spacr wrote:
Lox is clearly the way to go in terms of cost and performance.
However, until someone invests HEAVILY in Lox Valve R&D, it will
never be reliable.

That's caused by humidity. It is absolutely critical that the valve and
any gas/air in the valve be absolutely dry before passing LOX through it.

NASA don't seem to have many problems. NASA run, if I remember
correctly, nitrogen gas through their systems
for several hours before launch to dry the pipes out as part of the
chilldown procedure.

Frozen Lox valves are one of the most common causes of vehicle flight
failures in the entire history of rocketry.

That's more to do with the learning curve than anything else. It's a
subtle problem, even tiny amounts of moisture can produce rock hard ice
crystals- which are terrible for valve seatings. Additionally, moisture
can freeze out from the atmosphere and physically stop any external
mechanism from operating; again, the same technique of minimising the
moisture or controlling the temperature on the outside of the valve pays
dividends.

Jay Troetschel