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It's not easy to research a weapon that's still under a lot of secrecy. I had hoped that Russian-language articles would be closer to the source, as they usually are, but all are clearly the work of analysts not involved in the missile's development. This is the best I've managed to find. It confirms again that the missile uses a synthetic hydrocarbon fuel and "a special ramjet engine, which is characterized by supersonic combustion".
  Another article digs into listing every test, including launch footage, and attempting to estimate the missile's exact shape and dimensions from OSINT.

Mass-wise, it's not advertised, but considering it's smaller than the missile it replaces, it can't be more than 3,000 kg full. That leaves a best-case weight of 900 kg after acceleration, which falls short of 315 kg fuel, 300 kg warhead, 300 kg airframe and engine, 100 kg guidance ~= 1,000 kg best-case estimate.

So, it's possible to achieve the specifications achieved in early tests without using a scramjet engine. A combination of rockets or air-augmented rockets could reach this velocity and range. For the later tests, the warhead would have to be removed or greatly reduced.

However, this missile is a successor of a long line of ramjet missiles. The manufacturer doesn't explicitly state they're using a scramjet, but a ramjet would be ineffective at this speed. Its shape clearly indicates an airbreathing engine inside. Scramjet engines have been studied and tested extensively, in various forms.

Changing the equations to reflect a rocket to get to half speed, then a scramjet for the rest brings different math:

Of particular interest is the joint program between MBDA (French missile manufacturer) and MAI (Russian aviation research center) to develop a "wide range ramjet" and a "dual mode ramjet", covering the range from Mach 2 to Mach 12. The program was extensive on both sides, with a lot of prototypes. Testing at MAI's hypersonic wind tunnel has validated Mach 2-7.5 (sc)ramjet engines. It appears that this program has culminated in the Meteor missile with a wide-range ramjet and the Zircon missile with a dual-mode engine.

It's not easy to research a weapon that's still under a lot of secrecy. I had hoped that Russian-language articles would be closer to the source, as they usually are, but all are clearly the work of analysts not involved in the missile's development. This is the best I've managed to find. It confirms again that the missile uses a synthetic hydrocarbon fuel (JP-10 equivalent) and "a special ramjet engine, which is characterized by supersonic combustion". That would be the definition of a scramjet. Another article digs into listing every test, including launch footage, and attempting to estimate the missile's exact shape and dimensions from OSINT.

Mass is not advertised, but considering that the Zircon is a bit smaller than the missile it replaces, it can't be more than 3,000 kg total. That leaves a maximum weight of 900 kg for the second stage. With 35% for fuel and 30% for airframe+engine, its weight budget falls 20-25% short of that required to fit a 300 kg warhead and at least 100 kg in avionics.

So, it's possible to achieve the 350-450 km range from early tests without using a scramjet engine. A combination of rockets or air-augmented rockets could reach this velocity and range. For the later long-range tests, the warhead would have to be removed or greatly reduced.

Overall, a rocket engine is very unlikely. This missile is a successor of a long line of ramjet missiles. Its shape clearly indicates an airbreathing engine inside. The manufacturer doesn't explicitly state they're using a scramjet, but regular ramjets or other engines can't run at Mach 7. Scramjet engines have been studied and tested extensively, in various forms, and could realistically be used there.

Changing the equations to use a rocket to get to half speed, then a scramjet for the rest brings different math:

Of particular interest is the joint program between MBDA (French missile manufacturer) and MAI (Russian aviation research center) to develop a "wide range ramjet" and a "dual mode ramjet", covering the range from Mach 2 to Mach 12. The program was extensive on both sides, with a lot of prototypes. According to the article, testing at MAI's hypersonic wind tunnel has validated Mach 2-7.5 wide-range ramjet engines. It appears that this program has culminated in the Meteor missile using a wide-range ramjet and the Zircon missile using a dual-mode ramjet engine.

More likely, the missile uses a dual-mode ramjet, rather than a straight scramjet. Dual mode ramjets are a cross between ramjet and scramjet, able to operate in both subsonic and supersonic combustion modes. A DMR allows an earlier takeover from booster to primary engine, improving range and payload capacity. This article comes from trusted sources, having been published by MBDA, the Von Karman Institute, and republished by NATO.

Of particular interest in the article is the joint program between MBDA (French missile manufacturer) and MAI (Russian aviation research center) to develop a "wide range ramjet" and a "dual mode ramjet", covering the range from Mach 2 to Mach 12. The program was extensive on both sides, and testing at MAI's hypersonic wind tunnel has validated Mach 2-7.5 ramjets. It appears that this program has culminated in the Meteor missile with a wide-range ramjet and the Zircon missile with a dual-mode one.

With a dual-mode ramjet, the range estimate changes as follows:

• The missile's specifications are consistent with a scramjet engine.
• A dual-mode ramjet is also a possibility, consistent with the missile's specifications, history and pictures.
• Test results are not consistent with other engine types. They could be replicated with rockets, but with no room for a warhead.
• The engine appears to be a "live prototype" that's still being improved. It falls short of the theoretical abilities of a scramjet, being more similar to an early iteration.

More likely, the missile uses a dual-mode ramjet, rather than a straight scramjet. Unlike the rest I've had to refer to, this article comes from a trusted source, having been published by the Von Karman Institute and republished by NATO.

Dual mode ramjets are a cross between ramjets and scramjets, able to operate in both subsonic and supersonic combustion modes. A DMR allows for an earlier takeover from booster to primary engine, improving range and payload capacity.

Of particular interest is the joint program between MBDA (French missile manufacturer) and MAI (Russian aviation research center) to develop a "wide range ramjet" and a "dual mode ramjet", covering the range from Mach 2 to Mach 12. The program was extensive on both sides, with a lot of prototypes. Testing at MAI's hypersonic wind tunnel has validated Mach 2-7.5 (sc)ramjet engines. It appears that this program has culminated in the Meteor missile with a wide-range ramjet and the Zircon missile with a dual-mode engine.

With a dual-mode ramjet, range estimates change:

• The missile's specifications are consistent with a scramjet engine.
• A dual-mode ramjet is also a possibility, consistent with the missile's specifications, history, appearance, and research programs.
• Test results are not consistent with other engine types. Rockets could replicate them, but with no room for a warhead.
• The engine appears to be a "live prototype" that's still being improved. It falls short of the theoretical abilities of a scramjet, being more similar to an early iteration.

It's not easy to research a weapon that's still under a lot of secrecy. I had hoped that Russian-language articles would be closer to the source, as they usually are, but all are clearly the work of analysts not involved in the missile's development. This is the best I've managed to find. It confirms again that the missile uses a synthetic hydrocarbon fuel and "a special ramjet engine, which is characterized by supersonic combustion". Another article digs into listing every test, including launch footage, and attempting to estimate the missile's exact shape and dimensions from OSINT.

From the articles about the missile, it sounds more probable that it uses a dual-mode ramjet, rather than a straight scramjet. Dual mode ramjets are a cross between ramjet and scramjet, able to operate in both subsonic and supersonic combustion modes. A DMR allows an earlier takeover from booster to primary engine, improving range and payload capacity.

It's also be more consistent with evolutionary development of ramjets and modest statements about a "special ramjet supporting supersonic combustion", rather than an entirely different engine. The linked article makes specific references to dual-mode ramjets in active development by MBDA (French missile manufacturer) and MAI (Russian aviation research center), which would logically culminate in Meteor and Zircon missiles.

With a dual-mode ramjet, the range estimate changes as follows:

That version is consistent with current and intended range, is on the lower end for booster size, and leaves margins for heavier guidance or lighter launch weight, 2850 kg assumed here. It's also possible that the ramjet takes over a little later, maybe 1 km/s, driving a larger booster.

It's not easy to research a weapon that's still under a lot of secrecy. I had hoped that Russian-language articles would be closer to the source, as they usually are, but all are clearly the work of analysts not involved in the missile's development. This is the best I've managed to find. It confirms again that the missile uses a synthetic hydrocarbon fuel and "a special ramjet engine, which is characterized by supersonic combustion". 
Another article digs into listing every test, including launch footage, and attempting to estimate the missile's exact shape and dimensions from OSINT.

More likely, the missile uses a dual-mode ramjet, rather than a straight scramjet. Dual mode ramjets are a cross between ramjet and scramjet, able to operate in both subsonic and supersonic combustion modes. A DMR allows an earlier takeover from booster to primary engine, improving range and payload capacity. This article comes from trusted sources, having been published by MBDA, the Von Karman Institute, and republished by NATO.

Of particular interest in the article is the joint program between MBDA (French missile manufacturer) and MAI (Russian aviation research center) to develop a "wide range ramjet" and a "dual mode ramjet", covering the range from Mach 2 to Mach 12. The program was extensive on both sides, and testing at MAI's hypersonic wind tunnel has validated Mach 2-7.5 ramjets. It appears that this program has culminated in the Meteor missile with a wide-range ramjet and the Zircon missile with a dual-mode one.

A DMR is also more consistent with modest statements about a "special ramjet supporting supersonic combustion", rather than an entirely different engine.

With a dual-mode ramjet, the range estimate changes as follows:

That version is consistent with current and intended range, is on the lower end for booster size, and leaves margins for heavier guidance or lighter launch weight. 2,850 kg is assumed here, but it could be as little as 2,000-2,400 kg with a lighter airframe or guidance package. It's also possible that the ramjet takes over a little later, maybe at Mach 3, driving a larger booster and a smaller second stage.