Ok I built the stove and the thermal battery. Before i put all the mud and mass in the thing drew. I am working alone and I am afraid I misset the container on the heat riser. The thing worked for about a month in the summer then started to backdraft after I assume it completely dried.

I will type some dimensions here to see if anyone sees an obvious problem. I think I am going to have to rebuild.

Using the key letters from the Bible Rocket Mass heater book by Ianto

A=5X8 I loosley tried to shorten that and found no change in draft
B is 12 inches  when I was shaping the cob around the combustion unit I added 6 inches to the height of that.Today I chopped out the front of that when I was trying to tighten A and held a lit match in the mouth of the burn tube and the flame stood straight up
C=5x7
D might be 10 inches but I have to check the photos of my layout
d is 16 inches
F is 6 inch stove pipe inside an 8inch pipe  now I am wondering about this
E is 41 inches but at this point it includes G which I set at 1.5 inches
    i put the core of the hot water heater eccentrically over the heat riser it is domed so I wonder if  the slope has caused a constriction at that point
J is a crescent about 10 inches long with a maximum gap of 3 inches at the peak

K is 8 inch stovepipe. The total length is about 37 feet with 4 right angles. it does not lay flat but rather slopes up 4-6 inches from the beginning to the end

I will send another mail and attach some photos

Pics would be excellent!

If you don't get your info in a day or two, speak up and I'll check with erica.

I went on and tore out the heat riser today. after reviewing  I decided the 16.5 inch water heater core and 6 inch diameter heat riser did not match the needs of the 8 inch duct. In the process I  found some clay obstruction at J which was probably too tight anyway.I also think there was some settling and the size of G was 2 inches or maybe less

I think I will be able to retrofit a regular steel drum in the space and will have the room to make a brick riser in the barrel Have to double check to see if that would need to be insulated or the brick is adequate alone.

I will take some pictures of the process.

Its a learning process. building the stove and posting the pictures

paul wheaton wrote:
Pics would be excellent!

If you don't get your info in a day or two, speak up and I'll check with erica.

Hey guys, My name is Matt & I have been looking for a good efficient way to heat my shop (roughly 22,000 Cubic feet)

I have always been interested in alternative ways to do things, as is my dad... So I was surfing the web looking for some sort of efficient way to heat all this space & found Pauls vids on YouTube

So after reading about as much as I can find to read on Rocket Mass Heaters, I have a few questions for the experts

Has anyone tried running these heaters off of creasote coated wood (cross tie splinters to be exact)

I think it would work well & get hotter, if it can burn all the soot that they produce

Thanks, Matt

I've not heard of anyone doing that. 

I would choose to not do that - but I'm just one guy concerned about toxic gick.  Clearly, you are not so concerned.

I would think it would be far better than a conventional fireplace or woodstove - because I would guess it would get burned more completely.  I cannot think of any problems that might arise with a rocket mass heater that would not be with something more conventional.

nizhoni1 wrote:
I went on and tore out the heat riser today. after reviewing  I decided the 16.5 inch water heater core and 6 inch diameter heat riser did not match the needs of the 8 inch duct. In the process I  found some clay obstruction at J which was probably too tight anyway.I also think there was some settling and the size of G was 2 inches or maybe less

I think I will be able to retrofit a regular steel drum in the space and will have the room to make a brick riser in the barrel Have to double check to see if that would need to be insulated or the brick is adequate alone.

I will take some pictures of the process.

Its a learning process. building the stove and posting the pictures

You spotted what I was going to say about your initial post:
your areas are all different. 
the 8" duct has a 50" cross-sectional area
the 6" heat riser has a 30" cross-sectional area
that alone can make a stove really sluggish, as we did it once by accident.

then the burn tunnel has a 35" cross sectional area,
and the feed tube has a 40" cross section.
Ideally, you want all of these to match the ducting's area, so if you're keeping the 8" bench ducting then enlarge these other elements to a 50" area.

Insulating the heat riser is definitely important.  Insulate the burn tunnel too if you can.
The difference in temperature between the inside of the heat riser, and the outside of the barrel, is important for the draft.

Hope that helps and good luck.  Post a link to pictures when you have them.

I moved into my little cob hut halloween night.It is only 12x12 by maybe 16 feet high.
It was freezing as I put the finish layers over the final structure and water was dripping from the interior windows as the cob sweated as I laid it on,

There have been several nights recently that were 14 degrees.I was sleeping with the window open and just cooked a meal on the top today. It is taller than the original and I have compensated by being able to step up on the bench to stand next to it. The the rocks in the floor surrounding are all toasty warm.

What a arelief.

I posted pics somewhere else and they said they were HUGE. What are the recommended limits for size here?

Images are limited to 800 pixels wide and 1000 pixels high.

hello, im new here. and have a few questions. i have not bought the book yet but plan to in the next few days.  i just discovered the rocket stove concept a few days ago and have been brainstorming on how to heat my trailer ever since. 

will a smaller version of the rocket mass heater work with smaller pipe as long as everything is proportionate? or is 8 inch pipe absolutely necessary? theres a video that briefly mentions that 4inch pipe is too small but doesnt really explain why.  im also not sure what you mean by cross sectional area, but i guess thats probably in the book.

with pocket rockets,  does the exhaust pipe get very hot? could i use an existing old furnace chimney  to connect the exhaust or would this be too dangerous?

also,  why use pipe in the cob mass at all?  why not use chicken wire or something as a skeleton for a system of cob tunnels?  just a thought. 

any help would be greatly appreciated, thanks!

The crossectional area means the area if the pipe (pi.radius)squared as well as the dimensions of the fire box. If you cut through each section with a knife the area if each section must be equal.That way the force of the gasses/pressure is the same and they leave the exit instead of the entrance.

I used huge pipe because I didnt want to think about obstruction in the flue once all that mass surrounds it.You could probably carefully build a tunnel with cob around chicken wire but I would worry about constrictions from the weight of the mud in the building process.I would also wonder if any gasses could seep out through the breathable cob.

They weren't very clear about why the 4 inch flue was too small.There has to be a system mismatch since there are smaller "pocket rocket" stoves but maybe the difference lies in that they are used for cooking and we are talking mass heaters here.
the exhaust pipe at the exit is just warm after almost 40 feet of travel. I have a cleanout marked by a rock 10 feet from the firebox and it gets too hot to touch.


The

has anyone tried using concrete blocks joined together as mass and ducting all in one?  there is a huge pile of concrete blocks and other concrete/brick debris right by my place, trying to figure out a good way to use them. 

also,  trying to think of a more compact mass heater. i have a mobile home and am worried about how much weight in can support.  has a sort of coil been attempted with the ducting?  kind of like a radiator? would the tight back and forth of the flue affect the draw?  i guess a pocket rocket with mass around it would be a good and easy solution to this problem. 

brainstorming and experimenting on this with my grandfather... excellent male bonding.

I defer to those with more experience to discuss how to make a more compact version of a rocket stove. retrofitting and not going up in flames sounds like a challenge to me.

I used concrete rubble around the flue of my rocket stove. I don't even want to think about how much it weighs. probably more than two tons but I built a bench along two sides of my house.

4 inch pipe:  My impression is that all attempts have ended up with poor results.  I have a theory .....  I suspect that a four inch system could work fine provided that the pipe is no longer than 12 feet with no more than two bends.  It would seem that the 4 inch systems I've heard of had an awful lot of pipe and the combustion chamber struggles to push air through that much pipe in a four inch system.  But a six inch system can have, maybe, 30 feet with five turns.  And an eight inch can have 60 feet with six turns.  (I am just making these numbers up on the go - and I'm certainly no expert)

pocket rocket:  I suspect that the exhaust would get really hot.  That's where you would probably get a great deal of the heat.  The downside is routing the exhaust immediately outside before getting most of the heat out.  And that is a sad waste.

Concrete blocks:  my guess is that they would be pretty good for part of the system.  The exhaust would go down one channel and back the other channel. But I would worry about the exhaust taking shortcuts.

Lightweight heater:  I've been thinking a lot about the same thing.  Something radiator-esque.  Lightweight.  Easy to put in and take out.  I'm thinking of something that has a proper combustion chamber, but that there will be a pile of duct that will go back and forth to get all of the heat out before the exhaust goes outside.  So it would be massless - but still way better than a wood stove or fireplace.

My stove is 8 inch with 3 turns.No problems.

nizhoni1 wrote:
My stove is 8 inch with 3 turns.No problems.

How long is the exhaust?

about 30 feet

nizhoni1 wrote:
about 30 feet

If you were doing, say, 80 feet and had no problems, then I would be impressed.  But 30 seems like the norm - no biggie.

just giving information not bragging
sorry you took it that way

No prob.  I thought we were talking about pushing the limits.

planning to push some limits next year when I build mine. oh yes.

it's going in the 'basement', and should have about 3.5 cu yards of mass, I expect Ill have some thermal calcs to do as I have about 40 feet of riser, so i should get 25 feet of heated horizontal after the burn, and that puts me at 65 feet... its all 8" pipe, but i dont plan on adding height to the existing pipe, just going down a story with the stove and building it from earth.

is that the limit pushing were refering to? od do I have to rebuild my exterior piping to accomdate more interior horizontal?

so i built a version of Woodman's  modified pocket rocket (see thread titled 'modified pocket rocket').  i took pictures but i cant find the damned cord to connect my camera to the computer. its basically exactly the same but  i used a good heavy steel pipe and welded an elbow to use for the woodfeed/burn chamber.  it works great so far but it seems like all of the heat is radiated off of the feed tube. i tried stacking fire brick all around the elbow, and they seemed to store some heat but the radiated heat was mostly lost, as the barrel didnt seem to get very hot.   anyway, ive been sitting around thinking about how to make this more efficient and radiate more heat.    heres what i got...



would this work? ??  seems like it would boost efficiency and give more room for the exhaust to heat up the air in the room. 

hopefully i can get pictures up soon of what i have so far.

took a picture on my phone.  heres what i have going so far, it works and heats the room quite well, but i think it could be much more efficient. 

do you plan to cob around it? heat transfer to thermal mass (TM) works best when the TM is in direct contact w/ the heated metal. this kind of transfer is called conduction.

all those bricks are TM, but have lots of surface area despite their mass. almost heat they pick up is heat that is convected through air,and because air moves, much of the stoves heat never sinks into the bricks. rather than conducted through mass, your TM is charged weakly and dissipates quik- ie, inefficient. the rest of the heat transfer is done through radiance, and in this case a thats an even weaker transfer due to reflectivity and other consequences of matter...litterally, as radiation travels best in a vaccum. in the presence of matter it quickly reflects and bounces about, and when it is absorbed, the spectrum of radiation were concerned with turns into conducted and then convective heat so fast that it litterally bounces of whater it hit as heat that gas or liquid simply dissipates-


so  you have a choice-insulate or build thermal mass. or both.

the surface are of the brick TM you're using is a large area for heat transfer, but convected heat absorbs poorly into TM vs. Conductive.  The mass/surface area ratio of cob that is in contact with the heat source compared to the bricks is significant.. tha less moving air per TM volume = a bigger battery, deeper heat sink, slower charge and release. all that ends up becoming what we call more effcient.

youre well on your way, it seems.

theres more here than we need to know, but its good reads

http://en.wikipedia.org/wiki/Heat_transfer

http://en.wikipedia.org/wiki/Thermal_mass

D

i get the whole heat transfer and thermal mass thing,  i was kind of thinking of just going for a heat radiation effect and maybe working toward some kind of mass later.  the problem right now is that the barrel seems useless, id be better off using just a big burn tube.  thats why im trying to figure out if i can put an internal heat riser in the barrel and get more 'bang for my buck' with the wood(see previously posted drawing idea)  im just worried i wont get enough draw.  bought the book today,  doing more research.

Wow, so many ideas!  Let's see if I can squeeze all my responses into one post.
I'll pick on Paul, because I think he likes it.  Hope that's true, Paul
(and, How's Montana treating you?)

paul wheaton wrote:
4 inch pipe:  My impression is that all attempts have ended up with poor results.  I have a theory .....   I suspect that a four inch system could work fine provided that the pipe is no longer than 12 feet with no more than two bends. 

Smaller systems suffer from laminar flow constriction - in other words, the gas goes too slow because the pipe is too small, and it gets stuck along the sides.  Especially at the bends.  Shorter pipe, fewer bends - good place to start.

Also, it's hard to build and maintain a decent fire in a space with a 12.5" cross-sectional area - that's about 3 1/2" by 4".  Can't hardly fit a stick in there.  So fuel mix might be different: more bark, more twigs, quicker-burning kindling.
 
And the friction / bumps on the sides get more significant as the wood gets smaller and lighter, so it won't self-feed downward.  More fiddling.

And... and...
To solve all the problems that crop up at 4", you're basically inventing a different animal. 
If anyone makes a nice miniature Rocket Mass Heater at 4", please let us know.

paul wheaton wrote:
pocket rocket:  I suspect that the exhaust would get really hot.  That's where you would probably get a great deal of the heat.   The downside is routing the exhaust immediately outside before getting most of the heat out.  And that is a sad waste.

Hey now, don't you remember the Pocket Rocket demo?
The hottest part of a genuine Pocket Rocket is at the bottom of the can; Ianto proudly points out that it was designed to keep your feet warm. 

Here's a Pocket Rocket picture:

The red "blush" areas on the dark grey exterior are my attempt to show where the hot spots tend to be on the ones I've built and burned.

This is one of the options I'd try for the trailer-home situation, rather than trying to build a mass-heater without any mass.  Flame in direct contact with the barrel/bucket makes it HOT (red glow is literal reality) so use an insulated sandpan & bricks underneath.  When you're dependent on radiant heat, the hotter it is, the more effective it will be at reaching across those distances, or transferring into nearby mass.

Note that when the flames go up inside the container, they easily find the upper opening to the vertical stovepipe.  The combined heights of container and stovepipe create a continuous drafting thermo-siphon, pulling air and flames through the system, and keeping the smoke from coming back out the fuel feed.

The insulated heat-riser on a full-scale Rocket Mass Heater creates a hotter, miniature thermo-siphon.  This allows the apparent defiance of the laws of physics and 'pushes' the exhaust down and around the remainder of the system.  Without a thermo-siphon, squiggly pipes = no draft.

The heat lost in the exhaust is 'wasted', true.  But for good reason:
"better out than in" when it comes to smoke in the house.

That's one of the dangers of a squiggly 'exhaust radiator' inside a house: it can mess up your draft, creating more potential for smokeback and/or leaks from all the extra joints.

There are other reasons to think twice about re-routing a stovepipe around inside your house to capture waste heat.  I posted them on the "conventional fireplace" thread, where the suggestion seemed singularly ill-suited to the original question:

Erica Wisner wrote:
Paul, you sound excited about this idea of an exposed-pipe radiant heater.  (New thread?)

Do you know that this was tried historically?
...
...

Please read up on the historic versions before attempting this one at home.


...

nizhoni1 wrote:
about 30 feet

30 feet is definitely the norm; it's a decent, efficient 8" system.

FYI the record as far as I know on Rocket Mass Heater length is about 70' run on an 8" system; that's with a horizontal exhaust.  But it didn't work so good.

Dale H (the masonry heater guy) said the masonry-heater rule of thumb is keep the heat-exchange path to about 20 feet, and the exhaust temp above 90 degrees (to draft vertically up a chimney). 

90C is just under 200 F; water vapor is still (mostly) steam.  I suspect that keeping the water vapor from condensing into a fat dense plug is a big factor in keeping vertical exhausts happy. 90F just seems too cool to me; a horizontal exhaust would be needed, as the CO2 and H2O are both denser than air.

Ianto's book sez:

"To rescue most of the heat, a 6-inch system should have at least 20' of ducting and a thermal mass of about three tons.  An 8-inch system needs 30 feet of duct and should weigh about 5 tons." (p.25, Rocket Mass Heaters). 

A well-proportioned rocket mass heater can push exhaust 30, 40 feet or more. 
But heat transfers most effectively when there's a large difference in temperature.   So the further you go, the more the benefit diminishes.

A stove that can draft at 50' if the wind and weather are cooperating, or a stove that reliably drafts at 30' no matter the weather.... which of these beasties would you want in your home?

Now, if it's a prototype in a barn, go for it.  Set a new record.  And send us pictures.



What'd I miss?

brett, imho, yeah, a riser. i understood wrong, thought it was as the dwg, but now get that its not. from what ive seen (ive built two proto types, but nothing permanent, just playing) that riser will make good changes.

erica- you and ernie kick ass. thanks for all you do in this field!

Erica,  any time I say anything that is even slightly inaccurate, I do hope you will correct me.

A lot of my thinking comes from something I read about 20 years ago:  a 90 degree turn in a dryer exhaust has the same friction as five feet of straight dryer exhaust.  And the longer your dryer exhaust is, the less efficient your dryer will be.

I think the key to a lot of this is what you said about the insulated combustion chamber:  that provides the "push" - once you have that, then you can set about extracting all of the heat out.  And the pocket rocket does not have that!  Therefore, the pocket rocket DEPENDS on a warm chimney - so if you don't have heat leaving the system, then it will not properly expel exhaust!

Please read up on the historic versions before attempting this one at home.

Link?

if you click on the bold text in ericas quote it takes you to a thread that explains that s curved chimneys can be deadly.

something i wanted to add...  people say that one of the drawbacks of a rocketstove is that you dont get the pleasing view of the flame,  but the light that comes out of my wood feed while burning dances on the ceiling in an amazing aurora-borealis-like effect when all of the lights are out.  its quite nice, some friends and i spent a couple hours last night just watching the light show.

brett hunter wrote:
if you click on the bold text in ericas quote it takes you to a thread that explains that s curved chimneys can be deadly.

And there's the rub:  are we talking about a chimney or are we talking about exhaust?  I think most of the concerns for a chimney do not apply when you use an insulated combustion chamber.  No creosote.  Very little smoke, if any. 

that is indeed the trick.
Exhaust radiators or fancy masonry heaters with hard-to-clean intestines work best on systems with very clean exhaust.  They also greatly increase drag, as you pointed out, so they call for sophisticated balancing of the system to maintain adequate draft.

On a home-built barrel stove that's not insulated.... I'm glad to hear it has clean exhaust, but I wasn't sure of that from the pictures...
Even when it's capable of a clean burn, I tend to suspect that at some point, users of any metal stove are going to be tempted to smolder wood overnight to keep warm since there's not much thermal mass in the typical American woodland retreat.

I just like to throw the cautions in there, because all these ideas sound good one at a time, but they're only safe or reasonable in certain combinations.

The best predictor of any combustion system's behavior is another working model the same size, shape, etc. 

Gotta go - landlady's family looking at our rocket stove

-Erica

Here's another question:  If I build a rocket stove in our garage, as I plan to do, I can run the exhaust out either the east side or the west side.  The east side is *usually* (not always) down-wind; west side is *usually* up-wind.  But the east side is the front of the house and would be visible from the road.  Will the exhaust draw better on the down-wind side (I would think so, but....).  I'd rather not have it visible from the front of the house, though, if it would work the other way.

Kathleen

Kathleen Sanderson wrote:
Here's another question:  If I build a rocket stove in our garage, as I plan to do, I can run the exhaust out either the east side or the west side.  The east side is *usually* (not always) down-wind; west side is *usually* up-wind.  But the east side is the front of the house and would be visible from the road.  Will the exhaust draw better on the down-wind side (I would think so, but....).  I'd rather not have it visible from the front of the house, though, if it would work the other way.

Kathleen

My experience is that going out the downwind side is usually worth it.
A dryer-vent style cap on the vent can make it inconspicuous as far as neighborhood aesthetics.

If you have windows on both sides, you could try sending the exhaust out the upwind side through the window, and see if it works OK in typical wind conditions. 

Once you are pretty sure it will work, you can do the permanent install through the wall.

It may be that while the morning wind makes it smoke back, the evenings are OK, or vice-versa. Or you may be able to create enough wind baffling to compensate for the wind.  (There's a funky H-shaped Bernoulli cap among the case studies in the back of the Rocket Mass Heaters book that might help.)  Getting the exhaust above the roofline by a few feet can help, too.

The big issue is how big is the pressure difference between the two walls.  If the downwind side (and house) are at low pressure in the lee of the wind, it will be very had to push the exhaust into the high pressure area created on the windy side of the house.  Baffles can only do so much.  In that case the last resort is a good-sized fresh air intake on the upwind side; run it behind the bench so it pre-heats from the thermal mass, and you might solve the problem. 

Or move the exhaust to the downwind side, which makes the low and high pressure work for you.

Good luck with the project.  If any of these suggestions work for you, or if you find a new solution, we'd love to hear about it.

Thanks,
Erica Wisner

when you have a heat riser,  does a tall chimney outside still benefit the draw?  or is the exhaust all being 'pushed' out rather than pulled?   

My feeble response:  I think a tall "chimney" (exhaust tube) would do more harm than good because the idea of a chimney is to deal with hot (or very warm) smoke.  This is not smoke and it is not even warm, really.  So then you just have a bigger pipe to push exhaust through.

  hello all. after reading all of your post i had a few questions.
can you use a gas burner in the rocket stove if you run out of things to burn ?
could you add a cast iron grate for coal, pellets, charcoal, and corn cobs?
are bends in the exhaust required to slow air movement or could you just use all straight pipe with thermal mass?

I would caution anyone trying to use things other then stove pipe or pipe.  Carbon monoxide needs only heat and carbon to form.  Hopefully you will burn that bad stuff up. But late at night who knows what may happen.  You really do not want some small crack to leak gas into the house that you may not smell.

carbonout wrote:
   hello all. after reading all of your post i had a few questions.
can you use a gas burner in the rocket stove if you run out of things to burn ?
could you add a cast iron grate for coal, pellets, charcoal, and corn cobs?
are bends in the exhaust required to slow air movement or could you just use all straight pipe with thermal mass?

My first thought is that if I answer in any way other than "if you try that you will die" then I will expose myself to lawsuits.

So:  if you try any of that you will die.

Now that that's out of the way:  those are some awesome questions.  I think a fully accurate answer would be "it depends."  I would actually like to add to the question pile with "junk mail, newspaper, pizza boxes, paper packaging, lousy books, oily paper towels, sawdust, wood chips, pine straw, cooking oil that has gone a bit off,  and the dried bones of mean people."

It seems it would be pretty awesome to burn just about anything and get heat out of it.  While I remember hearing about Ianto and Linda heating their home in a pocket rocket one winter with junk mail - I'm not sure how big that home is/was nor whether it was exclusively junk mail. 

So .... I'm thinking about the corn cobs, coal, pellets and the like ....  I'm guessing that the primary problem might be that the air goes right by the coals without putting a lot of air to the fuel.  I suppose one might cobble something together that you could drop into the feed tube to help focus some of the air to the fuel. 

As for "gas" - I suppose this question could be broken up into gasoline, diesel, methane, propane, biogas, etc.  I suspect each one would need something to facilitate the burning.    I can imagine a propane heater being rigged up just under the combustion chamber.  As the combustion chamber warms, air will be pulled in via the feed tube and pushed through the mass.  It seems like it would work.

carbonout wrote:

are bends in the exhaust required to slow air movement or could you just use all straight pipe with thermal mass?

My impression is that it could be all straight pipe.

brett hunter wrote:
when you have a heat riser,  does a tall chimney outside still benefit the draw?  or is the exhaust all being 'pushed' out rather than pulled?   

It depends on how hot your exhaust is, and how hot the chimney can get.

On most rocket systems longer than the 'minimum' described in the book (20-30 feet) the exhaust is getting cool enough that it is a "push" rather than a natural upward draft at the exit. 
The really efficient stoves that Ianto and Paul favor, tend to have exhaust temperatures more like a dryer vent, maybe 90-110 degrees F.  So that tends not to benefit from vertical chimney except in certain conditions.

If it's cold outside, the natural draft of warm air is better; if it's hot outside, your 'warm' exhaust is effectively colder than the outside air, and will not rise without mechanical intervention.

A cold chimney with cool exhaust (under 90 degrees, or in some cases anything under 200 F) will not draft upward.  Both CO2 and water vapor are denser than air, and condensed wet fog even more so.  If you plan to suck all the heat out of your exhaust, give yourself an option to exhaust it safely sideways or downward instead of up.

A cold chimney with hot exhaust will warm up quickly, and then the chimney will aid the exhaust moving out.  Hot exhaust rises due to its buoyancy compared to the surrounding outdoor air.  If the chimney stays cold (like an exposed chimney with a lot of wind chill), then there may be a tendency for the exhaust to cool and sink back down at certain points in the chimney, causing intermittent problems.

A hot chimney with cold exhaust doesn't occur by itself in nature.  You can create something similar with either a chimney heater, or install the chimney in the center of the house instead of outside in the cold.  Black-painted chimneys also soak up a little sun and warm up in the daytime too.  These can help what would otherwise be a problem chimney, to draft better.

carbonout wrote:
   hello all. after reading all of your post i had a few questions.
can you use a gas burner in the rocket stove if you run out of things to burn ?
could you add a cast iron grate for coal, pellets, charcoal, and corn cobs?
are bends in the exhaust required to slow air movement or could you just use all straight pipe with thermal mass?

Paul's answer was pretty much accurate.

I think they did use 100% junk mail in a Pocket Rocket just to prove that they could.  They said it was a very transformative experience and they have never looked at junk mail quite the same way again.  Or signed up for so many catalogs voluntarily.

I know some of the researchers have played around with adding oily fuels, sometimes just to get rid of the oily pizza boxes, other times to attempt destructive testing of prototypes.  Oily fuels smoke a lot, but small amounts sometimes burn cleanly when combined with normal wood fuel.  Loose fuels like paper or oily paper towels can cause smokeback when they flare up, momentarily clogging the burn tunnel while burning quickly in both directions. 

Destructive testing using oil and hardwood fuels to super-heat the systems revealed that the barrel can get cherry red or even mostly white-hot without failing.  Over-firing is much more likely to crack the bricks in the burn tunnel, especially the first brick in the bridge.

If there is firewood at all, then these heaters will require less of it than you would expect from any other type of stove.

If your region does not have a lot of naturally occuring wood fuels, it might be better to design a heating system around the available fuels in the first place.

Rocket researchers are definitely interested in the notion of a fuel cage or hopper-fed system for pellets, corncobs, pine cones, etc.  As far as I know, nobody's gone and built one yet.   It's hard for me to imagine a hopper-fed system where the hopper wouldn't also serve as a potential chimney for a deadly backflash.  An iron grate or cage such as you suggest, might be a better option.  It's all about getting the right airflow to fuel ratios, if you want to use the same system for alternative fuels.

Bends in the exhaust are not required and do in fact slow down the exhaust.  You don't want any obstructions you can avoid, which is one reason the smooth pipes work better than rough brick or stone for the heat-exchanger.

In order to fit 20-30 feet of exhaust into a typical living-room bench, bends are most people's best option.  The other is trying to split the exhaust, and have it run evenly through several smaller, straight pipes. 
I've heard of that being done, a 12" system splitting into aboiut 8 4" pipes.  Apparently it worked pretty well, although flow dynamics would predict uneven heating.  There were unrelated problems that kind of made the whole question of uneven heating a moot point.  (Don't use beeswax to finish a heated earthen floor.)

Like Paul said, doing these things injudiciously could cause
- pain,
- injury,
- death, or
- embarrassment.

Any advice I give, should be taken with the same caveat: 
proceed at your own risk and pleasure.

If you decide to test any fire-powered ideas indoors for some reason, a carbon monoxide detector would be a very good investment.