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.