Back Engineering Oreshnik

I don’t know if anyone has done any back engineering yet, so I’ll start. The following is a first pass, speculative, mostly qualitative. It’s done without crunching very many hard numbers or running simulations, informed by some background in aerospace engineering and in hypersonic flight. One major problem in this kind of retro-engineering is the lack of trustworthy data. Military secrets are closely guarded and surrounded by flak.

As of this writing, a few days after the attack of Novemeber 21, 2024, there is very little information or reliable analysis available on how this weapon works. It appears to be a species of multiple-warhead, kinetic-kill munition launched by one of the Russian ICBM or IRBM systems, possibly the S-26_Rubezh. Which particular launch system was used is of secondary interest, what’s interesting is the final stage design. It didn’t appear to use explosives- though a variant probably could- relying instead on the energy of large projectiles moving very much faster than bullets.

The information at present suggests there were six groups, or salvos, of six projectiles each for a total of up to 36 hypersonic “bullets”. They may have hit the ground at the Ukraine factory at about Mach 10, or about 3.4 km/s. We see large bright streaks, which are the ionized air of the plasma sheaths that always result from pushing objects through the air at such speeds. We also see very tight groupings of both the individual salvos and of the overall pattern of the six salvos, which gives us some clues.

Still shot of 6 incoming projectiles of one of the six salvos from X

What was the actual trajectory?

What was the total delivered mass?

The correct answers to these two questions will determine whether the Oreshnik is a real “game changer” or merely a spectacular fireworks display. The Russians are understandably unhelpful in this regard, though they do imply that a phalanx of Oreshniks would be functionally equivalent to a tactical nuclear bomb. The yield calculations suggest otherwise on the face of it, but Oreshnik-like munitions deliver precision destruction over a much wider area than a single-point nuclear explosive can. It’s more akin to an unstoppable rocket or artillery barrage delivered on point as if a bolt from the blue.

The following are some speculative design ideas, which I expect would be some of many different concepts the ‘Russian Aerospace Design Committee’ would have looked at over the past few decades. Regardless of the particular design details, the scaling laws, together with the restrictions described by the rocket equation, limits this type of weapon to relatively short ranges, with the 800 km throw of the Dnipro attack being within its possibilities. A truck-mounted rocket can only be so big; a siloed ICBM can only cost so much, a hypersonic missile can only be so small.

A MIRVed missile deploys its warheads one-by-one while still in the vacuum of outer space. The cone shape isn’t designed to travel through the lower atmosphere at Mach 10.

The idea that the projectiles were like re-purposed MIRV re-entry vehicles doesn’t stand up. The cone shape isn’t suitable for low drag at hypersonic speeds; the accuracy imparted by the bus mid-course corrections from 400 km away and 200 km up probably can’t account for what was seen.

A naive calculation of the ballistic trajectory in a flat Earth model without air. Maximum altitude is 200 km.. Entry angle is wrong, vertical component of the drop-in velocity is too low at 2 km/s.

There are two conflicting design requirements for this type of weapon. First, the rocket should go as fast as it can over to the target and second, the projectiles should drop down as fast and as nearly vertical as possible to minimize the hypersonic flight path in the thick lower atmosphere.

There are several ways to resolve this conflict. A stick-figure model would have the rocket travel horizontally over to the target as fast as it can, then a big retro-rocket kicks in to slow its speed to zero. Another big rocket pointing down then drives it into the ground. This design, though it illustrates the problem, was probably discarded early on.

Another method is to launch the rocket more nearly vertical so that it travels high into outer space. The RS-26 has a claimed terminal speed of 6.81 km/s. Launching at 85.1 degrees from horizontal from a flat earth, it would rise to 2,346 km and take a leisurely 23 minutes to arrive at its destination. Once it got there, it would only be able to deliver an unimpressive 800 kg, since it took three stages and spent all of its energy making speed.

A third way would be to react against the atmosphere to turn the vehicle in the manner of an airplane or a hypersonic Avangard. Someone at Southfront also wrote “It is possible that the rocket was equipped with an Avangard-R hypersonic glider.” If the turn radius of the glider was 100 km, it would take about 70 km of altitude to make a 45-degree turn while pulling about 8 g’s.

Another possibility is an apogee retro-burn from a putative (liquid fueled) third stage to reduce the horizontal velocity component. And/or a third stage burn is done just before entering the atmosphere, angled downward. These kinds of add-ons are probably too wasteful.

The actual trajectory may have been a combination of rising higher than the minimum-energy ballistic path and then the use of the glider to finish the turn. This would add more speed while reducing the angle of the turn. There are design trade-offs in all of this which make the optimal trajectory solution different for each launch. The glider loses energy due to drag; the higher altitude loses payload capacity because more fuel has to be expended.

Given an impact speed for the warheads of 3.4 km/s (Mach 10) and guessing another 0.6 km/s for drag loss brings the launch speed up to 4 km/s. Launching at 75.3 degrees gives a 13 minute time-of-flight with apogee at 760 km above the surface. The hypersonic turn angle is reduced to about 10 degrees, a much less strenuous maneuver with less drag loss. That is one possible answer to the first question above. A burnout speed too much faster than 4 km/s would require adding a third stage, which would restrict the size of the payload, so there’s a clue to the second question.

There’s not much information about the booster or even which type of rocket was used. If it was a 36,000 kg RS-26, the mass is the same as the Minuteman III. Its first two stages should be similar, though slightly larger in diameter and shorter in length. It may be more robust due to the tube launch from a mobile platform, so the mass ratios and top speeds are lower.

A Minuteman III ICBM. 3 stages, all solid rocket motors. Launch weight is 36,000 kg, top speed is 7.83 km/s, where the RS-26 Rubizeh is 6.81 km/s.

If the masses, mass ratios, and other parameters are available for the Minuteman III, these could be used to get a better number for the total third stage, or payload, mass. In lieu of that, a ballpark guess can be put together using a few rudimentary considerations. It’s somewhere on the order of 5000-10,000 kg, maybe 7500 kg.

The hypersonic glider doesn’t need much. It’s a hollow shell like a miniature Space Shuttle, longer and thinner with a similar type of skin and some control surfaces. The less it has to pull g’s to turn the smaller the wings or lifting body in plan view. Inside is a payload dispenser which probably ejects the six warhead sub-buses out the back, where the air density of the hypersonic wake is almost zero. The warheads have a higher ballistic coefficient so they will pull ahead of the glider after doing a stochastic walk through its turbulent wake. We can see from the videos that the rate of release was about one set of six warheads per second.

The 36 warheads are probably tungsten or iron rods with some tail fin, not cone-shaped MIRVs. Tungsten-coated iron rods would be cheaper at the cost of doubling the cross section and the drag. They’re not traveling very far nor subjected to heating for very long. As an embellishment, the warheads themselves may have been pre-cooled with liquid nitrogen in order to buy a few more milliseconds or meters of atmospheric travel. The glider would point at the target in order to minimize or eliminate the need for elaborate warhead maneuvers. They travel in basically straight lines after release. A ten-second flight from 35 km altitude would only add 100 m/s to the theoretical impact speed, mostly in the downward direction and only a few m/s sideways.

A no-frills design

The launch pad is a truck-mounted tube similar to a Russian ICBM launch system. The IRBM has two solid rocket motor stages and an unpowered hypersonic glider for the third stage.

The missile is launched at a high angle, near vertical, without curving over in the way an ICBM and most other ballistic missiles would. After second-stage burnout, the glider coasts up to an apogee several hundred kilometers above the Earth and halfway over to the target then continues in its orbit back down to the surface.

The glider enters the upper atmosphere traveling about 4 km/s in a near-vertical drop. It is oriented in roll and pitch to be able to execute a turn downward, bringing the trajectory closer to vertical as it falls. At less than 100 km altitude it is still in the turn with 25 seconds left before impact.

At 15 seconds to impact and 50 km above and out, the glider is pointing directly at the target. At this point the remaining trajectory is practically a straight line downward. The release sequence begins for the six bundles of tungsten-tipped iron rods. Each bundle of six is close-packed around a hollow center tube and held together with the high-end plastic zip ties or similar. The zip ties keep the bundle together just long enough for it to clear the wake of the glider and pull ahead of it, then they disintegrate. A more elaborate design might incorporate a central bus for the bundles with its own guidance and control to allow a release from further away.

At nine seconds to impact the 36 rods are flying on their own and slowly separating from each other. The empty glider decelerated at an increasing rate during the release sequence, so maybe the sequence had to start from a higher altitude. It might have a mechanism that causes it to break up in the hypersonic wind. There would be a rain of hypersonic-glider debris quite some time after the main event.

With all of that, the total delivered load of all 36 warheads is guessed to be somewhere between 3600 and 7200 kg, or between 100 and 200 kg per rod.

One benchmark number is the speed at which an arbitrary mass has the same energy content as it would if it were a mass of TNT. With the energy of 1 kiloton of TNT being defined as 4.184 megajoule, that speed is 2.893 km/s, regardless of whether the mass is feather pillows or golf balls. With kinetic energy varying as the square of the speed, an object moving at 3.400 km/s has 1.381 times the energy content of TNT, a dimensionless ratio of kinetic to chemical energy content.

For the 3600 kg total, the total explosive equivalent yield would be 36*100*1.38 = 4.97 tonne, or about 0.005 kiloton, and 0.01 kiloton for the larger number.

Conclude what you will.