Would a heat pipe behind the LED a good idea?
Apparently, yes.
I ordered one today, hope it will arrive soon so i can check it out.
…you mean with a pump and a radiator? 
heat pipes by themselves can not handle anything, they only move heat, not dissipate it.
there is always a heatsink and fan on the other side of heatpipes, also heatpipes work well only when hot end is lower than cold end, with something stationary like a computer, you can have radiator\heatsource mounted with height difference, but a flashlight that is not stationary it is not the best solution at all.
I think a heatpipe is probably a good solution for getting heat away from the LED. Even if it is not oriented optimally, it’s probably a lot better than a solid piece of Cu of the same size. I could be wrong about that, but heatpipes in laptops, for example, are not oriented vertically.
I think everyone is aware that the heatpipe is just a way to transfer heat and not dissipate it. Of course it would be connected to a heatsink.
Heatpipe is the best solution to cool a led above 15W
I’m going to make my test light without active cooling: the XP-G2 will be producing about 14W of heat, let’s see if I can get that guided away fast enough, I think I can 
To make this a bit more quantitative:
The thermal resistance of a piece of material with cross sectional area A, length x, and thermal conductivity k, is
R=x/(A*k)
So for a copper bar 0.5cm x 1cm and 7cm long, with k=385 W/m*K, the thermal resistance is 3.6 K/W. This is quite high when compared to other flashlight shapes/designs. For example, I did some thermal testing of a couple flashlights and found their thermal resistances (from MCPCB shelf to outside of flashlight head) were almost 10x less. Whether it’s too high depends on the power of the LED and the size of the heatsink and how hot we allow the heatsink to get.
It’s supposed to transfer the heat from the LED to the head (and body) of the light in this case.
I didn’t know that it uses height differences.
If so, the drawing in opening post is upside down.
I think i should have ordered more than one heat pipe maybe…
(edit) Just ordered a 2nd one.
Dang, i wish i had a mirror i could use…
…but when you say the XP-G2 will produce 14 Watts of heat, you’re apparently gonna push some 7 Amperes through it…??
I think the height difference is not necessary, it just helps. If the hot side is lower, the liquid that recondenses on the cold side then doesn’t have to diffuse/wick back to the hot side against gravity. Just a guess.
That's really high. Could you post a picture or link of your setup plz? :)
OP is not going to do that for costs reasons and convenience, but when I do in the future, I'll definitely put a build log on the forum ;)
The actual light efficiency of an LED drops when you give it more current, so even if the LED is 150lm/W at stock it might only be 100lm/W or less when running near max.
https://en.wikipedia.org/wiki/Luminous_efficacy
So you get about 15-20% efficiency.
I think people overestimate how efficient LEDs are, there is still a ton of energy being turned into heat.
I think people don’t realise how totally inefficient incandescent lights are.
That too.
10x better efficiency than 2% is still only 20% :P
I thought LEDs did better than 20% though, Djozz’s XP-G2 will probably survive then.
The led will be reflowed on the middle of a copper bar, 3mm thick, on average 15mm wide, and 90mm long, so the heat can go in two directions 45mm far. After that it is spread out over an aluminium plate. I’m ok with the led running at 100degC I guess….
Sounds like it is made already but I have done nothing yet 
But 95mm isn't even close to the diameter of the reflector?
A 95mm heatpipe would be good from the center to the edge, but not the full diameter, if you're talking about the 6" one.
BTW thanks for the info on these cheap mirascope reflectors :) checking some out right now.
I’m not going to use the full 6 inch, but a 9mm section (~3.5 inch). I have a feeling that that is a sort of optimal size/throw combination for this reflector.
So for the 9inch version the optimal size would be 135mm, and that may even get close to the 1 Mcd (well, 900 kcd). But as said, can’t get that one for cheap, let’s first try this one.
I hope you’ll try if more will work too, because the idea is to collect / catch as much light as possible / sensible / worthwhile.
Collecting that last bit of light (the less intense light) will not add that much extra light (lumens), but the extra area of the reflector (going from 90 to 120mm) will add a lot more throw, assuming the reflector is shaped well enough to be focused. The light reflected from the outer portions of the reflector would make a very small hotspot.