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ecos
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[*] posted on 30-3-2016 at 08:03
High-energy composite (HEC) propellants


I was reading about solid fuel rocket

I noticed that double-base (DB) propellants has medium performance:

Quote:

nitroglycerin is dissolved in a nitrocellulose gel and solidified with additives. DB propellants are implemented in applications where minimal smoke is required yet medium-high performance (Isp of roughly 235 s) is required. The addition of metal fuels (such as aluminum) can increase the performance (around 250 s)


while composite propellants has higher Isp :

Quote:

Ammonium Perchlorate Composite Propellant often uses aluminum fuel and delivers high performance (vacuum Isp up to 296 s with a single piece nozzle or 304 s with a high area ratio telescoping nozzle)


I was expecting that DB propellant should have higher Isp since it consists of high explosives !! , any explanation to that?

the funny thing is , Composite modified double base propellants have higher Isp :


Quote:

Composite modified double base propellants start with a nitrocellulose/nitroglycerin double base propellant as a binder and add solids (typically ammonium perchlorate and powdered aluminum) normally used in composite propellants. The ammonium perchlorate makes up the oxygen deficit introduced by using nitrocellulose, improving the overall specific impulse. The aluminum also improves specific impulse as well as combustion stability.


for high-energy composite propellants :

Quote:

Typical HEC propellants start with a standard composite propellant mixture (such as APCP) and add a high-energy explosive to the mix. This extra component usually is in the form of small crystals of RDX or HMX, both of which have higher energy than ammonium perchlorate. Despite a modest increase in specific impulse, implementation is limited due to the increased hazards of the high-explosive additives.


I don't know why RDX and HMX specifically but do you know if nitro erythritol would be a good choice ?
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[*] posted on 30-3-2016 at 09:30


Performance is largely dependent on the gas composition the fuel generates. Low molecular weight products is by far the most important parameter. Double base propellants form mostly carbon dioxide which is a relatively heavy gas. RDX and HMX form more nitrogen and water.

My thermochemical code can accurately model CHNO rocket propellants. Here are some calculated specific impulses (68 bar chamber pressure, 1 bar nozzle exit pressure):
ETN: 229.4 s
Nitroglycerin/nitrocellulose 50/50: 240.8 s
HNIW: 255.9 s
RDX: 259.9 s
Dinitromethyltetrazole: 282.8 s

As you can see, RDX gives higher Isp than HNIW despite being a weaker HE.
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Marvin
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[*] posted on 30-3-2016 at 09:55


High explosives do not have the highest energy density of fuels, not even close. It's just that they can release the energy they do have very quickly.
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[*] posted on 30-3-2016 at 16:51


Quote: Originally posted by Dornier 335A  
Performance is largely dependent on the gas composition the fuel generates. Low molecular weight products is by far the most important parameter. Double base propellants form mostly carbon dioxide which is a relatively heavy gas. RDX and HMX form more nitrogen and water.

My thermochemical code can accurately model CHNO rocket propellants. Here are some calculated specific impulses (68 bar chamber pressure, 1 bar nozzle exit pressure):
ETN: 229.4 s
Nitroglycerin/nitrocellulose 50/50: 240.8 s
HNIW: 255.9 s
RDX: 259.9 s
Dinitromethyltetrazole: 282.8 s

As you can see, RDX gives higher Isp than HNIW despite being a weaker HE.


it is interesting to know about your code. how you calculate those parameters ?

if RDX has Isp of 259.9 s and AP has Isp of 296 s, this means the composite should have Isp value between 259.9 and 296 s !

how come combining high Isp substance (AP) with lower Isp substance give high composite propellant?

I thought RDX is more energetic than AP as propellant !

[Edited on 31-3-2016 by ecos]
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[*] posted on 1-4-2016 at 04:44


HE and rocket fuel has different measurement of their performance.
For rockets, one want high thrust and low rate of fuel consumption:

specific impulse/effective exhause velocity = newton/(kg/second) = meter/second

which is a unit of velocity. 0.5v^2 is (effective kinetic energy)/mass, which is proportional to the (chemical energy)/mass of the fuel. Specific energy(energy/mass) is thus proportional to the square of SI. So in rocket fuel, it is the Specific energy that matters.

For high explosives, CJ pressure of the detonation wave is the way to evaluate its performance. Higher the CJ pressure, better the shattering power. Using a model of a spherical HE initiate from the center, one can conclusion that:

pressure of the detonation wave = constant x density x specific energy
= constant x volumetric energy density

therefore in HE, both density and (energy per mass) have a say in the performance of an explosive.
..............










TLDR.
rocket fuel: higher energy/mass = better, density of the fuel don't matter
HE: higher energy/volume = better
a good HE doesn't necessary makes a good rocket fuel vice versa
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[*] posted on 2-4-2016 at 00:36


Quote: Originally posted by ecos  
it is interesting to know about your code. how you calculate those parameters ?

if RDX has Isp of 259.9 s and AP has Isp of 296 s, this means the composite should have Isp value between 259.9 and 296 s !

how come combining high Isp substance (AP) with lower Isp substance give high composite propellant?

I thought RDX is more energetic than AP as propellant !

[Edited on 31-3-2016 by ecos]


The specific impulse code uses the equilibrium engine I originally wrote (and still develop) to predict the performance of high explosives. It works by repeatedly solving the equilibrium composition of the gases during isentropic expansion. When the selected exit pressure has been reached, the exit velocity is given by v = sqrt(H0-H1)/2, where H is the energy of the gases in the chamber/at the nozzle.

RDX gives a good gas mixture (low molecular weight gases) when it combusts. On the other hand, APCP has really good energy density thanks to the aluminium content, but has higher mean molecular weight of the products. Combining them therefore yields a better propellant than any of them alone.
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[*] posted on 6-4-2016 at 16:33


I came across this old report that describes the use of solid explosives for pulse detonation propulsion of a rocket.

http://www.dtic.mil/dtic/tr/fulltext/u2/750554.pdf (Skip to page 84)

My question is, what explosives would have the highest performance for this application? Other sources I've found have said that the exhaust velocity of the detonation products can be approximated with the Gurney velocity, but the molecular weight of the products would also play a huge role.

My gut feeling is that the specific impulse of an explosive used for unconfined pulse detonation propulsion would be roughly the same as in Dornier's program if the chamber pressure was drastically increased.
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[*] posted on 11-4-2016 at 16:05


Ive been reading on a similar topic.

IMO the a good feasible composite propellant better than APCP could be consisted of energetic high gas producing oxidizer, ADN for example, a decent energetic binder, Nitrated HTPB, GAP, GNP, for example, with energetic plasticizers such as DEGDN or TEGDN, and a metallic fuel such as atomized aluminum or a hydride such as AlH3.

[Edited on 12-4-2016 by DubaiAmateurRocketry]
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[*] posted on 18-4-2016 at 11:51


I wonder why NG/NC deflagrate not detonate inside rocket motor while temp/pressure increase suddenly?
I think NG detonate if temp increase suddenly over 218 deg.
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[*] posted on 18-4-2016 at 22:08


Quote:
I wonder why NG/NC deflagrate not detonate inside rocket motor while temp/pressure increase suddenly?
I think NG detonate if temp increase suddenly over 218 deg.


Wouldn't the combustion gases carry heat away from the fuel grain via forced convection? Solid fuel grains are pretty sensitive to spikes in chamber pressure, though.
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[*] posted on 18-4-2016 at 23:30


@Canerican, I think the temperature inside rocket motor is around 1000 degree and 60 pars of pressure !
this is enough for detonation of solid fuel in my humble idea. I still wonder why it doesn't explode !
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[*] posted on 19-4-2016 at 00:11


You think that is impressive? In a gun the pressure can exceed 4000bars and 2000degress C. Your humble idea is evidently wrong :P





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[*] posted on 19-4-2016 at 00:15


Ecos, the whole fuel grain is not at the combustion temperature (around 3400 K). The heat is slowly transferred to the next burning layer. Slowly heating the whole rocket engine would of course lead to a rapid unplanned disassembly if not high order detonation, but that's a whole other story. Neither will pressure alone initiate detonation - remember, we're talking secondary explosives here - a strong shock is required. Since the chamber pressure is constant during the combustion, no such shocks are present.
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[*] posted on 26-4-2016 at 09:09


Quote: Originally posted by Dornier 335A  
Ecos, the whole fuel grain is not at the combustion temperature (around 3400 K). The heat is slowly transferred to the next burning layer. Slowly heating the whole rocket engine would of course lead to a rapid unplanned disassembly if not high order detonation, but that's a whole other story. Neither will pressure alone initiate detonation - remember, we're talking secondary explosives here - a strong shock is required. Since the chamber pressure is constant during the combustion, no such shocks are present.


Thanks all for your explanations.

It make sense that the pressure inside the rocket motor should be constant or this would be like a detonation shock.
I also understand that heat transfers from layer to layer but lets assume that the surface area of the grain will face sudden increase in the pressure and temperature.
(intial pressure = atmospheric air pressure , initial temperature = room temperature) ===after ignition of the solid grain ====> (temp = very high , pressure = very high).

the sudden change in the temperature should detonate NG/NC gel since the final temperature should be very high.

I really feel confused. :(


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[*] posted on 27-4-2016 at 02:40


Quote: Originally posted by ecos  
Quote: Originally posted by Dornier 335A  
Ecos, the whole fuel grain is not at the combustion temperature (around 3400 K). The heat is slowly transferred to the next burning layer. Slowly heating the whole rocket engine would of course lead to a rapid unplanned disassembly if not high order detonation, but that's a whole other story. Neither will pressure alone initiate detonation - remember, we're talking secondary explosives here - a strong shock is required. Since the chamber pressure is constant during the combustion, no such shocks are present.


Thanks all for your explanations.

It make sense that the pressure inside the rocket motor should be constant or this would be like a detonation shock.
I also understand that heat transfers from layer to layer but lets assume that the surface area of the grain will face sudden increase in the pressure and temperature.
(intial pressure = atmospheric air pressure , initial temperature = room temperature) ===after ignition of the solid grain ====> (temp = very high , pressure = very high).

the sudden change in the temperature should detonate NG/NC gel since the final temperature should be very high.

I really feel confused. :(




This will happen if the exhaust hole is too tiny...and this explains rocketry is full of dimensional calculations and trial-error type of real experiments to masterize all aspects of the problem.
This also explains that even if it has been long an extensively studied that CATO/explosion-detonation of space rockets-shuttles stil happens to th expand of living astronauts :(.


[Edited on 27-4-2016 by PHILOU Zrealone]




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[*] posted on 27-4-2016 at 16:15


ecos there are burn-rate modifiers to prevent accidents. most propellant burn rate have a positive feedback as pressure increases and burns even faster, which might result in a detonation, however many burn-rate modifiers can, and different binders and plasticizes has some kind of effect on the burn rate, obviously the appropriate ones are chosen.
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[*] posted on 28-4-2016 at 08:37


@DubaiAmateurRocketry and @PHILOU Zrealone , thanks for the info. it was helpful.

the heat transfer is my issue. the grain surface area get suddenly hot , specially the first layer and this should be enough to cause detonation !

from wiki :
Quote:

Strong heating may lead to explosions. Complete reactions leave no residue. Pure crystals cannot sustain a flame below the pressure of 2 MPa.
AP is a Class 4 oxidizer (can undergo an explosive reaction) for particle sizes over 15 micrometres[6] and is classified as an explosive for particle sizes less than 15 micrometres


milling AP to a small sizes seems a very bad idea ! :o

it seems there are a lot of parameters that surrounds how to make a grain that burns in constantly and without detonation.
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[*] posted on 29-4-2016 at 04:08


Quote: Originally posted by ecos  
@DubaiAmateurRocketry and @PHILOU Zrealone , thanks for the info. it was helpful.

the heat transfer is my issue. the grain surface area get suddenly hot , specially the first layer and this should be enough to cause detonation !

from wiki :
Quote:

Strong heating may lead to explosions. Complete reactions leave no residue. Pure crystals cannot sustain a flame below the pressure of 2 MPa.
AP is a Class 4 oxidizer (can undergo an explosive reaction) for particle sizes over 15 micrometres[6] and is classified as an explosive for particle sizes less than 15 micrometres


milling AP to a small sizes seems a very bad idea ! :o

it seems there are a lot of parameters that surrounds how to make a grain that burns in constantly and without detonation.

Heat transfer is a hard problem:
-Convection
-Radiation
-Heat of reaction/decomposition/deflagration/detonation
-Latent heat of fusion, vaporization, sublimation




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[*] posted on 2-5-2016 at 04:50


It make sense that solid propellant is really a challenge but it has very simple motor design.
I think the safest thing would be AN+Al propellant since it is hard to ignite(minimum hazard) , hard to detonate ,... but low Isp ~ 250

DubaiAmateurRocketry mentioned ADN as a better oxidizer. I think its synthesis is not that easy.
any other options ?

[Edited on 2-5-2016 by ecos]
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[*] posted on 7-5-2016 at 05:19


I plan now to build AP rocket to get the max from it before I jump to the next level. what is the composition that would make me reach Isp = 290 ?
I found many compositions over internet but none specify the Isp for each.
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[*] posted on 7-5-2016 at 09:05


Quote: Originally posted by ecos  
I plan now to build AP rocket to get the max from it before I jump to the next level. what is the composition that would make me reach Isp = 290 ?
I found many compositions over internet but none specify the Isp for each.


The time scales we're talking about inside a rocket motor are hard to comprehend, where thousands of a second is considered "A long time"

from ignition to initiation there is a "slow" progression of events.

You need to toss out the window what you think the time scales are and do more reading befor you'll under stand.


For grains you want a very fine homoginous fuel mix and that can be packed solidly with out bubbles or cracks, you need to experiment with the inside bore and pattern to get best thrust/burn rate.

Start with learning basic model rocket engines, I am a poor text based communicator, if I had all the gear I once had and you where here it'd be easy to show you (frustrating not being able to get the idea out on screen)

Engines are not a simple thing as they seem to the eye once packaged!
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[*] posted on 13-5-2016 at 01:08


I am working now to get my high power solid propellant as my first step.
I left my perchlorate cell (NaCl ---->NaClO4 ) running and waiting for the results
I will then use NH4OH to get my lovely NH4ClO4 :)
I know that NH4ClO3 is not stable and I need to get high quality of NaClO4 to avoid hazards.

any way to get high purity perchlorate ?

[Edited on 13-5-2016 by ecos]
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[*] posted on 13-5-2016 at 01:47


NH4ClO3=Chlorine Dioxide and other things. Bad.
Get an EPP license and buy NaClO4 online.

[Edited on 13-5-2016 by Eosin Y]
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[*] posted on 13-5-2016 at 06:13


I don't have such license in my country.

I think there should be a way to purify.

I thought to purify depending on solubility in water but chlorate and perchlorate are very close to each other :(
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[*] posted on 13-5-2016 at 06:27


Quote: Originally posted by ecos  
I don't have such license in my country.

I think there should be a way to purify.

I thought to purify depending on solubility in water but chlorate and perchlorate are very close to each other :(

HCl is the way to go...while ClO3(-) is sensitive to it, ClO4(-) is not.
5HCl + HClO3 -->3Cl2(g) + 6H2O

Then afterwards add some AgClO4 to precipitate Cl(-) as AgCl.

About purification, you may be surprised by the high solubility of NaClO4 > NaClO3 >> NaCl in organic solvents like aceton...

[Edited on 13-5-2016 by PHILOU Zrealone]




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