How much N2O is used...

[als57]

Originally posted by edwardw
The reason that when you chill a flight tank to fill it that it doesn't burst is that you are filling it by MASS not by VOLUME. That way you get the same amount in each time. 1 lb 100F nitrous is the same as 1 lb 40F nitrous.

Edward

Now if you said weight I'd agree. Mass does vary with temp ; liquified gases are rather a good example.

That's why most C02 containers have a weight stamped on them. If the filled weight exceeds the stamped weight its overfilled.

BTW I not real happy with the idea of relying on a burst disc. A personal example. My 4500 PSI CF air tank was mistakely supplied with a burst disc for a 3000 PSI aluminum tank. After 4 years of use the 3000 PSI disc is still good. So 4 years at 50% overload makes me wonder.

Al

 

[Art Upton]

Originally posted by kgholloway
OK, I've read the thread and understand the problems that low/high N2O temperatures can cause but here is a question I've never seen addressed.

When filling and launching a Hybrid you sometimes run into problems with ignition systems. This usually results in a "dump" of the N2O in the loaded motor. The dump should result in the motor cooling due to expansion of the liquid N2O.

What would be a minimum time delay after a dump for 38mm and 54mm Hybrids to allow them to return to ambient temperature? If the flight tank is immediately refilled after the dump it would seem to me that you could get into a situation where the N2O pressure is low at launch resulting in lower thrust and possible flashback.

With standard APCP/BP motors the HPR Safety Rules specify 60 seconds must elapse prior to approaching the rocket to re-install an igniter. But I've never heard of a rule of thumb as to how long after a N2O dump that the motor should "sit" before refilling. I've seen several spectacular CATO's of Hybrids and, as I recall, they all happened after a dump and refill.

Ken Holloway
NAR #78336, L-II

Hi Ken,

I have seen at least more then 15 dumps, then rapid fills on both RATT and Skyripper motors with no CATO.

In fact they got improved thrust as the N20 was more dense.

A frosted grain, not a dump/fill is what can give a hypertek problems.

I can not comment on any other manufactures motors that might have experianced a CATO from a rapid dump/fill cycle.

But it does not cause problems at our launch sites on the motors above.

 

[OARJeepr]

Originally posted by als57
Now if you said weight I'd agree. Mass does vary with temp ; liquified gases are rather a good example.

That's why most C02 containers have a weight stamped on them. If the filled weight exceeds the stamped weight its overfilled.

BTW I not real happy with the idea of relying on a burst disc. A personal example. My 4500 PSI CF air tank was mistakely supplied with a burst disc for a 3000 PSI aluminum tank. After 4 years of use the 3000 PSI disc is still good. So 4 years at 50% overload makes me wonder.

Al

Sorry but that's just wrong. Mass does not vary with temperature. Weight, the force exerted by earth's gravity on an object, is directly proportional to the mass of an object. If the mass changes then the weight has to. If the mass doesn't change then neither can the weight (as long as we're close to the earth). If I have a kilogram of nitrous in a tanker truck or in paintball cylinder it's always a kilogram. Doesn't matter if it's -40F or 100F it's still a kilogram of nitrous.

If a tank can handle one pound of nitrous at ambient (say 70F) then filling it 32F is not going to cause an overpressurization when it warms back up to ambient, because you didn't overfill it.

Now density does change with temp. So if you took that same tank, chilled it, filled it until you couldn't stuff another drop of nitrous in there, you'll probably have a problem. It'll warm up, want to expand, the pressure will rise, and the tank may go. The difference here is that you didn't fill by weight and ended up with more than a pound of nitrous in the tank.

 

[als57]

Bill:

Your right I was thinking density not mass.

Your last point is what I was trying to get across.

BTW do the nitrous tanks (used in hybrids) have a weight stamped on them so you can tell if they are overfilled?


Al

 

[Gambit7]

These temp issues are all volumetric, not weight related. a liquified gas that exists at a temp. higher then its boiling point is merely kept liquified by pressure.

If you heat up ANY gas (which invarably there's always gas in an N2O cylinder) its volume increases and explodes.

The whole point of keepin cylinders cool is for more dense gas/liquid (more power), and less gas-off when that pressure is released.

Of course, if you fill your tank by weight and pay attention.. there shouldnt be an overfill assuming the tank can handle the pressure. That said weight of liquified gas will take up "x" amount of space at "x" temp. Regardless, once that temp goes up that liquid will expand and/or boil off to an equilibrium pressure inside the tank.

Assuming there's air inside your tank (not a vacuum), once this cooled/dense liquid increases its temp of course all that stuff is goin to expand. BUT, since it's taken up less space to begin with it shouldnt be a problem (because they weighed it).

Anyways, I love this physics talk... this is why I'm into hybrids

I can actually learn and apply it to something enjoyable.

 

[Gambit7]

Originally posted by als57
Bill:

Your right I was thinking density not mass.

Your last point is what I was trying to get across.

BTW do the nitrous tanks (used in hybrids) have a weight stamped on them so you can tell if they are overfilled?


Al

yes, 10, 15, 20lb... etc.

 

[jderimig]

Originally posted by OARJeepr
Sorry but that's just wrong. Mass does not vary with temperature.

Now density does change with temp. So if you took that same tank, chilled it, filled it until you couldn't stuff another drop of nitrous in there, you'll probably have a problem. It'll warm up, want to expand, the pressure will rise, and the tank may go. The difference here is that you didn't fill by weight and ended up with more than a pound of nitrous in the tank.

But mass changes with speed though...

 

[Gambit7]

Originally posted by jderimig
But mass changes with speed though...

can we really say mass changes? or does is simply exist in many forms? i.e. energy, etc. Either way, they all take up differing amounts of volume.

 

[cjl]

Mass does change with speed. Not much though, unless you go absurdly fast

 

[OARJeepr]

Originally posted by jderimig
But mass changes with speed though...

Sure it does but for the speeds we fly at the change is neglible. I'd rather spend my time calculating something useful.

 

[edwardw]

Originally posted by als57
Now if you said weight I'd agree. Mass does vary with temp ; liquified gases are rather a good example.

That's why most C02 containers have a weight stamped on them. If the filled weight exceeds the stamped weight its overfilled.

BTW I not real happy with the idea of relying on a burst disc. A personal example. My 4500 PSI CF air tank was mistakely supplied with a burst disc for a 3000 PSI aluminum tank. After 4 years of use the 3000 PSI disc is still good. So 4 years at 50% overload makes me wonder.

Al

You really aren't 'relying' on the burst disc. You are not operating the tank in the range of the burst disc - you are below it. The burst disc is also a proven product and is used industry wide. A high pressure air tank is different than a CO2 tank also. Those tanks are just filled with compressed air which can expand, but not nearly at the rate that CO2 will. CO2 is liquid which changes to gas. Air is just compressed air. I'm wondering if the reason the disc holds is that the pressure increase is 'gradual' as it is pumped up and never really had a delta spike to burst it.

Edward

 

[als57]

Originally posted by Gambit7
yes, 10, 15, 20lb... etc.

Actually I was wondering about the tank used on the motor itself. For example the C02 cylinders used in paintball have a weight stamped on the tank that includes the weight of the cylinder. For example a 3.5oz tank might be stamped 12oz. If the weight exceeds the 12oz ; you have an overfilled condition.

I'll have to look at my 20lb tank to see if its actually labeled. Last time it was filled it took 19.8 lbs.


Al

 

[als57]

Originally posted by edwardw
I'm wondering if the reason the disc holds is that the pressure increase is 'gradual' as it is pumped up and never really had a delta spike to burst it.

Edward

Good point. Most of the time it has been filled by a Bauer compressor. So that would qualify as a gradual fill. But it has been to the dive shop a few times and that would be a faily quick peak to about 4350 (all the shop can supply).

Al