Mini Nuclear Pulse Probes

[Airbrush]

Does anyone know how small a nuclear explosion can be?  The smallest I could find is the Davy Crocket.

"The smallest, known deployed nuclear bomb was the W54, which had a blast yield equivalent of between 10 and 20 tonnes of TNT (in the neighborhood of 1/1000 the power of the bombs used on Hiroshima and Nagasaki)."

Is it possible to scale this down even more so you have tiny nukes that don't weigh very much but they can put a huge impulse on a pusher plate of a small, unmanned, interstellar probe?

"Daedalus would be constructed in Earth orbit and have an initial mass of 54,000 tonnes including 50,000 tonnes of fuel and 500 tonnes of scientific payload. Daedalus was to be a two-stage spacecraft. The first stage would operate for two years, taking the spacecraft to 7.1% of light speed (0.071 c), and then after it was jettisoned, the second stage would fire for 1.8 years, taking the spacecraft up to about 12% of light speed (0.12 c), before being shut down for a 46-year cruise period."

"Daedalus would be propelled by a fusion rocket using pellets of a deuterium/helium-3 mix that would be ignited in the reaction chamber by inertial confinement using electron beams. The electron beam system would be powered by a set of induction coils trapping energy from the plasma exhaust stream. 250 pellets would be detonated per second, and the resulting plasma would be directed by a magnetic nozzle."

https://en.wikipedia.org/wiki/Project_Daedalus

Can this be miniaturized?  It seems that IF you can reduce the mass of the probe, and the size of nuclear explosions, and maybe even use shaped-charge nukes, and scale down the cost of the tiny bombs, we could send MANY mini probes to MANY nearby stars.  How much mass does the science payload need in order to study the destination star and planets, and then relay the info back to Earth?  Maybe the "Golden Age" of exoplanet study will begin when the first such probes start reporting back to Earth in 50 to 100 years.

"Unlike Daedalus, which used an open-cycle fusion engine, Longshot would use a long-lived nuclear fission reactor for power. Initially generating 300 kilowatts, the reactor would power a number of lasers in the engine that would be used to ignite inertial confinement fusion similar to that in Daedalus. The main design difference is that Daedalus also relied on the fusion reaction to power the ship, whereas in the Longshot design the internal reactor would provide this power.^[1]

"The reactor would also be used to power a laser for communications back to Earth, with a maximum power of 250 kW. For most of the journey, this would be used at a much lower power for sending data about the interstellar medium; but during the flyby, the main engine section would be discarded and the entire power capacity dedicated to communications at about 1 kilobit per second.

"Longshot would have a mass of 396 tonnes (873,000 lb) at the start of the mission including 264 tonnes of helium-3/deuterium pellet fuel/propellant. The active mission payload, which includes the fission reactor but not the discarded main propulsion section, would have a mass of around 30 tonnes.

"A difference in the mission architecture between Longshot and the Daedalus study is that Longshot would go into orbit about the target star while the higher speed Daedalus would do a one shot fly-by lasting a comparatively short time."

https://en.wikipedia.org/wiki/Project_Longshot

In theory, if you can miniaturize the science payload and the fuel supply, you could accelerate a probe to 10% light speed, and also slow down to orbit the destination star.  If we could send out a large number of these probes to possible earth-like solar systems, we would have another reason to survive to see the results of a multitude of probes that would start reporting back to earth in about 50 to 100 years.  As technology improves we can send out more and more, cheaper and more capable probes every year into the future.

Edited July 5, 2020 by Airbrush

[swansont]

17 hours ago, Airbrush said:

Does anyone know how small a nuclear explosion can be? 

You need to clarify what constitutes an explosion. Is one fission inducing another enough? How many steps of a chain reaction are enough?

Part of the limit on size would have to be from ensuring the reactions take place, rather than the neutrons just leaking out and rapidly going subcritical. You could make it smaller than was demonstrated, but it might not actually explode reliably.

[MigL]

Nothing can go wrong with that.

Shooting off nuclear bombs at exoplanets that may have intelligent, or even advanced, lifeforms.
Next thing you know …  Star Wars !

( sorry, sometimes I'm a little silly, first thing in the morning )

[Airbrush]

15 hours ago, MigL said:

Nothing can go wrong with that.

Shooting off nuclear bombs at exoplanets that may have intelligent, or even advanced, lifeforms.
Next thing you know …  Star Wars !

( sorry, sometimes I'm a little silly, first thing in the morning )

Also that is a great way to announce to every intelligent ET in a thousand light years that we are traveling at 10%C with nuclear explosions pointing directly to Earth.  That way they know how to find us to take our planet.

[swansont]

6 hours ago, Airbrush said:

Also that is a great way to announce to every intelligent ET in a thousand light years that we are traveling at 10%C with nuclear explosions pointing directly to Earth.  That way they know how to find us to take our planet.

If they had that capability already, what are the odds they don't know about us? And what is the gain to be had of taking our planet, if that can't happen for up to 10,000 years? (1000 LY at 0.1c)

It's an interesting conundrum. You launch a takeover bid, but the target has many generations to innovate and invent while you are on your way. You might have been the more advanced species when you initiated the attack, but that may not be the case when you arrive.

[MigL]

Reminds me of Hannibal of Carthage, crossing North Africa, the Iberian Peninsula, and France.
By the time he got to the Alps with his elephants, it was the middle of winter.
Not an easy way to attack Rome.

[Airbrush]

On 7/7/2020 at 3:19 AM, swansont said:

If they had that capability already, what are the odds they don't know about us? And what is the gain to be had of taking our planet, if that can't happen for up to 10,000 years? (1000 LY at 0.1c)

Do you think that any aliens that could detect mini-nuke blasts from an interstellar probe, would probably already have detected full-scale nuclear tests on Earth?   Because there have been many multi-megaton blasts, including Tsar Bomba at 58 megatons.

There is a way we can mask our location.  Start the probe in one direction, then change course before accelerating to cruising speed.  That way the aliens cannot align the nuke blasts to trace back to the location of Earth. 

A nuclear pulse probe should use the smallest nuclear blast possible.  If the Davy Crocket was 1/1000 of a Hiroshima blast, that seems quite small and useful for nuclear pulse propulsion.  How many of those would it take to reach 10%c, assuming a rocket total mass (including fuel pellets) of a few hundred tons?

Why not use multiple methods of propulsion in a single probe?  For example, get up to a high speed using nuclear pulses, then switch over to a propulsion method that scoops up interstellar hydrogen as fuel.

Edited July 8, 2020 by Airbrush

[swansont]

26 minutes ago, Airbrush said:

Do you think that any aliens that could detect mini-nuke blasts from an interstellar probe, would probably already have detected full-scale nuclear tests on Earth?   Because there have been many multi-megaton blasts, including Tsar Bomba at 58 megatons.

Yes, that was one thing I was thinking. We’ve also been beaming radio waves into space for 100 years. This means, of course, that aliens >~100LY away haven’t gotten signals yet, and could not have detected us this way.

 

[Airbrush]

Does anyone know how small a nuclear fission explosion can be for nuclear pulse propulsion?   It seems that you would want a "tiny bomb" with the smallest mass possible.  You want to give enough push to your pusher plate but you don't want the tiny bombs to add too much weight to your space probe.  The smallest I could find is the Davy Crocket:

"The M-28 or M-29 Davy Crockett Weapon System was .... one of the smallest nuclear weapon systems ever built, with a yield between 10 and 20 tons TNT equivalent [1/1000th Hiroshima]."

https://en.wikipedia.org/wiki/Davy_Crockett_(nuclear_device)

ONE of the smallest?  What are the others?  By now there must be smaller ones, after 70 years of improved fission technology.  Are there "suitcase nukes?"   The Davy Crocket weighed 51 pounds.  Maybe this yield of 10 to 20 tons TNT is practical for pushing a probe to Alpha Centauri at 10%c?

How many pulses of a Davy Crocket bomb does it take to get a probe with a total mass of 100 tons going 10%c?  I think the science payload can be a few tons and most of the mass would be a robust frame, a huge pusher plate, and the fuel bombs.

When an unmanned probe is moving 10%c do you think it should have a streamlined shape to minimize contact with bits of matter of various sizes, that may exist between us and Alpha Centauri?

Edited July 11, 2020 by Airbrush

[MigL]

Here is a list of American nuclear warheads and bombs since WW2

https://nuclearweaponarchive.org/Usa/Weapons/Allbombs.html

Some are sub-KT.

[Airbrush]

12 hours ago, MigL said:

Here is a list of American nuclear warheads and bombs since WW2

https://nuclearweaponarchive.org/Usa/Weapons/Allbombs.html

Some are sub-KT.

Thank you.  On your list the Davy Crockett is the lightest weight.  Anything smaller would probably be top secret, right?

[Curious layman]

2 hours ago, Airbrush said:

Thank you.  On your list the Davy Crockett is the lightest weight.  Anything smaller would probably be top secret, right?

Not sure if these will help, but

https://en.m.wikipedia.org/wiki/Suitcase_nuclear_device

Quote

Nuclear weapons designer Ted Taylor has alleged that a 105 mm (4.1 inch) diameter shell with a mass of 19 kg is theoretically possible.^[3] Conversely, reduction beyond the size of the W54 means that linear implosion designs must be employed and neutron reflectors dispensed with ("bare core"), so a much larger mass of fissile material is required and explosive yield is reduced dramatically.

https://www.globalsecurity.org/wmd/systems/w54.htm (the bottom of the page)

Quote

Special Atomic Demolition Mines (SADMs) were developed for employment by Soviet special operations forces, known as Spetsnaz. While the numbers of SADMs developed for possible use by Soviet forces was unclear, former Red Army intelligence personnel have written that these weapons were designed to have between a 0.8 and 2.0 kiloton yield, and were man-portable. Research suggested that the Soviets investigated applying "boosted fission" technology to their SADMs, which would provide 98% of the yield of fusion weapons.

 

 

[Airbrush]

 

1 hour ago, Curious layman said:

https://en.m.wikipedia.org/wiki/Suitcase_nuclear_device

https://www.globalsecurity.org/wmd/systems/w54.htm (the bottom of the page)

Thank you Curious Layman, you get a +1 for that.  The theoretically possible 19 kg is about 42 pounds, only 10 pounds lighter than the Davy Crockett.  Smaller than that is probably not cost effective, according to your article.

The Russian SADMs may have a yield of up to 2 kilotons,  or 2000 tons, or 4,000,000 pounds of TNT yield, and it is "man-portable".  Compare that to the Davy C at only 15 tons, or 30,000 lbs of TNT yield and weighs 51 lbs.  The Davy C calculates to 588 pounds of TNT per pound of mass (30,000lbs/51lbs) .  The SADM calculates up to 40,000 pounds of TNT per pound of mass (4,000,000lbs/100lbs= 40,000).   I suppose a guy could carry a suitcase weighing 100 pounds.  Something is not adding up.  How can the Russian SADMs be man-portable (100lbs) and also be 68 times the yield per pound (40,000/588=68) as the Davy C?  Where is my math wrong?

Edited July 12, 2020 by Airbrush

[swansont]

23 minutes ago, Airbrush said:

 

Thank you Curious Layman, you get a +1 for that.  The theoretically possible 19 kg is about 42 pounds, only 10 pounds lighter than the Davy Crockett.  Smaller than that is probably not cost effective, according to your article.

The Russian SADMs may have a yield of up to 2 kilotons,  or 2000 tons, or 4,000,000 pounds of TNT yield, and it is "man-portable".  Compare that to the Davy C at only 15 tons, or 30,000 lbs of TNT yield and weighs 51 lbs.  The Davy C calculates to 588 pounds of TNT per pound of mass (30,000lbs/51lbs) .  The SADM calculates up to 40,000 pounds of TNT per pound of mass (4,000,000lbs/100lbs= 40,000).   I suppose a guy could carry a suitcase weighing 100 pounds.  Something is not adding up.  How can the Russian SADMs be man-portable (100lbs) and also be 68 times the yield per pound (40,000/588=68) as the Davy C?  Where is my math wrong?

Is that total weight or the weight of the fissile material?

And, as Curious layman’s quote indicates, an implosion device is more efficient than a linear device, so the mechanism matters.

[Curious layman]

2 hours ago, Airbrush said:

Russian SADMs may have a yield of up to 2 kilotons,  or 2000 tons

Oops, reading too fast, thought it meant 0.8-2 kilos not 2000 tons. 

 

[Airbrush]

8 hours ago, swansont said:

Is that total weight or the weight of the fissile material?

And, as Curious layman’s quote indicates, an implosion device is more efficient than a linear device, so the mechanism matters.

I was trying to get a ratio of TNT yield per pound of device.  For example, the Davy C can yield 15 tons (30,000 pounds) of TNT, and the device weighs 51 pounds.  So that means 588 pounds of TNT per pound of device (30,000/51=588....that doesn't sound right).

My next question is how much of an impulse will an explosion of the Davy C at the ideal distance from the pusher plate?  What can you accelerate the 100-ton mini probe to with such an impulse?  The first pulse will occur far away from Earth of course.  I need to know how many Davy C's you will need to accelerate your 100-ton probe to 10%c and yet not exceed a mass limit of 100 tons.

Edited July 13, 2020 by Airbrush

[swansont]

9 hours ago, Airbrush said:

I was trying to get a ratio of TNT yield per pound of device.  For example, the Davy C can yield 15 tons (30,000 pounds) of TNT, and the device weighs 51 pounds. 

My question is about the data being provided.

The device will have a lot more overhead if it's an implosion-style device, but then, it will likely have a much higher yield per mass of the fissile material. The mass of the rest of the bomb probably doesn't increase rapidly as the yield goes up with a larger core.

9 hours ago, Airbrush said:

So that means 588 pounds of TNT per pound of device (30,000/51=588....that doesn't sound right).

Why doesn't that seem right? That's roughly 1.3 kT per kg

1 kg of U-235 is 1/235 of a mole, and each fission releases ~200 MeV, with ~180 MeV of this prompt (plutonium not much different), so (assuming it all fissions, which it won't) that's about about 72 gigajoules, or ~18 kT of TNT. If the bomb is 20% efficient, then that's 3.6kT per kg of core. Ultimately, the efficiency will depend on the design of the device. (e.g. Fat Man was 21 kT with 6.4 kg of Pu)

Meaning you can have a couple of kg of support material per kg of core, and have that yield.

(we don't have to settle for "that doesn't sound right" when there's trivial physics that can be applied)

 

9 hours ago, Airbrush said:

My next question is how much of an impulse will an explosion of the Davy C at the ideal distance from the pusher plate?  What can you accelerate the 100-ton mini probe to with such an impulse?  The first pulse will occur far away from Earth of course.  I need to know how many Davy C's you will need to accelerate your 100-ton probe to 10%c and yet not exceed a mass limit of 100 tons.

Her's a question for you: have you studied the project Orion information at all?

[Enthalpy]

The mass of plutonium stays roughly constant as the explosion yield decreases. The chain reaction needs a minimum amount of fissile material. The smaller explosion results from a design voluntarily less efficient, supposedly where the fissile material is expelled earlier from the reaction zone, so a smaller fraction of it detonates.

Fission energy can't attain 0.1*c because it's not concentrated enough. Even the fission fragments themselves don't. One fission frees 100MeV while one plutonium atom weighs 200GeV. But 0.1*c is a kinetic energy equalling mass/200. A factor of 10 is missing even before including other masses and huge losses. The Log of initial and final masses can't compensate for that, not even with many stages.

Beware most proposals for travel to stars are just plain cr*p. Where the author works and where he gets published changes nothing to that.

By the way, D-³He fusion is much more difficult than D-T, and electron beams are no serious candidates for inertial confinement.

[Airbrush]

"A preliminary design for a nuclear pulse unit was produced. It proposed the use of a shaped-charge fusion-boosted fission explosive. The explosive was wrapped in a beryllium oxide channel filler, which was surrounded by a uranium radiation mirror. The mirror and channel filler were open ended, and in this open end a flat plate of tungsten propellant was placed. The whole unit was built into a can with a diameter no larger than 6 inches (150 mm) and weighed just over 300 pounds...."

https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)

Imagine that each little bomb is only 6 inches across and weighs 300 pounds!

The smallest example given for an Orion vehicle is 880 tons.  I was hoping for a much smaller design for an unmanned probe, maybe 100 tons, but perhaps that is not possible?

The article talks a lot about the difficulties of launching from the Earth's surface.  Certainly such a vehicle should begin pulsing nuclear bombs FAR away from Earth.  It should be constructed in orbit.  A safer form of propulsion can push it far enough away from Earth before it starts pulsing nuclear.

https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)#/media/File:Project-Orion_propulsion-module_section.png

Does anyone agree that an unmanned vehicle to Alpha Centauri should have a streamlined shape?  Who knows what size of particles are floating around between stars.  They will all be battering the space probe at 10%c.  There could even be some micro-black holes between Earth and Alpha Centauri with the mass of Mt Everest compressed into a tiny speck.  That will punch a hole clean thru any space craft.

Edited July 13, 2020 by Airbrush

[Airbrush]

"In late 1958 to early 1959, it was realized that the smallest practical vehicle would be determined by the smallest achievable bomb yield. The use of 0.03 kt (sea-level yield) bombs would give vehicle mass of 880 tons. However, this was regarded as too small for anything other than an orbital test vehicle and the team soon focused on a 4,000 ton "base design".

https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)

The smallest achievable bomb yield is given at 0.03 kt, which means 30 tons of TNT per bomb.  The Davy Crockett yields 15 tons of TNT and the bombs weigh only 51 pounds rather than the 6" diameter 300 pound bomb pellets.  Maybe the Davy Crockett is newer technology?

The 880 ton vehicle was considered too small for interstellar travel.  Maybe a smaller, less costly vehicle, maybe 100 tons, could achieve 10%c and also decelerate upon arrival so the probe can study the alien solar system for a long time?

"Dyson calculated that the properties of available materials limited the velocity transferred by each explosion to ~30 meters per second independent of the size and nature of the explosion."

Does that mean that each nuclear explosion accelerates the vehicle by another 30 meters per second?

The article mentions a pusher plate miles across.  Could a pusher plate be a thin metal foil stretched across a web of supports so it could by miles across and yet not very massive?

 

Edited July 15, 2020 by Airbrush