Implications of Adhesion Studies for Dust

Comprehensive NASA studies validated the effectiveness of nylon brush in preventing dust migration.

nasa-logo

To quote a study by James R. Gaier and Stephen P. Berkebile, of the Glenn Research Center, Cleveland, Ohio:  ‘NASA/TM—2012-217213 10, from March 2012–AIAA–2012–0875:

‘Implications of Adhesion Studies for Dust Mitigation on Thermal Control Surfaces

Brushing Experiments

Concern was raised during the Apollo program about the effect that lunar dust would have on thermal control surfaces, particularly with regard to the radiators on the LRVs which were used on the last three missions.

A study was published that evaluated the effectiveness of different types of brushes in removing lunar soil and dust. Using soil returned from Apollo 12, they concluded that a nylon bristle brush would be effective to remove dust from the LRV radiator, and return it to nominal function.

But when the brushing was used on the LRV radiators on the lunar surface during Apollo 15, 16, and 17, it was found to be almost wholly ineffective (Ref. 4).

Exactly why there was a discrepancy between the brushing effectiveness during the ground tests and the lunar surface was never explained.

It is presumed that the lunar environment simulation conditions during the tests were not of sufficient fidelity.

One of the priority projects of the NASA’s Exploration Technology Development Program has been to develop dust mitigation technologies to remove dust from thermal control surfaces.

A logical starting place is with the technology used on Apollo, the nylon bristle brush, but using the highest fidelity lunar simulation chamber available, the Lunar Dust Adhesion Belljar (LDAB).

The goal of this study was to quantify the effectiveness of the nylon bristle brush to remove dust from thermal control surfaces in the LDAB as a baseline.

Five different nylon bristle brushes were tested that varied in bristle diameter, bristle length, brush configuration, and bristle packing density.

In addition, four other bristle materials, PTFE Teflon (DuPont), Thunderon (Nihon Sanmo Dyeing Company Ltd.), fiberglass, and carbon fiber were tested for comparison.

These also varied in bristle diameter, bristle length, brush configuration, and bristle packing density.

Preliminary brushing effectiveness tests were run under bench-top conditions, and the most promising candidates were tested in the LDAB. Details of this study have been published previously (Ref. 17), so only the results will be recalled here.

A four stage investigation into the effectiveness of brushing of thermal control surfaces was undertaken.

In Stage 1, strip brushes of three bristle types were used to remove NU-LHT-1D lunar simulant from AZ93 and AgFEP thermal control surfaces under ambient laboratory conditions.

The nylon bristle removed more than 90 percent of the dust and PTFE bristle removed nearly 80 percent of the dust from AZ93, as determined by particle counting.

The Thunderon bristle brush was ineffective. On the AlFEP surface, the Thunderon bristle brush removed more than 90 percent of the dust, and the nylon bristle removed two-thirds of the dust, and the PTFE bristle brush was ineffective.

In Stage 2, none of the brushes proved to be effective under simulated lunar conditions.

A nylon bristle brush was not very effective in restoring the  of thermal control surface on the Apollo LRV, so perhaps it is a validation of the fidelity of our lunar simulation facility and protocol that the brushing was not effective, as opposed to the study of Jacobs (Ref. 2) that indicated otherwise.

In Stage 3 of the investigation, seven additional brushes made up of three materials, two brush designs, and seven bristle lengths were tested for their effectiveness to remove dust from thermal control surfaces under ambient laboratory conditions.

The carbon bristle brush was found to be ineffective, but fiberglass and nylon brushes were found to be equally effective.

Both the fan brush and round brush designs proved to be more effective than the strip brushes tested in the first stage.

Longer bristles were found to be more effective at removing dust than shorter bristles, though the effect seems less important than brush material.

Two brushes, the nylon Escoda fan brush and the round fiberglass Zephyr brush removed more than 90 percent of the dust from AlFEP surfaces under ambient conditions, with as few as 40 strokes.

Both brushes were also able to remove more than 99.5 percent of the dust from AZ-93 thermal control paint with as few as 120 strokes.

In Stage 4 the Zephyr and Escoda brushes were tested for their effectiveness at removing lunar simulant dust from thermal control surface materials under simulated lunar conditions.

Both proved effective, restoring more than 80 percent of the pristine for both thermal control surfaces after 20 strokes, and more than 90 percent after 200 strokes (Figs. 8 and 9).

The Escoda brush performed slightly better than the Zephyr on AgFEP, though perhaps within the error of the experiment.

Although the brushes removed more dust from the AZ93 surfaces than from the AgFEP surfaces, the dusted-then brushed AgFEP surfaces still out performed the dusted-then-brushed AZ93 surfaces, when judged by.

Both brushes were judged effective at removing dust and restoring optical properties.

The effect of environment on the brushing effectiveness is summarized in Table 4, where It is noted that the brushing was consistently more effective in the bench top environment than in the simulated lunar environment of the LDAB.

This can be explained by noting that in Figure 5 that the adhesion between the dust and the surface drops as the pressure increases from 10–10 to 10–4 torr.

The LDAB tests were run in the 10–8 to 10–7 torr range, and the bench top at atmospheric pressure. In addition, the LDAB samples were cleaned in two RF plasmas (air and H-He) and so the surfaces had a much lower level of surface contamination of water and organic contaminants than did the bench top tests.

Figures 8 and 9 show that this also has a large impact on the adhesion. So the adhesion tests identify specifically why testing brushing effectiveness under standard laboratory conditions only gives a lower bound.

That is, brushing effectiveness under these conditions is necessary, but not sufficient to demonstrate effectiveness under lunar conditions.

csp Tue 24Mar2015

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

w

Connecting to %s

%d bloggers like this: