DARPA's hypersonic scramjet achieves successful flight • Hacker News Board

Re: DARPA's hypersonic scramjet achieves successful flight

#141

Post by jhgb » Sun, Oct 03, 2021, 10:49 PM UTC

sudosysgen wrote: post #133

jhgb wrote: post #117

Earlier quoted context omitted.

They're too fast to catch up with (i.e., from behind). But if someone sprints straight into your fist, that person will be hurt even if your fist itself is fully stationary. In fact this probably makes them somewhat more vulnerable to an interceptor's warhead's shrapnel: even lesser damage, survivable for a slower target, might be much more serious for a target flying at high Mach speeds (not the mention the imminent…

That only works if your rate of tangential acceleration is quite low. The reason why you'd make an airbreathing hypersonic instead of a ballistic missile is so it could manoeuvre.

Your tangential acceleration is zero in a sustained flight. So it works, then.

Re: DARPA's hypersonic scramjet achieves successful flight

#142

Post by JshWright » Sun, Oct 03, 2021, 10:51 PM UTC

cdash wrote: post #130

_2d30 wrote: post #102

Earlier quoted context omitted.

In fairness, emergency department is more sensible. They generally aren't a single room. Soldier to warfighter seems unnecessary. EDIT: It seems possibly using "warfighter" is more accurate as marines and pilots aren't soldiers?

Sure but then you get into the situation where even though the full name is Emergency Department they still refer to it as the ER and not the ED.

That's just a bit of lag though. ED is becoming increasingly common.

Re: DARPA's hypersonic scramjet achieves successful flight

#143

Post by JumpCrisscross » Sun, Oct 03, 2021, 10:51 PM UTC

sudosysgen wrote: post #132

JumpCrisscross wrote: post #121

Earlier quoted context omitted.

As a former aerospace engineer, I’m sceptical of the strategic value of hypersonics. Tactical? Sure. They’d have a better chance of taking out SAMs, light radar, et cetera . But the talk about these being carrier killers is, based on everything we’ve seen, off the mark. (Spending the cost of one hypersonic on a swarming attack would probably do more damage.)

The biggest difference between hypersonics and swarms of slow missiles is that the hypersonics can get there fast enough that you might not need high-quality targeting information. That's definitely a strategic advantage! Also the 3M22 is estimated to only cost around 2 million dollars.

> hypersonics can get there fast enough that you might not need high-quality targeting information

The strategic systems hypersonics are hyped to hit have been hardened against ballistics. Hypersonics are slower than ballistics. (Their plasma envelope also makes them easier to track by satellite.)

This is a tactical evolution. Meaningful. Helpful. But not a strategic shift.

Re: DARPA's hypersonic scramjet achieves successful flight

#144

Post by jhgb » Sun, Oct 03, 2021, 10:55 PM UTC

sudosysgen wrote: post #132

JumpCrisscross wrote: post #121

Earlier quoted context omitted.

As a former aerospace engineer, I’m sceptical of the strategic value of hypersonics. Tactical? Sure. They’d have a better chance of taking out SAMs, light radar, et cetera . But the talk about these being carrier killers is, based on everything we’ve seen, off the mark. (Spending the cost of one hypersonic on a swarming attack would probably do more damage.)

The biggest difference between hypersonics and swarms of slow missiles is that the hypersonics can get there fast enough that you might not need high-quality targeting information. That's definitely a strategic advantage! Also the 3M22 is estimated to only cost around 2 million dollars.

> that you might not need high-quality targeting information

I don't see how this would work, unless of course you're talking about nuclear weapons. With a conventional warhead you still need to hit your target.

Re: DARPA's hypersonic scramjet achieves successful flight

#145

Post by JumpCrisscross » Sun, Oct 03, 2021, 10:55 PM UTC

sudosysgen wrote: post #136

jhgb wrote: post #118

Earlier quoted context omitted.

But maneuvering at high velocities won't be that easy. As far as I understand it, your lift/drag ratio decreases appreciably with increasing Mach numbers. So either you'd have to limit your maneuvering, or you'd have to have a propulsion unit with ridiculous amounts of thrust - at a situation when any amount of useful thrust is hard to obtain, since we still don't know really well how to do propulsion in this regime.

Thrust is not the issue, the issue is net thrust. Essentially the missile/engine is compressing so much air that trying to get more air to produce more thrust doesn't work, you have to manage to increase efficiency, by increasing compression, without melting or blowing apart your engine Once you manage to make significant net thrust making a bit more isn't that big of deal, it's really a critical point you have to pa…

> once you manage to make significant net thrust making a bit more isn't that big of deal

You’re still limited by your fuel and flight envelope. The latter which bleeds the former.

Hypersonic missiles will be harder to intercept than cruise missiles. Due to their novelty, they’ll also be harder than ballistics. (Ballistic projectiles don’t continuously illuminate their flight path. That permits for subterfuge options an air-breathing missile forfeits.)

Re: DARPA's hypersonic scramjet achieves successful flight

#146

Post by JumpCrisscross » Sun, Oct 03, 2021, 10:57 PM UTC

trhway wrote: post #129

amelius wrote: post #6

US's answer to Russia's Zircon missile? https://www.military.com/equipment/weapons/why-russias-hyper... > U.S. Aegis missile interceptor systems require 8-10 seconds of reaction time to intercept incoming attacks. In those 8-10 seconds, the Russian Zircon missiles will already have traveled 20 kilometers, and the interceptor missiles do not fly fast enough to catch up.

No. Russian Zircon is to deal with the next after nuclear strategic threat to Russia - US aircraft carrier groups. US pursue hypersonic weapons for different purpose - fast global much less than nuclear strike capability, ie. being able to strike any given target anywhere in like under half an hour. And thus not really much of hypersonic development in US - the cheap ballistic would do it better, especially with Spac…

> US pursue hypersonic weapons for different purpose - fast global much less than nuclear strike capability

Ballistic missiles fly much, much faster than hypersonics. Speed is not their advantage.

Re: DARPA's hypersonic scramjet achieves successful flight

#147

Post by JumpCrisscross » Sun, Oct 03, 2021, 10:58 PM UTC

speedybird wrote: post #94

allenrb wrote: post #12

For a moment, reading only the first paragraph, I was hopeful that the problem of accelerating from subsonic to hypersonic had been solved. Alas, further down it makes clear that a booster was involved. Presumably a small solid rocket that accelerates to Mach 3-5(?) prior to scramjet ignition. Fine for a weapons system but will never be useful as transportation. Is anyone aware of progress toward being able to reach…

Unless you're trying to go to space, there really isn't much reason to go this fast in the first place. Even relatively modest supersonic passenger flight has proven to be a commercial failure (good luck to Boom Tech inc... you guys will need it.) If you're going to space, an SSTO with scramjets would be interesting. But I think if SpaceX's reusable Starship succeeds without that tech, it would be hard to justify the…

> relatively modest supersonic passenger flight has proven to be a commercial failure

Tried it once in the 70s. Didn’t work. Everyone go home.

Re: DARPA's hypersonic scramjet achieves successful flight

#148

Post by moralestapia » Sun, Oct 03, 2021, 10:59 PM UTC

pengaru wrote: post #92

Gravityloss wrote: post #82

Earlier quoted context omitted.

The SR-71 was very complex. For example it had multiple heat exchangers to get rid of heat, to transfer it to the fuel before it was combusted. The titanium structure was complex to maintain. Welding needed to be done in "bubble" work stations under protective gas.

> The titanium structure was complex to maintain. Welding needed to be done in "bubble" work stations under protective gas Operational complexity is orthogonal, and tends to be inversely proportional to simplicity in implementation... For instance they needed to refuel the thing immediately upon reaching operating temperature in flight, since it leaked like a sieve on the runway until everything expanded. Rather than…

>Operational complexity is orthogonal, and tends to be inversely proportional to simplicity in implementation ...

I'm not sure what your point is with this. All complexity is operational complexity.

Re: DARPA's hypersonic scramjet achieves successful flight

#149

Post by rsj_hn » Sun, Oct 03, 2021, 11:01 PM UTC

nimbius wrote: post #112

corndoge wrote: post #87

Is it just me or has the Pentagon and the complex largely replaced the word "soldier" with "warfighter"? Feel like I've seen it a lot lately and I'm not sure what the change implies

War fighter showed up to replace soldier about the same time the space force was announced. It's meant to be a blanket term to satisfy categorization from rifle carrying traditional soldiers to drone pilots in an Idaho parking lot.

No, I heard this in 2002 by our marketing guy who was in communication with people in the military bureaucracy (of course to sell them our products).

Re: DARPA's hypersonic scramjet achieves successful flight

#150

Post by sudosysgen » Sun, Oct 03, 2021, 11:01 PM UTC

jhgb wrote: post #134

sudosysgen wrote: post #123

Earlier quoted context omitted.

Interception against an actively manoeuvring target is a question of kinetic energy. If the intecepting missile is smaller all the hypersonic missile has to do is zig-zagging randomly and you're toast. If you're going to wait for the hypersonic missile to be close enough that this isn't a problem, you have the issue that your missile won't be able to manoeuvre right and might still be building up velocity. Plus there…

> Interception against an actively manoeuvring target is a question of kinetic energy. If the intecepting missile is smaller all the hypersonic missile has to do is zig-zagging randomly and you're toast. What you're describing is not a matter of kinetic energy (clearly if it were about kinetic energy, this would be defeated by making the interceptor weigh 30 tonnes) but a matter of transverse acceleration. On that ma…

>What you're describing is not a matter of kinetic energy (clearly if it were about kinetic energy, this would be defeated by making the interceptor weigh 30 tonnes) but a matter of transverse acceleration. On that matter, as far as I understand it, current AAMs/SAMs are somewhere in the 50g region. I strongly suspect that hypersonic vehicles due to their lower L/D are nowhere near that -- hell, many of them seem to have a hard enough time flying straight, if US military's experience is of any relevance here.

Not so. AAMs/SAM can only pull anywhere close to 50g at gigantic cost in drag and can only do that once and briefly. It is a question of kinetic energy because the missile that can regenerate kinetic energy and that loses the least kinetic energy relative to it's mass is at a massive advantage.

>Also, due to low L/D ratio in the high Mach region, if your missile is zig-zagging randomly, it won't stay fast for long. And if it's not zig-zagging randomly, then the question is how it gets the information how to zig-zag, since the nose of a hypersonic vehicle seems to be an extremely poor sensor platform -- which you admitted yourself. It's virtually certain that it won't be able to see the interceptor.

Traditional aerodynamic designs suffer from poor L/D ratios, yes. Waverider designs probably have much fewer issues. Given the advent of HGVs, which are made to turn multiple times without any engine to back them up, we can be fairly confident that this is not the case.

As far as being a poor sensor, this is an oversimplification of a complex mechanism. Plasmas block radar up to a certain frequecy depending on their temperature. So you make it so that the radar is somewhere where the air is slightly colder and make its frequency higher. You don't even have to put the radar in the nose if the target you're trying to hit is sluggish and large enough.

It doesn't need to maneuver in response to the interceptor. The missile knows when it is in LOS to the target, and knows that the interceptor will be fired soon after. So it already knows when the missile will be fired and thus when the missile would hit. The window for interception is pretty short. It would have to do evasive maneuvers at maybe 4G for maybe 30 seconds, all the while gaining back Ke from reducing in altitude in combination with engines.

>It would seem that the easiest way to distinguish a flare would be to look if it's in a controlled flight? Unlike a flare, the hypersonic vehicle will continue flying, and the difference is readily apparent. Besides, you can't have that many flares on it, so you can't keep firing them continuously -- again, the only thing that's available to you if you can't see the incoming interceptor.

This is a REALLY hard problem, and it's the reason why imaging infrared guidance was invented. It takes a lot of time to be certain that a flare in a ballistic trajectory at high speed is different from a missile coming towards you! Those are precious seconds you can't afford. Remember that you have no way of estimating distance.