Updated: May 7
As China's ambitions grew to superpower level, the quest for a survivable, compact, launch-on-demand ICBM became inevitable.
In the 60's, the U.S Polaris SLBM was the system that accomplished all three above requirements.
Its MRBM-class range was no hurdle, the basic technology fundament on which it was built clearly offered growth potential up to ICBM-class range.
Back then in the 60's, just like France, China set itself the goal to reach the technology level to build a equivalent to the Polaris.
In the 60's China had just began mastering Soviet second generation liquid-fuel technology level ballistic missiles and was still very far away to achieving something like the Polaris.
Chinese intelligence/engineers saw the Polaris quickly evolving from the -A1 to the -A-3 and then to the IRBM-range class Poseidon.
This attractive technology path was strengthen to be the best approach as Chinese witnessed France going the same path with their M1, M2 and M20 SLBM.
Two key technologies had to be mastered:
A high strength steel allow, well weldable, in order to reduce casing weight and achieve the required structural-ratio/mass fraction.
A thrust vector control system to avoid fins and hence safe on weight and allow container/silo launch, fulfilling future SLBM requirement.
Other technological hurdles were the required cold launch, the thrust termination and the staging of the second stage from the first stage that still created thrust after nominal burnout.
Most other technological issues, such as guidance, had been already mastered by the liquid fuel program.
The star of Chinese missile engineering
The product of this super-project was the JL-1 SLBM of the 1980's and its later DF-21 mobile TEL based MRBM variant.
Like the French, Chinese opted for a conservative approach on the key issue of TVC: four, relative easy to manufacture, relative small length nozzles for compactness.
Three TVC options were available:
Hinged, movable nozzles which were complex and hard to master, famously applied in the Minutman up until today.
Rotating nozzles which had lower technology risks and were first applied in the Polaris A-3
Jetavators which was the lowest risk approach but relatively heavy and created thrust losses
A forth one, jet vanes were well known to the Chinese and DF-11 and DF-15 programs of same development generation applied them, in their solid fuel optimized variation.
However the main problem with Jet vanes beside increased length, was twofold; they created thrust losses of 2-5% like Jetavators, but they also had the requirement of a low corrosive solid fuel.
The combined losses would reduce the Isp of the stages so much, that the two future goals of compactness and IRBM-ICBM-range would hardly be achievable.
Up until today we don't know whether hinged or rotating nozzles were used in the JL-1/DF-21 but rotating nozzles are the most likely candidate. The French are also believed to have used this TVC method until they mastered the single nozzle TVC of the 80's vintage M4.
At this point it should be noted that beside the compact Soviet liquid fuel SLBM school and the superior U.S FBM program, only China and France were participating in this technological race.
In the field of casings, Chinese metallurgy had reached a level that could support the program by the 1980's.
At that point U.S SLBMs already used fiberglass and even Kevlar for their composite casings
The baseline DF-31
The range/throw-weight performance of the JL-1/DF-21 was not high; at below 2000km it could be compared to the 1960 vintage Polaris A-1, representing a gap of 25 years.
Looking at the few players still in this game, it was nonetheless a great achievement for the Chinese engineers and created a motivation to reach beyond the DF-21.
The DF-31 replaced the digit 2 with a 3 as it was the evolved form of a technology, that had been mastered in the DF-21 at all its bottlenecks.
Hence the development strategy was simple:
Increased diameter steel casing to a size, that would allow intercontinental range performance
Add a modified DF-21 stage as third stage on the two new, increased diameter, first and second stages
Use the mature rotating four-nozzle TVC system on all three stages
The program went according to the plan: Instead of the more than 15 years development time of the JL-1/DF-21, the DF-31 development was finished in the 90's.
However the range/throw-weight performance was, as already anticipated, low and the two main contributors, beside nuclear warhead miniaturization technology level, were clear:
High steel casing weight
Lowered Isp due to the four nozzle design, compared to a efficient large single-nozzle.
While a small portion of CONUS could be target by the mobile, nuclear tipped DF-31, it was clear that the next version would have to introduce new technologies in order to improve the range.
Technology enabled: The DF-31A
Like the improvement program of the Minutman I to II and III, the first stage remained unchanged in the next generation DF-31A.
Casing materials were improved but also remained steel for all three stages.
The development strategy was to improve what had the greatest impact on range/throw-weight; concentration on new second and third stages.
A departure from the four-nozzle rotating TVC design was necessary to improve stage Isp/efficiency by using single large nozzles.
TVC technology that would have enabled this and known from U.S examples, were either nozzle liquid-injection or a "flexseal" flexible nozzle.
Mastering one of the two would have created following issues:
Nozzle liquid-injection was risk-wise achievable, but normally increased complexity/maintenance and involved toxic liquids, that combined, would have decreased the robustness of the land-mobile system and its future SLBM variant.
A flexible nozzle would have high risks involved at Chinas 2000s technology level
The solution for the third stage is known and represents a unique and novel approach in terms of missile technology worldwide:
The nozzle design was a large fixed one allowing high Isp, close to what the solid fuel is able to optimally deliver.
TVC is achieved without involving hot parts by changing the center of gravity of the stage. The mass of the relative large re-entry vehicle is moved by a hydraulic system, which in turn creates a change of the effective thrust vector.
This elegant "cold" TVC approach required intensive roll maneuvers, which in turn required a relative large/heavy thruster system. This adds some complexity and potentially maintenance requirements, if it is liquid fuel based.
The TVC system represents the first main missile technology innovation of the Chinese, who up to that point relied on foreign, proven solutions.
The second stage achieves its required Isp improvement by a large fixed nozzle which like the thrid stage nozzle is made of lightweight carbon-carbon, or one step lower technology, carbon-phenolic composite. This improves structural ratio even though steel remains the casing material.
TVC of the second stage remains an open question.
During the U.S Polaris programs it was theorized based on experience that the second stage, after having passed max q and with very thin atmosphere remaining at stage-1/stage-II separation altitude, could omit a TVC system.
Did Chinese experience with flight data from the JL-1/DF-21 program indicate something similar?
Maybe in combination with the following approach:
After stage-I burnout, the shroud of the DF-31A would eject itself from stage-III, via its top small rockets.
The same TVC principle of stage-III would then be utilized for limited TVC action on stage-II, by hydraulically moving the relative large RV mass.
Another technology innovation in the DF-31A was the removal of the third stage thrust termination system of the DF-31.
This additional weight saving is achieved by continuously re-calculating the resulting trajectory and do necessary corrections by the stage-III TVC system. This technology, called GEMS by the U.S, was first employed in the Trident-I and requires computing power and more sophisticated guidance algorithms.
Result was that the DF-31A stage-III could completely burn out the last remaining fuel and still maintain the right trajectory for low CEP RV impact.
All these improvements allowed a range/throw-weight improvement that allowed targeting the U.S westcoast.
The new system also became smaller, allowing the development of its JL-2 SLBM variant which has to pay a range/throw-weight penalty for being a SLBM due to necessary re-designs.
Up to Minutman III range: DF-31AG
As a kind of benchmark for ICBM range/throw-weight performance, the Minteman III's 13000km range would allow to put all of the U.S, into the range of Chinese land-mobile ICBMs. Not only from the peripheral regions but from deep inside central China where protection against the opponent is highest.
Since no photo of the DF-31AG airframe exists in the internet, very few information about it is available. It is somewhat longer than the DF-31A.
A largest contribution to a higher performing system would be to put aside the U.S role model of the Minutman III, which up until today retains the early 1960's vintage, four nozzle, steel casing first stage.
However a more logical conclusion could be the utilization of potentially scaled or not, DF-41 technology in the upper stages.
Even less is known about the DF-41 but if the overall diameter is not uniform, its possible that a 2m diameter upper stage of the DF-41 is used as second, or third stage in the DF-31AG.
At Chinese 2010's technology level the technological improvements could be based on the following three:
Filament casing at least for upper stages made of Aramid/Kevlar or carbon fiber.
Single flexible nozzle for TVC
More energetic solid fuel
It would be striking to expect a steel casing used for upper stages like the DF-31A in a Chinese strategic-only system developed in the 2010's. A MIRV bus on the DF-31AG is a possibility if the third stage diameter was increased to make the missile uniform. However, if as expected, the obsolete first stage is retained, the larger diameter third or lighter composite second stage would only translate into a improved range/throw-weight performance. It is expected that Chinese re-entry vehicle size and weight is still behind Soviet/Russian and U.S level as indicated by the 2000's vintage RV of the DF-31A. Therefore a counter-value single warhead role for the DF-31AG appears most likely.