A successful Japanese trial of a ramjet engine designed for Mach‑5 aircraft
bgr.com217 points by rmason 21 hours ago
217 points by rmason 21 hours ago
Ramjets were developed right after the second world war and Mach 4+ was reached in the fifites. It's complicated but not extremely. See Antonio Ferri or Lockheed X-7.
It turned out out solid fuel rockets are operationally more practical for the use cases like air defence, long range missiles that are ballistic instead of cruising in the atmosphere and so on. And jet engines are more efficient for subsonic cruise missiles. Ramjets are still used in some missiles like the long range mach 3 air-to-air Meteor.
There have been air-to-air missiles using rocket-ramjet combinations: they start as a rocket, then once up to speed an inlet opens and they transition to ramjet mode (using the same chamber and nozzle). It extends range. Ramjets are best for maintaining speed rather than accelerating to speed.
EDIT: I see this was referenced above (Meteor).
For many many reasons this engine only has one economic application - delivery of a nuclear payload in a way which is very hard for missile defences to stop.
ICBMs can go faster than this already but as I understand it they go higher allowing for earlier detection and they follow a more predictable trajectory which makes interception more realistic.
I find super fast missiles far scarier than advanced AI. I suppose they maintain the "mutually assured destruction" which might be the main reason there hasn't been a nuclear war since WW2, but it's not a huge comfort.
My take is also a missile, but as an interceptor, not offensive purposes. North Korea has shown that they can deliver a payload to Japan if they want. If the NKs do decide to launch something at Japan, it's not going to be tipped with conventional explosives. It'll be a nuke. Hypersonic interceptor seems the more likely application is this tech to me.
I think that hypersonic interceptors would use a rocket motor and not a ramjet, but I'm not sure.
I think this is for offence because I understood the advantage of this engine (compared to a rocket engine) is that you don't carry oxygen so you can carry more fuel and get more range. I think range is much more useful for offence than defence.
I am possibly making myself look foolish now as I'm not an expert on either rockets or ramjets and I might have incorrectly dismissed defensive applications.
judging by what we saw so far in Russia, Ukraine, and Israel, defensive missiles don't require particularly sophisticated offensive missiles to overwhelm.
yeah, but those are using conventional explosives, only countries like the US and Russia have enough nukes to do the same with nuclear explosives, a country like japan would need a sophisticated missile instead
I find fast missiles with AI to be pretty scary. Heck a slow drone with AI targeting chasing me is bad enough...
> I find fast missiles with AI to be pretty scary
Not sure I entirely appreciate the use-case vs classical targeting. I'd imagine you're going so fast that you don't really have the opportunity to engage in thought that is particularly useful.
You only need to draw a fake moustache on yourself to fool it, or something.
like a backpack from which a cardboard version of a soldier springs out, with the operator being shielded with an aluminum foil pane
Well here's an economic application without nuclear tipped warheads.
The Wiki pages says top speed is about Mach 4. There are already multiple rockets from US, Russia, and China that can achieve the same (or more) but with a solid-fuel rocket motor. What is the advantage of a ramjet here? It just seems way more complex and much less well tested (in labs and in combat). Also, has this missile (Meteor) been used in any combat scenarios? To be clear, the max speed for any fighter jet is about 2.5 Mach. Once one (or two) of these missiles has locked on, you are done. I read some funny commentary once about how to shoot down a modern fighter jet: Two missiles. They can dodge the first one, but sacrifice so much speed, that the second one can easily find its target.
Missiles versus aircraft is a fight between very high kinetic energy in the missile, and relatively low kinetic energy in the plane, but with the ability to generate more kinetic energy. Missiles don't have a lot of fuel, so they need to generate a lot of kinetic energy to still be effective by the time they reach the target. Typically a missile will accelerate to its top speed in the first few seconds of flight and coast the rest of the way. At very long ranges, all the energy generated when launched has bled off, so there's two common solutions for long-range missiles to generate more energy: a "dual pulse" motor is basically a second rocket motor that fires later in the course; or a ramjet, which can be throttled up and down and is more fuel efficient than a rocket engine.
From a science and engineering point of view, I root for this.
From an environmental point of view, I hope this won't materialise for some time.
It can be nuclear powered, so you don't have CO2 emmisions /S
You just need to add heat to the air at the point where the diameter is the lowest.
The russians are maybe using this idea for one of their new cruise missles named Burevestnik, although for them the nuclear emmisions are likely a positive side effect.
Is a Mach-5 passenger aircraft actually the goal of this project?
Seems more likely that Japan is designing this engine for a hypersonic cruise missile program, and the passenger aircraft concept is somewhat of a cover.
IMO, there is no point in a Mach-5 Aircraft (other than cruise missiles). There is potentially some point in Mach 2-3 aircraft, (not that we have ever made them commercially viable) but at the boundary to hypersonic, you might as well just switch to a suborbital hop concept.
A suborbital hop gets you to anywhere in the world within ~90min, avoids issues of supersonic overflight and you don't need to worry about the massive engineering issues caused by sustaining hypersonic flight. And as a bonus, the passengers get a hour of weightlessness.
> Is a Mach-5 passenger aircraft actually the goal of this project? > Seems more likely that Japan is designing this engine for a hypersonic cruise missile program, and the passenger aircraft concept is somewhat of a cover.
Case of China's got them, and can't rely on the Orange Emperor and his heirs to have their backs.
> (not that we have ever made them commercially viable)
Concorde was commercially viable at Mach 2.2 in supercruise (although there's a common misconception that it was not).
However, its overheads were very high, and its applicability was severely limited by fears around the sonic boom (most particularly in the US, which banned supersonic flight overland, possibly largely because they wanted to kill off foreign competition).
The individual aircraft could be operationally viable on certain routes, but the whole program was not commercially viable.
Air breathing engines don't need the oxidizer tank, so like the 2/3 of a rocket just goes away before even touching Tsiolkovsky math. That improves payload mass fraction massively.
Also, this doesn't scale down to Mach 3-4 and under. This thing uses scramjet, or supersonic combustion ramjet. It REQUIRES intake air to be at high supersonic speeds for it to work.
> It REQUIRES intake air to be at high supersonic speeds for it to work
This is why I am highly sceptical it can be part of a commercial supersonic passenger jet: how do you get from subsonic -> supersonic without also tacking on some kind of conventional jet engine?
Japan, Italy and UK have a program for a competing F35 design, GCAP. And Japan is focusing mainly on the engines.
Given there will at some point be the need to deliver competing cruise missiles for this platform, and after the crisis of the US not being able to keep demand with Israel's and Ukraine's orders they greenlighted SK and Japan to enter the European defense market, to answer your question yes, this is of course a defense related project.
There's been an industry request to develop native defense components on these matters within the EU following pressures and contrasts with the US (on a report to the EC for the ReArm campaign, EU's biggest playes of aerespace industry made a joint report estimating 60-80% of their components and tech are sourced from the US).
> you might as well just switch to a suborbital hop concept.
One is not exclusive to the other.
Skylon was expected to use air breathing engine up to Mach5+ and switch to rocket engine beyond it.
You can probably do the same for a suborbital airliner if you are insane enough.
90 minutes is a full low Earth orbit cycle. For a suborbital hop it should be about half of that at maximum for any 2 points on Earth.
I didn't initially believe these numbers, but if you look at some real life stats, you are probably right.
Nominal SECO for the last starship mission was at ~8 minutes and it took ~20 minutes from deceleration started (well, from air resistance outweighed the forces of acceleration) to landing. So basically 30 minutes of flight is just the "getting up to speed" and "slowing down" part. Both account for some distance traveled, but still. ~45 minutes is probably a good bet.
Do note however that you may have to go around the world "the wrong way" to get some places due to launch constraints. But living in a world where going around the world "the wrong way" is the easier path is interesting. Imagine that.
90 minutes is a low earth orbit period.
A suborbital craft won’t be travelling at that speed.
Like, you could do a partial orbit & then drop down over the destination. But it would need much more delta-v & an orbital class heat shield.
It was proposed as nuclear warhead delivery method though: https://en.wikipedia.org/wiki/Fractional_Orbital_Bombardment...
Unless a suborbital trip is nearly at orbital velocity, it will involve a high, arcing trajectory. This will make the deceleration at the end unacceptably (lethally) high for all but short arcs. Some of the Mercury suborbital missions involved deceleration of 15 gees, if I recall correctly.
That was only an issue because they were fired pretty much straight up; They only went 500km down range.
You can also reduce peek deceleration forces by using aerodynamic lift to stretch out the reentry over a longer period.
No, it's an issue for most arcing trajectories. Lift doesn't help much if you're coming in at a steep angle. Reentry from orbit only works well because the entry is almost flat; there even a little lift helps a lot.
If the capsule/rocketplane has some lift & preferably steerable aerosurfaces then you can compensate the purely ballistic deceleration somewhat.
But yeah, if it is going down almost vertically then this will not be enough.
And all but rather short ballistic trajectories (well below orbital speed) will come in at a steep angle.
Unless one has seriously variable aerodynamics, the vehicle will have to swerve to nearly horizontal over a distance of about 1 scale height of the atmosphere, which is about 10 km. The exponentially thinning atmosphere goes from "too thin to matter" to "brick wall" over a short distance.
I actually learned about what a ramjet is after looking up the definition of “scramjet” when watching the _Top Gun: Maverick_ movie with my son. This is at the beginning of the movie when he is flying the Dark Star plane designed in conjunction with Skunk Works from Lockheed Martin. Well, we are obviously a ways away from Mach 10 reached in the film by the SR-71 Blackbird descendant, the new technology pushing Mach 5 and into high hypersonic is pretty impressive.
> A scramjet is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow. As in ramjets, a scramjet relies on high vehicle speed to compress the incoming air forcefully before combustion, but whereas a ramjet decelerates the air to subsonic velocities before combustion using shock cones, a scramjet has no shock cone and slows the airflow using shockwaves produced by its ignition source in place of a shock cone - Wikipedia
As I understand, the problem with anything over Mach 4 (or 5), the metal begins to disintegrate. It is fine for a one way missile, but not a reusuable aircraft. Without some alien tech (see 1990s game "X-Com 2"), I cannot believe that we can build a reusuable aircraft that can reliably and safely fly for long periods over Mach 5.
Ramjets are limited in the speed they can operate because slowing the air to subsonic speed in the engine causes it to become hot, and the temperature increases rapidly with speed (the kinetic energy of an incoming air parcel is proportional to the square of the vehicle's speed, and most of that energy is being converted to heat.)
The original submission didn't mention ramjet.
I always thought it was an underutilized design that could be improved for practical applications.
Improved enough, it could become cheaper and more environmentally-friendly than current aviation whereas regular supersonic jets are never going to achieve that.
Interesting, but assuming they can get the engine to work as intended, the question still remains how the passenger jet would get up to Mach 5 so the engine can start working. A solid-fuel rocket booster that would then drop off?
It is a safe bet that the first applications will be missiles and there won't be any passengers to worry about.
I may be wrong but I think a ramjet doesn't need Mach 5 to ignite, Mach 3 to 6 is just where it is most efficient at high altitudes.
E.g. early German experiments during WW2 based on the Lorin tube (https://en.wikipedia.org/wiki/Ren%C3%A9_Lorin) only had to get to 320 km/h to start working:
> At that elevation at Mach‑5, air around the nose and leading edges can reach temperatures exceeding 1,000 degrees Celsius (1,832°F), a challenge the U.S. Air Force has struggled to overcome with its own hypersonic jets.
> To handle that level of heat, engineers constructed an advanced thermal‑protection system that maintained the aircraft's interior near normal operating temperature, allowing the onboard avionics and control electronics to function normally.
Hindenburg 2.0 waiting to happen
> Hindenburg 2.0 waiting to happen
Fortunately there's no hydrogen in that plane.
We have a lot of experience with heat shields from cosmic reentries now, though. This is probably doable.
I kind of think today's air travel is in an uncanny valley.
I'd prefer either: just as uncomfortable or even more so but very fast, like multiple mach numbers fast. I live in the Midwest and go to the West Coast often, and it usually takes 5 hours -- I'd cram into a less comfortable seat for 2 hours instead to get there at mach 2.5.
OR... airships. Big cushy seats. A lounge with fast wifi and desks. A coffee shop. You can walk around. The trip takes all day, or overnight with a sleeping cabin. Trains could fill this niche too, but airships could go overseas.
Go faster or go roomier.
People say this like it's a simple engineering problem.
No. By itself, a new hypersonic engine can't make 2-hour flights between Japan and the US a reality. We are not even close to being able to build an aircraft that can do that - we don't even have the materials for that. What seems "easier" (as in "less impossible") is a hypersonic glider design that enters a suborbital trajectory and does shuttle-like aerobraking while it glides to its destination, before reengaging propulsion prior to landing on an airstrip (because passenger planes need to be able to abort landings and do multiple attempts). Not sure how reverse thrust would work there - variable geometry rocket bells?
How long of a weightlessness period does this entail?
Maybe not complete weightlessness, because at 80-90 km the atmospheric drag is still noticeable. But it should be enough of a unique experience.
As a Canadian who travels to Europe about once per month I am very excited for this :D
PG celebrates Boom's pivot to power generator supply.
I've always wanted someone to bring back the Avro Arrow to use the Iroquois engine for freight, but I don't think anyone has the knowledge to even pull it off anymore.
I’d love to have a big die cast model of one, but that’s about as close as we’ll ever get to the Avro flying again.
I don’t think you’d be pushing much freight on an Arrow (though I’d love to fly one!).
An XB-70 with modern engines? Now that would be interesting.
Boy, that's an evergreen headline.
that headline uses a subjunctive verb mood "_could_ <verb>" and those titles are seemingly always clickbait
I imagine passengers will be exposed to very high noise levels during flight.
I don't have a good intuitive feel for that.
At 25km altitude, with 1% of normal atmosphere, maybe you're close enough to vacuum that it can get really quiet?
The engine noise can still be conducted through the body of the plane. With the kind of ramjet being talked about, I think that's still likely to be significant even at very high altitude.
This effect would help mitigate the noise heard by people outside of the plane (but I suspect that noise is negligible compared to the sonic boom), but in the plane the sound would mostly be propagated though the fuselage of the plane, not through air, so it's not going to help.
Cool science. But the article fails to take even a cursory stab at contextualising the plan against the economic, environmental and political backdrop - doesn’t even mention that there’s already been one failed supersonic commercial flight programme. This is as pie-in-the-sky as it gets.
I think a lot of the Concorde failure is tied to its status as a British-French project. Trans-Pacific flights are much longer and there's a lot of money in PEK -> LAX than in JFK -> LHR.
Qantas wanted to offer London to Sydney, but they couldn't fly supersonic over land. Mainland China or Japan to Australia is a feasible route for high-margin, low-capacity supersonic flights.
If you could make the flight from Beijing to California take less than 5 hours that seems like a premium product many ultra wealthy people would spring for. Dubai to SFO is also a possible route.
I was pretty sure the whole Concorde thing failed because people don't like it when you sonic boom an entire city dozens of times a day. And that all attempts to reduce the sonic booms necessarily resulted in flight times that aren't significantly faster than traditional subsonic flights, rendering the entire thing moot.
It was impractical due to physics, not some weird racism. You simply can't push a supersonic shockwave over inhabited areas, and the only way to not do that is to fly subsonic over land. Even if the oversea leg is supersonic, the tickets were much more expensive for not very much shorter flights. It wasn't a valuable proposition for most people.
1) The flight markets are different now. There's been a large increase in both transatlantic and transpacific flights, especially the latter. These change the economics of considering only these types of flights, flying only over uninhabited regions.
2) The technology has changed. We're much better at dealing with sonic booms now. You can't get rid of them entirely, but you can reshape them. You can't send everything "up" but the longer of a tail you can make the more the sound dissipates by the time it hits the ground. There's lots of research around this and as you can imagine, incredibly important for the military. You can't fly fast spy aircraft if they are just announcing their position while flying around. Sure, there are satellites, but those are predictable by the enemy, you'll always need aircraft to do this.
However, there are markets where you don't have to fly supersonic over land, the distance is long enough for the speed to matter, and there is massive amount of demand. The only problem is, such markets require a longer range than what the Concorde was capable of. Notably, all the very frequently traveled trips over the Pacific.
Concorde’s sonic boom was astonishingly loud. The night flights would go supersonic outside the Bristol Channel at around 9pm to 10pm. It was still audible over 60 miles away and sounded like a muffled barn door slamming outside.
Far louder though — it would wake all the pheasants up just as they’d gone to roost.
England in the '80s didn't give a shit about little people. Had it been really profitable, Concorde would have continued operations. It just did not make sense economically, particularly once they stopped making new airframes.
It failed because the market dried up due to economic reasons, and they couldn't fill seats.
There is a lot of money in NYC-LHR, that's why Concorde continued to fly that route and profitably too, once they realized how high they could yank the prices and still fill the plane.
Also, Concorde's maximum range was 4,488 mi, which was calibrated to allow trans-Atlantic but not much more. Trans-Pac was not an option and even Australia to North Asia would be a stretch.
I think they are agreeing with you re: the range.
There is money in NYC-LHR (it brings BA alone $1B in revenue annually) but the market for supersonic basically vanished. In the 70s when Concorde started flying, it was certainly a step up. However, the market niche basically disappeared when the lie flat seat was developed; for a lot cheaper, you could have a sleep for six hours in a really cushy lie flat, or you could spend a crapton more to be in a much louder, more cramped cabin for only about three hours less. If you were halving a 12-16 hour journey instead, there would still be a market left, but Concorde just didn't have the ability to do so.
You can also essentially work remotely in an airplane now. I haven’t tried videoconferencing, but I easily do all my other software work on trips. So a couple extra hours might even be a benefit: more time with no distractions to wrap up that slide deck, maybe a 1:1 or two, get your free drinks from premium/business class, doze off to a movie, wake up for an early start at your destination.
12 hours on a plane is 12 hours on a plane. And there's currently no amount of ticket money that can make that shorter.
Shorter, no, but having a private cabin with a shower, and a lounge with a bartender on the plane, not to mention Starlink, would make those 12 hours a lot more bearable vs 12 in an economy seat.
> having a private cabin with a shower
Wasn’t Concorde like 20-50% more expensive than a normal first class ticket for the same itinerary?
So any hacker considering a SST flight should also be able to afford the first class cabin.