The research is in three areas which are: (1) techniques were devised for achieving the required levels of helium (He) saturation in liquid propellants (limited to monomethylhydrazine (MMH) and nitrogen tetroxide (NTO)); (2) the values were evaluated for equilibrium solubilities of He in liquid propellants as currently used in the industry; and (3) the He dissolved in liquid propellants were accurately measured. Conclusions drawn from these studies include: (1) Techniques for dissolving He in liquid propellants depending upon the capabilities of the testing facility (Verification of the quantity of gas dissolved is essential); (2) Until greater accuracy is obtained, the equilibrium solubility values of He in MMH and NTO as cited in the Air Force Propellant Handbooks should be accepted as standard (There are still enough uncertainties in the He saturation values to warrant further basic experimental studies); and (3) The manometric measurement of gas volume from a frozen sample of propellant should be the accepted method for gas analysis

Moran, Clifford M.

Yavrouian, A. H.

English

NASA Technical Reports Server

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HELIUM SATURATION OF LIQUID PROPELLANTS 
A. H. Yavrouian
Space Materials Science and Engineering Section

C. M. Moran
Propulsion and Chemical Systems Section 
Jet Propulsion Laboratory 
California Institute of Technology 
Pasadena, California 
ABSTRACT 
The work described in this paper is threefold: 
(1) Devise techniques for achieving the required levels of helium (He) saturation in liquid 
propellants. This paper restricts its discussion to monomethylhydrazine (MXII) and 
nitrogen tetroxide (NTO). 
(2) Evaluate the values for equilibrium solubilities of He in liquid propellants as currently 
used in the industry. 
(3) Accurately measure the He dissolved in liquid propellants. 
Conclusions drawn from these studies include: 
(1) Techniques for dissolving He in liquid propellants depend upon the capabilities 
of the testing facility. Verification of the quantity of gas dissolved is essential. 
(2) Until greater accuracy is obtained, the equilibrium solubility values of He in MMII and NTO 
as cited in the Air Force Propellant Handbooks should be accepted as "standard." There are 
still enough uncertainties in the He saturation values to warrant further basic experimental 
studies. 
(3) The manometric measurement of gas volume from a frozen sample of propellant should be the 
accepted method for gas analysis. 
Approved for public release; 
distribution is unlimited 
The studies described in this paper were carried Out by the Jet Propulsion Laboratory, California 
Institute of Technology, under contract with the National Aeronautics and Space Adininistration.... 
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INTRODUCTION 
The renewed interest in liquid bipropellant systems, pressurized directly by He has resulted 
in the expression of some concerns regarding the role of dissolved He. At JPL the Galileo and 
Mars Observer spacecraft both have He pressurized liquid propellant feed systems. 
This paper details the studies performed by JPL to assure that the propellants used in the 
thruster qualification tests for the above programs had been satisfactorily saturated with He. 
DISCUSSION 
SATURATION TECHNIQUES 
The rate of solution of a gas in a liquid is highly dependent upon the extent of agitation 
and mixing; gas diffusion through an immobile liquid can take months to equilibrate. The 
configuration of the propellant supply tanks at a test facility will determine the most efficient 
saturation technique for its system. JPL has had experience with several saturation techniques. 
Figures 1, 2 and 3 illustrate the saturation of liquids by aeration, bubbling and stirring. These 
mixing techniques combined with a systematic incremental pressurization with He until test 
pressure is achieved can provide satisfactorily saturated propellants. Depending upon the size 
of the run tanks, the saturation procedure may continue for several hours. 
SAMPLING TECHNIQUES 
Samples of propellant for gas analysis should be obtained at the pressure and temperature 
of the holding tanks. The best technique places the sampling cylinder as part of a pressurized, 
recirculating loop. However, many test systems cannot accommodate such a loop. A simpler 
technique is the filling of an evacuated sampling cylinder. If care is taken to allow time for 
redissolving of any He released during the propellant transfer this can be a satisfactory 
technique. 
If the propellant samples are to be shipped to another location for analysis, each sample 
must be partially discharged into another evacuated cylinder. This satisfies the DOT mandate for 
an ullage in cylinders containing propellants. 
REFERENCE VALUES FOR He SATURATION OF PROPELLANTS 
The standard saturation values for He in liquid propellants are usually cited from the Air 
Force Propellant Handbooks (Ref. 1, 2). However, a search of other pertinent literature reveals 
some discrepancies. 
L The Propellant Handbook value (Ref. 2) for He saturated MMH (see Table I) is based 
entirely on the work of E. Chang of the Aerospace Corp. (Ref. 3, 4). Her tests were made at about 
one atmosphere of He pressure. This value, extra polated to 250 psig (to reflect the conditions 
in various propulsion systems), is 0.21 cc STP*/g. An earlier study at the Marquardt Co. (Ref. 
5) yields a value of 0.18 cc STP/g. However, tests at Aerojet General Corp. (Ref. 6, 7) in 1966 
(and cited by I. Smith of WSTF in Ref. 8) produced a value of 0.13 cc STP/g. During saturation 
testing on the Viking propellants, JPL determined a value of 0.12 cc STP/g, and this value was 
used by Messerschinitt-Bolkow-Blohm (MBB) during their recent testing of the Galileo thrusters. 
As noted in the Table II, actual values from recent testing by JPL and MBB lie mainly between 
the high and low numbers. These tests were done in support of Galileo thruster tests at MBB, 
Galileo alternate thruster tests and the Mars Observer thruster qualification program. These are 
single point measurements and considering the experimental conditions during the tests, it is not 
known if a true equilibrium was actually reached. Therefore, these numbers should not be used 
as "theoretical" saturation values. 
The Propellant Handbook value (Ref. 2) for He saturation of NTO is derived from an 
unpublished study at Martin Marietta. This number, 0.32 cc STP/g at 250 psig and 25° C, agrees 
with those from WSTF (Ref. 9), TRW (Ref. 10) and the JPL Viking tests. The Marquardt Co. (Ref. 
* STP = Standard Temperature and Pressure, 0° C and 760 mm Hg, (273 K and 101 kPa). 
350
NTO (cont.) 
5) and Aerospace Corp. (Ref. 3, 11) values are about 25% higher - 0.39 to 0.41 cc STP/g. 
The saturation data cited above came from-tests made at least 20 years ago. A critique of 
each data point would reveal weaknesses in the test techniques. The main weakness being the 
limitations of sensitivity in the pressure measuring devices. Modern manometer sensors have 
sensitivities of one to two orders of magnitude better than those used 20 years ago. Another 
major weakness is the sampling technique used to obtain the liquid propellant samples prior to 
gas analysis. The JPL Viking propellant samples were withdrawn from the storage tanks into sample 
cylinders pressurized with argon. This procedure allows He to escape from solution. The Handbook 
MMH value is almost double that found by JPL. However, the JPL NTO value agreed with the Handbook 
value. 
The best sampling technique was used by WSTF, viz., the sampling cylinder was part of a 
pressurized, recirculating loop. Done correctly, this method should provide valid samples. 
However, their MMH value was 40% lower than the Handbook value. 
The best overall technique for determining saturation value was used by E. Chang at Aerospace 
Corp. An accurately weighed propellant sample was placed in a container of accurately known 
volume. He gas was metered into the container, and the subsequent pressure drop was measured. 
Careful control of the temperature and stirring of the propellants insured uniformity during the 
many test cycles. For each propellant - NTO and MMH - this method yielded saturation values 
higher than those found in any of the other studies. However, these measurements were made at 
about one atmosphere of He pressure and it was necessary to use a linear extrapolation to actual 
propulsion test conditions.
EXPERIMENTAL 
GALILEO 
Saturation. Propellants for use in the Galileo alternate thruster program were stored in 
30-40 gallon run tanks equipped with stirring paddles. The propellants had originally been stored 
in their original shipping containers under dry nitrogen pressure and contained considerable 
dissolved nitrogen. The propellants in the run tanks were "conditioned" prior to saturating by 
pressurizing with He to 450 psi and then- blowing down to 20 psi. This cycle was repeated for 
ten times while stirring the propellants. Subsequent analysis indicated that most of the nitrogen 
had been blown off. Calculations (and later analysis) showed that the NO in the NTO is not 
seriously affected. 
At this point, the stirred propellant was pressurized to 475 psi with He and held at this 
pressure for two hours at ambient temperature. The run tank pressure was reduced to 250 psi, and, 
after a half hour, the propellant was ready for sampling. 
Sampling . Propellant samples were obtained by pressure filling into evacuated 150 cc 
stainless steel cylinders (see Figure 4). After allowing about 10-15 minutes for equilibration, 
the valve to the run tank was closed and the sampling cylinder opened into another evacuated 
cylinder. This step provided adequate ullage (necessary to comply with DOT regulations for 
shipment) without losing any of the dissolved gases. Each set of cylinders (sample and ullage) 
was shipped to JPL (as one sample). The cylinders were weighed before and after filling to 
determine the exact quantity of propellant sampled. 
Gas Measurement. The apparatus for evolving and collecting the dissolved gases is shown in 
Figure 5. The propellant is introduced into the degassing tube and then frozen with LN 2 . The 
evolved gases are pumped (using a mercury-filled toeppler pump) into an accurately known volume. 
The propellant is thawed to free any trapped a gas and then refrozen with LN 2 . This gas is also 
pumped into the measured volume. 
The volume of evolved gas can then be calculated from the pressure and temperature in the 
measured volume. Gas samples are then withdrawn for analysis by mass spectrometry. As seen in 
Table II these values were very close to the measurements made by MBB in their tests of the ION 
engine.
351
MARS OBSERVER 
Saturation. Two sets of run tanks were used for propellant supply - small tanks of about 
20 gallon capacity, and large tanks of 400 gal capacity. None were equipped with stirring 
paddles. In each case the He saturation of the MMH and NTO was accomplished by the stepwise 
addition of He gas through bubbles at the bottom of the tanks. To assure adequate exposure to 
the He and sufficient stirring for uniform mixing of the propellants, the He gas was added in 
increments of about 50 psig for 10 steps (with a 5-10 minute pause between steps) until a total 
pressure of 450 psig was obtained. This pressure was held for a minimum of two hours and then 
lowered to the test pressure of 250 psig. 
Sampling. Samples were taken as before, except that they were not intended for shipment, 
and, were not divided between two sample cylinders. The contents of each cylinder was confirmed 
by weighing. 
Gas Content. The volume of gas dissolved in the liquid propellant was measured as indicated 
above. The propellants in the small run tanks were easily saturated (see Table II). The large 
tank containing NTO oversaturated with He to about 140% of the Handbook value and required at 
least several days to equilibrate. The large tank containing MMII was difficult to saturate and 
a multiported bubbler was finally used to provide adequate agitation during the introduction of 
the He. 
RECOMMENDATIONS FOR FURTHER STUDIES 
Definitive Gas Solubility . The uncertainty in the equilibrium value for the solubility of 
He in MMII, and, to a lesser extent, in NTO warrants a thorough study to establish accurate values 
particularly at thruster test pressure and temperature. Extrapolation of low pressure values to 
10-20 ATM may be valid, but experimental evidence at the higher pressures would be desirable. 
(See also Ref. 1) 
Propellant Sampling Errors. No systematic study has been made of the errors inherent in the 
sampling of liquid propellants under high pressures. The testing done by JPL for GLL and Mars 
Observer were single point measurements, and it is not known if true equilibrium was actually 
attained in all of the tests. Duplicate samples generally yielded gas measurements within ± 10%. 
However, these duplicates were simultaneously obtained and can be considered as one sample divided 
into two. Sampling of propellants by filling evacuated cylinders is straightforward and 
convenient, but it is not known how consistent it is. Repetitive sampling of a single source of 
propellant would yield the information needed to establish the error limits. 
352
REFERENCES 
1. USAF Propellant Handbook, Hydrazine Fuels, Vol. 1, AFRPL-TR-69-149 (1970). 
Prepared by Bell Aerospace Textron. 
2. USAF Propellant Handbook, Nitric Acid/Nitrogen Tetroxide Oxidizers, Vol. II, 
AFRPL-TR-76-76(1977). Prepared by Martin Marietta Corp. 
3. E. T. Chang, et. al., "Solubilities of Gases in Simple and Complex Propellants," 
J. Spacecraft, 6 1177-1180 (1969). 
4. E. T. Chang, et. al., "Thermodynamic Properties of Gases in Propellants II. Solubility of 
Helium, Nitrogen and Argon Gas in Hydrazine, Methylhydrazine, and Unsymmetrical 
Dimethyihydrazine," J. Phys. Chem 72, 638-642 (1968). 
5. D. K. Fulkerson, "A Method for Saturating Propellants with Helium for Testing SSRCS 
Thruster," Marquardt Report no. TM05-A048 (1977) (An analysis of data originally presented 
in TMC Report No. ML2421, dated 1967). 
6. Aerojet-General Corp.," Pressurization Systems Design Guide, Vol. uS, Design Procedures and 
Data," Report No. 2736 (1966). 
7. Aerojet-General Corp., "solubility of Helium and Nitrogen in Liquid Propellants," Report 
No. LM 0696-03-7 (1966). 
8. I. D. Smith, "Determination of the Helium Saturation Level of Monomethlyhydrazine and 
MOM-3 Nitrogen Tetroxide," WSTF Document TR-244-001 (1979). 
9. I. D. Smith, "Determination of Helium Saturation Level of Aerozine-50 and Nitrogen 
Tetroxide," NASA TN D-6249 (1971). 
10. M. P. Gardner, et. al., "Study of Pressurant Gas Solubility in Nitrogen Tetroxide and 
Aerozine 50," TRW Report 01827-6205-R0-00 (1968). 
11. E. T. Chang, "Thermodynamic Properties of Gases in Propellants and Oxidizers I. Solubilities 
of He, N21 02, Ar, and N20 3 in Liquid N204 ," 3. Phys. Chem. jQ, 2394-2399 (1966).
353
Table I. Literature Values for Helium Solubility in Liquid 
Propellants Normalized to 25 C and 250 psig 
cc STP/g	 Reference 
MMH	 0.21'	 Aerospace (3, 4) (Extrapolated value) 
0.21'	 Air Force Handbook (1) 
0.18	 Marquardt (5) 
0.132	 WSTF (8) 
0.132	 Aerojet (6, 7) 
0.12	 JPL Viking Tests 
II9	 0.41	 Aerospace (3, 11) (Extrapolated value) 
0.39	 Marquardt (5) 
0.34	 JPL Viking Tests 
0.33	 WSTF (9) 
0.32	 Air Force Handbook (2) 
0.32	 TRW (10) 
'The MMH data in the Handbook is based entirely on the work of E. Chang at Aerospace Corp. 
2The MMH data in the White Sands report was referenced as being derived from the earlier Aerojet 
report.
Table II. Helium Dissolved in Liquid Propellants at 25CC 
and 250 psig as Measured During Recent Saturation 
Tests for Galileo (GLL) and Mars Observer (MO) Programs 
cc STP/a	 Reference 
KMH	 0.13-0.17'	 MBB/CLL 
0.15-0.17	 JPL/GLL 
Alternate Thruster 
0.16-0.19	 JPL/M0 large tank 
0.22-0.23	 JPL/M0 small run tanks 
NTO	 0.26-0.40'	 MBB/CLL 
0.31	 JPL/GLL 
Alternate Thruster 
0.27-0.28'	 JPL/M0 large tank 
0.29-0.40	 JPL/MO small run tanks 
11488 data normalized to 25 C and 250 psig. 
'Initial MMH saturation tests in the large tank indicated only 20-40% saturation. 
'Initial NTO saturation tests in the large tank indicated 1401 oversaturation. 
354
REGULATED —4-
HELIUM SUPPLY 
PROPELLAI 
RUN TANK 
FROM —4-
PROPELLAI 
STORAGE TANK
PROPELLANT...
HELIUM 
RUN TANK 000000 000000 000000 000000 000000 000000 POROUS 000000 000000 BUBBLE EMITTER 000000 
—P- ULLAGE VENT 
HEAD 
—4- TO ENGINE 
Figure 1. Gas Saturation of Liquids by Aeration 
REGULATED -4- 	 i I - Ii	 —4- ULLAGE VENT HELIUM SUPPLY 
FROM —0-  
PROPELLANT [•X:J	 —4 TO ENGINE 
STORAGE TANK 
Figure 2. Gas Saturation of Liquids by Bubbling
355
REGULATED —Po-
HELIUM SUPPLY 
PROPELLA 
RUN TANK 
FROM -oo-
PROPELLAI 
STORAGE TANK
OUPLING 
lIVEN 
ER 
- ULLAGE VENT 
—4- TO ENGINE 
Figure 3. Gas Saturation of Liquids by Stirring 
FUEL 
RUN 
TANK	 GHE 
FREG 
D
I'
P51 
TEST REG 
ITEM 4
P2 GN2 
S 4
CS 
P6£I 
Si )-I1 
SV1 J\ P 
SCRUBBER G 1 VAC 
FILL
"7S
PUMP 
LINE	 %	 P	 V 2%
Figure 4. Apparatus Sampling of Propellant by Pressure Filling of Cylinders 
356
TURBO 
PUMP 
SECONDARY (MEASURED
COLD 
TRAP 
4-PROPELLANT 
INLET 
Figure 5. Apparatus for Evolving and Collecting Dissolved Gases
357


Helium Saturation of Liquid Propellants

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