16.522, Space Propulsion Lecture 5
Prof. Manuel Martinez-Sanchez Page 1 of 12
16.522, Space Propulsion
Prof. Manuel Martinez-Sanchez
Lecture 5: Chemical Thrusters for In-Space Propulsion
HYDRAZINE
Hydrazine was first isolated by Curtius in 1887, and in 1907 a suitable synthetic
method was developed by Raschig. Anhydrous hydrazine is a clear, colorless,
hygroscopic liquid with an odor similar to that of ammonia. Anhydrous hydrazine is a
strong reducing agent and a weak chemical base. Aqueous hydrazine shows both
oxidizing and reducing properties. Although potential data show hydrazine to be a
powerful oxidizing agent in acidic solutions, reactions with many reducing agents are
so slow that only the most powerful ones reduce it quantitatively to ammonium ion.
Hydrazine will react with carbon dioxide and oxygen in air. When hydrazine is
exposed on a large surface to air, such as on rags, it may ignite spontaneously due
to the evolution of heat caused by oxidation with atmospheric oxygen. A film of
hydrazine in contact with metallic oxides and other oxidizing agents may ignite.
Hydrazine is an endothermic compound and will decompose spontaneously in a
similar way to hydrogen peroxide. The reaction of hydrazine with the oxides of
copper, manganese, iron, silver, mercury, molybdenum, lead or chromium may be
particularly violent. The spontaneous or artificially induced decomposition of
hydrazine does not follow the reaction N
2
H
4
= N
2
+2H
2
, but a more exothermic one
such as 2N
2
H
4
= 2NH
3
+N
2
+H
2
.
SUMMARY AND CONTENTS
PHYSICO-CHEMICAL
PROPERTIES
METRIC VALUE ENGLISH REFERENCE FIGURE
Molecular Formula N
2
H
4
(1)
Molecular Weight 32.04 (1)
Freezing Point
1.5 C
null
34.7 F
null
(6)
Freezing Point Diagram
with Additives
1
Boiling Point
113.5 C
null
236.3 F
null
(10) 4
Critical Properties
C
P = 145 atm.
c
T =380C (716F)
nullnull
d
c
= 0.231 g/cc
(5)
(5)
(5)
3
3
3
Density, liquid
1.0045 g cc @ 25 C
null
8.482 lb gal @ 77 F
null
2, 3
Density, vapor and liquid (5) 3
Vapor Pressure
14.38 mm @ 25 C
null
0.0189 @ 77 Fatm
null
(6) 4
Surface Tension
66.67 dynes/cm @ 25 C
null
62.32 dynes/cm @ 35 C
null
.004568 lb ft @ 77 F
null
.004270 lb ft @ 95 F
null
(7)
(9)
Viscosity, liquid
0.90 centipoise @ 25 C
null
.000605 lb ft -sec
@ 77 F
null
(11, 12) 5
Heat Flux at q
ul
/ Pressure (8) 6
Heat Flux at q
ul
/
Temperature
(8 7
Heat Flux at q
ul
Velocity (8) 8
16.522, Space Propulsion Lecture 5
Prof. Manuel Martinez-Sanchez Page 2 of 12
Heat of Fusion
3.025 kcal/mole @ 1.5 C
null
37.51 Btu lb @ 34.7
null
(6)
Heat of Vaporization
9.600 kcal/mole @ 113.5 C
null
54.0 Btu lb @ 236 F
null
(3)
Heat Capacity (liquid)
Temp.
23.62 cal/mole- C @ 25 C
nullnull
.737 Btu lb- F @ 77 F
nullnull
(6) 9
Heat of Combustion
N
2
H
4
(l)+O
2
=N
2
+2H
2
O (l)
148.6 kcal/mole @ 25 C
null
8.346 Btu lb @ 77 F
null
(20)
Heats of Formation at
25 C (77 F)
nullnull
N
2
+2H
2
= N
2
H
4
(g)
N
2
+2H
2
= N
2
H
4
(liq)
N
2
+2H
2
+H
2
O = N
2
H
4
.H
2
O
N
2
+2H
2
+ aq = N
2
H
4
. aq
22.750 kcal/mole
11.999 kcal/mole
10.300 kcal/mole
8.140 kcal/mole
1278 Btu/lb
675 Btu/lb
579 Btu/lb
457 Btu/lb
(20)
(20)
(20)
(20)
Index of Refraction, D
1.4644 @ 25 C (77 F)
nullnull
(9)
Dielectric Constant
51.7 @ 25 C (77 F)
nullnull
(19)
Electrical Conductivity -6 -1
310 ohm @ 25C (77F)×
nullnull
11)
Flash Point (open cup)
52 C
null
126 F
null
(18)
Explosive Limits (in air, 1
atm.)
4.7% lower
100% upper
(18)
(18)
B. Materials: The following table gives an evaluation of the compatibility data that
are available for numerous metals, plastics, elastomers, and miscellaneous material:
COMPARATIVE COMPATIBILITY OF VARIOUS MATERIALS
WITH HYDRAZINE AND HYDRAZINE MIXTURES (Ref. 28)
A-Material is acceptable for general service.
B-Material is acceptable for limited service.
C-Material which must be avoided.
MATERIAL ANHYDROUS
HYDRAZINE
HYDRAZINE
HYDRATE
HYDRAZINE-
HYDRAZINE
NITRATE-
WATER
MIXTURES
Metals
Aluminum
2S A A B
2SO A
2SH A A B
3S A
3SH A A B
24ST A A
40E B B B
43 B
52ST A A A
16.522, Space Propulsion Lecture 5
Prof. Manuel Martinez-Sanchez Page 3 of 12
61ST A A A
75ST A
MATERIAL ANHYDROUS
HYDRAZINE
HYDRAZINE
HYDRATE
HYDRAZINE-
HYDRAZINE
NITRATE-
WATER
MIXTURES
XA-545 B B B
716 B
Brass B B B
Cobalt C C C
Copper C
Inconel B B B
Inconel X B B B
Iron C C C
Lead C
Magnesium C C C
Manganese
Molybdenum C C C
Monel B B C
Nickel B C
Nickel-chrome alloys
(Chromel-A, Nichrome)
B
B
C
Silver B B
Steel
Mild C C C
Stainless
302 B B B
303 C C C
304 A A A
315 C C C
316 C
317 C C C
321 B B B
329 C C C
347 A A A
410 B B B
416 C C C
420F C
430 B B B
430F C C C
440A C
440C C C C
W B B B
Stellite B
Tantalum A A A
Tin C C C
Titanium A A A
Zinc C
Plastics and Elastomers
16.522, Space Propulsion Lecture 5
Prof. Manuel Martinez-Sanchez Page 4 of 12
Cellulose acetate C C C
Diallyl phthalate C C C
MATERIAL ANHYDROUS
HYDRAZINE
HYDRAZINE
HYDRATE
HYDRAZINE-
HYDRAZINE
NITRATE-
WATER
MIXTURES
Epon B B B
Ethyl cellulose B B B
Furane resin B B B
Hycar B
Kel-F B B B
Lactopreme C C C
Lucite B B B
Melamine
formaldehyde
B
B
B
Nylon B
Phenolic B B B
Polyester C C C
Polyethylene A A A
Polystyrene and
polydichlorostyrene
C
C
B
Polyvinyl alcohol C C C
Polyvinyl chloride
(Koroseal, Vinylite,
etc.)
B
B
B
Rubber
Natural gum C C C
Synthetic B B B
Saran C C C
Silastic B B B
Teflon A A A
Tygon B B B
U.S. Rubber
Plastic
L7825 B B B
M20995 B
Veloform C C C
Miscellaneous Materials
Asbestos B B B
Glass
Soft A A A
Pyrex A
Graphite B B B
Graphitar B
Pipe-joint compounds
AN-C-53 B B B
Oxyseal B
Thread-Tite B B B
Rags C C C
16.522, Space Propulsion Lecture 5
Prof. Manuel Martinez-Sanchez Page 5 of 12
Silicone lubricants
DC-200 series B B B
MATERIAL ANHYDROUS
HYDRAZINE
HYDRAZINE
HYDRATE
HYDRAZINE-
HYDRAZINE
NITRATE-
WATER
MIXTURES
DC-550 B B B
DC-710 B
Plug-cock grease B B B
Solder
Lead-tin B B B
Silver B
Varnish B B B
Wood C C C
Wool C
C. Equipment: The selection of materials of construction of equipment for use with
hydrazine should be limited to those previously listed as acceptable for general
service.
Hydrazine and hydrazine rockets -
(1) Properties.
Hydrazine monopropellant rockets have become standard for NSSK and other on-
board maneuvering needs. This is because of the simplicity of a monopropellant
rocket, storability of hydrazine over many years, and relatively good performance
achievable. Also, the technology of hydrazine engines is well developed. There
had been other monopropellants used or considered before (H
2
O
2
, nitromethane)
but they are inferior either in safety or in performance.
Formula: N
2
H
4
Clear, very similar to water in physical properties
(
3
=1g cmρ , freezes at 1.5 C
null
, boils at 113.5 C
null
, surface tension = 66.7 dyn/cm
at25 C
null
, viscosity = 0.9 cp at 25 C
null
, specific heat 0.72 cal/g C
null
at 25 C
null
). Other
properties listed in the given table.
Chemically, however, N
2
H
4
is very different from H
2
O. It will react with CO
2
and
O
2
in air; if exposed to a large surface of air, as in wet rags, it may ignite; it may
also ignite in contact with metallic oxides. Its combustion is very exothermic:
24 2 2 2
Kcal
NH+O N 2HO+149
Mole
→+
and so hydrazine is a good fuel for a bipropellant rocket (Hydrazine + O
2
).
Variations are the monomethyl hydrazine (MMH) NH
3
CH
3
and the unsymmetric
dimetyl hydrazine (UDMH, NH
2
(CH
3
)
2
), which are very similar but somewhat
16.522, Space Propulsion Lecture 5
Prof. Manuel Martinez-Sanchez Page 6 of 12
inferior thermochemically, but have a wider liquid range. A popular bipropellant
combination is A50 (50% hydrazine, 50% UDMH).
These cannot be used as monoprops, because they poison the catalyst.
Hydrazine evolves NH
3
, and smells like it. Its vapors damage the eyes and the
pulmonary tract. The liquid, if pure, is fairly inert, but vapors form flammable
mixtures in air for vapor pressures corresponding to T >40C
null
. The liquid
decomposes exothermically if catalyzed by iron oxide, copper oxide, or oxides of
Pb, Mn, Mb, Ag, Hg or Cr. It is not sensitive to friction or impact.
Materials compatibility is an important consideration, especially for long term
storage and for parts of thrusters with long life exposed to it. Materials which are
OK are aluminum, stainless 304 or 347, titanium, tantalum, rhenium and
platinum. Also glass, Teflon and polyethylene plastics. To be avoided are copper,
cobalt, iron, lead, magnesium, manganese, molibdenum, mild steel, high Mo
stainless (416, 303), most plastics (except as noted), wood, rags, paper. See
table handed out.
Oils are OK for lubrication, but not if there is a catalytic bed, since it gets
poisoned by the oil. Hydrazine is expensive ($ 50-60/lb for MMH, as of 1994).
(2) Thermochemistry.
When catalyzed either by an oxide or by a hot platinum surface, N
2
H
4
decomposes. Since at low temperatures ammonia, NH
3
, is stable, the preferred
end products would be NH
3
+N
2
:
24 3 2
3N H 4NH N→+ (a)
However, this is a very exothermic reaction, and the equilibration T would be
1650 K,
null
at which temperature NH
3
is not stable anymore. Hence, the final
equilibrium composition would contain very little NH
3
, due to
322
2NH N +3H→ (b)
and would be at intermediate T, since the latter reaction is endothermic. In
practice reaction (a) is very fast (less than 1 msec) if catalyzed, while (b) is slow.
Hence, for small decomposition chambers and high flow rates, when the
residence time
gas ch
ρ Vm
i
is short, reaction (b) proceeds only partially, the extent
being controllable by the design conditions. The final composition and overall
reaction assuming a fraction x of NH
3
decomposes is
24 3 2
3
322
41
N H NH + N (1)
33
Form (1)+ x(2) ; x = fraction of NH that decomposes
42
NH N +2H (2)
33
?
→
?
?
?
?
→
?
?
() ()
24 3 2 2
41
N H 1- x NH + 1+2x N +2xH
33
→
16.522, Space Propulsion Lecture 5
Prof. Manuel Martinez-Sanchez Page 7 of 12
For an adiabatic combustion (no heat loss, no heating), we must have (starting
from liquid N
2
H
4
at 298 K
null
)
()() () ()() ()
24 3 2 2
NH NH N H
41
h ,298 1-x h T 1+2x h T 2xh T Equation for T
33
=+ +→
null
null
The respective molar enthalpies can be fitted by
()
3
2 cal
NH
K T
h T = -16.83+12.35 +0.983 =
mole 1000 K
??
θθθ
??
??
null
()
2
2
N
h T = -2.83+7.75 +0.183 θθ
()
2
2
H
h T = -1.967+6.6 +0.367 θθ
( )
300 T 4000 K≤≤
null
(Notice
()
p
ccalmoleC
null
) is
dh
dθ
, with h in Kcal/mole)
From the table given,
()
24
NH
h ,298 K 12 Kcal/mole=
null
null . We can now solve for T at
various arbitrary values of x:
()
2
139.3 - 20.75x +8.45x - 9.525+0.95x
=
1.372 - 0.455x
θ
x (fraction of NH
3
decomposed) 0 0.2 0.4 0.6 0.8 1
T( K),
null
adiabatic temperature 1659 1502 1343 1182 1023 863
16.522, Space Propulsion Lecture 5
Prof. Manuel Martinez-Sanchez Page 8 of 12
Equilibrium Hydrazine Decomposition
If we allowed “infinite” time for the reaction, hydrazine products would reach an
equilibrium with little ammonia left (depending on pressure). The product, N
2
, H
2
,
NH
3
, must then satisfy
()
3
22
NH
p3
1
22
NH
P
=K T
P P
;
()
6289
-1
T
-6
p
K1.089×10e atmnull (P’s in atm)
and the pressure is
322
NH N H
P=P +P +P
To conserve moles of H and N, starting from (arbitrarily) 3 moles of N
2
H
4
, we must
have
× ?
?
?
×
?
?
32
32
3232
NH H
NH H
NH NNH N
H: 3n +2n =3 4=12
3P +2P
dividing, = 2
P+2PN: n + 2n = 3 2 = 6
Define
3
NH
P
y=
P
(ammonia mole fraction).
Then
2
2
H
N
3y+2P P
=2
y+2P P
and
22
HN
PP
y+ + =1
PP
Solving,
2
H
P
25
=1-y
P3 4
??
??
??
2
N
P
11
=1-y
P3 2
??
??
??
16.522, Space Propulsion Lecture 5
Prof. Manuel Martinez-Sanchez Page 9 of 12
and substituting into the equilibrium law,
p12 32
y
=KP
11 25
1- y 1- y
32 34
????????
????????
????????
()
12 32
py12 32
y1
=PKTK
33
15
1- y 1- y
24
????
≡
????
????
????
????
????
So, given P and T, we can solve this for
3
NH
P
y=
P
.
But T itself must be consistent with energy conservation
()() () ()
32224
NH N H N H
41+2x
1-x h T + h T +2xh T =h
33
where
24
NH
h would be 12.0 Kcal/mol if we start from liquid hydrazine at 298K, or it
could include the external heating energy due to an electrical heater, as in the case
of an Electrothermally Augmented thruster (in fact, in this case the equilibrium
assumption is realistic, given the long residence time in the heater, and the higher
temperature).
Now x is the fraction of NH
3
decomposed after the initial fast reaction
24 3 2
41
NH NH + N
33
→ .
To relate it to y, which is the mole fraction of NH
3
in the final products, we write
()
()
()
4
1-x
41-x
3
y= =
41+2x
5+4x
1-x + +2x
33
or
()
4-5y
x=
41+y
(notice for x=0 we have
4
y=
5
, i.e.
1
5
of the products is N
2
while for x=1, y=0).
So, to complete the computation, we could iterate as follows:
(1) Given P, guess T
16.522, Space Propulsion Lecture 5
Prof. Manuel Martinez-Sanchez Page 10 of 12
(2) Compute () ( ) ( ) ( ) ( )
322
py NH N H
K T , K T, P , h T , h T , h T
(3) Solve
y31
22
y
=K
y4
1- 1- y
25
??? ?
??? ?
??? ?
for y
(4) Calculate
()
4-5y
x=
41+y
(5) Calculate ()
322
prods. NH N H
41+2x
h=1-xh+ h+2xh
33
and the “error”
24
prods. N H
h-h
(6) Use this error to generate a new T guess, go back to (2)
Some results (with no external heating)
which, indeed, shows
minimal NH
3
present
For propulsion purposes, the important thing is not T, but I
sp
. This depends also on
the molecular weight of the gas (and somewhat on γ ); since the gas gets lighter as
NH
3
decomposes, this compensates for the lower T, and I
sp
is very insensitive to x for
x<0.4.
~
Assuming frozen flow (constant γ , constant M), and a pressure ratio P
e
/P
o
,
the exit velocity is
-1
e
e0
0
PR
u= 2 T 1-
-1M P
γ
γ
??
??
γ
??
??
γ
??
??
??
()
()
41+2x
17x 1- x +28 +2x.2
33
M=
41+2x
1-x + +2x
33
96
M=
5+4x
and the area ratio is
P(atm)
NH
3
P
y=
P
T(K)
2 0.00098 864.8
5 0.0024 867.3
10 0.0046 871.3
20 0.0086 878.4
50 0.0185 895.5
100 0.0306 916.3
200 0.0479 945.0
16.522, Space Propulsion Lecture 5
Prof. Manuel Martinez-Sanchez Page 11 of 12
()
+1
2-1
e
*
-1
1
ee
00
A -1 2 1
=
2+1A
PP
1-
γ
γ
γ
γ
γ
??γ
??
γ
??
?? ??
?? ??
?? ??
and so, in vacuum
*
eee e ee e ee
sp *
0
mu +P A u P A u P AFc
I= = = + = +
gggA
mg mg mg
i
ii i
with
()
+1
*
2-1
0*0
*
RM TPA
2
m= ,c = ,T=
T+c
γ
γ
??
γ
??
γ
??
i
Take P
e
/P
0
=0.0012 (about A
e
/A
*
=50).
Using chamber composition (pretty good),
()
()
() ( )
()
41+2x
17 1- x +28 +4x
68 1- x +28 1+2x +12x
96
33
M= = =
41+2x
4 1- x +1+2x+6x 5+4x
1-x + +2x
33
1.986
()( ) ()()
()( ) ()()
θθθ
γ
41+2x
1- x 12.35+2×0.983 + 7.75+2×0.183 +2x 6.6+2×0.367
=
1- x 10.36+1.966 + 5.76+0.366 +2x 4.61+0.734
33
x = 0.4 =1.343→θ
()()()
()()()
θ
γ
41.8
0.6 12.35+1.966×1.343 + 7.75+0.366×1.343 +0.8 6.6+0.734
33
=
0.8 10.36+ _ _ _ +0.6 5.76+ _ _ _ +0.8 4.61+ _ _ _
γ =1.234 (for comparison, at the throat T, γ
*
=1.238)
As a practical matter, one wants T
0
as low as possible if the performance is
acceptable. A very common choice is
sp
x = 0.4 I 250 sec.→ null (ideal)
16.522, Space Propulsion Lecture 5
Prof. Manuel Martinez-Sanchez Page 12 of 12