To vent or not to vent ... #2
Well, I may not have found (yet) any picture of the Soyuz Launch Vehicle venting, but over at the Energia web site, I found an interesting video (RealPlayer needed) which shows the rocket venting away before liftoff ... in a very Saturn 5 fashion :) In fact this isn't really a surprise as the propellant used in the Soyuz launcher is a mix of LOX and Kerozene, quiet similar to the US moon rocket. In order to stay in liquid form the LOX must be keept (super) cold (−219 °C) and most of the rockets do not contains any special apparatus (cryogenic systems) to maintain this kind of temperature (a side from some thermal protection on the tank). Once the LOX reach −183 °C it will boil-off and be vented out. The amount of LOX that will be lost to boil-off between the time the tank are fully filled and the actual liftoff is likely to be taken in consideration during the design of a rocket. For example, the second stage (Block B) of the N1 rocket was supposed (according to Astronautix.com) to lose 1000kg of propellant from liftoff to ignition. Knowing that the burn time of the first stage (Block A) was of about 125s, the boil-off rate is of 8kg per second. My guts feeling is telling me that the rate is probably a function of a complex equation which depends on the amount of fuel and the ambiant temperature, but for a space simulator addon a static rate such as the one we calculated above is good enough I will think.