Solid fuel questions

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Viv
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Re: grain temp

Post by Viv » Fri Feb 23, 2007 6:00 pm

heada wrote:Does grain temp pre-ignition really matter that much? Once the motor ignites, the CC is going to be above 1500 degrees C in a matter of less than 1 second (I'd hazard to guess that it would be up to temp in much less time, something along the lines of once the motor comes up to pressures)

Even if pre-heating the grain helps, you'd be walking a very fine line up to the self ignition temp of the grain.

-Aaron
This is why I wondered about the energy/mass addition from the oxidizing metal wires/rods/staples, as for preheating and combustion chamber temperatures, its tricky, preheating would give heat to the grain and improve the initial combustion but with combustion heating I think flame front speed is faster than the conduction rate of the grain.

Ok maybe that makes no sense, try again, the temperature in the active region of the combustion chamber is lets say a 1000c plus but the temperature of the fuel grain lets say 1 or 2 mm deeper is going to at ambient temperature, the heat conduction rate of the grain is going to cause this plus maybe ablative cooling of the active combustion surface.

The difference between silver and aluminum intrigues me as I would expect ali to give better performance due to its oxidizing performance.

For the self ignition point maybe include a wire bead thermocouple:-)

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Greg O'Bryant
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Post by Greg O'Bryant » Fri Feb 23, 2007 6:51 pm

Grain temp does make a difference. I have never actually measured just how much but I know a hot glob of rocket candy burns faster than a cold one. I think the variable nozzle is your best bet. Instead of having it round make it square and just have one of the sides swing in and out to very the KN value. Some propellants won’t function properly unless they are under a lot of pressure. Like Scott Fintells erythritol rocket propellant. In fact you would have a hard time to get it to ignite at ambient pressure. So I am thinking an end burner type engine with 2 propellants. One propellant that has a very slow burn rate but is not pressure sensitive to act like a fuse or igniter surrounded by a pressure sensitive propellant. As you would very the KN and pressure the pressure sensitive propellant would ignite and extinguish. All the while the non-pressure sensitive propellant would stay lit to function as the igniter. Last for another idea I remember a post some while back about a missile that had a rocket/ram hybrid engine. The rocket fuel was very fuel rich. This would enter into the ram jet and would be burned further. The thrust was variable based on how much air was allowed into the ram jet section of the engine.

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Chambered Grains

Post by jthompso » Fri Feb 23, 2007 9:49 pm

I just began reading the first volume of the collected works of Robert Goddard, and lo and behold, in his first rocket patent he describes a device that mechanically chambers a fresh fuel grain. Here's a link to the patent online:

http://www.invention-protection.com/pdf ... 103503.pdf

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Re: Chambered Grains

Post by Viv » Fri Feb 23, 2007 10:12 pm

jthompso wrote:I just began reading the first volume of the collected works of Robert Goddard, and lo and behold, in his first rocket patent he describes a device that mechanically chambers a fresh fuel grain. Here's a link to the patent online:

http://www.invention-protection.com/pdf ... 103503.pdf
Bums:-) he wins on prior art:-)

Funny thing to see and nice in a way that one of the fathers of all this went through the same process of developing a magazine motor, from first read it looks a lot like a semi auto mechanism so for us to use the metal storm as a starting point is a nice similarity.

Our major plus point is reduced mass and complexity though

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jthompso
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Goddard

Post by jthompso » Fri Feb 23, 2007 10:27 pm

Actually in the book there is a good deal of information about the development of the "multiple-charge rocket". In fact, this might sound familiar to you Viv:

"It impressed me that the simplest plan would be to use a long tube from which the cartridges were shot into the combustion chamber by small charges of powder--in other words, a Roman-candle type of rocket. This was also made and tried at Mt. Wilson, and operated with as many as three cartridges, 25-gram cartridges being thus shot into a combustion chamber of but 1/8-inch thickness of wall. Defects in the cartridges, as there made, caused the method to be uncertain."

"An alternative way of setting off the charges the charges of black powder in the Roman-candle rocket would be to send an electric current through fuse wires in the successive charges of powder, having copper wires extending through the wrappings of the cartridges, and sliding contacts on each cartridge that could be easily pulled off when the cartridge rose."

"The chief advantage of the Roman-candle rocket lies in the absence of mechanism; and the chief disadvantage lies in the fact that every cartridge must be contained in a tube strong enough (and therefore heavy enough) to withstand a black-powder discharge. This, incidentally, was the reason for not developing it in preference to the inertia-loading rocket during 1918-1922." --Robert Goddard

The device is nearly the same, the best point made is in the last paragraph. For a "MetalStorm" motor the entire casing has to be made thick enough to withstand the chamber pressure. When you couple this with the weight of the propellant that isn't being used to contribute thrust (waiting to be burned) you see a sharp decline in the thrust-to-weight ration. The magnitude of this decline is where things get interesting.

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Rocket-Plane

Post by jthompso » Fri Feb 23, 2007 10:32 pm

In addition, if this type of motor were used for a plane, we could be talking about a pretty considerable change in the center of gravity over the course of the flight. I haven't done any rc flying, so whether this would be an issue or not will hopefully be answered by somebody with first-hand experience.

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Post by Viv » Fri Feb 23, 2007 11:00 pm

He seems to have covered all the basis so far:-) very interesting and pretty good fit for what we have been talking about, the metal storm motor always was going to suffer from the mass problem but how much do you think modern materials and techniques make up for the problems that Goddard saw?

The shifting mass issue I also was looking at, what crossed my mind was the same as Aaron, having multiple short tubes feeding a chamber.

What is becoming clear is if the long chamber is a mass problem we move back towards a small combustion chamber and loader mechanism, again Goddard has blazed a trail for us but we have the advantage of modern materials and manufacturing techniques the poor guy could never have dreamed off.

Maybe the upper section of a rocket just becomes a fuel pellet hopper feeding a loader/motor in the base, incidentally this comes closer again to maybe using the motor as a gas producer for a small turbine?

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Re: Chambered Grains

Post by Viv » Sat Feb 24, 2007 3:06 am

jthompso wrote:I just began reading the first volume of the collected works of Robert Goddard, and lo and behold, in his first rocket patent he describes a device that mechanically chambers a fresh fuel grain. Here's a link to the patent online:

http://www.invention-protection.com/pdf ... 103503.pdf
Is it worth posting the details of this book J" ? maybe an ISBN number or an amazon link, maybe Mike can get an advertising click from it to support the forum.

I seem to remember Goddard also patented the centrifugal rocket motor as well? or was that some one else?

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Al Belli
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solid fuel pellet rocket

Post by Al Belli » Sat Feb 24, 2007 1:42 pm

Hi,

Curtis Wright corporation produced small vernier rocket engines using a roll of "caps" similar to a toy cap gun.

Al Belli
Last edited by Al Belli on Sat Feb 24, 2007 6:54 pm, edited 1 time in total.

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Re: solid fuel pellet rocket

Post by Viv » Sat Feb 24, 2007 3:56 pm

Al Belli wrote:Hi,

Curtis Wright corporation produced small vernier rocket sngines using a roll of "caps" similar to a toy cap gun.

Al Belli
Oh! thats just perfect Al:-) now all we need is a set plans to see how they did it:-)

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Curtiss Wright thruster

Post by Al Belli » Sat Feb 24, 2007 7:34 pm

Hi Viv,

A small divergent nozzle was attached to a mechanically operated breech block assembly that contained the drive sprockets which fed the cap containing tape through the system. The cap was electrically fired by a small heating coil.

The assembly was very small with a nozzle outlet of about 5 CM. diameter.

I think the system was for attitude control on an early satellite platform.

The example that I saw was in a local museum exhibit of Curtiss Wright technology, in Paterson, NJ. This was around 30 years ago.

Al Belli

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Book

Post by jthompso » Sun Feb 25, 2007 1:52 am

Unfortunately I can't find an ISBN for the book, I'm even having trouble finding mention of it online. It's called "The Papers of Robert H. Goddard" and is a three volume set--really an interesting read, Goddard's research was very broad.

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Goddard

Post by heada » Sun Feb 25, 2007 3:06 am

Would it be either of these?

http://www.amazon.com/Papers-Robert-H-G ... F8&s=books

http://www.amazon.com/including-Smithso ... F8&s=books

The first one being about $300 and the second being $100

Both of those I found on Amazon.com I found this link which describes the first link from Amazon.

http://www.polybiblio.com/antiqbot/120126.html

I think this is the one that jthompso is referring to. It has a list price of $450 so Amazon.com's price of $300 isn't that bad

-Aaron

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Post by jthompso » Sun Feb 25, 2007 4:31 pm

Yup, that's the one. At that price I don't think anybody's going to be getting it though, it was a lucky find at my library.

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Post by Viv » Mon Feb 26, 2007 11:52 pm

jthompso wrote:Yup, that's the one. At that price I don't think anybody's going to be getting it though, it was a lucky find at my library.
For $300 bucks it looks like you will just have to read and then tell us all about it afterwards:-)

But to my mind the book is worth it as it has already saved a lot of time by letting us know about his patent on the cartridge engine.

Its always a good day when you get to stand on the shoulders of giants;-)

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