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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numpad0 29 minutes ago | parent | next [-]

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.

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adev_ an hour ago | parent | prev | next [-]

> 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.

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m4rtink 2 hours ago | parent | prev [-]

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.

	▲	petterroea 2 hours ago \| parent \| next [-]
		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.
	▲	thrownthatway 2 hours ago \| parent \| prev [-]
		90 minutes is a low earth orbit period. A suborbital craft won’t be travelling at that speed.