your destination, this jumps to 75 kg of fuel per kg of payload.

This is impractical.

Fusion is the better option since it converts a larger percentage of the mass into energy, thus giving you a higher exhaust velocity, which decreases the fuel to payload ratio needed to reach any given velocity.

Can you please explain why the highlighted section is true? I'm curious why slowing down didn't also use roughly 8kg of fissile fuel per kg of payload thus increasing the fuel needed to 16kg per kg of payload, rather than 75kg.

Edited by zapatos
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38 minutes ago, zapatos said:

Can you please explain why the highlighted section is true? I'm curious why slowing down didn't also use roughly 8kg of fissile fuel per kg of payload thus increasing the fuel needed to 16kg per kg of payload, rather than 75kg.

I'd like to know the Janus' explanation, but my guess is that to decelerate at the end, you need to accelerate that deceleration fuel in the beginning. So, you'd need about 8 kg of fuel per kg of payload for the deceleration at the end, but much more than that at the beginning.

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Just imagine the stuff that you would need to take along with you, if by some technological marvel, you could get the theoretical travel time down to 1,000 years.  What would be the minimum number of humans that would constitute a worthwhile manned expedition? And how much food etc would you need to load, to make it even survivable? You could of course be growing food, using nuclear energy, and recycling everything. But what's the weight of such a nuclear plant, including all of the components needed to run it for a thousand years? And you would need living and exercising space for the humans, and a hospital, and all of the medical equipment and drugs for three thousand generations of humans.

The size of the craft needed would be absolutely impossible to accelerate this end, and decelerate at the other. So you have to conclude that humans ever going to the nearest star is an absolute impossibility, even with technology that is only being imagined at the present.

Miniature space probes, with cameras, robotics and radio communication are slightly more feasible, but they would be relatively pointless from a practical point of view. They would be on a par with a space telescope. Interesting from an academic point of view, but with no kind of payoff at the end of it, so the incentive to invest in it would be much lower than a manned mission. Who's going to put billions into a probe that will, if all goes well, be sending back data in a thousand years time? It just won't happen.

So even though stellar travel is an attractive idea, it won't get to reality.  Probably never ever.

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1 hour ago, Genady said:

I'd like to know the Janus' explanation, but my guess is that to decelerate at the end, you need to accelerate that deceleration fuel in the beginning. So, you'd need about 8 kg of fuel per kg of payload for the deceleration at the end, but much more than that at the beginning.

Right. So instead of 1 kg payload, you have 9 kg you need to initially accelerate for each 1 kg of actual payload.

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7 hours ago, mistermack said:

Just imagine the stuff that you would need to take along with you, if by some technological marvel, you could get the theoretical travel time down to 1,000 years.  What would be the minimum number of humans that would constitute a worthwhile manned expedition?

For a viable longterm colony, the 50/500 rule of population biology would apply.  For a human population to retain evolutionary potential, to remain genetically flexible and diverse, 500 is considered an approximate minimum.

For exploration, I've heard of studies that monitor glucocorticoids (stress hormones) in groups and found optimal sizes ranging from 6-25,  however I wonder if there is still a shortage of empirical data on this (especially where a rigorous selection process is used as would be the case with astronauts).  And, in the situation of exploring an entire strange new world (boldly going where no one has gone before), the demand for many kinds of expertise could mean higher numbers regardless of what would be optimal group dynamics.  It wouldn't be like a moon trip where a couple astronauts can confer constantly with JPL while doing a small set of activities of limited scope.

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3 minutes ago, TheVat said:

For a human population to retain evolutionary potential, to remain genetically flexible and diverse, 500 is considered an approximate minimum.

Doing it the old traditional way, yes. But there is a way round it, for those prepared to take it. A smaller party could carry frozen eggs and sperm from a wide cross section of humanity, and use them as donors instead of each other's samples to maintain genetic diversity in the next generation. Keeping the stored gametes super cold wouldn't cost anything in energy or equipment terms.

They might even develop artificial wombs in the future, that can carry a fetus all the way from fertilization to birth. So in theory, you could send a small party, with billions of sperm, and millions of eggs, and they could start a new colony with healthy genetic diversity.

Of course, all of the humans and eggs and sperm would need to be fully shielded from cosmic radiation, to prevent damage over a very long period. And shielded from collisions with space dust, which would be very destructive at significant fractions of c.

And of course, there wouldn't be much point in just sending humans. You would want a big store of plant seeds, and animals. Don't know how you would go about all of that. The mind boggles at the complexity and sheer weight of it all, were it really to be attempted to go to the nearest star on a mission to create a colony.

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On 9/16/2023 at 7:42 AM, Gian said:

Since at least WW2, scientists and science fans have been speculating about how to accelerate a spaceship fast enough to reach other star systems.

We already have spacecraft that move fast enough to reach other star systems. There is no required minimum velocity required to travel that far. What is required (among other things) is an appropriate amount of time. Most velocities we achieve will ensure that whomever embarks on the journey will die before reaching the destination. Since we would already need to figure out how to survive for generations in space for such a journey, does it really matter if that is three generations or 100?

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Three generations and one can be happy knowing ones grandchildren will step on a new world.  100 and any future involving another world and your descendants would be pretty abstract.  How would many generations that know they are only placeholders....footnotes in history feel about their lives?  I guess it's the topic that generation ship novels explore.

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4 minutes ago, TheVat said:

How would many generations that know they are only placeholders....footnotes in history feel about their lives?

How do you feel about being a placeholder for your descendants 100 generations from now? You were born and raised on earth, got an education, a job, ate food, etc. Basically you lived your life with the built in constraints that come with your environment. Being born on a starship would be similar; you live your life based on the constraints of your environment. If you leave earth you are embarking on a journey for a brand new life. Just like many of those did who crossed the Atlantic hundreds of years ago. Even the second generation will not likely long for earth as they have never known it.

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9 hours ago, zapatos said:

How do you feel about being a placeholder for your descendants 100 generations from now? You were born and raised on earth, got an education, a job, ate food, etc. Basically you lived your life with the built in constraints that come with your environment. Being born on a starship would be similar; you live your life based on the constraints of your environment. If you leave earth you are embarking on a journey for a brand new life. Just like many of those did who crossed the Atlantic hundreds of years ago. Even the second generation will not likely long for earth as they have never known it.

I've thought about that effect of acceptance of what one is born into, and it's certainly an element of generation ship novels (like Harry Harrison's Captive Universe).  You could argue the ship would just be your world, and seem normal, but I wonder how a starship compares to an entire planet, especially if you are young and restless and find out what planets are.  Much would depend on the shipboard culture that evolves over the dozens of generations.  And the question also gets back to Mac's comments on size.  How much ship does it take to satisfy any human desire for a "world"?

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23 minutes ago, TheVat said:

I've thought about that effect of acceptance of what one is born into, and it's certainly an element of generation ship novels (like Harry Harrison's Captive Universe).  You could argue the ship would just be your world, and seem normal, but I wonder how a starship compares to an entire planet, especially if you are young and restless and find out what planets are.  Much would depend on the shipboard culture that evolves over the dozens of generations.  And the question also gets back to Mac's comments on size.  How much ship does it take to satisfy any human desire for a "world"?

For some it only takes a village, for others the world is never enough, we can only vet the first generation; after that who cares?

"Because," said Lord Henry, passing beneath his nostrils the gilt trellis of an open vinaigrette box, "one can survive everything nowadays except that. Death and vulgarity are the only two facts in the nineteenth century that one cannot explain away."

– Oscar Wilde

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23 hours ago, zapatos said:

Can you please explain why the highlighted section is true? I'm curious why slowing down didn't also use roughly 8kg of fissile fuel per kg of payload thus increasing the fuel needed to 16kg per kg of payload, rather than 75kg.

Others have already basically answered your question in that it takes fuel to accelerate the fuel you'll use during the acceleration.

mathematically it works out to MR= edV/Ve

dV is the total change in velocity

Ve is the exhaust velocity

As far as fuel usage goes, accelerating up to 2V is no different than accelerating up to V, then decelerating back to 0.

1 hour ago, dimreepr said:

For some it only takes a village, for others the world is never enough, we can only vet the first generation; after that who cares?

"Because," said Lord Henry, passing beneath his nostrils the gilt trellis of an open vinaigrette box, "one can survive everything nowadays except that. Death and vulgarity are the only two facts in the nineteenth century that one cannot explain away."

– Oscar Wilde

Before we ever reach the point of sending a generation ship out to the stars, we will likely already have spent a good deal of time learning how to build and maintain artificial environments in the form of orbital space colonies in our own system.  This is turn means we will already have populations more attuned to this type of life.  It will be these people that crew these generation ships.

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In keeping with my pessimism about grand space dreams...

I think the only way humans could reach another star may be the slow way - migrating from deep space object to deep space object and building a new industrial base at each stop. 10,000 generations later, maybe... if they remember why or don't find their artificial habitats perfectly satisfactory. Or maybe if such a form of human civilisation can thrive it could become an expanding sphere that ultimately reaches other stars as an inevitability, without any set destination. Unfortunately I think establishing an industrial economy anywhere in space - let alone within a single ship - is extraordinarily difficult, next to impossible.

We can have knowledge in a can - libraries, archives, training courses, AI virtual experts - but how big a population to support having living expertise capable of not just maintaining what already exists but expanding on it? Any specialty on Earth is going to have at least some extraordinarily capable individuals with living knowledge that goes deep enough for serious problem solving. I worked at a plastic factory for a time, that had problems with an extruder, that ultimately required an engineer from the manufacturer to fly to Australia to diagnose and plastic extruders aren't that complicated; I think having the designs and  the availability of that kind of living knowledge underpins the viability of our industries. At least asteroid/comet/planetoid civilisations wouldn't have such extreme resource or population limitations and might achieve sufficient size to do more than struggle to maintain even a pared down, optimised technological minimum within a planned economy. Of course not all would go on the next migration.

Edited by Ken Fabian

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