rocket forge: steel insertion, plus a crucible (aluminum) a design to last 20 years

Something that can be moved around with a hand truck.

This is a placeholder so design discussion can begin.

Here is a thing that ernie and erica made in 2012



https://permies.com/t/rocket-canner-fryer-forge

Pretty much a j-tube "engine".   Ernie would drop a stick of steel down in it and it would quickly get to yellow hot and then he would bang on it and twist it.  

You can see the deets in the movie (movie #4 in the first 4-movie set).

That creation also served as a rocket canner/stove and other stuff.  It had a skirt to put a pot on/in.  And it was something built to last a year or two.  

I guess I would like to figure out a design that might last 20 years.  And facilitate a blacksmith a bit more.  Something for forging steel/iron and something that could also take a crucible and melt aluminum.

I have now ordered this crucible https://amzn.to/3l9MWrn



.....

Starting with an idea for a crappy design ...   8 inch j-tube "engine" but with a hole at "the sweet spot" on either side so metal can be inserted.  The hole needs to be able to be capped.  So maybe a small door with a hinge?  And maybe one of the little doors is actually in an even bigger door - and the bigger door is where you might put the crucible.  

Only how is the crucible held in place?

I am not an accomplished blacksmith but I've built two forges and wailed away at metal a little bit.

From a blacksmithing perspective I think you want a door that is at least 3" by 3".  8" wide by 4" high would be even better.  Having a way to open the door, stick the metal bar in and close the door mostly shut is needed.  In my experience it's uncommon to put a piece of metal in the forge and close the door behind it.

Normally your metal sits on a fire brick in the forge (if propane) or in the coals in a coal forge.  So unless the "hot spot" is in the horizontal part of the J, the metal you're heating will be suspended.  This will require extra support outside the forge.

Could the crucible sit on the floor of the horizontal part of the J?

The best designs I have seen are knife doors with foot operated openers.  You already need three hands to blacksmith.

A way to use a blower to add extra air when needed would be handy.  While still being able to keep a low oxygen environment as needed.

Holding a crucible is three or four "bumps" in the walls of the tube that the crucible nestles in.  Trick is to leave enough volume around it to not choke the rocketry.

I've spent some time thinking about this today as well.  While I am a professional metalsmith doing a lot of "forging" work hammering sheet metal into bowl forms I have only a very tiny bit of experience with traditional blacksmithing.  I work in non-ferrous metals and can anneal my metal with a torch.  I don't use a forge.  I say this so no one will put too much weight behind my suggestions.  They are really just speculations based on extremely limited knowledge.

As Mike noted I think you do want the opening to potentially be of a decent size.  Sure it could be small to stick a rod in, but unless you are just going to swage down the rod, keeping it straight you will need a wider opening to handle eventual bends and curves in the piece you are forming.  Basically think of the difference between what a bar of steel might need and what a horse shoe made from that bar would need, as just a simple example.  I wonder if this opening could be formed by a space just slightly shorter than a standard firebrick on each side.  Say 8" instead of 9".  That way you could cap across the top of it in the next row of the build with a simple firebrick.  It could perhaps be closed off simply if there was a wide enough ledge to allow a firebrick or two on the outside to be slide in front of the opening from one side or both.  R. Scott's idea of knife doors sounds way better, but also likely more complicated to make for an initial test model.

To set a crucible in things get trickier.  My basic understanding of rocket stuff is that you need to keep the cross sectional dimension from being reduced.  Ideally you want it the same (right?).  So a chamber for the crucible would need to have it's volume enlarged to allow for a free flowing cross sectional area equal to the rest of the run when the crucible is in place.  I feel like a container of (hopefully) molten metal should probably also be sitting on a secure surface.  When I've done bronze or aluminum pours in the past it always sat down into a furnace with the flames swirling around.  Granted this was a larger scale operation using a different style of tongs to lift it up and out.  An idea I thought of was to essentially make a side jog in the heat riser such that a ledge is formed where the crucible would sit.  I think their would need to be a weird opening shifting the flames/heat from the initial heat riser section over to the second functional one where the crucible is.  I'd want to see the transition be narrower but wider to keep the same cross sectional size but able to focus the heat and gasses to the bottom of the crucible first to hopefully more evenly heat it.  (Maybe this would be completely unnecessary)

I also thought about the blower idea R Scott mentioned.  I believe this is generally a way to increase the heat when needed, but then allow it to reduce when not needed, which also reduced the fuel consumption.  How to do this with a rocket furnace?  Could one get something that blows more air into the burn chamber?  Instead, would making the heat riser chimney section taller increase the draw, thus increasing the air flow across the burning wood?  If this second idea would work then how about a top heat riser section on an arm that could swing away to add or remove it.  At the same time when it was swung away it could perhaps make an opening from which to insert or pull out the crucible.

I don't know if I'm making sense with verbal descriptions.  I'll add a really crappy drawing to try and convey a bit of this.  In my drawing the "hatch openings" are the slightly shorter than firebrick areas opposite each other so long rods could be inserted.  The swing away chimney reduces back to the original cross sectional area.

David, you potentially solved two problems at once! Bigger sweet spot for larger pieces and the crucible cross section.  Add a dummy/empty crucible if you need to keep that cross section the same.

Funny, I never even thought about a dummy crucible to maintain the cross section.  That seems like a good idea and could probably be made with a firebrick or two if one didn't want to purchase an extra crucible just to fill the space.

How about a 10" batchbox with viewing window and a hand crank blower.
We can melt some metal with one of those!

I wanna try this without batch box on this try.  After all, the thing that E&E made was a j-tube and it got plenty hot.

expanding on david's stuff a little ...

adding some doors and a shelf.  kinda thinking that the doors need to go in the left-right way instead of the front-back way in order to dodge the wood feed.

If that heat zone is 8+ inches across, you may be able to do away with the second small door to reduce complexity. If you ever need to make a sword, then you'd want to add that door but for normal work I don't think it would be used hardly at all.

You may not need to do a sword, but I think there would be quite a few times you need to get to the middle of a long bar to do stuff.  Hinge point for tongs, bends in levers, etc.  Often that second door is only a loose brick because it is either in or out for that burn.

Agree that the feed needs to not be under the door, but maybe only 45 degrees off, it really depends on what would be an efficient workspace on site.

I agree that having the second small door to allow for longer lengths of rod seems worth the effort, even if it is only a firebrick plug.  

I also agree that shifting the sides the doors are on so that a rod inserted through them would run perpendicular to the line of the J tube is the way to go.  No need to create obstructions to feeding the fire.

David Huang wrote:I agree that having the second small door to allow for longer lengths of rod seems worth the effort, even if it is only a firebrick plug.  

I also agree that shifting the sides the doors are on so that a rod inserted through them would run perpendicular to the line of the J tube is the way to go.  No need to create obstructions to feeding the fire.

I second this idea. In addition to a pass-through for long work, it also allows access to the forge from both sides, so that two people could each work on small projects without getting in each other's way.  The forge and a vise could be between two anvils and two could work at once.

I've seen quite a few forges where the "doors" are just two (or two halves) firebricks that sit on a wide shelf, which is also deep enough to be a support for small workpieces. The smith just parts them enough to fit the work inside (thus retaining more heat, and not letting so much heat further down the workpiece).

Sticking to 90* faces is simpler dealing with ceramic boards and bricks.

For the crucible, I've seen/used a gas furnace that the crucible sat on a plinth to raise it up off the bottom, into the hotter part of the furnace. Maybe something similar?  A pillar of soft firebrick, maybe round not bigger than base of crucible, for it to rest on?

I like the jog in the riser/chimney. I imagine it creating a hot spot where the heat "dwells" a bit, before exiting. and that would be a good spot to get work hot.
"Sweet spot"? could be highest heat? could be choice of reducing/neutral/oxidizing atmosphere...? (unburnt fuel/full combustion/excess air)
The idea that I had for a rocket forge is a marriage of rocket-forge and charcoal retort. Forging would be combined with charcoal making (which could later be used in a conventional forge).

More thoughts having actually read the whole thread so far...

David's idea of the swing away riser/chimney is interesting. Most foundry furnaces that I've seen load from the top, and have a swing away lid with a center hole for the exhaust. After the initial charge, additions to the melt are often through this hole, but if a larger "ingot" is to be added, the lid can be swung aside momentarily. (we're not thinking large enough melts to necessitate a crane, but that's another reason...)

Paul's sketch shows the crucible over a gap between two bricks, but it may be better if it sat on just one brick in the center (just wide enough for the crucible) and possibly corbelled out into the flame path a bit.

The crucible within the widened section, reminds me of the riser inside of the barrel of a RMH, rules for CSA between riser and barrel likely apply to CSA between crucible and this furnace section? It might not matter if the crucible is absent (forge mode) since this might just perform like a bell (not a restriction)?
My idea for the rocket forge/retort would would have a bell-like chamber for the forge/furnace (borrowing a bit from wood fired ovens) before continuing up to the retort (borrowing flue path idea from the rocket oven, just cooking charcoal instead of pizzas).

Wood gas produced would be fed into the J-tube to be burnt. Maybe this would require a blower to provide adequate air?

Another option for "more air" which I think also means we're burning "more fuel" would to take a two-sided forge one more step and make a second feed tube opposed to the first, sharing the same riser. Maybe it needs a bigger riser? more draw?
Maybe this second J-tube is offset to the side? now the riser section is combined (space for crucible/forge) then into one chimney? This might create a swirling effect as the two burns enter the riser at opposite corners. (CW or CCW? hmm...)

I’ve been thinking about this for years. Glad your going to try, I’ve not had the time yet. The best design I’ve come up with was to build a large chimney and have three batch burners feed it. The rocket pizza oven I built in Amsterdam has two batch burners feeding it. And we noticed the huge flames last about a half hour. So we started alternating the feeding of the oven to always have one blasting in flame during the pizza night. You could cook for the next four days in the thing.
So I thought with three rotating the feeding you could have a constant swirl of flame around the crucible on a plinth. And shutter doors and maybe a ten feet high chimney/riser. I also thought of using Rob’s pellet feeder to the three chambers, which are all set at a tangent, by the way, to promote a cyclone style burn. Anyway, that’s what I’ve been dreaming up. The three chambers for power and for space to work in between. I guess the trick would be to find the sweet spot that is hottest. Our high temp firebrick warped about two feet above the bottom of the port in the risers. So I guess 2’ is a good starting point.
Good luck! And thank you for working on a design for this.

This is from Rodney. He would like to use a venturi effect from a shopvac to boost the airflow. I would like to see instead a large insulated chimney pulling on this thing.

The shop vac goes.

I think that mounting the door on a hinge might be a bit of an ask at this point.  So my guess is that the door will be blanket wrapped around that orange firebrick stuff.

I also think that the opening for the crucible now needs to be at the side.

I' not so familiar with the current state of J tube rockets.
How tall would the heat riser need to be, and is the hottest spot at the top of the riser or lower down?
Depending on the answer, sinking the j into the earth or elevating the work plate form might be necessary.
Neither accommodation is good for portability.

My own abortive experiment in this field is an old propane BBQ lined with refractory.
I planned on using a single  4 " J tube, but that was before I knew how finicky they can be.
I was also considering adding up to 3 more in a row, side by side, to heat longer pieces.

As noted above, a rockets flame could turn a bed of wood into char very quickly.
A bed of glowing char is probably better than a firebrick for heat transfer.
The intriguing idea is the rocket acting a draft inducer.
A second rocket exhausting into the chimney after the smelting/ forgoing chamber could be also be used to induce draft, but that seems messy.


I've noted  of the reluctance to use induced draft wood burners here at Permies, and in this case especially, it makes extra good sense.
Wood fired smelters and charcoal forges that use fans are known technology.
Using a rocket stove with a fan, well the heat riser would be new, and it would add efficiency and a good bit of emissions control.
Still, rocket stoves are natural draft stoves, its an important part of the resilience they contribute.


A note on the crucible.
One of the smelters I've seen tosses their aluminum directly into the flames and collect it when runs out of the burn chamber.
This seems very effective.

William Bronson wrote:
A note on the crucible.
One of the smelters I've seen tosses their aluminum directly into the flames and collect it when runs out of the burn chamber.
This seems very effective.

I don't know if this tidbit is of any use here or not, but many years ago I was invited to visit and watch an iron pour.  The approach here was similar to what you are describing William.  The iron was in the burn chamber and they would keep tossing bags of coke into what I guess would be the chimney as a way to add the fuel.  Once it was all ready they'd break a plug at the bottom of the apparatus and the molten iron would come pouring out into the crucible to then be brought to and poured into the molds.  Meanwhile someone would jam a stick with a soft clay plug on the end into the hole to stop the molten iron from continuing to drain out until they were ready to break the plug again for another round.  The plug didn't always hold the first time so extras were ready on hand!  It was dramatic to watch, but seemed rather wasteful of iron to me.  On the other hand it was a pretty low tech operation.

Chris McClellan wrote:This is from Rodney. He would like to use a venturi effect from a shopvac to boost the airflow.

The nozzle for the shopvac can't be plastic downstream of the burn tunnel.  I think putting the nozzle upstream of the burn tunnel and having a door for the feed tube would be better from a pressure standpoint.

Wikipedia

Wikipedia wrote:increasing the pressure ratio increases the compressor discharge temperature

pressure ratio = burn tunnel pressure/atmospheric pressure
compressor = shopvac

Edit: Note the position of the compressor. I don't think the engineers got it wrong.

Well, I dunno if this rocket forge design will last 20 years, but it certainly works! See for yourselves.