What is a rocket mass heater?

I wonder what the efficiency is compared to a modern "rocket stove" build [1] Mass rocket stoves are _incredibly_ efficient. So much so that I've always wondered why more similar designs weren't used in times when the primary heat sources were wood, etc. 1 - [VIDEO] (skip to 1:50 if you want to see a full one and not the ms paint diagrams)

Best fireplace in terms of efficiency is a rocket mass heater. It burns the wood gas in a secondary burn chamber and then routes the exhaust through a lot of thermal mass. One 2 hour fire a day will usually heat 2000 sq feet of house. 3m video explains the concept; longer videos will better explain it [VIDEO]

You can do a hybrid approach with the hot metal burn chamber for wood gas and earthen materials to slowly radiate the heat. This is called a rocket mass heater. The exhaust temperature is usually below 100C and often smokeless (a complete burn). Using a fan to extract heat from the final vertical stack (pipe around the exhaust with air running by) can give you an exhaust temperature of around 40C. Here's a quick 3 minute video that explains the concept; there's other YouTube videos that go into greater details if you wish to research this topic more. [VIDEO]

The rocket mass heater is the modern replacement for this. [VIDEO] Uses about 1/10th the fuel, and fuel can be wood unusable in other stoves. Very little exhaust / smoke. Super efficient.

Quick Video about the rocket mass heater. I'm guessing the cost to heat with wood, would be even lower if your house was built around this. [VIDEO]

A few things: Clay weighs significantly more than sand. For the same volume of material, clay will store significantly more thermal energy. The big question is- is the hot air on a closed loop or an open loop? I don't know of too many fans comfortable cycling 600C air, which means that you need an open loop of air (fresh air > fan > heater > sand > exhaust). If the sand isn't drawing heat off of the hot air fast enough, the heat won't actually "sink" in through the sand. If we are talking a closed loop of air (fan > heater > sand > fan) then the sand will get fully heated. My reference to "residential" is the aformentioned rocket stove mass heater- see [LINK] or [VIDEO] and [LINK] if you prefer videos. These types of things typically stay warm for about 24 hours, so they're burned once a day, maybe once every other day depending on how the house is insulated. Since these are all wood fired, they must be open loop, and sand is almost universally acknowledged as a terrible idea to use for thermal storage.