Low weight RMH for caravan

Would it be possible to build a low weight rocket mass heater? The obvious answer would be no, as even the name includes the word MASS. But my think is, could the mass be avoided during transportation? If the mass was tanks of water, they could be emptied before transport, and refilled from any water source when parked.

Whats the preferred way to transport the heat from the rocket to the water mass? Long air ducts through water tanks has two issues - it has to be watertight, and the ducts takes up quite a bit of volume that should have been water. Could this be solved by winding a coil of piping around the steel drum and let a small pump flow water from the tanks, through that coil and back to the tanks?

I've read some posts here where people have used superwool for casting riser. That sound like a quite low weight construction. Could the batchbox be built in similar way?

How big is your caravan? A batch box is a powerful heater, perhaps a 4” batchbox would fit in ok.
Yes water is a good way to store heat, you may not need that much room  (mass) depending on the size of space you want to heat.
Personally I would steer clear of ceramic material  especially in a small enclosed space but there are other lightweight alternatives like insulating fire brick.

Hi, Jane

We have had several folks asking about putting an RMH in a caravan aka trailer or tiny house...

Here are some threads that might be useful to you or others:

https://permies.com/t/128743/Mini-Mouse-Camper

https://permies.com/t/170894/rocket-heater-fit-tiny-house

https://permies.com/t/152567/Uncle-Mud-Tiny-Home-Trailer

Thanks for the links. I've read them before. I feel like I've read all rmh build threads here and my questions are a result of that. I haven't seen any rmh that aims for low weight and water heat storage. (Those aiming for low weight use rocket stoves.)

The caravan is about 10m (33ft) long, with an side tent, insulated. If it gets too hot inside, we could leave the door open to let the tent be heated as well. I know the rmh is powerful, but this is also a cold climate. We typically reach -25C (-13F) for a couple of weeks. I'm thinking, if there is enough mass to heat, it will help to average out the temperature. Maybe it's ok to fire up less than once a day. If the mass radiate its heat too fast, there is always the option to slow it down by insulating around it.

I dry stacked some bricks (not firebricks) like shown in a drawing for a 6" system in one of the threads here. I did this just to get a feel for the size. It felt really heavy. I think firebricks are even heavier. To drive this caravan legally on roads, there can't be much weight added. That's why I'm looking for ways to make this as light as possible.

You would use lightweight insulating firebricks for the case described, they weigh very little especially for a 4" batch box system. If tightly secured in a metal shell they should be able to stand road vibration.

I came up with a concept for a water-mass 4" batch box system using a ~30 gallon barrel upside down inside a 55 gallon barrel, making a donut of water all around the heat riser (also water on top of the riser.) The barrels would be supported above the firebox, and exhaust collected from around the bottom of the barrel and directed to a chimney. Yes it makes less compact storage, but you need the small barrel-shaped enclosure for the riser anyway. You could add more water barrels or whatever containers with piping connected top and bottom so hot water circulates to all of them. If you want it permanently installed inside the space, you could use two 55 gallon barrels stacked, which would give a lot of heat storage in a simple system yet be very light when drained.

Hi Jane;
Your options are limited when weight and travel are an issue.
A 6" J tube would be large and heavy.
As Fox suggested a small batchbox might be a better choice in your situation.
How much traveling do you do and in what condition are the roads?

I'm thinking a 4 or 5" batch.
That would be small enough that even with heavy firebrick weight should not be an issue, but still crank out a lot of heat.
You would need a metal containment such as angle iron or perhaps a metal box to keep your stove together when traveling.

As far as using water tanks to be a mass.  
This is a good idea but implementing it into your space could be problematic.
I'm sure it can be done but I do not have any valid suggestions as to how you would accomplish this.
On-site evaluation is the best way to come up with a plan.

Have you visited Peter Bergs  Batchbox website?
Here is the link.  
http://batchrocket.eu/en/

Great idea with barrel inside barrel, @Glenn. How did you connect them? Did you cut a huge hole in the bottom of the outer barrel to put the inner barrel inside and weld them together around the hole? I've never tried welding anything watertight before. Will welding hold up over time with the heat? How hot does the outer barrel get during fire? Is there a risk the water will boil?

All the fire bricks I've seen are heavier than the regular bricks. Where do you find the insulating fire bricks?

The plan is to move the caravan every 3-6 months. If the road is rough, the speed will be low.
Would it be enough the secure the outside of the heater with steel? What about the riser? Any way to secure it from falling over?

I didn't actually build the design, just a mockup of the batch box and riser. I did have the two barrels on hand to verify that the dimensions would work. You could weld the two barrels, or make tabs and bolt and waterproof the joint. If bolting the joint, I would cut a hole a couple inches smaller than the 30 gallon barrel in the floor of the 55 gallon barrel, bend tabs up to fit inside the smaller barrel rim, and secure with stainless carriage bolts from the water side to the fire side so tightening and threaded parts are accessible and not exposed to water. Some sort of durable rust-resistant (and heat-resistant) coating would be needed on the inside of the barrels. There is an automotive engine gasket material in a tube that is good to 700 F for sealing the joint. I used it to seal a seam in the bell of my RMH several years ago.

Honestly, that didn't sound like the seam would be watertight. I'm familiar with the liquid gaskets used in car engines. I haven't taken apart a lot of engines, but from those I've been working on, I've noticed that any place the liquid gasket is used, there is no standing water or oil. In this dual drum seam the seam will be under at least 80cm (30in) of water - while the water expands.
In a location where water leak will cause problems, I wouldn't trust this solution. It could be worth trying to weld the seam. Or the safer way would be to wrap a pipe a number of turns around the barrel. It will take a lot of piping though, and be way less efficient heat transfer than the dual barrel solution.