Hybrid fuel consumption

[lyuv]

Hi all,

This is my first post in these forums. I just built my first hybrid (after years in pyro), and many ideas and questions occupy my mind. But one at a time:

I have seen many references to O/F ratio in hybrid, but I dont understand how this works, or matters. The fuel grain "just lies there" as the oxidizer flows through it. The oxidizer oxidizes whatever it can.

As far as I understand, we do not control the o/f, unlike in solids of liquid fuel. (Sure, we could deprive the oxidizer of fuel, but to what end?)

So as far as there is enough fuel, what is the meaning of O/F figures in hybrids?

And a thought - if the fuel port is LONGER than is necessary, will I benefit from vaporization of fuel (and extra gas volume) or lose due to heat loss?

Thanks,

 

[bobkrech]

Welocme to TRF.

Hybrids utilize nitrous oxide as an oxidizer and a plastic as the fuel.

Combustion occurs in the gas phase above the surface of the plastic so the effective O/F ratio is determined by the flowrate of nitrous oxide through a control orifice located between the nitrous tank and the thrust chamber and the pyrolysis rate of the plastic fuel grain.

In addition to being an oxidizer, nitrous oxide is also a monopropellant. So 2 N2O + heat ==> 2 N2 + O2 making the oxygen that will support the compustion of the plastic fuel grain. The heat from the combustion pyrolizes the plastic fuel grain into hydrocarbon gases that burns with the oxygen to make hot steam and carbon dioxide. (CH2)n + 1.5 n O2 ===> n CO2 + 0.5 n H2O + heat making the overall reaction 3 N2O + CH2 ===> CO2 + H2O + 3 N2 so the O/F ratio is approximately 3 * 44 / 14 ~ 9.4

If there is not enough nitrous oxide flow, you can't sustain the pyrolysis and combustion reactions so the motor will out. If you flow too much nitrous, you risk rapid pressure oscillations in the thrust chamber resulting in an overpressurization of the the thrust chamber and a backflash of hot gas flow back into the nitrous tank resulting in a detonation of the nitrous and cato of the motor.

Bob

 

[Larry Curcio]

As I understand it, as the core of the motor burns, it regresses from the center axis of the motor. Consequently, more and more oxidizer just blows right out the nozzle of the motor, never touching and oxidizing any fuel. Thus, near the beginning of the burn, the fuel has lots of oxidizer near it and the motor has a desirable O/F ratio, but near the end of the burn, the fuel is starved of oxidizer. The long fuel grain is used to give the oxidizer more time to interact with the fuel. Research has shown that hybrids can be imporved by drilling a helix into the fuel grain (as opposed to a simple circular shaft) and using injector nozzles that introduce a spin into the oxidizer; we think that these measures improve the O/F ratio throughout the burn of the motor, but we aren't completely sure how or why they work.

Another remedy is an oxidizer tube inserted well down into the axial hole within the fuel cylinder. As combustion proceeds, the tube is slowly withdrawn at a rate faster than the average linear regression rate of the fuel. Doing this exposes more fuel surface for combustion. The result is a burning cone of constant size, which recedes from back to front.

FWIW, The last time I checked (years ago - maybe things have got better?) HPR hybrid Isp's were very low - lower than BP, in fact. The low Isp was thought to be caused by inefficient combustion. The issue is a big concern among professionals and amateurs alike.

 

[bobkrech]

As I understand it, as the core of the motor burns, it regresses from the center axis of the motor. Consequently, more and more oxidizer just blows right out the nozzle of the motor, never touching and oxidizing any fuel. Thus, near the beginning of the burn, the fuel has lots of oxidizer near it and the motor has a desirable O/F ratio, but near the end of the burn, the fuel is starved of oxidizer. The long fuel grain is used to give the oxidizer more time to interact with the fuel. Research has shown that hybrids can be imporved by drilling a helix into the fuel grain (as opposed to a simple circular shaft) and using injector nozzles that introduce a spin into the oxidizer; we think that these measures improve the O/F ratio throughout the burn of the motor, but we aren't completely sure how or why they work.

Buring occurs in the gas phase. Flow is turblent. Nitrous orifice controlls the flowrate of the nitrous and the pyrolisis surface area of the propellant grain determines the fuel flow rate. Surface recession rates are about 1/3 that of APCP so you need about 3 times more surface area with a hybird to get equivalent thrust of an APCP motor under ideal conditions.

Another remedy is an oxidizer tube inserted well down into the axial hole within the fuel cylinder. As combustion proceeds, the tube is slowly withdrawn at a rate faster than the average linear regression rate of the fuel. Doing this exposes more fuel surface for combustion. The result is a burning cone of constant size, which recedes from back to front.

FWIW, The last time I checked (years ago - maybe things have got better?) HPR hybrid Isp's were very low - lower than BP, in fact. The low Isp was thought to be caused by inefficient combustion. The issue is a big concern among professionals and amateurs alike.

Good injector design and turulent mixing eliominate the need for a long injector tube. Isp can be equivalent or better than APCP solids when properly designed.

Bob

 

[lyuv]

. (CH2)n + 1.5 n O2 ===> n CO2 + 0.5 n H2O + heat making the overall reaction 3 N2O + CH2 ===> CO2 + H2O + 3 N2 so the O/F ratio is approximately 3 * 44 / 14 ~ 9.4

Bob

If I got you, than in FULL combustion, for every 9.4 gram of N2O, 1 gram of fuel will be consumed. So in a less than perfect motor, where some oxidizer is lost, even less than 1 gram fuel is taken.

However, in my microhybrid, for every 8 grams of N2O, the fuel grain (nylon 6/6) looses 1.3 grams. What is the meaning of this? Isn't it suppose to be impossible?

 

[CarVac]

Incomplete combustion. Some of the carbon becomes carbon monoxide instead of carbon dioxide, thus consuming more fuel per unit oxidizer.

 

[lyuv]

Incomplete combustion. Some of the carbon becomes carbon monoxide instead of carbon dioxide, thus consuming more fuel per unit oxidizer.

Is that good or bad? Should I strive to get complete combustion? (and how?) Or should I be content that I get more gas for the same oxidizer? (more bang for the buck )

 

[CarVac]

I'm actually not sure. Incomplete combustion creates more gas volume per unit of solid propellant, though it probably generates less heat. I couldn't tell you which is the dominant effect, though. And it's not like there's much you can do to change it, either (I think...).