Help Modifying Hybrid Motor

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prospect_1

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Hello,
First post! So, for a school project, I am looking to decrease the total impulse of a Rattworks H70 Hybrid motor by decreasing the volume of the oxidizer chamber. I'm hoping to be able to get to the desired impulse with only two static tests, so trial and error is not really an option.

I'm having trouble figuring out the exact volume of oxidizer to remove. I don't want to just assume its proportional but I'm not sure what else to do. I read somewhere that the oxidiser-to-fuel ratio varies along the fuel grain, which makes things more complicated. Can anyone point me in the right direction as far as figuring out the math behind my problem? Here is some info that might be helpful:

Diameter: 29mm
Length: 18" (457mm)
Average Thrust: 15.4 lbs. (68.5 N)
Peak Thrust: 34.5 lbs. (153.5 N)
ISP: 167.3 sec.
Burn Time: 2.57 sec.
Weight (Empty): 207.5 g
Weight (Loaded w/o N2O): 251.5 g
Weight (Loaded w/ N2O): 352.7 g
N2O Weight (750 PSI): 0.223 lbs. (101.2 g)
Tank Volume: 8.29ci (135.8cc)
Polypropylene fuel grain

Any help would be greatly appreciated!
 
prospect_1 said:
Hello,
First post! So, for a school project, I am looking to decrease the total impulse of a Rattworks H70 Hybrid motor by decreasing the volume of the oxidizer chamber. I'm hoping to be able to get to the desired impulse with only two static tests, so trial and error is not really an option.

I'm having trouble figuring out the exact volume of oxidizer to remove. I don't want to just assume its proportional but I'm not sure what else to do. I read somewhere that the oxidiser-to-fuel ratio varies along the fuel grain, which makes things more complicated. Can anyone point me in the right direction as far as figuring out the math behind my problem? Here is some info that might be helpful:

Diameter: 29mm
Length: 18" (457mm)
Average Thrust: 15.4 lbs. (68.5 N)
Peak Thrust: 34.5 lbs. (153.5 N)
ISP: 167.3 sec.
Burn Time: 2.57 sec.
Weight (Empty): 207.5 g
Weight (Loaded w/o N2O): 251.5 g
Weight (Loaded w/ N2O): 352.7 g
N2O Weight (750 PSI): 0.223 lbs. (101.2 g)
Tank Volume: 8.29ci (135.8cc)
Polypropylene fuel grain

Any help would be greatly appreciated!
Click to expand...
It's really not that simple. The Mass of the Nitrous will be determined by the temperature of the liquid Nitrous in the propellant tank and how you will it. What kind of school project?

Bob
 
Like Bob says, it's not quite that simple but you can start with this. The H70 and I80 use the same fuel grain. Only the nitrous volume changes. The specs for the two motors are here.



Look around and try to find a copy of HDAS by Todd Moore. I found a link for it that Bitdfender didn't like but I think that's because it's a spreadsheet with macros.
 
Last edited:
So I don't know if my computer messed up or I just didn't click send but apparently I didn't reply to this comment last week like I thought I did. Thank you so much for that info! The fact that the H70 and the I80 differ only by the volume of nitrous is very interesting! Using the info from the rattworks site, it seems that doubling the amount of nitrous did indeed double the total impulse (but only increased the burn time by about 85%), so maybe it is proportional? Also, does that imply that the H70 does not use all of its solid fuel grain?

I've looked at that spreadsheet. It's a little overwhelming but I'm gonna keep looking at it and see if I can figure something out. Thanks again for the info and sorry for the late response!
 
Yes, most hybrids do not use all of the fuel grain as the grain doubles as a insulator to the casing wall.

If you want to decrease the impulse of the motor the simplest way is to decrease the nitrous fill volume. A very simple way to do this with the H70 is to place and object in the tank to displace some volume, not react with the N2O and not plug the flow into the injector. An aluminum tube that will fit inside the H70 case would do the trick.

For estimation since we know that doubling the N2O volume in a 29mm rattworks motor doubles the impulse, and the impulse with 0 N20 will be zero, a simple linear interpolation will get you in the ball park.
 
If you add the aluminum tube, make sure it is very clean. No grease, no oil. Adding either to nitrous makes a more explosive mixture.

Gerald

PS - Of course for flight purposes it would now be EX.
 
John,
That is exactly our plan, though we haven't decided on a material for the tubing. We don't want to add too much weight nor do we want to interfere with the motor reactions. My main concern right now is with costs. We'll need to perform two successful tests, and I'd rather not do a third unless we really need to. That's why I want to make sure there isn't other factors that might impact the results. That being said, it does indeed look like a linear interpolation would get me close.

JT
 
For tubing we know nylon would work (I haven't seen any nylon fill tubes exploding into flames yet). Probably most plastic tubing would be safe.
 
G_T said:
If you add the aluminum tube, make sure it is very clean. No grease, no oil. Adding either to nitrous makes a more explosive mixture.

Gerald
Click to expand...

Isn't whipped cream an oily hydrocarbon?
 
N2O needs to be treated carefully no doubt. But I think the greater hazard is one that is shared by all compressed gases. In all the industrial, medical and gear head uses there doesn't seem to be alot (any?) of instances of unexpected decomposition of N2O. The Mojave desert incident was a structural tank failure root cause if I am not mistaken.
 
Medical N2O usage is as often as not nitrox, not nitrous. That is a close to equal blend of nitrous oxide and O2. The O2 dilutes the N2O sufficiently that the energy released from its decomposition is no longer sufficient to heat the surrounding gas to the decomposition temperature. It is a better oxidizer, and for medical usage, prevents asphyxiation. But it is not what we are normally using in our hobby hybrids. IMHO from the research I have done it is a superior mildly cryogenic oxidizer. Though it may not be explosive by itself, as long as it is not contaminated with a fuel source, it IS a room temperature oxidizer, a high pressure gas, and/or a cryogenic liquid. So it has its own handling and safety considerations.

Gerald
 
jderimig said:
N2O needs to be treated carefully no doubt. But I think the greater hazard is one that is shared by all compressed gases. In all the industrial, medical and gear head uses there doesn't seem to be alot (any?) of instances of unexpected decomposition of N2O. The Mojave desert incident was a structural tank failure root cause if I am not mistaken.
Click to expand...

I heard it was a valve pressure spike induced decomp that travelled upstream into the tank, but that was also a large volume of nitrous.
 
You can "safely" premix N2O and Ethylene and/or other similar fuel gases. https://en.wikipedia.org/wiki/Nitrous_oxide_fuel_blend

This mixture has been patented and is being commercialized. The monopropellant mixture is "stable" but when it reacts is can not stopped unless you use very fine sintered metal quenching filters in the fuel lines and fuel injectors. Since N2O is a monopropellant as well as an oxidizer, I'm confident that it will under go violent sustained and uncontrolled decomposition under conditions where self-quenching can not happen. Specifically, in a system that has undergone a sudden depressurization in the presence of shocks you have an enormous number of vapor bubbles in the liquid with a very large surface to volume area. A large delta T can occur at the liquid gas interface on shock passage causing large energy release that could produce an exponential pressure increase that will rupture the container. I'll argue it's somewhat analogous to a dust explosion in a grain elevator.....

Bob
 

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