All astronomical gamma-ray observations are done from high-altitude balloons (typically >120,000 ft altitude) or from satellites because the earth’s atmosphere blocks gamma rays effectively. So for the same reason, I think gamma rays from a nuclear-powered low-altitude cruise missile would not be detectable from a satellite. It would be detectable from a few miles away, but only if you have a clear line of sight.
Of course-- I should have remembered that.
There are probably still plenty of ways to very easily detect them: Subtlety is not among their strengths. For instance, I imagine that something flying at hypersonic speed over an ocean would leave a telltale signature on the water’s surface. And there’s always good old radar.
For a one-time sneak attack? Maybe, but honestly, anyone who’s thinking a one-time sneak attack in the nuclear era needs to have their head examined. Putin may be a dictator, but I suspect he’s not suicidal. In addition to that, Russia has a military-industrial complex in many ways similar to the US’s, and “disguised airliner procurement program” doesn’t tick all the boxes generals and defense industry shareholders like in the way “new cruise missile procurement program” does. Finally, Tom Clancy fiction aside, wars don’t generally start according to a years-long planned out process - they emerge from the actions of different nations on a shorter timescale. Even Japan, famous for its planned attacks on Port Arthur and Pearl Harbor (the oil embargo in the summer of 1941 was the precipitating event for Pearl Harbor, and the breakdown in negotiations over Korea as a demilitarized buffer in 1904 was the precipitating event for Port Arthur,) likely didn’t know it was going to be involved in those wars a year before they occurred. The timescale of military procurement is years to decades, while the timescale for planned surprise attacks is on the order of months, so you generally don’t have the desire for surprise attacks driving the military procurement process.
As a regular deterrent? The US and Soviets gave up on regularly flying armed nuclear bombers 24/7 decades ago because of the expense and accidents, and painting the airplanes differently isn’t going to change that. In addition, the “disguised airliner” program only works if it’s secret, and the longer these secret nuclear bomber/airliners are flying (and the more of them,) the less secret it’s likely to be. Even nations on friendly terms with Russia would be less likely to allow its airlines to fly there if it were widely known that airliners originating from Russia were frequently armed with nuclear weapons.
wevets, I think you are agreeing with me in dismissing the whole “nuclear-powered missile disguised as conventional aircraft” idea?
I also agree secret disguised airliner is a questionable concept because the idea of new types of nuclear weapon isn’t necessarily to fight nuclear wars more effectively but to appear to have more capability, as a bargaining chip in times of international tension, or at the nuclear disarmament negotiating table.
I say not necessarily because the Cold War era mentality did at one time, arguably on both sides, feature the idea of more effectively fighting nuclear wars not just gaining the psychological upper hand below the nuclear threshold. But the particular idea of disguised airliner wouldn’t work as well once a war was underway.
On the actual weapon, nuclear powered air breathing missile, it seems generally believed the idea is to chip at US confidence in deterrence, since the weapons with their virtually unlimited range (if they work as advertised) could approach the US from any point on the compass launched from Russian territory. US early warning is notoriously weak for weapons approaching from the south, same reason fractional orbit ballistic missiles might be effective bargaining chips. Although it’s not unlimited potential since lack of full southern coverage could be addressed for a bearable cost.
Although hypersonics in general also have the advantage of lowered warning time against ground radars, can stay below their radar horizon much longer, but much faster than subsonic a/c which also can do so. Again that could potentially be addressed by airborne (or space, conventional IR, doesn’t have to be gamma ray detecting) sensors but it might cost relatively more to do that than build the new weapons.
So even under the reassuring ‘nobody would ever really launch a nuclear war’ assumption (one which unfortunately comes with no gtee) you might build and demonstrate a hypersonic nuclear powered cruise missile to bargain it away (its deployment at least) in return for something different you want the US to give up in some future negotiation. Or, just to lower US confidence if someday there’s a ‘spontaneous anti-NATO uprising’ in one of the Baltic republics that ‘Western propaganda’ says is somehow linked to the Russian govt, etc.
Conventional radar is line-of-sight, so it’s not good at detecting missiles flying at low altitude and can be coming from any direction. There is such thing as over-the-horizon radar but I don’t know if those can detect missiles flying at low altitude.
Also, do we have effective ways to shoot down low-altitude supersonic aircraft?
Errr, sorry, but you base that on what? The Russians have been upgrading their ICBM’s pretty constantly.
No need to speculate on why they want such weapons, I mean beside the fact they have always been partial to exotic technologies and apparatuses. Their doctrine has always called for near enemy coast launched SLBM and air launched weapons like the AS-3 to attack enemy RADAR sites, command centres and airbases, to ease up the work of the ICBMs.
Yes, sorry - sometimes it’s hard to get the tone of a question in a text-only format. No offense was intended. 
One big advantage of the nuclear-powered ramjet is that it gets Russia something like a FOBS capability without violating the letter of SALT II.
Its range allows it to be launched over the south pole instead of the north pole, and it would fly on an atmospheric trajectory. This would easily bypass existing US missile radar and defenses, all of which are more or less pointed northward at space.
SALT II never came into force.
Honestly I am not clear on the treaty status of FOBS. I know the US never ratified SALT II, but both sides honored it, and this was the reason given for the Soviets scrapping their FOBS system (also it turned out to be unworkable).
But that’s beside my main point, which is that the nuclear-powered missile would provide a FOBS-like avenue of attack from the southern hemisphere. It would sidestep any treaty prohibition on FOBS (since it doesn’t orbit), and presumably it wouldn’t have the same technical problems. (I mean obviously it does have technical problems, apparently it blew up on the launch pad last month. But presumably, being a different system, it wouldn’t have the FOBS-specific problems).
Is FOBS or FOBS-like even necessary if you’ve got a credible submarine launched ballistic missile threat? Maybe the Ruskies have lost confidence in their subs.
I am not sure that I understand how this works. I think the nuclear heating comes on after the missile gets to about Mach 3 (by traditional propellants) and then the nuclear reactor is turned on for ramjet propulsion.
How does the heat transfer happen at such high speeds of air traveling through the heating media ? Heating air is certainly not fast but maybe air behaves differently at high compression ratios?
Also - is it possible to turn the nuclear heating down and go back to conventional propellant at low speeds or once on, the missile is always Mach 3 or more ?
It only takes one getting through to make for a bad day.
The Americans are building new stealth bombers and new long range ALCMs. They have lost confidence Rio?
FOBS idea was to deliver nuclear payload from unexpected directions and with little warning.
People keep talking about ramjets, but they also keep talking about low-altitude flight (as in low enough to be stealthy). Ramjets aren’t happy unless they’re going Mach 3+. There are missiles that fly that fast, but they don’t usually spend very long doing so at low-altitude. If you’ve got a missile that’s supposed to cover thousands of miles at stealthy-low altitude, you’re going to roast it within a few minutes after launch if you’re doing those kinds of speeds. The Wikipedia page for cruise missiles lists the Burestevnik as being supersonic; if it’s supposed to be stealthy, then it’ll have to fly low, in which case I’ll bet it’s not supersonic by much.
Might the engine be a standard turbojet, except with the heat being provided by a nuclear reactor instead of by burning jet fuel??
Sounds like you are wondering about the same thing I am.
How does a ramjet REALLY work?
I hate it when there are so many layperson explanations of tech on the net but never including concrete quantitative numbers, so they really leave you none the wiser anyway.
My speculation using high school physics:
You need some tens of Megawatts of heating in a ramjet to make the principle work.
A run of the mill performance of a (shell and tube) heat exchanger is 1kW/sq.m.*K
If the temperature of the inlet is ~1300K and the temperature of the reactor is >2300K then you only need a heat exchange surface area of some tens of sq.m.
That’s not an excessively large heat exchanger (reactor). So I can easily see it work, if these numbers are somewhat realistic.
I think we are wondering around same lines. The overall heat transfer coefficient you quoted are for heat exchange where liquids are involved. For gas/gas exchanger the number is around a 1/10th of that.
I doubt there is a exchanger involved, the air must be going through directly through the reactor carrying all the radioactive leftovers with it - but even then it’s a big heat transfer problem.
Yes, the air goes straight through the reactor, IOW, the reactor is the heat exchanger. The same calculations apply.
The Wiki Project Pluto page mentions 500,000 pencil-sized ceramic fuel elements. I guess we’re talking 1000’s of air channels. Assuming a 1cm diameter and 3m length we could be talking 100’s of sq.m. surface area. That could work with your lower heat transfer performance.
What’s the stagnation temperature for ~2,000 knots true airspeed at low-level, say under 1,000 feet?
From this NASA site, it looks like it is around 1500 Rankine. (https://www.grc.nasa.gov/WWW/BGH/stagtmp.html) Could a flying structure be created that would be largely unaffected by multiple hour flight at 1000 degrees Fahrenheit?
The drag from a Pluto-style heat exchanger must be enormous.