Since the 1980’s I’ve heard that the advance warning of an SLBM attack was as little as 6 minutes, and I took it as self-evidently correct, since the subs might be launching from a much shorter distance, potentially right off the coast. My recollection is that this was “common knowledge”, though I may have misunderstood.
After some thought and research, I have some doubts about that. I know these are solid-fueled rockets, so they can’t be shut off. That means you can’t just use a shorter burn to get a shorter range. It seems like even if a Russian sub is right off the coast of DC, it still needs to use a highly lofted trajectory to hit DC, which would take about the same flight time as if it were launched from maximum range.
So the questions are:
Do current subs have an actual tested depressed-trajectory capability? (inferring as best we can from non-classified sources, since obviously this wouldn’t be announced)
Is there any other way they can decrease the flight time of their missiles, using existing capabilities and tactics?
What was the deal about the 6-minute flight time assumption? Was this simply talking about theoretical DT flight time, or just the fact that SLBMs have a generally shorter range than ICMBs, was it a different mechanism, or was it misinformation, or misunderstanding?
Well, Wikipedia says that Russia has more than zero active, operational nuclear-powered cruise missile submarines. It also says that a Zircon missile has a range of 135 to 270 nautical miles (155 to 311 mi; 250 to 500 km) at low level, and up to 400 nmi (460 mi; 740 km) in a semi-ballistic trajectory, and that it can travel at Mach 8. A crude calculation: at a range of 400 km and an average speed of 2.7 km/s, that’s not a long flight, certainly less than 6 minutes if you count only the flight phase.
Of course, cruise missiles also aren’t the same thing as ballistic missiles, so while that might be relevant, it’s not exactly what the OP is asking about.
Yes, SLBMs are typically fired in a depressed trajectory, that is, the trajectory is shaped during the boost phase to minimize the apex altitude and have a shorter flight time to target.
Practically speaking, no. the D-5 ‘Trident II’ ICBM is a three stage booster with the first two stages designed to booster the vehicle up out of the atmosphere, the third stage to shape the trajectory for targeting, and then the liquid post-boost vehicle (PBV) which provides final accuracy refinement and to separate the trajectories of the RVs. The ‘physics package’ (euphemism for the nuclear weapon) is not armed until late in flight to prevent any kind of accidental arming, and separating the RVs before end of boost is neither practicable nor would give good accuracy even if you could
SLBMs have a shorter distance and generally lower apex, so they have a shorter flight time (although 6 minutes is really at the low end of that). There is a minimum practical range (technically you could fly the weapon straight up with just enough arch to account for the rotation of the Earth and bring it back down arbitrarily close to the launcher, albeit with degraded accuracy), and frankly if you had a submarine just off the cost of Washington D.C., it would make more sense to perform a ‘decapitating strike’ with cruise missiles that with good planning likely wouldn’t even be discriminated as threats until they already struck their targets.
However, getting a Russian ballistic missile submarine anywhere that close to the American Eastern Seaboard is highly unlikely, not only because of the difficulty passing GIUK Gap without being detected by underwater surveillance (formerly SOSUS and IUSS) but because Russian submarines just don’t have long patrol durations even when they are able to sail. in the Soviet era, the Northern Fleet tended to say in the Barents Sea, occasionally edging out into the Norwegian and North Seas but almost never without detection (if NATO propaganda is to be believed) and trailed by US fast attack submarines. And per deterrence doctrine, even such a first strike isn’t gong to prevent a response by US land-based ICBMs, the Sea Based Strategic Deterrent (Fleet Ballistic Missile), and the B-52 (carrying the AGM-86 ALCM cruise missile), B-1, and B-2 (carrying versions of the B61-4,-7,-11) bomber fleets. Even with retirement of the LGM-30F ‘Minuteman II’ and LGM-118A ‘Peacekeeper’ ICBM fleets, and mostly restricting the LGM-30G ‘Minuteman III’ to single MK21A RVs, there is still enough retaliatory firepower to assure a catastrophic response from the ICBM fleet alone, assuming everything works as designed.
Fer sure. A cruise missile is a whole other thing and I would think has a minimum range.
I’d think that, in theory, a ballistic missile could be programmed to land on the ship that launched it. It’s in the name…ballistic. Just program the missile to fly a short trajectory and it’ll do that. Of course, it still probably has to go very high and then come back down so “short” is relative and there is a minimum flight time involved but if you can draw a ballistic arc from A –> B then I see no reason they could not do it.
Is this actually true? The sources I’m finding seem to suggest that although fielding a depressed-trajectory SLBM is very much achievable, there’s an absence of any description of any actually existing DT capability, and quite a bit of information about what would be needed to create one, and some language about why creating one would be a very destabilizing development for the balance of deterrence. So this suggests to me that no actual DT SLBM has been fielded.
Of course an SLBM will inherently have some depression of trajectory relative to an ICBM, just because it’s shorter-range and will have less energy, but my impression is that this is just a lower-apogee suborbital shot, as opposed to purposely burning more fuel to spend more time horizontally boosting in the atmosphere.
For transparency, what’s on my mind right now is Trump’s public suggestion that he’s moved a couple of subs closer to Russia, implying they should be worried about this. This might deserve its own thread, but I’ve concluded that this poses no added threat to Russia, because it seems clear that the flight time of a solid-rocket missile can’t really be decreased by being closer to target, hence the flight time to (say) Moscow must be about the same whether it’s fired from the Azores or the Baltic Sea.
I was thinking ballistic missiles, but part of my question was about why I’m thinking of a 6-minute flight time, so sub-launched nuclear cruise missiles might be relevant (if those are or were a thing, I’m not clear on the status with all the different treaties and whatnot).
“Depressed trajectory” means that the trajectory is shaped such that it is not flying a high arc to achieve maximum range but rather a lower apex for shorter range and flight time. The D-5 SLBM could actually strike at Europe or Centra Asia even with the maximum RV load currently carried, so to hit targets that are closer during a mid-ocean deterrence patrol it would either need to fly a higher arc or a depressed trajectory to manage excess energy from the boost (lower stage) motors.
I think the “very destabilizing development for the balance of deterrence” are highly depressed trajectories using hypersonic maneuverable reentry vehicles (MaRVs), which can fly lower and difficult to track trajectories that reduce the time for a mid-course-phase interception and could potentially evade terminal-phase defense systems. The extent to which that is practically destabilizing is pretty questionable since satellite-based launch detection systems will see the launch of salvos of missiles regardless of whether the trajectories are depressed or not, and unless there is the first strike elimination of virtually all retaliatory capability or command & control authority any strike is going to provoke an all-out response that will eliminate all parties involved.
That is more or less the case. Ballistic missile submarine patrol areas are defined in terms of staying in deep water where they are difficult to detect and could (at least in theory) survive post-launch versus just being as close as possible to the target. It would be inadvisable to move these submarines into littoral regions where maneuverability is constrained and bottom scatter could help conceal an opposing fast attack submarine; in their deterrence patrol patterns they are already where they need to be in order to execute a prompt launch order.
Right, that’s my question. Of course it’s possible for a given SLBM to hit a shorter-than-maximum range using a lofted trajectory and the same flight time. But is the D-5 in its current form able to fly the depressed trajectory to achieve both a shorter range and a shorter flight time? I’m talking, get down from say a 12-minute flight time to a 5-minute flight time. This is not clear to me, but it seems there’s not a lot of scope for variation there.
I’m not going to talk about specific numbers but if you want to imagine a trajectory think of an ellipse that starts at the end of powered flight and intersecting the slice of the Earth in that azimuth (which for simplicity we’ll take to be a circle even though it is actually an oblate sphereoid). For a high apex trajectory, it is flown almost straight upward forming a very eccentric arc where it spends most of its impulse going up and then down instead of ‘forward’ along its trajectory. This is also a more efficient use of impulse, and thus gives the maximum ‘throw weight’ for a given boost vehicle configuration. By contrast, a depressed trajectory will expend more impulse going forward, forming an ellipse that is less eccentric. The path length of the trajectory will be less and while that isn’t a perfect relationship to ballistic flight time before impact it will give an idea of why a depressed trajectory is somewhat faster. The tradeoff is less throw weight, although if you are firing at some fraction of your maximum range that probably isn’t a consideration, and more uncertainties, although if you are able to dial in your trajectory with the PBV you can still get far more practical accuracy than you need, especially for the W88 with a reported yield of ~475 kTtnteq.
Shut off no, but the motor casing can be breached by linear shaped charges or blowing out a port as part of a thrust termination system. The Pershing I and II missiles had such a system (vents). I’m sure there were others.
If they’re British SLBMs they can get the flight time down to just a few seconds.
Cold War talk in the UK always mentioned the ‘four minute warning’. My understanding was that this was the time between identification of the threat and it hitting us. The actual flight time would be a lot longer, but it takes time to identify if it’s a missile or a rocket and if it’s heading for us or not.
Test flights of ICBMs, targets, and space launch vehicles employing solid propellant rocket motors are equipped with a flight termination system which incorporates explosive charges (typically linear shaped charges (LSC) in the raceway or sometimes mounted to the dome, or else directional penetrating charges firing into the forward dome) which abruptly terminate thrust and combustion by splitting the case open. This is an uncontrolled event that typically results in the payload and other structures tumbling wildly. A few motors like Stage 3 motors in different versions of the LGM-30 A/B/F/G ‘Minuteman’ ICBM incorporate thrust termination, either by releasing covers to ports built into the case (M57 on the Minuteman I/II) or using LSC to cut ports into reinforced areas of the dome (SR73 on the Minuteman III) into stacks that direct the exhaust forward and outboard. In both cases (and with the Pershing), thrust termination is intended for precise targeting and can only be used after a certain point in motor burn where the propellant grain has burned back to expose the case and insulator, and only safely when the pressure has dropped below a certain threshold so it really can’t be used to significantly reduce time in flight. Both Stumpf and Sutton have diagrams showing how this works.
Right, I get all that. And there are other tradeoffs such as increased aerodynamic forces that may actually damage the vehicle. But while the theoretical capacity is there, it’s not clear to me that DT trajectories have actually been tested on Trident missles, that the crews are actually authorized or capable to program that kind of firing solution. In short, that it’s an actual tested and trained capability rather than something that ought to be possible.
Because given that solid rocket boosters can’t be shut off in mid-flight, it seems this approach would rely on atmospheric friction and payload weight management to shed enough energy that a DT trajectory doesn’t overshoot the target, not to mention the added risk of damaging the vehicle from added atmospheric friction from the depressed trajectory.
In short, while I think this capability probably could be implemented without a lot of effort, it’s not clear to me that it’s actually been tested and trained, nor that it’s simple as altering the trajectory and weight. I don’t think I’m hearing that anyone else knows this for a fact, either.
The use of depressed trajectories for the D-5 ‘Trident II’ has been extensively studied. Until recently treaty stipulations prohibited testing depressed trajectories on SLBMs (or the development of MaRVs that would be required for use with them). But that may have changed:
While “atmospheric friction” (drag), has a small effect on the performance of an ICBM early in first stage boost, most of the trajectory (even for a depressed trajectory) is going to be sufficiently above the most dense part of the atmosphere as to be almost irrelevant. Ballast could certainly be added to the payload to absorb some of the impulse and shorten the maximum range but it is far easier from a design standpoint to shape the powered portion of the flight trajectory to ‘waste’ energy that would normally go into achieving a longer range, i.e. instead trying to loft the trajectory as high as possible to achieve maximum range, arching it over to expend impulse and shorten the flight time (hence, a ‘depressed’ trajectory, one that is lower than could be achieved). This is done all of the time with space launch where a payload needs to reach a specific set of orbital parameters and energy state which is significantly less than the booster capability. It is also possible to do guidance-directed maneuvers (termed Generalized Energy Management System or GEMS) to expend extra energy as calculated by the guidance system looking at thrust performance and estimating how much additional energy and duration the solid motor has left.
That’s the same link I shared earlier. That paper is entirely a hypothetical discussion of how it might work, not a description of any fielded capability.
It may not have. That’s a cockpit video that someone claims is a depressed-trajectory Trident launch. It might be, but I can’t seem to find corroboration elsewhere, and I’m not sure about the reliability of “The War Zone” or whoever that is.
Moreover I’d think that if the Russians actually observed evidence that the US tested DT capability, they’d be making all sorts of noise about it, as would other OSINT researchers. It wouldn’t just be a single blog saying “this might be it”.
Right, but reaching orbit is just about reaching a certain velocity, not about reaching a certain place on the earth’s surface, which is a different problem.
In any case, I’ve already stipulated to the point of exhaustion that it probably wouldn’t be very costly or difficult to field a DT capability, so I’m not really interested in retreading that. It’s a doable thing. The question is whether that capability has actually been designed, tested, trained, and fielded. I’m still not seeing it.
Getting to a specific orbit isn’t just about velocity; it is having to shape the trajectory to achieve all of the osculating elements (inclination, longitude of the ascending node, argument of periapsis, et cetera). It is essentially the same problem albeit you usually have a somewhat larger ‘box’ (unless you are trying to intercept something else in orbit) and you don’t have to worry about the dynamics of reentry.
Reading back through the thread I think something that isn’t clearly stated but should be understood is that the time of powered flight for an ICBM is only somewhere between 3-4 minutes for the main booster (not counting the post-boost vehicle or any other dynamic elements after separation), and only the first minute or so of that is in significant atmosphere. Once the booster is all expended, the PBV or unitary payload is flying a purely ballistic trajectory which is determined by the end state at separation. By shaping the trajectory during that few minutes of boost it is possible to hit a wide range of distances along that azimuth and lengthen or shorten the ballistic time in flight by making the trajectory more or less lofted, and flying a ‘non-optimal’ (compared to maximum range) trajectory can allow the payload to impact shorter and sooner than it would otherwise. So, for a six minute flight, the vehicle could just fly with a constant turning vector or just perform maneuvers to ‘waste’ excess energy to get a shorter, less lofted ballistic path.
As to whether it has been tested by the US Navy there is no official word; for the traditional ‘deterrent’ use of SLBMs it isn’t needed because all of the Soviet targets were far inland from the deep water patrol grids in the North Atlantic and Pacific oceans. The Soviets definitely did some testing of depressed trajectory vehicles pre-START (as well as fractional orbit trajectories for surprise bombardment, another prohibited capability) but all it really requires from a design standpoint is shaping the trajectory in simulation and, for extremely depressed trajectories developing a maneuvering reentry vehicle for precise targeting with high angle of attack.