North Korea’s New HS-18 Makes a Solid but Incremental Contribution to the ICBM Force

North Korea launched its first solid-propellant intercontinental ballistic missile (ICBM), the road-mobile “Hwasongpho-18” (Hwasong-18 or HS-18), on April 13. The new ICBM is a notable achievement that underscores North Korea’s prowess in solid-propellant technology, but not an inconceivable one given its previous work on solids, the well-understood nature of what is now a 60-year-old technology, and probable acquisition of Russian and Chinese know-how.

The HS-18 is not more mobile than at least the current liquid-propellant HS-15 road-mobile ICBM, but it is safer and easier to operate in a mobile mode because liquid missiles use toxic propellants that require special handling and impose operational burdens on their crews in the field. The lack of additional support vehicles needed for liquids also makes solid ICBM units easier to conceal when deployed to the field and thus harder for an adversary to detect and attack. But the North’s liquid, mobile ICBMs are already hard to find and thus highly survivable.

It would not be surprising if North Korea deploys the HS-18 based on this one successful flight test, although it may still conduct one or two more. However, as with all  the country’s recent new missiles, there is no indication as to how many HS-18s the North plans to deploy. It may also decide to base some HS-18s in other modes (such as rail-mobile), to use various combinations of its three stages to create medium- or intermediate-range missiles, and to eventually modify the HS-18 and develop more capable follow-on solid ICBMs as its technology improves. Yet it is highly likely to retain liquids in its ICBM force as well to take advantage of their greater range/payload capability and to capitalize on its long liquid experience and substantial infrastructure.

Based on this analysis, the addition of solid-ICBMs is not a “game-changer” and does not substantially boost the North’s ICBM threat to the US. The US homeland has been under threat from liquid ICBMs since 2017, and liquid ICBM deployments are still likely to increase. The HS-18 will thus add incrementally to this threat, to the extent North Korea’s nuclear weapons and solid-propellant production capacities, and resource allocation decisions allow.

First Flight of Solid ICBM

North Korea launched a ballistic missile on the morning of April 13, flying about 58 minutes on a highly-lofted trajectory to a range of about 1000 km, according to the Japanese and South Korean governments. Both governments initially termed it a possible ICBM, but a subsequent White House statement confirmed it was an ICBM. The next day, North Korean press announced[1] the successful first flight-test of the “Hwasongpho-18” (HS-18) solid-propellant ICBM. This was backed up by photographs and later video showing the launch of a canisterized three-stage solid ICBM from a road-mobile transporter-erector-launcher (TEL) and indications of in-flight stage separation.

This was North Korea’s first flight test of a solid-propellant ICBM; the previously tested HS-14 (apparently never deployed), -15, and -17 are all liquid-propellant. Based on the launcher and launch canister seen, the HS-18 is the same system unveiled at the February 8, 2023 military parade. The North did not report the missile’s maximum altitude (South Korean sources report it was less than 3000 or less than 5000 km) which, when coupled with the unusual trajectory and apparent delayed upper-stage separation and ignition (as reported by North Korean media), means there is not currently a good open-source basis to estimate the HS-18’s range/payload capability.

The photographs published in North Korean media provide a good basis for estimating the HS-18’s dimensions. According to one analyst, the HS-18 is 26.95 m long, its first stage is 2.21 m in diameter, and its second and third stages have a 1.9 m diameter. The length and maximum diameter are larger than previously estimated based on an analysis of the paraded launch canister (26.5 x 2.1 m), underscoring assessments at that time that the HS-18’s motors are less powerful than what we now know to be the similarly configured but smaller US Minuteman-III and Russian SS-27 Mods 1 and 2 solid ICBMs.[2] The HS-18’s first-stage diameter is consistent with that estimated from photos of the large solid rocket motor North Korea reported static (ground) testing last December 15, which analysts assessed was appropriate for an ICBM first stage.

Notable but Understandable Progress in Solid Technology

Solid rocket motors are more difficult to develop and to produce reliably as they increase in size, particularly diameter. The North’s largest previously-flown motors were the 1.5 m diameter Pukguksong-1 and -3 submarine-launched ballistic missiles (SLBMs) and Pukguksong-2 road-mobile medium-range ballistic missile (MRBM); the earliest of these was flight tested in 2015 (unsuccessfully) and 2016 (successfully). Pyongyang’s ability to successfully fly a three-stage ICBM using 1.9 m and 2.2 m diameter motors some 7-8 years later is a notable achievement, but not an inconceivable one. For example, the US first flight tested the 1.88 m diameter Minuteman-I solid ICBM in 1961, just five years after the first flight of the 0.79 m diameter Sergeant short-range ballistic missile (SRBM).

The basic technology underlying solid ICBMs is now some 60 years old, fairly widely diffused, and reasonably well understood. We do not know from open sources how long North Korea has been developing solid ICBMs, although doing so has long been regarded as a logical next step. We also do not know if it conducted other ICBM-related static rocket motor tests beyond the one in December 2022, including for the upper stages. It seems fairly clear based on other aspects of the North’s missile program that it almost certainly has obtained substantial solid-propellant know-how from Russian entities since the early 1990s, and probably obtained at least production-related equipment and various material inputs from Chinese entities.

What Solid ICBMs Add…and Don’t Add

Not more mobile, but easier to operate. Press reports routinely regard solid ICBMs as “more mobile” than liquids, but it is more accurate to say mobile solids are safer and easier to operate than mobile liquids. As one analyst put it, “it’s just easier to use solid-fuel missiles” in mobile applications. Initial reports that the HS-18 was launched from a grassy area rather than the concrete pads used previously for liquid ICBMs, and thus had better off-road capability, were disproven in commercial satellite imagery revealing that the HS-18 launch area had been graded, concreted, and covered with grass since last December and a concrete access bridge installed.

At some 40-50 metric tons fully fueled, North Korea’s HS-15 liquid ICBM probably weighs less than the HS-18 (the smaller Russian SS-27 Mod 2 weighs 49.6 metric tons), and thus would be at least as easy to move around on a road-mobile TEL. Although the much larger North Korean HS-17 liquid ICBM, estimated at 80-110 metric tons fully fueled, would obviously be much harder to move around by road than the HS-18, it is on a par with the 104.5 metric ton former Soviet SS-24 solid-propellant rail-mobile ICBM.

Beyond their weight, however, liquid missiles use toxic propellants that require special handling and impose operational burdens on their crews. Fueled liquid missiles also cannot tolerate rough transport as well as solids. In order to prevent leaks of the toxic propellants, operators would need to choose routes and speeds to limit sloshing in the liquid missile’s propellant tanks and avoid disrupting plumbing in the propulsion system. Solid propellants cannot leak, although overly rough handling and transport risks introducing cracks in the solid propellant grain that could inhibit missile performance or even lead to a launch or in-flight failure.

Nevertheless, North Korea has some 35 years experience operating liquid-propellant road-mobile missiles in the field, which suggests it can adequately address the challenges of operating road-mobile liquid ICBMs. Indeed, it may well already have been doing so, since the road-mobile HS-15 probably has been operationally deployed since 2017.

No reaction time advantage over modern liquids. Most press coverage of the HS-18 launch characterized solids as being able to launch faster than liquids, providing no warning time to an opponent. But many of the prerequisites for launching quickly without warning are available to both liquids and solids if the North provides them. These include being located in an area suitable for launch, mated with a warhead, crewed, and in communication with launch authorities. We do not know if North Korea keeps any ICBMs in such launch readiness on a day-to-day basis, or if it would instead increase readiness in a crisis or if it decided to prepare for a first strike.

The real reaction time difference concerns whether the missiles are fueled with propellants. While solids are loaded permanently with fuel as part of the production process, liquids must be fueled at some point prior to launch. Once fueled, a liquid can launch as quickly as a solid.

Thus, the key question is:  how far in advance can a liquid ICBM be fueled? According to much press commentary, liquids supposedly must be fueled just hours prior to launch, with fueling taking an hour or more, during which liquid missiles supposedly are vulnerable to detection and attack. Liquids also are often characterized as supposedly being unable to remain fueled for prolonged periods due to the corrosiveness of their propellants, also forcing them to be fueled shortly before launch.

But such assessments do not take account of the fact that the current generation of North Korean missiles (the HS-15 and -17 ICBMs and the HS-12 intermediate-range ballistic missile [IRBM]) use liquid propellants specifically developed by the US and USSR in the 1960s so ICBMs could stay deployed and immediately ready to launch for years at a time. At a minimum, therefore, North Korea could decide to fuel its liquid ICBMs early in a crisis (or if contemplating a first strike), days or weeks before dispersing them to the field prior to hostilities beginning.

Moreover, in the September 2021 launch of the Hwasong-8 “hypersonic missile,” which used a shortened version of the HS-12 booster, the North referred to its use of a “missile fuel ampoule.” North Korean media coverage of the February 18, 2023 HS-15 launch also referred to the use of “the system of fuel ampoule.”[3] Based on these indications, it is likely that the HS-15 ICBM is fueled in the factory before being sent to operational units, with its propulsion system sealed with membranes to separate the engines and fuel plumbing from the propellant tanks. Thus, it would be ready to launch without the need for potentially detectable in-field fueling.

We do not know if the larger HS-17 uses the “ampoule” approach, but the similarly-sized Soviet SS-19 liquid ICBM apparently did, and the North has “noted the military significance of turning all missile fuel systems into ampoules.[4]” The possibility that the HS-17 was too large to be safely erected from transport (horizontal) to launch (vertical) position while fueled, thus requiring it to be fueled only after being erected, appears to have been disproven by North Korean video of the erection and launch of an HS-17 on March 16.

Less detectable, so more survivable…to an extent. A road-mobile solid-propellant ICBM unit would not need as many support vehicles—such as fuel, oxidizer, and propellant-handling vehicles—when deployed, meaning that a solid ICBM unit should be easier to conceal when deployed to the field and thus harder for an adversary to detect and attack.

A unit of “ampoulized” liquid ICBMs probably would not need as many specialized vehicles as a unit deploying with unfueled missiles either, but probably would still include propellant-monitoring and consequence-management vehicles that are unnecessary in a solid ICBM unit. Either way, however, a larger logistic tail for liquid fuel missiles does not make them “easy targets for any adversary trying to take them out.” Iraq showed a remarkable ability to conceal Scud-class liquid missile units during the 1990-91 Gulf War despite Coalition air superiority, a dedicated “Scud hunting” effort, and desert terrain harder to hide in than North Korea.

Thus, it would be an overstatement to call the future HS-18 force “the closest thing that North Korea will have to a relatively secure second-strike capability”—the existing road-mobile liquid ICBM force was already well on track to fulfill that role.

Potentially greater reliability. In general, solid missiles have the potential to be more reliable than liquids (fewer missiles fail at launch or in-flight) because solids do not use large numbers of moving parts or high-pressure liquid flows. North Korea has demonstrated remarkable success in its flights of the current generation of solid SRBMs (KN-23/-24/-25).

Although the HS-18’s larger-diameter motors may be more prone to production, quality-control, and handling problems than the smaller SRBMs, the high success rate even of apparent SRBM operational troop training launches suggests the North knows how to minimize and manage those kinds of issues in its solid ICBMs. The outcomes of future HS-18 launches will shed more light on this, but the above factors suggest the missile will not experience “a relatively high ‘dud’ rate” and that hedging against such failures will not be the reason Pyongyang retains liquid ICBMs (see below).

Looking to the Future

The advent of solid-propellant ICBMs suggests the following for North Korea’s future missile force and the threat it poses to the US homeland:

• Deployment announcement soon. North Korea’s April 14 statement suggested the HS-18 was not yet deployed, but it would not be surprising if North Korea soon deems the system deployable based on its single successful flight test, just as was the case for the HS-15 in 2017. Even if it does conduct further tests before deployment, it would be reasonable to suspect there would only be one or two.

• Force size unknown. North Korea’s April 14 announcement claimed the HS-18 would be “the future core pivotal means of the strategic force of the DPRK” and “the most powerful, pivotal and principal means in defending the DPRK.” This will depend heavily on how many HS-18s are produced and deployed. One limiting factor will be the size of the North’s overall nuclear stockpile, which it apparently intends to distribute across SRBMs (including five new-generation systems), MRBMs, IRBMs, land-attack cruise missiles, and both liquid and solid ICBMs, possibly along with SLBMs, lakebed-based missiles, and perhaps even unmanned underwater Other limiting factors will be North Korea’s capacity to produce modern solid propellants, which are needed across all of its new solid ballistic missile systems (including the hundreds of SRBMs needed for conventional warfighting), and its capacities to cast and cure large-diameter solid motors and to produce large motor cases.

• Other versions and future follow-ons possible. North Korea may decide to use various combinations of the HS-18’s three stages as the basis for new solid propellant MRBMs and/or IRBMs, just as the USSR used the first two stages of the SS-16 solid ICBM to create the SS-20 IRBM. It may also decide to base some HS-18s in other modes, such as rail-mobile (for which the missile would be quite well-suited, and which might compensate for any shortage of road-mobile TEL chassis) or less likely in silos (which are much more vulnerable to detection and attack) or lakebed submersible launchers. The HS-18’s successful flight-test also augers well for any future North Korean development of solid SLBMs; the displayed but yet-to-be-tested Pukguksong-4 and -5 are each about 2 m in diameter (close to the HS-18’s second and third stage), and an even larger displayed SLBM (presumably Pukguksong-6) has about the same diameter as the HS-18’s first stage. Finally, Pyongyang can be expected over time to develop modifications of the HS-18 to take into account lessons learned in flight testing and deployment, as well as more capable follow-on ICBMs as the North’s propulsion and warhead technology improves.

• Liquids probably here to stay. Even if the HS-18 takes on a larger role in the North Korean ICBM force, it is highly unlikely that North Korea will entirely abandon liquid-propellant missiles. Liquid propellants are more energetic pound-for-pound, meaning that a given-size liquid will have more range/payload capability than a solid. The North probably will continue to want this capability to carry the “multi-warheads” and “super-large hydrogen bomb” Kim Jong Un reports are under development, as well the hypersonic glide vehicle (HGV) it tested in September 2021 if it is intended for deployment. Moreover, after some 35 years North Korea has built a substantial liquid missile infrastructure and deep development, production, and road-mobile deployment experience. It probably regards liquids as capable, reliable, and survivable enough to merit a place in the missile force for years to come.

Not a Game-Changer or a Big Threat-Booster

Despite some press commentary that the HS-18 is “a huge leap in the North’s nuclear capabilities” and “more threatening to the US than a liquid-fueled missile,” the above analysis substantiates other analysts’ assessments that North Korea’s new solid ICBM “does not amount to anything like a game-changer.” The US homeland has been under threat from the HS-15 since 2017, and the HS-17 probably since this March, and deployments of both systems are likely to increase. The HS-18 will add incrementally to this threat, to the extent North Korea’s nuclear weapons and solid-propellant production capacities and resource allocation decisions allow. It will be somewhat more survivable in the field than the already-survivable road-mobile liquid ICBMs, and should be at least as reliable. The new ICBM does underscore North Korea’s prowess in solid-propellant technology, and makes good on Kim Jong Un’s January 2021 solid ICBM development goal.

1. [1]

   “Another Mighty Entity Showing Continuous Development of Strategic Force Unveiled in DPRK: Respected Comrade Kim Jong Un Guides First Test-Fire of New-Type ICBM Hwasongpho-18 on Spot,” Korean Central News Agency, April 14, 2023, http://kcna.kp/en/article/q/1764e4431f2d91ad94dd8be97a2f8760.kcmsf.

2. [2]

   The Minuteman-III is 18.2 x 1.85 m, the SS-27 Mod 1 is 21.9 x 1.9 m, and the SS-27 Mod 2 is 22.5 x 2.0 m. See Missile Defense Project, “Minuteman III,” Missile Threat, Center for Strategic and International Studies, September 19, 2016, last modified August 2, 2021, https://missilethreat.csis.org/missile/minuteman-iii; Missile Defense Project, “RT-2PM2 Topol-M (SS-27 Mod 1 “Sickle B”),” Missile Threat, Center for Strategic and International Studies, August 10, 2016, last modified August 2, 2021, https://missilethreat.csis.org/missile/ss-27; and Missile Defense Project, “RS-24 Yars (SS-27 Mod 2),” Missile Threat, Center for Strategic and International Studies, August 10, 2016, last modified August 12, 2021, https://missilethreat.csis.org/missile/rs-24.

3. [3]

   “Press Statement of Kim Yo Jong, Vice Department Director of C.C., WPK Issued,” Korean Central News Agency, February 20, 2023, http://kcna.kp/en/article/q/59ff167348aa7bab60621a1741c8f65d3f3324ff29161b4ee6a300ec434bcf58.kcmsf.

4. [4]

   “Hypersonic Missile Newly Developed by Academy of Defence Science Test-fired,” Korean Central News Agency, September 29, 2021. http://kcna.co.jp/item/2021/202109/news29/20210929-05ee.html