A proposal for an Iranian ICBM

Prelude

In early November 2020, two event coincided: The results of a historic U.S election became known and the anniversary of the Bidgane accident that killed the "father of Irans missile program" Hassan Tehrani Moghaddam.

These two events led to three disclosures: Deputy general of the Revolutionary Guard aerospace forces Mousavi in his first publiced speech, announced that beside short-, mid- and long-range missile capabilities, his force also has advanced Irans intercontinental capabilities.

This claim was a first for Iran and was later censored, to create plausible deniability. Iran had previously voluntarily restricted itself to a 2000km range limit.

The second disclosure was the unveiling of a new launch//basing method for Iran's so called underground "missile cities". The message was that of Iran's possession of a nuclear blast hardened, second strike missile capability. See my blogpost about it: https://patarames.blogspot.com/2020/11/irans-path-to-second-strike-capability.html

The third disclosure is significant for OSINT missile performance analysis: The payload capability of the Qased SLV, launched in April 2020. U.S intelligence certainly knew about this parameter, but for OSINT it is the enabler to calculate the performance of the Salman second stage motor.

Significance of Salman

Qased SLV with Salman second stage compared to a Ghadr BM

Salman is a very compact size stage, that could be a ideal add-on package for the existing IRGC-ASF missile arsenal, but only if it performs high enough, that such a retrofit effort makes sense.

The Salman upper stage motor has following features:

• Solid fuel
• Lightweight all-carbon-filament woven composite casing
• Flexnozzle, gimballed composite nozzle with electro-mechanical TVC actuators

As no performance parameters were disclosed on it, indirect ways were required to calculate these.

To calculate it, following parameters need to be known or confidently approximated:

• Burnout speed (known from the boost phase graph in the video)

• Propellant/motor/nozzle combination Isp (approximated with published Fateh-110 data)

• Payload (approximated via official payload data, approximated mass of third stage, guidance package, cold-gas bus system and estimated mass of shroud/fairing)

• Aerodynamic losses (estimated from similar aceleration, tip/stepping geometry and length/diameter ratio missiles/SLV)

• Fueled mass (known via geometry/dimension data from Qased SV launch)

• Empty mass (major unknown)

The available data and reasonable approximations allowed to calculate the performance of the Salman upper stage: When used with the Qased SLV, it is the amount of additional speed, delta-v it can generate while hauling the mass that sits on top of it; the total impulse.

The lower the weight of the motor that delivers this total impulse, the higher its performance.

Key here is the structural ratio of the Salman, which is the empty mass divided by the fueled mass. Its performance within the three stages of the Qased SLV allows to determine the structural ratio necessary to deliver that kind of performance.

Intentionally leaked delta-v/time graph of Qased launch

The Qased's first stage, a Ghadr-F ballistic missile, is well known and its performance can be modeled via its range-payload ratio. This gives an idea on what it has to haul, what sits on it, via its published burnout velocity. 

The Salman above it lacks range-payload ratio, but its burnout velocity is known and what mass it hauls can be estimated well. 

How? Via the sum of delta-v the first two stages generate, plus the third stage must have a performance that allows the 20kg Noor satellite to be delivered to a 430km low earth orbit. So the third stage and its necessary delta-v can be modeled to tell what needs to be known: What, or how much is the Salman second stage hauling?

Finally via this mass parameter an estimate for the empty mass and the structural ratio of the Salman stage can be made.

The answer is between 0,012 and 0,016, or 12%-16% of the total Salman mass is that of the motor itself and the rest is fuel. A more precise estimation would require knowledge about the exact weight of the shroud/fairing and the avionic package.

For details on the calculations, read this blogpost: https://patarames.blogspot.com/2020/06/trident-ii-and-df-31-comparing-data.html

With the performance of the Salman upper stage approximated, one question arises:

The options for upgrade

MaRV equipped Khorramshahr-2 at ascend

In terms of size, the Salman is very compact and could be even stored in a secure warhead storage container, together with the warhead. In operational conditions, hence fitting a existing missile with a Salman and its warhead should be feasible. If launcher interface and structural reserve margins allow for it.

• The Qased SLV uses Ghadr MRBM first stage, which is sufficiently robust designed to take the additional weight. This combination creates a lightweight SLV or ASAT weapon. Used as a ballistic missile with Salman as second stage and legacy guidance system removed, it would have a range of ~5.200km with a reduced weight warhead of 400kg.

• The Sejil solid fuel MRBM is another candidate for the Salman, here in the role of the third stage of an all-solid fuel ballistic missile. This system would have the tactical advantage of being able to launch on short notice and being independently road-mobile. The range with Salman as third stage and legacy guidance system removed would be ~7.200km with a reduced weight 400kg warhead.

• Here the topic of this article becomes relevant. The Khorramshahr MRBM is a trivial choice as the highest performance ballistic missile in Irans arsenal. It is a variant of the Soviet R-27 SLBM, which was far ahead of its time and the technological marvel of the Makeyev OKB. The latest variant, the Khorramshar-2, with its guidance system removed would have a range of ~10.100km with a reduced weight warhead of 400kg.

The last range value of 10.100km is equivalent to the distance of Tehran to Washington D.C, Iran's major adversary.

So while a Ghadr upgraded with a Salman upper stage and reduced warhead assembly becomes a (long range) IRBM and a Sejil-2 equipped accordingly a (short range) ICBM, the Khorramshahr becomes a true ICBM.

The key enabler beside the compact Salman upper stage, are the new generation fiberoptic inertial navigation system that drastically saves weight on the guidance and avionics package.

Khorramshahr-Salman ICBM

Up to scale details of the layout

The economic attractiveness and the reasoning of a conventional ICBM were questioned in the past and already the R-27 and its more potent Khorramshahr variant are highly complex, costly systems.

As a single stage ballistic missile, the Khorramshahr has a complicated staged combustion motor that is immersed inside the fuel tank. It has common bulkheads and divided tanks for center of gravity management. The tanks of the R-27 are made of an aluminum-magnesium alloy that is chemically milled. The Khorrmshahr may achieve similar structural strength and rigidity by using flow formed tanks, having higher structural requirements due to larger size and payload. Additional complexity is due to pressurizing gas from the motor being injected back to the oxidizer tanks to avoid cavitation.

R-27's 4D10 Staged combustion engine: SpaceX is yet to
move to this advanced cycle with its Raptor engine

This all illustrates how far beyond R-17 Scud-B level technology the Khorramshahr is and how difficult it was to master it and make it a mature missile system.

This effort made sense for the IRGC-ASF because the payload capacity of the Khorramshahr was almost three times higher than the Ghadr-F. So replacing three Ghadr-F with a single Khorramshahr was worth the effort. 

While Khorramshahr represents a more economic solution than the Ghadr, mastering it took some time for Iran. DPRK's R-27 variant, the HS-10 and the two HS-13 ICBM variants based on it, became failed projects and the North Koreans simply gave up on it after a series of failed flight tests.

Size comparison of Iran's Khorramshahr, DPRK HS-10, and original R-27.
The Iranian and Korean variants are very different to each other in detail.

Iran however pursued this technological path as its range requirements were lower than that of the DPRK and the compactness of the design made it more useful in terms of launch survivability/basing.

Was the option for a Khorrmashahr-Salman ICBM an unintended path, or did one technological breakthrough, in form of the Khorramshahr, await another breakthrough in form of the Salman to enable a light ICBM?

In terms of system cost a Khorrmashahr-Salman conventional light ICBM can be quite attractive. Just two stages with two motors, with the large first stage being storable for long periods of time, having an excellent life-cycle-cost relationship.

However would a light weight conventional 400kg re-entry vehicle warhead make economic and military sense? 

At this point it should be noted that Iran's Fateh-110/-313 SRBM, which was used against Kurdish separatists and the U.S base at Ain-al-Assad Iraq, used light weight 448kg warheads, but with 10m CEP precision.

At the velocities of intercontinental range re-entry, trajectory corrections with a MaRV become immensely more difficult. Without MaRV for trajectory corrections, a CEP of 100-300m would be possible in the best case scenario. The value would likely be closer to 300m CEP and above, since only highly evolved RV designs are able to reach numbers near 100m, via their high speed sharp nose-tip and body designs.

300-500m CEP with a 400kg warhead would still be a valuable asset against the opponents high value targets of large dimensions. True military capability that comes close to a counter-force capability however would not be achievable.

Such a light weight warhead would make use of the highest grade explosives to compensate the gap to a standard Iranian 650kg unitary warhead. However the key to overcome this shortcoming is only increased precision. 

An option could be an light weight aerodynamic steering module, that corrects re-entry trajectory in the uppermost atmospheric layers and detaches once velocity and angular errors are corrected. This would allow more blunt, initial generation, intercontinental range RVs to close into the 100m CEP region.

In conclusion such a light weight, unitary, 400kg warhead, would need to compensate the lack of mass via improved explosive power and higher precision. Alternatives could be using submunition or thermobaric warheads. The latter solutions are easier and more feasible but are restricted to area targets and are less destructive against targets which are hardened.

The most likely candidate would be a submunition warhead and with an estimated CEP of above 500m. A counter-value asset of intercontinental range.

Conclusion

Salman: The technological enabler

If the Salman upper stage could be fitted like a different warhead to the existing Iranian ballistic missile arsenal to enable a whole new set of range capabilities, it could be described the ultimate force-multiplier. A smart, modular capability, preserving previous investments in the missile arsenal and made possible by key technological breakthroughs.

Logistics and handling, would not get significantly more complicated since Salman with warhead could be handled as a 2,2 ton heavy warhead, not compromising explosive security procedures of Irans "missile cities". At just 14-14,5m length the Khorramshahr-Salman ICBM would fit well into the unveiled missile tunnels and wagon. With a weight of 20 ton, its light enough for the R-27 4D10 motor to acclerate and the existing Khorramshahr TEL to transport.

Also in terms of costs, it would make much sense to reduce payload in order to reach intercontinental range.

True military relevance however, would require precision strike capability like e.g Irans Fateh family. In more distant future, the range-payload relation could be significantly improved by a hypersonic glide vehicle, the growth potential here is given.

The unveiling of the Qased SLV, an legacy Ghadr-F with Salman fitted to it, was a message of a new capability, one that can be applied to the existing arsenal. The impact this compact 2,2m length, 1m diameter upper stage has on the range capability of the existing arsenal, is immense.

At times of increased tensions, the unveiling of nuclear blast hardened, underground missile complexes equipped with a large number of light intercontinental missiles, has a heavy effect on the decision making table. It is a latent intercontinental range capability whether to turn existing MRBMs to ICBMs, threatening U.S mainland, or not.

Once a dream: A young Hassan Tehrani Moghaddam in front
 of a Chinese missile, when Iran has no missile program yet

It's a late gift from Hassan Tehrani Moghaddam; mastering the key technologies of the Salman upper stage was what lead to his death.

Iran's path to a second strike capability

The concept

Interpretation of Irans new basing concept

Non-nuclear states can have the highest conventional military capability but are always regarded as irrelevant, once the joker function of nuclear warfare is employed.

This intuitive notion is based on two main ideas:

• There is no credible defense against nuclear weapons

• Conventional weapons are unable to establish significant counter-value and counter-force deterance

Irans missile warfare doctrine of the 21st century relies on three main capabilities:

• Ballistic missiles that are sufficiently accurate to perform point-strikes (~50m CEP) on the opponents key objects. These can be counter-value targets such as powerplants and other critical infrastructure or counter-value targets like missile silos or BMD early warning radars.

• Ballistic missile launch and basing options that can survive an opponents nuclear counter-force decapitation strike and carry out the mission to inflict heavy damage on the opponent

• The inflicted damage on the enemy is done in a sufficiently high pace to deny any counter-campaign aimed to neutralize Irans missile forces to bear fruits. Once that capability can be disabled, it has already spent its ballistic missile arsenal and has no further function in the conflict.

Once such a capability is reached, the nuclear option of the opponent is neutralized due to two realities:

• A nuclear counter-force strike will not disable Irans counter-strike capability and what survives is sufficient to cause a scale of damage that can't be tolerated by the opponent

• The scale of damage due to point-strike capability on critical and high value targets, reaches a level high enough, to be compared to what was previously only possible with nuclear weapons or at least complete air dominance.

Survivable basing

In August 2020 Iran revealed a novel soft basing method for solid fuel ballistic missiles; buried missile containers. This concept is based on ambiguity of true and decoy sites, as well as large areal distances such so called "missile farms" can have. 

For a nuclear counter-force strike to neutralize these low hardened assets, large areas need to be hit, increasing the number of nuclear weapons needed.

Other solid fuel ballistic missiles in Irans arsenal are either: 

• Road-mobile, single autonomous vehicles, which can launch the missile quickly after receiving the order (e.g Sejil). 

• Or off-road capable transporter erector launchers that can hide in the terrain ( e.g Dezful).

Irans mountainous terrain is especially well suited for the latter of the two launch methods. Deep valleys deny line of sight for stand-off reconnaissance assets which requires it, as well as blast deflection of conventional and nuclear weapons.

Another mean Iran employs for increasing the survivability of its TELs is to design them small enough to disguise them as civilian trucks, e.g by using a plastic cover.

Dezful solid-fuel BM off-road twin-missile launch TEL

For liquid fueled missiles, the situation is more complicated.

Iran prefers to keep its liquids fuel ballistic missiles and even expand on it, despite the availability of solid fuel missiles for three main reasons:

• The higher performance/ISP of liquid fuel ballistic missiles allows heavier payloads and longer ranges at same size.
• They can be safely stored without any risk of explosion, or catastrophic cascade event, making them attractive for deep tunnel basing. That's primarily due to the physically separate location of fuel and oxidizer tanks as well as warheads
• If stored in unfueled, dry condition, their lifetime without significant refurbishment is immense. The investment in the arsenal is hence a multi-generation one and life-cycle costs makes such conventional liquid fuel ballistic missiles attractive.

The problems liquid fueled missile cause are however also significant:

• A well and continously trained crew is needed for fueling and handling the missile
• Irans new generation missiles have fuel/oxidizer combinations which can't be used at high temperatures, typical in Iran

So while a solid fueled Sejil-2 TEL can travel hundreds of kilometres, disguised as civilian truck and hide anywhere, liquid fuel TELs can't operate very far from their home base.

Missile cities

The concept of storing the valuable ballistic missile arsenal in deep tunnel complexes is not new and China, North Korea and Iran are the countries that employ this basing concept most prominently. Iran calls this kinds a complexes "missile cities" due to their large scale.

These bases are often so deeply buried into mountains, that the arsenal is secure against nuclear weapon strikes. However nuclear strikes at the entrances of such bases can potentially disable them for the rest of a high intensity conflict or reduce their significance as they are not able to operate until they are fixed.

Missiles and TELs stored in a typical missile city tunnel

In conventional attacks against such missile tunnel complexes, excavator machines and other constructing machines will either re-open the entrance in a short period of time or create alternative emergency exits if necessary.

The entrance sections also employ high-grade tunnel lining with high-performance composite concrete.

Damage caused by conventional bunker busters against such entrance structures are lower than often intuitively anticipated. This is true for bunker busters that can be delivered by survivable airpower, as well as bomber delivered special assets like the GBU-57 massive ordnance penetrator.

So whereas dozens of meters of soft rock can be penetrated by such weapons or several roofs made of normal-grade concrete, the values dramatically diminishes to a few meters or lower against hard granite rock formations or high-performance composite concrete.

The effects on critical assets inside tunnel complexes via such conventional weapons is almost non-existent due to the rock formation overburden of typically 50-100m for the least hardened of these.

Hence a credible capability for fast repair and continued operation is guaranteed against conventional bunker busters.

At this point its necessary to stress the time-critical nature of operation of such missile tunnel complexes. With their sole goal being the launch of the complete stored ballistic missile arsenal and heavy conventional bunker busters having airpower as their only delivery platform a dilemma is created: In order to deliver the bunker buster, the opponents integrated air defense system and its assets must be degraded sufficiently to allow the chance of successful delivery of the weapon.

• The range of Irans ballistic missiles allow missile cities to be based deep in the central areas of Iran, which is a vast country.
• The distance airpower must survive, flying through hostile airspace, without a mission-kill or total loss, is hundreds of kilometres. Evasive maneuvering with such heavy payloads often equates to mission-kill.
• The time criticality of ballistic missile warfare does not allow for a significant SEAD/DEAD campaign to disable the opponents IADS
• Irans IADS and its assets have reached a technological level that enables effecitvley countering low observability techniques primarily employed by western airpower against high value targets. 
• Irans task would be the protection of a point target (missile city), which is magnitudes easier to defend than random area targets. Subsonic cruise missile delivered warheads would be most severely affected in such a point-defense engagement secenaro.

Countering missile cities

A solution to the threat presented by such missile city basing today would be nuclear warheads delivered by accurate ballistic missiles. Future options could be hypersonic conventional "bunker busting" missiles or low-yield earth penetrating warheads delivered by ballistic missiles.

Granite rock formations can withstand contact fuse 300 kiloton yield thermonuclear ballistic missile re-entry vehicle delivered warheads (~100m CEP) if the depth/overburden is around:

• 300m if low-end rockbolt and mesh tunnel lining is employed. This is typical for very deep missile storage areas and low-risk transit tunnel sections which can tolerate damage.
• 100m if high end backpacked concrete lining is employed. This is typical for critical forward sections with sensitive equipment and personel.
• 30-50m if high-performance-concrete structures for entrance areas are present. Such transit sections can tolerate damage and spall/rubble and just need to remain passable.

Rockbolt and mesh liner type: Handling area
with polymer spall protection, transit area without

Entrance areas which nearly always have less granite overburden than those mentioned 30-50m, suffer great damage due to risk of complete tunnel collapse. To break free and clear such entrance sections for continued launch operations, a significant amount time is necessary. Multiple hits over time can permanentley disable the missile city for the duration of the war.

So while the missile arsenal can't be eliminated by the opponents nuclear weapon systems, a mission-kill for the conflict is possible against these kind of missile tunnel complexes.

Nuclear hardened missile cities

Missile cities that can withstand nuclear strikes and continue launch operations are of the cavern shaft type.

This concept does not need an entrance area or a opening large enough for a TEL to operate and instead launches missiles from inside of the mountain via a vertical shaft that can be dozens of meters in length.

As long as the shaft remains clear, launch operations can continue and the nature of this concept allows it to take several strikes of accurate (e.g. Trident II re-entry vehicles) counter-force-rated nuclear weapons and remain operational.

Cavern shaft launch of a Qiam SRBM

Since Irans lacks a nuclear triad and expensive delivery platforms such as SSBN's, it can concentrate its resources on developing such a complex and expensive basing method as cavern shaft basing.

During the trails for a basing method for the U.S MX Peacekeeper ICBM, cavern basing was assessed to be the most survivable basing concept in terms of hardness but also one of the most expensive ones.

Only Iran is known to employ the cavern shaft launch concept and potentially North Korea.

The rock overburden of such Iranian complexes are however probably not suitable for multiple nuclear weapon hits due to the relative low shaft depth.

Displayed examples of such complexes, show that they shot one missile at a time which works in counter to the time parameter that is of greatest important in high intensity missile warfare.

Early November 2020 Iran presented a novel ballistic missile launch and loading concept. A hardened-mobility electrical rail wagon, that carries 5 ready to launch liquid fuel missiles, which are loaded in a semi-automated erector system. It can be best described as a multiple-launch carousel-magazine loading and launch system which allows continuous parallel loading operations.

IRGC aerospace forces did not show how and where the missiles are launched.

The highest end launch method would be the combination with the previously unveiled cavern shaft launch concept, potentially with a vertical shaft with more than 100m depth.

However, it seems that Iran has selected a lower-end, much less complicated and resource-intensive concept that maintains nuclear hardness requirements.

The concept is based on following features:

• An open-air pit, just deep enough not to be exposed to high blast over-pressure and thermal heat levels.
• Small enough for very low risk of a direct nuclear RV hit into the pit.
• Deep enough to require weapon trajectories with steep angle of attack, in order to hit the critical lower part of the pit, where the rails and tunnel door are located.
• Location deep inside a valley to complicate weapon trajectories and make use of natural barriers for blast deflection

Detailed graphic showing how the new concept probably functions

The launch pit is drilled into hard granite rock and re-enforced and lined with high-performance concrete structures. Such relative small structures can be heavily protected against nuclear weapons, just like ICBM silos are designed to remain intact with a nearby hit of a thermonuclear warhead (~300m).

In contrast to a ICBM silo, the launch pit neither houses sensitive equipment nor has any other function than to house the door and the rails. Hence its tolerance to damage is not only higher but it can also be repaired with crude means if necessary.

The goal of the launch pit is to keep nuclear blast caused erosion low enough to allow the door and rails to remain intact. Ground shock levels, the primary defeat mechanism of nuclear weapons against hardened targets, need to remain low enough for the last 10-30m of the missile transit tunnel not to collapse irreparably. Vibration levels and suspension requirements in this tunnel section can be neglected as just collapse and door malfunction must be avoided.

Multi megaton yield nuclear warheads have more adverse effects but are not used for counter-force strikes, mainly since there are either fewer or heavier. Earth penetrating nuclear bunker busters are mainly restricted to airpower delivery as ballistic missile RV impact causes very high deceleration forces.

The newly unveiled complex

Loading chamber of new launch concept, note scale

Beside this new basing and launch concept the tunnel complex also has TELs as secondary launch option. The vast complex has deep missile storage sections with rockbolt and mesh lining, sections with polymer spall liners where sensitive equipment is handled and ground shock effects are likely to cause low damage. Warhead mating and TEL loading areas where the warhead is a risk for a catastrophic cascade effect and hence needs to be separrated from the rest of the complex.

Transit tunnel sections, where damage and spall is tolerable and rockbolt and mesh lining applied.

Critical sections and entrance areas with low overburden where high-end lining is applied.

Automatic electrical missile wagon in
transit tunnel section

Beside these details, a total firepower estimate would require to know how many loading areas are present, how many launch wagons as well as how many launch pits.

If it is a large complex as expected, hundreds of different ballistic missiles can be stored, which is done in the most deep sections. Those rockbolt and mesh low-cost lined sections have granite overburdens of 300-600m and are hence considered immune to nuclear attacks.

Liquid fuel missile types that are likely present and compatible with small modification are:

• 800km range Qiam SRBMs with pin-point strike maneuverable re-entry vehicles (MaRV) as well submunition warheads. Primarily for use against airbases of neighboring counties to destroy specific assets or render operations impossible due to random interval strikes by submunition warheads that require to be cleared fist due to the risk of unexploded ordnance and random location.

• 1700km range Emad MRBM with pinpont-strike MaRV warhead for regional opponents key military and high value objects.

• 2000km range Khorramshahr heavy MRBM with submunition and MaRV warhead options. With a payload capability that is around 3-times higher than the Emad and Ghadr-H missile, this new ballistic missile with heavy warhead(s), will become available in higher numbers. In terms of cost-effect, a single Khorramshahr with a 1,8 ton submunition warhead inflicts the same damage as three Ghadr-H, which is a significant advantage.

In terms of firepower generation, depending on how many parallel loading areas are available in the complex to load an unmanned, automated wagon magazine, the value can vary strongly. As site location of this static system is known and the modern missiles employed do not require a turntable for azimuth alignment, loading is done relatively fast.

Compared to Irans tactical solid fuel missiles, the liquid arsenal is launched against predetermined targets, hence if the conflict is of high intensity, missiles will be launched as fast as possible.

Conclusion

In the past Iran achieved nuclear blast hardening with its cavern vertical shaft basing concept for its liquid fuel missile arsenal.

The new 5-missile launch and loading concept can also be applied to that concept, but it is expensive and a very mature missile system with highest reliability is necessary.

Instead it is expected that a sufficient degree of nuclear blast hardening is reached in combination with the open launch pit concept.

It allows existing missile cities to be retrofitted with this capability at a affordable cost and reasonable timescale.

This and the differing lining methods, shows that the IRGC aerospace force is very confident in what is needed to reach a certain objective, at high cost efficiency.

Working area and warhead mating

The dynamics at play for the creation of this basing method are very different to that of other nations:

• Iran is not a nuclear power; it needs a vast amount of precise firepower to make a difference in a conflict and ultimately deter the opponent from a nuclear counter-force first strike. Here single-missile silos, make no sense for as each theater requires several hundred to up to thousands of ballistic missiles.
• The lack of nuclear weapons, as well as different basing concepts for it, with the highest capability and most expensive option, SSBNs, allows resources to be concentrated on the conventional missile forces.
• Iran has the necessary low-populated regions as well as highly mountainous topology and suitable rock formations to allow for such a basing method

The concept brings Iran closer to achieve a capability no other country has: Deter nuclear powers from a preemptive counter-force strike by conventional means. Iran has restricted itself to about 2000km missile range. This range used to be the maximum that could be achieved by a relatively cost efficient single stage missile.

With the technological level of Irans missile program improving quickly, at some point a two stage liquid fueled ICBM to deter the U.S mainland directly may become reality and for such a missile, the missile basing method shown, is likely already sufficiently large in its current state.

Sources for the values on tunnels:

Book: Effects of Nuclear Earth-Penetrator and Other Weapons

https://www.nap.edu/catalog/11282/effects-of-nuclear-earth-penetrator-and-other-weapons

https://www.nextbigfuture.com/2016/05/a-hundred-super-ground-penetrator-bombs.htm