Amount of Nitrous for a Contrail M motor

[Dave A]

Anyone know how many pounds it takes for a full fill for flight of the M Contrail?

 

[crossfire]

I am sure info is on the Contrail site.

 

[jjhdnd]

The 75 mm Contrail M's use a 3200 cc nitrous chamber.
Using 1.98 g/cc for nitrous, that comes to just at 14 pounds.

 

[G_T]

Ummmm, your nitrous density is off by quite a bit. Plus the 3200cc figure may not include losses due to ullage.

View attachment 316247

Gerald

 

[AlphaHybrids]

Ummmm, your nitrous density is off by quite a bit. Plus the 3200cc figure may not include losses due to ullage.

View attachment 316247

Gerald

That density graph looks off - doesn't nitrous get more dense as it is colder? That says it is less dense when it is cold.

Edward

 

[eggplant]

The blue line shows higher specific gravity with lower temperature, which what I would expect to see.

 

[TheAviator]

That density graph looks off - doesn't nitrous get more dense as it is colder? That says it is less dense when it is cold.

Edward

Density is the blue line, vapor pressure is the pink line.

 

[jason-and-his-toys]

It takes about 5lbs at about 800Psi NOT including losses. Average loss is a couple pounds if I remember correct. DO NOT try to fill out of a 10Lb tank. By the time it is full, if it does actually vent liquid, the pressure would be very low. Use a 20lb tank for a 3200cc motor.

Jason <<May have flown a few of these over the years!

 

[Dave A]

I was wondering since I flew 1 off my 20# bottle.

 

[Dave A]

Regardless the temperature, a liquid is not easily compressed. The volume of liquid would be the same regardless of temp, correct?
To ensure a good and quick fill, I thought of using one of those 12v racers warming blankets on the bottle when filling the motor, isn't that ok?

 

[dhbarr]

Regardless the temperature, a liquid is not easily compressed. The volume of liquid would be the same regardless of temp, correct?
To ensure a good and quick fill, I thought of using one of those 12v racers warming blankets on the bottle when filling the motor, isn't that ok?

There are many kinds of bottle warmers, some I would feel comfortable with and some not.

I'd suggest discussing your GSE with the RSO of wherever you're planning to fly

 

[AlphaHybrids]

Dave,

That would work if the liquid behaved like water. After ~97 degrees, nitrous is supercritical and is neither a liquid or a gas. Also, as you heat it, more of it becomes a gas and becomes a less dense liquid. When you cool it more of it becomes a liquid and less gas.

Edward

 

[G_T]

Dave,

If you look at the chart I linked to, you'll see that the density of nitrous varies tremendously with temperature. As Edward said, it is not much like water.

Gerald

PS - If you increase the pressure of the supply tank, then yes the flight tank should fill faster. There will be some pressure loss through the fill line because of resistance to the nitrous flow. There will be some fractional pressure drop at the flight tank through the vent line. The net result though will be higher pressure in the flight tank. Higher pressure yields higher equilibrium temperature and lower equilibrium density (per the chart I posted). The motor will have less total mass of nitrous in the flight tank but will run at a higher pressure.

The net result is changes in supply tank pressure end up altering the burn curve of the motor - thrust and duration, and total impulse. That's for a classically vented flight tank. You want to keep the nitrous supply tank pressure within the 650 - 900 psi range (verify via gauge) to stay within reasonable range. Too low and the fill will take a long time and it is possible the flight tank won't have enough pressure for reasonable flow through the nozzle. I could give some numbers but it is likely out of bounds for this forum. Too high a pressure makes nitrous much more dangerous - it IS a monopropellant after all. Higher temperature greatly decreases the energy required to make it go boom. So stay in the 650-900psi range, which is also the range the motor should have been certified for.

EX of course, would be up to you and the RSO.

 

[karlbaum]

A 3200cc tank takes 5.24 pounds of nitrous. If you have a tank with a syphon tube don't count on using the last couple of pounds of nitrous.
Being in California I have never needed to heat my fill tank. In fact if it is not winter I have to cool the tank. I wrap it with a wet towel and place it in a bucket. I also try to launch in the morning. As discussed earlier in the thread heat is very, very bad for hybrids. If you want to increase your fill times you need larger diameter fill hose and solenoid. Wilson manifolds has the largest solenoids that I have found.

 

[karlbaum]

The 75 mm Contrail M's use a 3200 cc nitrous chamber.
Using 1.98 g/cc for nitrous, that comes to just at 14 pounds.

No, it is more like 5.24 pounds or 2.38 kg for the 3200cc motor

 

[AlphaHybrids]

Karl - what temperature and density is that at? It can vary vastly depending on those variables. I have a 75 pound tank and I can never get the last 5-7 pounds out of the tank. I do use the tanks when empty of liquid as a starter to pressurize the system then I transfer over to a tank with liquid so I can conserve some liquid.

Edward

 

[karlbaum]

Karl - what temperature and density is that at? It can vary vastly depending on those variables. I have a 75 pound tank and I can never get the last 5-7 pounds out of the tank. I do use the tanks when empty of liquid as a starter to pressurize the system then I transfer over to a tank with liquid so I can conserve some liquid.

Edward

Are you talking about getting all the liquid out of the tank? If you are using a dip tube, which most of us are, then you can only use about 90 % of it probably because the dip tube can't reach it all. That has just been my experience. So I refill my tanks when they get down to a quarter tank. That's why I would recommend at least a 20 pound tank for M size motors.

 

[karlbaum]

Dave,

That would work if the liquid behaved like water. After ~97 degrees, nitrous is supercritical and is neither a liquid or a gas. Also, as you heat it, more of it becomes a gas and becomes a less dense liquid. When you cool it more of it becomes a liquid and less gas.

Edward

It is very easy for your fill tank or your flight tank/rocket to reach hi temps in the summer sun. One of my ideas which I have not got around to testing is to put foam insulation between the motor tube and the main body tube to keep the motor from heating up in the sun. A hot hybrid motor is not an efficient rocket motor.

 

[kramer714]

I think the tanks get cold from the boiling and venting. I know they get cold when you launch them see condensation forming during the launch ofthe twin, you can see it better on the purple motor.
[video=youtube;xirKHQjEHng]https://www.youtube.com/watch?v=xirKHQjEHng"]https://www.youtube.com/watch?v=xirKHQjEHng[/video]

 

[djs]

I've seen that video a few times- any pictures of the whole rocket? it looks cool from this angle.

 

[kramer714]

I've seen that video a few times- any pictures of the whole rocket? it looks cool from this angle.

https://www.flickr.com/photos/paytonkramer/22790418098/ other rocket pictures are on Payton's aka 'the boy' flickr https://www.flickr.com/photos/paytonkramer/

Mike K