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Comment by phire | Hacker News Reader

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.

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> (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?

And you’d need the conventional jet to survive

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).

> 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.

> 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.

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.

The acceleration for turning is v^2/r; for v = 5000 m/s and r = 10 km this is 250 g.

Acceleration also limits how rapidly one can reenter from beyond Earth orbit. At > LEO velocity, the vehicle has to use (downward) lift to stay in the atmosphere, and if v is too high the required acceleration is too high.

It would still be possible to fly a predominantly ballistic trajectory, yet use rocket propulsion to decelerate whilst still outside the atmosphere. It would require a huge amount of extra fuel compared to a purely ballistic trajectory, but perhaps still less than achieving a full orbit and de-orbiting again for some destinations.

At global distances a fraction orbital trajectory would use less total rocket delta-V.

More practical would be a trajectory that was a series of small suborbital arcs with skipping off the atmosphere (perhaps with some airbreathing propulsion during the skips.) The thermal protection can cool by radiation between the skips.

That would be very cool to witness! SABRE sounds like it would be an ideal engine for that application.

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.

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