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Comment by superkuh | Hacker News Reader

How long did it travel at hypersonic speeds while operating it's air breathing engine? I'm going to guess it was shorter than the thermal soak time of whatever material they were using for the inlet. The non-simple conical form of a scramjet means shockwave impingment has to happen in there somewhere and you can bet there's no material on Earth that's going to last more than a couple minutes.

Hypersonic air-breathing craft that generate lift just aren't a reasonable thing to make. Hypersonic "gliders" and other rocket lofted things that get up to speed outside of the atmosphere and then just briefly travel through it make sense. But all this 'research' towards lift generating air-breathing hypersonics is just a trap to get nation states to dump money and effort.

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No one cares about 'just' a hypersonic per se.

The world's militaries want a *homing* hypersonic missile, to avoid AEGIS missile defense and attack a US Carrier moving at 35MPH, trying to actively dodge out of the way.

That means steering and thrust on the way down, likely staying at hypersonic speeds until it impacts the moving target.

The world's militaries are trying to kill the US Carrier Strike Force.

There's no need for air breathing, or other, thrust for this. Maneuverable reentry vehicles have existed for quite a while. We can call them hypersonics for nationalistic reasons but the speed is only achieved after the atmosphere and very briefly in it going down.

If the goal is to avoid looking like an ICBM trajectory to avoid the other nation states thinking it's a nuclear attack then it makes sense. But it's still not achieveable for long periods of flight so the ICBM is required regardless.

> "[Hypersonic] Maneuverable reentry vehicles have existed for quite a while"

I keep reading this comment on HN. It's like saying "What's the deal with Chuck Yaeger breaking the sound barrier, we've had supersonic bullets that do that for a long time". There is zero overlap between the two.

Maneuverable reentry vehicles can wobble around a bit while falling, whereas the point of hypersonic weapons is maneuverability for a significant part of the trajectory.

Not just maneuvearable, but actively maneuverable, aka able to sense some type of input inside the cone of visibly superheated air around it. This is different than an ICBM RV drifting around with RCS before re-entry or a cruise missile idling about before it's accelerated to attack speed.

Unrelated but that's also why any talk of stealth with hypersonic is stupid; everything around the projectile is going to give off more of a signature than the projectile itself

> Not just maneuvearable, but actively maneuverable, aka able to sense some type of input inside the cone of visibly superheated air around it.

For ICBMs these are called MARV warheads and they were first demonstrated in the early 1980s, and every major power has had them, though many don't have any operational now.

> avoid AEGIS missile defense and attack a US Carrier moving at 35MPH

They want a propaganda tool. As the other comment mentions, manoeuvrability is cheap. And projectile defenses’ vulnerability to swarming is well know.

But launching tens of thousands of explosive-laden drones sounds low tech, so we get Xi and Putin touting hypersonics and our military-industrial complex smelling money in the counter.

You do not need to manuver to hit a carrier? 35mph is "slow" relative to a missle, and a carrier takes literally minutes to speed up / slow down / turn. You can math out and shoot a literal cannonball and hit a moving 35MPH carrier within a few inches...

Two problems:

First, you need accurate, current location info on the carrier. That's not easy.

Second, you need something that can launch your cannonball. That's not easy, either, because if you're within cannonball range of a carrier, then you are within range of an enormous amount of return fire from the carrier strike group, and you're dead long before you get a chance to fire your cannonball.

The problem is getting current targeting information to a launcher that's 1000 miles away from the carrier, and then the carrier still moves enough to really matter during the time that your missile travels that 1000 miles.

Using satellite imagery or a swarm of drones would make it easy to know and predict the rough location of a carrier within a couple of miles

Well, let's see. Say "hypersonic" is 4000 MPH. 1000 miles/4000 MPH is 1/4 hour. At 35 miles/hour, in that time the carrier can move 8.75 miles.

If it's traveling more-or-less in a straight line, you can predict where it will be to within a couple of miles. If not...

Where does that "swarm“ of drones come from? Any drones cheap enough to use in a swarm have very limited endurance and weak sensors. Possibly useful in the littorals, but a total joke for blue water operations.

Satellites are limited and impossible to hide. The carrier strike force knows exactly when they are in view of an adversary's reconnaissance satellite. In any future major conflict the satellites will be the first casualties. There is a very active arms race in anti-satellite weapons.

> swarm of drones

That swarm of drones will almost certainly be spotted by the AEGIS air defense network (the cruisers and destroyers that escort the carrier), and shot down. Cruisers and Destroyers serve as the "screening" role for the Carrier, you will always see the cruiser/destroyers first in the Carrier Strike Group no matter what angle you approach from (and vice versa, they will see you first before you get to the Carrier).

This gives the cruisers/destroyers a chance to shoot down the drone swarm (or whatever air platform you're using). If its too much for the sea-based air defense systems to handle, they'll radio for assistance from the Carrier and F18 / F35 jets will be dispatched, beyond the horizon, before the swarm even sees the Carrier or pinpoints its location.

That's the difficulty of fighting a Carrier. Not the Carrier itself per se, but the entire team that works together. AEGIS cruisers have the best radar systems the US Navy can offer, and their large power plants can make their RADAR systems more sensitive / higher power than anything you can possibly hope to put into a drone.

That's not how it works. The launching platform will never have a sufficiently precise target track to hit a carrier with an unguided weapon from any significant distance. Flight time will be at least several minutes so even a slight course deviation is enough to generate a miss. The new generation of hypersonic missiles will have some onboard sensors and ability to seek out a target, but only within narrow limits.

> and a carrier takes literally minutes to speed up / slow down / turn

No it doesn't.

https://www.youtube.com/watch?v=M-Q8P049-2Q

The US Aircraft carriers are powered by two nuclear powerplants, with massive rudders/turbines that can propel, accelerate, and move the craft at surprising speeds.

In fact, one of the problems of the US Supercarriers is that at these high speeds / acceleration, an unprepared sailor can fall off the ship. Its legitimately one of the most common ways to die on a US Supercarrier.

How long did it travel at hypersonic speeds

300 miles was the reported test distance.

https://www.darpa.mil/news-events/2022-07-18

After release from an aircraft, the first stage boosted the vehicle to the expected scramjet ignition envelope. From there the missile’s Northrop Grumman scramjet engine fired up and propelled the cruiser to speeds greater than Mach 5 (five times the speed of sound) for more than 300 nautical miles and reaching altitudes higher than 60,000 feet.

So ~2 hours then: https://www.wolframalpha.com/input?i=300+nautical+miles+%2F+...

I think more like 5.5 minutes: https://www.wolframalpha.com/input?i=300+nautical+miles+%2F+...

Yeah, I just realized that number seemed way off and was about to edit my comment. ~5 minutes is correct.

So just regeneratively cool that portion like you would in a rocket engine (or an ablative or whatever). No big deal.

Why this assumption that aerospace engineers are just idiots and they didn’t think about designing it to last longer than thermal soak?

You can read the full reasoning here as it is too long to fit in a HN comment: https://exrocketman.blogspot.com/2022/06/about-hypersonic-ve...

Read it. None of that justifies the absurd claim that no materials exist and therefore they can only fly to thermal soak.

Your only comment about heat load is that it's the most important factor and then I should look at some picture. There's zero reasoning in that picture.

The British are still investing in development of the Reaction Engines SABRE air-breathing hypersonic engine concept. They have a novel design to address engine cooling but it's unclear whether it will really work.

https://reactionengines.co.uk/advanced-propulsion/sabre/

How would you get the gliders above the atmosphere? Or rather, how would the military?

> and other rocket lofted things

> How would you get the gliders above the atmosphere? Or rather, how would the military?

A rocket. I believe the those hypersonic gliders are ICBM warheads.

They have options, including platforms such as https://en.m.wikipedia.org/wiki/Boeing_X-37

So why are they investing in it then?

to make Americans feel better about the fact that Russia also has them. Plus any research in the field is likely to provide benefit elsewhere.

The USA invested a lot in them in the 1960s/70s. That research pretty much conclusively found lift-generating thrust-producing in-atmosphere hypersonic craft weren't feasible. So the US let it drop.

Fast forwards 50 years and other nation states are looking for ways to achieve ICBM-like performance without being mistaken for an ICBM. They foolishly fund these projects and then have to save face. The "hypersonics" are lauded as impressive "new" weapons for propaganda.

The US, seeing this propaganda, has researchers that realize they too could use the funding again even if the end goal is silly. They could get in on the propaganda too. I assume the decision makers re: funding just don't understand the physics.

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