Calculations of Hydrazine Thruster Contamination for the Genesis Solar Wind Collectors

Eileen Stansbery¹, Benton C. Clark², and Timothy J. Girard²

August 1997

This is a trade study, conducted during the Genesis Discovery 5 Feasibility Study, with the goal of determining whether cold gas was needed during solar wind collection to assure sample surface cleanliness, or whether hydrazine could be used.

Studies of thruster exhaust plumes indicate that thrusters scatter a very small fraction of the ejected mass at angles greater than 90° off of the thruster centerline. Typically, the amount of mass ejected at higher angles is less than one part in 10³, but this is dependent on the specific thruster design. On-orbit measurements and laboratory tests indicate that hydrazine exhaust does not collect on surfaces warmer than about -45°C. Consequently, deposition from hydrazine thruster plumes will not be a problem for most non-cryogenic surfaces. Since the contamination levels we are interested in are generally below the level of resolution of previous measurements we have undertaken this assessment to verify that contamination from the station-keeping thrusters is insignificant.

Station-keeping thrusters are a potential source of contamination to the collectors during the mission. There are four potential issues: (1) what is a reasonable thruster plume profile? (2) what is the exhaust composition of ultra-pure hydrazine? (3) what is a reasonable sticking fraction for the effluent? (4) what design features can be used to further minimize thruster impact to the collectors? To address these questions we have performed significant analyses during the feasibility study.

Note that because thrusters are well out of the line of sight of the collection surfaces, this is a worst-case study which actually determines the contamination to the back sides of the collection areas. Our conclusion is that even contamination to the back sides meets our requirements.

Plume Profile

The most important input to thruster contamination analyses is the thruster plume profile. There are few good plume models for the backflow region of small thrusters. The profile chosen for this assessment corresponds to the Rocket Research Co. MR-103 (0.2 lb_f thruster) RAMP2/SFPGEN calculated density as a function of angle from plume centerline for various axial distances (figure 1). As expected, the density fraction in the backflow region increases as you get further away from the nozzle (molecular spread). Since the plume profile calculations were not extended beyond 155°, we have taken the conservative assumption of using the same density fraction at 180° as calculated at 155°.

Exhaust Composition

The thrusters operate by the catalytic decomposition of hydrazine (N₂H₄) into ammonia (NH₃), nitrogen (N₂), and hydrogen (H₂). Cold starts, pulsed operations, aged catalyst beds, or high fuel flow rates can cause ammonia dissociation to vary from 2 to 80%. In addition, there can be an expulsion of 0.4 to 5% unburned fuel. The following table shows typical monopropellant thruster exhaust compositions and the associated species condensation temperatures (at which the bulk evaporation rate is 10^-7 g/cm²/sec).

Sticking Fraction

Not all of the molecules which impinge on the collector surfaces will remain there. Unfortunately the theory of molecular sticking is not well developed. The sticking fraction for return flux is usually calculated from a simple temperature relationship derived from limited materials testing. This relationship has the form (T_j - T_i)/200, where T_j = source temperature and T_i = surface temperature. However, the collision processes and their impact on the ability of the contaminant molecule to adhere must be accounted for. Persistent sticking fractions can also be calculated by dividing measured sensor mass deposition by total engine plume mass flow delivered to the sensor location (calculated using plume models such as that discussed above). The sensitivity of any technique used to measure sticking fraction is limited by the signal to noise ratio, the total mass flow in the plume gases over the sensor, and the resolution of the sensor. Most sticking fractions are measured with larger engines, cooler sensors, and dirtier thrust gases than are appropriate for the Genesis mission. Bipropellant exhaust constitutes a larger contamination concern than hydrazine monopropellant. Measured sticking fractions for the MIR 13 kg and the Shuttle PRCS engines are 0.006±0.05 % and 0.007±0.006 % respectively.

Existing test data is inadequate to establish a general relationship for the persistent sticking fraction. It is, therefore, necessary to select a value for this parameter based upon scientific judgment of the analysis being conducted. If the deposition sensitive surface operates at warm temperatures (>50°C), the sticking coefficient will be relatively low (i.e., < 0.1). Table B gives the sticking fractions used for this analysis.

Analysis

In determining the contamination associated with the ultra-pure hydrazine thrusters I have assumed a fuel burn of 20 grams for the short, daily attitude control burns and 100 grams for the longer monthly state adjustment burns.

Analysis Geometry

Four specific geometric cases were analyzed corresponding to the minimum distances for:

1. axial thruster to exposed array (95 cm, 157°)
2. axial thruster to array inside canister lid (131 cm, 122°)
3. roll thruster to exposed array (82 cm, 111°)
4. roll thruster to array inside canister lid (121 cm, 84°)

As examples of the geometry, figure 3 below shows the distances and angles associated with the axial thrusters.

Determine Fluence

The Fluence (moles/cm²) as a function of distance and angle was determined by taking the plume density profile as function of angle and properly normalizing the spread of the exhaust density over solid angles.

where

N is the normalization factor

n_hydrazine is the moles of hydrazine used

p(Ø,D) is the plume density profile

Attitude Control Burn fluence data

State Adjustment Burn fluence data

Determine Contamination

The amount of contamination of any particular exhaust specie impinging on the collectors is assessed by taking the fluence and multiplying it by the exhaust specie mole fraction, Avogadro`s number, the atoms per molecule (not counting H) for each specie, the sticking coefficient, shield factor, and back-scatter factor. The shield factor refers to physical barriers on the spacecraft between the thruster and the wafer. The back-scatter factor is credit taken for molecules needing to turn a corner to reach the "top" of the wafer (molecules must reverse direction to hit the sensitive collector surface). For this assessment we have assumed no benefit associated with either the inherent spacecraft shielding or the fact that the sensitive surface is facing away from the field of view between thruster and collector. This assessment, therefore, calculates the contamination that would form on the backsides of the collector surfaces if there were no shielding from the spacecraft between the thruster and the collectors. This is an extremely conservative assumption. The contamination that would be expected to form on the sensitive surfaces of the collectors would be much less than the calculated values.

Results

Attitude Control Burns

The results of the calculations for the 20 gram daily attitude control firings result in contamination estimates varying from 10⁹ to 10¹³ atoms/cm².

The maximum allowable contamination for CNO is 10¹⁵ atoms/cm². The maximum contamination for Fe is assumed to be the calculated 2-year solar wind fluence of 1.7x10¹² atoms/cm². As you can see from the table below, the contamination associated with attitude control is insignificant.

State Adjustment Burns

The results of the calculations for the 100 gram monthly state adjustment firings result in contamination estimates varying from 10⁹ to 10¹⁴ atoms/cm².

As stated previously, the maximum allowable contamination for CNO is 10¹⁵ atoms/cm². The maximum contamination for Fe is assumed to be the calculated 2-year solar wind fluence of 1.7x10¹² atoms/cm². As you can see from the table below, the contamination associated with station adjustment is insignificant except for the calculations of Fe when using the roll thrusters. However, the roll thrusters are not used for the majority of the state adjustment burn and as such is not deemed significant.

Design Considerations

To add further conservatism beyond the worst-case analysis, the spacecraft design was modified during the Feasibility Study such that the thrusters are mounted under the equipment deck rather than on a boom to the solar arrays. The reconfigured flight system is shown in figure 4. In this configuration we have inherent shielding of the sensitive surfaces from the thruster plume by the spacecraft equipment deck. We will continue with further contamination analyses in Phase B.

References

Carre, D.J., and Hall, D.F., "Contamination Measurements during Operations of Hydrazine Thrusters on the P78-2 (SCATHA) Satellite," J. Spacecraft, Vol. 20, no. 5, p.444, 1983.

Chen, P., and Thompson S., personal communication, July, 1997.

Etheridge, F.G., Garrard, G.G., and Ramirez, P., "Plume Contamination Measurements," Rockwell International, SSD84-0073, June 1984.

Koontz S. (1996) Neutral External Contamination Quick Look Report: DTO-829 (STS-74) Plume Impingement Contamination, Space Environments and Effects Related NASA Publications, 1996.

1. NASA - Johnson Space Center, Houston, TX 77058
2. Lockheed Martin Astronautics, Denver, CO

Other Contamination Documents:

• Contamination Control Plan
• Phase A Contamination Presentation
• Spacecraft Outgassing Contamination Analysis
• Micrometeorite Background