SAILING TO THE STARS WITH NUCLEAR

[Gian]

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

[Janus]

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.

[swansont]

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 

[mistermack]

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??

[Janus]

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 September 16 by Janus

[Gian]

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🙂×

[Janus]

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)

[Gian]

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 Monday at 03:52 PM by Gian

[mistermack]

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. 

[Bufofrog]

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.