Sciencemadness Discussion Board

Higher density or higher specific impulse? AlH3 vs Al powder in formulations

DubaiAmateurRocketry - 28-3-2014 at 10:12

There have been a lot of research on high specific impulse density propellants, and I have asked my self if density matters more or specific impulse. For same specific impulse in two formulations, the higher density is preferred, however what if one had higher impulse however lower density?

This situation is seen in many formulations. CL-20 give impressive density of 2.04 however due to its lower gas production, it give lower specific impulse than RDX and HMX. RDX and HMX have same products so HMX obviously out performs RDX. However, CL-20's specific density (Isp*density) is higher than HMX formulations[1]. Another example is aluminum (III) hydride. It have much lower density(1.477g/cm3) than aluminum powder(2.7g/cm3). In APCP, the aluminum powder enhances the combustion and gives a specific impulse around 245s, however replacing the aluminum powder aluminum (III) hydride gives around 9-11% higher Isp, which is around 270s[2]. so what do we choose?

Aluminum powder(Al) VS Aluminum hydride (AlH3)

NASA's SRB contains 500,000tons of APCP, and the take-off weight is 590,000 tons. So Ill bring the things down. Assuming 500g of APCP, and the casing to be 90 grams. Assuming that nozzle design are perfectly expanded de-laval and same for both. We have a rocket that have total weight of 590 grams, 500g APCP, 90 for empty mass. We will also assume that the 500g will burn out in 5,and 10 seconds for 2 different calculations using equation from [3](I know this might not be the best reference but its enough to tell us which choice shall we make). Calculated density for 70 15 15 APCP is 1.7211, while for 70 15 15 APCP with aluminum hydride instead of aluminum powder is 1.5861

Normal APCP formulation (similar to NASA's SRB booster)
Calculated results:
17.17 km.(10 seconds burn-out)
Burn out velocity: 3.43km/s
618km (apogee)

87.08 km.(5 seconds burn-out)
Burn out velocity: 3.48km/s
627km(apogee)

Now that the aluminum powder content in the APCP in changed to AlH3, the specific impulse is around 270s. The same volume to fit 500g of APCP now will drop to 460.8 grams due to the lower density.
APCP with aluminum powder replaced by AlH3
Calculated results:
18.55 km (10 seconds burn-out)
Burn out velocity: 3.71km/s
721km (apogee)

9.4km (5 second burn-out)
Burn out velocity: 3.76km/s
730km (apogee)

Note : All calculation does not take into the accounts of air friction drag, in reality, it obviously will not reach 600km and this is a free motor without payload, so the dead mass can fly easily.

So, these calculations showed that specific impulse is more important than density. Although AlH3 have a terrible density of 1.477, its high hydrogen content(gas production) generate higher specific impulse, and if replaced aluminum powder in APCP(only 15% of propellant), It increases the burn-out velocity by 8.2%. The replacement of aluminum powder also lower the combustion temperature according to GUIPEP and PROPEP's thermo calculations which is preferred. Aluminum hydride shows some promising results. If one insists, I can also calculate CL-20 versus HMX for a given formulation.

[1]Combustion, Explosion, and Shock Waves, Vol. 41, No. 2, pp. 121–132, 2005, Hexanitrohexaazaisowurtzitane (CL-20) and CL-20-Based Formulations (Review)
[2] Theoretical analysis of hydrides in solid and hybrid rocket propulsion
- Volume 37, Issue 2, January 2012, Pages 1760–1769. 10th International Conference on Clean Energy 2010. International Journal of Hydrogen Energy
[3]http://www.nakka-rocketry.net/articles/altcalc.pdf

Dany - 28-3-2014 at 16:42

DAR,

The comparison between specific impulse (Is) and density is not correct. it is like trying to say: what is more important? density or detonation velocity? this is nonsense. better than comparing density and specific impulse, you should explain or try to compare what is more important to reach high Is values. It is known that Is is proportional to (Tc/M)0.5, where Tc and M are the adiabatic flame temperature and average molecular weight of combustion gases, respectively. In your conclusion you showed that density is not a good factor for high Is which in theory, is not correct. If you can produce a propellant with high density and one that generate low M (average molecular weight of combustion gases) this would be a very interesting propellant because you can achieve high Is (because of low M) using a small volume of this propellant (because of it's high density). In practice, the high specific impulse propellant are found in low density propellant. A more striking example than the one of Al and AlH3 is the liquid bipropellant systems. For example, the NH3 and F2 bipropellant has the following characteristics [1]:

ρ= 1.16 g/cm3, M= 19, Is= 303 s (s= second)

for H2 and F2:

ρ= 0.32 g/cm3, M= 8.9, Is= 373 s (s= second)

the lower M in the second case make high Is, as the formula above predicts. Also, as is evident from the formula, increasing Tc increase Is. However, if one would choose between increasing Tc or decreasing M than, it is better to decrease M rather than increasing Tc. Increasing the combustion temperature poses serious engineering problem related to the rocket structure. At high Tc values materials will loose their strength and begin to fail under the high temperature encountered in the combustion chamber which will lead to missile structural failure during flight.

I suggest to read reference [1].

Reference

[1] ORDNANCE ENGINEERING DESIGN HANDBOOK, BALLISTIC MISSILE SERIES-PROPULSION AND PROPELLANTS, 1960.

Dany.



andy - 28-3-2014 at 17:53

Lower density products are better at being accelerated, with F = v2*M, they get more bank for the buck, same with temperature, can't remember of the top, but it has a exponential increase.
The impulse governs how much mass can be sent into orbit, whether its lead or gas hydrogen, for the fuel, you want high hydrogen content.

Will try and print out some pages if you have something specific in mind.

[Edited on 29-3-2014 by andy]

macckone - 28-3-2014 at 20:39

You should consider the safety aspects.
Aluminum is much less likely to go boom than aluminum hydride.
One of the key aspects of rocket engines is they don't blow up.

DubaiAmateurRocketry - 29-3-2014 at 06:43

Dany, I am not comparing Isp with density, I am proving which one is more preferred when used in a propellant formulation. Very low density is not beneficial, since the total weight vs fuel weight becomes too extreme. Which is why UDMH or kerosene is used over liquid hydrogen.

I will read the reference you posted and thanks for that.

[Edited on 29-3-2014 by DubaiAmateurRocketry]

Dany - 29-3-2014 at 08:29

Quote: Originally posted by DubaiAmateurRocketry  
Dany, I am not comparing Isp with density, I am proving which one is more preferred when used in a propellant formulation. Very low density is not beneficial, since the total weight vs fuel weight becomes too extreme. Which is why UDMH or kerosene is used over liquid hydrogen.

I will read the reference you posted and thanks for that.

[Edited on 29-3-2014 by DubaiAmateurRocketry]


I repeat what i mentioned above in my reply:

"If you can produce a propellant with high density and one that generate low M (average molecular weight of combustion gases) this would be a very interesting propellant because you can achieve high Is (because of low M) using a small volume of this propellant (because of it's high density)"

It is important to say that for long range missile like Intercontinental Ballistic Missile (ICBM), the increase in specific impulse may have a dramatic effect on the range of these missile. Quote from ref.[1]:

"A 1% increase in specific impulse increases the ICBM range by 365 nautical miles (for a 5000 nautical miles ICBM) or more than 7%. A 5% specific impulse increase results in a nearly 45% range increase..."

Reference:

[1] S. S. Penner, The Chemistry of Propellants: A Meeting Organised by the AGARD Combustion and Propulsion Panel.

Dany.



[Edited on 29-3-2014 by Dany]

DubaiAmateurRocketry - 29-3-2014 at 09:16

Quote: Originally posted by Dany  
Quote: Originally posted by DubaiAmateurRocketry  
Dany, I am not comparing Isp with density, I am proving which one is more preferred when used in a propellant formulation. Very low density is not beneficial, since the total weight vs fuel weight becomes too extreme. Which is why UDMH or kerosene is used over liquid hydrogen.

I will read the reference you posted and thanks for that.

[Edited on 29-3-2014 by DubaiAmateurRocketry]


I repeat what i mentioned above in my reply:

"If you can produce a propellant with high density and one that generate low M (average molecular weight of combustion gases) this would be a very interesting propellant because you can achieve high Is (because of low M) using a small volume of this propellant (because of it's high density)"

It is important to say that for long range missile like Intercontinental Ballistic Missile (ICBM), the increase in specific impulse may have a dramatic effect on the range of these missile. Quote from ref.[1]:

"A 1% increase in specific impulse increases the ICBM range by 365 nautical miles (for a 5000 nautical miles ICBM) or more than 7%. A 5% specific impulse increase results in a nearly 45% range increase..."

Reference:

[1] S. S. Penner, The Chemistry of Propellants: A Meeting Organised by the AGARD Combustion and Propulsion Panel.

Dany.



[Edited on 29-3-2014 by Dany]


Yes I understand your point. Few weeks ago I told you about nitroaminoguanidine nitrate, it produces more water and nitrogen than RDX(lower overall mw), while having 1 extra oxygen atom to be used per molecule(this is amazing) and it is significantly denser than RDX and just beat HMX slightly.

While being less impact/friction sensitive. Its synthesis also uses materials which are already producing world wide on industrial scale. Which is why I was really excited about that compound.

[Edited on 29-3-2014 by DubaiAmateurRocketry]