The beauty of this site is whatever the problem someone knows how to solve it.

From his earlier pictures, I think "retirement-hobby" makes static models rather than functioning power models...

For a static model I'm not sure why solder is being considered - though it is a good strong way to join brass. Superglue and plastic pipe might be adequate....

If your using an electric soldering iron avoid " active" fluxes if your willing to use a flame then good old fashioned " killed spirits" is as fine a flux as your going to find for soft soldering.
Thinking about things if your using an IC engine and your making a prop shroud then please consider hard ( silver ) solder. Just think about the effect of a failed joint and a rapidly spinning prop. if your joining tubes at app 90 degrees think about drilling the tube your going to join to and fitting a spigot to solder the joining tube to. Making a good mechanical joint is an almost essential requirement for a sound long lasting soldered joint.

Sound advice from Haverlock and Dodgy geezer!
I have found La-co flux (from plumbers merchants) a fabulous "active " flux. La-co spray-on heat mat ( a gel) prevents heat travel away from heated area.
An unsupported butt joint is inherently weak. I would suggest some sort of supporting insert into the tube to maintain the flush appearance,but with added strength.Eg: a smaller close fitting tube of smaller diameter/section.

As mentioned before, I'm a great fan of plumbers acid flux. Yes, you have to take care where you put it, but it goes into small cracks and just dissolves the oxide layer, making soldering a doddle.

For pipework you will need adequate heat. it is hard to get heat out of a contact soldering iron onto a rounded surface. Gas flames are MUCH better at heating pipes.

The small gas torches come with a small pencil flame which is easy to control and direct. Things like this, for instance: https://www.amazon.co.uk/Butane-Micro-Soldering-Welding-Blowlamp/dp/B004U8UALW

Remember that metal conducts heat. if you solder at one end of a tube while the other is in contact with, say, polystyrene foam, then the foam will melt...

First rule of soldering is clean it all then clean it again ohh and then clean. Seriously use an abrasive to make sure joint areas are clear of oxides. Presuming your using soft solder get some rosin flux https://www.ebay.co.uk/itm/Soft-Solder-Paste-Soldering-Flux-Grease-Soldering-Accessories-D01850/111141455814?hash=item19e08c07c6:g:-cYAAOSwnFZXUiS8:rk:13:pf:0 one example. Try and make your joints really good fits. Solder will wick into a close fitting joint. if possible " Tin" the individual parts first that is getting a layer of solder onto the area to be soldered before final assembly. Then try to fix your parts together in some kind of clamp they need to be held still until the solder cools. NEVER cool the joint quickly! Allow it to cool naturally that way the joint will be at its strongest. Make sure your heat source is strong enough the solder should melt and run freely. if your soldering brass tube honestly I would use a small butane torch but also I would if possible use hard ( silver) solder since if you chose the right one it is a match to brass and makes the joint look like one piece. Hope this is of some help.