Hybrid Motor ignition and combustion

[edwardw]

I have been pondering some points of hybrid motors and wanted some input and advice and just to see if I'm on the right track.

First, hybrid motors burn in diffusion flame. First I have to melt my solid fuel to a liquid, then heat it to a gas and then it mixes with the oxygen released in N2O breaking down and then it burns. How fast all of this happens is dependent on my material, acrylic has a lower regression rate than wax. So, would it be beneficial to have a higher regression rate or lower regression rate for the highest ISP? I know ISP is average thrust over massflow...but I don't know how energy content/density figures in.

Also, for igniting hybrids it seems you want a material with enough energy to bring your fuel up to a liquid and gas, and also burn through your burst disk/fill line. But you don't want it to burn too long then your just dripping newton-seconds down the core and out the nozzle before it lights. So for this would I need something that releases a lot of energy in a short time, or just enough energy over a longer period of time to get the nitrous flowing?


Edward

 

[DynaSoar]

Originally posted by edwardw
I have been pondering some points of hybrid motors and wanted some input and advice and just to see if I'm on the right track.

First, hybrid motors burn in diffusion flame. First I have to melt my solid fuel to a liquid, then heat it to a gas and then it mixes with the oxygen released in N2O breaking down and then it burns. How fast all of this happens is dependent on my material, acrylic has a lower regression rate than wax. So, would it be beneficial to have a higher regression rate or lower regression rate for the highest ISP? I know ISP is average thrust over massflow...but I don't know how energy content/density figures in.

Also, for igniting hybrids it seems you want a material with enough energy to bring your fuel up to a liquid and gas, and also burn through your burst disk/fill line. But you don't want it to burn too long then your just dripping newton-seconds down the core and out the nozzle before it lights. So for this would I need something that releases a lot of energy in a short time, or just enough energy over a longer period of time to get the nitrous flowing?


Edward

I've only been picking up on the latter point in my explorations, leaving the other to the motor designer.

"Just enough" is still a fair amount. But it does need to be longer burning than a common ignitier. I've seen recommendation for a full second burn on a smaller motor. I think it'll be dependent on the particulars of a given motor, but it'll still be relatively long.

 

[edwardw]

For my preheater grain I have been using the standard sugar grain. I have been using a full diameter 3/4" long in the casting tube around the burst disc. Would I have any benefit of going to an AP grain?

Edward

 

[Daedalus]

Check this reference -

https://www.aspirespace.org.uk/aspireold/TechSeries/TechSeries.html

Apparently for a hybrid the best perfromance is from a low regression rate.

 

[MattEx]

Originally posted by edwardw
For my preheater grain I have been using the standard sugar grain. I have been using a full diameter 3/4" long in the casting tube around the burst disc. Would I have any benefit of going to an AP grain?

Edward

I wouldn't have thought that going to an APCP grain would have much benefit. Sugar makes lots of solids on combustion (40% IIRC), mostly potassium carbonate. This would help in heat transfer to the hybrid grain. Gases aren't very good at transferring heat.

However, if you used a highly aluminized APCP formulation you would get the same effect (Al2O3) ... and it would burn very, very hot. Using KP instead of AP might also be a good idea. That way you get fast burnrate, solids from the K and KP is pretty dense so you could make a pourable formulation for making preheaters if you used say 80-75% solids.

 

[edwardw]

Originally posted by Daedalus
Check this reference -

https://www.aspirespace.org.uk/aspireold/TechSeries/TechSeries.html

Apparently for a hybrid the best perfromance is from a low regression rate.

After reading it, the best performance is from the lowest regression rate that will still provide the needed max flux for your application. Otherwise you might have to have larger hardware.

Edward

 

[bobkrech]

The combustion in a hybrid motor is definately not a difusion flame. You simply couldn't get the required mixing and energy release rates to sustain the combustion if this were the case. It is a highly turbulant subsonic process.

Convective heat transfer and radiative heat transfer are both important in vaporizing the fuel and insuring rapid combustion. If you don't think gases are very good at transfering heat, stick your hand in front of a cutting torch. (Don't actually do this, or you'll find out how wrong that statement is.)

Read the referenced papers to get a pretty good idea of what happens inside a hybri

Bob Krech