Solid fuel questions

[Viv]

Ah ha:-) now you see what I saw and so picked my interest:-)

Yes to the separation between pellets being an issue and what it could be? it may be as simple as airojel? its an insulator with low strength so will pop through the nozzle rather than as you say "cato"

The magazine had me intrigued for a long time but complexity and weight kept it off the list so far.

Now let me ask a simple question, why does the chamber volume increasing by N number of charges lower thrust? what is the difference between this discrete staged increase in volume and that of a progressive volume increase in a normal fuel grain and chamber?

Viv

[heada]

I've been giving this some thought and I think that the only real way you'd be able to vary the throttle on a solid rocket motor would be to vary the nozzle.

It would vary the chamber pressure which would vary the thrust. It would also vary the burn time some, but not much I think. One huge thing to worry about would be over pressurizing which would be "very bad" HOW to vary the nozzle would be vary hard. I was thinking something along the lines of the vectored thrust nozzles from a jet fighter, but I don't know if they could be made small enough or strong enough to withstand the temps/pressures that a rocket motor sees. Also, you would need a very long burn motor for it to even be worthwhile.

What about a hybrid system? Not a normal hybrid (N2O + fuel) but something like taking a very long burning, low thrust motor and adding a N2O or O2 injection when additional thrust is needed?

Or why not just use multiple motors and have an R/C relay to air-start them on demand? As long as the plane could take an additional 8 grams per motor plus the weight of an igniter/relay/battery you could add 12N peak and 2.35N average for 0.85 seconds per motor (Estes A10T-0) Make then parallel staged(next to each other) rather than linearly staged(fore to aft to fore, etc) You can pickup a 4-pack of A10T motors for less than US$5 at just about any hobby store and sometimes even WalMart.

-Aaron

[jthompso]

Now let me ask a simple question, why does the chamber volume increasing by N number of charges lower thrust? what is the difference between this discrete staged increase in volume and that of a progressive volume increase in a normal fuel grain and chamber?


In a normal fuel grain there is an increase in volume, but there is also an increase in the surface area that is burning--more surface area means more gas being generated so the larger volume is counteracted. In a MetalStorm type motor that is just a simple tube with a nozzle, the volume of the combustion chamber will increase but the surface area will remain constant which will lower the pressure (thrust).

However, I think the easiest way would be a traditional bates grain with the lengths of the grains getting progressively longer to counteract the loss in thrust.

[Viv]

Bangs head on desk for not thinking of surface area! now that makes perfect sense I can start thinking about how to try and overcome it, most of the other factors I had all ready seen from just working on paper but this is why forums work for this kind of thing:-)

Having just done a super fast read of the Bates grain and taking in to account what you have said I agree! knowing the length of the tube burn rates can be charted to get the varying pellet lengths, maybe adding to this a simple scale up of the centre hole for initial burn area?

the real issue is the pressure drop and its effect on burn rate in between charge firings, if we let the pressure drop to much we lose performance.

but maybe an answer is two very different grains, imagine a longer slow burning Bates grain with short fast burning grains in-between, the fast burn spacers would boost pressure for the longer burn pellet to burn properly.

With wings its not an issue to have thrust in pulses.

Viv

[Viv]

Heada,

It sounds like your talking about a pintle engine? but you have correctly mentioned all the other points that could cause a problem;-)

I did start off thinking along these lines but the pressure containment was not acceptable and the nozzle would have been an interesting challenge.

This is why I have gone towards a digital rocket engine:-)

Viv

[Ray]

Now let me ask a simple question, why does the chamber volume increasing by N number of charges lower thrust? what is the difference between this discrete staged increase in volume and that of a progressive volume increase in a normal fuel grain and chamber?


In a normal fuel grain there is an increase in volume, but there is also an increase in the surface area that is burning--more surface area means more gas being generated so the larger volume is counteracted. In a MetalStorm type motor that is just a simple tube with a nozzle, the volume of the combustion chamber will increase but the surface area will remain constant which will lower the pressure (thrust).

However, I think the easiest way would be a traditional bates grain with the lengths of the grains getting progressively longer to counteract the loss in thrust.

HUH? Combustion chamber volume isn't even a part of the equation when you calculate thrust...I suppose if your combustion chamber is abnormally large for the propellant volume it *could* make a difference. Volume loading of the chamber (for solid fuels) is usually greater than 75%. Once the motor is up to pressure the pressure remains pretty constant...you have an equal amount of gas being generated as is leaving the nozzle...

Normal grains do not increase in area substantially as they burn, if you are talking about a BATES grain...they are designed to be as neutral burning as possible. The thrust curve will rise in the middle slightly and fall at the end. You can tailor your thrust curve by lengthening a grain to get a progressive thrust profile, or shorten them for a regressive profile.

Viv, if you want a throttlable rocket motor, go hybrid...its easy to do.

[Viv]

Hi Ray

Yes every one else is mentioning the hybrid as a solution but the experiment is to see if we can do it simpler with just solid fuel pellets and a very simple motor configuration, yes hybrid still figures but what can we do by looking at the problem again from a different perspective.

For some thing along the lines of the metal storm we would end up with a high chamber volume at the end of the run but the real kicker is that we are firing individual fuel charges on throttle command.

A digital burn if you will, so that will give wildly fluctuating pressures and a expanding volume..

Another point is this motor would stop and start:-)

Viv

[heada]

If you are very serious about it being a solid motor, look into a program called burnsim. I think there is a 14 day eval of it. Here is a site that has a copy.

http://burnsim.com/download.asp

It will let you simulate a solid rocket motor with all the variables you'd ever want.

If you are putting multiple motors into a tube and firing one after another from the aft to the fore, you might run into a separate problem called the Krushnic Effect. Here is what is said about it:

A very dramatic phenomenon where your rocket makes a tremendous amount of noise and smoke but doesn't go anywhere! This happens when the motor is recessed into the body tube by more than one tube diameter. If so recessed, the cylindrical volume below the motor forms a secondary expansion chamber which allows the exhaust gases to expand below atmospheric pressure before leaving the rocket. Surrounding air aspirated into the exhaust stream causes turbulence which negates much of the thrust, along with creating the characteristic roar. A multi-stage model that ejects its booster motor, but not the airframe, is a perfect example. Very damaging; it almost always destroys the lower body tube beyond use. Named for Richard Krushnic, the rocketeer who characterized the effect in the late '60s. Not to be confused with Suction Lock

For a true throttable motor (and restart if needed) I'd suggest either a hybrid or a bi-propellant motor (LOX + fuel) Not small nor light nor cheap....but thats where you can throttle it with ease.

-Aaron

[heada]

What about a linear aerospike that could put aft pf the motor and moved up and down into the exit throat of the nozzle? It would vary the exit diameter (well, exit area anyway) of the nozzle and change the chamber pressure and thus thrust. It would have to be made of something that could withstand the high temps, +2500 degrees C (nickel?) Again, too far forward and the motor over pressurizes and "very bad" things happen.

I'm just brainstorming here. Again, for simplicity, alot of smaller solid motors parallel staged would give you the power only when desired and remove all of the issues around trying to vary a single motor and how to ignite several in the same tube.

-Aaron

[Zippiot]

Ray
Could you have a square rocket motor with 2 inclined planes attatched to servos (pistons hydraulics etc..), this could vary the nozzle ratio.

here is my very bad picture of what I mean

[Zippiot]

Sorry for 2x psot...

Problem I see is the short burn time for solid rockets. Could it also work with liquid or hybrid rockets?

[Viv]

Hi Aaron

Burnsim looks pretty interesting and useful, the Krushnic effect makes sense too but weather I would have spotted it first or in hindsight is another question:-) looking at a tube with for to aft firing the space in front of the burning charge is clear to the nozzle, I am assuming the Krushnic effect happens due to the velocity/expansion of the nozzle and the secondary expansion space formed by the recess, (so outside our multi charge tube?).

The airospike motors really interest me and I would gladly build it in I could:-) graphite is a suitable spike material but the engineering problem of holding it central is the issue, there is a nice video of a spike motor failing on the internet due to asymmetric pressure in the slot, the problem of moving it and keeping it central is pretty huge.

Brainstorming is the goal of this thread so we are not losing any thing by throwing the ideas back and forth, some one may see what we do not and have a eureka moment:-) its why I decided to do this in the public domain instead of in my own company.

A single motor tube with charges was first, then multiple tubes side by side, then a single chamber with a magazine loader based on a roots or gear pump, as you can see I have been working my down the possibilities:-)

I like the idea of the motor ultimately feeding a power turbine, I think you mentioned that, as an energy store it would work well to smooth out the power delivery, I have had this vision of a tiny jetex powered turboprop ever since it was mentioned:-)

Viv

[Viv]

Ray
Could you have a square rocket motor with 2 inclined planes attached to servos (pistons hydraulics etc..), this could vary the nozzle ratio.

here is my very bad picture of what I mean

Same problems as a square pulse jet, dead spaces in the corners, plus I would now imagine an odd burn surface/pressure/time curve.

The sides of your nozzle would be very hard to seal, any pressure lost is not expanded for thrust, sliding seals are a bitch generally but at these temperatures even harder.

Ratio between throat and cone angle/profile would pose an interesting challenge to get right through the adjustment range.

Viv

[leo]

Hi Viv maybe a linear aerospike will be a possibility no central positioning problems here.
google search

What about a solid piece of oxidiser and a solid piece of fuel almost thatching each other on the burning area, controlling the burn rate by the distance between the them.

[Viv]

Hi Viv maybe a linear aerospike will be a possibility no central positioning problems here.
google search

What about a solid piece of oxidiser and a solid piece of fuel almost thatching each other on the burning area, controlling the burn rate by the distance between the them.

If you have a look along the top edge ether side of the linear motor you will note its actually a series of normal DeLavel liquid fuel rocket motors, its what they refer to as a thrust unit.

I also thought of feeding a two component fuel but you have a few sealing issues at a 1000 PSI plus to solve, we also run back in to that "keep it simple" requirement of the thread.

Maybe this is what we will end up with or similar but I like to simplify things as much as possible when I can.

Viv