BROACH warhead

This is the principles of BROACH warhead according to BAE

The AGM-154C uses an Imaging Infrared (IIR) terminal seeker with autonomous guidance. The AGM-154C carries the
BROACH warhead. This two stage warhead is made up from a WDU-44 shaped augmenting warhead and a WDU-45
follow through bomb. The weapon is designed to attack hardened targets. It entered service with the US Navy in
February 2005. The BROACH warhead is a multi-stage warhead. The two stage warhead is made up from an initial
shaped charge, which cuts a passage through armour, concrete, earth, etc., allowing a larger following warhead to
penetrate inside the target

1-How come the precursor charge doesn't destroy the follow through bomb?
2- HEAT warhead leave very small hole compared to their diameter, so how is it possible to have a hole big enough for the follow through bomb to get in?

This is the principles of BROACH warhead according to BAE

The AGM-154C uses an Imaging Infrared (IIR) terminal seeker with autonomous guidance. The AGM-154C carries the
BROACH warhead. This two stage warhead is made up from a WDU-44 shaped augmenting warhead and a WDU-45
follow through bomb. The weapon is designed to attack hardened targets. It entered service with the US Navy in
February 2005. The BROACH warhead is a multi-stage warhead. The two stage warhead is made up from an initial
shaped charge, which cuts a passage through armour, concrete, earth, etc., allowing a larger following warhead to
penetrate inside the target

1-How come the precursor charge doesn't destroy the follow through bomb?
2- HEAT warhead leave very small hole compared to their diameter, so how is it possible to have a hole big enough for the follow through bomb to get in?

Because the precursor charge is highly directive and pretty much all the energy goes away from the follow through bomb. Anti-tank missiles have used this kind of technology for decades in tandem warheads with two or even threee shaped charge warheads one after the other.

Shaped charge warheads can be designed to leave larged hole with designing shallower shaping. Naturally penetration will suffer then, but that's just balancing the design. Really shallow shaping is called Explosively Formed Penetrator (EFP) whic does not produce jet but rather a really hot and large slug. This can create a large hole in target material.

This picture from Innovative Defense shows the difference in shaped charge design goals:


http://innovativedefense.net/super-caliber-charges.php

Because the precursor charge is highly directive and pretty much all the energy goes away from the follow through bomb.

I understand how can they focus the energy at the front but how could they contain the blast wave in the back?

Because the precursor charge is highly directive and pretty much all the energy goes away from the follow through bomb.

I understand how can they focus the energy at the front but how could they contain the blast wave in the back?

That follow through bomb is likely pretty tough and there is some kind of shield between the precursor charge and the bomb. Here is patent about how tandem shaped charge warheads have been shielded so that the warheads do not interfere with each other:

https://patents.google.com/patent/US7493861B1/en

There are probably differences in exact method as the bomb works quite differently from shaped charge warhead, but the basic shielding requirements are pretty similar. I think with hard nosed and relatively heavy bomb the shielding can be pretty light and thin. Shaped charges are likely a lot easier affected by the precursor charge effects without proper shielding.

2- HEAT warhead leave very small hole compared to their diameter, so how is it possible to have a hole big enough for the follow through bomb to get in?

It is important to remember how armor or concrete works against kinetic energy weapons. It doesn't stop the shell, it absorbs some of the energy by fragmenting the hard outer layer of armor and deforming the more ductile layer or powdering the outer layer of reinforced concrete and then redirects the shell so it ricochets instead of penetrating. Besides helping to break up the outer layer, the the precursor charge acts like a pilot hole for a screw, it ensures the warhead doesn't ricochet.

2- HEAT warhead leave very small hole compared to their diameter, so how is it possible to have a hole big enough for the follow through bomb to get in?

It is important to remember how armor or concrete works against kinetic energy weapons. It doesn't stop the shell, it absorbs some of the energy by fragmenting the hard outer layer of armor and deforming the more ductile layer or powdering the outer layer of reinforced concrete and then redirects the shell so it ricochets instead of penetrating. Besides helping to break up the outer layer, the the precursor charge acts like a pilot hole for a screw, it ensures the warhead doesn't ricochet.

This is very true. A bit like drilling even a very small pilot hole helps a ton when drilling a large hole to hard materials like concrete, metal or hard wood. Shaped charge is also going to make a larger hole to concrete or other construction materials compared to armour in tanks.