How Ramjets Work

Update 23 March 2024:  For the readers of this and other similar articles about ramjet propulsion,  be aware that GW’s ramjet book is finally available as a self-published item.  Its title is “A Practical Guide to Ramjet Propulsion”.  Right now,  contact GW at gwj5886@gmail.com to buy your copy. 

He will,  upon receipt of payment by surface mail or Western Union (or similar),  manually email the book to you as pdf files.  This will take place as 9 emails,  each with 3 files attached,  for a total of 27 files (1 for the up-front stuff,  1 each for 22 chapters,  and 1 each for 4 appendices).  The base price is $100,  to which $6.25 of Texas sales tax must be added,  for an invoice total of $106.25. 

This procedure will get replaced with a secure automated web site,  that can take credit cards,  and automatically send the book as files.  However,  that option is not yet available.  Watch this space for the announcement when it is.  

GW is working on a second edition.  No projections yet for when that will become available.

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A ramjet scoops up air,  decelerates it,  and increases its pressure by the pitot ram effect.  It then burns fuel with that air,  at the elevated pressure,  which accelerates its speed some.  Expanding that stream back to ambient pressure in the nozzle requires that elevated combustion pressure,  and produces a really fast exhausted stream.  That nozzle thrust less the ram drag of the ingested air is the “net jet thrust”.  That net jet thrust must exceed the airframe drag plus all the propulsion-related drags.  

Ramjets prefer critical-to-supercritical inlets to maximize captured airflow,  in order to maximize thrust.  For a ramjet,  subcritical spilled air is not only added drag,  it is also lost thrust.  Gas turbines,  on the other hand,  actually need a subcritical inlet in order to match the captured airflow to the throughflow demanded by the gas turbine at any given rotor speed and altitude.  I've got all that well described in detail in the "Fundamentals of Inlets" article dated 9 November 2020.  

The RJ-43 ramjets that pushed the old Bomarc missile used V-gutter stabilization.  The old Talos missile had a ramjet with an inverted can-type combustor,  sort of like the usual can-type combustors in gas turbines,  but reversed,  with radial flow outward instead of inward. 

Figure 1 – Basic Notions

For more details about any of this stuff,  plus related information,  please go look at some of my ramjet articles on "exrocketman".  There's a sort of catalogue article with many of the articles listed with titles and dates,  by topic area.  That one is "Lists of Some Articles By Topic Area",  dated 21 October 2021.  There is a navigation tool on the left side of the page.  Click on the year,  then the month,  then the title if need be.  That's how to get to the one you want,  the fastest.  Just jot down the titles and dates you want from the catalogue article,  then go right to them.  

Flameholding is a stable recirculation of partly-combusted hot gas behind some wake-producing feature,  where it mixes with fresh air and fuel,  reaching a mixed temperature high enough to cause fuel-air ignition.  That recirculating wake zone is required because the flow speed is far higher than the flame propagation speed.  This process has to happen continuously.  I have an article on "exrocketman" titled "Ramjet Flameholding" and dated 3 March 2020.  It's got a whole lot more detail.  

Too high a speed in the inlet diffuser air can literally blow out the recirculation flame like blowing out a match.  Too high a flow speed in the combustor can also blow it out.  Too low a diffuser air temperature,  too low a diffuser and combustor pressure (as at high altitude),  and too small a wake-creating feature,  reduces flame stability and can preclude ever getting any ignition at all.  If the flameholder goes out,  so does the whole engine!  These things are very configuration-specific,  and they vary from fuel to fuel,  too.  There are no general rules. 

It's bad enough in subsonic flow in a ramjet.  These flameholding things get really "iffy" in the supersonic flow inside a scramjet.  And yes,  flameholding indeed applies to scramjet,  until and unless the supersonic air static temperature greatly exceeds the fuel autoignition temperature. 

The same subsonic flameholding physics applies to gas turbine engine afterburners,  too.  Turbine combustor cans are a little different,  as up in the forward end,  mixture is rich and flow speed is less than flame propagation speed.  That's the flameholder.  About midway down,  mixture is leaning down near stoichiometric and flow speed is greater than flame speed.  The forward end is what pilots that mid-can burn.  The aft end of the can is not burning,  the hot gas is just being diluted down with excess air to a tolerable turbine inlet temperature.  The perforated can liner is air-cooled on its outside,  with air introduced into the interior through the pattern of holes.  Fuel injection is generally through the forward dome,  and the spark ignition to start it off,  is located fairly close by that forward dome.  

Figure 2 – About Flameholding

If you fly too fast,  the gas turbine combustor can liner melts,  because the captured and compressed air is so hot that it is no longer "cooling air".  Same goes for the compressor and turbine blading.  Most gas turbine engines are limited to about Mach 2.5 speeds.  There have been a few that reached Mach 3,  but those are extraordinary designs.  There was one that could reach Mach 3.5 in the Mig-25,  but it had a very short lifetime ~ 500 hours,  and you didn't overhaul it,  you replaced it entirely. 

That same overheated-air problem is why a ramjet (or a gas turbine afterburner) cannot use a simple V-gutter flame stabilizer above about Mach 3.5-ish.  Those are bathed in very hot flame on the downstream side,  and cooled by the inlet air on the upstream side.  Too fast,  and the inlet air is too hot to qualify as “cooling air” anymore.  I ran the numbers for a simple ramjet V-gutter flameholder at 40,000 feet altitude on a standard day,  with a 1-inch leg length for the stabilizing angle-type element.  Those numbers verify what I said about a top speed for any ramjet that uses V-gutter (or can-type) stabilization.  An insulated sudden dump flameholder is simply required to fly Mach 4+.

Figure 3 – Overheat Limits on V-Gutter

Update 12-6-2022:  

I forgot to discuss the heat protection scheme for the combustor and nozzle of a ramjet (again similar to a gas turbine afterburner,  if you look at air-cooled technologies). Because of the high-speed "it's not cooling air anymore" problem,  the air cooling technologies common in afterburners and early ramjets cannot be used for flight speeds in the stratosphere above about Mach 3 to 3.5.  The captured air is simply too hot to serve that cooling function.  At lower altitudes,  that speed limit is even lower.

For flight at Mach 3.5+,  you simply have to use ablative technologies!  The fiber-reinforced rubber compounds that serve well in solid rockets are inadequate for that function in ramjets.  The exposure times,  and the mass fluxes that drive heating by scrubbing action,  are far too severe in the ramjet.  The strength and durability of the insulation needs to be roughly factor-10 better than the rubber-based rocket insulations.

Of the rubber compounds,  silicone rubber (specifically PDMS,  poly dimethyl silicone),  has the higher charring temperature.  It is still silicone at 600-700 F,  and is charring to carbon at around 1000 F.  It forms a carbon char layer,  but that layer sheds too easily,  and fractures too easily.  However,  if you add short carbon fiber to the formulation,  the char layer is a little stronger,  and adheres to the virgin rubber very much better.  

Adding silica powder to the formulation produces a sticky viscous silica melt that further strengthens the char layer against fracture,  and by a very significant amount.  Adding silicon carbide powder to the formulation strengthens both the virgin rubber and the char layer by the solid aggregate effect,  similar to the stones in concrete.  That silicon carbide aggregate does not melt at ramjet flame temperatures,  although it probably would in solid rockets.   

The result of all these additions is a char layer that resists fracture from applied air forces,  and that resists being sheared off the virgin material beneath.  That char layer has a little bit higher thermal conductivity than the virgin material,  but it still has an insulating effect.  If you also mechanically retain it,  you can continue to use it as insulation,  even when the virgin material is gone. 

There are two materials available,  that are formulated like this.  One is Dow Corning's DC 93-104 material.  The other is "Type 0" from Shin Etsu Silicones.  Being intended as military products,  neither is advertised to the public,  although you can buy them.  I have tested them both in full scale engine hardware as combustor insulators.  They perform quite similarly to each other.  Impregnating carbon cloth with PDMS polymer does not perform as well,  because it lacks the strengthening effects of the silica and silicon carbide powders.   I have tested that,  too. 

The shearing effect of the mass flux scrubbing is higher in the ramjet nozzle (same massflow,  smaller flow cross section area),  but not as high as in the solid rocket.  It is not necessary to use graphite throat inserts in the ramjet nozzle,  the way it is required in solid rocket nozzles.  The approach to the nozzle throat and the supersonic expansion cone,  may be silica phenolic ablative,  in both the ramjet and the rocket.  Those choices are well-tested,  and they work.  The ramjet nozzle can be a monolithic chunk of silica phenolic.  Correct fiber orientation is important to getting best performance out of it.   

This ablative construction was adequate for a long stratospheric cruise burn at Mach 4 in the ASALM design in ground testing and in flight test,  and it survived all the way up to Mach 6 on a short transient,  in one ASALM-PTV flight test.  This is just how you have to build them,  if you want to fly faster than about Mach 3 to 3.5.