In the latest (Danish) blog post CS's new theory is that the LOX had
gotten too warm due to peculiarities of their LOX vent system, and as
such they got two phase flow which led to reduced thrust. I still
think they would have been better off with a dip tube in the LOX tank
to ensure consistent LOX loading.
You can't use a flowmeter to load LOX and expect a high fidelity
result because a significant (and variable) portion of the liquid is
boiled off to initially chill the tank and then continues to boil off
while the vehicle sits with the vent open.
Though regarding flowmeters, turbine flowmeters with thermocouples are
what I've seen most often. For highest accuracy you can get a dual
turbine flowmeter; the turbines spin in opposite directions, removing
error from spin the in the fluid, and can give W-dot accuracy well
under 1%. If the engine doesn't throttle and you have little interest
in the startup and shutdown transients, you can instead use a
cavitating venturi flowmeter.
One challenge with LOX is that it has an incredibly low viscosity that
is impossible to calibrate the flowmeter to. But for development
purposes repeatability is often more important than accuracy; you want
to know if your improvements are really improvements.
Ben
On Thu, Jul 28, 2016 at 5:57 PM, Zachary Martinez <znm3m8@xxxxxxx> wrote:
Hello All,
I was thinking about the recent Copenhagen Suborbitals launch and was
wondering what ways can fluid flow be measured accurately. I have heard of
various flow meters used to monitor mixture ratio and overall mass flow but
I am curious about the cost and accuracy of these including turbine flow
meters, magnetic flow meters, or any other ones you have experience with.
Also when measuring how full tanks are what are the best ways to do this?
Obviously using a load cell and just weighing it did not lead to the most
accurate results with Copenhagen. Would it have been better to use a flow
meter when filing or would that have introduced too much more
weight/complexity and had similar error? Using blow down pressure system
would require a fairly accurate amount of propellant but I'm not sure how
much it matters if using a pressure regulator.
Thank you,
Zachary Martinez
Aerospace Engineering | Missouri S&T