Oxidizer Tanking Procedure

[josborne]

I wanted to ask what others opinions are on their flight tank filling procedure. Specifically, I'm trying to debate between a vent valve and no vent valve on the top of the tank. Basically my procedure will be as follows:

- Fill and vent flight tank to lower temperature
- Fill with N2O until liquid phase streams out of vent valve
- Plug vent valve
- Wait until tank increases in temperature/pressure until with desirable range
- Fire

My main concern with not having a vent valve would of course be not knowing how full the tank is, and my concern with having a vent valve would be a) difficulty plugging it and b) if we don't plug it then the tank won't build up enough pressure to fire properly. What are the thoughts of others on this?

[Yiqing Wang]

So your main concerns are:
1. Measuring Oxidizer mass
2. Reliable method of plugging the vent

To measure oxidizer mass, my idea would be to build a jig that involves a measuring device such as a strain gauge that can be mounted to the launch rail/stand. The best solution is definitely a custom launch stand but that involves quite a bit of work.

To plug the vent, I would start with a default to close type solenoid valve. Automotive racing may have something for it. And since the nitrous involved here is gaseous, automotive grade solenoid valve would suffice. Though weight is always a concern.
Another way is of course just leave the vent hole open which according to our experience at IREC 2013 should still give you around 700PSI.
If you really want high pressure I would suggest looking into using nitrogen, CO2 pressurant which I plan on research. The down side is of course mass penalty. But it does have the benefit of removing the pressure variation due to temperature and when coupled with a pressure regulator can provide a constant pressure.

[josborne]

I like the idea of a strain gauge to measure mass. I had a thought also about using the flight tank temperature to infer the mass of the flight tank. If you have vent valve that allows for up to (let's say) 80% full of liquid N2O, and then you measure the temperature of the oxidizer tank right after you filled it, based on the density of N2O at that temperature you could calculate what the mass of N2O you have. Of course, this is sort of a moot point because if you know it's 80% full you know what volume you have, and in that case all you really want is the temperature of the N2O anyways. I guess what I'm thinking would be a strain gauge and thermocouple in the static test assembly and you could compare the two of them to see how accurate these two different ways of measuring N2O mass are. Then on flight you can just carry up a thermocouple, which is a lot lighter and simpler than a strain gauge. Last year we had a wireless communication system and we had live data on a few parameters of our rocket, so it would be relatively simple to have a system return flight tank temperature readings from the thermocouple. 

Do you think a solenoid would be strong enough for the vent valve? I was more just thinking of a ball valve that you close manually when you are full. Assuming you try to avoid getting a face full of freezing cold nitrous. My main concern with leaving it open isn't actually the loss of pressure, but the decrease in tank temperature that results from the constant venting and the potential for freezing the actuation valve. My concern is that if you vent/purge your tank to chill the nitrous, your tank is going to be cold enough to freeze your actuation valve leading to the engine. If you leave your vent valve open then as the nitrous streams out it will slowly chill your flight tank. My hunch is that the rate that the vent hole chills the tank is greater than the rate at which the flight tank heats up due to ambient temperatures. I remember at IREC2013 when you guys tried to launch the second time the valve froze, and it wasn't until the flight tank was nearly empty that the valve unfroze and opened up. Another thought on that issue, is that if your flight tank increases in temperature (due to ambient), there is a corresponding rise in pressure, and as the pressure rises, there is a corresponding increase in the rate at which oxidizer exits through the vent valve, and thus there is a greater decrease in temperature of the flight tank. Where equilibrium lies I have absolutely no idea, but not plugging the vent valve has it's concerns for me.  

[Yiqing Wang]

I like the idea of using volume and temperature to derive mass as it does simplify the instrumentation. Though there may be an analog way to replace the strain gauge, something similar to a spring.

Solenoid valve should be ok for handling gaseous nitrous, we have some automotive ones that worked fine until we tried to put liquid nitrous through them.

Regarding keeping vent open or close them. You're right that a continuously vented tank will probably freeze your valve leading to combustion chamber. The vent hole on our 2013 rocket was drilled with No.64 drill bit which is the smallest we could get. And it still vented fast enough to freeze our valve. So a closed vent hole is probably better if you're using a valve between tank and chamber.

Then the problem about nitrous heating up in a closed tank leading to pressure increase. I don't have any good solution to this for you. A pressure relief valve can be set to vent at a certain pressure but that means loss of mass which is not ideal. But to keep the mass constant means higher pressure which leads to problem with injection rate. Also if the nitrous is not cooled, supercritical nitrous can possibly form depending on exact temperature and pressure.

My current idea to get around that is to use a floating injector/UC valve with a tank that is continuously vented. I'll make another thread about that later so I don't completely derail this one, and to get some feed back on that idea.

[josborne]

I'm not familiar with the floating injector/UC valve, so would love some info. on this.

To get around the risk of overpressurizing your tank, my thought it is have a thermocouple on the tank so you know what temperature it is. Then in your procedures you have a range of tank temperatures for which you are able to fire (this would likely be around 15-20C). Once it heats up into that range (assuming a vent/purge procedure) your window to fire opens up and you have to fire. If you exceed 20C you need to vent your tank and refill it. The way to have it so that you don't risk bursting the tank is to have a burst disk relief valve that opens at around 1000-1200 psi or so. That way you don't risk rupturing your tank. That's sort of what I'm thinking of doing for this year. Correspondingly you could also do this with a pressure transducer, and instead of the 15-20C window you have a window of like 650-750 psi or so. Both would likely work, and it might be work it to use both for some redundancy.