Solid fuel questions

[Viv]

Although I don't want to build a rocket I have a few questions for those experienced with solid fuel.

How much residue is left after combustion is complete (ash)

How long is the average burn time and is that burn time easily controllable through control of the recipe/mixture

What is required for ignition? how much energy/time does it take to bring the fuel to proper combustion levels

Probably have some more questions later, and yes I know I could look up a lot of this but I want people who have played with the stuff to answer to get the benefit of their experience.

Thanks in advance

Viv

[Ray]

How much residue is left after combustion is complete (ash)

Very little, most of the ash I see in a motor is from the casting tubes and liner...I sometimes find a small amount of Al2O3 on the convergent portion of the nozzle, but that depends on formula and pressure. Combustion is usually very complete. I have spent most of my time on AP mixtures...not much if any experience with other oxidizers.

How long is the average burn time and is that burn time easily controllable through control of the recipe/mixture

"typical" burn time is 1-4 seconds...but again, its very dependent on motor size, formula and pressure. Larger diameter motors tend to burn longer (thicker web), higher pressure leads to shorter burn times. Burn rate can be controlled by formulation, particle size and addition of burn rate enhancer, usually oxides of metals. Burn rate is very difficult (impossible) to predict with any accuracy, however experience will lead to one being able to guess pretty close. Most formulas burn at less than 0.6 ips at 1000psi. 1 ips is considered to be VERY FAST, a slow formula is around 0.25 ips.

What is required for ignition? how much energy/time does it take to bring the fuel to proper combustion levels

Ignition is usually accomplished by an igniter...The igniter is a conductive bridge with a pyrogen mixture over that. The pyrogen is usually a pretty aggressive oxidizer mixed with metal and other fuels. It usually takes 0.5-1.5 seconds to get the motor to pressure. Some folks use a thermite mixture to ignite the motor. Thermite igniters tend to be "instantaneous". Most recommend using 0.5 g/1000NS for thermite igniters.

[Viv]

Hi Ray,

Thanks for the detailed answers, you seem to have also answered a large number of the follow on questions and a large amount that I had not got round to formulating:-)

This is a thought experiment over why we do not use solid fuel motors for RC craft and how to make it so we can, its no good for pure rockets but with wings it may be practical.

Liquid fuel is out I think as the complexity is high for such a small scale engine, my first thoughts were to break up the solid fuel charge in to small discreet charges with their own igniters and use a common plenum chamber to feed the nozzle, this would allow simple throttle/thrust control by sequential firing or group firing.

The fuel charge burn rate/chamber pressure relationship would have to be watched closely due to the accelerating nature of the reaction from what you have said:-)~

Have a look at the metal storm web site for an interesting idea, multiple bullets stacked inside a single barrel.

Viv

[jthompso]

One idea that I've thought about (not very seriously) is a solid fuel turbojet. Now, I'm not talking about a solid fuel as in coal dust or something similar, I mean a formed fuel grain inside a turbojet engine. I don't think it would have much practical value (maybe useful for a drone engine) but I think it would be kind of neat anyway. I'm sure you've read of them but the only resource I can think of are the Jetex brand of planes--also estes has its rocket powered rc gliders. Any chance of a little more detail on what you're thinking about?

[Viv]

One idea that I've thought about (not very seriously) is a solid fuel turbojet. /// I can think of are the Jetex brand of planes--also estes has its rocket powered rc gliders. Any chance of a little more detail on what you're thinking about?

Yes the same thing occurred to me regarding turbines (and probably a lot of others people too:-), I was starting by looking at the problems of using a rocket on an RC plane, so throttle control, run time, simplicity and lightness are some of the considerations.

From there I started by looking at a basic solid grain engine and how to make it do what I wanted, my first thought is to break up the grain in to fuel pellets like the JetEx engine you mentioned, fitting these in to some kind of arrangement to allow individual staged firing would then give control over burn time/ thrust output.

Using a large common plenum chamber feeding a normal nozzle would smooth out the pressure pulses from burning the fuel pellets this way.

Now your idea of a turbine could be added to this very nicely as the nozzle/s could feed a power turbine, that would open up a whole lot more possibilities albeit at the expense of complexity.

A possible configuration could be a simple miniature turboprop!

Was it WebPilot who posted the rocket turbine info?

Viv

[Zippiot]

Solid fuel ramjets exist, roofing tar can be formed and stuck to the walls lining the combustion chamber. It works but makes a bunch of ugly smoke...

If you are interested in rocketry buy a book on the subject. Even if you make the "beginners" Candy propellant a book still gives good rules of thumb, safety procedures and design info.

http://skylighter.com/mall/books.asp

Ray told me to get Experimental Composite Propellant and it was a great buy!! Amateur Rocket Motor Construction looks good too...

[Viv]

Solid fuel ramjets exist, roofing tar can be formed and stuck to the walls lining the combustion chamber. It works but makes a bunch of ugly smoke...

If you are interested in rocketry buy a book on the subject. Even if you make the "beginners" Candy propellant a book still gives good rules of thumb, safety procedures and design info.

http://skylighter.com/mall/books.asp

Ray told me to get Experimental Composite Propellant and it was a great buy!! Amateur Rocket Motor Construction looks good too...

Well thanks for the advice I will have to look and see if I can find a nice book on the subject.

Solid fuel ram jets I know about but they are not a good fit for what we are talking about in this thread as they have the same drawbacks as the basic solid fuel rocket engine we started the thread with.

The goal is a thought experiment to come up with some thing different.

Viv;-)

[Zippiot]

Just found a combined solid fuel rocket engine and solid fuel ramjet

Down the page a bit
http://www.islandone.org/LEOBiblio/SPBI104.HTM

[Viv]

Just found a combined solid fuel rocket engine and solid fuel ramjet

Down the page a bit
http://www.islandone.org/LEOBiblio/SPBI104.HTM

Any suggestions on throttle control of this solid fuel rocket/ram jet? what about scaling to fit the typical RC plane and low speed flight?

Viv

[Zippiot]

No low speed flight with a ramjet...200mph+ just to maintain thrust

Solid fuel stuff, generally, cannot be throttled. I dont really see a way around that, only example I can think of would be a Hybrid rocket where throttleing the N2O throttles the rocket somewhat.

[Viv]

No low speed flight with a ramjet...200mph+ just to maintain thrust

Solid fuel stuff, generally, cannot be throttled. I don't really see a way around that, only example I can think of would be a Hybrid rocket where throttling the N2O throttles the rocket somewhat.

In real terms that 200-mph is 500-mph plus my friend;-)

Yes its the sheer fact of the difficulty of throttling a solid fuel engine that interests me, the others I am familier with also pintle types.

Have another look up the thread for some possibilities

Viv

[Zippiot]

Maybe some method of bleeding off the intake air?

Could work with the turbine idea also, have a little valve or shutter that can be opened or closed to control the amount of air that reaches the CC.

Like on high performance car turbos, they adjust the wastegate based on altitude and how fast you want to go. Some racecars can dial in enough boost to blow and engine at sea level or keep things running strong in high altitudes.

[Rossco]

I did some reading on peroxide rockets a while back, i know, not solid fuel.
Peroxidepropultions is an excellent site for generating thought expariments on the subject, and the guy is contactable and very helpfull, he is/was around here on occasion too.

I wouldnt think that monopropelants would be considered as complicated, and you have the added bonus of throttlability.

Just check out the rotor tip rockets there, and have a look at the vtol gyro video's and prop rockets, very interesting!

Just my 2 cents... which doesnt go real far these days.

Rossco

[Viv]

Hi Rossco, how ya doing

Yes I looked at peroxide but rejected it for this project, plenty of stuff around on the subject but it has sourcing difficulties and engineering complexities that I wanted to try and stay clear of, having said that this little project could end up just as complex in the engineering but one of my targets is that the fuel should be as easy to get and cook up as the low power rocketry boys and girls do already.

Its just to see if we can come up with a different way of doing things, this is why I was interested in the Metal storm gun barrel idea and then just taking the bullets out and putting a nozzle on the end;-)

No longer than a PJ for RC planes so no trouble mounting just a simple long tube with a number of fuel pellets in and individual igniter wires coming out to a controller.

I blame the guys drawing those rather nice 3D planes in the other threads:-)

Viv

[jthompso]

Here's the biggest problem I see, in order to get a non-regressive thrust profile you'd have to do some pretty complex casting of the fuel charges or have some sort of spring loaded mechanism (think magazine) to push the charges forward in the top.

Essentially what you would have would be a bunch of little end-burning fuel grains, the problem is, as each one burns away, the volume of the combustion chamber increases decreasing the thrust. The ways to combat this would be casting progressively bigger cores in to the fuel grain (you would then need to increase the lengths of the grains as you went down the tube too), having a cone shaped combustion chamber/grains, or keeping the cc volume constant by some sort of mechanism.

Another problem involves chain ignition. In the MetalStorm the bullets themselves prevent the next charge from going off, so in order to stop this from happening in the rocket there would need to be some fire-resistant partition to isolate the grain ends from one another. The problem is that this would introduce a non-burning piece of material into the combustion chamber which could plug the nozzle causing a cato.

You've caught my interest!