A friend went through the equation for an open vented line from the pressure casing and found that the losses were much less than expected. To prove it, he runs a KJ-66 with the propane circuit open through a small tube (ie not even plugged with a check valve like most) and the engine still performs as expected after disconnecting the supply. Its been running that way for over a year. To partially refill a retract system, the flow would be nearly static, so the loss in performance would likely be that much more negligible. For that same reason, its really not all that important to have a perfectly air tight seal on the NGV clamp rings or the glow plug...

You're right about the heat problem though, if you assume the leak causes a significant flow rate of air. When testing a JG-100 in 2001, I hooked up a case pressure line to an off-the shelf pneumatic gage and ran the motor. The engine worked and the gage registered the pressure level expected. The pressure line we was just a simple yellow Festo hose plumed to a right angle 4mm bulkhead fitting on the diffuser cover, just like the propane and fuel lines used on other motors. I then made the mistake of switching the gage while the motor was running for a more accurate one I had on hand. Funny though, after I switched it neither gage worked... Ignorantly, I ran it for another few minutes and the performance still looked within the spec.

I shut the motor down and pulled off the fairing to see why the gage stopped working... In that few seconds of free venting through the 4mm hose, enough hot air passed through the fitting to melt the plastic right-angle portion and the section of hose immediately following it... In a lower flow rate, I'd assume we could plum the system with a length or two of aluminum or brass tubes to act as heat exchangers to the ambient air in the fuselage. Its added weight, but I don't think it would be too difficult to make it work, just don't expect to get 120psi out of it... If a leak did happen, the plane would probably still limp home. I think the AMT Netherlands engines use a system like this to propel smoke oil.

Kelly