How efficient is a Scramjet?

Question

I searched about the subject, but I couldn't find any place where the overall efficiency of the scramjet is compared to the other types of engines.

All they say is how the "Scramjet is more efficient than rockets inside atmosphere" and how they are "better than ramjets", but they lack the numbers for these claims.

Answer 1

A scramjet is more efficient than a rocket engine for the same two reasons that all air-breathing engines are more efficient than (chemical) rocket engines:

• Chemical rocket engines need to carry both their own oxidizer and their own fuel, whereas air-breathing engines only need to carry their fuel and take the oxidizer from the air. Therefore, for the same mass / volume, they can carry roughly twice the amount of energy, or for the same required energy, they need to carry roughly half the amount of propellant. (It's not exactly 2:1, of course, because of different densities and stoichiometric ratios.)
• Rocket engines need to carry all their reaction mass with them, whereas air-breathing engines simply suck in reaction mass at the front and throw it out again at the back. This means that
  • For rocket engines, the reaction mass is finite, for air-breathing engines, the reaction mass is infinite.
  • For rocket engines, the reaction mass eats into the payload capacity, for air-breathing engines, it is "free".

These points are true for all air-breathing engines, not just scramjets. Air-breathing engines, in general, are more efficient than chemical rocket engines: ramjets, scramjets, turbojets, turbofans, turboprops, all of them.

Note: I only consider chemical rocket engines here, not nuclear-thermal, nuclear-fission, or nuclear-fusion ones.

As for the comparison between ramjet and scramjet, it is actually not true that scramjets are more efficient that ramjets, as a blanket statement. It depends on the speed:

• Below Mach 4, scramjets don't even work, so talking about their efficiency makes no sense.
• Between Mach 4 and Mach 5, ramjets are more efficient than scramjets.
• Between Mach 5 and Mach 9, scramjets are more efficient than ramjets.
• Above Mach 9, ramjets no longer work, so talking about their efficiency makes no sense.

(All numbers rough approximations.)

See the $I_{sp}$ graph from the accepted answer to the related question Is the compressor required on jet engines? Can air be rammed into the turbine? for details:

Answer 2

Scramjets use super compressed air from the atmosphere, while a rocket engine carries its own oxidiser. This main difference means that all weight and volume required for the oxidiser and reaction mass in a rocket, can be used for fuel only in a scramjet. So all being equal, the range of the scramjet is larger than that of a rocket.

But all is not equal: scramjets can only operate at speeds far exceeding Mach 5, while a rocket engine thrust is instantaneous. Provide enough volume, and the range is all the way to the moon, without requiring any oxygen at all.

And this is the main difference, not so much the increase in efficiency described in this answer. The scramjet flies superfast in the atmosphere, and can therefore use conventional flight controls to control the trajectory - it is basically a cruise missile.

Answer 3

Scramjet has the advantages of simple structure, light weight, low cost Unit thrust (thrust generated by propellant per unit mass flow) high and fast. Compared with rocket engine, scramjet does not need to carry oxidant. Therefore, it has a larger payload and is suitable for the power of hypersonic cruise missile, hypersonic aircraft, transatmospheric aircraft, reusable space launcher and single-stage orbit space aircraft. Due to its important Scramjet has attracted much attention all over the world.