Get Acquainted Info: High Speed Vehicles

This article is for people who know little about high speed flight vehicles.  It gets across some key concepts about:

#1. frontal thrust density and top speed capabilities, 

#2. how the same inlet components are used quite differently in ramjet versus turbojet installations, 

#3. why achieving combined cycle engine designs can be so difficult,  and

#4.  how heat protection is the true driving issue for high-supersonic and hypersonic flight.

There are other articles posted here and available elsewhere,  that go into considerably more detail about these topics.  But this one tries to illustrate the basics,  to get started.

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Get Acquainted Info: High Speed Vehicles  

There are many concepts to understand about high-speed flight.  Frontal thrust density is a very important issue.  And,  there is no “magic” to waveriders.  See these 2 illustrations:

 

The number of propulsion nozzles at the back of a vehicle also seriously affects frontal thrust density.  This applies to both rockets and airbreathers (of any type).  See:

The over-simplified behavior of inlets on a supersonic ramjet vehicle is shown: 

 Bear in mind that pitot-normal shock inlets,  which have no shock-on-lip behavior,  actually have 6 behaviors to understand,  and external-compression feature-fitted inlets have 9 different behaviors to understand.   You do not initially need to understand all that detail!

But,  it is the basic as-illustrated inlet behavior above,  that drives supersonic ramjet performance.  Ramjet takeover from the booster needs to occur no lower than shock-on-lip speed.  The lower the shock-on-lip speed is,  the smaller the booster can be,  leaving more room for ramjet fuel and the nonpropulsive items.  Considerably higher speed is still efficient:

For supersonic flight,  gas turbine engine installations use the same supersonic inlet components,  but they use them quite differently!  These are usually low-bypass “turbojets”,  and they are usually fitted with afterburners. 

Unlike the ramjet,  which when operating properly,  accepts a fixed scooped air massflow from the inlet,  the turbojet demands a variable air massflow corresponding to its rotor speed(s),  determined in turn by the throttle control setting.  The turbojet inlet has to vary the captured air massflow to match engine demand,  which inherently requires subcritical inlet operation,  with variable-but-significant amounts of spillage around the cowl lip. 

The dominant pressure-rise feature in a turbojet installation is the compressor,  not the inlet!  (The only pressure rise feature in a ramjet is the inlet.)  See:

High speed flight involves lots of aero-heating.  Adjacent and captured air temperatures are high.  As you go hypersonic,  shock impingements multiply heating rates substantially.  See:

Shown just below are the heating rates to,  from,  and within,  any given piece of exposed material.  There is steady-state equilibrium (applicable to hypersonic cruise),  and there is transient behavior (applicable to atmospheric entry),  to worry about. 

Radiation occurs only when there is a view of something hot or cold from the affected surface.  The emissivity “e” can make radiative transfer either inefficient if low,  or efficient if high.  It varies between 0 and 1.  (The sigma represents Boltzmann’s constant.)

For convective transfer,  heating rates can be to,  or from,  the surface.  The “film coefficient” h is larger near stagnation zones,  and smaller on lateral skins.  The values of h all decrease as the air thins drastically at very high altitudes. 

Thermal conduction can be to,  from,  or within the piece.  The conduction within acts to set the temperature distribution of the piece from one end to the other.  The other two determine how much heat enters or leaves the piece.  See:

It should now be obvious that the main enabling factor for high supersonic,  or especially hypersonic,  flight is really thermal management,  more so even than propulsion.

And “scramjet propulsion”,  whether combined-cycle or not,   does not make your job any easier,  because it is geometrically incompatible with ramjet and gas turbine,  including even most of the inlet.  In fact,  combining any of these propulsive cycles,  including rocket,  is difficult at best,  because of the severe geometric incompatibilities,  not to mention the speed-of-application differences.  See:

The two that do combine well are rocket and ramjet,  for the “integral rocket ramjet” (IRR):

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