Solid rocket propellants and their properties

A solid propellant is characterized by that the fuel and oxidizer are stored in a condensed, solid state of matter. We distinguish:

  1. Black powder or gun powder is a low explosive, composed essentially of a mixture of potassium nitrate or sodium nitrate, charcoal, and sulfur. It is hygroscopic and subject to rapid deterioration when exposed to moisture. It is also one of the most dangerous explosives to handle because of the ease with which it is ignited by heat, friction, or spark.The use of black powder as a propellant has ceased except for fireworks.
  2. Homogeneous propellants are propellants characterized by that the fuel and oxidizer belong to the same molecule (compare liquid monopropellant). We distinguish:
    • Single-base propellants: Single base propellants include compositions that are principally gelatinized nitrocellulose (a white fibrous material also referred to as guncotton) and contain no high-explosive ingredient such as nitroglycerin. It is used in for example electric primers and in electrically initiated detonators.
    • Double-base propellants: When combining NC with nitroglycerine (NG) we obtain a "double base" propellant, which is more powerfull than a single-base propellant.The nitrocellulose (NC) serves as the binder and the nitroglycerin (NG) causes it to gel. For highest specific impulse, NG content should be about 85%. Since NG is an oily liquid plasticizers are needed to enhance the mechanical properties. With diethylphthalate as plasticizer and diphenylamine and ethyl centralite as stabilizers, , it is difficult to include more than 43.5% of NG in a double-base propellant. To achieve good stability during storage, the NG percentage is often even lowered to 25%. Typical examples of double base propellants are Ballistite and Cordite, see below.
    • Triple-base propellants: Same as double-base propellants, but with nitroguanidine added.

  3. Heterogeneous (or composite) propellants which consist of a separate fuel and oxidizer. Ordinary composite propellants generally consist of an organic fuel that also serves as a binder and a solid oxidizer. High-energetic composite propellants also include combustible metal particles which on combustion increase the energy available for propulsive purposes.  The organic fuel is usually a hydrocarbon polymer, which initially is in a liquid state. The oxidizer and metallic fuel are then added in the form of small particles which are a few to a couple of hundred microns in diameter. After mixing with the liquid organic fuel, the mixture is cured to allow the binder to solidify.
  4. Composite/double base: Combinations of composite and double-base propellants.

Below some detailed information on black powder, homogeneous and heterogeneous solid propellants is given.

Black powder or gunpowder

The use of black powder or gunpowder dates back to the early 14th century. By the end of the 18th century the composition of gun powder was fairly well  standardised at 75%  potassium nitrate, 15% charcoal, and 10% sulphur. Its mass density is about 1750 kg/m3. The molar mass of the combustion gases is about 34,75 gram/mol and the combustion temperature is about 2600 K. Regression rate is about 9 mm/s @ 1 MPa and increases with 9 mm/s/MPa. A disadvantage of black powder is that it produces over half its weight of solid residue. For example, the combustion of 1 kg of black powder gives about 400 gram of gases and 600 gram of solid (condensed) materials. Black powder remained the only solid propellant available for rockets until the middle of the 19th century. Since then it has been replaced by modern propellants. Current use is mostly for firework, low velocity guns and in rocket motor igniters.

Homogeneous propellants

Homogeneous propellants date from 1845, when a German chemist discovered nitrocellulose. A satisfactory propellant was first produced in 1884 by gelatinizing nitro-cellulose with an ether-alcohol mixture giving a single-base propellant. Alfred Nobel produced one the first double-base propellants by using  nitroglycerine instead of ether-alcohol. Typical double-base propellants have a NC content of 50-60% and an 30-40% NG content. Depending on the NG content, double-base propellants can be extruded, cast or pressed into shape. For extrusion and casting a high NG content is required, whereas a low content is required to allow for pressing the propellant into shape. The properties of double base propellants are sometimes improved by adding a certain amount of finely ground crystals of an inorganic oxidizer or even an explosive compound. Aluminum, magnesium of other metallic powders can also be added. This gives a composite modified double-base propellant.

Table 1: Compostion of some homogeneous propellants [2, 3, 4]

Propellant

Composition

Single-base

Nitrocellulose dissolved in ether and alcohol

Double-base

Ballistite

Nitrocellulose (51.5%), Nitroglycerine (43.0%), Plasticizer (1.0%), Other (4.5%)

Cordite (English)

Nitrocellulose (56.5%), Nitroglycerine (28.0%), Plasticizer (4.5%), Dinitrotoluene (11.0%)

ATO (German)

Nitrocellulose (67.7%), Diethyl Guanidine (29.3%), miscellaneous (6.0%)

S.D. (French)

Nitrocellulose (66%), Nitroglycerine (25.0%), Ethyl centralite (8%), miscellaneous (1.0%)

AFU (French)

Nitrocellulose (54.5%), Nitroglycerine (23.1%), Triacetin (18.1) Ethyl centralite (0.6%), miscellaneous (3.7%)

1) All % are mass %

Table 2: Typical properties of double-base propellants [2]

Propellant (mass %)

Mass density
[kg/m3]

Flame temperature
[K]

Molar mass
[kg/kmol]

Specific heat ratio
[-]

Regression rate
[mm/s]

Nitrocellulose (50-60%)
Nitroglycerine (30-40%)

~ 1600

2100-3125

22-28

1.21-1.25

15-25


Table 3: Effect of composition on double-base propellant properties [1]

Type of propellant

Double base

Extruded and Cast Double Base

Pressed Double Base

Composite Modified Double Base

Specific impulse (s) @ 69 atm with  ideal expansion to 1 atm.

170 to 220

170 to 220

240 to 260

Regression rate (mm/s)

5 to 20

1 to 4

7 to 25

Flame temperature (K)

1900 to 3000

1600 to 2300

over 4000

Temperature sensitivity (% / K)

0.1 to 0.5

0.5 to 0.9

0.5 to 1.1

Mass density (kg/m3)

1495 to 1635

1495 to 1690

1660 to 1855


Table 4: Properties of specific double-base propellants [3, 5]

Propellant

Mass density
[kg/m3]

Flame temperature
[K]

Molar mass
[kg/kmol]

Specific heat ratio
[-]

Regression rate
[mm/s]

Temperature sensitivity
 (% / K)

Ballistite

1620

3125

26.4

1.215

21.4 @ 98.1 bar

0.38

French S.D.

1590

2170

22

1.26

0.7 @ 68.6 bar

0.32

French AFU

1514

1693

21.55

1.26

52.1@  293 K and 6.9 Mpa

 1) Ballistite has a low pressure limit for stable burning of about 40-50 bar

Heterogeneous propellants

A wide variety of fuel-binders has been used, including the early asphalt and modern polymers like polyester, epoxy, synthetic rubbers, polyvinyl, polyethylene, polyurethane, polystyrene, and polybutadiene. Currently mostly poly-butadiene binders, like hydroxy-terminated poly-butadiene (HTPB) in the Space Shuttle and Ariane 4/5 solid rocket boosters and carboxy-terminated poly-butadiene (CTPB) or poly-butadiene acrylonitrile (PBAN) in older designs, are used.  As oxidizer, an inorganic salt is used like ammonium perchlorate (AP), ammonium nitrate (AN), or potassium perchlorate (PP). As metal fuel mostly aluminum and sometimes magnesium or boron are used.

The choice of fuel-binder does not normally have a strong influence upon the specific impulse of the propellant. It is much more associated with propellant processing. Ordinary composite propellants burn relative clean with not much solid matter in the combustion products. Metallized (high-energetic) composites on the other hand may have upto 30% of solid mater in the exhaust [3]. The table 5 compares some important parameters for ordinary (non-metalized) and high-energetic (metallized) propellants. Specific propellants and their characteristics are given in the tables 6 and 7.

Table 5: Characteristics of ordinary & high-energetic heterogeneous propellants [1]

Type of binder

Plastic and polymerizable binder types

Plastic and polymerizable binder types (high-energy)

Specific impulse (s) @ 69 atm with  ideal expansion to 1 atm.

170 to 230

240 to 260

Regression rate (mm/s)

1 to 38

5 to 38

Flame temperature (K)

1400 to 3000

3000 to 3800

Temperature sensitivity (% / K)

0.1 to 0.5

0.2 to 0.5

Mass density (kg/cubic meter)

1580 to 1770

1660 to 1855


From the above table, we clearly find that the specific impulse attainable with high-energetic propellants is about 30-70s higher.  This is mostly associated with the difference in flame temperature due to the combustion of the metal particles.

Table 6: Characteristics of specific high-energy heterogeneous propellants [5, 6, 7]

Application (propellant designation)

Ingredients 

Mass % 

Regression rate 

Density
[kg/m3]

Adiabatic flame temperature
[K]

Molar mass
[gram/mole]

Specific heat ratio
[-]

Temperature sensitivity
[%/K]

Minuteman

AP

70

8.9 mm/s @  6.9 MPa  

1760  

3472  

   

   

Al

16

Binder + additives

14

Orbus 6

AP

68

7.0 mm/s @ 6.9 MPa  

1758  

3672  

   

   

Al

18

HTPB + additives

14

Ariane 5 SRB

AP

68

8.9 mm/s @ 300 K and 6.9 MPa  

1770  

3328.9  

27.4  

1.142  

Al

18

HTPB

14

(Flexadyne RDS-556)

AP

69

37.8 mm/s @ 6.9 MPa  

1747  

3246  

26.92  

1.2  

Al

14

CTPB + additives

17

Ariane 3 & 4 SRB (Flexadyne CTPB16-13)

AP

71

   

   

   

   

   

Al

16

CTPB + additives

13

Space Shuttle SRB

AP

69.9

7.4 mm/s @ 6.9 MPa, 294 K

1772    

3480    

27.8    

1.18    

0.20

AL

16

ECTV

1.5

Fe Oxide

0.1

PBAN

12.5

Star 48 & Star 37 (TP-H-3340 )

AP

71

6.5 mm/s @ 5 MPa
5.4 mm/s @ 2.75 MPa 

1800  

3396  

   

   

0.18

AL

18

HTPN

11


Table 7: Characteristics of specific ordinary heterogeneous propellants [2, 3]

Propellant

Mass density
[kg/m3]

Flame temperature
[K]

Molar mass
[kg/kmol]

Specific heat ratio
[-]

Regression rate
[mm/s]

Ammonium nitrate (80%)
Organic fuel  + additives (20%)

1550

1755

22

1.26

1-4

Ammoniumperchlorate (65-85%)
Polyester fuel (35-15%)

1520-1750

1600-3050

19-28

1.22-1.3

2-10

Potassiumperchlorate (50-80%)
Organic fuel (50-20%)

1660-1940

1875-3075

25-35

1.24-1.27

10-30

Potassiumperchlorate (70-78%)
Asphalt (30-22%)

1740

2475

~ 30

~ 1.25

32-43

1) all % are mass%.

Typical thermochemical and other data of specific solid propellant constituants can be found in the next table.

References

  1. Solid-Fuel Rocket propulsion by J.E. Daboo
  2. H. Wittenberg, TU-Delft, LR-memorandum M-144, 1970.
  3. Rocket Propulsion, M. Barrere et al., Elsevier Publishing Comp., 1960.
  4. Military Ballistics - A Basic Manual, C.L. Farrar, E.W. Leeming, Brassey's Publ. Ltd. , 1983.
  5. Rocket Propulsion and Spaceflight Dynamics, Cornelisse J.W.,Schoyer H.F.R.and Wakker K.F., Pitman Publ. Ltd., 1979.
  6. Space propulsion Analysis and Design, Humble R.W. et al, McGraww-Hill, 1995.
  7. Rocket Propulsion Elements, 6th edition, Sutton, G.P., John Wiley and Sons Inc., 1992.

Comments

Mail to: b.t.c.zandbergen@remove-this.tudelft.nl

Last updated: 14 September 2004 (Added: Propellant proerties of some specific solid propellants + link to thermochemical properties)
Created: 18 December 2002

 

Name author: B. Zandbergen
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