This article -- which expands on material appearing in the July 1999 issue of Orbital Debris Quarterly discusses the rationale for an externally applied patch, requirements influencing patch design, patching procedure and developmental status.

External Repair Rationale
The decision was made to develop a kit for external patching for several reasons: time constraints, accessibility, work envelope, collateral damage and EVA suit compatibility.

A primary risk factor in repairing punctured modules is the time constraint involved. Even given the relatively large volume of air within the Space Station upon assembly completion, analyses have shown that a 1-inch-diameter hole can cause pressure to drop to unacceptable levels in just one hour. In that timeframe, the crew must conclude a module has been punctured, determine its location, remove obstructions restricting access, obtain a repair kit and seal the leak. This action would be a challenge even if the crew was not injured and no significant subsystem damage had occurred.

And in the months before completion of ISS assembly, when the total pressurized volume of the station is much less, depressurization is even more rapid. The same is true -- whatever the timeframe -- for punctures over 1 inch in diameter. With such tight time constraints, it may be wiser for the crew to isolate the damage, retreat to a safe area, stabilize subsystems and allow the damaged module to depressurize.

A second factor is accessibility. Though some ISS modules house standardized racks, which fold down for access to interior pressure module walls, about 30 percent of the interior walls remain inaccessible. Some wall surfaces are blocked behind utility runs in standoffs. In the end cones of certain modules, there are no fold-down racks, so access is even more limited. Others lack the standard racks entirely. In these modules, subsystem and scientific equipment is attached directly to secondary structures, and is not designed to be removed in orbit. In these modules, up to 90 percent of the wall surface is inaccessible.

A third reason to patch externally is to exploit the larger work envelope generally available outside the damaged module. If you fold down a standard rack to get to a hole, the cavity vacated by the rack is only 37 inches wide, 75 inches tall and 40 inches deep. This work envelope can be particularly tight and confining in a pressure-loss situation, when repairs must be made wearing a space suit. Outside the station, however, work envelopes on module surfaces are less restricted, providing good lateral, vertical and depth clearances for repairs.

Despite protective measures designed to protect both structure and crew, there is an inevitable risk of collateral damage received during an impact. A puncture can generate particulate debris within the affected module; this can be hazardous to the crew, whether module repairs are to be done in "shirtsleeves" or a protective space suit. Collateral damage also can cause subsystems to behave erratically or in degraded modes, forcing the crew to stabilize vehicle systems as a first priority. Assessment of collateral damage may require significant time, thereby increasing the likelihood of module depressurization.

A final reason for external repair is that neither the EMU nor the Orlan space suits are designed to operate effectively in depressurized modules. Though Russians in the Orlan suit entered the damaged Spektr module aboard Mir in August 1997, they planned to repair the module externally. Another complication with using an EVA suit inside a depressurized module is the need to depressurize an adjacent module to enter the one that is damaged.

Patching Requirements
There are several requirements for an ideal external patch kit such as KERMIt. These requirements primarily address size, function and compatibility.

Meteoroids and other space debris vary in size, shape, and composition, and the same is true of the holes these objects can produce. Patch size and performance requirements are derived from a study of previous on-orbit impacts and ground-based meteoroid/debris impact simulations.

Thus, patches must be capable of sealing holes up to 4 inches in diameter, and cracks with a maximum length of 8 inches. Damage beyond such limits is highly improbable; it is also significantly more difficult to repair damage exceeded those limits.

An ideal external patch also must be able to seal a hole for a minimum of six months, permitting the crew plenty of time to analyze damage and make more permanent repairs as needed.

Finally, the patch must be compatible with a permanent patch, if the crew determines such a procedure is necessary to restore structural strength to original levels.

Patch Kit Design
Marshall researchers intend the KERMIt Patch Kit to meet these specific requirements. The kit consists of three components: patches, tools and adhesive. The patch design (as illustrated in the July 1999 Orbital Debris Quarterly) is a clear lexan disk with a toroidal seal on one side, a toggle bolt through the center and fittings for injecting adhesive. Several hand tools are provided for surface preparation, hole measurement and marking, and adhesive injection. The adhesive is a white, two-part epoxy glue, packaged in cartridges that snap into the injector like a double-barreled caulking gun.

Repair Operations
The patching operation begins with a crew EVA to locate and examine the leak site on the exterior of the depressurized module. Any damaged debris shields and thermal insulation obstructing the hole must be removed. Surface preparation tools are then used to clean surrounding exposed areas. A special tool is used to determine the size and shape of the hole and to mark reference points. Upon completion of the EVA, the crew uses data on the hole to select properly sized patch components tailored to the size of the damage.

A second EVA is undertaken to deliver the patch, adhesive injector and cartridges to the work site. The toggle bolt is inserted through the hole. A Zipnut on the toggle bolt is tightened, compressing the toroidal seal against the damaged module wall. Next, the adhesive is injected into fittings on the clear disc, filling the cavity formed by the disc, ring and punctured wall. When the cavity is filled, the injector is removed. The adhesive cures, forming a cast plug that seals the hole. Curing takes two to seven days. Afterward, the module may be repressurized in stages to verify proper function of the seal.

Development Activities
Development and testing of the KERMIt Patch Kit is underway at the Marshall Center. Extensive leak tests have been done to assure the patch can hold a one-atmosphere pressure differential. In September 1998, KC-135 tests were conducted comparing adhesive flow in reduced gravity with one gravity flow.

The kit underwent a Preliminary Design Review in February 1999, and in June, Marshall conducted tests in the Neutral Buoyancy Laboratory to examine the adequacy of crew interfaces. A six-month life test of the patch is expected to be conducted in coming months. The operational patch kit is slated to be delivered in September 2000.

From October 1999, Vol. 4 Issue 4, of Orbital Debris Quarterly News.