Big Ivan, The Tsar Bomba (“King of Bombs”)

The World's Largest Nuclear Weapon

 

The device offically designated RDS-220, known to its designers as Big Ivan, and nicknamed in the west Tsar Bomba (and referred to as the Big Bomb by Sakharov in his Memoirs [Sakharov 1990]) was the largest nuclear weapon ever constructed or detonated. This three stage weapon was actually a 100 megaton bomb design, but the uranium fusion stage tamper of the tertiary (and possibly the secondary) stage(s) was replaced by one(s) made of lead. This reduced the yield by 50% by eliminating the fast fissioning of the uranium tamper by the fusion neutrons, and eliminated 97% of the fallout (1.5 megatons of fission, instead of about 51.5 Mt), yet still proved the full yield design. The result was the "cleanest" weapon ever tested with 97% of the energy coming from fusion reactions. The effect of this bomb at full yield on global fallout would have been tremendous. It would have increased the world's total fission fallout since the invention of the atomic bomb by 25%.
The nickname Tsar Bomba is a reference to a famous Russian tradition for making gigantic artifacts for show. The world's largest bell (the Tsar Kolokol) and cannon (the Tsar Pushka) are on display at the Kremlin [Kalinin 1994; pg. 33]. Having come to power by overthowing and assassinating the last royal family of Russia, the Soviet leadership would never have countenanced such a royalist name, but this designation has become popular in Russia since the collapse of the Soviet Union.
The test was conducted by air dropping the bomb from a specially modified Tu-95N "Bear A" strategic bomber piloted by mission commander Major Andrei E. Durnovtsev. It was released at 10,500 meters, and made a parachute retarded descent to 4000 meters in 188 seconds before detonation. By that time the release bomber was already in the safe zone about 45 km away. The drop area was over land at the Mityushikha Bay test site, on the west coast of Novaya Zemlya Island, above test field D-2, near Cape Sukhoy Nos. [Podvig et al 2001; pp. 466, 498], [Khalturin et al 2005]. Durnovtsev was immediately promoted to lieutenant colonel and made Hero of the Soviet Union. The Tu-95 was accompanied by a Tu-16 "Badger" airborne laboratory to observe and record the test. The time of the test is given by [Adamsky and Smirnov 1998] as 11:32 AM Moscow Time; it is listed in [Podvig et al 2001; pg. 498] as occurring at 06:33 Moscow Decree time.
The test location was about 55 km north of the Severny settlement and 250 km north of the headquarters at Belushya, from where it was observed by the State Commission. The bomb design team and the test supervisors, headed by Major General Nikolai Pavlov, Chairman of the State Commission, monitored the test at the airfield near Olenya station on the Kola Peninsula 1000 km away. Observers were also at many other locations. Among these were Soviet Minister of Medium Machine Building Efim Slavsky and Marshal of the Soviet Union Kirill Moskalenko, deputies to the 22nd Congress of the CPSU then in session, who had arrived by plane on the day of the test to observe the explosion. They observed the test aboard an Il-14 "crate" at a distance of several hundred kilometers from ground zero. Sakharov himself stayed by the phone, presumably at Arzamas-16, waiting for a call from Maj. Gen. Pavlov.
The effects were spectacular. Despite the very substantial burst height of 4,000 m (13,000 ft) the vast fireball reached down to the Earth, and swelled upward to nearly the height of the release plane. The blast pressure below the burst point was 300 PSI, six times the peak pressure experienced at Hiroshima. The flash of light was so bright that it was visible at a distance of 1,000 kilometers, despite cloudy skies. One participant in the test saw a bright flash through dark goggles and felt the effects of a thermal pulse even at a distance of 270 km. One cameraman recalled:

The clouds beneath the aircraft and in the distance were lit up by the powerful flash. The sea of light spread under the hatch and even clouds began to glow and became transparent. At that moment, our aircraft emerged from between two cloud layers and down below in the gap a huge bright orange ball was emerging. The ball was powerful and arrogant like Jupiter. Slowly and silently it crept upwards.... Having broken through the thick layer of clouds it kept growing. It seemed to suck the whole earth into it. The spectacle was fantastic, unreal, supernatural.

Another observer, farther away, described what he witnessed as:

... a powerful white flash over the horizon and after a long period of time he heard a remote, indistinct and heavy blow, as if the earth has been killed!

A shock wave in air was observed at Dickson settlement at 700 km; windowpanes were partially broken to distances of 900 km. All buildings in Severny (both wooden and brick), at a distance of 55 km, were completely destroyed. In districts hundreds of kilometers from ground zero, wooden houses were destroyed, and stone ones lost their roofs, windows and doors; and radio communications were interrupted for almost one hour. The atmospheric disturbance generated by the explosion orbited the earth three times. A gigantic mushroom cloud rose as high as 64 kilometers (210,000 ft). Despite being exploded in the atmosphere, it generated substantial seismic signals. According to a bulletin of the U.S. Geological Survey it had seismic magnitude mb = 5.0 to 5.25. The blast wave was detected circling the world.[Khalturin et al 2005]
Some time after the explosion, photographs were taken of ground zero. "The ground surface of the island has been levelled, swept and licked so that it looks like a skating rink," a witness reported. "The same goes for rocks. The snow has melted and their sides and edges are shiny. There is not a trace of unevenness in the ground.... Everything in this area has been swept clean, scoured, melted and blown away."
[Adamsky and Smirnov 1998]
The radio blackout created by ionization from the explosion gave immediate indication to the command post on the Kola Peninsula that the explosion had occurred, but kept them from receiving any reports on the degree of success, or the fate of the bomber and the Tu-16 "Badger" airborne laboratory accompanying it for 40 minutes. Only when radio contact with Novaya Zemlya was reestablished were they able to request information on the altitude of the cloud, and it became clear that the bomb had worked as designed.
The Tu-95 was painted with a special white reflective paint to protect it from the thermal radiation of the fireball. The airborne laboratory plane was also covered with the same paint. In clear air, the 50 Mt test was capable in principle of inflicting third degree burns at a distance of up to 100 km.
The area of effectively complete destruction extended to 25 km, and ordinary houses would be subjected to severe damage out to 35 km. The destruction and damage of buildings occurred sporadically at much greater ranges than this due to the effects of atmospheric focusing, an unpredictable but unavoidable phenomenon with very large atmospheric explosions that is capable of generating localized regions of destructive blast pressure at great distances (even exceeding 1000 km).

Origin, Development, and Test Preparations

Like the entire 1961 test series in which it was conducted, the creation of the Tsar Bomba was the result of political calculation by the Soviet leadership, especially of Premier Nikita Khrushchev. A de facto moratorium had existed between the U.S., USSR and UK since the conclusion of the last U.S. and Soviet test series in 1958, and two years of discussion had been conducted regarding formal limitations on nuclear testing. But the Cold War continued at high pitch, with the occasional reductions in tension being only partial and transitory phenomena. Many high-stakes cards remained to be played by the Soviets - the erection of the Berlin Wall and the deployment of missiles to Cuba being notable examples. The decision to break the moratorium with a "testing spectacular" that coincided with the Twenty Second Congress of the Communist Party of the Soviet Union was a move cast in the same mold.
The Soviet weapons scientists had spent the three years since the last test series in 1958 developing new concepts and refining old ones, but they had not been preparing for a new test series per se until Khrushchev called a meeting with the "atomic scientists" - the leaders of the weapons program - on 10 July 1961. There was no discussion of whether more tests were necessary or desirable, which Sakharov, the senior weapon designer, very much doubted. Khrushchev simply began the meeting with a speech declaring that tests would resume in the fall to 'show the imperialists what we could do', a decision that came as a surprise to the scientists present. Khrushchev specifically cited as the primary motivation a political rather than a technical justification - his view that the international situation was deteriorating [Sakharov 1990, pg. 215]. From there on until the end of the test series it was an all-out effort to ready as many designs, concepts, and devices for testing as possible.
Available sources do not make it clear where the idea of the 100 megaton device test originated. Sakharov does not mention this device being proposed at the 10 July meeting, but first refers to it in connection with a mid-August review: "Khrushchev was already familiar with the test program, and in particular with our plan to explode a device of record-breaking power", implying that the idea of this test spectacular originated with the weapons team [Sakharov 1990, pg. 218]. Comments by Reed and Kramish [Reed and Kramish 1996] conversely indicate that the development and test of this device was a directive from Khrushchev at the July meeting. The detailed account by Adamsky and Smirnov [Adamsky and Smirnov 1998] do not address this at all. They do state that the development of the device began in the middle of July (i.e. immediately after the meeting) and that "We knew that the culmination of the series of tests planned in the USSR would be the explosion of the 50-Mt device, which was designed to produce explosions of up to 100 megatons" but do not indicate how they came to know this.
There was no previously existing military requirement for a 100 megaton weapon - such weapons are virtually useless for military purposes. The Soviet Union had only one delivery system capable of carrying a weapon of this size - a handful of the relatively slow prop-driven Tu-95 bomber - and it was incapable of intercontinental range with a payload this large. A 100 Mt weapon can level urban areas in a zone 60 km wide, cause heavy damage in a zone 100 km across, cause 3rd degree burns in a region 170 km across (only a bit smaller than the width of West Germany) and eye damage to 220 km. Such a weapon can only be used as a means of destroying an entire urban region - a major urban complex including suburbs and even neighboring cities. This scale of destruction is much larger than any discrete urban area in Western Europe. With its dense settlement, use of such a weapon in Europe is equivalent to an attack on a major portion of an entire nation and its population. Fallout from a low altitude or surface burst in central England could produce lethal exposures extending into the Warsaw Pact nations; a similar explosion in West Germany could create lethal fallout as far as the Soviet border. Even in the United States there were only three urban regions at that time large enough to conceivably merit attack with such a weapon - New York, Chicago, and Los Angeles. On any smaller target it would be simple overkill. Even if the Tu-95 were able to reach Chicago, the closest plausible U.S. target, (which is doubtful given the enormous payload, far in excess of normal for long-range missions, and the added drag from the belly bulge required to house the bomb) it would have been detected crossing the North American early warning line and then been over U.S. and Canadian territory for 8 hours - ample time for jet fighters to intercept and shoot it down [Zaloga 1993].
Since preparation of the 100 megaton bomb only began after the 10 July meeting at which Khrushchev ordered the test series be held, no more than 112 days elapsed from initial concept to detonation - exactly 16 weeks.
Upon returning to Arzamas-16, the secret nuclear weapons laboratory in the Urals, after the meeting Sakharov selected a team to develop the 100 megaton device. He included Viktor Adamsky, Yuri N. Babaev, Yuri Trutnev, and the newly arrived Yuri Smirnov, then 24 years old ([Adamsky and Smirnov 1998], [Khariton 1993]). Sakharov indicates that the lead responsibility for the project lay with Adamsky and V.P. Feodoritov [Sakharov 1990, pg. 220].
Every aspect of the development was rushed. The mathematical analysis normally conducted by the Soviet weapon scientists for a new thermonuclear weapon design was skipped, substituting estimates and approximations of various kinds. This created uncertainties about the system performance that cropped up late in the preparations - leading to eleventh hour doubts, and last minute design modifications even while assembly was underway.

By the mid-August review, held after 13 August (Sakharov states that is was 'after the Berlin Wall had been built') and thus after about 4 weeks of work, Sakharov had decided to test a reduced yield "clean" version of the device with a yield of 50 megatons. At this review Khrushchev said that he had already disclosed the planned test of this device to visiting dignitaries from the U.S.. Khrushchev identified the dignitary as an unidentified U.S. senator (and his grown daughter), but Sakharov speculates that it was actually presidential adviser John McCloy [Sakharov 1990, pg. 218].
Khrushchev went public regarding the planned superbomb test with the announcement of the new test series issued simultaneously with the first shot fired on 1 September 1961 [Time 1961], [Adamsky and Smirnov 1998]. By pre-announcing the event, Khrushchev exhibited great confidence in his weapon development team, and also placed extreme pressure on them. In any ordinary test of a new weapon design a failure results in only a delay in successful completion (and the cost of the materials expended). Now any marked deviation in yield would result in the loss of the planned propaganda value in which Khrushchev placed so much emphasis. The make-or-break character of this test was heightened still further by its scheduling to coincide with the final sessions of the Twenty-Second Party Congress.
The weight of this bomb - 27 tonnes - was nearly equal to the Tu-95's maximum payload, and two and a half times its normal weapon load [Zaloga 1993]. Special attachment and release hardware thus had to be developed and installed. Since the bomb's dimensions - 2 meters wide and 8 meters long - were larger than the bomb bay could accommodate part of the fuselage had to be cut away, and the bomb bay doors removed. The bomb was partially recessed in the plane, but not enclosed, with over half of it protruding in flight [Adamsky and Smirnov 1998]. A special parachute had to be developed to slow the bombs descent. The fabrication of this massive parachute disrupted the Soviet nylon hosiery industry [Reed and Kramish 1996]. Even special ground handling equipment had to be developed to lift the bomb for attaching to the aircraft.
Assembly appears to have been conducted in parallel with the design effort - that is, they began building the device even while developing its design. The bomb was assembled on a railroad flatcar in a special workshop built over a railroad line. After completion, the workshop was dismantled and the flatcar was camouflaged as a regular freight-train car. The bomb was taken by train all the way to the airfield where it was loaded directly into the delivery aircraft [Adamsky and Smirnov 1998], [Sakharov 1990, pg. 219].
At the beginning of October Sakharov travelled to Moscow to discuss calculations for the 100 megaton bomb. After he returned to Arzamas-16, with the device almost ready for shipment, serious doubts about its design arose. This would have been about the middle of the month, no more than two weeks before the test.
The device had 'some risky new features' (according to Sakharov) and Evsei Rabinovich had become convinced that the device would not work. Rabinovich communicated his concerns to the rest of the project staff, without at first notifying Sakharov. His arguments were evidently persuasive, and could not be easily set aside. Sakharov was pulled into the debate, and he, with Adamsky and Feodoritov, developed counter-arguments that refuted Rabinovich's conclusions. Since both parties relied on approximations it was difficult to discern which was correct.
Sakharov explains his response to this crisis:

I decided to introduce some changes into the design of the Big Bomb, trying to minimize the margin of error in calculating the subtle processes which worried Rabinovich. I hurried off to David Fishman, the head of the design department, who did not even bother to complain -- the matter was too serious. The designers did not go home that night until they had handed in revised blueprints; the actual design changes were made the following day.

[Sakharov 1990, pg. 220] Adamsky and Smirnov comment on the uncertainties experienced by the team:

"From time to time, we would naturally have doubts: would the device deceive us, would it fail at the moment of testing?
Alluding to this, Sakharov said: "If we don't make this thing, we'll be sent to railroad construction."

[Adamsky and Smirnov 1998]. This was however a marked improvement over the days of Stalin when nuclear weapon designers ruminated over the prospect of being shot! By October 24 (only 6 days before the actual test) the final report was complete, including the proposed design of the bomb and the theoretical and design calculations. The specifications in the report were sent to design engineers and bomb assemblers. The report was co-authored by Andrei Sakharov, Viktor Adamsky, Yuri Babaev, Yuri Smirnov, and Yuri Trutnev. Adamsky and Smirnov, two of the reports authors have recently quoted the following statement from the report: "A successful result from the test of this device opens the possibility of creating a device of practically unlimited power" [Adamsky and Smirnov 1998].
According to [Adamsky and Smirnov 1998] "even if the parachute system had failed during the test, the bomber's crew would not have been endangered, as the bomb contained a special mechanism which triggered its detonation only after the plane had reached a safe distance". This suggests that the bomb was rigged with a proximity fuze (which could either be a timer, or a barostatic or radar altimeter) that would detonate it close to the ground (the pictures of the bomb do show nose mounted probes that have been identified as a radar altimeter [Janes Defense Weekly 1992]). Even with this technique, the free fall time to the ground was less than 60 seconds (46 seconds neglecting air resistance), allowing the Tu-95 release plane to get no more than 30 km from ground zero (since this requires maximum speed, and a virtually instantaneous turn after release, the real separation would have been less).

Was it 50 Megatons or 57?

Shortly after the 30 October test the U.S. estimated the yield at 57 megatons. This value then circulated for 30 years as the actual yield of this device, quoted by Western sources and by the Soviet government. In his 1974 memoirs Khrushchev recollects: "Our scientists calculated in advance that the force of the bomb would equal 50 million tons of TNT. That was in theory. In actual fact, the explosion turned out to be equivalent to 57 million tons" [Khrushchev 1974; pg. 71]. However, all Russian sources since 1991 have consistently used a figure of 50 megatons, not 57. This includes the official Russian listing of all nuclear tests ([RFNC-VNIIEF 1996]), the personal account of the Arzamas-16's accomplishments by its long-time director Yuli Khariton ([Khariton 1993] ), and the account of this device given by its developers Viktor Adamsky and Yuri Smirnov [Adamsky and Smirnov 1994].
In preparing its estimate of the bomb's yield the U.S. had data about the test that was collected surprisingly close at hand. With the advance notice of Khrushchev's announcement, and the other tests in the series, a crash program code-named Speedlight was organized at the behest of Hebert Scoville (Joint Atomic Energy Intelligence Committee chairman) and Gerald Johnson (assisstant to the Secretary of Defense for atomic energy). A KC-135 Stratotanker was modified to carry broadband electromagnetic and speical optical equipment (which would have included a high-speed photometer called a "bhangmeter"). The modification was carried out under the supervision of Doyle Northrup by an Air Force unit headquartered at Wright-Patterson AFB called "Big Safari." The plane was ready for overseas deployment to its staging base by 27 October. Crossing over the Arctic Ocean, Speedlight was able to get quite close to the detonation point; close enough that the fuselage suffered scorching (suggesting it was closer than the 45 km separation of the Tu-95 drop aircraft).
The light emission profile of the explosion collected by the "bhangmeter" would have been used to calculate yield; the electromagnetic monitoring equipment would have detected signals generated by each stage of the bomb as it ignited, allowing the interstage timing to be measured. The data was analyzed by the Foreign Weapons Evaluation Panel (better known as the Bethe Panel, after its chairman Hans Bethe) which assigned the yield estimate of 57 Mt.[Richelson 2006].
The discrepancy may be explained if the test were actually 50 megatons, but the U.S. estimate was high by 14%. This difference would not be an unusual deviation between actual and estimated yield. For example authoritative estimates of the yield of the Hiroshima bomb have varied from 12 to 16 kt, a 25% (or 33%) difference, despite U.S. advantages in knowing the detailed device design, and having conducted exhaustive studies of its effects on the ground. In the case of the 50 megaton test, the U.S. did not have the benefit of detailed information about the device. Nonetheless, given the up-close high quality data provided by Speedlight the yield magnitude of the discrepancy remains puzzling.
The reasons why the Soviets might use this high foreign estimate instead of correcting it with the actual lower figure are clear. The test was intended to be a spectacular demonstration of awesome Soviet capabilities. For this purpose the higher the yield the better. The Soviets had no reason to want to provide a more accurate, but lower, yield. Further, the underlying pathologies of the Soviet system encouraged self-deception. The capricious and very political nature of Khrushchev's decision making, and the fear and apprehension of the weapons scientist about the consequences of failure (even if less extreme than during the Stalin years) illustrate how the system hardly encouraged feedback and truth-telling to the Soviet leadership. If Khrushchev heard of Western estimates (as he surely did) and was pleased with the weapons team "exceeding their quota" as it were, they could hardly be expected to risk themselves in disabusing the leader of the party and state of cherished notions. Further, it is not unusual for governments to use inaccurate and unofficial figures developed by others in public discourse, if the accurate official figures are classified. It was even more typical for the CPSU and the Soviet government to refuse to ever acknowledge error. If once upon a time, the leader of the USSR publicly accepted a yield of 57 megatons, then this figure was unlikely to be corrected in subsequent statements.
After the fall of the USSR, and the dethronement of the Communist Party as the monopolistic holder of state power, then these motivations to continue with inaccurate estimates disappeared.

Was it Ever a Weapon?

A test device, even one that is air-dropped like a operational weapon, is not suitable for normal military stockpiling (although it could be employed as a weapon in an emergency). Entry into a nation's weapon stockpile requires considerable engineering effort and planning to ensure a satisfactory stockpile lifetime, provision for required maintenance, a variety of safety and security mechanisms, development of suitable delivery techniques and equipment for combat use, development and approval of operational doctrine, institution of a suitable training program, development of a list of suitable targets and operational plans, etc., etc.
Given the overtly political nature of the development and test of this device, the dubious military usefulness of a weapon of this size, and the extremely compressed development effort, it is not a foregone conclusion that this device would ever be manufactured in quantity or accepted into the stockpile of the Soviet Union. The question thus arises: "Was it ever really a weapon?"
On 16 January 1963 Khrushchev made an explicit claim that the Soviet Union was in possession of a 100 megaton bomb, claiming that it was located in East Germany.
Further, Reed and Kramish [Reed and Kramish 1996, pg. 32] seem to indicate that it was weaponized over a period of time (but not as quickly as Khrushchev claimed):

Smirnov spent four years at Arzama-16, long enough to oversee the transformation of that bomb [the 50 Mt device] into a practical weapon. He then turned his attention to the peaceful uses of underground explosions.

Yet in Smirnov and Adamsky's own account ([Adamsky and Smirnov 1994])they state that this was a one-off test device, never weaponized:

In fact, the 50-Mt bomb tested on 30 October 1961 was never a weapon. This was a one-of-a-kind device, whose design allowed it to achieve a yield of up to 100 megatons when fully loaded with nuclear fuel.

Lev Feoktistov, bomb designer at the competing weapons laboratory Chelyabinsk-70, in recent remarks has also denied that this bomb was ever weaponized, saying that the "100-megaton giant - the pride of Arzamas-16 - was made only once - for the test". [Feoktistov 1999].
Given the lack of any plausible military role for the 100 Mt device (and professional militaries everywhere are loathe to take ownership of a weapon they consider useless), and the authority of Smirnov and Adamsky, supported by Feoktistov, the most likely conclusion is that this device was never weaponized and stockpiled, at least in the full yield form.
The 50 Mt clean version would have been a plausible weapon though, since it could be delivered by Tu-95 in Europe, and the reduced yield and the relative lack of fallout would have made it much easier to find targets in Europe where it could be used without devastating effects on the Warsaw Pact itself.
Another Russian publication ([Spassky 2000], p. 389) asserts that tests of a version with a maximum yield of 50 Mt were carried out over Novaya Zemlya in 1963 at half-yield, and that a Tu-95 variant was created to carry it. This model, the Tu-95-202, carried the bomb exteranlly, suspended under the fuselage.
The test referred to in this last source was actually conducted on 12 December 1962, with a yield of 24.2 Mt and has been confirmed in other sources as being a 50 Mt design. Whether it should be considered a weaponized version of Big Ivan is debatable. Since it was scaled down in maximum yield it was likely reduced in size and weight as well to make it more easily deployable, and really a different but related design.

Notable Features of the Explosion Sequence

A well known phenomenon in atmospheric explosions is the "double flash": an initial rapid peak in brightness that quickly drops, followed by a much slower rise to a second peak in luminosity that lasts much longer. The two peaks are similar to total luminosity, but as the seocnd peak lasts 100 times as long, it accounts for 99% of the emitted light and thermal radiation. In small nuclear explosions, like the 20 kt Trinity test, the first peak passes so quickly that it cannot be seen (unless captured by a high speed camera). The first peak is reached, and the luminosity plunges to its minimum point in only 10 milliseconds. The human eye sees only the second peak, which is reached at 140 milliseconds. But the time scale stretches out as yield increases and in the 50 megaton test the first peak occurs at more than half a second (560 milliseconds), and the minimum occurs at 7 seconds. This is easily visible in the test footage.
Another interesting feature is the effect of the shock wave reflected from the ground striking the bottom of the fireball. Simply from fireball radius scaling laws, one would expect the fireball to reach down and engulf the ground around the hypocenter ("ground zero"). In fact, the shock wave reaches the ground before the fireball expansion can, and bounces upward, striking the bottom of the fireball, flattening it and driving it upward, thus preventing actual contact with the ground.

Technical Discussion

It is safe to assume that the 100 Mt bomb was a very conservative design - one that pushed no technical envelopes save for size. The two principal reasons for thinking this are the extremely compressed development schedule, and the very high profile of the test.
Taking the second of these first, the fact that Khrushchev made this shot a public-relations centerpiece of an overtly political test program, going so far as to begin boasting about it only weeks after planning began, meant that the developers had to follow a failure-proof design approach. The political capital invested in the test series, and one of the principal pay-offs expected from the huge expenditure on the dozens of tests, would be largely lost if this device fizzled, or fell substantially short of its design yield. The cost paid by the scientists and the lab would be heavy if Khrushchev felt that they had failed him. Whatever they developed would have to be a very reliable design.
The extremely short schedule for designing and building the device imposed stringent conditions on the design. In the U.S. a device would often be on the drawing board for two years before testing, not 16 weeks. The use of approximations where precise calculations were normally employed meant that a very robust design, insensitive to variations in the design values would be needed. Since the schedule permitted no dead-ends or back-tracking, and only minor mid-course adjustments if design problems surfaced, the design approach had to have a high confidence at the outset for producing a successful device. In addition whatever they designed would have to be built with tools, processes, and techniques already at hand. There would be no time for shaking out production processes for fabricating new massive weapon components.
The largest device tested by the Soviet Union before the 1961 test series had a yield of 2.9 megatons (there had been three with yields of 2.8 to 2.9 Mt). It would be very unlikely that Arzamas-16 would (or could) attempt to create a unitary thermonuclear stage that was over 30 times larger than the largest device tested so far under these kinds of constraints. An alternative approach to creating a huge single third stage (containing as much as two tonnes of lithium deuteride) would be for the third stage to consist instead of an array of thermonuclear fuel capsules instead of just one. Several capsules, each with a yield of 10-20 Mt, could provide the necessary power.
It is likely that devices with yields in this range had already been under intensive development for some time that would provide the basis for the new design. In fact a bomb with a yield of 12.5 Mt was tested in the 1961 series on 23 October, a week before the Tsar Bomba. Assuming this was a full yield version, eight capsules similar to the 12.5 Mt device secondary stage could have been used to build the 100 Mt design. If this were the case then the successful 12.5 Mt test would have provided increased confidence in the 50 Mt test, or conversely a failure would have given a warning that redesign was needed.
Support for this idea may be drawn from the shape of the Tsar Bomba, a fat 2 m wide bomb, too wide to fit inside the Tu-95 bomb-bay. A wide bomb body is what would be expected from a cluster of stages, presumably positioned at the widest part of the bomb.
It is even possible that the secondary stage also consisted of multiple fuel capsules. The circular cluster of cylinders visible through the rear access panel in the images above might possibly be secondary stages.
Though not terribly revealing, comments by Lev Feoktistov on this bomb emphasize the pedestrian nature of the Tsar Bomba's design:
At the beginning of 1961 we, who worked in the Urals, had word that our competitors in Arzamas-16 had thought of a new super-bomb. Pretty soon it turned out that it was not some super-discovery, but merely an increase in weight and size. Did that make sense? Building up yields in this simple fashion looked to us both trivial and useless. In those days, we were obsessed with a very different idea - miniaturization, which I have already described.

At the same time (and I must honestly confess this) the fuss over the super-bomb idea could not leave us untouched. We were professionally jealous. We looked into the problem and at once spotted two weaknesses in our competitor’s design: their product would be too complicated and too heavy. It could not be squeezed into any of the delivery vehicles - already existing or those still on the drawing boards.