Carbon Fiber versus Titanium | Rabbi Yirmiyohu Kaganoff | Beit Midrash

Torah Portion and Tanach
Para

קטגוריה משנית

133

Question #1: Interruptions
When is it good to interrupt?

Question #2: Flying through Air
Is an entertainer swinging from a flying trapeze an ohel?

Question #3: Ohel Zaruk
Why would anyone throw a tent?

Question #4: Carbon Fiber versus Titanium
What difference does it make, halachically, whether an airplane is manufactured from aluminum, titanium or carbon fiber?

Foreword:
The laws of tumas ohel, defined below, are taught at the beginning of parshas Chukas, which we read this Shabbos as our maftir, parshas Parah. Technically, these laws are not germane exclusively to kohanim, but are a subset of the laws of tumas meis, the laws of tumah that result from contact with a dead person, which apply to all people. However, since we are all currently tamei meis, and without parah adumah we cannot become tahor again, the laws of tumas meis primarily affect kohanim. I say primarily because, in fact, everyone is required to make sure that a kohein not become tamei. So, for this reason, we should all be fully familiar with the laws of tumas meis. For our purposes, I will subdivide the laws of tumas meis into four general categories:

1. Maga -- touch
This is tumah spread through physical, tactile contact. This method of spreading tumah is not unique to tumas meis, but applies to virtually all tamei sources, including neveilah (dead animals), sheratzim (certain varieties of dead, small creatures) and people who contract tumah (see Keilim 1:1). However, there is no prohibition for a kohein to become tamei because of either neveilah or sheratzim, and, therefore, the laws of these tumos are, for the most part, not that applicable until we again have korbanos, the Beis Hamikdash and the ashes of parah adumah.

2. Masa -- lifting
Tumas masa is generated when a person lifts a tamei item. This is also not limited to tumas meis, but applies to most varieties of tumah (see Keilim 1:2). Perhaps the most common case today of becoming tamei through tumas neveilah is someone who lifts or moves a non-kosher piece of meat in a supermarket. Since the animal died without the benefit of shechitah, the meat is neveilah and therefore tamei. Someone who moves the neveilah becomes tamei, even if he did not touch the meat itself, but only lifted or moved the package.

3. Ohel – "under cover"
Ohel literally means tent, but tumas ohel means tumah from a meis that spreads underneath an extended roofed area and thereby conveys tumah to any person or vessel that is also under the extended ohel area. This will be the main topic of this article.

4. Other related tumah considerations
There are various other categories of tumas meis, such as golel, dofek, kever, kever sasum, and cherev harei hu kechalal, each of which has its own, highly detailed laws that I will not be discussing in this article. Most of these -- golel, dofek, kever, and kever sasum -- concern either parts of a grave, or different methods of burial. Cherev harei hu kechalal is a type of tumas meis conveyed via items (according to many rishonim, only metal items) that, themselves, contracted tumah via a meis. Most rishonim rule that the prohibition of a kohein contracting tumas meis does not include coming in contact with cherev harei hu kechalal (see Tosafos, Nazir 54b).

Ohel
Although the word ohel translates as "tent," or "roof," tumas ohel has much broader connotations. Tumas ohel is conveyed via almost any item that covers at least a tefach (about three inches) cubed, regardless of how high it is above the meis or above the kohein. A ledge of a building, an umbrella, or a branch that is a tefach wide and overhangs a grave or corpse conveys tumah onto anyone or any vessel susceptible to tumah positioned directly beneath the ohel. Tumas ohel spreads from one ohel area to any other ohel that overlaps or connects, even if the different ohel "roofs" are of very different heights. It also spreads from one area to another adjacent area through an open door, window or other break in a wall, even if it is as small as a tefach by a tefach. Thus, a series of overlapping or connecting roofs, ledges, caves, umbrellas, tree branches or even people, can create a continuous ohel that transfers tumah for great distances. Indeed, that which appears to be separate buildings or structures may be one large ohel connected by open doors and windows (under certain circumstances, even through closed ones), ledges or tunnels, and tumah in one building may spread across an entire complex of buildings. This is particularly common in hospitals, museums, shopping malls, university campuses, subway systems and airport terminals, where human remains in one building may spread tumah throughout the entire complex or airport -- notwithstanding that those complexes appear to be several separate buildings -- via interconnecting tunnels or other passageways.

An airplane that is partly over a grave or meis and partly over a branch, umbrella or person will also convey tumas ohel. We will soon discuss if this is true only if when the airplane is stationary or even if it is in flight.

In the modern world, numerous teshuvos have been published discussing whether tumas meis extends to an entire train or vehicle, when part of it passes through a cemetery or under a tree that overhangs a cemetery (see, for example, Shu’t Maharam Schick, Yoreh Deah #353; Shu’t Birchas Retzei #12; Shu’t Melamed Leho’il 2:133 and in many more recent publications). Responsa concerning whether a kohein may fly in an airplane whose route takes it over graves or cemeteries appeared as early as the 1930’s, in the very infancy of commercial air travel.

Many common situations can create a halachic problem for a kohein, because of the laws of tumas ohel. For example: carrying human remains into an airport terminal or medical facility that connects to a subway station could convey tumah throughout the entire subway system and prohibit any kohein from remaining anywhere in the subway, since the entire system qualifies as one large ohel. Therefore, someone dying in a Bronx subway station contaminates a kohein awaiting his commuter train in Penn Station! These more complicated ohel situations can be easily rectified during construction or refurbishing of the buildings – however, they require input of a knowledgable expert in these matters to explain how to avoid the problems. There are hospitals in Israel in which these tumah problems were rectified, because care was taken during renovation to consult rabbinic authorities how to remedy the problem.

This article will be discussing tumas ohel as spread through keilim, which I will translate loosely, but not that accurately, as "vessels," and an important concept of tumas ohel called chatzitzah, blocking or interrupting tumah.

Blocking tumah
Although tumas meis spreads throughout the building in which it exists, it usually does not spread through the ceiling of the room in which it is located. These halachos are derived from the posuk in parshas Chukas (19:14) that implies that, although tumah spreads under and throughout the roofed area in which it is currently found, it is blocked from spreading above, below, or outside that ohel area. A ceiling usually is a barrier blocking tumah from spreading (Ohalos, Chapter 9).

There are three ways to provide a barrier to block tumah:

1. An item situated directly above the tumah might block tumah from penetrating above and through it.

2. An item situated directly below the tumah might block the tumah from penetrating below and through it.

3. Closing an opening in a room or building, thus preventing tumah from moving laterally from one roofed area to an adjacent roofed area.

What blocks tumah
As a rule of thumb, anything that is not mekabel, susceptible to, tumah will be able to block tumah. What materials are mekabeil tumah? There are several categories of utensils (defined here as receptacles that can contain an item) depending on the type of material of which they are manufactured. For our purposes in this article, we will discuss three categories:

A. Never mekablei tumah
Materials that do not become tamei. Indeed, there are many such materials. In the time of the Mishnah, these included most unfired vessels made of earth, and those made of stone. According to many authorities, today these would include vessels made of plastic materials and, potentially, might include materials made of carbon fiber or fiberglass.

B. Always mekablei tumah
There are materials that become tamei when they are complete utensils, regardless of their size. In general, metal items, or at least those made of the six metals mentioned in the Torah as susceptible to tumah – gold, silver, copper, iron, tin and lead (Bamidbar 31, 22) are always mekabeil tumah. Steel, the most common metal used today in manufacture, is predominantly iron, and the Mishnah implies that an alloy has the halachic status of its majority constituent (Keilim 11:4). Thus, although there are hundreds of steel alloys containing a wide variety of other components, for halachic purposes, steel is iron. Similarly, both bronze, an alloy of predominantly copper and tin, and brass, an alloy of predominantly copper and zinc, are halachically treated as copper; pewter, an alloy of predominantly tin with either lead or antimony, is treated as tin. Therefore, items made of steel, bronze, brass or pewter all become tamei and do not block tumah.

C. Depends on size
This category consists of materials that become tamei when they are manufactured into small vessels, but do not become tamei when manufactured into large vessels, which are not meant to be moved when full. For these purposes, a "large vessel" is defined as one that can hold sixty se’ah, which, by my estimate, is between 150-250 gallons. (For comparison purposes, a standard wine barrel holds 31.5 gallons.) This category includes wood and most natural cloth.

Interruptions
At this point, we are in a position to appreciate our opening question: "When is it good to interrupt?"

The answer is when we are interrupting tumah, i.e., blocking tumah so that an adjacent area will not be forbidden for kohanim to enter. In this case, interrupting is definitely a welcome action. A vessel made from material in category A, or a large item in category C, can serve as a tumah blocker.

With the greatest of ease
Does the daring young entertainer swinging from a flying trapeze qualify as an ohel?
The Mishnah states: "The following items neither convey tumah nor block it… someone jumping from one spot to another, a bird flying overhead, a garment fluttering in the breeze, or a boat sailing on the water" (Ohalos 8:5). The reason why tumah does not spread underneath the person, bird, garment or boat is because it is not at rest, unlike an ohel (Sefer Hayashar #275). Thus, the daring young man on the flying trapeze does not qualify either as an ohel to convey tumah or as an interrupter to block it. (Of course, this is relevant only if he is flying outdoors on his trapeze, and the meis is not underneath any other ohel. Otherwise, the "big tent" conveys tumas ohel.)

However, this is only if the item is not at rest. Should you tie down the garment or chain the boat in place, it becomes an ohel and spreads tumah underneath itself and contaminates anything both above and below itself (see Ohalos, Chapter 9).

Ohel zaruk
Let us now explore the third of our opening questions: Why would anyone throw a tent?
Allow me to introduce a concept called ohel zaruk, which literally translates as a "thrown tent," and is the subject of a dispute between the tana’im, Rebbi and Rabbi Yosi berabbi Yehudah. Rebbi asserts that an ohel zaruk, a moving ohel, such as a large cabinet being transported by animals, does not block tumah, whereas Rabbi Yosi berabbi Yehudah rules that it does (Eruvin 30b; Chagigah 25a; Nazir 55a; Gittin 8b). Their dispute applies in the case of a large vessel [category C above], which is not mekabeil tumah, and therefore can potentially block tumah. When such a vessel is stationary, all agree that it blocks tumah; the dispute between Rebbi and Rabbi Yosi berabbi Yehudah concerns whether it blocks tumah while moving.

According to several early acharonim, this dispute is only a rabbinic issue. In the opinion of these poskim (Shu’t Shevus Yaakov, Yoreh Deah 1:85 and 2:88, Penei Yehoshua, Sukkah 21a s.v. Uve’ikar), all tana’im agree that, min haTorah, an ohel zaruk blocks tumah. The dispute between Rebbi and Rabbi Yosi berabbi Yehudah is whether Chazal made a takanah that ohel zaruk does not block tumah, Rabbi Yosi berabbi Yehudah contending that they did and Rebbi contending that they did not.

Aluminum, titanium, zinc and chrome
The entire discussion regarding whether airplanes can block tumah is only if we assume that they are not mekablei tumah (see Ohalos 2:1). To clarify this topic, we need to analyze yet another major issue. What is the halachic status, in respect to the laws of tumah and taharah, of metals that have been discovered or rendered practically useful since the times of Chazal, including zinc, chrome, manganese, nickel, magnesium, platinum, aluminum, titanium and many others? The Tiferes Yisroel assumes that they have the same halachic status as the six metals mentioned in the Torah, and therefore they are mekablei tumah min haTorah (Yevakeish Daas #44). As such, they could never block tumah, as explained above.

However, there are poskim who dispute this conclusion of the Tiferes Yisroel and contend that only the six types of metal that the Torah mentions are mekabeil tumah, and not any of the newly discovered ones (Shu’t Igros Moshe, Yoreh Deah 2:164; Sefer Tevilas Keilim page 243). We should also note that Rav Avraham Shaag, the rebbi of Rav Yosef Chayim Sonnenfeld, seems to hold that all these materials will be mekablei tumah miderabbanan, which would preclude their blocking tumah (Shu’t Ohel Avraham #24).

The primary metals used for airplane manufacture today are aluminum and titanium. Only small amounts of steel are used, since it is very heavy. Most of our readers are familiar somewhat with steel and aluminum, but not with titanium, which is almost as strong as steel, but much lighter, and is resistant to heat and corrosion. The Lockheed SR-71 Blackbird, the world’s fastest jet-propelled aircraft, is made of titanium. The Boeing 747 is made predominantly of aluminum. Newer aircraft are being made from composite materials, such as graphite-epoxy, also called carbon fiber, which are very strong, but much lighter than titanium or aluminum. More than half of the materials used to make the Boeing 787 Dreamliner are carbon fibers.

Carbon fiber versus titanium
At this point, it is appropriate to discuss the last of our opening questions: "What difference does it make, halachically, whether an airplane is manufactured from aluminum, titanium or carbon fiber?

Assuming that we rule that the entire airplane is considered one item for kabalas tumah purposes, and that 51% of the component materials of an airplane determine whether it is mekabeil tumah or not (see Keilim 11:4, see also Keilim 13:6), a Dreamliner manufactured from carbon fiber might have more potential resolutions to our halachic issues of blocking tumah than a plane manufactured from titanium or aluminum. However, since I am aware that there are rabbonim who dispute my assumptions, I will simply instruct our kohein to ask the question of his posek.

Conclusion
Although it is beyond our ability to fathom the reasons for the mitzvos, we can and should attempt to glean a taste of Hashem’s mitzvos, in order to grow from the experience of observing them. Thus, it behooves us to attempt to explain why, under normal circumstances, the Torah bans a kohein from having contact with a meis. Rav Hirsch, in his commentary on Vayikra 21:5, provides us with a beautiful insight into this mitzvah. In most religions, fear of death and what happens afterward are the major "selling points." Thus, the role of the priest is most important when dealing with death. However, the Torah’s focus is how to live like a Jew—to learn Torah and perform mitzvos, and devote our energies to developing ourselves in Hashem’s image. To emphasize that the Torah is the blueprint of perfect living, the kohein, who is the nation’s teacher, is excluded from anything to do with death. The kohein’s role is to imbue us with the knowledge and enthusiasm to live!!

This Shiur is published also at Rabbi Kaganof's site