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Is fuel efficiency the only reason why hypersonic missiles don't use rocket engines?

And why are rocket engine missiles relatively slow compared to scramjet missiles? In theory rocket engines can produce more than enough thrust for 24Mach speeds?

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    $\begingroup$ Any Department of Defense requirement that needs a hypersonic missile to fill a strategic or tactical mission role will not have fuel efficiency at the top of the list of design criteria. $\endgroup$ Apr 23, 2022 at 19:00
  • $\begingroup$ @MichaelHall but fuel efficency leads to short range.. $\endgroup$
    – user207141
    Apr 23, 2022 at 19:12
  • $\begingroup$ Liquid propellant rockets are quite complex to keep in readiness, much more so than scramjets. If you’re going to the trouble of building an ICBM then you’ll factor this in, but for a relatively mass-produced device an air-breathing engine is much more attractive. $\endgroup$
    – Frog
    Apr 23, 2022 at 21:53
  • $\begingroup$ I think most of the hyper-sonic missiles do use rocket engines. Remember a hyper-sonic missile, in practice, is a solid-motor ballistic missile with a waverider warhead. To accelerate to hyper-sonic speed and climb to that altitude you have to use a rocket booster, but once you are there however, the a cruise engine isn't entirely necessary. $\endgroup$ Apr 24, 2022 at 0:04
  • $\begingroup$ Not an answer but useful data point en.wikipedia.org/wiki/Sprint_(missile) which was a mach 10 pure rocket with a starting mass around three and a half tonnes for final payload around 500kg. flight time of 15 seconds to around 40km. Not a particularly useful platform for a cruise missile. $\endgroup$ Apr 24, 2022 at 9:07

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Hypersonic missiles fly through the atmosphere and remain manoeuvrable until impact, making it much harder to defend from them. And since they're flying through the atmosphere they use atmospheric air for combustion.

enter image description herePic source

The main advantages of rocket engines:

  • They don’t require air to maintain combustion, so they can fly trajectories that take them outside of the atmosphere while still providing thrust.
  • Since they carry everything they need on board, they can be launched from practically anywhere, even from submarines.

The main disadvantage of rocket engines:

  • They need to carry their own oxidiser, thereby reducing the maximum fuel weight. The Tsiolkovsky equation teaches us the extent of limitations of payload as a proportion of empty weight and fuel weight - but a large enough rocket can still carry men to the moon.
  • Long range rockets need to employ multiple stages, ejecting bits of empty fuselage weight with its engines, and continue flight with other engines.

enter image description herePic source

Hypersonic missiles only need to carry fuel and payload, which they oxidise with air which is also used as a reaction mass. Without having to eject bits of its construction with perfectly functioning engines, and using wings to create lift and flight control surfaces to control flight path just like an aeroplane. But launching them is an issue: flying faster than Mach 5 they are propelled by scramjets, which cannot be operated from a standstill like rockets can. So they need to be launched by either a rocket or from an aeroplane, preferably a really fast one.

Is fuel efficiency the only reason why hypersonic missile don't use rocket engines?

It's the main reason, yes. While exclusively flying trough the atmosphere it makes no sense to not use the unlimited amount of reaction mass of atmospheric air for combustion.

And why are rocket engine missiles relatively slow compared to scramjet missiles? In theory rocket engines can produce more than enough thrust for 24Mach speeds?

Whether rocket propelled or scramjet propelled, when flying through the atmosphere the missile heats up and therefore there are limits on the maximum attainable velocity. A rocket propelled missile can definitely reach the same speed as a hypersonic missile, there is nothing slow about them.

The two pictures linked to a from a site that explains the basics of missile threats, a worthwhile read.

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  • $\begingroup$ You mentioned reaction mass shortly, but maybe you could repeat that in your summary paragraph starting with "It's the main reason, yes." Basically, the value proposition of a jet engine over a rocket motor is "Free, unlimited oxidizer and free, unlimited reaction mass". Even ion engines which don't need fuel or oxidizer are still limited by their reaction mass. $\endgroup$ Apr 24, 2022 at 6:54
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Mach 24 in the atmosphere? Not for long. Ask any meteor.

Developing airbreathing technology may improve fuel efficiency, but oops, the drag of the intake and heating from compression may cancel any advantages airbreathing technology gives. In short, try as they may, hypersonic jets do not seem to be advancing beyond the research stage.

The Kh-47M2 Kinzhal air launched solid fueled rocket is a very practical solution, but one that could be answered with one of these, also carried by an aircraft, if needed.

Speed vs maneuverability rages on.

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  • $\begingroup$ It is my sincere hope that aviation research continues for $peaceful$ purposes. $\endgroup$ Apr 24, 2022 at 0:21
  • $\begingroup$ @Roberto So why they claim that missile speed is around 20mach? $\endgroup$
    – user207141
    Apr 24, 2022 at 8:38
  • $\begingroup$ @JurgenM to sell it. Orbital and suborbital craft commonly make that speed outside the drag of the atmosphere. $\endgroup$ Apr 24, 2022 at 17:19

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