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Imagine a certain nation tested a hypersonic nuclear-capable missile and it was designed to make a few trips around the Earth at low altitude before striking the surface. Don't they need to worry about the prolonged period of time the missile needs to survive the temperature of aerodynamic friction from air colliding with the missile? It increases exponentially with speed and at hypersonic velocities (5-10 times the speed of sound) the temperature around the missile should melt most metals known to science.

My question is why risk a nuke since so far no one is capable of intercepting it anyway? Why not just plot a direct route and let it scram at low altitude until it crashed into the target zone if efficiency is all that matters?

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    $\begingroup$ Most of the presumptions here are untrue. A stagnation point in fluid dynamics is the point where the fluid goes on either side of the moving body, and has nothing to do with the temperature. Tantalum, tungsten, rhenium and other metals have melting points well above the temperatures generated by friction in hypersonic flight, and can be used in alloys, or ceramics can be employed as they were in Project Pluto or reentry vehicles. $\endgroup$
    – jdunlop
    Mar 22, 2022 at 8:25
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    $\begingroup$ @jdunlop the major problem is heat propagation through the heatshield. A lovely refractory coating stops your missile/re-entry vehicles burning up, but it will still conduct heat in and it will still be ablated away... its a delaying technique, and the OP's suggestion probably requires it to delay for too long. Also, the stagnation point is where the temperature is gonna be highest. See also: stagnation temperature. $\endgroup$
    – Starfish Prime
    Mar 22, 2022 at 10:26
  • $\begingroup$ @jdunlop also, I believe project pluto and its modern equivalents are supersonic, but are not hypersonic. I'm not 100% certain on that though, so if anyone knows otherwise I'd happily be corrected. $\endgroup$
    – Starfish Prime
    Mar 22, 2022 at 10:28
  • $\begingroup$ Possibly a duplicate... Related broader question, with applicable answers though, was put yesterday: worldbuilding.stackexchange.com/questions/227075/… $\endgroup$
    – Goodies
    Mar 22, 2022 at 12:38
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    $\begingroup$ The actual problem here is that you need a world which has hypersonic nuclear missiles which orbit earth at low altitudes several times before hitting their target? And we need to justify these world-touring missiles? Someone tell me how this isn't the actual story, because a story which needs this device in its world escapes me. $\endgroup$
    – Vogon Poet
    Mar 22, 2022 at 15:35

5 Answers 5

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  1. No need to orbit in the atmosphere. The ballistic trajectory goes outside the atmosphere, where there is no air, no friction.

  2. No way a rocket could fly a few trips around the planet in the atmosphere anyway. Energy-wise, it will run out of fuel going against such drag.

  3. Even if re-entry is to be endured for a long time, we have materials for that. Ceramics and heavy metals can withstand it. The temperature of plasma near the ship will be larger than the melting point, but the surface will not be as hot.

  4. It is actually easy to intercept low orbit stuff. The energy required to intercept a low orbit is about 10 times less than the energy needed to go into that orbit. Even much less technically advanced nations can intercept an orbit, even if they can't go there. Lots of our anti-air rockets designed to hit a plane can intercept a low orbit, actually, with very few changes. The chances for a successful interception are much lower than for a plane unless changes are significant, but still, numbers can help, and no bleeding edge tech is needed.

The reason why orbital weapons are so deadly is that they can stay in orbit for years. Not just 3 turns, but thousands of turns. Then it is more of a weapon storage, that can be utilized at any point in time. If there is no war yet, and a nation puts nukes in orbit, then another nation needs to either intercept them right now, before they activate, or just accept that they can't do it later. Once a nuke decided to drop from orbit onto a target - it cannot be intercepted, not even by most modern tech. Nukes can only be intercepted in the speed-up phase, or if they are staying in orbit.

If you know that a nuke is going to hit in 20 minutes, it is somewhat better than not knowing when it is going to hit at all. A nuke from orbit can wait for 3 years, and then drop in just 3 minutes. Normal nukes with their 20 minute delivery time give at least some time to run to a bunker, or launch a response. But orbital nukes with their delivery time of just a few minutes give no such chances. You either live in a bunker, or just accept that you will not have a chance to get to it.

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    $\begingroup$ Do you have a source for "drop in just 3 minutes"? Prompt re-entry requires big engines, and big heatshields. If you're referring to the Soviet-era fractional-orbit bombardment system, be aware that the FOBS had an orbit low enough that it couldn't be sustained for years, as atmospheric drag would sweep it out of orbit in fairly short order. $\endgroup$
    – Starfish Prime
    Mar 22, 2022 at 17:42
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    $\begingroup$ @StarfishPrime current rockets that go to orbit do so in 3 minutes, thats their active phase. I assumed about the same time to go down. Using engine to actively slow down. They likely will not need as much fuel, as a lot of slowing down can be made by reentry friction. There is no such system, so I cant give you a link. From basic calculations, 2km/s delta v will allow to drop the orbit by 360 km in 3 minutes. And once in atmosphere, 50km or so,can dive. mathworks.com/matlabcentral/fileexchange/… $\endgroup$
    – Surprised Seagull
    Mar 22, 2022 at 17:51
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    $\begingroup$ @SurprisedSeagull Dropping the orbit is something different than dropping out of orbit. Any reasonably efficient burn can only change the altitude of the opposite side of the orbit. You then have to wait for half an orbital period to get to that part of the orbit to change your actual altitude. Indeed if you look at current rockets, they don't go into orbit in 3 minutes, but they burn for 3-5 minutes, then coast for ~20 minutes to get to the orbital altitude, then burn again to get to orbital velocity. The same time is needed in reverse. You can do it faster but only using much more fuel. $\endgroup$
    – mlk
    Mar 23, 2022 at 14:35
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    $\begingroup$ @mlk mars.nasa.gov/images/launch_sequence_diagram-full.jpg some example. 4 min 47 sec for 5.6 km/s. Opposite side of earth would require 40 minutes, and spend 100 m/s delta V, which is even longer than ICBM delivery for most targets. Why would military care about optimal deorbiting, if it makes this worse than land based ICBM? Why would military leave only 100 m/s delta V if 2km/s delta V is still quite possible? it is expensive. But it is the only way to make it better than land based ICBM. Time for delivery is critical for such a system. They will not care about optimal deorbiting. $\endgroup$
    – Surprised Seagull
    Mar 23, 2022 at 14:51
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    $\begingroup$ @SurprisedSeagull You are looking at the wrong burn. The initial burn of a space launch is mostly to get the velocity from 0 to something hypersonic, you don't even need to reverse that for a weapon on the way back down. The point is that a stable orbit is only reached at MECO2 after 40 minutes. So if you start from orbit, you need to start there and work backwards. And military may care about somewhat optimal deorbiting, since the rocket equation means they get several times as many missiles that way for the same weight they initially launch to orbit. $\endgroup$
    – mlk
    Mar 23, 2022 at 15:21
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Loitering

First, a hypersonic-capable missile need not remain at hypersonic speeds for its entire flight. Many missiles differentiate thrust between cruising and final approach, saving sprint speeds for the last leg.

Further, if the missile uses a nuclear ramjet as opposed to conventional rocketry, it could have a tremendous amount of endurance (neglecting all the reasons that's a bad idea), so in a major war situation, these missiles could be launched and set into holding patterns, circling the planet (or the target country) until commanded to go into their attack patterns.

Because they're so fast, the enemy has no way of intercepting them, so they provide a credible, and threatening, second-strike capability. But the time in loiter mode means that they can also be aborted without hitting their targets.

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  • $\begingroup$ So replacement for missile submarines? $\endgroup$
    – DKNguyen
    Mar 22, 2022 at 21:53
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    $\begingroup$ Wear is extreme. We are talking minutes, hours at best. Even if reactor could work for months, wear from plasma will erode everything. We can shoot it down, firing to a place where the rocket will be, from the nose, as opposite to a more usual tail chase mode, both modes can be used, nose mode is less efficient, but not impossible. They are significantly slower than actual ballistic missile, because at orbital speed erosion will be even more extreme. And main point: it is not actually loitering. Destruction of a shockwave of this machine is more destructive than a nuke it carries. Seriously. $\endgroup$
    – Surprised Seagull
    Mar 22, 2022 at 22:41
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Sending a strong message

Of course there is no need for the missile to circle around Earth before striking its target. Furthermore, as other answers and the question itself had mentioned, cruising at low altitude present enormous technical challenges which no nation had been able to solve so far.

Then why the attacker should take such a long time before the strike? The answer is to let the opponent know about its intentions. Traditional ballistic missile, or traditional cruise missile (or "traditional" hypersonic missile, for that matter) cannot be called off. It can not change its initial target either. The only option for the attacker is to self-destroy the missile mid flight if the attack needs to be aborted - and this option is often not available.

Thus, when the traditional missiles had left their silos, the world is already past the point of no return. Even if world leaders suddenly come to their senses and want to fix everything, it is hardly possible to do so.

Here comes a world cruising hypersonic missile. This missile is more akin to traditional nuclear bomber which can be called off - only much, much harder to intercept. After the launch, opponents would quickly know that there is an impending strike - but there is nothing that they can do except to negotiate with the attacker.

This answer assumes that cruising hypersonic missile is able to communicate with its flight control center.

P.S. I realized that my answer is essentially a variation of @jdunlop's answer.

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Why not just plot a direct route and let it scram at low altitude until it crashed into the target zone if efficiency is all that matters?

  1. That's how you get intercepted mid-flight.

  2. Define efficiency. If a 500,000 dollars missile can demoralize the enemy with one shot when 50,000 dollar ones can't with 100 shots, which one do you prefer?

In the ongoing invasion of Ukraine, Russia is using their new Khinzal hypersonic missiles. They are not relevantly faster than other missiles that NATO, China and India have available - but they are extremely maneuverable (when compared to regular missiles, they still turn like any other rocket). In other words, they are not ballistic. They make random turns while in flight to evade anti-missile barrages. Some variants do go up to the higher atmosphere and possibly beyond.

The Khinzal spends most of its time flying at Mach 4, but can accelerate to Mach 10 at sprints. For comparison, India had the Shaurya which goes to Mach 7.5 in low altitudes, but is ballistic, since 2011 at least. The US might soon have a hypersonic missile capable of reaching Mach 20. And China has anti-satellite missiles, which necessitate hypersonic flight and high maneuverability. So while this is all literally rocket science...

Don't they need to worry about the prolonged period of time the missile needs to survive the temperature of aerodynamic friction from air colliding with the missile? It increases exponentially with speed and at hypersonic velocities (5-10 times the speed of sound) the temperature around the missile should melt most metals known to science.

... That same rocket science has already solved those problems time and again.

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    $\begingroup$ "they are extremely maneuverable" - Not in the usual sense. A Mach 10 missile, roughly 3 km/sec, maneuvering at 10 g's, will have a turn radius of about 100 km. $\endgroup$
    – WhatRoughBeast
    Mar 22, 2022 at 17:07
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    $\begingroup$ @WhatRoughBeast In terms of banjaxing an intercept firing solution they're pretty damned maneuverable :P $\endgroup$
    – Corey
    Mar 22, 2022 at 22:47
  • $\begingroup$ @WhatRoughBeast maneuverability in terms of evasion is not just about how big of a turn you can pull, but how quickly you can move your cross section out of your expected path. If you try to intercept a missile, and it moves over 1 meter in the last moment before you can correct your own course, it does not matter if that last moment included 10 meters for forward movement or 1000. $\endgroup$
    – Nosajimiki
    Mar 23, 2022 at 16:28
  • $\begingroup$ @Nosajimiki - Think of it in terns of lateral rate vs range. Let's say a target is approaching at closing rate of 1 km/sec, and when 1 km away begins a 10g turn, At intercept (after 1 second), it will have deviated 50 meters from its original path. With a closing rate of 10 km/sec, the deviation at impact will be .5 meters. This will be much easier to hit assuming a proximity fuse is used. $\endgroup$
    – WhatRoughBeast
    Mar 23, 2022 at 17:43
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    $\begingroup$ Dodging a fast moving missile is easier because of its larger turn radius, but intercepting a fast moving missile is harder. Khinzal missiles are Air-to-Surface missiles meaning you don't have a lot of opportunity to dodge them. $\endgroup$
    – Nosajimiki
    Mar 23, 2022 at 18:38
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Mass panic puts time pressure on negotiations

Q: "My question is why risk a nuke since so far no one is capable of intercepting it anyway? Why not just plot a direct route and let it scram at low altitude until it crashed into the target zone if efficiency is all that matters?"

For some reason, the agressor decided to let the missile fly multiple rounds in the atmosphere.. at Mach-5, that will take about 6-7 hours for a round trip, depending on latitude of the flight path. It could be up there, for a day or two. I'll skip the feasibility analysis and technical frame challenges "it's a waste to do this", other answers have covered that. This is an attempt to improve on Alexander's answer. Why would this weapon be the perfect way, to put pressure?

Extremely low orbit will not allow secrecy to prevent panic

Say 200-500 feet altitude, modern guided missiles can do that. All the time, this frightening nuclear missile will fly over your populated areas, multiple times, visible to everyone in the target areas. It will be in all the newspapers. And feel like Russian roulette (what's in a name)

Uncertainty: no tactical or strategical response, it could even be fake

The defending army would get intimidated - air defense will do 3-4 attempts to intercept the thing.. and no attempt will succeed. That would undermine army moral. Especially at high command levels.. your country is a super power, you have many places around the world this missile could be intercepted, but it does not succeed. To deploy this missile is a new strategy that's not in the books, there is no response either. The nuclear war head mounted on the missile could even be bluff, nobody knows for sure.

Time pressure on negotiations

Suppose you have to negotiate with the aggressor. His missile flying around for a day now, during the negotiations. The aggressor has the means to challenge you with a turn-key acute threat. This nuke can (and should) be cancelled at any moment, the aggressor needs to do that. If it does not happen, you can't prevent the damage and the aggressor has not even indicated a target. This would put enormous pressure on negotiations to give in, and meet demands quickly. You'll beg them to crash this hell machine into the Pacific ocean.

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  • $\begingroup$ The problem with causing panic is that panic responses are unpredictable and often unreasoned. The response could be 'hit them while we can' first strike. This also assumes that the guided missile actually cannot be intercepted; the response does not prove whether it can be -- look up "Coventry" in WW II; a city which was allowed to be destroyed rather than showing the Germans that their secret communications were being intercepted and read, which traded a city against winning the war. And then there's the rain of cruise missiles or bunker busters on the dictator's last known location... $\endgroup$
    – Technophile
    Mar 24, 2022 at 4:52
  • $\begingroup$ One huge mistake in war is to assume that you know what will happen. Well, it's probably safe to say "things you won't like will occur." $\endgroup$
    – Technophile
    Mar 24, 2022 at 4:55

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