In the thread about what one would write on a nuclear bomb, I came across this picture. I’ve seen similar ones before, just like the name says, they are independently targeted. But how do they steer them? There’s no obvious fins or retrorockets, what’s up with that?
They’re ballistic. The targeting happens before and on release:
Maybe thats another reason that MIRV’s made ABM’s unfeasible, besides sheer numbers? The warheads may come on multiple trajectories and you don’t know which one until pretty late in the game?
As suggested, operational US MIRV warhead/RV’s themselves are unguided. Rather the ‘bus’ which holds them maneuvers outside the atmosphere using rocket thrusters and then releases the warheads on pure ballistic trajectories to their various targets.
However a single warhead/re-entry vehicle could maneuver. This is what MAneuvering Re-entry Vehicles of ICBM’s did or do, starting with US experiments back in the '60’s (but never operationally deployed), as well as the Pershing II (actually deployed) later Russian land attack guided BM’s, and what antiship ballistic missile RV’s must do. The US ICBM MARV’s are believed to have had some kind of flaps on the after body (I don’t think they’ve even been seen in open sources), IOW to maneuver only inside at least the outer reaches of the atmosphere. The Pershing RV (which has often been seen in open sources) had cruciform fins, as do unofficial depictions of Chinese MARV’s.
I’m surprised they can get such accuracy out of ballistic projectiles. Are we sure of that, is there a solid source? I would have guessed that those holes near the bottom were nozzles for maneuvering rockets.
The actual accuracy of long-range ballistic weapons is very well known, because they’re actually tested. For ICBMs, generally launched from Vandenberg AFB towards Pacific missile ranges (like Kwajalein). All the components of the missiles are monitored carefully the entire flight by various active and passive methods, as are the splash sites for the payloads.
Downrange facilities are also the logical place to test terminal-phase defense systems like AEGIS BMDS.
The ports you see near the base of the Mk 21 RVs are for inserting arming plugs and controls. At the speeds the RVs are going at reentry, forward or laterally firing thrusters would be completely unstable. (Apollo and Gemini did have thrusters that it used for guidance and stability in the forward section but it entered base first and were naturally stable.) As gnoitall notes, the RVs (with inert ballast in place of the physics package) are tested.
The picture shown in the o.p. contains the same Mk 21 RV used on the LGM-118A ‘Peacekeeper’ missile during its operational life, and since fitted to the LGM-30G ‘Minuteman III’ fleet to replace the Mk 12A. However, the warhead bus and vehicle described below are not seen because they were destroyed per treaty stipulations. Both Peacekeeper and Minuteman III have what is referred to as a post-boost vehicle (PBV), which consists of a storable liquids propellant system, a guidance system or ‘wafer’, a warhead bus (the structure and electronics which control and release the RVs), and then the RVs and whatever countermeasures are carried (decoys and chaff). The PBV fires somewhere before apogee in order to give the system a certain momentum, releases an RV, and then maneuvers again, repeat and rinse, allowing each RV to have a different target within a certain downrange and crossrange target box.
The Peacekeeper was capable of carrying up to 10 RVs, while Minuteman III carried only 3 Mk 12A or 1-2 Mk 21 RVs, but the Soviets put this to shame with their ginormous R-36M (NATO reporting name SS-18 ‘Satan’) which could have potentially carried 25-30 RVs, although the Mod 4 and 5 carried only 10 (but many decoys) which does, in fact, make antiballistic missile defense enormously challenging. The Soviet systems were never terribly accurate; Janes’ Defense estimated the circular error probable (CEP) of the SS-18 to be about 500 meters, while the Minuteman III and Peacekeeper were advertised to be less than 150 meters CEP. However, given the size of the warheads and that most were used in air burst against open targets like cities and military/industrial installations, even the larger CEP is quite adequate for deterrence value. Maneuverable reentry vehicles (MaRVs) have been extensively studied and considered, but not so much for increased accuracy as an ability to hit targets that would be difficult on a purely ballistic path or to increase strike range by using hypersonic lift. MaRVs are technically very challenging but most nuclear-armed nations are in the process of studying hypersonic maneuvering technology for future deployment in both nuclear and tactical weapons.
BTW, the above mentioned R-36M, as well as the Minuteman and Peacekeeper missiles have found a post-Cold War second life as space launch and sounding rocket vehicles. The Dnepr, based on the R-36M, is used as a medium lift vehicle and for multiple satellite deployment, actually using its PBV (which, I found out at a conference a few years ago, actually fires backward which is kind of perfect for deploying large strings or flocks of smallsats). The Minuteman II and Peacekeeper are used in the Minotaur family of launch vehicles with the addition of upper and kick/apse stages, all flown with 100% success to date, including a Minotaur V (Peacekeeper) mission which flew a lunar examination probe (LADEE). It is kind of a great reversal of fortune that these tokens of political and ideological conflict were not only retired without waging the apocalyptic war they were designed to execute, but now provide some genuine utility.
Stranger
This is another reason why the RVs previously shown are not MaRVs designed for increased accuracy. How would it know which way to maneuver? An ICBM warhead is sheathed in conductive plasma and cannot receive any radar signals, photographic updates, etc. It is in a total communications blackout. In that era the only useful reentry guidance would have been inertial, and it already got a good internal fix and fine delta V adjustment from the bus right before reentry.
The INS is one of the most expensive and delicate parts of an ICBM, and they wouldn’t put one on each warhead: https://s-media-cache-ak0.pinimg.com/736x/27/1b/1a/271b1a85493e1c1aed3e176bea05369d.jpg
The RV could not have received GPS even had those satellites existed back then. The only solution is downlink updated nav data from higher-flying specialized satellites on an angle that avoids the plasma sheath on the lower side. That is how the U.S. space shuttle communicated during reentry once TDRS was available and is similar to how late-generation MaRVs are thought to work.
None of that existed when the RVs previously shown were developed, so that’s one way we know they were not MaRVs.
True, no operational US ICBM RV’s have been MARV’s, as far as is known from open sources. However the US did develop them, the Mk.500 Evader tested a number of times in the '70’s was apparently workable. However as the name suggested, the idea in that case was to evade ABM’s by flying pre-programmed evasive maneuvers around a pure ballistic trajectory to a predetermined endpoint, with self contained inertial navigation. Such a feature decreased rather than increasing accuracy, and was eventually judged unnecessary in view of the numerical preponderance of non-maneuvering MIRV’s against any feasible ABM defence in a general nuclear war, and ‘penetration aids’ like decoys could make that problem even more daunting for ABM systems.
As far as the feasibility of guiding BM RV’s with non self-contained guidance (GPS, terminal seeker etc) that increases accuracy, that depends on the application and limiting parameters which are not publicly known, especially considering the possibility of advancements in countries other than the US that US experts don’t know about and might mistakenly believe aren’t practical (eg. like pure oxygen torpedoes were believed possible but not practical outside Japan in the 1930’s). At below full ICBM speed/range it’s definitely possible: again the deployed US Pershing II RV had a terrain correlation radar seeker. At SLBM speed/range it’s apparently possible to use GPS during enough of an RV’s flight to be effective, since the US apparently successfully tested such RV’s for a conventional version of the Trident missile, at what range/speed isn’t publicly known AFAIK. And the Chinese antiship BM’s must have terminal guidance, though again sub-ICBM speed/range. Another technique would be to aerodynamically slow an RV to whatever the maximum feasible speed is for a particular form of (non-inertial) guidance.
It is probably not how it is done, but if they were spin stabilized, it is possible to steer them by shifting weights within the body, and using the lift of the body. This is done with some ship launched missiles that I am aware of.
Conventional RVs such as the Mk 12a and Mk 21 are not spin stabilized, and trying to spin them would likely result in a decrease in accuracy. The ablative behavior of the RV and its impact on aerodynamics during terminal descent is very well characterized.
Stranger
To be clear, the OP question was how are ICBM MIRVs steered (they aren’t), and follow up questions asked why not steer them to improve accuracy. This is perfectly understandable since it’s difficult to believe ICBMs with unguided, non-steering RVs achieve such accuracy. Supposedly the MGM-134 Midgetman would have achieved 90 meters CEP without any terminal guidance: Encyclopedia Astronautica Index: 1
The MGM-31 Pershing II is mostly unrelated to the OP question. It is a medium-range (about 1000 miles), Mach 8 missile. Its peak trajectory altitude was only about 100,000 feet. Reentry blackout happens above roughly Mach 10, so the Pershing II never experienced that and could easily use radar imaging for terminal guidance.
Re SLBMs, they are more akin to ICBMs in range, speed and reentry dynamics. The Trident II D5 has a peak velocity of about Mach 24 and a range of about 7,500 miles. It uses (or used) stellar navigation to update the INS and GPS has been tested. However this is for a mid-course positional update, not for steering during reentry which was the OP’s question. In general SLBMs are just like ICBMs from a reentry plasma dynamics standpoint.
It is theoretically possible today an ICBM or SLBM RV could receive navigation updates during reentry via satellite, and this may have been investigated for Prompt Global Strike: Conventional Prompt Strike - Wikipedia
but the OP’s question was about existing ICBM RVs (Peacekeeper RVs were shown).
As already stated there are apparently some medium-range missiles that use terminal guidance such as the DF-21: DF-21 - Wikipedia However that is a Mach 10 missile, not remotely close to an ICBM.
So for nuclear ICBMs or SLBMs reentry steering isn’t really needed since they are already so accurate. Due to the reentry blackout issue it also wasn’t possible until more recent years. For a conventional ICBM, reentry steering is more important but those have various military destabilization issues since they look just like a nuclear ICBM from a trajectory standpoint, hence they’ve never been deployed.