How long does it take for a drop pod to reach the surface from orbit?

The International Space Station is at 340km. If a drop pod travels at 15,000 kph, then it can reach the surface in 80 seconds.

I'm a little unsure on how drop pod insertions work in 40k.

I'm assuming you have the RT supplements, because I believe the stats, including speed, of drop pods are in one of the books (can't remember which one off the top of my head, and I've not got my books on me).

How they work, meaning what? The tactics and strategies of it? The science or technology of it?

In Rites of Battle (by FFG) it is stated an Astartes Drop Pod travels at 12,000 km per hour (in atmosphere). Thats ~Mach 11.4 or therearbouts. Tactical Speed is 200AU or so

A starship with a drop pod launch facility gets into orbit, shoots the pods at the planet and peels off. Because Space Marines rarely outnumber their foe they strike at vital command and control, leadership assets, supplies et cetera, rather than trying attrition warfare. The pod is controlled by a cigotor's machine spirit and it is armed with a storm bolter to lay down suppressive fire upon landing. The pods get them to the target quick so they can strike with as much surprise and force as can be had. At the very last instant thrusters ignite to slow the pod to a crushing halt on the ground, survivable by an Astartes but not humans (though in fluff some pods have carried exceptional humans into combat - but we don't know if it was slowed to do so).

There are other things about pods but thats the essential bit: insert a squad of astartes or a single dreadnaught into combat as fast as you can.

Pods need to be recovered as they cannot take off again on their own. The Chaos Legion version, Dreadclaw, can take off from a planet after it has landed and is specially equipped to be used as an assault lander against starships - I think it has a plasma cutter built in for the purpose of cutting a way in for its occupants to use in boarding.

bobh said:

That's actually covered in the RT supplements - they have additional dampening hardware that helps to decrease the inertia felt by the occupants, as well as additional padding and breaking systems.

With Drop Pod operations, the thing to keep in mind is what their function is. Specifically, they're very good at landing a small number of troops on the ground despite the fact that there's armed resistance to a landing which would make landing with the slower but higher-capacity Dropships hazardous or suicidal. This means that what they accomplish is stranding, at most, a couple of hundred troops with limited vehicle support behind enemy lines.

The only way this will ever make sense is if the troops being landed are Elite and equipped with top-rate wargear, allowing them to present a threat to the enemy that far exceeds what their numbers suggest. This is why drop pods are typically only associated with Space Marines.

It takes more than just a ship component to make drop pod operations worthwhile. You also need troops to put in them. Sisters of Battle or Imperial Guard Stormtroopers, while not at Astartes standards, could probably fill in for them in a Rogue Trader's army.

Other potential occupants that a Rogue Trader might find in the expanse could be Kroot. While not as good as Astartes in combat, as long as the Rogue Trader didn't mind not getting the drop pods back, dozens of Kroot with plenty of ammo could be inserted into enemy territory, where they could go to ground and mount a very nasty guerilla campaign.

Combat Servitors would be another option- fill the pods with them, and drop them behind the lines as a one-use terror weapon. Again, this has a high risk of not getting the servitors or drop pods back, but it'd be a nasty shock to the enemy it was used against and could turn a battle if used right.

There's one other use of drop pods worth considering- infiltration. Drop pods could insert agents onto a hostile world very easily. This option is likely more popular with the Inquisition or the Assassins, but a Rogue Trader who feels espionage and infiltration are more viable against a Heathen world may do this.

You forgot to mention orks as a viable source of 'ammunition' for a drop pod. As far as terror weapons go they do have a certain shock and awe value quite distinct from even the Astartes.