Our plan to successfully throttle Hephaestus hinges on a significant amount of cold-flow testing to obtain a flow curve that we will use to calibrate our throttle valves. However, cold-flow testing at nominal pressures will result in inaccurate data because there is no chamber pressure, which would result in a much larger dP across the injector than there would be during hotfire. To accurately characterize flow response, we need to enforce the same flow through the system during a cold flow test as there will be during a hotfire.
Currently, we are thinking of two ways to do this. The first is to make "mock injectors" that have smaller orifices to account for the increase in dP. So, we would run fluid through the system at nominal pressures, but with a smaller orifice area. The orifice area can be sized to offset the greater dP, such that a smaller total injector area perfectly offsets the larger dP. The other method we are thinking of is to cold-flow test at off-nominal conditions, i.e. with manifold pressures equal to injector dP during hotfire + atmospheric pressure so that the dP across the injector for this cold flow is the same as the hotfire. We prefer the second option much more than the first, as we wouldn't need to make mock injectors with even smaller orifices (since our engine is small, it might not even be possible to make our fuel annulus smaller). But we haven't seen people do it this way, so we need to look deeper into whether there's anything wrong with this method before we decide to do it. I don't see anything wrong with the strategy at the moment though – as long flowrates in a cold flow are equal to the flowrates during a hotfire, I would expect valve calibration for that coldflow to also work similarly for a hotfire.