HARM missile nuke warhead question

That question up is beyond esoteric and the expertise of this forum. Why not put whatever you want to be the size? The accuracy is irrelevant to the story. You’re not writing a technical manual so what harm would whatever you write be? You clearly have military or munitions background so you’re the expert here.

You speak only for yourself when you proclaim your ample ignorance.

Others here are not nearly as clueless as you are. You do not speak for us.

Without testing, 50 kt.

With underground nuclear tests to apply our most advanced ideas to warheads of that particular weight, 100 kt.



The term three stage has been used to mean two very different things. The answer to your question is yes for one meaning, and no for the other meaning. I'm not sure which meaning you are using, so I'll explain both meanings and you can pick which answer fits the meaning that you are using.


Some people refer to "clean nukes" as two stage (fission-fusion): a fission device compresses fusion fuel to set it off. These people refer to standard nukes, where that fusion fuel is surrounded by fissionable material, as three stage (fission-fusion-fission) weapons.

Note that in this usage of the term, two stage and three stage versions of the same nuke weigh the same. Fusion fuel needs to be surrounded by heavy shielding no matter what (it needs to be protected from the nuclear explosion that is being set off right next to it to compress it). This heavy shielding will weigh the same whether it is fissionable or inert.

If that is what you mean by three stage, then yes. The lightest possible multi-stage weapon would just barely be able to fit on a HARM.

The HARM is listed as having a warhead weighing: 66 kilograms (146 lb)
https://en.m.wikipedia.org/wiki/AGM-88_HARM

The W66 is listed as having a weight of: approximately 150 pounds (68 kg)
https://en.m.wikipedia.org/wiki/W66

Although the wiki article doesn't give a yield, the W66 is often quoted as being 20 kt. And the W66 being a neutron bomb, the fusion fuel will not surrounded by any fissionable material.

Unless you need the warhead to be a neutron bomb for your story (note that neutron bombs don't exactly work the way pop culture pretends), presumably your fusion fuel will be surrounded by fissionable material to dramatically increase the explosive yield.


Some people consider "fusion fuel surrounded by fissionable material" to constitute a single stage. In their eyes, the above weapon would only be two stage even if the fusion fuel was surrounded by fissionable material. They would use the term three stage to refer to a weapon where a "fission device" (stage one) provides the energy to compress "fusion fuel surrounded by fissionable material" (stage two), and that then provides the energy to compress "an even larger mass of fusion fuel surrounded by fissionable material" (stage three).

If that is what you mean by the term three stage, then no. Such a device would be far beyond anything that could be carried by a HARM missile. It could only carry the lightest possible two stage warhead.

That was an outstanding answer, and is exactly the sort of information I was looking for. Thank you very much for your time.

I see that the W66 was much wider in diameter than a HARM, so in my fictional universe I guess my engineers will be hard at work designing a 10" diameter fission-fusion-fission device.

In my story the warhead will be intended to have maximum blast effect, as opposed to the enhanced radiation effect of a neutron device, and will be used in the Anti radiation role to take out over the horizon EW launch detection radar systems.

Thank you again. If there is anything else you feel is relevant, please feel free to add it.

Welcome to A2K, we always wanted to meet oral's mother.

Well, I'll chip in here. Just make sure all parties are using either metric or imperial, NOT a combo of both. Get that? We've already had one disaster because of that.

And segue, why is it that Brits sell their 'petrol' in litres, but their road signs are in miles? Absolutely boggles the mind.

Life is a cacophony of mysteries Mame, when things get dicey, break out the Merlot.

Litres and kilograms all were down to the EU.

People were not happy about it at the time.


However, it’s always been the case that at most greengrocers and butchers you can get your produce weighed in imperial or metric if you want.

Some Brexit idiots have gone right over the top, some even want to go back to old money which is the stupidest thing I’ve ever heard. A penny is worth naff all as it is, and that’s when there’s 100 to the pond, let alone two hundred and forty.

Well, our stores use either/or kg or lb. Because of dealings with the US, our lumber is still imperial (2x4, etc). We're all over the map. Most of us talk about how many square feet our homes are, our height in feet and inches, and our weight in pounds. But hospitals use metric.

I don't know why it's been so difficult to adapt, but I get really irritated when I want to buy a picture frame and it's in metric because then I have to get the calculator out. It's an 8x10! They should put it in both.

Weight in pounds?

In stones, 14 pounds to the stone.

I have no children that are rocket scientists. You'll have to keep on looking.

The metric system is only used by nations that have never landed men on the moon.

Not countries who were crushed by the Vietnamese then.

That's a pretty long list. The Vietnamese are badasses at war.

Pretty sucky at space travel though.

There are implosion fission devices narrower than the one the W66 apparently uses for its primary. But I don't know if there are any narrow enough to let you get a multi-stage nuke on a HARM missile.

The thinnest staged warhead (of a typical design, where the fission primary is compressed by implosion) that I know of is the W80 that we use on our cruise missiles. It is listed as being 11.8 inches wide. I really doubt you can make a regular staged nuke any narrower than that.

https://en.m.wikipedia.org/wiki/W80_(nuclear_warhead)


There are however thinner devices that don't use traditional implosion in the fission primary. We use them in atomic artillery shells.

Instead of a traditional implosion where a shock wave converges on the plutonium from all directions and compresses it to a greater than normal density, the plutonium starts off shaped like a football (American football) and the explosive pressure reshapes it into a sphere. There is less surface area on a sphere than there is on a football shape, so fewer neutrons escape once the plutonium is spherical. Retaining these extra neutrons makes the core supercritical and causes an explosion.

They try to help this out a little bit by having voids in the plutonium that are collapsed by the explosive pressure, and by having the plutonium alloy be compressed into a denser arrangement of atoms, but the plutonium still remains at normal everyday density (whereas in a real implosion the converging shockwave would squeeze the plutonium into a much higher density than normal). Remaining uncompressed at normal density means you need a greater amount of plutonium in order to achieve a supercritical mass and explode.

You can read a little about this technique in section "4.1.6.3.2 Linear Implosion" here:
https://nuclearweaponarchive.org/Nwfaq/Nfaq4-1.html#Nfaq4.1.6.3

And also the section on "Two-point linear implosion" here:
https://en.wikipedia.org/wiki/Nuclear_weapon_design#Two-point_linear_implosion


These devices weigh more for a given yield than a traditional nuke (that relies on implosion for the fission primary) would weigh. A staged version would probably be too heavy for a HARM missile to carry.

Note the W79 artillery fired neutron bomb. The secondary achieves only 0.3 kt of fusion. It weighs 200 pounds, already too heavy for the HARM even with such a tiny yield.

https://en.m.wikipedia.org/wiki/W79_Artillery-Fired_Atomic_Projectile

To get within the size and weight limits of 10 inches diameter and 150 pounds, you will probably have to use the technology of our artillery shells, but not make it a multi-stage device.

Staging really adds a lot of weight to a nuke, because you need a heavy shell to protect the fusion fuel from being destroyed by the explosion that is compressing it, and another heavy shell to surround the explosion to hold the energy in so that the fuel gets compressed. Without staging, a weapon can be made much lighter.


You can still get a respectable yield even without it being a multi-stage device if you use deuterium/tritium boosting to increase the yield. And the lower efficiency of the weapon, requiring a greater amount of plutonium to reach criticality, actually helps you here. In general once half the atoms in the fissile material have been split, the fission fragments start impeding the chain reaction enough to bring it to a halt. If you have more plutonium in your device, that means you can split more atoms before you reach 50%. You might even want to have your engineers surround the plutonium with a neutron absorber instead of a neutron reflector in order to maximize the amount of plutonium in the device.

Note section "4.2.4 High Yield Weapons" here:
https://nuclearweaponarchive.org/Nwfaq/Nfaq4-2.html#Nfaq4.2.4

"In very large fission bombs (hundreds of kilotons) the major disadvantage of HEU, its lower maximum alpha, disappears. This is because the race between the exponential growth in energy release and the disassembly of the core stops being the limiting factor in efficiency. Instead the problem of dilution of the fissile material by the fission products comes into play as the limiting factor. This limits efficiency to a maximum of about 50%."


Such a device should be able to produce a yield of 100 kt if you provide enough deuterium/tritium gas to boost it that high.

I base my 100 kt estimate on the following:

Section "4.2.3.1 Minimum Size" suggests that it will require about 13 kg of uncompressed unreflected plutonium to produce a significant explosion:
https://nuclearweaponarchive.org/Nwfaq/Nfaq4-2.html#Nfaq4.2.3

"Since the critical mass for alpha-phase plutonium is 10.5 kg, and an additional 20-25% of mass is needed to make a significant explosion, this implies 13 kg or so."

If you boost 13 kg of plutonium all the way to the point where 50% of the plutonium atoms have been split, that means you've split 6.5 kg of plutonium.

This page gives the energy content of plutonium fission as 17.3 kt/kg:
https://nuclearweaponarchive.org/Nwfaq/Nfaq12.html

6.5 x 17.3 gives you a theoretical maximum yield of 112.45 kt.

Boosting a weapon from a very low yield all the way to 100kt will require much more tritium than is normally used in a weapon, but it is doable.

Before France developed staged thermonuclear weapons, they once had warheads that used U-235 (which has a much larger critical mass than plutonium, so they could split many more atoms before reaching 50%) that they boosted all the way to a half-megaton yield:
https://en.m.wikipedia.org/wiki/MR_41

Just a thought, but there are rockets with sections that are greater in diameter than the rest of the rocket.

Note this rocket here:


Why not just have your engineers give your HARM missile a slightly wider section in the warhead area?

That way your missile can carry a 150 kiloton W80 warhead, or even the explosive section of a B61 bomb. The B61-0 tested at 365 kilotons in the Flintlock Halfbeak test. The B61-1 and B61-7 have the same yield.

If you choose to widen a section of your missile and have it carry the explosive section of a B61, these details may be useful:

The first 365 kiloton B61s were the B61-0s. Then they moved on to building 365 kiloton B61-1s. Then they moved on to building a variety of B61s with lower maximum yields for tactical uses.

Later they decided to disassemble the B61-0s and modernize the B61-1s. The B61-1s had modern safety features added to them and became B61-7s.

So your engineers will have a choice between taking a B61-7 out of the existing US arsenal or reassembling the pieces of an old B61-0. The second choice might be the better one as it does not diminish the US arsenal. However, with current arms treaties drawing down our number of deployed weapons, there are also now a lot of B61-7s in cold storage (although they are kept in good condition just in case we need to build our arsenal back up quickly).